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5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th -14th, 2014,
IIT Guwahati, Assam, India
302-1
Resistance Welding of Austenitic Stainless Steels (AISI 304 with
AISI 316)
A.B.Verma1*, S.U.Ghunage2, B.B.Ahuja3
1*Shree Ramchandra College of Engineering, Pune,412207,[email protected] 2College Of Engineering,Pune,411005,[email protected] 3College Of Engineering,Pune,411005,[email protected]
Abstract
Resistance spot welding (RSW) has a very important role as a joining process in the automotive industry and a
typical vehicle contains more than 3000 spot welds. The quality and strength of the spot welds are very
important to the durability and safety design of the vehicles. The development of the new materials results
constantly in the resistance spot welding tasks with new materials or combinations of them. The lack of
experience with the new materials or combinations of them often results in the use of the welding parameters
which are not optimal. A few common guideline values and weldability diagrams for spot welding of steels exist
and most of the guidelines are for non stainless steels each spot welding is not performed on the same condition
because of the alignment of sheets and electrodes as well as the surface condition. For that reason, a spot
welding process needs the optimum process condition that can afford allowance in parametric values for good
quality of welding. In this paper ASS 304 and ASS 316 is used and its tensile strength and hardness is studied
by using Taguchi approach and ANOVA while microstructure is studied by Schaeffer diagram.
Keywords: ANOVA, Austenitic stainless steel, Taguchi method.
1 Introduction
Resistance spot welding (RSW) is widely used in
sheet metal fabrication as an important joining process.
It is a simple process that uses two copper electrodes to
press the work sheets together and force high current to
pass through it. In electric resistance spot welding, the
overlapping work is positioned between the water-
cooled electrodes and then the heat is obtained by
passing a large electrical current for a short period of
time. Resistance spot welding is a widely used joining
process for fabricating sheet metal assemblies such as
automobiles, truck cabins, rail vehicles and home
applications due to its advantages in welding efficiency
and suitability for automation. For example, a modern
auto-body assembly needs 7000 to 12,000 spots of
welding according to the size of a car, so the spot
welding is an important process in auto-body
assembly. Over the last few years, the weight of
automobiles has increased considerably due to the
addition of safety related items, such as impact
resistance bumpers and door impact beams, emission
control equipment and convenience items, such as air
conditioning. At the same time fuel consumption has
increased significantly primarily due to emission
control equipment.
Dissimilar metal welds are common in welded
construction and their performance is often crucial to
the function of the whole structure. Dissimilar metal
welding involves the joining of two or more different
metals or alloys. There are several types of dissimilar
metal welds and the most common type is the joining
of stainless steel to non stainless steel. In arc welding
filler metal is typically used. However, in resistance
spot welding, the use of filler metal is very rare. In
resistance spot welding, the parameters which
control the weld strength are the amount and duration
of electric current, electrode force, the shape and
material properties of electrode, the surface condition
and alignment of sheets. Thus, the quality of weld
strength in resistance spot welding process greatly
affects overall quality of the entire welding structure.
Vural et al. investigated the effect of nugget diameter
on the fatigue strength of resistance spot welded
joints of galvanized steel and austenitic stainless
steel (AISI304) welded as lap joints. Bouyousfi et al.
have studied the effect of spot welding process
parameters (weld current, welding duration and
applied load) on the mechanical properties and
characteristics of the spot joints between two
stainless steel sheets (304ASS) having the same
thickness. Micro hardness and tensile test results
have shown that the weld resistance is important and
highly correlated to the value of the process
parameters especially the applied load. The applied
load is control factor of the mechanical
characteristics of weld joint compared to the welding
duration and the current intensity of welding. Esme
has studied optimization of RSW process parameters
for SAE 1010 steel using Taguchi method. Esme
investigated that increasing welding current and
electrode force are prime factors controlling the weld
strength. He concluded that Taguchi method can be
Resistance Welding of Austenitic Stainless Steels (AISI 304 with AISI 316)
302-2
effectively used for optimization of spot welding
parameters. The development of the new materials
results constantly in the resistance spot welding tasks
with new materials or combinations of them. The level
of importance of the welding parameters on the tensile
shear strength is determined by using ANOVA. Based
on the ANOVA method, the highly effective
parameters on tensile shear strength were found as
welding current and electrode force, whereas electrode
diameter and welding time were less effective factors.
The results showed that welding current was about two
times more important than the second ranking factor
(electrode force) for controlling the tensile shear
strength. The lack of experience with the new materials
or combinations of them often results in the use of the
welding parameters, which are not optimal. A few
common guideline values and weldability diagrams for
spot welding of steels exist and most of the guidelines
are for non stainless steels. In general, an unlimited
number of weld metal compositions can be obtained in
the dissimilar metal welding, depending on the
combination of the base and filler metals and the
welding process. Dissimilar resistance spot welding is
of complex nature due to different thermal cycles
experienced with each metal.
2 Experimental procedure
In this study, different grades of austenitic
stainless steel sheets (AISI 304 and AISI 316) of 0.6
mm thickness were spot welded by KEJE spot welding
machine (TSP 30). Nominal chemical composition and
mechanical properties of the sheets are given in Table
1. The sheets were cut parallel to the rolling direction.
The dimension of sheet are 140 mm length (L), 40 mm
width (w) and 0.6 mm thick (t) (Figure 1). Overlap is
equal to width of the sheet as per AWS standard. Sheet
surfaces were chemically cleaned by acetone before
resistance spot welding to eliminate surface
contamination. The tests were carried out using a
current and time controlled electric resistance spot
welding machine. The electrodes material was
Chromium alloy with end diameter 5 mm. This
machine was equipped with a pneumatic pressure
system. Welding, squeezing and holding cycles were
manually selected.
Figure 1 Dimension of specimen
Three process parameters viz. Current, Electrode
Force and Weld cycles were selected as given in Table
2. The parameters which kept constant are electrode
Table 1 Chemical Composition
Wt % C Mn Si Ni Cr Mo
AISI
304 0.07 0.54 0.3 9.4 18.4 0
AISI
316 0.07 0.55 0.31 9.56 16.5 2.25
diameter and electrode material. Experiments were
conducted according to the test conditions specified
by the Taguchi L9 Orthogonal Array (OA). The
parameters used in the resistance spot welding of the
sheets are given in table 2.
Table 2 Taguchi orthogonal array
Tensile shear tests were carried out on a
electronic Tensometer (Model TM-ER3) having
20KN capacity with the tension speed of 20 mm/min.
For Micro-hardness measurement, Vickers
hardness measurement machine (Future-Tech FM-
700) was used under the 500g load acting over a
period of 10 sec. Microhardness measurements
across the weld nugget were performed on each
sample. The individual indents were made
horizontally along the weld line through HAZ and
the weld nugget. The distance between the indents in
the weld metal was 25 μm.
For metallographic examinations, the
specimens were cut mechanically using wire EDM
machine in the direction parallel to thickness and hot
mounted in bakelite. Grinding was carried bu using
rough and fine emery followed by polishing
operation. These specimens were etched in a
chemical solution of 15 ml HCl, 5 ml HF and 80 ml
water. Microstructures of the specimens were
examined using a Nikon Epiphot 200 type optical
microscope.
3 Results and Analysis
3.1 Tensile shear strength
Spot weld failure mode is a qualitative measure
of the weld quality. In the resistance spot weld
(RSW) failure occurs in two modes: interfacial and
Run
No.
Current
(KA)
Time
(Cycle)
Pressure
(kg/cm2)
1 5 5 2
2 5 6 2.2
3 5 7 2.5
4 6 5 2.2
5 6 6 2.5
6 6 7 2
7 7 5 2.5
8 7 6 2
9 7 7 2.2
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th -14th, 2014,
IIT Guwahati, Assam, India
302-3
pullout. In the interfacial failure (IF) mode, failure
occurs via crack propagation through fusion zone. In
the pullout failure (PF) mode, failure occurs via nugget
withdrawal from one sheet. Pull out mode of failure of
the welded specimen was observed as shown in figure
2.
Figure 2 Pull out failure of welded specimen
The tensile-shear test is the most widely used test
for evaluating the spot weld mechanical behaviors in
static condition. Peak load, obtained from the tensile-
shear load displacement curve, describes mechanical
behavior of spot welds. The experimental results of
tensile shear strength are shown in the table 3.
Table 3 Tensile shear strength results
Run 304-304
(N)
316-316
(N)
304-316
(N)
1 4207.2 4050.29 4030.68
2 4060.13 3667.82 3373.61
3 3805.12 3559.94 3824.73
4 3932.61 4011.06 3618.78
5 3765.89 3579.56 3510.91
6 4011.06 3677.63 3942.41
7 4462.19 4197.4 4187.59
8 4276.01 4452.38 4305.27
9 4364.12 4540.82 4557.26
The experimental process involves the test
conditions to compartivley assess the mecahncial
behaviour of spot welded sheets of AISI304-AISI304,
AISI304-AISI316 and AISI304-AISI316. To analyse
the given results, Taguchi methodology and ANOVA
technique have been used. Taguchi recommends the
use of the Signal to Noise (S/N) ratio to measure the
quality characterstics deviating from the desired
values. The main principle of measuring quality is to
minimize the variability inthe products performance in
response to Noise factors while maximize the
variability in response to Signal factors. Noise
321
73.2
72.9
72.6
72.3
72.0
321
321
73.2
72.9
72.6
72.3
72.0
Current
Me
an
of S
N r
atio
s
Time
Pressure
Main Effects Plot for SN ratiosData Means
Signal-to-noise: Larger is better
Figure 3 S/N Ratio of AISI304- AISI304
321
13.0
12.5
12.0
11.5
321
321
13.0
12.5
12.0
11.5
A
Me
an
of S
N r
atio
s
B
C
Main Effects Plot for SN ratiosData Means
Signal-to-noise: Larger is better
Figure 4 S/N Ratio of AISI316- AISI316
321
72.8
72.4
72.0
71.6
71.2
321
321
72.8
72.4
72.0
71.6
71.2
Current
Me
an
of S
N r
atio
s
Time
Pressure
Main Effects Plot for SN ratiosData Means
Signal-to-noise: Larger is better
Figure 5 S/N Ratio of AISI304-AISI316
Resistance Welding of Austenitic Stainless Steels (AISI 304 with AISI 316)
302-4
factors are those that are not under control of the
operator of a product and the Signal factors are those
that are set or controlled by the operator of the product
to make use of its intended functions. Therefore, the
goal of quality imporovement effort can be given as to
maximize the Signal to Noise (S/N) ratio for the
product.
The results are converted into S/N ratios for
Taguchi analysis. The higher the better criterion has
been used to obtain the S/N ratios of the tensile
strength. The main effects of process parameters for
raw data and S/N data are plotted. The response curves
(main effects) are used for examining the parametric
effects on the response characteristics. The analysis of
variance (ANOVA) of raw data and S/N data is
performed to identify the significant parameters and to
quantify their effect on the response characteristics.
The most favorable conditions (optimal setting) of
process parameters in terms of mean response
characteristic are established by analyzing response
curves. The tensile strength result is as shown in table
3.
The summery of ANOVA results for AISI304-
AISI304, AISI316-AISI316 and AISI304- AISI 316
are as shown in table 4.
Table 4 Summary of ANOVA results
Parameters 304-304
(N)
316-316
(N)
304-316
(N)
R2 0.9912 0.9339 0.9501
F(current) 86.46 11.07 13.41
F(time) 5.08 0.79 3.6
F(pressure) 20.49 2.26 2.02
P(current) 0.011 0.083 0.069
P(time) 0.165 0.559 0.217
P(pressure) 0.047 0.307 0.331
As shown in above table, value of R2 is greater
than 0.8, therefore the model established by ANOVA
is acceptable. Also, F-estimated is greater than 3.44
(Ref: statistical tables). Therefore model is validated
and it concludes that all factors have significant effect
on Tensile strength.
3.1.1 Optimsation
The optimum conditions from Taguchi analysis
for spot welding of similar material AISI 304-AISI 304
and AISI 316- AISI 316 are is found to be current at
level 3 (7 KA), time at level at 1(5 cycles) and pressure
at level 1(2.2 Kg/cm2). However, for dissimilar grades
of austenitic stainless steels, AISI304 to AISI316, the
optimum conditions are found to be current at level 3
(7 KA), time at level at 3 (7 cycles) and pressure at
level 1(2.2 Kg/cm2).
From Table 5, 6 and 7, percentage contribution
(% C) of various parameters affecting the tensile shear
strength is clearly indicated. For welding of similar
Table 5 Results of ANOVA for Tensile Shear
Strength AISI304
Table 6 Results of ANOVA for Tensile Shear
Strength AISI316
Table 7 Results of ANOVA for Tensile Shear
Strength of AISI304 to AISI316
sheets (AISI304 to AISI304 and AISI316 to
AISI316), amongst the three factors chosen for the
study that governs tensile shear strength, current is
ranked first, pressure applied electrodes is ranked
second factor and weld time is ranked third factor.
However for the welding of dissimilar sheets of
austenitic stainless steels (AISI304 to AISI316),
current is most influencing factor having highest
contribution of 66.94 %, followed by weld time and
pressure applied having contributions 17.97 % and
10.10 % respectively. This may due to the reason
that more heat is required to cause sufficient solid-
molten pool of weld at the interface of AISI304 with
AISI316 and electrical resistivity of AISI316 is
comparatively more than AISI304. Therefore, more
amount of heat needs to be generated, which is
accomplished by passing current for more amount of
time. This results into more heat generation and
value of contact resistance decreases with increasing
values of temperatures. As AISI 316 has higher
hardness value than AISI304, it has higher contact
resistance at the constant welding force. It is also
observed that for a factor with a high percentage
contribution, a small variation may greatly influence
the output characteristics.
Source DF SS MS F P % C
Current 2 672748 336374 86.46 0.011 76.50
Time 2 39521 19760 5.08 0.165 4.49
Pressure 2 159411 79706 20.49 0.047 18.13
Error 2 7781 3891 - - 0.88
Total 8 879461 - - - 100
S = 62.3749 R-Sq = 99.12% R-Sq(adj) = 96.46%
Source DF SS MS F P % C
Current 2 833692 416846 11.07 0.083 73.24
Time 2 59393 29696 0.79 0.559 5.22
Pressure 2 169869 84935 2.26 0.307 14.92
Error 2 75295 37647 - - 6.61
Total 8 1138249 - - - 100
S = 194.029 R-Sq = 93.39% R-Sq(Adj) = 73.54 %
Source DF SS MS F P % C
Current 2 808489 404245 13.41 0.069 66.94
Time 2 217061 108530 3.6 0.217 17.97
Pressure 2 121971 60986 2.02 0.331 10.10
Error 2 60295 30148 - - 4.99
Total 8 1207817 - - - 100
S = 173.631 R-Sq = 95.01% R-Sq(adj) = 80.03%
5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12 th -14th, 2014,
IIT Guwahati, Assam, India
302-5
3.2 Micro-structure of dissimilar SS304/316 RSW
In any welding process, the properties and
performance of the weld are characterized by careful
examination of the microstructure. This in turn is
determined by the thermal cycle of the welding process
which can normally be varied by changing the welding
parameters. Therefore, selection of welding parameters
is crucial that gives best possible microstructure and
that allows welds to be made free from defects and
other undesirable features.
Microgrpah of a dissimilar resistance spot weld
between AISI304 and AISI316 is shown in fig. 6.
Three distinct structural zones are observed in the joint
region:
i) Fusion Zone (FZ) or weld nugget,
ii) Heat Affected Zone (HAZ), and
iii) Base Metal (BM).
Figure 6 Micrograph of AISI304- AISI316
From the micrograph, it is observed that the
welded joint is asymmetrical. The fusion zone size on
the AISI316 side is larger than the fusion zone on the
AISI304 side. On the basis of macrostructure analysis,
it can be stated that the higher the welding current for
the welding, the larger is the fusion zone.
According to Schaeffler diagram, a martensitic
structure is expected to form in the FZ, as confirmed
by the higher hardness of the FZ relative to the BMs.
An ellipse nugget was formed between two sheets, and
HAZ was found around the nugget. Microstructure of
fusion zone (FZ) was found fully austenitic. The
change in microstructure from FZ to BM depends on
the highest temperature reached at each region. The
final solidification structure consists of dendrite
morphology with directional solidification onward the
center. The changes during the solidification mode is
from planar to cellular, cellular to columnar dendrite,
and final equiaxed dendrite. Due to extreme high heat
inputs, some grain coarsening could be observed at
HAZ. The grain size in HAZ is clearly larger than that
of the BM. Since, the HAZ was heated to temperature
approaching the solidus temperature of the alloy; many
of the alloys that were present in the BM may dissolve
at this high temperature. This can led to a super
saturation of the austenite matrix during cooling.
3.2 Hardness Variation of dissimilar RSW sheets
As can be seen, the micro hardness of the FZ is
higher than the micro hardness of both BMs. In AISI
304, the increase in hardness in FZ was from a value
of 178 HV, measured in the base metal, to hardness
of 220 HV (measured in the weld metal). The
hardness measured in AISI 316 steel increased from
a value of 218 HV to a value of 220 HV. Weld FZ
microstructure of dissimilar ASS RSWs can be
predicted by constitution diagrams e.g., Schaeffler
diagram. It should be noted that the application of
this diagram might be inaccurate due to the very high
cooling rates of RSW process. The FZ microstructure
of dissimilar ASS RSWs depends on the chemical
composition of the BMs and the dilution (defined as
the carbon steel to the weld nugget volume ratio).
Dilution is controlled by welding parameters. In the
applied welding conditions the dilution was
measured as 40%.
Figure 7 Hardness variation of welded specimen
Conclusion:
In this study, the properties of resistance spot
welds of dissimilar steels have been studied.
Attempts were made to link a weld’s quality to its
attributes under tensile-shear testing. The influence
of the welding parameters on the weld metal size has
been evaluated. The use of Taguchi method provides
a systematic and effective means to deal with the
multivariable nature of characterizing a spot weld.
The findings can be summarized as follows:
1) Tensile shear strength for different grades of
Austenitic Stainless Steels (AISI304 to
AISI316) was found to be comparatively
more than compared with similar sheets
(AISI304 to AISI304 and AISI316 to
AISI316).
2) Weld current is major governing factor
affecting the tensile shear strength of the
resistance spot welded specimens. As the
weld current increases, size of weld nugget
also increases. This results into increased
values of tensile-shearing strength.
3) For dissimilar RSW between austenitic
stainless steels, asymmetric fusion zone was
obtained due to their different electrical
resistivity and coefficient of thermal
expansion.
Resistance Welding of Austenitic Stainless Steels (AISI 304 with AISI 316)
302-6
4) Hardness of the welded zone is greater than
the hardness of the unwelded zone for AISI
304 and AISI 316 joints, but for dissimilar
joint, there was marginal increase in hardness.
5) For the welding of dissimilar sheets of
austenitic stainless steels (AISI304 to
AISI316), current is most influencing factor
having highest contribution of 66.94 %,
followed by weld time and pressure applied
having contributions 17.97 % and 10.10 %
respectively.
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