Optics and Lasers in Engineering 51 (2013) 1143–1152
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Optics and Lasers in Engineering
journal homepage: www.elsevier.com/locate/optlaseng
Correlation analysis of the variation of weld seam and tensile strength
in laser welding of galvanized steel
Amit Kumar Sinha a, Duck Young Kim a,n, Darek Ceglarek b
a
b
School of Design and Human Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulsan, Republic of Korea
International Digital Laboratory, WMG, University of Warwick, West Midlands CV4 7AL, United Kingdom
art ic l e i nf o
a b s t r a c t
Article history:
Received 11 February 2013
Received in revised form
17 April 2013
Accepted 17 April 2013
Available online 15 May 2013
Many advantages of laser welding technology such as high speed and non-contact welding make the use
of the technology more attractive in the automotive industry. Many studies have been conducted to
search the optimal welding condition experimentally that ensure the joining quality of laser welding that
relies both on welding system configuration and welding parameter specification. Both non-destructive
and destructive techniques, for example, ultrasonic inspection and tensile test are widely used in practice
for estimating the joining quality. Non-destructive techniques are attractive as a rapid quality testing
method despite relatively low accuracy. In this paper, we examine the relationship between the variation
of weld seam and tensile shear strength in the laser welding of galvanized steel in a lap joint
configuration in order to investigate the potential of the variation of weld seam as a joining quality
estimator. From the experimental analysis, we identify a trend in between maximum tensile shear
strength and the variation of weld seam that clearly supports the fact that laser welded parts having
larger variation in the weld seam usually have lower tensile strength. The discovered relationship leads
us to conclude that the variation of weld seam can be used as an indirect non-destructive testing method
for estimating the tensile strength of the welded parts.
& 2013 Elsevier Ltd. All rights reserved.
Keywords:
Laser welding
Variation of weld seam
Tensile strength
Part-to-part gap
1. Introduction
In the automotive industry, there has been a growing interest in
the use of laser welding technology for joining new materials to
meet the increasing demand of corrosion resistant, lightweight and
durable auto-body parts. Laser welding has many advantages over
conventional joining methods such as high speed and non-contact
welding, deep penetration, low heat input per unit volume, low heat
affected zone, effective integration with industrial robots, and the
capability of joining materials by single side access [1–3].
Tensile strength, hardness, fatigue and toughness are the four
major mechanical properties that are widely used to judge the quality
of welding. In particular, tensile strength is most popularly used in
practice due to the fact that it includes very important information
about the breaking point, maximum load, fracture position and the
percentage elongation of weldment. In order to measure the tensile
strength of weldment, it is necessary to carry out the tensile test
which requires time, cost, and wastage of materials. Therefore the
estimation of the tensile strength in a non-destructive way is a very
challenging task for academician and practitioners.
Several studies have been made to scrutinize the effects of the
changes in welding process parameters on the resulting geometry
n
Corresponding author. Tel.: +82 52 217 2713; fax: +82 52 217 2709.
E-mail address: dykim@unist.ac.kr (D.Y. Kim).
0143-8166/$ - see front matter & 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.optlaseng.2013.04.012
of weld seam (usually called weld pool or weld bead interchangeably in the literature). For example, Chen et al. [4] and Furusako
et al. [5] investigated relations between the geometry of weld
seam and tensile strength using conventional destructive techniques for measuring the geometry of the weld seam and tensile
strength. Interestingly, some researchers [6,7] reported that the
width of weld seam is correlated with welding quality.
However, little attention has been given to analysing the
relationship between the variation in the width of weld seams
and tensile strength in a non-destructive way. Therefore, we
conduct laser welding experiments to examine the statistical
correlation between them, using 1.8 mm and 1.4 mm thick galvanized steel parts in a single lap joint configuration. We make partto-part gaps purposely between the upper and the lower parts to
examine the effects of zinc vapour on the tensile shear strength of
weldment. Two quality characteristics namely, variations in the
width of the top weld seam and tensile shear strength (hereafter
called ‘tensile strength’ in short), and three welding parameters:
laser power, welding speed, and part-to-part gap are considered
for the experiment.
2. Related work
Many studies in the domain of laser welding have focused on
empirical search of welding process parameters in order to
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A.K. Sinha et al. / Optics and Lasers in Engineering 51 (2013) 1143–1152
maximize welding quality by minimizing the formation of porosity, spatter and intermetallic brittle phase [8]. On the basis of
observing the state (welding defects) around the weld-zone,
Kawamoto et al. [9] obtained ranges of laser welding parameters
which will improve the tensile strength of laser welded aluminum
alloy in a lap joint configuration. They concluded that heat input
per unit length and part-to-par gap are the two important factors
for deciding the tensile strength of laser welding in a lap joint
configuration. Acherjee et al. [10] proposed a sequentially integrated optimization approach for predicting the optimal level of
weld strength and the width of weld seam. Their optimization
approach is developed based on Taguchi method, response surface
methodology (RSM), and desirability function analysis.
Laser welding joints are not axisymmetric, and thus the geometry
of weld seam affects the strength of laser welded joints [11–13]. In
other words, the fatigue levels, the depth of penetration, the length of
weld seam and the width of weld seam are yet another important
factor for the prediction of laser welding quality. Stoschka et al. [14]
obtained the optimum value of feasible fatigue levels of laser welding
joints. Lawrence et al. [15] identified that fatigue resistance of laser
welding mainly depend upon the weld seam geometry. They also
proposed a mathematical model on the basis of weld seam geometry
for predicting the fatigue resistance of welds.
Siva Shanmugam et al. [16] predicted the weld seam geometry
(in terms of the depth of penetration, and the length and the width
of weld seam) in laser welding of stainless steel by using a finite
element method (FEM)-based simulation model. Their simulation
model is developed based on the calculation of the total heat input
by assuming three dimensional conical Gaussian heat sources. They
also considered the effects of latent heat of fusion, the convective
and radiative aspects of boundary conditions for simulation.
Chen et al. [4] evaluated the quality of full penetration laser
welding of 5A90 Al–Li alloy on the basis of the width of weld seam.
They found that the ratio of the width of the bottom (root) weld
seam to the width of the top (face) weld seam influences the
tensile strength, and further, if this ratio is larger than 0.4 in full
penetration laser welding then a stable formation of keyhole, less
defect and higher tensile strength can be expected. They concluded that this ratio has a polynomial relationship with the
tensile strength, and follows a normal distribution with the
percentage elongation of the laser welded joint. Furusako et al.
[5] established an ad hoc model to predict the tensile shear
strength of a laser welded lap joint based on the width and the
length of weld seam. They identified three fracture types of laser
welded lap joints. The first one is the fracture at the base metal;
the second is the fracture at the weld seam; and the last is the
fracture at the curvature in between the base metal and the
weld seam.
Acherjee et al. [17] developed a RSM-based optimization model
to predict the weld strength and the width of weld seam of laser
welding. This model, by selecting an appropriate combination of
laser power and welding speed, determines an energy threshold
where the weld strength reaches the maximum. Zhao et al. [18]
also utilized the RSM for investigating the effect of changes in laser
welding process parameters (i.e. laser power, welding speed, partto-part gap and focal position) on the geometry of weld seam (i.e.
the depth of penetration, the width of weld seam and the surface
concavity) of galvanized steel in a lap joint configuration. Ghosal
and Chaki [19] developed an Artificial Neural Networks (ANN)
model for predicting the depth of penetration. Sathiya et al. [20]
further investigated the relationship between laser welding process parameters (laser power, welding speed and focal length) and
response variables (the width of weld seam, the depth of penetration and tensile strength) by using the optimized process parameters obtained by a Genetic algorithm-based ANN model.
In general, the size of weld seam, particularly, the width of
weld seam decreases with the increasing welding speed because
heat input per unit length decreases. Benyounis et al. [21] found
that higher heat input does not guarantee the higher tensile
strength of a laser lap welded joint because higher heat input
usually facilitates the formation of a wider heat affected zone.
Neither can welding with relatively lower laser power and higher
welding speed, guarantee a higher tensile strength due to the lack
of full penetration.
A number of laser welding experiments have been carried out
to study the effects of process parameter change (e.g. welding
speed, focal length, laser power, and the flow of shielding gas) and
different materials on welding quality. Several reports have
detailed the role of the weld seam geometry on the quality of
welding. In particular, some studies reported that the width of
weld seam is an important property of welded joints. However, we
found that it is necessary to take a close look at the variation in the
width of weld seam, rather merely than the width itself. In this
regard, we examine the importance of the variation of weld seam
for the indirect estimation of laser welding quality.
3. The experiment
3.1. The laser welding system
The laser welding system (see Fig. 1) used for the experiment is
a gantry-based automated welding system that delivers a laser
beam from IPG YLS-2000-CT fiber laser source with a maximum
output discharge of 2 kW in the TEM01 mode of laser radiation.
A laser beam of 35 mm diameter was focused by a parabolic mirror
Fig. 1. The laser welding system (2.5 axis gantry robot, maximum power of 2 kW).
A.K. Sinha et al. / Optics and Lasers in Engineering 51 (2013) 1143–1152
with a focal length of 168 mm. Table 1 lists the laser technical
parameters used in the experiment. The intensity distribution of
the focused beam was measured using PRIME’s FM-35 beam
analyser; the focused spot size was approximately 600 mm diameter which contained 86% of the total power passed. The laser
welding head is mounted directly on a 3-axis gantry system that
consists of belt-drive linear modules, travel limit/home position
sensors, and servo motor system.
3.2. The experimental materials
Laser welding experiments are conducted on the lap joint of two
different galvanized steels: SGARC440 (lower part) and SGAFC590DP
(upper part). The dimensions (length width thickness) of the
lower and the upper parts are 130 mm 30 mm 1.8 mm and
130 mm 30 mm 1.4 mm, respectively. The amount of Zinc coating on the lower and the upper parts are 45.5 g/m2 and 45.4 g/m2,
respectively. Chemical compositions and mechanical properties of
the tested materials are listed in Tables 2 and 3, respectively.
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the gap of galvanized steel sheets is too small, it may then create
weld defects such as porosity, spatter, intermetallic brittle phase
and discontinuity formed by zinc vapour entrapment in the
welding joints [23,24]. These types of weld defects occur usually
because the boiling point of Zn (906 1C) is lower than the melting
point of Fe (1538 1C) [25].
On the other hand, if the gap is too large, it is generally difficult
to obtain good bonding at the interface between sheets because
laser welding does not induce any forces on the parts to be joined,
as does resistance spot welding [6]. According to Akhter [26], the
effective gap through which zinc vapour can escape from the weld
pool should be in between 0.1 to 0.2 mm with respect to the
thickness of the upper and the lower parts as well as the thickness
of zinc coating. Particularly, when we apply clamping force to the
joint, we should then increase the gap in a certain percentage [27].
Akhter et al. [28] proposed a mathematical relationship
between part-to-part gap and sheet thickness, given by:
g
Avt Zn
¼ 3=2
t
t
where
3.3. Part-to-part gap
Strict assembly tolerance between the upper and the lower
parts to be joined needs to be guaranteed during the joining
operations of laser welding. In general, the gap can be controlled
within 10% of the thickness of the upper part upon which the laser
beam is incident [22].
This tight part-to-part gap control is even more critical in laser
lap welding of galvanized steel since we have to additionally allow
for the minimum gap between the parts, so that the vaporized zinc
produced by the welding heat can be exhausted through the gap. If
Table 1
Laser technical parameters.
Parameters
Value
Maximum laser power
Laser mode
Focus length
Operation mode
Switching ON/OFF time
Emission wavelength
Output power Instability
BPPn(1/e2) at the output of fiber
2 kW
TEM01
168 mm
CW
80 ms
1070 nm
2.0%
2.0 mmnmrad
Table 2
Chemical composition (wt%) of the test materials.
Tested material
C (%)
Si (%)
Mn (%)
P (%)
S (%)
SGARC440 (1.8 mm thickness)
SGAFC590DP (1.4 mm thickness)
0.12
0.09
0.5
0.26
1.01
1.79
0.021
0.03
0.004
0.003
Table 3
Mechanical properties of the test materials.
Tested material
SGARC440 (1.8 mm
thickness)
SGAFC590DP (1.4 mm
thickness)
Tensile test
Yield strength
(N/m2)
Max-tensile strength Elongation
(N/m2)
(%)
327.5
451.1
38
413.8
625.7
28
g: Part-to-part gap
t: Sheet thickness
A: Empirical constant
v: Welding speed
tZn: Thickness of Zinc coating on each part at the interface.
They presented that if the values of g/t is approximately in
between 0.2 and 0.3, then we can expect good quality of laser
welding of galvanized steel.
Several techniques have been developed for part-to-part gap
control in laser welding of galvanized steel in lap joint configuration. Shims (e.g. metal gauge type) are inserted in between the
upper and the lower parts to create a fine gap that provides a
required clearance for zinc vapour [29]. Pre-stamped projection
technique creates V-shaped tabs in the lower part which act as gas
venting channels that allow zinc vapour to escape during laser
welding process [30].
The laser dimpling technique is a practical method to create a
required part-to-part gap in between the upper and the lower
parts by humping effects [31]. The main advantage of this
technique is that dimples can be produced with the same laser
system used for the laser welding [32].
‘Fill the part-to-part gap with a porous metal’ is yet another
technique to maintain part-to-part gap where a porous power
metal will provide room for zinc vapour to pass through. It is
unfortunately difficult to implement this technique in a real
production environment due to its long processing time.
‘Pre-drilling vent hole along the welding line’ allows zinc
vapour to escape through the weld zone without causing
expulsion of molten metal [1]. A time consuming preprocess,
pre-drilling is necessary by nature.
‘Prior zinc removal from the weld area on a part joint’ is also an
innovative technique to control part-to-part gap in laser welding
of galvanized steel in lap joint configuration [33]. In order to
provide corrosion protection, the zinc removed zone could be
further coated with nickel, which has a higher vaporization
temperature as compare to the fusion temperature of steel.
2.5 kW pulse CO2 laser welding of galvanized steel in lap joint
configuration without a tight part-to-part gap control with
visually sound welding has been proposed first by [34]. Kennedy
et al. [35] also claimed successful laser welding of galvanized steel
without a tight part-to-part gap control by using pulsed Nd:YAG
laser. Tzeng et al. [36] reported successful lap welding of galvanized steel with porosity/spatter free welds by using a 400 W
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A.K. Sinha et al. / Optics and Lasers in Engineering 51 (2013) 1143–1152
pulsed Nd:YAG laser. They suggested that by careful control of
pulse energy, pulse duration, peak power density, mean power,
and welding speed, zinc gas can be reduced in the pulsed mode
and effectively exhausted by stabilized keyholes. However,
the limitation of low laser power causes slow welding speed of
less than 2.4 mm/s, which is hard to maintain in an industrial
environment.
Bilge et al. [37] claimed that if the metal parts are positioned
and moved vertically during welding while the laser beam is static
and applied to parts horizontally, then continuous wave CO2 laser
Table 4
Part-to-part gap control techniques in laser welding of galvanized steel.
Industrial
solutions
Description
Shim
Insert shims in between sheets
insertion
Pre-stamped A preprocessing creates V-shaped tabs in the
projection
lower part which act as a gas venting channels
that allow zinc vapour to escape during laser
welding process
In this technique a preprocessing is carried out
Laser
in which the laser beam generates dimples to
dimpling
maintain a part-to-part gap.
technique
Fill the part- The porous powder metal allows zinc vapour to
to-part gap escape without disturbing molten metal
with a
porous
powder
metal
Pre-drilling Pre-drilling vent holes along the welding line
allow zinc vapour to escape without causing
vent hole
techniques expulsion of the molten metal
Remove zinc coating from welding zone and
Prior zinc
then coat the treated zone with nickel in order
removal
techniques to provide corrosion protection
Low power
pulsed laser
welding
Altered joint
geometry
techniques
Dual beam
hybrid
technique
Addition of
oxygen as a
shielding gas
to argon
Vertical
positioning
of metal
parts
Synchronous
rolling
techniques
Advantages
Disadvantages
Intuitive and easy to control the required
clearance
Useful for hem joints (special case of lap joint
configuration)
Needs additional work and tool for shim
insertion
Needs preprocessing
Dimples can be produced with the same laser
system used for the laser welding
No need to remove the porous powder metal
after welding
Needs two-step process: (i) preprocessing by which dimples will
generate and (ii) actual welding
Difficult to implement in a real
production environment
No need to provide part-to-part gap
Time consuming and expensive process
[33]
Preprocessing is necessary
[34]
The limitation of low laser power causes
slow welding speed of less than 2.4 mm/
s, which is hard to maintain in an
industrial environment
[37]
Intentionally, we need to create altered
joint geometry in the form of either
concave or convex on the top surface of
the metal part
Needs additional complex equipment
[39]
Nickel coating not only provides good corrosion
resistance but also removes the problem of zinc
vapour because nickel has a higher vaporization
temperature as compare to the fusion
temperature of steel
Literature survey reveals that both CO2 and Nd:
By careful control of pulse energy, pulse
duration, peak power density, mean power, and YAG pulsed laser provide porosity/spatter free
welding speed, zinc gas can be reduced in the welds
pulse mode and effectively exhausted through
stabilized keyholes
Altered joint geometry offers channels between This technique is very useful when the
the metal parts to exhaust zinc vapour
dimensional variation in between part-to-part
gap is high
The first beam creates a slot as an effect of
preheating and the second beam performs
actual welding process
Addition of a small amount of oxygen (2–5%) as
a shielding gas to argon facilitates the zinc to
react with oxygen and reduce the effect of
vaporized zinc
Metal parts are positioned and moved vertically
during welding while the laser beam is static
and applied to parts horizontally
The first beam facilitates vaporization of the
zinc that will prevent weld defects
No need to provide part-to-part gap
Vertical position of metal parts allow zinc
vapour to escape through the weld zone
Decrease the formation of brittle intermetallic
The additional roller generates pressure
compound like zinc oxide
between part-to-part gap as well as creates
favorable conditions for rapid heat transfer from
the upper part
The flow rate of shielding gas must be
optimized otherwise plasma will
dissipates and appears as oxides porosity
on the surface of the weldment
Due to difficult positioning and
movement of parts this idea has been
rarely applied in industry
Needs additional roller during welding
Fig. 2. Weld joint configuration (material compositions are presented in wt%).
Reference
[29]
[30]
[31,32]
[33]
[40], [41],
[42]
[38]
[43,44]
A.K. Sinha et al. / Optics and Lasers in Engineering 51 (2013) 1143–1152
welding on a lap joint produces better results. However, precise
positioning and movement of parts are required which are not so
easy in practice.
The altered joint geometry techniques, which offers controlled
channels between the metal parts to exhaust zinc gas, was used by
[38] in laser welding of galvanized sheet steels. In general, altered
joint geometry is usually created in the form of either concave or
convex on the top surface of metal part. This technique is very
useful when the dimensional variation in between part-to-part
gap is high.
Dual beam hybrid technique (called also as bi-focal hybrid or
shaped beam techniques) [39,40] that will give rise to preheating
effects to eliminate zinc, is another most utilized technique in
industry [39]. This technique however needs additional complex
equipment.
The addition of a small amount of oxygen (2–5%) as a shielding
gas in the argon facilitates the zinc to react with oxygen and
reduces its explosive effect [41]. However, the flow rate of
shielding gas must be optimized otherwise plasma will dissipate
and appear as oxides porosity on the surface of weldment Table 4.
In this experiment, we have intentionally created part-to-part
gap by inserting a conventional metal thickness gauge (thickness:
Fig. 3. Part-to-part gap created by metal thickness gauges.
1147
0.1 mm, 0.2 mm, 0.3 mm) in between the upper and the lower
parts to be joined as shown in Fig. 2 and Fig. 3.
3.4. Weld pool geometry
Fig. 4 illustrates the formation of weld pool during laser
welding of zinc-coated steel sheets with a part-to-part gap. The
thickness of the upper part (t1) and the lower part (t2) are 1.4 mm
and 1.8 mm, respectively. The details of weld pool geometry are
illustrated in Fig. 5. In the literature, weld pool geometry (weld
seam geometry or weld bead geometry) includes the width of the
top (face) weld seam (W1), the width of the bottom (root) weld
seam (W2), the depth of penetration (D), up sunk height on the top
weld seam (L1), up protruded height on the top weld seam (L2),
down sunk height on the bottom weld seam (L3), down protruded
height on the bottom weld seam (L4), and aspect ratio (D/W1)
[44,45]. There have been many research efforts to utilize the weld
pool geometry for measuring the welding quality indirectly, for
example, Mistry [46] found that the tensile strength of laser lap
weld joints is proportional to the undercut depth of the top part,
given by 0.15 (t1+part-to-part gap).
It is known that the width of weld seam is usually correlated
with welding speed. In general, W1 is greater than W2 although
the difference between these two values depends on welding
speed and incident laser intensity [6]. Kimara et al. [47] reported
that the difference between W1 and W2 decreases with welding
speed at the constant laser power. Cline and Anthony [48]
developed a simulation method to approximate a set of weld pool
geometry curves that provide a good estimation of penetration
depth versus laser power and welding speed. Akhter et al. [49]
developed a mathematical model to show that the power
absorbed by the laser welded plate (i.e. amount of heat content)
is correlated with the width of weld seam and the depth of
penetration.
According to Zhang [7] the width of weld seam and the depth
of penetration are the most important factors that determine the
welding quality. They have shown that the width of the bottom
weld seam is the direct physical parameter reflecting the depth of
penetration. However, in practice, it is not easy to inspect the
width of the bottom weld seam directly, and thus, they estimated
the width of the bottom weld seam according to the shape of the
top weld seam [7]. For this reason, this paper also investigates the
Fig. 4. Schematic diagram for formation of weld pool during laser welding of zinc coated steel plate in a lap joint configuration in existence of part-to-part gap.
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A.K. Sinha et al. / Optics and Lasers in Engineering 51 (2013) 1143–1152
variation in the width of the top weld seam to estimate the tensile
strength of the weld joint.
3.5. Experimental design
The major welding parameters that affect the quality of laser
welding of galvanized steel in the lap joint configuration are
summarized in Fig. 6. It has been reported that several welding
parameters such as laser power, welding speed, part-to-part gap,
clamp pressure, and focused position have the major effect on the
width of weld seam during laser lap welding [17,45]. In general,
laser power is proportional to the width of weld seam. In other
words, the width of weld seam is increased as we increase the
laser power up to a certain limit. Welding speed, on the other
hand, is inversely proportional to both the width of weld seam and
the width of heat affected zone [45]. It is well-known that the
part-to-part gap for laser lap welding must be controlled within a
certain threshold [48] otherwise the two parts would not be
joined correctly. This threshold value of part-to-part gap is
correlated with the width of weld seam and the thickness of
parts. Part-to-part gap has a negative effect on the depth of
penetration but it is proportional to the width of weld seam [50].
Welding speed has a negative effect on tensile shear strength
but both laser power and clamp pressure have a little positive
effect on the tensile shear strength [17,51]. In the laser welding of
zinc coated steel, shielding gas also play an important role for
deciding the weld quality [52]. Chen et al. [53] observed uniform
hardness as well as higher tensile strength throughout weld
seams, when they use Nitrogen gas (N2) as shielding gas. Welding
speed is the most important factor for residual stress which arises
in the heat affected zone while focus position is not so relevant
with residual stress [54]. The tensile strength of laser welded lap
joints depends mainly on the width of weld seam, the depth of
penetration and the weld seam length [5].
According to the literature survey, laser power, welding speed
and part-to-part gap are the most important welding process
parameters for the weld pool geometry, (particularly, the width of
weld seam) and the resulting mechanical properties (particularly,
tensile strength) of laser-welded lap joints [54]. Therefore, in this
Table 5
Full factorial design with three welding process parameters each at three-levels.
Code unit
−1
0
1
Experimental factor
Laser power (W)
Welding speed (mm/min)
Gap (mm)
1600
1800
2000
1200
1500
1800
0.1
0.2
0.3
Fig. 5. Weld pool geometry.
Fig. 6. Cause and effect diagram for laser welding of galvanized steel in a lap joint configuration.
A.K. Sinha et al. / Optics and Lasers in Engineering 51 (2013) 1143–1152
experiment, these three factors are involved at three levels each in
the mechanical response of tensile strength, namely, three factors
three levels full factorial design.
In order to find the initial values of welding process parameters, trial weld runs (approximately 200 experiments) were
performed a priori by changing one of the process parameters at a
time. Absence of clear welding defect, visual soundness of the top
and the bottom seams are the criteria of specifying the initial
ranges of the welding process parameters. Table 5 shows the
welding process parameters considered in the experiment, their
coded and actual values.
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The analysis of variance (ANOVA) for the laser welding experiment, by considering maximum tensile strength as a response
variable is summarized in Table 7. We conclude from the P-values
that laser power, welding speed, part-to-part gap, and the interaction between laser power and gap have statistically significant
effects on the maximum tensile strength at the 0.05 level of
significance.
Statistical correlation analyses are carried out to clarify the
relation between weld seam and welding quality. First, as shown
Table 6
The laser welding experimental data.
3.6. Variation of weld seam
The procedure to estimate the variation of weld seam is as
follows: we first take a series of magnified pictures of a top weld
seam in section wise by Olympus LEXT OLS 3100 confocal laser
scanning microscope at the total magnification of 160 . Since the
length of a top weld seam is approximately 20 mm and the
maximum field of view of the microscope is 2560 μm 2560 μm,
we create panorama images of top weld seams, computed from
about eight weld seam patch images each, as illustrated in Fig. 7.
We then import the top weld seam image into CATIA V5 to extract
the two boundary curves of the top weld seam efficiently. Finally,
we measure the variation in the width of the top weld seam
by means of the deviation analysis between the two extracted
curves.
4. Analysis of experiment
The result of laser welding experiments is summarized in
Table 6. Maximum tensile strengths are measured by the INSTRON
5582 universal testing machine and the mean and the variation of
the width of each weld seam, listed in the last two columns, are
estimated by the procedure described in the previous section.
Fig. 8 shows panorama images of all the 27 specimens. The
width of weld seam and their variation are listed with corresponding process variables: laser power, welding speed, and partto-part gap.
No Runorder Laser
power
(W)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
3
11
6
5
13
17
12
7
1
23
18
20
22
4
27
25
15
9
2
24
26
21
16
19
8
14
10
1800
1800
1600
1800
1800
1800
2000
2000
1600
2000
1600
1800
1600
1800
1600
1800
1600
1600
2000
1800
1600
2000
2000
2000
2000
1600
2000
Welding
speed
(mm/min)
1200
1200
1200
1800
1200
1800
1800
1800
1800
1500
1500
1800
1200
1500
1800
1500
1500
1800
1500
1500
1200
1200
1800
1200
1500
1500
1200
Topweld seam
width (μm)
Gap
Maximum
(mm) tensile
strength
(MPa)
mean
0.1
0.3
0.1
0.3
0.2
0.2
0.2
0.3
0.2
0.1
0.1
0.1
0.2
0.1
0.3
0.3
0.2
0.1
0.3
0.2
0.3
0.2
0.1
0.1
0.2
0.3
0.3
1528.00 3.48
930.64 7.324
1550.80 2.451
934.14
3.444
1721.40 0.591
1358.39 2.731
1480.17
0.522
762.57 6.027
731.93 6.511
1438.16
2.13
1339.16
1.257
1355.35 1.991
1352.12 1.937
1335.01 1.883
555.48 12.243
1047.93 4.44
1051.72 4.359
1319.14
2.451
931.07 5.159
1702.29 1.652
817.32 8.075
1641.91 2.332
1338.06 1.728
1625.75 2.96
951.07 4.729
649.90 11.249
950.38 6.511
Fig. 7. Definition of the width of the top weld seam by using the two extracted boundary curves.
161.30
101.77
161.50
124.00
156.30
148.00
162.00
114.70
123.33
165.69
134.51
141.24
140.60
141.73
34.43
137.90
145.40
134.48
159.95
156.19
90.90
181.00
153.02
170.10
158.90
57.52
163.00
variation
1150
A.K. Sinha et al. / Optics and Lasers in Engineering 51 (2013) 1143–1152
Fig. 8. Variations of top weld seams.
Table 7
ANOVA table for thelaser welding experiment.
Source
Sum of squares
DF
Laser power (x1)
Welding speed (x2)
Gap (x3)
Laser power (x1) welding speed (x2)
Laser power (x1) gap (x3)
Welding speed (x2) gap(x3)
Error
9280
2,086.3
10,896.5
924.8
4,227.6
524
1711
2
2
2
4
4
4
8
Total
29,650.1
26
Mean square
F
Prob4F
4,640
1,043.14
54,448.26
231.2
1,056.9
130.99
213.87
21.7
4.88
25.47
1.08
4.94
0.61
0.0006
0.0412
0.0003
0.4267
0.0265
0.6656
A.K. Sinha et al. / Optics and Lasers in Engineering 51 (2013) 1143–1152
in Fig. 9, the following regression model describes the relation
between the average width of the top weld seam ‘w’ and
maximum tensile strength ‘T’:
T ¼ 86:248 lnðwÞ−469:56;
R2 ¼ 0:6472
1151
The coefficient of determination R2 shows that 64.72% of the
variation of maximum tensile strength is accounted for by the
above regression model with the transformed average width of the
top weld seam (log-transformation). The computed P-value of
‘4.2478E-7’ indicates that the investigated positive correlation
between T and ln(w) is statistically acceptable at the 0.05 level
of significance.
In the same way, the observed relation between the variance of
the top weld seam width ‘v’ and maximum tensile strength ‘T’ is
described by the following regression model (see Fig. 10):
T ¼ −0:8087 v2 þ 158:63;
R2 ¼ 0:7867
The coefficient of determination R2 and the negative coefficient
of ‘v2’ explain a strong negative correlation between T and v2. The
computed P-value of ‘7.2109E-10’ indicates that the investigated
negative correlation is statistically acceptable at the 0.05 level of
significance.
In order to visually verify the discovered trends in the weld
seam, we select the specimen 5, 22, 2 and 17 as shown in Fig. 11;
the first two have small variation in the width of top weld seam
with high tensile strength, while the last two have large variation
with low tensile strength.
Fig. 9. A positive correlation between the log-transformed average width of the top
weld seam and maximum tensile strength.
5. Conclusion
Fig. 10. A negative correlation between the variance of the width of the top weld
seam and maximum tensile strength.
We have investigated the correlation between (i) the shape
complexity of the top weld seam, that is, the variation in the width
of the weld seam, and (ii) welding quality, for the case of laser lap
welding of galvanized steel. Laser power, welding speed, part-topart gap and their interactions were taken as the experimental
factors, and tensile tests have been made to determine the welding
quality of each specimen.
We observed that if the heat input per unit length is not
sufficient due to low laser power, large part-to-part gap or high
welding speed, weld seam is not formed sufficiently from the initial
melting zone which is usually created nearby at the mating plane of
the two metal parts to be joined, outwardly onto the top surface of
the upper part. Consequently, unstable top weld seam is often
created, so that the variation in the width of the top weld seam is
relatively large. We found that laser welded parts having larger
variation of the top weld seam usually have lower tensile strength,
and vice versa. This result leads us to conclude that the variation of
Fig. 11. Top weld seams: (a) and (b) have lower variations with higher maximum tensile strength, while (c) and (d) have higher variations with lower maximum tensile
strength.
1152
A.K. Sinha et al. / Optics and Lasers in Engineering 51 (2013) 1143–1152
weld seams can be used as an indirect non-destructive testing
method for estimating the tensile strength of the welded parts.
Further experiments will be conducted to verify the results
in terms of hardness, toughness, surface roughness and fatigue.
Furthermore, in this paper, we manually extracted two boundary
curves of the top weld seam to calculate its variation. It is however
necessary to develop a more programmed way of measuring the
variation in order to evaluate the quality of welded parts
efficiently.
Acknowledgments
The research reported in this paper is supported by Korea
Institute for Advancement in Technology (EUFP-M0000224) and
the European Commission (FP7 Project 285051).
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