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Short Communication Weldability of Al 4 C 3 –Al composites via diffusion welding technique Halil Arik a, * , Mustafa Aydin b , Adem Kurt a , Mehmet Turker a a Department of Metallurgy, Faculty of Technical Education, Gazi University, 06500 Ankara, Turkey b Department of Metallurgy, Faculty of Simav Technical Education, Dumlupinar University, Kutahya, Turkey Received 1 March 2004; accepted 23 July 2004 Available online 11 September 2004 Abstract In this study, Al–Al 4 C 3 composites, produced by powder metallurgy in situ techniques, were joined by diffusion welding tech- nique at 250 MPa pressure with various welding temperatures and durations. Microstructures and shear strengths of the joined areas were determined. Al powders were mixed with 2% carbon black and milled in a high energy ball mill (mechanical alloying) for up to 20 h. In order to obtain cylindrical blanks with 10 mm in diameter and 15 mm in height, powders were compacted in a single action press at 1000 MPa. Samples were sintered in Ar atmosphere at 650 °C and metal matrix composite (MMC) containing 8% Al 4 C 3 particles were produced. Products were then joined to each other by using diffusion welding techniques. Scanning electron micros- copy examination was carried out on the welded interfaces and shear tests were conducted to the sample interfaces to find out the effect of welding temperatures and duration on the weldability properties. It was found that high welding temperatures resulted in increase of both joined strength and shear properties. However, increase in welding duration did not make any detectable changes. Results indicated that MMC could be joined by diffusion welding technique successfully with the 88% strength of base material. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Mechanical alloying; Composite material; Al–Al 4 C 3 ; Diffusion welding; Mechanical properties 1. Introduction Aluminium and its alloys have found very wide appli- cation at aerospace and automotive industries due to their high specific strength, corrosion and wear resi- stances. For this reason number of scientist has concen- trated on it [1]. In order to increase the mechanical properties of this materials different strengthening mech- anisms such as: mechanical alloying, in situ technique, extrusion and reinforcing with small and hard refractory particles have been used [2–4]. It is possible to produce composite materials by applying one of those processes and obtaining better mechanical properties than those obtained at plain alloys especially at high temperatures [5–7]. MMC can be produced by liquid or solid state. In the liquid state, ceramics particles are added to liquid metal by stirring before casting, but the resulting distri- bution of reinforcing elements is generally inhomogene- ous. In the solid-state rote, known as the powder metallurgy technique, reinforcing powders are added to the metal powders and then blended, pressed and sin- tered to produce block composite materials. One of the most important problems encountered in this technique is obtaining unsatisfactory strength between the metal powders and the reinforcing particles interfaces. An- other way of producing fine and homogeneous distribu- tion of hard particles (Al 2 O 3 , SiC, Si 3 N 4 , etc.) in the matrix is the in situ method which is generally achieved by reaction milling and annealing processes. In this method very strong interface between the matrix and particle can be assessed. Many researches have been car- ried out by using this method [8,9]. When adjoining of these materials is required, weldability gains particular importance. Then the materials can be joined by either 0261-3069/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2004.07.017 * Corresponding author. Tel.: +90 312 212 6820; fax: +90 312 212 0059. E-mail address: [email protected] (H. Arik). www.elsevier.com/locate/matdes Materials and Design 26 (2005) 555–560 Materials & Design

Weldability of Al4C3–Al composites via diffusion welding technique

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Page 1: Weldability of Al4C3–Al composites via diffusion welding technique

Materials

www.elsevier.com/locate/matdes

Materials and Design 26 (2005) 555–560

&Design

Short Communication

Weldability of Al4C3–Al composites via diffusion welding technique

Halil Arik a,*, Mustafa Aydin b, Adem Kurt a, Mehmet Turker a

a Department of Metallurgy, Faculty of Technical Education, Gazi University, 06500 Ankara, Turkeyb Department of Metallurgy, Faculty of Simav Technical Education, Dumlupinar University, Kutahya, Turkey

Received 1 March 2004; accepted 23 July 2004

Available online 11 September 2004

Abstract

In this study, Al–Al4C3 composites, produced by powder metallurgy in situ techniques, were joined by diffusion welding tech-

nique at 250 MPa pressure with various welding temperatures and durations. Microstructures and shear strengths of the joined areas

were determined. Al powders were mixed with 2% carbon black and milled in a high energy ball mill (mechanical alloying) for up to

20 h. In order to obtain cylindrical blanks with 10 mm in diameter and 15 mm in height, powders were compacted in a single action

press at 1000 MPa. Samples were sintered in Ar atmosphere at 650 �C and metal matrix composite (MMC) containing 8% Al4C3

particles were produced. Products were then joined to each other by using diffusion welding techniques. Scanning electron micros-

copy examination was carried out on the welded interfaces and shear tests were conducted to the sample interfaces to find out the

effect of welding temperatures and duration on the weldability properties. It was found that high welding temperatures resulted in

increase of both joined strength and shear properties. However, increase in welding duration did not make any detectable changes.

Results indicated that MMC could be joined by diffusion welding technique successfully with the 88% strength of base material.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Mechanical alloying; Composite material; Al–Al4C3; Diffusion welding; Mechanical properties

1. Introduction

Aluminium and its alloys have found very wide appli-

cation at aerospace and automotive industries due to

their high specific strength, corrosion and wear resi-

stances. For this reason number of scientist has concen-trated on it [1]. In order to increase the mechanical

properties of this materials different strengthening mech-

anisms such as: mechanical alloying, in situ technique,

extrusion and reinforcing with small and hard refractory

particles have been used [2–4]. It is possible to produce

composite materials by applying one of those processes

and obtaining better mechanical properties than those

obtained at plain alloys especially at high temperatures[5–7]. MMC can be produced by liquid or solid state.

0261-3069/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.matdes.2004.07.017

* Corresponding author. Tel.: +90 312 212 6820; fax: +90 312 212

0059.

E-mail address: [email protected] (H. Arik).

In the liquid state, ceramics particles are added to liquid

metal by stirring before casting, but the resulting distri-

bution of reinforcing elements is generally inhomogene-

ous. In the solid-state rote, known as the powder

metallurgy technique, reinforcing powders are added

to the metal powders and then blended, pressed and sin-tered to produce block composite materials. One of the

most important problems encountered in this technique

is obtaining unsatisfactory strength between the metal

powders and the reinforcing particles interfaces. An-

other way of producing fine and homogeneous distribu-

tion of hard particles (Al2O3, SiC, Si3N4, etc.) in the

matrix is the in situ method which is generally achieved

by reaction milling and annealing processes. In thismethod very strong interface between the matrix and

particle can be assessed. Many researches have been car-

ried out by using this method [8,9]. When adjoining of

these materials is required, weldability gains particular

importance. Then the materials can be joined by either

Page 2: Weldability of Al4C3–Al composites via diffusion welding technique

556 H. Arik et al. / Materials and Design 26 (2005) 555–560

fusion welding (ark welding, shielding gas welding, laser

welding, electron beam welding, etc.) or solid-state weld-

ing (soldering, explosive welding, friction welding, and

diffusion welding). In fusion welding, many problems

may occur at the vicinity of welding where particularly

at the heat affected zone [10,11]. During the welding of

aluminium an oxide film, which covers the surface of

Al, makes the welding difficult [12]. In case of using aprotective gas during welding the possibility of forma-

tion of an oxide surface film is reduced and therefore

better welding quality is achieved. Since between 50%

and 80% of the melting point of the materials is chosen

for diffusion welding, almost no phase transformation or

microstructural change can occur during welding [13–

16]. It is reported that if suitable welding parameters

such as: temperature, duration and atmosphere are cho-sen correctly; very high joining strength can then be ob-

tained [1]. In this study, Al4C3–Al composites were

produced by mechanically alloying and then character-

ised by determining the shear strength and microstruc-

tural evaluation. Samples were joined by diffusion

welding method under various welding parameters and

mechanical and microstructural properties of weld inter-

face were examined to find out the most suitable weldingparameters for better mechanical properties.

2. Experimental procedure

2.1. Production of composite material

Gas atomized Al powders were produced by GaziUniversity PM Lab. Maximum particle size of 150 lm(�100 mesh) powders were produced from 99% pure

Al ingots which were supplied from ETIBANK (Turkish

Aluminium Producing Co.). Carbon black of 99% purity

and mean powder size (agglomerate size) of 2.4 lm were

obtained from YARPET (Turkish Petrochemical Trade

Co.). Forty nine g of Al powders, 1 g of carbon black

and 300 g of steel balls, with the diameter of 10 mm,were placed into the 750 cm3 capacity tank of a high-

Fig. 1. Mechanical alloying set-up.

energy attritor (Fig. 1). Then 2 wt% stearic acid was

added to the mixture to prevent sticking of Al powders

to the balls and to the walls of the milling tank. Powders

mixture was milled in attritor for 20 h. In order to elim-

inate the oxidation of aluminium powders during MA,

the process was conducted under argon environment.

Ar gas was purified from residual oxygen by passing it

through Cu chips heated to 650 �C. The tank of theattritor was water-cooled during MA. In order to deter-

mine whether any carbide formation or any other struc-

tural changes had taken place during MA, powder was

analyzed by using a Rigaku–Geigerflex X-ray diffrac-

tometer (with Cu Ka radiation at 40 kV and 30 mA)

after milling and the peaks in the XRD traces were com-

pared with those obtained from the elemental powders.

Powder sizes were measured by using a Malvern MasterSizer E version 1.2 b laser scattering machine and their

shapes were analyzed by using a Joel JSM 6400-Noran

Instruments Series II Scanning Electron Microscope

(SEM) before and after MA processing. Mechanically

alloyed powders were compacted at 1000 MPa pressure

to produce blocks (B10 · 15 mm in size). Blocks were

then put on a graphite boat which was placed in an

atmosphere controlled tube furnace and heated to testtemperature in a flowing Ar atmosphere for predeter-

mined durations. The heating rate of the furnace to

the desired temperature was approximately 5 �C min�1.

The furnace was held at 650 �C for 20 h with the accu-

racy of ±5 �C, and then cooled to room temperature at 5

�C min�1. The sintered composite samples were charac-

terized by measuring the densities, hardness together

with XRD and SEM analysis.

2.2. Preparing of diffusion welding of samples

Areas of concern on surfaces of sintered samples,

with the size of B10 · 15 mm, were polished by 0.4 Ra

surface roughness and then cleaned by alcohol. Surface

roughnesses of the samples were measured by Taylor

Hopsen instrument. In order to obtain suitable weld

Fig. 2. Welding couple.

Page 3: Weldability of Al4C3–Al composites via diffusion welding technique

Table 1

Diffusion welding parameters used in this study

Temperature (�C) Pressure (MPa) Atmosphere Welding

time (min)

625 2.5 Ar 90–150

650 2.5 Ar 90–150

675 2.5 Ar 90–150

Fig. 3. Schematic illustration of diffusion welding device.

Fig. 4. Schematic illustration of shear test apparatus.

Fig. 5. SEM micrograph of mechanical alloyed powder showing

irregular shapes of fine powder particles after 20 h mechanical alloying.

Table 2

Properties of mechanically alloyed powder

Mean powder

particle

size (lm)

Morphology

of

powders

Transformation

of

Al4C3

Green density

under the

1000 MPa (%)

21.9 globular – 88

H. Arik et al. / Materials and Design 26 (2005) 555–560 557

couples, surfaces of the samples were fully contacted to

each other as shown in Fig. 2. Samples were then placedin the centre of the diffusion welding equipment (Patent

Number: TR 2002 02710 U) as illustrated Fig. 3. In or-

der to eliminate the oxidation problem Ar gas was intro-

duced into the test chamber before welding. About 2.5

MPa pressure was applied to the samples before weld-

ing. Temperature was risen up to test temperature with

5 �C min� 1 and samples were kept there for certain test

durations and then cooled to room temperature in Aratmosphere. Welding parameters for this experiment is

given in Table 1.

2.3. Shear test

One of the five welded samples for every temperature

and durations were prepared metallograpically for scan-

ning electron microscopy examinations. Remains weresubjected to shear test with the speed of 1.5 mm s�1by

using MFL universal test equipment (Fig. 4) and the

average of the results was taken as a shearing test result.

3. Result and discussion

Properties of alloyed powders are given in Table 2.During mechanical alloying process powders were

trapped between the colliding steel balls and size of them

decreased considerable (Fig. 5). Furthermore, due to the

cold deformation of the powders during the proces,

powders were work hardened and lost their compacta-

bility. Thus, powders compacted at 1000 MPa showed

lower green density then expected. According to XRD

results, there was not any detectable phase transforma-

tion in the Al-C system during mechanical alloying proc-

ess. XRD result and SEM examinations indicated thatabout 8 wt% of Al4C3 phase had been synthesised after

sintering and the densities of the samples increased con-

siderably up to 92%. Fig. 6 shows the distribution of

Al4C3 particles in the matrix after sintering.

In all diffusion welding applications, it is important to

obtain full contacts in between the welding surfaces.

When this is not provided, it is almost impossible to ob-

tain the sufficient joint strength where in contrast verylow shear strength values can hardly be achieved. In fact

Page 4: Weldability of Al4C3–Al composites via diffusion welding technique

Fig. 7. Effect of interface fitting on the joinability of sample at 625 �Cfor 1.5 h.

Fig. 8. SEM micrographs of samples welded at 625 �C for 1.5 h: (a)

general appearance of the welded interface; (b) high magnification of

the welded interface.

Fig. 9. SEM micrographs of samples welded at 650 �C for 1.5 h.

Fig. 6. SEMmicrograph showing the distribution Al4C3 particles in Al

matrix.

558 H. Arik et al. / Materials and Design 26 (2005) 555–560

this situation is clearly shown in Fig. 7. SEM micro-

graphs of the samples welded at 625 �C show very thin

oxide film formation at the joining interface. This filmactually acts as a barrier for diffusion welding, since

welding pressure is not enough to break this thin oxide

film. As can be seen through Fig. 8(a)–(b), in case of

the break down of the oxide layer, then diffusion weld-

ing can occurs within this region. From these result it

can be concluded that welding temperature of 625 �Cis not enough for this study. With increasing the welding

temperature to 650 �C a considerable increase in diffusedareas occurred (Fig. 9). However, the best welding inter-

face was obtained on the sample welded at 675 �C,where most of interface lines disappeared and resembled

to that of a single block sample (Fig. 10). From the SEM

investigation and micrographs taken from the interface,

it can be said that welding temperature 675 �C is enough

for diffusion welding of these materials. Similar SEM

micrgarphs were also taken from the sample which werewelded for 2.5 h. (see Table 3).

Shear test results of the sample welded at different tem-

peratures and durations are given in Fig. 11. Samples

welded for 1.5 h at temperatures of 625, 650, and 675

�C showed the 66%, 81% and 85% of the shear strength

of the original un-welded materials, respectively.

These results were found to be 68%, 83% and 88% for

the samples welded at the same temperatures for 2.5 h.

Samples welded at 700 �C however exhibited some local

melting together with high deformation at the weldinginterfaces. Thus maximum welding temperature was

Page 5: Weldability of Al4C3–Al composites via diffusion welding technique

30

32

34

36

38

40

42

44

46

48

675 700Welding Temperature (˚C)

1,5tsaah.

2,5tsaah.

600 625 650

Shea

r st

reng

th (

MPa

)

Fig. 11. Shear test results of samples welded at various temperatures

and durations.

Fig. 10. SEM micrographs of samples welded at 675 �C for 1.5 h.

Table 3

Properties of composite materials after sintering

Sintered

density (%)

Al/Al4C3 Hardness (Hv) Transverse rupture

strength (MPa)

92 92/8 314 183

Polished surfaceprepared for welding

Residual carbon or aluminalayers covered the surfaceof powders orAl-Al4C3particles surface.

Fig. 12. Schematic illustrations of residual carbon and aluminium

oxide covered the surface of the particles in composite materials.

H. Arik et al. / Materials and Design 26 (2005) 555–560 559

chosen as 675 �C for these samples. Shear strength of thewelding of these samples was found to be quite high

compared to those mentioned in the literature [17].

The reason for this may be attributed to the sintering

behaviour of the materials, which affects the shearing

behaviour. It is known that one of the most important

parameter for shear strength of blocks is the diffusability

of the atoms of the powder particle to each other during

sintering. During sintering, formation of Al2O3 films orresidual carbon covers the surfaces of the powder parti-

cles and hence hinder the mobility of atoms. This prob-

lem tried to be overcome by high compacting pressures

and also high sintering temperatures (Fig. 12). In order

to obtain oxide free, clean and smooth surface before

welding, the significant surfaces of blocks were polished

before sintering. Removing the oxide films before sinter-

ing caused easy diffusion of the atoms and consequently

better welding quality was achieved.

4. Conclusions

In this study, Al–Al4C3 composites were successfullyproduced by powder metallurgy in situ techniques from

elemental Al and 2% carbon blacks. During the diffusion

welding of composite materials, the most important

welding parameter was found to be the welding temper-

ature. For this study, 675 �C was found to be the most

suitable welding temperature. About 1.5 h welding dura-

tion was found to be enough for these materials. How-

ever, increasing the welding duration to 2.5 h showeda slight increase in shear strength of the welding

interfaces.

Acknowledgements

The authors thank to Gazi University Research Fund

for the financial supporting of this research work. (No.:07/2000-06).

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