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University of Waterloo Faculty of Engineering
Department of Mechanical and Mechatronics Engineering
Hybrid Laser Welding
Prepared by
Shahnawaz Lodhi
ID 20194907
20 December 2014
iv
Abstract
Hybrid laser welding (HLW) is a technology that utilizes both a laser heat source and an
additional (secondary) arc welding source in welding applications in a synergistic
manner. This combined approach results in a welding process that can deliver faster
processing speeds, deeper penetration, and greater control of the weld pool. This
technology is of particular importance in the automotive industry, railway industry,
aircraft industry, shipbuilding industry where the focus is weight reduction and use of
aluminum alloys allows this due to its nature of having low density and high-specific
strength. The technology utilizes a very high energy intensity from the laser to rapidly
reach the vaporization temperature of the work piece forming a vapor cavity. The
resulting cavity acts as a radiation blanket leading to the buildup of radiative energy and
acting directly (in combination with the filler wire) onto the work piece. In this paper, we
will discuss the several categories of hybrid welding including laser-gas tungsten arc
welding and laser-gas metal arc welding. We will also study the primary laser heating
sources along with relevant operational parameters.
v
Table of Contents
Abstract ........................................................................................................................................... iv List of Figures ................................................................................................................................. vi 1 Introduction .............................................................................................................................. 1 2 Types of lasers ......................................................................................................................... 3 3 Types of laser arc hybrid welding processes ........................................................................... 6 4 Shielding gases......................................................................................................................... 7 5 Gap tolerance and depth penetration ...................................................................................... 11 6 Welding Direction .................................................................................................................. 14 7 Power and Beam Spot Diameter ............................................................................................ 17 References ...................................................................................................................................... 20
vi
List of Figures
Figure 1 Comparison of welding processes (B. Ribic, 2009) .......................................................... 2 Figure 2 Comparison of arc, laser and hybrid welding processes (B. Ribic, 2009) ......................... 2 Figure 3 Quality of a laser beam: Beam Parameter Product (Paleocrassas, 2010) ......................... 4 Figure 4 illustrating feature comparison for typical materials processing laser sources (Olsen, 2009)
......................................................................................................................................................... 5 Figure 5 Overview of the HLW Process (Kah, 2011) ...................................................................... 7 Figure 9 A sectional view of hybrid welds (P Kah, 2011) ............................................................... 8 Figure 10 Function and effect of different shielding gases used in laser and arc welding (P Kah,
2011) ................................................................................................................................................ 9 Figure 6 Welding parameters for this experiment (Ming Gao, 2007) .............................................. 9 Figure 7 the arrangement parameters of gas nozzle (Ming Gao, 2007) ......................................... 10 Figure 8 Plasma shapes of laser-TIG under different shielding gas parameters (Ming Gao, 2007)
....................................................................................................................................................... 10 Figure 11 Comparisons of cross-sections of YAG-MIG hybrid welds produced at various laser-
focused point and MIG wire target (FUJII, 2007) ......................................................................... 11 Figure 12 X-ray inspection results of YAG-MIG .......................................................................... 12 Figure 13 Hybrid welding conditions (Jing-bo Wanga, 2009) ...................................................... 13 Figure 14 Effect of laser-MIG distance on arc current at (Jing-bo Wanga, 2009) ........................ 14 Figure 15 the welding parameters (WangZhang, 2014) ................................................................ 15 Figure 16 The weld pool profile (WangZhang, 2014) ................................................................... 16 Figure 17 the detail information of weld pool (WangZhang, 2014) .............................................. 16 Figure 18 Effect of laser beam diameter on bead shape (T U E Y A M A, 2004) ......................... 17 Figure 19 Depth of penetration as a function of welding speed (Hilton, 2005) ............................. 18
1
1 Introduction
Laser welding is a popular choice that has developed over the course of the last two
decades for use in many industries. The valuable features of hybrid fusion welding is the
combination of two heat sources with different energy densities which makes it possible
to utilize efficiently the welding and technological special features of each method and, at
the same time, eliminate their shortcomings (Bernadskiy, 2014). In hybrid welding, the
laser and arc are arranged in a manner such that a common interaction zone is achieved.
This interaction zone yields high welding speed, deep penetration, and improved weld
quality with reduced susceptibility to pores and cracks and excellent gap bridging ability.
HLW can be categorized into three main categories: (1) laser-gas tungsten arc (GTA)
welding; (2) laser-gas metal arc (GMA) welding; and (3) laser-plasma welding (H.L.,
2012).
Prior to commencing the study of hybrid laser welding (HLW), it is important to
understand what is laser welding and arc welding. Laser welding is a technology that
utilizes a laser as a power source (Nd:YAG or CO2 amongst others) to apply heat to the
work pieces such that the melting temperature is exceeded resulting in a small heat
affected zone (Welding Procedure Services, n.d.). The result is a weld with narrow, high
depth of penetration and high welding rates. Arc welding technology most applicable in
context of HLW are the TIG-Tungsten Inert Gas and MIG-Metal Inert Gas. TIG process
uses a non-consumable tungsten electrode that delivers the current to the welding arc.
The tungsten and weld puddle are protected and cooled with an inert gas, typically argon
(Gomez, 2013). The MIG process uses a wire connected to a source of direct current acts
as an electrode to join two pieces of metal as it is continuously passed through a welding
gun. A flow of an inert gas, typically argon, is also passed through the welding gun at the
same time as the wire electrode. This inert gas acts as a shield, keeping airborne
contaminants away from the weld zone (Gomez, MIG Welding: The Basics for Mild
Steel, 2014).
Figure 1 below illustrates very accurately the weld pools of arc, laser and the HLW
processes. Figure 1c (arc welding weld pool) is shallower than both the laser (a) and
hybrid weld pools (b). This shows the advantages of HLW (keyhole penetration and
controlled gap coverage) over both traditional processes.
2
Below is a summary showing the important characteristic of welding. It is apparent that
HLW process is the optimal method that allows the process to achieve benefits of both
arc and laser welding without their negative effects.
Figure 2 Comparison of arc, laser and hybrid welding processes (B. Ribic, 2009)
The use of aluminum as a structural component is widely increasing in numerous
industries. However, by itself as an element it has low mechanical strength and corrosion
resistance. But to improve these properties, different concentrations of differing elements
are mixed with aluminum. The following are the alloying elements used to reinforce
aluminum (Modern technologies of welding aluminium and its alloys, 2012):
1. Copper
Figure 1 Comparison of welding processes (B. Ribic,
2009)
3
2. Silicon
3. Manganese
4. Zinc
5. Magnesium
The alloying elements mentioned above are crucial to the acceptance of aluminum for use
in industry. Zinc and magnesium have a tendency to increase the corrosion resistance and
improve overall mechanical strength. Elements such as copper, chromium, iron,
zirconium, silicon, vanadium, bismuth, nickel and titanium are also added. They have the
benefit of improving the susceptibility to heat treatment, as well as strength and corrosion
resistance. Small amounts of these elements are usually not taken into account when
assessing the weld ability of aluminum alloys. But often it is these elements that have a
significant impact on their weld ability.
Another very important consideration when HLW of aluminum is the very high reflection
of the laser radiation and a tendency for porosity and cracks to occur in the welds. It is
especially difficult to start the process of laser welding due to this high reflectivity and a
large power input is required to penetrate into the substrate. The power density must be
high enough above such that “a gas-dynamic capillary tube is created, filled with gases
and metal vapor, surrounded by a layer of liquid metal” (Modern technologies of welding
aluminium and its alloys, 2012). This will ensure that the laser radiation is absorbed by
the metal vapor contained in the capillary through multiple reflections of the laser beam
from the capillary walls covered with liquid. For aluminum and its alloys, due to their
much higher thermal conductivity and high reflection coefficient of laser radiation, the
threshold power density is approximately 1.5 x 10^6W/cm2 and not less than 2x10^6 to
have a stable process. Comparatively for iron alloys the power density is typically 0.15
x10^6, and for a stable process (Modern technologies of welding aluminium and its
alloys, 2012).
2 Types of lasers
The selection of a laser is an extremely important topic to understand prior to
commencing the study of HLW. An important parameter is beam quality which is of
major relevance in materials processing with high power laser. The beam quality is
defined by ISO 11146:1999 by the beam parameter product (BPP) or the M2 factor, this
assess the ability to focus a laser beam. The beam quality of a solid state laser is often
specified by the beam parameter product (BPP) defined as follows:
4
Low values of BPP express good quality beams or the ability to focus a beam to a small
spot. As mentioned previously aluminum is one of the best reflectors of light. In
addition, many aluminum alloys contain magnesium or zinc, which are easily vaporized
forming plasma that blocks the incident beam. Additionally, aluminum also by its nature
absorbs very little energy and the surface must be properly prepared. Typically the laser
must have a minimum power density of 3kW to overcome the reflectivity of aluminum
(L. Quintino, 2012).
The major types of lasers used in HLW are the: carbon dioxide (CO2) and Nd:YAG
(solid state).
The CO2 laser is a gas laser emitting at a wavelength of 10.6 microns with higher output
powers (near 50kW) and efficiencies near 20% with a good beam quality but the light
cannot be transferred via fiber optics. However, a major drawback is that the wavelength
is not readily absorbed by the aluminum as the majority is absorbed/reflected by the
plasma created during the arc/laser interaction (Eboo, 1978). This results in reduced
energy reaching the substrate and a reduction in important characteristics such as
penetration depth occurs.
Nd: YAG lasers are widely used in the HLW of aluminum because their wavelength of
1.06 microns is easily absorbed by aluminum even though their overall power output is
much lower than CO2 (<10kW). However, they can still deliver significant power to the
substrate as the plasma does not absorb much of the laser energy. For this reason the Nd:
YAG can deliver same penetration with a lower power output.
Figure 4 below illustrates the types of lasers and their important parameters as discussed
above.
Figure 3 Quality of a laser beam: Beam Parameter Product (Paleocrassas, 2010)
5
Figure 4 illustrating feature comparison for typical materials processing laser sources (Olsen, 2009)
6
3 Types of laser arc hybrid welding processes
There are numerous types of laser-arc welding processes that exist and are summarized
from the research article (Kah, 2011).
Laser-MIG/MAG welding process: This process combines the use of a laser as a primary
source and the MIG arc as secondary source. This process allows for larger gaps to be
covered via the filling of the gap with a filler metal. The main benefit of this method
benefit is that the microstructure and mechanical properties of the weld metal can be
improved by controlling its chemical composition when using an appropriate filler
material. This approach can be used with either a pulse or continuous laser.
Laser-TIG welding process: This process combines the use of a laser as a primary source
and the TIG arc as a secondary source. This process is very fast and can be operated
either with or without filler metal addition. This process is mainly used in thin sheet
applications (0.4-0.8mm). However, since the electrode is non-consumable heat is
consumed from the arc-laser zone to reach the melting point of the filler wire and deposit
into gap.
Laser-PAW process: This process combines the use of a laser and a high frequency
microwave based source which induces a plasma jet. The plasma creates (upon
application of a current between substrate and nozzle) sufficient heat to ionize the air gap.
In this process, the laser beam is surrounded by a plasma arc. As with the TIG and MIG
process, a gas is used to prevent exposure of the substrate to the environment’s damaging
effects.
The figure below illustrates the experimental setups of the above mentioned processes.
As mentioned, for the purpose of this report we will only be investigating the one electric
arc and laser source setup (similar to a) below.
7
4 Shielding gases
Shielding gas selection and its distribution into the heat source–material interaction zone
is one of the most important parameters in a welding process and can deeply affect the
quality and reliability of the welded joints. This consideration is particularly true if hybrid
laser-GMAW processes are considered, because the synergist effect between the two heat
sources may be achieved only if a trade-off condition in gas composition is obtained.
Aluminum for example readily reacts with the environment in its molten state which may
lead to the formation of porosities, oxides or nitrides inclusions and thus to performance
reduction of the welded joints. Shielding gases need to fulfill different requirements: (a)
composition: the gases have to be inert with respect to the working materials (in this case
aluminum) also at high temperatures, but some oxygen content is useful for improving
arc stability; (b) flow: the gas must be capable of moving the laser induced plasma plume
away from the process zone but it must be low enough not to blow the liquid phase away
from the bead; (c) ionization potential: high ionization potentials help to reduce the
amount of plasma during welding and thus lead to deep penetration but they often lead to
arc instability (G. Campana, 2008).
In laser–arc hybrid welding, the shielding gas is supplied first to (as mentioned
previously) to isolate the molten metal from the ambient air. But it also has a very
important secondary function which is to suppress the laser-induced plasma, remove the
plume of metal vapor out of the keyhole, and to stabilize the metal transfer. Typically,
when welding with a carbon dioxide (CO2) laser, a phenomenon called plasma shielding
may cause problems in welding, when the evaporated metal plume prevents (shielding)
the laser beam to reach the work piece surface by reflection and absorption, causing an
unstable keyhole and thus lowering the penetration depth. In the case of autogenously
Figure 5 Overview of the HLW Process
(Kah, 2011)
8
laser welding, the velocity of the shielding gas is sufficient to displace the induced plume
but with arc welding this can change the direction of the filler wire deposition and
negatively affect the appearance. To overcome this plasma shielding effect, selection of a
type of laser to be used in the process and shielding gas is very important. As discussed
since CO2 lasers which have a very high wavelength (10.6 microns), it tends to reflect
energy back from the aluminum lowering the effectiveness of its plasma shielding effect.
helium and argon are used extensively as shielding gases. Helium because it has a high
ionization potential. (P Kah, 2011). However, helium is not conducive to promoting arc
stability due to its high thermal conductivity. In order to reduce its high cost, helium is
often combined with argon to enhance the overall performance of the blend while
minimizing its cost as well as achieving maximum efficiency. Also it is important that
when using helium, the gas flow has to be three times greater compared to argon in other
achieve an efficient shielding due to its lighter weight (Tusek, 1999) .
In Nd: YAG lasers however, a mixture of argon and helium can be used due it’s the
smaller wavelength and thus does not have the negative effect as a CO2 laser (Tusek,
1999).
In addition to argon and helium, numerous studies have found that adding up to 12%
CO2 gas which is reactive to a mixture of argon and helium can improve metal transfer.
However, above 16% leads to spatter during welding. Studies have also found that adding
between 5-10% of CO2 improves the appearance (shape of the weld). The figure 9 below
very well illustrates the interaction of the mixture of helium and argon through a study
performed with a 12kW CO2 laser. The effectiveness of the helium gas is obvious due to
its ability to prevent laser induced plasma formation. Furthermore, a small amount of
argon can be added to the mixture without significant reduction in the weld geometry (P
Kah, 2011).
Figure 6 A sectional view of hybrid welds (P Kah,
2011)
9
The summary below shows the different shielding gases and their relevant parameters.
As mentioned previously, plasma shielding is a phenomenon that occurs when using a
CO2 laser. In a study performed (Ming Gao, 2007), the effect of shielding gas parameters
were studied experimentally using a 5 kW Rofin-sinar TR050 CO2 laser together with a
Miller 300A conventional DCEN TIG welder. The shielding gases were the standard
argon and helium. The following were the weld parameters:
The experiment utilized the argon gas for the paraxial and coaxial nozzles and helium in
the tig torch nozzle offset as shown in figure 7 to study impact of the velocity and angle
of dispersion on the effectiveness of the shielding gas.
Figure 7 Function and effect of different shielding gases used in laser and arc welding (P
Kah, 2011)
Figure 8 Welding parameters for this experiment (Ming
Gao, 2007)
10
The results obtained below show the effect of the angle and velocity of the shielding gas
dispersion on plasma height (see figure 8 below). The higher the interacting plasma
height with the laser, the smaller the laser energy absorbed by work piece and the
shallower the weld penetration.
This shows that the shielding gas angle and velocity can be effectively used to change the
plasma height and thus increase weld penetration. This is quantified by the following eqn:
I(h) ≈ I0e(−βh)
Where I (h) is the laser energy absorbed by work piece, I0 the laser incident energy, β the
plasma absorption coefficient for laser energy and h is the plasma height interacting with
incident laser.
This resulting interaction between laser and arc plasma leads to further impact on
interacting plasma height and weld penetration depth. “When the laser induced plasma
Figure 9 the arrangement parameters of gas nozzle
(Ming Gao, 2007)
Figure 10 Plasma shapes of laser-TIG under
different shielding gas parameters (Ming Gao,
2007)
11
has high temperature and density of charged particle, which is usually far higher than that
of arc plasma. When the two plasmas encounter spatially, an electric channel is formed.
Through the channel, the charged particle of laser induced plasma will enter into arc
plasma, resulting in the dilution and reduction of charged particle density of laser induced
plasma. This will reduce the plasma defocusing effect for laser and enhances the laser
energy effectively absorbed by work piece. On the other hand, the increase of charged
particle in arc plasma will improve the ionizability of arc to decrease the resistance of
arc.” (Ming Gao, 2007)
5 Gap tolerance and depth penetration
In HLW, weld depth penetration and gap coverage is extremely important. The very
purpose of welding is to attach two similar or dissimilar pieces of metal together such
that the depth penetration and gap coverage between them is maximized. We will look at
various studies where parameters were modified to maximize the above mentioned.
In a literature study (FUJII, 2007), an experiment was performed using an A5052
aluminum alloy (plate thicknesses: 2, 3 and 4 mm) and A5356 MIG wire (1.2 mm
diameter). An Nd:YAG laser (maximum power: 4 kW) and a DC pulsed MIG welding
machine (maximum current 350 A) were employed as hybrid welding power sources to
study impact of process distance penetration depth and width. The process distance “d” is
the distance between the laser beam irradiation (spot) position and the wire target
position as shown in figure 11 below:
Figure 11 Comparisons of cross-sections of YAG-MIG
hybrid welds produced at various laser-focused point and
MIG wire target (FUJII, 2007)
12
The experiment wanted to study what impact if any the distance has on the resulting weld
formation.
The results showed that the deepest penetration occurred with a 0-2mm process distance.
As the process distance was increased, the penetration depth decreased and resembled
conduction style welding. The further the distance between the laser beam irradiation
(spot) position and the wire target position, the larger the process zone and greater energy
was dissipated. The experiment also looked at what happens when the welding speed is
increased. The hypothesis was that as the speed is increased not sufficient enough energy
would penetrate the substrate. The effect on penetration depth was predictably similar
also when increasing welding speed. As the welding speed was increased between 30-
90mm/s, partial penetration welds were apparent. Only between 30-40mm/s did the weld
resemble keyhole welding -see figure 12 below.
Figure 12 X-ray inspection results of YAG-MIG
hybrid welds produced (FUJII, 2007)
In another similar experiment performed (Jing-bo Wanga, 2009) using a Nd:YAG laser
and fiber laser with MIG welding, the effect of laser beam parameters on depth
penetration and gap tolerance were studied on an aluminum alloy. The figure 13 below
shows the parameters of the laser and MIG energy sources. The lasers used were a
continuous wave (CW) Nd:YAG laser (wavelength 1064 nm) and fiber laser (wavelength
1070 nm).
13
The experiments were performed where the lasers were irradiated directly on the wire
surface varying distance between -6 to +2 mm. The study found similar results as the
above experiment regardless of the laser power source and arc current. It was also found
(predictably) that if the distance was zero there was a reduction in the molten pool size
(due to pressure reduction from arc), making a reduction in the bead width and
penetration depth. However, the study was performed using a 2.7kW power laser. But
deviating from that (increasing power) it is unknown the effect on the process. It would
be speculated that wire melting rate tends to increase when the laser output power is
increased and it can be anticipated that the laser beam energy absorbed by the droplets
formed at the tip of the wire will also increase. If a droplet absorbs too much laser beam
energy, its temperature rise becomes too rapid, resulting in violent vaporization. The wire
feed rate was also studied as it was varied from 115 to 230mm/s for its effect on arc
current and laser-mig distance. It was found that if the wire feed rate was increased then a
large arc current was required otherwise the penetration depth would be negatively
affected as shown in figure 14 below.
Figure 13 Hybrid welding conditions (Jing-bo
Wanga, 2009)
14
This leads us into the discussion of the impact of welding direction on a range of
resulting weld characteristics.
6 Welding Direction
The HLW process can be oriented in two directions: arc leading or laser leading. This
means that the laser or arc is the power source that leads in the welding direction. The
difference in welding direction can produce different weld surface geometries and
penetration. To optimize the welding process and obtain the desired welding quality, a
fundamental understanding of the complex phenomena between the two process
orientations is essential. In a study performed (WangZhang, 2014), the effect of various
welding directions on the behavior of CO2 laser GMAW-P hybrid welding processes was
investigated. The laser was a TLF15, 000 CO2 laser (TLF 15000, TRUMPF) was used as
the laser source with a maximum output power of 15Kw. The filler wire was a steel
CHW-50C6, and is 1.2 mm in diameter. It was fed at a velocity of 4 m/min. A 15 mm
thick aluminum work piece was employed. Shielding gas was argon–helium mixtures
(50% argon and 50% helium), flowed at 40 L/min.
Below were the welding parameters shown in figure 15:
Figure 14 Effect of laser-MIG distance on arc current
at (Jing-bo Wanga, 2009)
15
The results of the experiment showed that in the arc leading welding process, the work
piece is firstly heated by the arc, and the arc has a lower peak power density distribution
than the laser. As Figures 16 and 17 show, “the molten pool can be divided into three
regions, and the outlines of these regions are shown as dashed lines. The outer region is
the preheating zone, in this area the work piece is at a semi-solid state, so only an outline
can be seen. The second part is the arc melting zone, which is melted by the arc power
density peak. Most of the molten metal is mainly from the filling droplets, in addition a
relatively shallow weld pool is formed. The third part is laser melting zone. Due to the
sharp and highly intense power density distribution of laser, a keyhole is produced, the
metal flows out from the keyhole, resulting in a deep weld pool.” (WangZhang, 2014)
It's interesting to note in this study that the penetration depth is the higher in an arc
leading process then in a laser leading process. A deeper penetration reached with a
leading torch can be explained by the fact that the arc is already melting the work piece
surface, and when the laser beam reaches the location of the molten material at an
elevated temperature, it is able to start penetrating the metal on an already warm surface.
The leading torch also ensures better weld quality.
Figure 15 the welding parameters (WangZhang,
2014)
16
Figure 16 The weld pool profile (WangZhang, 2014)
Revisiting the experiment we discussed in the section regarding maximizing depth
penetration and gap coverage. The experiment was performed (FUJII, 2007) using an
A5052 aluminum alloy (plate thicknesses: 2, 3 and 4 mm) and A5356 MIG wire (1.2 mm
diameter). A YAG laser equipment (maximum power: 4 kW) and a DC pulsed MIG
welding machine (maximum current 350 A) were employed as hybrid welding power
sources to study impact of welding direction on penetration characteristics and porosity.
The study was performed both as Nd:YAG leading and MIG leading. The results showed
that both kinds of welding exhibited similar porosity levels. However, as was found in the
previous experiment, the laser leading penetration was shallow as compared to the arc
leading. Similarly, the width coverage was the inverse relationship as should be expected
as the filler metal is able to spread out over the gap when not penetrating deeply.
However, there was an additional factor here of weld surface quality. The x-ray
Figure 17 the detail information of weld pool
(WangZhang, 2014)
17
inspection showed that in the case of the arc leading the formation of AlMgO particles
which were most likely blown onto the bead surface from MIG torch gas resulting in the
unclean appearance. Typically, laser leading HLW provides a cleaner bead appearance
as the arc following the laser beam the process gas would blow the oxide particles away
(Jing-bo Wanga, 2009).
7 Power and Beam Spot Diameter
The spot diameter of the laser beam is an important factor in determining the
characteristics of the laser beam. In Nd:YAG laser and CO2 laser, energy density is
increased by making the beam spot diameter small so that keyhole welding can be
realized. An experiment was performed to study the effect of beam spot diameter on bead
shape and penetration geometry and the laser beam diameter of the 2kW Nd:YAG laser
was varied from 1.09, 3.34, 5.14, 6.92mm. The analysis of the results showed that as the
beam diameter increases to 6.92mm, the penetration depth decreases and the width of the
beam decreases. Further analysis explains that the method of welding was conduction and
not keyhole (T U E Y A M A, 2004). This is the result of the defocussing effect of the
laser as shown in figure 18.
There is also a direct relationship between spot size and power density and welding
speed. In studies performed using 4 and 6mm thick 5083 aluminum with spot sizes of
0.14 and 0.40mm, the welding speed was approximately 60% and 21% higher with a
0.14mm spot size, compared with a 0.4mm spot size. Figure 19 below also shows, on the
other hand, that welding at fixed speeds of 5 and 2m/min, means that 5.0 and 6.8mm of
Figure 18 Effect of laser beam diameter
on bead shape (T U E Y A M A, 2004)
18
aluminum can be penetrated with the smaller spot, compared with only 4.3 and 6.4mm,
using the larger spot size. This is an increase in depth of penetration of 16 and 6%
respectively (Hilton, 2005).
8 Conclusion
An extensive literature study was undertaken to develop this report on hybrid laser
welding. The two most common laser sources are the CO2 and Nd:YAG. The wavelength
of the CO2 is higher than the Nd:YAG and as such there is reduced absorption of energy
into the substrate. To maximize the energy delivered and achieve excellent weld
appearance, a shielding gas is necessary. A shielding gas is important because of
its strong plasma-defocusing effect. For a constant combination of the laser beam and arc
sources, there should be an optimal combination, and composition of types of shielding
gases. The shielding gases must be inert with respect to the working materials, must
change the laser-induced plasma plume and must have high ionization potential. Argon
and helium are two most commonly used shielding gases in the welding processes due to
their cost and effectiveness. The process distance was shown to be an important
parameter. The results derived from the experiments showed that as the process distance
was increased, the penetration depth decreased and resembled conduction style welding.
The further the distance between the laser beam irradiation (spot) position and the wire
target position, the larger the process zone and greater energy was wasted. The effect on
penetration depth was predictably similar also when increasing welding speed. As the
welding speed was increased, partial penetration welds were apparent. Welding direction
it was found was very important in controlling depth penetration. It was found that
penetration depth is higher in an arc leading process then in a laser leading process. A
deeper penetration reached with a leading torch can be explained by the fact that the arc
is already melting the work piece surface, and when the laser beam reaches the location
of the molten material at an elevated temperature, it is able to start penetrating the metal
on an already warm surface. Furthermore, in the case of the arc leading the formation of
AlMgO particles which were most likely blown onto the bead surface from MIG torch
gas resulting in the unclean appearance. Lastly, it was found that there exists a direct
Figure 19 Depth of penetration as a function of welding speed
(Hilton, 2005)
19
relationship between spot size and power density and welding speed. In studies
performed, it was shown that increasing the spot size the energy density and speed are
reduced.
20
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