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Laser cladding using filler powder and wire
Productivity and quality
Bernardo Miguel Fonseca da Costa Alves Borges
Dissertation submitted for obtaining the degree of Master in
Mechanical Engineering
Jury
President: Prof. Dr. Pedro Miguel dos Santos Vilaça Silva
Supervisor: Prof. Dr. Maria Luísa Coutinho Gomes de Almeida
Co-supervisor: Prof. Dr. Rosa Maria Mendes Miranda
Member: Dr. Nuno Miguel Carvalho Pedrosa
September 2008
To my dear family
“Yesterday's dream, today's concept, tomorrow's reality”
Robert H. Goddard
I
Acknowledgements
First of all, I would like to thank to Professor Luísa Coutinho for the unique opportunity to
perform this thesis and for the constant knowledge and experience sharing. Her orientation,
availability, guidelines, advising, opinion, and constant support, were a key factor for the
completion of this work, and will also be useful for my professional future.
In a special way, to Professor Rosa Miranda for all the suggestions, constructive critics,
technical support and advice, her precious knowledge about materials’ science and for having
the time to answer all my doubts with enormous patience.
To Phill Carrs, for the opportunity to perform technical work in his company, Carrs Welding
Technology Lda, and for his hospitality. I also would like to thank the help of the company’s
employees during the tests.
An honest appreciation to Professor Inês Pires for the accessibility to the metallographic
laboratory.
To Professor Pedro Rosa for the possibility of using important equipment to perform this work.
I would like to express my appreciation to my dear friend João Lopes for all the encouragement
during the development of this work and for the time spent in many writing suggestions and
revisions.
I also want to thank to my colleague Luís Pinto for his friendship, useful suggestions, different
points of view, and the never ending discussions that allowed me to better understand several
concepts. In a more personal level, I want to thank him for the good company, and for giving me
strength that was essential for the completion of this journey.
A sincere thanks to my friends André Cabrita and Miguel Castilho for the special friendship and
a few funny moments provided during this work.
Finally, I also would like to thank to my family, and especially to my mother and brothers, for all
the support, caring, motivation and understanding throughout this last year. Their love and care
kept me going in the hardest times.
II
Resumo
A elevada competitividade e a constante evolução do mercado onde é aplicado o processo de
revestimento por laser, obriga a que os revestimentos obtidos por este processo atinjam uma
boa produtividade e qualidade. O objectivo principal deste trabalho foi comparar estes dois
factores quando são usadas diferentes formas de alimentação de material, pó ou fio, em
revestimentos laser. O trabalho incidiu sobre casos específicos seleccionados por uma
empresa inglesa que opera neste sector. Considerou-se como medida de produtividade a taxa
de deposição de material. Para a análise da qualidade definiu-se um factor que considera
diferentes parâmetros de qualidade, como sejam: o número de defeitos, a diluição, a forma de
molhagem, os ângulos e a razão largura/altura do depósito. Durante a realização deste
trabalho foram feitas experiências em diferentes materiais para se atingir o objectivo proposto.
A utilização de diferentes materiais neste trabalho originou a possibilidade de estabelecer uma
conclusão geral do processo de revestimento laser usando alimentação pó e fio,
independentemente do material usado. Analisaram-se os problemas obtidos e propuseram-se
metodologias a implementar para minimização ou eliminação dos mesmos.
Palavras-chave
Revestimentos laser
Material de adição em pó
Material de adição em fio
Produtividade
Qualidade
III
Abstract
The competition and constant evolution of the laser cladding market, as in any other industry
market, demands that a high productivity and quality of coatings are to be achieved. The main
goal of this work is to compare these two factors when laser cladding is used with different filler
processes: powder and wire. This work is focused on specific cases that characterise the type
of work in this sector. The material deposition rate was considered as a productivity indicator. A
factor to analyse the quality of the coatings is proposed, where the number of defects, dilution,
melting shape, clad angles and clad’s width-to-height ratio are considered. Experiments were
performed in an industrial environment with different materials to cover a broad set of different
cases and achieve a sound comparison of results. The use of different materials allowed to lead
a general conclusion about the laser cladding process used, powder or wire, independently of
the material used. Problems resulting from the laser cladding process are analysed and a
methodology for its avoidance or elimination.
Key-Words
Laser cladding
Powder filler
Wire filler
Productivity
Quality
IV
Contents
Acknowledgements ........................................................................................................................ I
Resumo ......................................................................................................................................... II
Palavras-chave .............................................................................................................................. II
Abstract ........................................................................................................................................ III
Key-Words .................................................................................................................................... III
Contents ....................................................................................................................................... IV
List of figures ................................................................................................................................ VI
List of tables ............................................................................................................................... VIII
List of abbreviations ..................................................................................................................... IX
1 Introduction............................................................................................................................. 1
1.1 Overview ........................................................................................................................ 1
1.2 Motivation and objectives .............................................................................................. 2
1.3 Thesis structure ............................................................................................................. 2
2 Literature survey .................................................................................................................... 3
2.1 Laser cladding ............................................................................................................... 3
2.1.1 Introduction ................................................................................................................ 3
2.1.2 Laser cladding ........................................................................................................... 3
2.1.3 Nd:YAG laser ............................................................................................................. 4
2.1.4 Pulsed mode .............................................................................................................. 5
2.1.5 Pulsed laser cladding and advantages...................................................................... 6
2.1.6 Powder feeding .......................................................................................................... 6
2.1.7 Parameters of feeding powder in continuous flow .................................................... 7
2.1.8 Advantages of powder feeding .................................................................................. 7
2.1.9 Wire feeding and its advantages/drawbacks ............................................................. 7
2.1.10 Parameters of wire feeding ................................................................................... 8
2.1.11 Wire feeding direction ............................................................................................ 8
2.1.12 Other modes of coating ......................................................................................... 9
2.1.13 Applications ......................................................................................................... 10
2.2 316 Stainless steel ...................................................................................................... 10
2.2.1 Introduction .............................................................................................................. 10
2.2.2 Stainless steel 316 .................................................................................................. 11
2.2.3 Physical and mechanical properties ........................................................................ 11
2.2.4 Application ............................................................................................................... 13
2.2.5 Cladding of stainless steel ....................................................................................... 13
2.3 H13 tool steel and AISI P20 tool steel ......................................................................... 15
2.3.1 Introduction .............................................................................................................. 15
V
2.3.2 H13 tool steel ........................................................................................................... 15
2.3.3 AISI P20 tool steel ................................................................................................... 17
2.3.4 Applications of tool steels ........................................................................................ 19
2.3.5 Cladding of tool steels ............................................................................................. 20
2.4 Summary of the literature survey ................................................................................ 21
3 Experimental procedure ....................................................................................................... 22
3.1 Equipment ................................................................................................................... 22
3.1.1 Powder deposits ...................................................................................................... 22
3.1.2 Wire deposits ........................................................................................................... 23
3.2 Experimental approach ............................................................................................... 24
3.2.1 Description ............................................................................................................... 24
3.2.2 Material .................................................................................................................... 26
3.2.3 Parameters .............................................................................................................. 27
3.2.4 Samples preparation ............................................................................................... 28
3.2.5 Macro graphic and microstructure analysis ............................................................. 29
3.2.6 Dilution calculation ................................................................................................... 30
3.2.7 Vickers hardness tests ............................................................................................ 30
4 Results and discussions ....................................................................................................... 32
4.1 Productivity analysis .................................................................................................... 32
4.1.1 Material deposition rate ........................................................................................... 32
4.2 Structural analysis of the clads .................................................................................... 35
4.2.1 Defects analysis ...................................................................................................... 35
4.2.2 Microstructural analysis ........................................................................................... 43
4.2.3 Dilution ..................................................................................................................... 46
4.2.4 Melting and clad shape............................................................................................ 48
4.2.5 Hardness analysis ................................................................................................... 53
4.3 Quality analysis ........................................................................................................... 55
4.3.1 Quality factor expression ......................................................................................... 55
4.3.2 Factors ..................................................................................................................... 57
4.3.3 Results and discussions .......................................................................................... 59
4.4 Theoretical powder rate ............................................................................................... 60
5 Conclusion and future developments ................................................................................... 65
5.1 Conclusion ................................................................................................................... 65
5.2 Future developments ................................................................................................... 67
6 References ........................................................................................................................... 68
VI
List of figures
Figure 2-1 – Typical laser cladding equipment [4]. ....................................................................... 3
Figure 2-2 – Laser cladding using filler powder. ........................................................................... 6
Figure 2-3 – The best direction of wire feeding [9]. ....................................................................... 8
Figure 2-4 – Clad layer surface when wire is correctly fed [9]. ..................................................... 9
Figure 2-5 – Pseudobinary section of the Fe-Cr-Ni system at 70% iron. .................................... 12
Figure 2-6 – CCT diagram of H13 tool steel. .............................................................................. 17
Figure 2-7 – CCT diagram of AISI P20 tool steel. ....................................................................... 19
Figure 3-1 – Carrs Welding Technology Ltd Company. .............................................................. 22
Figure 3-2 – Robot with a continuous wave laser used for powder deposits. ............................. 22
Figure 3-3 – Nozzle….. ............................................................................................................... 23
Figure 3-4 – Detailed nozzle. ...................................................................................................... 23
Figure 3-5 – The working station [4]..... ....................................................................................... 24
Figure 3-6 – Wire deposits…………….. ...................................................................................... 24
Figure 3-7 – First experience. ..................................................................................................... 24
Figure 3-8 – Lay-out (top and side views). .................................................................................. 25
Figure 3-9 – Cutting plan. ............................................................................................................ 28
Figure 3-10 – Schematic representation of the procedures adopted to calculate dilution [48]. .. 30
Figure 3-11 – Representation of the hardness line in cross-section of the coating. ................... 30
Figure 4-1 – 316 stainless steel, powder, 1 layer..… .................................................................. 36
Figure 4-2 – 316 stainless steel, wire, 1 layer……… .................................................................. 36
Figure 4-3 – AISI P20 tool steel, powder, 1 layer…… ................................................................ 36
Figure 4-4 – AISI P20 tool steel, powder, 3 layers…. ................................................................. 36
Figure 4-5 – H13 tool steel, powder, 1 layer..... .......................................................................... 37
Figure 4-6 – AISI P20 tool steel, powder, 2 layers. ..................................................................... 37
Figure 4-7 – 316 stainless steel, wire, 2 layers….. ..................................................................... 37
Figure 4-8 – H13 tool steel, wire, 1 layer............... ..................................................................... 37
Figure 4-9 – 316 stainless steel, powder, 2 layers……..... ......................................................... 38
Figure 4-10 – H13 tool steel, powder, 2 layers…………... .......................................................... 38
Figure 4-11 – 316 stainless steel, powder, 3 layers. ................................................................... 38
Figure 4-12 – H13 tool steel, powder, 3 layers. .......................................................................... 39
Figure 4-13 – AISI P20 tool steel, wire, 3 layers. ........................................................................ 40
Figure 4-14 – AISI P20 tool steel, wire, 1 layer..... ...................................................................... 40
Figure 4-15 – AISI P20 tool steel, wire, 2 layers (coating) .......................................................... 40
Figure 4-16 – Schaeffler diagram [29]. ........................................................................................ 41
Figure 4-17 – 316 stainless steel, wire, 3 layers. ........................................................................ 42
Figure 4-18 – H13 tool steel, wire, 2 layers. ................................................................................ 42
Figure 4-19 – H13 tool steel, wire, 3 layers. ................................................................................ 43
VII
Figure 4-20 – 316 stainless steel, powder, 3 layers…... ............................................................. 43
Figure 4-21 – 316 stainless steel, powder, 3 layers (coating) ..................................................... 43
Figure 4-22 – 316 stainless steel, wire, 1 layer..… ..................................................................... 44
Figure 4-23 – 316 stainless steel, wire, 1 layer (coating) ............................................................ 44
Figure 4-24 – H13 tool steel, powder, 2 layers..... ...................................................................... 44
Figure 4-25 – H13 tool steel, powder, 2 layers (coating) ............................................................ 44
Figure 4-26 – H13 tool steel, wire, 2 layers...….….. ................................................................... 45
Figure 4-27 – H13 tool steel, wire, 2 layers (coating).................................................................. 45
Figure 4-28 – AISI P20 tool steel, powder, 3 layers.. .................................................................. 45
Figure 4-29 – AISI P20 tool steel, powder, 3 layers .................................................................... 45
Figure 4-30 – AISI P20 tool steel, wire, 2 layers..... .................................................................... 46
Figure 4-31 – AISI P20 tool steel, wire, 2 layers (coating) .......................................................... 46
Figure 4-32 – Dilution in the 316 Stainless steel..... .................................................................... 46
Figure 4-33 – Dilution in the H13 tool steel………… ................................................................... 46
Figure 4-34 – Dilution in the AISI P20 tool steel. ........................................................................ 47
Figure 4-35 – Schematic diagram to show the geometrical parameters of clad shape. ............. 48
Figure 4-36 – 316 stainless steel, powder, 1 layer…….. ............................................................ 48
Figure 4-37 – 316 stainless steel, wire, 1 layer……….. .............................................................. 48
Figure 4-38 – 316 stainless steel, powder, 2 layers...... .............................................................. 49
Figure 4-39 – 316 stainless steel, powder, 3 layers. ................................................................... 49
Figure 4-40 – H13 tool steel, powder, 2 layers..... ...................................................................... 49
Figure 4-41 – H13 tool steel, wire, 2 layers……… ...................................................................... 49
Figure 4-42 – AISI P20 tool steel, powder, 3 layers.. .................................................................. 49
Figure 4-43 – AISI P20 tool steel, wire, 3 layers. ........................................................................ 49
Figure 4-44 – Diagram showing a possible mechanical stress situation. ................................... 52
Figure 4-45 – Hardness in 316 stainless steel…... ..................................................................... 54
Figure 4-46 – Hardness in H13 tool steel..........… ...................................................................... 54
Figure 4-47 – Hardness in AISI P20 tool steel. ........................................................................... 54
Figure 4-48 – The QF in the 316 stainless steel..... .................................................................... 59
Figure 4-49 – The QF in the H13 tool steel….......... ................................................................... 59
Figure 4-50 – The QF in the AISI P20 tool steel. ........................................................................ 59
Figure 4-51– Layout of the clad and the substrate material. ....................................................... 61
Figure 4-52 – Equivalent electrical circuit. .................................................................................. 62
VIII
List of tables
Table 2-1 – Physical and optical properties of Nd:YAG [14]. ........................................................ 5
Table 2-2 – Chemical composition of the 316 stainless steel, in percentage. ............................ 11
Table 2-3 – Physical/thermal properties of 316 stainless steel [31]. ........................................... 13
Table 2-4 – Mechanical properties of 316 stainless steel [30]. ................................................... 13
Table 2-5 – Chemical composition of H13 tool steel, in percentage [28]. ................................... 15
Table 2-6 – Physical/thermal properties of H13 tool steel [31]. .................................................. 17
Table 2-7 – Mechanical properties of H13 tool steel [31]. ........................................................... 17
Table 2-8 – Chemical composition of AISI P20 tool steel, in percentage [28]. ........................... 18
Table 2-9 – Tempering of AISI P20 tool steel. ............................................................................ 18
Table 2-10 – Physical/thermal properties of AISI P20 tool steel [31]. ......................................... 19
Table 2-11 – Mechanical properties of AISI P20 tool steel [31]. ................................................. 19
Table 3-1 – The most important characteristics of HP 124 P laser. ............................................ 23
Table 3-2 – Chemical composition of substrate and coating material (316 stainless steel), in
percentage. .................................................................................................................................. 26
Table 3-3 – Chemical composition of substrate and coating material (H13 tool steel), in
percentage. .................................................................................................................................. 26
Table 3-4 – Chemical composition of substrate and coating material (AISI P20 tool steel), in
percentage. .................................................................................................................................. 27
Table 3-5 – Powder parameters. ................................................................................................. 27
Table 3-6 – Wire parameters. ...................................................................................................... 28
Table 3-7 – Mechanical grinding, polishing and etching in each material [47]. .......................... 29
Table 4-1 – Deposition rate of powder and wire, in each substrate material (1 layer). ............... 32
Table 4-2 – Defects in 316 stainless steel clads. ........................................................................ 35
Table 4-3 – Defects in H13 tool steel clads. ................................................................................ 35
Table 4-4 – Defects in AISI P20 tool steel clads. ........................................................................ 36
Table 4-5 – The melting capacity of the powder and wire filler material for each substrate
material. ....................................................................................................................................... 41
Table 4-6 – Geometrical parameters of powder clad. ................................................................. 50
Table 4-7 – Geometrical parameters of wire clad. ...................................................................... 51
Table 4-8 – Stresses ratio between the two processes. ............................................................. 53
Table 4-9 – Weights for each factor. ........................................................................................... 56
Table 4-10 – The NP and NC values associated to the number of defects. ................................. 57
Table 4-11 – Values of the dilution factor (D). ............................................................................. 57
Table 4-12 – The RW/H values. ..................................................................................................... 58
Table 4-13 – The values of theoretical powder rate in each substrate material. ........................ 63
IX
List of abbreviations
A – Coating’s area
Al – Aluminium
ASurface – Heat transfer area of the surface
Ar – Argon
B – Area of the mixing between filler and substrate material
c – Specific heat capacity
C – Carbon
CA – Clad’s angles factor
CCT – Continuous Cooling Transformation
CO2 – Carbon dioxide
Cr – Chromium
Cu – Copper
CVD – Chemical Vapour Deposition
CW – Continuous Wave
D – Dilution factor
Eg – Generated energy
Ei – Input energy
EL – Lost energy
Es – Stored energy
f – Load applied during the hardness test
F – Force
FCAW – Flux Cored Arc Welding
Fe – Iron
GMAW – Gas Metal Arc Welding
h – Convective heat transfer coefficient
hf – Heat of fusion
H – Height of the clad
H* – Coating parallelepiped’s height
HAZ – Heat Affected Zone
HB – Hardness Brinell
– Powder clad’s height
HRC – Hardness Rockwell C
HS – Height of the substrate material
HV – Hardness Vickers
– Wire clad’s height
k – Thermal conductivity of the material
Kr – Krypton
L – Length of the run
X
m – Mass
M – Moment
MC – Melting capacity
Mo – Molybdenum
MS – Melting shape factor
MZ – Melting zone
Nd:YAG – Neodymium Doped Yttrium Aluminium Garnet
N – Nitrogen
Nb – Niobium
NC – Factor associated to the number of cracks and/or lacks of adhesion in the coatings
Ni – Nickel
NP – Factor associated to the number of pores and/or lacks of fusion in the coatings
P – Laser power
– Powder rate
– Real powder rate
– Theoretical powder rate
PVD – Physical Vapour Deposition
qConduction – Total rate of heat transfer by conduction
qConvection – Total rate of heat transfer by convection
qRadiation – Total rate of heat transfer by radiation
– Material deposition rate
QF – Quality factor
R – Cooling speed
R1, R2 and R3 – Resistances
RW/H – Clad’s width-to-height ratio factor
S – Sulphur
SAS – Submerged Arc Surfacing
Se – Selenium
Si – Silicon
t – Period of time taken to make a run with a specific length
tf – Fusion time
tSol. – Solidification time
T0 – Initial temperature of the material
TAir – Temperature of the air
Tc – Critical temperature
Ti – Titanium
TIG – Tungsten Inert Gas
TLiquidus – Temperature which starts the solidification process
TS – Substrate’s temperature
TSolidus – Temperature which finishes the solidification process
XI
TSurface – Temperature of surface’s coating
V – Vanadium
V – Linear speed
Vol. – Volume of the deposit
W – Tungsten
W – Width’s clad
– Wire rate
– Powder clad’s width
– Wire clad’s width
Xe – Xenon
Z – Section modulus
α1 and α2 – Clad angles
ΔT1 – Thermal interval achieved during the laser power application
ΔT2 – Solidification interval in steady state conditions
η – Laser’s output power efficiency
ρ – Density
ε – Emissivity;
ζ – Normal stress
– Normal stress in powder coatings
ζS-B – Stefan-Boltzmann constant
– Normal stress in wire coatings
ζ – Shear stress
– Shear stress in powder coatings
– Shear stress in wire coatings
1
1 Introduction
1.1 Overview
The invention of the first working laser by Maiman in the 1960’s was a breakthrough in science.
Immediately after this invention, scientists claimed that the laser was the answer to a multitude
of scientific problems that had not been even known before. One of the areas that benefited
from the lasers technology was material processing, which was rapidly developed in the 1970’s
when the efficiencies and power of commercial lasers increased. Among the laser material
processing technologies, laser cladding was used by Gnanamuthu at Rockwell International
Corporation in Thousand Oaks of California in the late 1970’s. At the same time, several
research groups around the world started projects to develop apparatus and systems for
development and improvement of the process. Among these groups at Imperial College of
University of London and at Liverpool University, the projects had a great impact on the
development of laser cladding technology, introducing laser cladding by powder injection to
academia and conducting a number of projects to evaluate the developed process [1].
A review of the literature shows that the number of papers and patents related to this
technology increased significantly in the 1980’s. The features of this technology received
attention from industry in the 1980’s as well. Laser cladding was identified as a process with a
significant edge over the conventional processes for wear and corrosion resistant coatings. The
technology was being accepted by leading engine manufactures such as General Electric, Pratt
& Whitney, Allied Signal, Rolls-Royce, Allison, Solar and MTU [1].
In the automotive industry, laser cladding technology was transferred to the market for the
engine valve seat coating by some European and Asian automotive companies, such as Fiat,
Toyota and Mercedes Benz [1].
In the components repair market, laser cladding brought a huge amount of consideration in the
1980’s [1].
Another application of laser cladding, rapid prototyping or layered manufacturing, received a
great deal of attention in the 1990’s and continues to be exploited in the new millennium. Many
researchers and development groups initiated projects to develop methods for prototyping
metallic parts based on laser cladding by powder injection [1].
In the last decade, laser cladding has been applied in many industrial sectors, such as moulds
repair, automotive and nuclear power stations. It has also been growing due to its good features
and it has been substituting many conventional processes, such as TIG (Tungsten Inert Gas)
and SAS (Submerged arc surfacing).
2
1.2 Motivation and objectives
Laser cladding is a process with several important features when comparing with others
conventional cladding processes. This is the reason why laser cladding has became so
important in many applications of different industries, leading to the need of understanding the
performance of this process in different materials. The market where laser cladding is applied,
as any other industry market, it is a very competitive market and is in permanent evolution,
therefore it is essential to assume a high productivity and quality in cladded products.
Laser cladding can be done with filler material in two different forms: powder or wire. It is known
that higher productivities can be achieved with powder, but this process is more prove to
problems than wire cladding.
The aim of the present work is to compare the productivity and the quality of the laser cladding
process using powder and wire filler material and to present methodologies to improve the
results obtained with both variants.
1.3 Thesis structure
This work was structured in five chapters. The first chapter presents the overview, the
motivation and the objectives of this work.
In the second chapter is described the state of the art regarding laser cladding using filler
powder and wire in different materials. Also in this chapter a small theoretical background,
which will help in the experimental activities carried out in this work, is presented. This chapter
starts with an overview of laser cladding process and continues with the main properties of the
materials used and the performance of these materials for laser cladding, as well as, for other
cladding processes.
The third chapter addresses the experimental procedure, which will focus on the equipment
used in the experiments and the different types of materials employed.
In chapter five the analysis of the results and its discussion are carried out.
Finally, in chapter six, the overall conclusions and future research finalises this work.
3
2 Literature survey
2.1 Laser cladding
2.1.1 Introduction
The word laser is an acronym for “light amplification by the stimulated emission of radiation” [2].
Laser light possesses several unique properties, among which are the facts that it is parallel,
highly concentrated (high energy densities), monochromatic, time and spacial coherence and
has low divergence. It can therefore be conducted, by mirrors or glass fibres, to a welding
position that is remote from the power unit [3].
Figure 2-1 – Typical laser cladding equipment [4].
Lasers have been promoted as potentially useful welding tools for a variety of applications. Until
the 1970s, however, laser welding had been restricted to relatively thin materials and low
speeds because of the limited continuous power available. By 1965, a variety of laser systems
had been developed for making micro welds in electronic circuit boards, inside vacuum tubes,
and in other specialized applications where conventional technology was unable to provide
reliable joining. The availability of high-power continuous-wave (CW) carbon dioxide (CO2) and
neodymium-doped yttrium aluminium garnet (Nd:YAG) lasers and the limitations of current
welding technology have promoted interest in deep penetration welding in the past 20 years
using these devices [5].
2.1.2 Laser cladding
Laser cladding is a laser surfacing process in which the objective is to cover a particular part of
the substrate (base material) with material which has superior properties, producing a fusion
bond between the two with minimal mixing (dilution) of the clad by the substrate, in other words,
4
the cladding process consists of obtaining an homogeneous surface layer with a strong
metallurgical bond to the substrate and only a low degree of dilution [6-8].
The process has received a lot of attention over the years and is now applied commercially in a
range of industries such as the automotive, mining and aerospace [8].
Laser cladding is used in two respects: production of parts of composite materials and repair of
worn parts [9]. In the production of parts of composite materials, this technique is used to
produce hard, wear-resistant and/or corrosion-resistant surface layers [10]. Among the different
surface treatments used to improve the corrosion and wear resistance of metallic materials,
laser cladding is an attractive alternative to conventional techniques due to the intrinsic
properties of laser radiation: high input energy, low distortion, avoidance of undesirable phase
transformations and minimum dilution between the substrate and the coating. Furthermore, the
advantages of laser cladding include great processing flexibility and the possibility of selectively
cladding small areas [11]. These advantages not only result in better quality products but also
offer significant economic benefits [12].
Repair by cladding is a common and standard practice in the die and mould industries, where
the life of loaded die elements and vital tool parts can be successfully extended by the timely
repair of damaged surfaces. The main advantages of repair using the cladding procedure are
well known: a short down-time and economic advantage compared to machining a new tool or
die part [13].
2.1.3 Nd:YAG laser
The most common types of welding lasers are the CO2 laser (gas-state) and the Nd:YAG laser
(solid-state), with the latter tending to be used for thinner materials and the former for thicker.
The laser beam may be either pulsed or continuously produced [3].
The Nd:YAG laser is by far the most commonly used solid-state laser [14]. The active substance
in this laser is neodymium (Nd), in the form of a doping agent in a transparent rod of yttrium
aluminium garnet (YAG) [3]. A doping agent is an impurity element added to a crystal or
semiconductor lattice in low concentrations in order to alter the optical/electrical properties of
the semiconductor [15]. Energy is supplied by a flash tube, of the same principal as used in
cameras. The light output wavelength is 1.06 µm, i.e. considerably shorter than that of the CO2
laser, but still within the invisible infra-red section of the spectrum. An important difference is
that the shorter wavelength enables the light to be carried by optical fibres and focused with
normal lenses. This gives substantial practical benefits and makes it possible to use the laser
for robot welding. Additionally metallic materials absorb more efficiently short wavelength
radiation.
Problems due to the presence of absorbing plasma are less when welding with Nd:YAG lasers,
and so argon and argon/CO2 gas mixtures can be used. Acceptable results can even be
obtained without shielding gas when welding spot welds or at low powers.
This type of laser is particularly suitable for welding difficult materials, such as tantalum (Ta),
titanium (Ti), zirconium (Zr), Inconel, and so on. A disadvantage is that it is not available with
5
such high power outputs as is the CO2 laser, and so tends to be limited to metal thicknesses up
to 6 mm. However, development is increasing the available outputs and in combination with the
ability to use optical fibres light conductors, makes this type of laser potentially very
attractive [3]. Some physical and optical properties of Nd:YAG are in Table 2-1:
Chemical formula Nd:Y3Al5O12
Weight % Nd 0.725
Atomic % Nd 1.0
Nd atoms/cm3 1.38 × 10
20
Melting point 1970ºC
Mechanical hardness 1320 kg/mm2
Density 4.56 g/cm3
Tensile strength 200 MPa
Modulus of elasticity 310 GPa
Poisson ratio 0.30
Thermal expansion coefficient
[100] orientation
[110] orientation
[111] orientation
8.2 × 10−6
/ºC
7.7 × 10−6
/ºC
7.8 × 10−6
/ºC
Linewidth 120 GHz
Fluorescence lifetime 230 μs
Photon energy at 1.06 μm = 1.86 × 10−19
J
Index of refraction 1.82 (at 1.0 μm)
Table 2-1 – Physical and optical properties of Nd:YAG [14].
2.1.4 Pulsed mode
The Nd:YAG laser absorbs light energy in the 0.81 µm region to produce the 1.06 µm laser
output. For pulsed Nd:YAG lasers, the flash lamps are specifically designed for the typical
repetitive high-peak-current electrical pulses that create the laser pulses. For Nd:YAG lasers the
typical gas fill for the lamps is krypton (Kr) or xenon (Xe). Both Kr and Xe lamps produce a
typical blackbody light output when pulsed but with a peak of light energy that covers the
0.81 µm pump band of the laser. This factor improves the laser’s efficiency. The choice of gas
fill is determined by their output of light in the pump bands for the peak currents delivered by the
power supply.
Because of the high peak currents in the lamp during a pulse, these flash lamps have special
design features to improve their reliability and life. Wall thickness is optimized for the
high-pressure spikes; the electrodes are shaped for repeatable arc production; and the mass
and the placement of the electrodes is optimized for minimal thermal stresses where the metal
electrode is sealed to the glass envelope. The gas fill pressure and the lamp’s internal bore
diameter must be carefully engineered to match the impedance of the lamp to the impedance of
the power supply to get maximum power transfer of electrical energy to the lamp with minimal
reflected electrical energy (ringing). The electrodes, especially the cathode, are designed with
specific shapes and alloy contents to optimize their operation and minimize sputtering of metal
from their surfaces during operation. Great care is taken during manufacturing to eliminate
impurities in all components [16].
6
2.1.5 Pulsed laser cladding and advantages
The laser used for laser cladding is normally a continuous wave (CW) laser, such as the CO2
and Nd:YAG. However, contrary to the continuous wave laser cladding, the pulsed laser
cladding offers a number of advantages for the repair of turbine blades. While heat build up
during cladding with a continuous wave laser is relatively low compared to other conventional
processes, it can be too high in some situations leading to the undesirable effects of high
dilution and cracking of the layer. Pulsed laser cladding is one possible solution to this problem,
offering significantly lower heat build up in the workpiece and therefore lower heat-affected
zone, dilution and tendency to crack. The laser power-off period between two pulses allows the
melt pool to solidify; therefore, the cooling rate is faster in pulsed laser cladding. Hence, the
microstructure produced by pulsed laser cladding is more refined. Another advantage is the fact
that the erosion resistance of the clad produced by a pulsed laser can be superior to that
produced by continuous laser, since the erosion resistance is dependent on the
microstructure [8]. In this project, the wire coatings were done with the pulsed laser.
2.1.6 Powder feeding
Since Rolls Royce first used the laser cladding process in 1981 showing the feasibility of the
technique, the processes of surface treatment involving powder addition has undergone
numerous improvements [6].
During the laser cladding process, the laser beam has to melt the powder introduced and a thin
layer of the substrate surface at the same time [17].
Figure 2-2 – Laser cladding using filler powder.
Different kinds of powders can be used, whatever the desired application. If we want to increase
hardness, wear or corrosion properties, we have to incorporate different powders [6].
These treatments can be done essentially in two ways: a powder is preplaced in the surface
and after that melted by the laser beam or a continuous flow of powder is sent, carried by a gas,
to the surface of the workpiece and at the same time the melting process takes place [18]. In
7
this last mode, Argon (Ar) is often used as the carrier gas that also shrouds the melt pool region
and prevents oxidation [19]. Blown powder feed technique for laser cladding can produce high
quality overlays with controlled dilution [20]. This was the method used in this work.
This process can produce thin surface layers of about 1.0 mm with low energy input [21].
2.1.7 Parameters of feeding powder in continuous flow
The optimization of the process requires the measurement and the control of some parameters
such as powder feed rate, process speed, laser power, beam diameter or even melting pool
temperature [6].
Research to date indicates that the particle critical bonding velocity is sensitive to multiple
variables, including not only the type of spray material, but also the powder quality, the particle
size and the impact temperature [22]. The powders used in this work are certificated by
ISO9001.
An increase in the speed processing would lead to a decrease in the surface temperature and
the tracks would not correctly bind to the surface. Inversely, for a slower speed processing the
surface would reach higher temperature, leading to a deeper penetration thus a higher dilution
and lower mechanical properties [6]. It was used a value of speed processing that avoids these
problems.
In the laser cladding process, larger powders, for instance 140-mesh powder, need more heat
for melting, which means that an increasing laser power or decreasing travelling velocity is
needed. Smaller particle-sized powders, for instance 300-mesh powder, are easy to melt and
when the particle size is smaller than 400-mesh, the fluidity of the powder during powder
feeding will be poor, so the feeding process will cause difficulties in the laser cladding
process [17]. Small particle-sized powders (250-mesh powder) are used in this project.
2.1.8 Advantages of powder feeding
Laser cladding with metal powder feeding has the following potential advantages: low dilution
and limited heat effects on the base metal, metallurgical bonding, and minimum distortion of the
components, low cracking susceptibility and suitable automation [7].
2.1.9 Wire feeding and its advantages/drawbacks
Compared to laser cladding by powder feeding, the laser cladding by wire feeding has some
special advantages. Metal wires are cheaper than metal powders or ribbons and wire feeding
wastes less material than powder feeding. It is a suitable method for automatic production. One
of the most important advantages for wire feeding is its adaptation to the cladding position. For
example, the cladding of inner wall of a tube can be realized by wire feeding. Drop transfer is a
problem of wire cladding. The liquid melted at the end of the wire did not flow smoothly and
continuously onto the workpiece. In cladding process with filler wire, satisfactory transfer occurs
8
when the fed wire is adapted to an impingement angle of 30º and positioned to the exact edge
of the molten pool [9]. This situation was verified in this work.
2.1.10 Parameters of wire feeding
Experimental results indicate that the wire feeding direction and position are important for wire
laser cladding. By adopting correct wire feeding direction and position, the difficulties in the
melting of fed wire and the transferring of molten drop for laser cladding with wire feeding can
be solved. In this case, wire can be plunged into the melt pool and be melted by the heat of the
molten metal.
Dilution is related to the cladding speed and cladding time. With the increase of cladding speed,
the dilution of clad layer can be reduced and the growth of the grain size of heat-affected zone
(HAZ) of the base metal can be limited. With the decrease of the cladding speed or increase of
the cladding time, the dilution of clad layer increases.
If proper cladding parameters are used, clad layers have good surface shape, sound
metallurgical bonding with the base metal, low dilution, and the effect of laser heating on
heat-affected zone (HAZ) can be limited.
2.1.11 Wire feeding direction
For the wire feeding direction of Figure 2-3, clad layer does not disturb the wire feeding, and
wire can be melted completely even though the wire feeding speed is high [9].
Figure 2-3 – The best direction of wire feeding [9].
If the laser beam cannot melt the wire in time of interaction, the wire will be plunged into the
melt pool and will be melted by the high temperature of the melt pool, as shown in Figure 2-4(a).
So the wire feeding direction showed in Figure 2-3 has larger tolerance of wire feeding speed
and position and it gives the best wire feeding direction. Figure 2-4(b) shows the surface of the
clad layer when correct wire feeding direction and position (Figure 2-3) are adopted, which has
good surface shaping [9].
30º
9
Figure 2-4 – Clad layer surface when wire is correctly fed [9].
2.1.12 Other modes of coating
Many techniques such as thermal spraying (flame spraying and plasma spraying), plasma
vapour deposition, physical and chemical vapour deposition (PVD/CVD), electro-deposition,
cold spray, brush-electroplate, deposited welding, TIG (tungsten inert gas) welding, submerged
arc surfacing (SAS) and oxyacetylene flame spraying have been employed to create the desired
coating or hard material structure [13;22-27].
The resistance of materials against mechanical wear, corrosion, oxygenation, thermal shock,
etc, can be improved by plasma-sprayed coatings on the surface of substrate, and this
technique has been widely used for space and civil components. However, some shortcomings
exist in the plasma-sprayed coatings due to the presence of micro pores, micro cracks and poor
adhesion properties between coatings and substrates. These faults largely restrict the
application of this technique. Recently, the coating qualities have been improved by modifying
the processing parameters and adjusting the heat treatment parameters, but the poor interface
adhesion properties have not been solved yet. The rupture usually takes place near the
interface where residual stress and defects exist due to the thermal and mechanical mismatch.
The improvement in the interface properties plays a key role in using plasma spraying
technology, because it significantly affects the global strength of the material [23].
Laser cladding/alloying, thermal plasma and cold spraying are the only techniques that can
produce surface layer thicknesses of up to a few millimetres and the thermal plasma spray
method is limited by the poor substrate-surface bonding often encountered. Cold spray is a
promising method, in that it produces no thermal damage or distortion to the substrate, but it is
not a fully tested or industrially accepted method [22].
Conventional repairing methods presently adopted mainly include mechanical machining,
brush-electroplate, deposited welding, TIG (Tungsten Inert Gas) welding and thermal spraying
(flame spraying and plasma spraying), but there are still many drawbacks, such as being time-
consuming and labour intensive, having limited thickness of deposition layers and machinable
10
times, poor bonding strength, large amount of porosities and cracks, or significant heat injection
and distortion of the substrates [25]. For example, in TIG welding process, repair has been
carried out by a welder holding a heat source in one hand, while adding a filler wire to a molten
pool to create a build-up of new metal [13].
Submerged arc surfacing (SAS) process is also used like a repair technique. In this process, the
mixing of the filler material and the base metal is kept to a minimum, which ensures favourable
mechanical properties of the cladding and the surfacing weld. The respective mechanical
properties of the cladding and the surfacing weld can be additionally improved by a subsequent
precipitation annealing. The micro-chemical analyses of the submerged arc surfaced (SAS)
specimen show good agreement between the desired and obtained compositions of the
surfaced layer [26].
Surface laser cladding can also be done using fibre laser. Fibre laser allows producing thinner
and narrower strips than lamp-pumped Nd:YAG laser with the geometrical dilution controlled via
the processing parameters. Fibre laser shows a high potential for the reparation of delicate
areas of damaged pieces in a precise manner [27].
2.1.13 Applications
In automobile industry, laser cladding has been used for cladding on selected areas of valves,
shafts, and other engine components to improve wear and high-temperature corrosion
resistance. The power utility industry has begun to use laser cladding for boiler tubes and water
walls in steam generators. Laser cladding has also been used for the repair of jet and power
turbine engine shaft and blade components [9].
Laser cladding can also be used to produce thermal barriers, to achieve layers suitable for
applications in nuclear power stations or to obtain surface layers that prevent stacking [10].
When multiple tracks are deposited on top of each other to build complex three-dimensional
structures, it is called inverse machining or laser direct casting. Using this technique makes it
possible to grow small metal parts, like 3D metal prototypes. Cladding of a preplaced polymer
foil by scanning the laser beam is another special application that is used, for example, inside
steel tubes. Laser cladding is also an important repair technique for moulds and cutting
tools [16]. These last applications are very common in the Carrs Welding Technology Company.
2.2 316 Stainless steel
2.2.1 Introduction
Original discoveries and developments in stainless steel technology began in England and
Germany about 1910. The commercial production and use of stainless steels in the United
States began in the 1920s. Only modest tonnages of stainless steel were produced in the
11
United States in the mid-1920s, but annual production has risen steadily since that time. Even
so, tonnage has never exceeded about 1.5% of total production for the steel industry.
Stainless steels are iron-base alloys containing at least 10.5% Cr. Few stainless steels contain
more than 30% Cr or less than 50% Fe. They achieve their stainless characteristics through the
formation of an invisible and adherent chromium-rich oxide surface film. This oxide forms and
heals itself in the presence of oxygen. Other elements added to improve particular
characteristics include nickel (Ni), molybdenum (Mo), copper (Cu), titanium (Ti), aluminium (Al),
silicon (Si), neodymium (Nb), nitrogen (N), sulphur (S), and selenium (Se). Carbon (C) is
normally present in amounts ranging from less than 0.03% to over 1.0% in certain martensitic
grades.
The selection of stainless steels may be based on corrosion resistance, fabrication
characteristics, availability, mechanical properties in specific temperature ranges and product
cost. However, corrosion resistance and mechanical properties are usually the most important
factors in selecting a grade for a given application [28].
2.2.2 Stainless steel 316
Stainless steels are commonly divided into five groups: martensitic stainless steels, ferritic
stainless steels, austenitic stainless steels, duplex (ferritic-austenitic) stainless steels, and
precipitation-hardening stainless steels.
Stainless steel 316 is an austenitic type. Austenitic stainless steels have a face-centered cubic
(fcc) structure. This structure is attained through the liberal use of austenitizing elements such
as nickel, manganese, and nitrogen. These steels are essentially nonmagnetic in the annealed
condition and can be hardened only by cold working. They usually possess excellent cryogenic
properties and good high-temperature strength. Chromium content generally varies from 16 to
26%; nickel, up to about 35%; and manganese, up to 15%. The 3xx types contain larger
amounts of nickel and up to 2% Mn. Molybdenum, copper, silicon, aluminium, titanium, and
niobium may be added to confer certain characteristics such as halide pitting resistance or
oxidation resistance. Sulphur or selenium may be added to certain grades to improve
machinability [28].
Chemical composition of the 316 stainless steel is shown in Table 2-2.
Elements (%)
C Mn Si Cr Ni P S Other
0.08 2.00 1.00 16.0−18.0 10.0−14.0 0.045 0.03 2.0−3.0
Mo
Table 2-2 – Chemical composition of the 316 stainless steel, in percentage.
2.2.3 Physical and mechanical properties
Austenitic stainless steel are formulated and thermo-mechanically processed such that the
microstructure is primarily austenite. Depending on the balance of ferrite-promoting elements,
12
the wrought or cast microstructure will be either fully austenitic or a mixture of austenite and
ferrite. This ferrite results from the segregation of ferrite-promoting elements (primarily
chromium) during solidification and thermo-mechanical processing. It is usually present in
relatively low volume fraction (less than 2 to 3%). Although not considered deleterious in most
applications, the presence in the wrought microstructure can reduce the ductility and,
potentially, the toughness of austenitic stainless steels. It can also be a preferential site for the
precipitation of M23C6 carbides and sigma phase, of which the latter is an embrittling agent in
stainless steel.
The transformation behaviour of austenitic stainless steels can be described using the Fe-Cr-Ni
pseudobinary diagram at 70 % constant iron.
Figure 2-5 – Pseudobinary section of the Fe-Cr-Ni system at 70% iron.
Primary solidification of austenitic stainless steels can occur as either austenite or ferrite. The
demarcation between these two primary phases of solidification is at approximately 18Cr-12Ni
in the ternary system. At higher chromium/nickel ratios, primary solidification occurs as delta
ferrite and at lower ratios as austenite. There is a small triangular region within the solidification
temperature range where austenite, ferrite and liquid coexist. Alloys that solidify as austenite to
the left of this triangular region are stable as austenite upon cooling to room temperature.
However, when alloys solidify as ferrite, they may be either fully ferritic or consist of a mixture of
ferrite and austenite at the end of solidification. Because of the slope of the ferrite and austenite
solvus lines, most or all of the ferrite transforms to austenite under equilibrium cooling
conditions, as can be seen for a nominal 20Cr-10Ni alloy where the structure becomes fully
austenitic upon cooling to 1000ºC. For the rapid cooling conditions experienced during welding,
this transformation is suppressed and some ferrite will remain in the microstructure.
However, the solidification mode can be affected by the solidification rate. At high cooling rate,
such as in laser processing, transformations can be suppressed [29].
A variety of precipitates may be present in austenitic stainless steels, depending on composition
and heat treatment. Carbides are present in virtually every austenitic stainless steel, since
chromium is a strong carbide former. Additions of other carbide formers, including Mo, Nb and
Ti, also promote carbide formation. The nature of carbide formation, including the effect of
composition and the temperature range of formation, is quite complex.
13
Precipitation of M23C6 carbides has received considerable attention because of its effect on
corrosion resistance. These carbides precipitate very rapidly along grain boundaries in the
temperature range 700 to 900ºC. At slightly longer times, the presence of these grain boundary
carbides can lead to intergranular corrosion when exposed to certain environments. This
precipitation reaction is accelerated in alloys that are strengthened by cold work [30].
Physical and mechanical properties of the 316 stainless steel are show in Table 2-3 and 2-4.
Density
(g/cm3)
Specific Heat
Capacity (J/gºC)
Thermal Conductivity
(W/m.K)
CTE (µm/m°C)
(25 – 95ºC)
7.99 0.500 16.2 16
Table 2-3 – Physical/thermal properties of 316 stainless steel [31].
Tensile
strength (MPa)
Yield strength
(MPa)
Modulus of
Elasticity (GPa)
Elongation
(%)
Reduction
in area (%)
515 205 193 40 50
Table 2-4 – Mechanical properties of 316 stainless steel [30].
2.2.4 Application
Austenitic stainless steels are used in a wide range of applications, including structural support
and containment, architectural uses, kitchen equipment, and medical products. They are widely
used not only because of their corrosion resistance but because they are readily formable,
fabricable, and durable. Some highly alloyed grades are used for very high temperature service
(above 1000ºC) for applications such as heat-treating baskets. In addition to higher chromium
levels, these alloys normally contain higher levels of silicon (and sometimes aluminium) and
carbon, to maintain oxidation and/or carburization resistance and strength, respectively.
It should be pointed out that the common austenitic stainless steels are not an appropriate
choice in some common environments such as seawater or other chloride-containing media, or
in highly caustic environments. This is due to their susceptibility to stress corrosion cracking, a
phenomenon that afflicts the base metal, HAZ, and weld metal in these alloys [30].
2.2.5 Cladding of stainless steel
The cladding of stainless steel onto carbon steel brings a nice solution to the problem of the
elaboration of a material which combines high level mechanical properties and good resistance
to corrosion [32].
The economics of stainless steel weld cladding are dependent on achieving the specific
chemistry at the highest practical deposition rate in minimum number of layers. The composition
and properties of clad metals are strongly influenced by dilution. Dilution reduces the alloying
elements and increases the carbon content in the clad layer, which reduces corrosion
resistance properties and causes other metallurgical problems. It also affects the ferrite content
14
in claddings, thus emphasizing the need and importance of having cladding procedures capable
of giving optimum dilution [33].
A possible materials solution to providing structural components which combine the attributes of
high strength and corrosion resistance is to clad the surface of the steel with a metallurgically
compatible corrosion resistant alloy. The characteristics desirable in such a cladding alloy are
reasonable strength, weldability to the steel, resistance to general and localized corrosion
attack, and good corrosion fatigue properties. A candidate material for cladding which has
excellent corrosion resistance and weldability is stainless steel [34].
Laser cladding of stainless steel can significantly improve the hardness of the filler material and
the pitting corrosion resistance [35].
Regarding laser cladding using powder stainless steel, it has been reported in the previous
studies that the hardness and wear resistance of the stainless steel clad was improved
markedly by incorporating ceramic particles in the alloy clad matrix. Improvement in the
hardness and wear properties of clads was mostly attributed to the formation of MMCs (Metal
Matrix Composites) due to the inclusion of ceramic particles in the cladding mixture [36].
Studies performed in GMAW stainless steel claddings showed that within the range of the input
parameters investigated, wire feed rate was found to be the most significant variable affecting
dilution, followed by open circuit voltage and welding speed [33].
Auxiliary preheating of the filler wire in GMAW process reduce base metal penetration, apart
from relatively smaller variations in other bead geometry parameters, due to significant drop in
the main welding current. This is also the main reason for reduced dilution obtained in this
process [33;37].
In FCAW stainless steel claddings, it was found that among the three parameters, only are two
factors that play a dominant role in determining a bead geometry parameter. For instance,
welding current and nozzle-to-plate distance have more significant influence on coefficient of
internal shape than the welding speed. Also it was observed that interaction effects have
considerable influence over the weld bead geometry and their effects cannot be neglected [38].
Among the various thermal processes that are used to elaborate coatings on stainless steel, the
submerged arcs and laser cladding offer high quality of purity, hardness and homogeneity in
their microstructures [32].
15
2.3 H13 tool steel and AISI P20 tool steel
2.3.1 Introduction
Tool steels are very important engineering materials exploited in practice [21], and are used to
make tools for cutting, forming, or otherwise shaping a material into a part or component
adapted to a definite use. The earliest tool steels were simple, plain carbon steels, but by 1868
and increasingly in the early 20th century, many complex, highly alloyed tool steels were
developed. These complex alloy tool steels, which contain, among other elements, relatively
large amounts of tungsten, molybdenum, vanadium, manganese, and chromium, make it
possible to meet increasingly severe service demands and to provide greater dimensional
control and freedom from cracking during heat treatment.
In service, most tool steels are subjected to extremely high loads that are applied rapidly. The
tools must withstand these loads a great number of times without breaking and without
undergoing excessive wear or deformation [28].
The main drawback of tool steels is cost. Components made of tool steels, especially large
ones, are relatively expensive, due to either the material cost or the fabrication expense [21].
2.3.2 H13 tool steel
H13 is one of the most widely used in Chromium hot-work steels. H13 has good resistance to
heat softening because of their medium chromium (Cr) content and the addition of carbide-
forming elements such as molybdenum (Mo), tungsten (W), and vanadium (V). The low carbon
(C) and low total alloy contents promote toughness. Higher molybdenum (Mo) contents increase
hot strength but slightly reduce toughness. Vanadium is added to increase resistance to
washing (erosive wear) at high temperatures. Silicon (Si) improves oxidation resistance at
temperatures up to 800ºC. All of the chromium hot-work steels are deep hardening , and are
especially well adapted to hot die work of all kinds, particularly dies for the extrusion of
aluminium and magnesium, as well as die casting dies, forging dies, mandrels, and hot shears.
Most of these steels have alloy and carbon contents low enough that tools made from them can
be water cooled in service without cracking [28].
Chemical composition of H13 tool steel could be seen in Table 2-5.
Elements (%)
C Mn Si Cr Ni Mo V
0.32-0.45 0.2-0.5 0.8-1.2 4.75-5.5 0.3 max. 1.1-1.75 0.8-1.2
Table 2-5 – Chemical composition of H13 tool steel, in percentage [28].
16
2.3.2.1 Heat treatments, physical and mechanical properties of H13 tool steel
The recommended heat treatments for this steel are:
Annealing → Heat to 845 to 900ºC. Use lower limit for small sections, upper limit for
large sections. Surface protection against decarburization by use of pack, controlled
atmosphere, or vacuum is required. Heat slowly and uniformly, especially for hardened
tools. Holding time varies from about 1 hour for light sections and small furnace charges
to about 4 hour for heavy sections and large charges. For pack annealing, hold 1 hour
per inch of cross section. Cool slowly in furnace at a rate not exceeding 28ºC per hour
until 540ºC is reached, when a faster cooling rate will not affect final hardness. Typical
annealed hardness, 192 to 229 HB.
Stress relieving → Optional. Heat to 650 to 675ºC and hold for 1 hour per inch of cross
section (minimum of 1 hour). Cool in air.
Hardening → Surface protection against decarburization or carburization is required by
utilizing salt, pack, controlled atmosphere or vacuum. For preheating, die blocks or
other tools for open furnace treatment should be placed in a furnace that is not over
260ºC. Work that is packed in containers may be safely placed in furnaces at 370 to
540ºC. Once the workpieces (or container) have attained temperature, heat slowly (no
faster than 110ºC per hour) to 815ºC. Hold for 1 hour per inch of thickness (or per inch
of container thickness if packed). If double preheating facilities, such as salt baths, are
available, thermal shock can be reduced by preheating at 540 to 650ºC and further
preheating at 845 to 870ºC. Austenitize at 995 to 1040ºC for 15 to 40 minutes. Use
shorter time for small sections and longer time for large sections. Quench in air. If blast
cooling, air should be dry and blasted uniformly on surface to be hardened. To minimize
scale, tools can be flash quenched in oil to cool the surface below scaling temperature
(about 540ºC), but this increases distortion. The procedure is best carried out by
quenching from the austenitizing temperature into a salt bath held at 595 to 640ºC,
holding in the quench until the workpiece reaches the temperature of the bath, and then
withdrawing the workpiece and allowing it to cool in air. Quenched hardness, 51 to 54
HRC.
Tempering → Temper immediately after tool reaches about 52 ºC at 540 to 650 ºC.
Forced convection air tempering furnaces heat tools at a moderately safe rate. Salt
baths are acceptable for small parts, but may cause cracking of large or intricate
shaped dies due to thermal shock. Temper for 1 hour per inch of thickness, cool to
room temperature, and re-temper using the same time at temperature. The second
temper is essential and a third temper would be beneficial. Approximate tempered
hardness, 53 to 39 HRC.
17
CCT (Continuous Cooling Transformation) diagram of H13 is shown in Figure 2-6. In this
diagram is possible to see the transformations of H13 tool steel according the temperature and
the cooling time.
Figure 2-6 – CCT diagram of H13 tool steel.
The main physical and mechanical properties of H13 are present in Table 2-6 and 2-7.
Density
(g/cm3)
Specific Heat
Capacity (J/gºC)
Thermal Conductivity
(W/m.K)
CTE (µm/m°C)
(25 – 95ºC)
7.80 0.460 24.3 11
Table 2-6 – Physical/thermal properties of H13 tool steel [31].
Tensile
strength (MPa)
Yield strength
(MPa)
Elongation at
break (%)
Modulus of
Elasticity (GPa) Hardness (HV)
1990 1650 9 210 549
Table 2-7 – Mechanical properties of H13 tool steel [31].
2.3.3 AISI P20 tool steel
Mould steels, or group P, contain chromium and nickel as principal alloying elements. AISI P20
tool steel normally is supplied heat treated to 30 to 36 HRC, a condition in which they can be
machined readily into large, intricate dies and moulds. Because these steels are pre-hardened,
no subsequent high-temperature heat treatment is required, and distortion and size changes are
avoided. However, when used for plastic moulds, type P20 is sometimes carburized and
hardened after the impression has been machined. All group P steels have low resistance to
softening at elevated temperatures. Group P steels are used almost exclusively in low
temperature die casting dies and in moulds for the injection or compression moulding of
plastics. Plastic moulds often require massive steel blocks up to 762 mm (30 in.) thick and
weighing as much as 9 Mg (10 tons). Because these large die blocks must meet stringent
18
requirements for soundness, cleanliness, and hardenability, electric furnace melting, vacuum
degassing, and special deoxidation treatments have become standard practice in the production
of group P tool steels. In addition, ingot casting and forging practices have been refined so that
a high degree of homogeneity can be achieved [28].
Chemical composition of AISI P20 tool steel is seen in Table 2-8.
Elements (%)
C Mn Si Cr Mo
0.28-0.40 0.60-1.00 0.20-0.80 1.40-2.00 0.30-0.55
Table 2-8 – Chemical composition of AISI P20 tool steel, in percentage [28].
2.3.3.1 Heat treatments, physical and mechanical properties of AISI P20 tool steel
The recommended heat treatments for this steel are:
Forging → Heat slowly and uniformly to 1050°C. Do not forge below 930°C. After
forging cool slowly.
Annealing → P20 should always be annealed after forging and before re-hardening.
Heat uniformly to 770/790°C. Soak well and cool slowly in the furnace.
Hardening → Heat uniformly to 820/840°C until heated through. Quench in oil.
Tempering → Heat uniformly and thoroughly at the selected tempering temperatures
and hold for at least one hour per inch (2.54 cm) of total thickness.
Tempering (ºC) 100 200 300 400 500 600 700
Hardness (HRC) 51 50 48 46 42 36 28
Table 2-9 – Tempering of AISI P20 tool steel.
Nitriding → Moulds machined from pre hardened P20 may be nitrided to give a hard
surface which is very resistant to wear and erosion. A nitrided surface also increases
the corrosion resistance. The surface hardness after nitriding at a temperature of 525°C
in ammonia gas will be approximately 650HV.
Tufftriding → Tufftriding at 570° C will give a surface hardness of approximately 700HV.
After hours treatment the hard layer will be approximately 0.01mm.
Hardening → In order to maintain maximum surface hardness AISI P20 tool steel may
be case hardened. Before case hardening is carried out, the steel should be annealed.
To carburise, pack with carburising powder into a cast iron or heat resisting steel box
and see that the articles are separated from the sides by at least two inches of
carburising powder. Lute the lid with fireclay. Heat to the carburising temperature of
880°C and soak for sufficient time to give the required depth of case. Cool to 800/820°C
and quench in oil. Tempering will then be necessary. Reheat to 200/300°C and allow to
cool in the air to give a final surface hardness of Rockwell C55/59.
19
Flame & Induction Hardening → AISI P20 tool steel can be flame or induction hardened
to a hardness of 50 to 55 HRC. Cooling in air is preferable. Smaller pieces may
however require forced cooling. Hardening should be immediately followed by
tempering.
Hard Chromium Plating → After hard chromium plating, the steel should be tempered
for approximately 4 hours at 180°C, in order to avoid hydrogen embrittlement.
CCT diagram of AISI P20 tool steel can be seen in Figure 2-7.
Figure 2-7 – CCT diagram of AISI P20 tool steel.
Physical and mechanical properties of AISI P20 tool steel are present in Table 2-10 and 2-11.
Density
(g/cm3)
Specific Heat
Capacity (J/gºC)
Thermal Conductivity
(W/m.K)
CTE (µm/m°C)
(20ºC)
7.85 0.460 29-34 12.8
Table 2-10 – Physical/thermal properties of AISI P20 tool steel [31].
Tensile
strength (MPa)
Yield strength
(MPa)
Elongation at
break (%)
Modulus of
Elasticity (GPa)
Hardness
(HB)
965-1030 827-862 20 205 300
Table 2-11 – Mechanical properties of AISI P20 tool steel [31].
2.3.4 Applications of tool steels
Tool steels are commonly used for manufacturing moulds, dies (cutting tool) and other
components that are subjected to extremely high load, to provide the required wear resistance.
Due to their contents of high carbon and chromium, for example, they are widely used for
making rolls, forming dies, burnishing tools, components for cold working applications [21],
20
machinery components and structural applications in which particularly stringent requirements
must be met, for example, high-temperature springs, ultrahigh-strength fasteners, special-
purpose valves, and bearings of various types for elevated-temperature service [28].
2.3.5 Cladding of tool steels
In some cases, tool steels are restricted to moulds of complicated design, which are very
expensive and hard to manufacture. In these cases, laser technology is highly recommended to
repair these parts in view of its reliability and relatively low cost. Further improvement in
corrosion resistance may be achieved by laser surface modification via homogenisation and
refinement of the microstructure, and/or the formation of new alloys on the surface [11].
Traditionally, repair has been carried out by a welder holding a heat source in one hand, while
adding a filler wire to a molten pool to create a build-up of new metal. The main repair process
has traditionally been tungsten inert gas (TIG) welding, but a new non-traditional approach is
emerging called laser cladding with filler wire [13].
In terms of the practical side of welding, the laser cladding using filler wire and TIG repair
techniques are similar, since both are based on manual skills. However, given the well-known
properties of the laser beam, the laser cladding (wire) process has greater potential than TIG
and promises to become the reference process in the future.
Indeed, since the laser beam can be easily focused on a small area, thin diameter wires can be
used to repair small elements and localized geometries such as borders, edges, intricate
profiles and pockets. Moreover, the high energy density of the laser beam minimizes the local
thermal damage in the base material, because the heat is dissipated mainly in the wire and
substrate fusion. Consequently, the preheating and post-heating precautions which are
common in the TIG repair process are not required with laser cladding using filler wire. Finally,
in comparison to the TIG, the laser beam is usually stable and repeatable in time, so allowing
accurate control of the heat transmitted to the wire and work-piece. Human factors affect the
clad shape only if operators lack the necessary skills. Indeed, at the end of a training period, the
clad shape is not affected by operator performance [13].
Despite the use of preheating in tool steel TIG coatings, the welds present regions with high
hardness values corresponding to martensitic microstructures. In this case preheating is
essentially used to reduce the risk of the material to crack during welding. In multipass welds
some of these regions were tempered by the repeated welding thermal cycles, lowering the
hardness values. However, this procedure did not bring the harness to base metal values.
The tempering treatment made in TIG cladding of tool steels has shown that it was possible,
with a post welding heat treatment, to obtain a uniform hardness profile across weld zone, base
21
metal and heat affected zone, even using a filer wire with a composition different from that of
the base metal [39].
Lack of fusions defects can occur in laser cladding of tool steel. These defects, that are not
detectable by the welder, show the need for adequate parameter setting. The choice of cladding
parameters should be based in experimental tests and should assure that complete fusion is
always achieved [39].
Preheating the substrate to 300ºC completely eliminated all the cracks in tool steel laser clad.
This is attributed to the reduction in the degree of the thermal mismatch between the clad layer
and substrate at higher preheating substrate temperatures [12;40].
Laser cladding using filler powder is suitable for plastics mould steel reparation. The optimum
parameter for cladding production allows homogeneous and continuous coatings, free of
defects and with perfect adherence, to be obtained. Surface analysis showed a similar
composition to the initial powders, indicating that the cladding process was carried out with the
appropriate parameters which reduce dilution from the base steel [11].
A wide range of alloys can be laser deposited on different substrates using the powder blowing
technique to produce layers with very low dilution, high integrity, very fine structures, and low
heat input into the substrate [20].
2.4 Summary of the literature survey
Regarding the performance of laser cladding using filler powder and wire, the
characteristics’ behavior of these materials and the performance of these materials in laser
cladding, as well as in similar processes, it is possible to perform a work where some problems
could arise and some procedures to avoid them. The mode to prevent problems associated to
laser cladding is fundamentally to control well the process parameters. The knowledge of the
main properties of these materials is important to explain problems and the performance in
specific conditions. Hence, it is allowed to suggest ways to avoid problems that were unstated
and also to suggest procedures to obtain the best performance of laser cladding using filler
powder and wire.
22
3 Experimental procedure
3.1 Equipment
The experiments on laser cladding were performed in the company Carrs Welding Technology
Ltd, in Kettering, England. This company is specialised in laser welding for moulds, tools repair
and production of high quality/precision parts.
Figure 3-1 – Carrs Welding Technology Ltd Company.
3.1.1 Powder deposits
Powder deposits were done with a continuous wave Nd:YAG laser, model HL 1006 D, from
TRUMPF, with a maximum output power of 1400 W and a maximum laser power of 1000 W,
meaning that it can be achieved 1000 W of power at the workpiece. The main characteristics of
this laser are: a wavelength of 1.064 µm, beam parameter product of 25 mm.mrad and a laser
light cable of 600 µm [41].
The laser is linked to a Kuka’s robot (1 - Figure 3-2) with 8 axes. It is the robot that controls all
the movements of system.
Figure 3-2 – Robot with a continuous wave laser used for powder deposits.
With the block on the table (2 - Figure 3-2), and using the console (6 - Figure 3-2), it is possible
to program the track of the laser beam on the block. This is also made with the help of a video
camera linked to the robot, which allows watching the evolution of the coating on the auxiliary
1
2
3
4
5
6
23
television. After that the coating process is started with powder feeding and the laser (4 - Figure
3-2) turned on.
The powder feeding system (3 - Figure 3-2) is elevated to help the powder’s flow.
In Figure 3-3 it is shown the top of the nozzle (5 - Figure 3-2), where is possible to see the exit
holes of the powder (8 - Figure 3-3) and the hole of the laser beam (7 - Figure 3-3). The powder
has to cross channels as it can be seen in Figure 3-4 (10).
The gas used for shielding and to help powder feeding, Argon (Ar), comes to the nozzle by the
blue channel (9 - Figure 3-3).
Figure 3-3 – Nozzle. Figure 3-4 – Detailed nozzle.
3.1.2 Wire deposits
Wire deposits were done using a pulsed Nd:YAG laser, model HL 124 P, from TRUMPF, with a
maximum pulse power of 5000 W. The most important features of this laser are presented in
Table 3-1 [42]:
Features HL 124 P
Wavelength of the laser light 1.064 µm
Maximum average power 120 W
Minimum pulse power 300 W
Maximum pulse power 5000 W
Pulse duration at maximum pulse power 0.3 – 10 ms
Pulse duration at reduced pulse power 0.3 – 20 ms
Maximum pulse energy 50 J
Maximum pulse repetition frequency 300 Hz
Beam parameter product 16 mm.mrad
Table 3-1 – The most important characteristics of HP 124 P laser.
All wire deposits were performed in a working station as the one in Figure 3-5. The process of
adding filler material is manual, where the worker holds the filler wire (4 - Figure 3-6) with his
hand and executes the coating process. The block material (2 - Figure 3-6) is placed on the x-y-
z table (1 - Figure 3-6) and the laser beam is applied to the block by the laser
7
8
10 9
24
nozzle (3 - Figure 3-6). This nozzle controls the movement of the process, since the block
material is stopped during the entire process.
Figure 3-5 – The working station [4]. Figure 3-6 – Wire deposits.
3.2 Experimental approach
3.2.1 Description
The procedure used was the same for every substrate material and consisted on the deposit of
3 (three) runs with different number of layers. Each layer was deposited on the previous one.
Each run has 3 (three) steps. The first run has 1 (one) layer of powder/wire deposit [A – Figure
3-7], the second run has 2 (two) layers of powder/wire deposited [B - Figure 3-7] and the third
run has 3 (three) layers of powder/wire deposited [C - Figure 3-7], as is shown in Figure 3-7.
Figure 3-7 – First experience.
1
2
3 4
25
With the aim of clarifying the terminology used, Figure 3-8 represents the lay-out used for the
experiments.
Figure 3-8 – Lay-out (top and side views).
Speed of wire deposition
To calculate the speed of wire deposition, an additional experiment was performed, for which, a
run with 63 mm of length was done and the duration registered. The wire deposition process
was done in 45 seconds. This coating has one layer of filler wire and one step. Hence, the wire
deposition speed can be obtained by:
Where:
V – The linear speed of material deposition (mm/s);
L – The length of the run (mm);
t – Represents the period of time taken to make a run with a specific length (s).
The determination of the speed is very important because this is needed for calculations that
are performed later in this work.
26
3.2.2 Material
The materials used as substrate were: 316 stainless steel, H13 tool steel (53S) and AISI P20
tool steel. In each of those substrate materials, a similar material was used for coating.
In the case of the 316 stainless steel, the powder filler material used has the commercial
designation: DELCROME@
316 ´WM´ Powder COCA, and the wire filler material was 316L
stainless steel with a diameter of 0.40 mm. The chemical composition of these materials is
presented in Table 3-2 [28;43].
316 stainless steel
Elements Substrate material Powder filler Wire filer
C 0.08 0.01 0.03
Mn 2.00 0.50 2.00
Si 1.00 1.70 1.00
Cr 16.0 – 18.0 18.3 16.0 – 18.0
Ni 10.0 – 14.0 13.1 10.0 – 14.0
P 0.045 0.005 0.045
S 0.03 0.003 0.03
Mo 2.0 – 3.0 2.70 2.0 – 3.0
Fe Balance Balance Balance
Table 3-2 – Chemical composition of substrate and coating material (316 stainless steel), in percentage.
For H13 tool steel (53S) as substrate material, it was used a powder filler material which has a
commercial designation of Gas Atomised Powder – H13. The wire filler material was a wrought
H13 tool steel with a wire diameter of 0.38 mm. In Table 3-3 is presented the chemical
composition of substrate and filler material used on H13 tool steel (53S) [28;44;45].
H13 tool steel
Elements Substrate material Powder filler Wire filler
C 0.40 0.39 0.32 – 0.45
Cr 5.00 5.20 4.75 – 5.5
V 1.00 1.09 0.8 – 1.2
Si 1.00 0.96 0.8 – 1.2
Mo 1.40 1.37 1.1 – 1.75
Mn - 0.40 0.2 – 0.5
P - 0.02 -
S - 0.023 -
Fe Balance Balance Balance
Table 3-3 – Chemical composition of substrate and coating material (H13 tool steel), in percentage.
27
The powder used as filler material in the AISI P20 tool steel, is a nickel base alloy (NiCrMo – 3)
with a commercial designation of LPW 625.The commercial designation of the wire filler material
used is: GS 3 WSG 3-GZ-45-T, and the diameter was 0.38 mm. All chemical composition of
these material are shown in Table 3-4 [28;45;46].
AISI P20 tool steel
Elements Substrate material Powder filler Wire filler
C 0.28 – 0.40 0.05 0.25
Mn 0.60 – 1.00 0.15 0.7
Si 0.20 – 0.80 0.3 0.5
Cr 1.40 – 2.00 22.5 5
Ni - Balance -
Mo 0.30 – 0.55 9 4
Ti - 0.2 0.6
Al - 0.2 -
Fe Balance 3 Balance
Table 3-4 – Chemical composition of substrate and coating material (AISI P20 tool steel), in percentage.
3.2.3 Parameters
This section is divided in two parts. The first one is reserved for laser parameters used on
powder deposits and the second one for wire deposits. In each subsection are shown the
parameters used in each substrate material. The values of these parameters are the ones used
by the company to perform coatings on these substrate materials.
3.2.3.1 Powder deposits
In Table 3-5 the powder parameters are shown.
Substrate material
316 stainless steel H13 tool steel AISI P20 tool steel
Linear speed (V) 10 mm/s 10 mm/s 10 mm/s
Powder rate ( ) 14 g/min 14 g/min 9.5 g/min
Powder feeder Argon (Ar) 4 LPM 4 LPM 4 LPM
Shielding gas (Ar) 15 LPM 15 LPM 15 LPM
Power (P) 700 W 700 W 700 W
Table 3-5 – Powder parameters.
28
3.2.3.2 Wire deposits
In Table 3-6 the wire parameters are presented.
Substrate material
316 stainless steel H13 tool steel AISI P20 tool steel
Focus (beam diameter) 1.2 mm 1.2 mm 0.939 mm
Power (P) 2.0 kW 2.0 kW 1.6 kW
Duration 5.4 ms 5.4 ms 5.7 ms
Frequency 8 Hz 8 Hz 8.5 Hz
Energy 10.8 J 10.8 J 12.7 J
Average power 86 W 86 W 68.8 W
Table 3-6 – Wire parameters.
3.2.4 Samples preparation
This part of the work was performed after all coating process being ready and it was realised at
Instituto Superior Técnico, in Lisbon, Portugal. The aim of sample preparation is to allow the
realisation of metallographic, dilution and hardness analysis to assess the quality and the
productivity of the deposit.
The preparation of the samples was done in the following steps:
1. Selection of the local for examination;
2. Cutting;
3. Cleaning;
4. Mounting (hot) and marking;
5. Mechanical grinding and polishing;
6. Etching.
In the second step of the samples preparation, the sample was cut in the perpendicular
direction of the coating process. In Figure 3-9, it is possible to see the plan of cut.
Figure 3-9 – Cutting plan.
29
In the fifth and sixth steps, the procedure was different for each material; this can be seen in
Table 3-7.
Mechanical grinding and
polishing
316 stainless steel
Abrasives sandpapers of SiC: 240, 400, 600, 800, 1000,
1200, 2400 and 2500.
Cloth RAM and 6 µm diamond particles.
Cloth RAM and 3 µm diamond particles.
Cloth HSB and 1 µm diamond particles.
H13 tool steel
Abrasives sandpapers of SiC: 240, 400, 600, 800, 1000,
1200, 2400 and 2500.
Cloth RAM and 6 µm diamond particles.
Cloth RAM and 3 µm diamond particles.
AISI P20 tool steel
Abrasives sandpapers of SiC: 240, 400, 600, 800, 1000,
1200, 2400 and 2500.
Cloth RAM and 6 µm diamond particles.
Cloth RAM and 3 µm diamond particles.
Etching
316 stainless steel
Substrate material: Vilella’s reagent (1 g piric
acid - C6H3N3O7, 5 mL HCl and 100 mL ethanol).
Coating material: Vilella’s reagent.
H13 tool steel
Substrate material: Nital 2% (2 mL nitric acid – HNO3 and
98 mL ethanol).
Coating material: Nital 2%.
AISI P20 tool steel
Sustrate material: Nital 2%.
Coating material (powder): Glyceregia (2 parts glycerol, 3
parts HCl and 1 part HNO3).
Coating material (wire): Vilella’s reagent.
Table 3-7 – Mechanical grinding, polishing and etching in each material [47].
3.2.5 Macro graphic and microstructure analysis
For macro photography and microstructure analysis it was used a hund-WETZLAR microscope,
model H600, which was linked to a Canon camera, model EOS 300D DIGITAL. With this
system it was possible to take digital photos with various amplifications. Macro photography
analysis was done with amplifications of 40x and microstructure analysis with amplifications of
100x and 200x.
30
The aim of macro photography was to find welding defects, such as pores and cracks, while the
microstructure analysis’ aim was to allow understanding the structure of the material deposited
and of the fused zone in the substrate.
3.2.6 Dilution calculation
The equation for the calculation of the dilution is [48]:
The area of the filler material used as coating is represented by A (mm2), while B represents the
area of the mixing between filler and substrate material.
Figure 3-10 – Schematic representation of the procedures adopted to calculate dilution [48].
The procedure used to determine the values of A and B was:
1. Etching the samples, which allowed to identify the coating and the substrate material,
as it is seen in Figure 3-10;
2. Taking photos;
3. Analysing the photos in CorelDRAW 12 software, which allowed obtaining the quantified
area measurements.
3.2.7 Vickers hardness tests
The measures of Vickers hardness were taken along the cross-section of the coating, as it is
shown in Figure 3-11, using a hardness tester from Struers, model Duramin. The procedure of
these tests was guided by the standard ISO 3878:1983 [49].
Figure 3-11 – Representation of the hardness line in cross-section of the coating.
31
In these tests, it was applied a load (kg) with a pyramidal-shaped indenter on the material. The
load used in each material was 2 kg (HV 2), or 19.6 N, and during a period of 15 s. The
indentation on the material had a pyramidal shape done by the indenter. It is possible to
determine the Vickers hardness, HV, by [50]:
Where:
f – The load applied during the test (Kg);
d – The average length of the diagonal left by the indenter (mm).
Each indentation was done with a minimum distance of 2.5 d from the previous one.
32
4 Results and discussions
4.1 Productivity analysis
An economic definition for the productivity is related with the production rate of the products or
services, and the resources used. Therefore, the material deposition rate is a good productivity
indicator of the laser cladding process.
4.1.1 Material deposition rate
The material deposition rate, (mm3/s), is given by the equation [51]:
Where A (mm2) represents the area measured on the sample of filler material used as coating,
Figure 3-10, and V (mm/s) the linear speed of material deposition.
The linear speed of material depositions for powder and wire are, respectively, 10 mm/s and
1.4 mm/s.
Table 4-1 shows the material deposition rate for the powder and wire experiments.
(mm
3/s)
Powder Wire
316 stainless steel 16.0 0.7
H13 tool steel 14.8 0.55
AISI P20 tool steel 7.2 0.4
Table 4-1 – Deposition rate of powder and wire, in each substrate material (1 layer).
In each substrate material, the results show that the powder deposition rate is always bigger
than the wire deposition rate. This means that, for the same time, a larger area of coating can
be achieved in the powder process. This is due to the fact that the linear speed of the material
deposition and the quantity of filler material per unit time is higher in the powder coating.
The linear speed of powder deposition is higher because the robotic system allows achieving
faster speed than a manual system, as is the case of the wire deposition process.
With the goal of comparing the filler material rate, which is deposited in the substrate material in
both processes, the wire rate, (g/min), and the real powder rate, (g/min), were
calculated.
33
The first step of this calculation is to determine the volume of a run, and for this calculation the
coating performed in the 316 stainless steel was considered. The equation to determine the
volume of the wire deposit, Vol. (mm3), is:
L (mm) is the length of the run.
Considering the density’s equation, it is possible to know the associated mass, m (g).
The density, ρ (g/cm3), for the 316L stainless steel is 8 g/cm
3 [31].
Hence, it is possible to determine an approximate value for the wire rate, .
In the expression, the value of t (min) represents the period of time taken to make a run with a
specific length, L.
Next calculations are done to obtain the real powder rate. The volume of the powder deposit is:
It is allow to know the real powder rate, , using the next expression:
The density of the 316 stainless steel powder, ρ, is 5 g/cm3 [52].
So, it is possible to prove that the quantity of filler material is higher in the powder case because
the value of real powder rate is higher than the wire rate ( ). Regarding this result, and
together with the fact that the linear speed of powder deposits is higher, it is possible to confirm
that higher material deposition rates and, consequently, higher productivity are achieved in laser
cladding using filler powder.
According to the values of obtained for each substrate material (Table 4-1), and considering
that the material deposition rate is the most important productivity indicator of laser cladding, it
is possible to verify that the productivity of powder coatings is approximately 23 times superior
than the wire coatings for the 316 stainless steel, 26 times for H13 tool steel and 18 times for
34
AISI P20 tool steel. Therefore, it is only possible to state that the productivity is higher in laser
cladding with filler powder, not being possible to obtain a real value for how much this
productivity is superior in this process, since it varies for each specific material. This situation
can be explained by the fact that different materials have different physical properties, as
density. The powder deposits performed on the 316 stainless steel and on the H13 tool steel
used the same powder rate ( ), but as these materials have different densities, different
volumes of powder material deposits are involved, and consequently, the material deposition
rates ( ) vary.
Considering that the powder deposits performed on the 316 stainless steel and on the H13 tool
steel, both have the same powder rate ( ), but different densities, it can be concluded that the
different density of the powder used on the laser cladding process is responsible for the
different material deposition rates.
The powder deposition rate of the AISI P20 tool steel is the lowest value of the powder cases,
due to the fact that it has the smallest value of powder rate ( ), as it can be seen in Table 3-5. It
is known that a small deviation of powder rate results in large variations of the geometry, and
therefore variations of area’s values, which result in an alteration of the material deposition
rate [6].
35
4.2 Structural analysis of the clads
In this section are performed important analyses that allow identifying the performance of the
laser cladding using both the powder filler material and the wire filler material.
4.2.1 Defects analysis
One of the aims of any welding process is to obtain sound welds, free of defects such as pores
or cracks. In these macrographic analyses these defects are quantified in all the samples and
the defects’ origin is discussed.
Several types of defects were observed, mainly: pores, cracks, lack of fusion and adhesion.
The defects present in 316 stainless steel clads are shown in Table 4-2.
316 stainless steel
Powder Wire
1 Layer No defects. No defects.
2 Layers
3 pores near to the surface;
Lack of fusion in the
interface.
Large pore near to the
surface; small pore in the
middle of the coating; 3
small cracks in the coating.
3 Layers
2 pores near to the side
surface; Lack of fusion in the
interface.
2 large pores; 5 cracks; 1
crack associated to the pore.
Table 4-2 – Defects in 316 stainless steel clads.
Defects in H13 tool steel clads can be seen in Table 4-3.
H13 tool steel
Powder Wire
1 Layer A small pore above the
interface zone.
4 cracks and 1 small pore in
the coating.
2 Layers
1 pore above the interface
zone and 1 pore in the
coating.
2 cracks in the middle of the
coating.
3 Layers Lack of adhesion. 2 small cracks; 1 crack
associated to the pore.
Table 4-3 – Defects in H13 tool steel clads.
36
The defects present in AISI P20 tool steel clads are shown in Table 4-4.
AISI P20 tool steel
Powder Wire
1 Layer No defects. Lack of fusion and 2 pores.
2 Layers 2 small pores above the
interface zone.
Lack of fusion and 1
large pore.
3 Layers No defects.
Lack of fusion; 1 crack in the
interface; 1 crack in the
coating; 4 pores.
Table 4-4 – Defects in AISI P20 tool steel clads.
From the overall samples, three with powder deposits are of good quality, while in the wire
deposits samples only one is of good quality.
Figure 4-1 – 316 stainless steel, powder, 1 layer. Figure 4-2 – 316 stainless steel, wire, 1 layer.
Figure 4-3 – AISI P20 tool steel, powder, 1 layer. Figure 4-4 – AISI P20 tool steel, powder, 3
layers.
37
In powder deposits the most common defects are small pores, lack of fusion in the interface and
lack of adhesion in the interface, while the main defects in wire coatings are pores, cracks and
lack of fusion. Some examples of these defects can be seen in figures below.
Figure 4-5 – H13 tool steel, powder, 1 layer. Figure 4-6 – AISI P20 tool steel, powder, 2 layers.
Figure 4-7 – 316 stainless steel, wire, 2 layers. Figure 4-8 – H13 tool steel, wire, 1 layer.
38
Porosity and lack of fusion in powder coatings can be due to fact that the metal powder could
not be fully melted [7].
Figure 4-9 – 316 stainless steel, powder, 2 layers. Figure 4-10 – H13 tool steel, powder, 2 layers.
Figure 4-11 – 316 stainless steel, powder, 3 layers.
Another reason for porosity in powder coatings is the gas trapped, which occurs because gases
produced during the cladding process, do not have enough time to escape out of the molten
pool. Porosity can be avoided by increasing the laser power. The higher the power, the longer
the solidification time, and thus a reduced amount of gas is trapped in the molten liquid [17].
This relation could be proved examining the following equations.
The cooling speed of the material, R (K/s), is given by [29]:
Where:
k – Thermal conductivity of the material [W/m.K];
P – Laser power (W);
R – Cooling speed (K/s);
Tc – Critical temperature (K);
T0 – Initial temperature of the material (K);
ΔT1 – Thermal interval achieved during the laser power application (K).
39
And it could also be given by:
In the equation (4.8), the solidification time is represented by tSol. (s).
The solidification interval can be estimated from the phase diagram for an alloy, in steady state
conditions, and is represented by ΔT2 (K):
Where:
TLiquidus – Temperature which starts the solidification process;
TSolidus – Temperature which finishes the solidification process.
Substituting the ΔT1 for ΔT2 in equation (4.7) is possible to see that increasing the laser power,
decreases the cooling rate. Considering that ΔT2 is constant, the solidification time increases
when R decreases, as it could be proved in equation (4.8). Therefore, to increase the laser
power leads to the increase of the solidification time, since cooling speed decreases.
This is in agreement with rapid solidification theory, where it is shown that increasing the
solidification time, deviation from steady state conditions occur and the equilibrium solidus curve
is displaced to lower temperatures [29].
The lack of adhesion found in H13 tool steel (powder and 3 layers) can be explained by an
incomplete fusion of the base material (substrate) [39], and can be prevented with the increase
of the laser power (P).
Figure 4-12 – H13 tool steel, powder, 3 layers.
None of these defects are results of inefficient gas protections (Ar) or powder feeder gas rate
(Ar), as both values are in agreement with other researchers, where good results were
achieved [17;26;27].
40
Porosity in wire coatings is due to the gas trapped in the molten pool and could be avoided by
increasing the laser power (P), as it was done in powder coatings.
Figure 4-13 – AISI P20 tool steel, wire, 3 layers.
The lack of fusion in the AISI P20 tool steel (wire) can be a result of a deficient wire melting
since the wire is more compact than powder. Additionally, the pulsed mode has periods of low
energy that can be insufficient for the melting process. This fact can also be used to explain the
existence of porosity.
Figure 4-14 – AISI P20 tool steel, wire, 1 layer. Figure 4-15 – AISI P20 tool steel, wire, 2 layers
(coating).
One way to verify the melting capacity of the filler material is to determine the necessary energy
to melt a specific volume of the filler material. This melting capacity could be quantify by,
MC (J/ºC.cm3):
The term c (J/g.ºC) in equation (4.10) represents the specific heat capacity of the filler material ,
and ρ is the density of the filler material.
41
The values of MC for each substrate material are presented in Table 4-5.
MC (J/ºC.cm
3)
Powder Wire
316 stainless steel 2.5 4.0
H13 tool steel 2.8 3.2
AISI P20 tool steel 2.5 3.5
Table 4-5 – The melting capacity of the powder and wire filler material for each substrate material.
Analysing the values for MC, these are higher in wire coatings than in powder coatings, so more
energy is required to melt a unit volume of clad material when it is in a wire shape.
Cracks can be hot cracks or cold cracks. These two kinds can be present in laser cladding
process using filler wire. Hot cracks are present in the 316 stainless steel coatings, and they are
essential due to the low value of δ ferrite (< 5%) and thermal stress created during the welding
process. Using the Schaeffler diagram it is possible to estimate the quantity of δ ferrite present
in the fusion zone [29].
Figure 4-16 – Schaeffler diagram [29].
The value achieved for the 316L stainless steel is inferior to 5%.
42
One way to prevent hot cracks is increasing the quantity of δ ferrite, and this could be done
increasing the cooling speed [29]. Decreasing the laser power it is possible to increase the
cooling rate, since the heat input decreases.
Figure 4-17 – 316 stainless steel, wire, 3 layers.
Porosity can be reduced by increasing the laser power (P), however this also increases the
probability of arising hot cracks. Both defects represent a limitation regarding the coatings
mechanical properties, but, as a general rule, hot cracks have grave consequences. Therefore it
is suggested to decrease the laser power (P).
In the H13 tool steel and AISI P20 tool steel are present cold cracks, and are a result of the
thermal residual stresses.
Figure 4-18 – H13 tool steel, wire, 2 layers.
Cold cracks appear when the clad layer is too thick and near the stress concentrated zones.
This is because large thermal residual stress is produced during rapid cooling of the clad layer.
The wire coatings done on these substrate materials present a high value of hardness, more
than 450 HV. Hard materials have low capacity of accommodating the thermo-mechanical
stresses, and as a consequence they crack. To prevent this cracking an annealing treatment
can be beneficial, immediately after the laser cladding process is finished [17].
43
Another cold crack case is when the clad sample is machined, because of improper cutting or
grinding processes. One improved method is to use spark cutting [17].
In wire coatings some cracks and pores can be seen together. The presence of a pore
originates a stress concentration point that can initiate a crack line.
Figure 4-19 – H13 tool steel, wire, 3 layers.
By examining the Tables and Figures presented in this section, it is allowed to conclude that the
quantity and dimension of defects in wire deposits is higher than in powder deposits.
4.2.2 Microstructural analysis
Macrographic analysis allows identifying defects as pores, cracks, lack of fusion and adhesion,
while in microstructure analysis is possible to see micro-defects and structures formed after the
cladding process.
4.2.2.1 316 stainless steel
Microstructure of powder coatings performed in the 316 stainless steel can be seen in Figure
4-20 and 4-21.
Figure 4-20 – 316 stainless steel, powder, 3 layers. Figure 4-21 – 316 stainless steel, powder, 3 layers
(coating).
44
The wire coatings microstructure of the 316 stainless steel is presented in Figure 4-22 and 4-23.
Figure 4-22 – 316 stainless steel, wire, 1 layer. Figure 4-23 – 316 stainless steel, wire, 1 layer
(coating).
Above the interfacial zone of the coatings done on the 316 stainless steel it can be observed a
fine cellular-dentritic structure (epitaxial) oriented along the heat flow direction, which is a
perpendicular direction to the interface line.
In the middle of the coatings the grains are coarser than above the interfacial zone, and are
presented with different directions (Figure 4-21). This fact could explain why it is observed a
decrease in the hardness values along the coatings (Figure 4-45).
A continuous phase, without micro-defects along the interface zone, allows having a good
bonding between the substrate and the coating material.
4.2.2.2 H13 tool steel
Microstructure of powder coatings done on the H13 tool steel is shown in Figure 4-24 and 4-25.
Figure 4-24 – H13 tool steel, powder, 2 layers. Figure 4-25 – H13 tool steel, powder, 2 layers
(coating).
45
Microstructure of wire coatings performed on the H13 tool steel can be seen in Figure 4-26 and
4-27.
Figure 4-26 – H13 tool steel, wire, 2 layers. Figure 4-27 – H13 tool steel, wire, 2 layers (coating).
A cellular-dentritic structure oriented with the heat flow direction, above the interfacial zone, is
observed in both coatings done on the H13 tool steel. In these coatings, the structure is fine
martensite, with hardness above 350 HV (Figure 4-46).
In powder coatings a structure with fine grains near to the surface is observed. This is due to
the fact that fast cooling speeds near to the surface are achieved, since it is easy to dissipate
the heat introduced.
There are no micro-defects in these coatings and a good bonding with a continuous phase in
the interface zone is achieved.
4.2.2.3 AISI P20 tool steel
Powder coatings microstructure of the AISI P20 tool steel is presented in Figure 4-28 and 4-29.
Figure 4-28 – AISI P20 tool steel, powder, 3 layers. Figure 4-29 – AISI P20 tool steel, powder, 3 layers
(coating).
46
Microstructure of wire coatings performed in the AISI P20 tool steel is shown in Figure 4-30 and
4-31.
Figure 4-30 – AISI P20 tool steel, wire, 2 layers. Figure 4-31 – AISI P20 tool steel, wire, 2 layers
(coating).
The structure in powder coatings is martensite, and near to the surface a structure with fine
grains is observed. This fact validates the small increase in the hardness values (Figure 4-47).
In wire coatings a ferritic structure is seen with equiaxed grains, which is due to the high content
of iron in this filler material.
A good bonding without micro-defects is achieved in coatings done on AISI P20 tool steel.
4.2.3 Dilution
The dilution in each substrate material is analysed in this section with the aim of comparing the
dilution obtained in the powder and wire clad.
One of the most important objectives in achieving a high quality clad layer is to produce a fusion
bond between the substrate and filler material with the minimal dilution [7;8]. The dilution of the
coating in the substrate material must remain as low as possible in order to conserve the initial
properties of the coating and the substrate, but high enough to guarantee full bonding to base
material and prevent lack of fusion [20;32].
The evolutions of dilution for each substrate material are shown in Figure 4-32, 4-33 and 4-44.
Figure 4-32 – Dilution in the 316 Stainless steel. Figure 4-33 – Dilution in the H13 tool steel.
0
10
20
30
40
50
0 1 2 3 4
Dilu
tio
n (
%)
Layers
316 Stainless Steel
Wire Powder
0
10
20
30
40
50
0 1 2 3 4
Dilu
tio
n (
%)
Layers
H13 Tool Steel
Wire Powder
47
Figure 4-34 – Dilution in the AISI P20 tool steel.
Due to the evolution of dilution in each substrate material it is possible to say that the value of
dilution in powder coatings is always lower than in wire cases, except in the AISI P20 tool steel
with 1 clad layer which is approximately equal. The reason for this could be given by the effect
on the penetration of the two most important factors: speed and power [20]. It is known that the
dilution decreases with the raise of cladding speed of the laser beam and increases
proportionally with the laser power [7;20]. Therefore, the dilution in laser cladding using filler
powder is lower than in filler wire, because in powder deposits it is achieved a higher speed and
lower power laser input on the substrate material than in wire deposits.
By examining the dilution evolution with the number of filler material deposited layers on the
substrate, it can be proved that the dilution decreases with the increase of the number of layers.
This behaviour could be explained because the depth of penetration into the substrate is
reduced when the subsequent layer builds on the previous layer in multi-layer laser cladding [6].
This is essentially due to the fact that the laser power input on the substrate is reduced, as most
part of the laser power is consumed in the bonding of the successor layers.
The microstructure analysis of powder and wire coatings shows that a good fusion bond
between the substrate and filler material is achieved. Therefore the values of dilution in powder
coatings are better than in wire coatings, since the goal of the laser cladding process is
achieved with the minimal risk of changing initial properties of the substrate and filler material.
0
5
10
15
20
25
30
0 1 2 3 4D
ilu
tio
n (
%)
Layers
AISI P20 Tool Steel
Powder Wire
48
4.2.4 Melting and clad shape
The clad bead shape and dilution which are governed by the bead geometry, plays an important
role in determining the mechanical properties of the clad. To obtain the desired welds quality, it
is essential to have a complete control over the relevant parameters to obtain the required bead
geometry (Figure 4-35) on which the integrity and quality of a weldment is based [38].
The first aim of any cladding process is to create a bonding between the substrate and the filler
material, and it is also important that this connection is maintained along the full width of the
run. Looking at the melting shape is allowed to see if an integral bonding is achieved.
Some applications in cladding processes need to have more than one filler material deposited in
layers on the substrate material. One way to achieve this last goal is having a clad shape which
allows the subsequent deposition of filler material. The clad’s angles and the width-to-height
aspect ratio (W/H) are important geometric factors to perceive if the clad shape supports an
additional layer of filler material [20;53].
A schematic diagram showing the geometrical parameters of clad shape are presented in
Figure 4-35.
Figure 4-35 – Schematic diagram to show the geometrical parameters of clad shape.
The clad’s angles are represented by α1 and α2 (º), the height of the clad by H (mm) and the
width’s clad by W (mm).
The figures below (Figure 4-36 to 4-43) show the coatings with the aim of allowing the analysis
of melting and clad shape.
Figure 4-36 – 316 stainless steel, powder, 1 layer. Figure 4-37 – 316 stainless steel, wire, 1 layer.
49
Figure 4-38 – 316 stainless steel, powder, 2 layers. Figure 4-39 – 316 stainless steel, powder, 3 layers.
Figure 4-40 – H13 tool steel, powder, 2 layers. Figure 4-41 – H13 tool steel, wire, 2 layers.
Figure 4-42 – AISI P20 tool steel, powder, 3 layers. Figure 4-43 – AISI P20 tool steel, wire, 3 layers.
In wire and powder coatings on the 316 stainless steel, it is possible to see that the deposition
of one layer of filler material has permanent melting along the full width of the coating. In the
cases of two and three layers of powder deposits it is detected a part of the width’s coating with
a very low penetration in the substrate. A consequence of this low melting could be seen in
Figure 4-11 and Figure 4-39, which is lack of fusion in the interface zone. This situation
happens in these both cases. The existence of this lack of fusion could be the result of an
insufficient powder melting with the substrate material, as it was written previously. Lack of
50
fusion in the interface zone is a dangerous situation because the crack can increase and result
in disaggregation between the coating and the substrate material in an extreme case. One way
to avoid this problem is to increase the laser power, since it will increase the mixing with the
substrate.
An integral melting along the full width of the clad with two and three layers of deposited wire is
achieved.
By examination of powder deposits on the H13 tool steel it is possible to see that there is a part
of width’s coating with a low melting in all the three cases of layers deposition. In the H13 tool
steel sample with three layers of deposited material it is seen a lack of adhesion between the
coating and the substrate, as it is shown in Figure 4-12. The consequence of this lack of
adhesion is the same as described above.
A good melting shape is observed in all the wire coatings done on the H13 tool steel.
In the AISI P20 tool steel it is achieved an integral melting in powder and wire deposits. The
high-quality of the powder melting shape could be due to the fact that the lowest value of the
powder rate ( ) is used, thus it is possible to have a good control over the melting shape, since
it is easier to melt a lower quantity of filler material for the same time and laser power. Another
reason for the high-quality of the powder melting shape is the compatibility between the
substrate and the powder material, as different materials with similar melting points have an
excellent adhesion. The melting points are respectively 1427ºC and 1257ºC.
The values of geometrical parameters of powder clad for each substrate material are shown in
Table 4-6.
Powder
α1 (º) α2 (º) W (mm) H (mm) Ratio W/H
316
stainless
steel
1 layer 72 90 1.91 1.00 1.91
2 layers 72 104 2.23 2.64 0.85
3 layers 103 90 2.11 3.55 0.60
H13 tool
steel
1 layer 90 115 2.55 0.90 2.84
2 layers 90 90 2.00 1.92 1.04
3 layers 113 60 2.05 2.71 0.76
AISI P20
tool steel
1 layer 126 145 2.37 0.49 4.84
2 layers 90 132 2.26 1.32 1.71
3 layers 90 122 2.15 1.88 1.14
Table 4-6 – Geometrical parameters of powder clad.
51
The values of geometrical parameters of wire clad for each substrate material could be seen in
Table 4-7.
Wire
α1 (º) α2 (º) W (mm) H (mm) Ratio W/H
316
stainless
steel
1 layer 153 150 2.54 0.31 8.20
2 layers 144 131 2.77 0.60 4.62
3 layers 122 136 2.69 0.82 3.28
H13 tool
steel
1 layer 154 154 2.88 0.19 15.16
2 layers 151 144 3.12 0.38 8.21
3 layers 135 153 3.17 0.60 5.28
AISI P20
tool steel
1 layer 143 154 2.26 0.20 11.30
2 layers 129 146 2.40 0.50 4.80
3 layers 114 124 2.39 0.82 2.91
Table 4-7 – Geometrical parameters of wire clad.
One way to obtain a clad shape which allows subsequent over-lapping layer with smooth
profiles is to obtain clad angles larger than 100º and a width-to-height (W/H) aspect ratio of
5:1 [20;53].
Looking at the values of clad angles in powder and wire coatings it is possible to prove that it is
achieved better results in wire coatings than in powder coatings. This explains why the wire clad
shape is more symmetrical than the powder clad shape. The consequence of having a
non-symmetrical clad shape is extra machining work.
The reason which justifies the values of clad angles and the clad shape obtained in wire
deposits is because the wire coating process is more focused than the powder coating process,
since in wire coating process it is used wire as filler material, which allows having a good control
over the relative position of the filler material/substrate. In powder coatings the powder is blown
thus it is difficult to determine where some part of the powder will reach the substrate and the
last deposited layer. So, one way to improve the clad angles and the clad shape in powder
deposits is decreasing the powder rate because this procedure allows having a concentrated
powder flow.
Observing the values of the W/H aspect ratio, it is possible to see that they are better in laser
cladding using filler wire material than using powder filler material, since they are close to the
desires values. Coatings with a 5:1 W/H aspect ratio have a good mechanical stress resistance.
This could be proved by interpretation of the analysis of a possible mechanical stress situation.
52
A possible mechanical stress situation can be seen in Figure 4-44.
Figure 4-44 – Diagram showing a possible mechanical stress situation.
The application of a force, F (N), in the coating, will create a maximum intensity moment in the
coatings’ base, M (N.m), as is shown in Figure 4-44 [54].
This moment result in a normal stress, ζ (MPa), in the coatings’ base [54]:
Where Z (m3) is called the section modulus.
The applied force also results in a shear stress, ζ (MPa), in the base of the coating [54]:
Considering a case where the applied force and the run’s length have the same values for both
the powder and the wire coatings, it is possible to obtain a ratio between the stresses, normal
and shear, achieved for both processes. These ratios are presented in Table 4-8.
53
316
stainless
steel
1 layer 4.3 1.3
2 layers 5.5 1.2
3 layers 5.5 1.3
H13 tool
steel
1 layer 5.3 1.1
2 layers 7.9 1.6
3 layers 8.2 1.5
AISI P20
tool steel
1 layer 2.3 1
2 layers 2.8 1.1
3 layers 2.5 1.1
Table 4-8 – Stresses ratio between the two processes.
Evaluating the values obtained for stress ratios, it is observed that they are never inferior to 1. It
means that the normal (ζ) and shear (ζ) stress are higher in powder coatings than in wire
coatings. Therefore, the normal and shear stress in wire coatings are better supported than in
powder coatings.
4.2.5 Hardness analysis
In Carrs Welding Technology, laser cladding process is used for components repair, such as
moulds, tools or guns. These components need to be repaired mainly for two reasons: wear
components parts or to cover the shapes of the moulds with material. In the first one it is good
to improve the wear resistance. Therefore, the increase of hardness is needed. In the second
case it is just needed to keep the same hardness of the substrate material. Hence, in any case
coatings with hardness lower than the components are undesirable.
The hardness profiles of each substrate material for both filler material are shown in Figure
4-45, 4-46 and 4-47. Coatings with three layers were chosen because to obtained more
hardness points. The vertical lines in these figures represent the line of the interfacial zone. It
means that in the left side of the vertical line the hardness of the coating material is
represented. While in the right side it is the substrate material.
54
Figure 4-45 – Hardness in 316 stainless steel. Figure 4-46 – Hardness in H13 tool steel.
Figure 4-47 – Hardness in AISI P20 tool steel.
In powder deposits on the 316 stainless steel is achieved a value of hardness in the coating
surface similar to the substrate material. This means that this powder material is a good choice
for applications where the primary aim is not to increase the wear resistance.
The hardness of the wire coatings done on the 316 stainless steel (Figure 4-45) is lower than
the hardness of the substrate material. This result is expected because the wire filler material
chosen is the 316L stainless steel (148 HV) which has a value of hardness more minor than the
316 stainless steel (190 HV) [31]. Therefore, this is not a good filler material to be used on the
316 stainless steel.
In Figure 4-46 is possible to observe an increasing of coating hardness done on the H13 tool
steel for wire and powder process. This proves that both filler material are an excellent option in
applications needing an improvement of wear resistance. It is also allowed to prove which the
coating hardness does not depend of the process type of the filler material used.
The powder coatings done on the AISI P20 tool steel achieve near to the surface of the coating
a value of hardness approximately equal to the substrate material. This fact could be a result of
150
160
170
180
190
200
210
220
0 1 2 3 4 5
HV
2
Distance from surface (mm)
Powder v.s Wire (3 layers - 316 Stainless steel)
Powder Wire
0
200
400
600
800
0 1 2 3 4
HV
2
Distance from surface (mm)
Powder v.s Wire(3 layers - H13 tool steel)
Powder Wire
0
100
200
300
400
500
600
700
0 1 2 3 4
HV
2
Distance from surface (mm)
Powder v.s Wire (3 layers - AISI P20 tool steel)
Powder Wire
55
the low percentage of the carbon in the filler material. So, this filler powder material must not be
used on the AISI P20 tool steel when the first aim is to increase the wear resistance.
By examination of the hardness profile of wire deposits in AISI P20 tool steel is allowed to say
that the wire filler material used can improve the wear resistance, since the hardness coating is
higher than the substrate.
Near to the right side of the vertical line (interfacial zone) is the well known heat-affected zone
(HAZ) and near to the left line is the melting zone (MZ). In the HAZ of all cases is seen an
increasing of hardness in relation of substrate. This is due to the formation of martensitic
structure in this zone resulting from the thermal gradients created during the laser cladding
process. In most cases is also possible to see the MZ with hardness higher than in the HAZ.
This fact results by the increasing of the carbon element in this mixing zone between the filler
and substrate material.
Analysing the hardness profiles in each substrate material, it is possible to conclude that the
coatings’ hardness is essentially due to the properties of the filler material and not due to the
material used in the deposition process (powder or wire). This means that a good hardness
profile in laser cladding can be achieved, independently of the feeding process: powder or wire.
4.3 Quality analysis
4.3.1 Quality factor expression
Through the examination of the analyses done in all samples, it is difficult to obtain an answer
for the best overall quality of the coating, since more than one factor needs to be considered.
With the aim to obtain a global quality factor, an empirical expression that considers the most
important factors in the definition of coating quality was developed. These factors are: the
number of defects in the coating (pores, lacks of fusion, cracks and lacks of adhesion), the
dilution of the coating in the substrate material, the melting and clad shape, and the width-to-
height ratio of the clad.
Defects represent a limitation in the reliability and in the quality of the coating, since a deficient
union among the elements origin low coatings mechanical properties. This is the reason why
factors associated to defects are considered in the coating’s quality.
One of the main goals of laser cladding is to obtain a minimal mixing (dilution) of the clad
material in the substrate material without the risk of occurring lack of fusion between these two
materials. An inadequate dilution can modify the initial properties of the materials used as
coatings and substrate. Therefore, it is important to achieve an adequate value of dilution to
obtain a good quality coating.
Another goal of any cladding process is to create a bonding between the substrate and the filler
material, and it is also important that this connection is maintained along the full width of the
56
run. The melting shape allows to verify if an integral bonding is achieved and it is considered an
important factor to the coating’s quality.
Some applications in cladding processes need to have more than one filler material deposited in
layers on the substrate material. One way to achieve this last goal is to have a clad shape that
allows the subsequent deposition of filler material. The clad’s angles and the width-to-height
aspect ratio (W/H) are important geometric factors to perceive if the clad shape supports an
additional layer of filler material. These factors are also important to obtain coatings with good
mechanical stress resistance and without stress concentration points in the coating. Hence,
these two factors are considered in the quality factor expression.
The quality of the coating increases proportionally with the value of the quality factor. The
quality factor, QF (%), is given by:
The factors written in bolt in the expression represent:
NP → The number of pores and/or lacks of fusion in the coatings;
NC → The number of cracks and/or lacks of adhesion in the coatings;
D → The value of dilution in the substrate material;
MS → The melting shape;
CA → The clad angles;
RW/H → The width-to-height ratio of the clad.
To each factor is associated a specific weight that is represented by the C letter:
CNP CNC CD CMS CCA CW/H
0.10 0.15 0.25 0.30 0.10 0.10
Table 4-9 – Weights for each factor.
The values attributed to the weight of each factor do not have a scientific rigour. They were
determined with the help of the developed experience during the realization of this project. The
same idea was considered to quantify the quality values associated to each factor, since none
of these factors has a defined value regarding its impact on the overall quality of the deposit.
57
4.3.2 Factors
The values of the NP (pores and lacks of fusion) and NC (cracks and lacks of adhesion) factors,
according to the number of defects present in the coatings, could be seen in Table 4-10.
Number of defects NP and NC
0 1
1 0.9
2 0.8
3 0.7
4 0.6
≥ 4 0.5
Table 4-10 – The NP and NC values associated to the number of defects.
The maximum value of quality of the NP and NC factor is achieved when the coating is free of
defects. If the number of defects in the coating increases the quality decreases.
It is shown in Table 4-11 the values of the dilution factor (D).
Dilution (%) D
0 - 5 0.4
5 - 10 1
10 - 20 0.85
20 - 30 0.7
30 - 40 0.6
≥ 40 0.4
Table 4-11 – Values of the dilution factor (D).
There is no perfect value of dilution, although it is good to obtain coatings with the minimum
dilution as possible. According to the acquired experience during this work, it is possible to say
that an excellent value of dilution is between 5 and 10%, since it can be obtained an integral
bonding along the full width of the coating without the risk of desegregation (D=1). This risk
could occur in coatings with substrate dilution between 0 and 5% (D=0.4). Another situation
where this low value is obtained is when the dilution is above 40%, since it could be seen a
significant change in the properties of the substrate material (D=0.4). This is proved by
observing the wire coating done on the 316 stainless steel where it is seen an alteration of
substrate hardness (Figure 4-45). In the others 3 groups of dilution (10-20%, 20-30% and
30-40%) the quality of the coatings decreases with the increasing of the dilution, because the
risk of changing important properties of the substrate material also increases.
58
MS factor can have two values:
1 → If an integral bonding along the full width of the run is obtained;
0.5 → If a non-integral bonding along the full width of the run is obtained.
A perfect melting shape has to guarantee a complete bonding along the full coating width. The
maximum value of quality is achieved when this requisite is assured. If a fraction of the coating
width has not an integral bonding could result in the appearance of dangerous defects in the
interface zone as it was described previously. The quality of coating could be seriously affected
in these cases. Hence, a low value is attributed to the MS factor.
The values of quality associated to the CA factor are:
1 → If α1 and α2 ≥ 100º;
0.85 → If α1 and α2 > 90º;
0.70 → If α1 or α2 ≤ 90º.
The quality associated to clad angles is related to the capability of the deposition of a
subsequent layer. This is important because many applications in laser cladding need to have
more than one deposited layer of filler material on the substrate. This maximum capacity is
obtained for clad angles above 100º, and consequentially, the maximum quality of the coating is
achieved. This capacity decreases proportionally with the clad angles.
RW/H factor values are present in Table 4-12.
W/H ratio RW/H
≤ 2 0.6
2 - 3 0.7
3 - 4 0.8
4 - 5 0.9
≥ 5 1
Table 4-12 – The RW/H values.
The width-to-height aspect ratio has two important roles in quality of the coating: a good W/H
ratio has an adequate capacity of deposition of a subsequent layer and an improved mechanical
stress resistance. The ideal value of W/H ratio is 5, in agreement with other researchers [20].
So, the maximum quality related to the RW/H factor is obtained for ratios equal or above 5. The
quality decreases for ratios below 5 due to the inferior capacity of the subsequent deposition of
filler material and reduction of mechanical stress resistance.
59
4.3.3 Results and discussions
The results of QF in each substrate material, for powder and wire coatings with different number
of deposited layers, are shown in Figure 4-48, 4-49 and 4-50.
Figure 4-48 – The QF in the 316 stainless steel. Figure 4-49 – The QF in the H13 tool steel.
Figure 4-50 – The QF in the AISI P20 tool steel.
Comparing the results of powder and wire coatings, it is possible to say that more wire coatings
with higher quality are obtained than in powder coatings. It can also be seen that the difference
of QF values is more significant in the cases where a superior wire coating is obtained. This
could be easily seen in the coatings, for instance: with 2 layers performed on the 316 stainless
steel (18.5%), 2 and 3 layers done on the H13 tool steel (13.5 and 29.3%, respectively), where
the values in parenthesis show the percentage of these differences. The powder coatings cases
with a superior QF are: 1 layer done on the 316 stainless steel (0.5%), 1 and 3 layers performed
on the AISI P20 tool steel (2 and 4%, respectively).
85,5
74 7585
92,5 87,5
0
20
40
60
80
100
1 2 3
QF
(%
)
Layers
316 stainless steel
Powder Wire
74,25 76
61,5
83 89,5 90,75
0
20
40
60
80
100
1 2 3
QF
(%
)
Layers
H13 tool steel
Powder Wire
91,5 87,259389,5 93,25 89
0
20
40
60
80
100
1 2 3
QF
(%
)
Layers
AISI P20 tool steel
Powder Wire
60
By observing the QF graphs in each substrate material, a similar value of QF in wire coatings is
seen, which is approximately 89%. In powder deposits this situation is not verified. Irregular and
low values of QF in coatings done on the 316 stainless steel and on the H13 tool steel are
indentified, while on the AISI P20 tool steel the values achieved are higher and regular. This
behaviour is explained by the melting shape obtained, since the MS factor has a relevant
importance in the QF value. The melting shape was worst in powder coatings done on the 316
stainless steel and on the H13 tool steel and it was better on the AISI P20 tool steel. As it was
stated before, this could be proved by the inferior value of the powder rate ( ) used in the AISI
P20 tool steel. Therefore, one way to acquire a superior quality in the 316 stainless steel and in
the H13 tool steel is to decrease the powder rate. This conclusion can be extrapolated for
generic powder coatings.
It is also curious to observe that powder coatings can achieve higher quality than wire coatings
when it is used an adequate powder rate, as it could be confirmed in the AISI P20 tool steel.
The most important result of the quality analysis is that the wire coatings have higher quality
than the powder coatings, but the quality of powder coatings can be improved with the adequate
powder rate. Therefore, the powder rate must be reduced in the 316 stainless steel and in the
H13 tool steel to achieve higher quality. Laser cladding using powder filler material with inferior
powder rate will decrease the productivity of the process. But, nevertheless, the productivity is
always higher in powder coatings than in wire coatings.
4.4 Theoretical powder rate
In this section it is developed a study based on the heat transfer theory to obtain a theoretical
powder rate, which is the one that could permit the total melting of all particles of the powder
material. Since an incomplete melting of the powder particles origins undesired situations, such
as, defects and bad melting shapes, that result in coatings’ quality decreasing.
This theoretical powder rate represents the capability of the process, with a precise laser power,
to melt the coating powder mass in a specific time, which is the one that guarantees the total
melting of the powder.
To perform this study, a parallelepiped coating shape was considered, nevertheless the same
coating transverse area of the real case was used. This particular shape was chosen due to the
fact that is the one closest to the real scenario, allowing some simplification in the calculation of
the theoretical powder rate. A simple layout showing this situation is presented in Figure 4-51.
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Figure 4-51 – Layout of the clad and the substrate material.
In Figure 4-51, H* and HS represent the height of the coating parallelepiped shape and of the
substrate material, respectively.
Taking into consideration the coating as a control volume of a system, where the mass and the
energy can pass thought, remaining constants, it is possible to apply the energy conservation’s
law (the first law of thermodynamics). For the time interval considered, the fusion time (tf) of the
specific mass of powder material, this law can be written as [55]:
Where:
Ei – Input energy (J);
Eg – Generated energy (J);
EL – Lost energy (J);
Es – Stored energy (J).
The input energy is associated to laser power during the fusion time. Hence:
Where η is the laser’s output power efficiency, approximately 70% [41].
In this case, Eg is considered to be zero for practical effects, however there are
exothermic/endothermic reactions during the materials transformation phases, which could be
considered as sources of energy.
The lost energy of the control volume is transmitted by three mechanisms of heat transfer:
conduction, convection and radiation, that are represented by qConduction, qConvection and qRadiation,,
respectively.
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The total rate of heat transfer by conduction, qConduction (W), is given by [55]:
Where:
k – Thermal conductivity of the material [W/m2.K];
TM – Melting temperature (K);
TS – Substrate’s temperature (K).
The total rate of heat transfer by convection, qConvection (W), is given by [55]:
Where:
h – Convective heat transfer coefficient of the process (W/m2.K);
ASurface – Heat transfer area of the surface (m2);
TSurface – Temperature of surface’s coating (K);
TAir – Temperature of the air (K).
In the calculation of the convective heat transfer coefficient, h, an association of resistances
was used, as is shown in Figure 4-52.
Figure 4-52 – Equivalent electrical circuit.
The melting temperature is represented by TM in Figure 4-52, and the resistances are
represented by R1, R2 and R3.
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Hence, it is possible to obtain an equivalent resistance. Regarding the analogy with an electric
circuit (Ohm’s law), the convective heat transfer coefficient can be determined [55].
Considering that the laser energy is converted to the clad material as conduction energy, an
estimative of the coating surface temperature, TSurface, can be obtained by manipulating
equation (4.20).
The total rate of heat transfer by radiation, qRadiation (W), is given by [55]:
Where:
ε – Emissivity;
ζS-B – Stefan-Boltzmann constant (5.67x10-8
W/m2.K
4).
The stored energy (Es) represents the energy required to melt a specific mass of powder
material [55].
Where:
m – Mass (Kg);
hf – Heat of fusion (KJ/Kg).
As all the terms of equation (4.15), it is possible to determine the fusion time of a specific
coating mass. Therefore, it can be determined the theoretical powder rate that the process with
a specific laser power of 700 W can melt.
The values of theoretical powder rate of each powder material used in this work can be seen in
Table 4-13.
(g/min)
316 stainless steel 9
H13 tool steel 9.8
AISI P20 tool steel 9.4
Table 4-13 – The values of theoretical powder rate in each substrate material.
64
These values represent a theoretical approach to obtain better quality results in powder
coatings, since some problems related to incomplete melting of the powder material, which
reduce the quality of the powder coatings can be avoided, as stated before. The theoretical
power rate can also be used as efficiency metric, since it represents the maximum mass that
can be melted by the process. If a higher value then this one is used, an incomplete melting of
some particles of powder can occur, resulting in defects on the coating, as it was stated earlier.
On the other hand, if a lower value is used, an increase of the penetration in the substrate
material can lead to an extra dilution.
The theoretical powder rate values obtained have an error associated, since some
simplifications were considered, such as, the parallelepiped shape of the coating and the
approximation of properties to the standard material (not in powder form). Nevertheless, the
theoretical powder rate represents a good approximation.
65
5 Conclusion and future developments
The main goal of this work was to compare the productivity and the quality of the laser cladding
process using powder and wire filler material. Some experiments were performed with different
materials to have a complete analysis regarding the desired purpose.
These specific materials were used because they are very important for the Carrs Welding
Technology company to be familiar with the behaviour of laser cladding process using both filler
material processes in these materials as laser cladding is very requested in these materials. It is
also important to have a general conclusion, independent of the material used, regarding the
laser cladding process: powder or wire.
Several analyses were performed with the aim of determining which laser cladding process
(powder or wire) has the best performance in productivity and in quality. Some interesting
results of these analyses are summarised in the conclusions of this work.
Some suggestions for future work aroused during the elaboration of this work. These ideas are
presented in the future development’s section.
5.1 Conclusion
Laser cladding using powder filler material has more productivity than laser cladding using wire
filler material, as a consequence of a higher material deposition rate ( ). It is possible to state
that the productivity is higher in laser cladding with filler powder, but it is not possible to obtain a
real value for how much this productivity is superior in this process, since it is different for each
specific material.
In powder coatings defects as pores, lacks of fusion or lacks of adhesion in the interface can be
observed, whereas the most common defects in wire coatings are pores, cracks (cold and hot)
and lack of fusion.
One way to avoid porosity in laser cladding resulting from the gases produced during the
coating process is to increase the laser power (P), since it reduces the amount of gas trapped in
the molten liquid. However, a high laser power can lead to an increase of the dilution, changing
the initial properties of the materials involved.
In ductile materials, such as the 316 stainless steel, hot cracks in laser cladding using wire filler
material can be reduced decreasing the laser power (P) because the cooling speed increases,
and consequentially the quantity of δ ferrite.
Both porosity and hot cracks in ductile wire materials represent a limitation regarding the
coatings mechanical properties, but, as a general rule, hot cracks have grave consequences.
Therefore it is suggested to decrease the laser power (P).
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Laser cladding using hard wire filler materials is vulnerable to cold cracks resulting from a large
thermal residual stress produced during rapid cooling. Cold cracks can be prevented with an
annealing treatment, immediately after the laser cladding process being finished.
According to the defects analysis undertaken, it is possible to conclude that the quantity and
dimension of defects in wire deposits is higher than in powder deposits.
The microstructure analysis allows concluding that both processes, powder and wire, have a
good fusion bonding in the interface line between the coating and the substrate material.
Evaluating the result achieved for dilution in both processes it is possible to conclude that the
values of dilution in powder coatings are better than in wire coatings, since the goal of the laser
cladding process is achieved with minimal risk of changing initial properties of the substrate and
filler material. One way to improve the dilution results in wire coating is to decrease the laser
power (P), since the penetration in the substrate material is reduced.
Laser cladding using wire filler material has always excellent melting shapes. The melting
shapes achieved in powder deposits performed on the AISI P20 tool steel show that when is
used an adequate value of powder rate ( ), a good melting shape is obtained. Hence, the
quality of the melting shapes can be increased in powder coatings done on the 316 stainless
steel and on the H13 tool steel by reducing the powder rate ( ).
Comparing the clad shapes obtained in powder and wire coatings, it is possible to conclude that
the wire clad shapes allow a better capability of deposition of subsequent layers. One way to
improve the clad shape in powder deposits is to decrease the powder rate ( ) because this
procedure has a concentrated powder flow.
It is also possible to prove that mechanical stresses, such as normal and shear stresses are
better supported in wire coatings.
Analysing the hardness profiles in each substrate material, it is possible to conclude that the
coatings’ hardness is essentially due to the properties of the filler material and not due to the
material used in the deposition process (powder or wire). This means that a good hardness
profile in laser cladding can be achieved, independently of the feeding process: powder or wire.
A quality factor to evaluate the coating quality was suggested considering a set o parameters
related to: the number of defects in the coating (pores, lacks of fusion, cracks and lacks of
adhesion), the dilution of the coating in the substrate material, the melting and clad shape, and
the width-to-height ratio of the clad.
According to the values of the quality factor proposed (QF), it is possible to conclude that the
wire coatings have higher quality than the powder coatings. However, the quality of powder
67
coatings can be improved or even be better with an adequate powder rate ( ), as it was proved
in the QF results obtained in the AISI P20 tool steel.
Laser cladding using powder filler material with lower powder rate decreases the productivity of
the process. Nevertheless, the productivity is always higher in powder coatings than in wire
coatings.
Regarding the main objective of this work, the laser cladding using powder filler material is more
productive but has less quality than using wire filler material, although the quality of powder
coatings could be improved with the correct powder rate ( ).
In this work it was developed a study based on the heat transfer theory to obtain a theoretical
powder rate ( ), which allows to achieve better results in the quality of powder coatings.
The theoretical powder rate values for each case are: 9 g/min for the 316 stainless steel,
9.8 g/min for H13 tool steel and 9.4 g/min for AISI P20 tool steel.
5.2 Future developments
As future work it would be interesting to perform the same project using an automatic system for
the wire feeding process because it would be possible to compare different filler processes
(powder and wire) with automatic feeding systems.
It would be important to perform more tests in each material used in this work to discover the
correct process parameters that allow obtaining coatings with higher quality.
In order to obtain conclusions applicable in any laser cladding process, more experiments using
different materials should be carried out.
Considering Fick’s law, it would be of great interest to develop a study to determine the diffusion
of the shielding gas (Ar) in the structure of each of the materials used, and additionally, the
cooling speed expression can be used to obtain the correct laser power that avoids the gas to
be trapped in the molten liquid.
It would also be interesting to make corrosion tests to observe the performance of the laser
cladding using both filler material, and to introduce an extra factor in the QF expression related
to the results of the corrosion analysis.
68
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