MATEC Web of Conferences 381, 02007 (2023) https://doi.org/10.
1051/matecconf/202338102007
MTME-2023
Evaluation of microstructure and mechanical
behavior of Aluminum 2024 and Stainless steel
304 GTAW joints
Asad Ali*, Muhammad Jawad, and Mirza Jahanzaib
Industrial Engineering Department, University of Engineering and Technology Taxila, 47080,
Pakistan
Abstract. This study aims to evaluate the microstructure and mechanical
behavior of aluminum 2024 and stainless steel 304 dissimilar joints. The gas
tungsten arc welding (GTAW) process has been employed to weld base
metals by inserting copper-nickel-based (Cu-10%Ni) filler metal. The
effects of GTAW parameters such as welding current, welding speed, and
gas flow on microstructure and tensile strength have been analyzed through
the Taguchi method. Results revealed that tensile strength is primarily
influenced by welding current, followed by speed and gas flow rate. The
excellent joint strength of 138 MPa has been achieved by using Cu-10%Ni
filler metal. The optimal combination of parameters, i.e., welding current of
level 2 (80 A), welding speed of level 1 (100 mm/min), and gas flow rate of
level 3 (10 l/min), has been obtained through SN ratio optimization.
Microstructure and EDS analysis depicted that the weld zone of a high-
strength joint contained fine dendrites and CuAl and NiAl solid solutions,
while the weld zone of a low-strength joint featured coarse dendrites and
brittle FeAl phases.
1 Introduction
A growing number of automotive and aerospace industries are focusing on the
development of lightweight structures with sound mechanical properties in order to
reduce production costs and enhance the product functionality [1]. In this regard,
dissimilar joints of aluminum (Al) and stainless steel (SS) offer a lightweight, highly
stable, and cost-effective hybrid structure option. However, the low compatibility
between these metals results in brittle Fe-Al intermetallic compounds (IMCs) that
degrade joint strength and durability. Therefore, the selection of the correct welding
process, the operating parameters, and the suitable filler metal is vital for preventing these
IMCs. The various researchers have attempted to optimize the tensile strength of Al and
SS dissimilar GTAW at different combination of parameters and filler metals. Nguyen et
al. [2] reported the influence of Al-12%Si filler metal on the SS 304L and Al 6061 GTAW
joints. Due to the high Si content in filler metal, Fe diffusion in weld pool was greatly
* Corresponding author: asad.ali2@students.uettaxila.edu.pk
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons
Attribution License 4.0 (https://creativecommons.org/licenses/by/4.0/).
MATEC Web of Conferences 381, 02007 (2023) https://doi.org/10.1051/matecconf/202338102007
MTME-2023
reduced, leading to a tensile strength of 208 MPa at welding current (I) of 60 A, welding
speed (S) of 6 mm/s and gas flow rate (GFR) of 14 L/min. A study by Kotari and Punna [3]
examined the effects of GTAW parameters and copper filler in the joining of Al 6061 and
SS 304. The copper (Cu) filler metal along with Al base metal demonstrated a highly crack
suppressant Al13(Fem Cun)4 solid bond and depicted a maximum tensile strength of 132
MPa at I of 100 A, S of 90 mm/min and GFR of 9 l/min. Song et al. [4] evaluated Al 5A06
and SS 321 GTAW joints with Al-12%Si and Al-6%Cu fillers. At the weld seam and SS
side interface the Al-6%Cu filler depicted an Al13(Fe Cu)4 solid bond, while Al-12%Si
displayed brittle IMCs which adversely impacted the joint’s mechanical properties.
Furthermore, Al-12Si filler exhibited a tensile strength of 100 MPa, whereas Al-6Cu filler
achieved a tensile strength of 155 MPa. Shah et al. [5] conducted a study on joining Al
6061 with SS 304 using a Al ER5356 filler and SS ER308LSi filler. The microstructure of
the joint welded with ER5356 filler revealed the Si particle enrichment, which enhanced
the joint properties and resulted in a tensile strength of 104.4 MPa, whereas the joint
with ER308LSi mainly consisted of chromium carbide brittle elements, which resulted in
tensile strength of 61.76 MPa.
The literature review evident that the researchers optimized the GTAW parameters
during joining of Al and SS by using Al, SS, and Cu fillers, but joining of Al and SS using
Cu-10%Ni filler has not yet been examined. Therefore, this study examines the effect of
GTAW parameters and Cu-10%Ni alloy filler on Al-SS joints using Taguchi’s method.
Parametric significant analysis has been performed by analysis of variance whereas
optimization has been carried out by Taguchi SN ratios optimization method.
2 Materials and Method
Aluminum 2024 and Stainless steel 304 of dimensions 100 x 50 x 3 mm have been selected
as base metals, while Cu-10%Ni wire of diameter 2.4 mm has been chosen as a welding
filler metal. The chemical composition of selected metals is presented in Table 1. The 45°
bevel angles have been made on both base metal surfaces prior to welding so that the
filler metal could penetrate deeply. Afterward, the oxide layer on base metals has been
removed using emery paper, and the metals are then washed with acetone. The Lincoln
Electric v270T plant has been used to perform GTAW, and integrity of the welding arc
has been ensured using a fine-pointed tungsten electrode. An automated welding
workstation equipped with a lead screw, DC motor, and potentiometer, has been utilized
to conduct the welding. The welding current has been controlled by the current knob on
the GTAW plant whereas welding speed has been adjusted by the potentiometer. Pure
argon gas has been used as shielding gas to protect the weld from the outer environment,
and the gas flow rate has been controlled by the flow meter attached on the argon gas
cylinder.
Table 1. Chemical composition of selected metals
Composition (wt. %) Cu Fe Al Ni Cr Si Mn Ti Mg
Al 2024 4.90 0.50 Bal. 0.10 0.50 0.90 0.15 1.80
SS 304 0.009 Bal. 8.113 18.08 0.419 1.228
Cu-10%Ni Bal. 1.5 11 0.75
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MATEC Web of Conferences 381, 02007 (2023) https://doi.org/10.1051/matecconf/202338102007
MTME-2023
The mechanical behavior of joints has been assessed through tensile tests. Tensile
specimens have been prepared using wire discharge machining, and the tensile tests are
conducted on an Instron testing machine with a 0.1 s -1 strain rate. The tensile specimen
according to ASTM E8/E8m-15a having a total length of 100 mm and gauge length of 40
mm is depicted in figure 1. The tensile strength has been further evaluated by
microstructure. The cross-section samples from the weld plate have been ground through
sandpaper (300–3000 grit), then polished, and finally etched with Keller’s solution.
Figure 1. Tensile specimen
3 Experiment Design
The three dominant parameters of GTAW, welding current (I), welding speed (S), and
gas flow rate (GFR), have been considered and their effects on ultimate tensile strength
are assessed. The range of parameters has been determined through preliminary
published research and trial runs. Minitab statistical software has been employed to
design experiments. Nine experiments have been performed using the L9 array. The
experiment’s design is depicted in Table 2.
Table 2. Experiments design
Experiment Input parameters Output
No. Welding Welding Speed Gas Flow Rate Tensile Strength
Current (mm/min) (l/min) (MPa)
(A)
1 70 100 8 116
2 70 110 9 110
3 70 120 10 112
4 80 100 9 138
5 80 110 10 131
6 80 120 8 118
7 90 100 10 111
8 90 110 8 98
3
MATEC Web of Conferences 381, 02007 (2023) https://doi.org/10.1051/matecconf/202338102007
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9 90 120 9 94
4 Results and Discussion
4.1 Main Effects Plots
The plots of the main effects of tensile strength means are depicted in figure 2. From the
plots, it appears that the increase in I from 70 to 80 A resulted in rise in the tensile
strength but a further increase up to 90 A caused in decreasing the tensile strength. A low
I cause a low heat input, which leads to incomplete brazing defects on the SS side, thereby
decreasing the tensile strength. Similarly as a result of excessive I, more filler and
aluminum melt, causing a greater heat affected zone on the Al side and again resulting in
poor tensile strength [6]. Also, it is clear from the plot that decrease in S up to 100
mm/min produced a high tensile strength, whereas high welding speed (120 mm/min)
caused a low tensile strength. Accordingly welding at a low S level leads to proper
penetration of filler metal, whereas welding at a higher S failed to melt the SS side which
resulted in incomplete fusion and weak joint strength [7, 8]. An increasing trend of tensile
strength can be seen with the GFR. Higher GFR provides a greater shield against outer
atmospheric contaminants and porosity, resulting in improved tensile strength [9].
Figure 2. Main effects plots for tensile strength means
4.2 ANOVA
Analysis of variance with 95% confidence intervals has been performed to determine
critical process parameters and their contribution towards output response. The ANOVA
in table 3 indicates that welding current is the most significant tensile strength
influencing parameter, with a P value of 0.012. The percentage contribution percentage
of 75% for current shows that it is most contributed factor followed by welding speed
(18.2%) and gas flow rate (5%). Model adequacy is evaluated based on adjusted R 2 [10]
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MATEC Web of Conferences 381, 02007 (2023) https://doi.org/10.1051/matecconf/202338102007
MTME-2023
and predicted R2 values [11]. In this model, R2 adjusted is 0.962, and R2 predicted is 0.807,
indicating a high degree of accuracy.
Table 3. ANOVA
Source DF Adj SS Adj MS F value P value Contribution %
Welding Current 2 1186.8 593.4 79.72 0.012 75
Welding Speed 2 286.8 143.4 19.27 0.049 18.2
Gas Flow Rate 2 80.8 40.4 5.43 0.155 5
Error 2 14.89 7.4
Total 8 1569.5
R-sq. 99.05%, R-sq.(adjusted) 0.962, R-sq.(predicted) 0.807
4.3 Signal to Noise ratios Optimization
Parametric values where an optimal response is achieved are determined by signal-to-
noise ratios. Signal to Noise ratios measures the impact of noise factors on performance
characteristics [12]. Tensile strength dictates effective joint quality, therefore the larger
the better performance characteristic has been chosen. Equation (1) depicts the larger the
better SN ratio.
n
1 1
SN= -10 log( ∑ 2 ) (1)
n yi
i=1
Where yi refers to the response value of ith run in the set of n experiments [13]. The
plots of SN ratios in figure 3 clearly indicate that level 2 of I (80 A), level 1 of S (100
mm/min), and level 3 of GFR (10 l/min) are the optimal levels for getting the better tensile
strength. Furthermore, the better signal quality at these levels has been confirmed by
their SN ratios (highlighted) in table 4.
Figure 3. SN ratios plots
Table 4. SN Ratios
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MATEC Web of Conferences 381, 02007 (2023) https://doi.org/10.1051/matecconf/202338102007
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Level Welding Welding Gas
Current Speed Flow Rate
1 41.03 41.66 40.85
2 45.19 41.00 41.03
3 40.06 40.63 41.41
Delta 2.13 1.04 0.56
Rank 1 2 3
4.4 Microstructure Analysis
The microstructure and elemental composition of the weld zones have been examined on
the high-strength sample (experiment no. 4) and the low-strength sample (experiment
no.9). A welding current of 80 A has been used in experiment no.4, which causes a high
cooling rate of the molten metal, thus resulting in fine dendritic structure consisting of
Cu-Ni as shown in figure 4 (a) which improved tensile strength. Alternatively, a welding
current of 90 A (experiment no. 9) slows down the cooling rate of molten metal, resulting
in coarse dendrites of Fe (dark phase) and Al (light phase) which reduced the joint
strength, as presented in figure 4 (b). Tanmay and Panda [14] have also found the fine
dendritic grain structure of Cu and Fe which contributed to high tensile strength.
(a) (b)
Coarse dendrite Fe-Al
phase
Fine dendrite Cu-
Ni phase
50 µm 50 µm
Figure 4. Optical microstructure of the weld zone (a) Experiment No. 4 (b) Experiment No.9
The SEM photographs of experiment no. 4 and experiment no. 9 are shown in figures
5 (a) and 5(b) and their corresponding elemental compositions are summarized in EDS
analysis table 5. In experiment no.4 the low heat input of 80 A current led to limited
melting of the SS side that resulted in possible solid solutions of Cu-Al and Ni-Al
(Spectrum 2). Contrary to this, I of 90 A increased the melting of SS during welding,
resulting in the possible brittle IMC FeAl as well as solid solutions of CuAl, and NiAl
(Spectrum 1). Moreover, CAO et al. [15] show that the formation of CuAl and NiAl in
weld zone enhances the dissimilar Al and SS welds strength.
(a) (b)
Al
Al
Weld Zone
Weld Zone
SS
SS
2 mm 2 mm
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MATEC Web of Conferences 381, 02007 (2023) https://doi.org/10.1051/matecconf/202338102007
MTME-2023
Figure 5. SEM photographs (a) Experiment No. 4 (b) Experiment No.9
Table 5. EDS analysis results
EDS spectrum (wt. Cu Fe Al Ni C O Mn Possible Phase
%)
Spectrum 2 52.13 3.33 21.35 8.31 11.56 2.46 0.85 CuAl, NiAl
Spectrum 1 34.86 29.81 16.58 7.87 8.70 0.419 1.11 FeAl, CuAl,
NiAl
The joint mechanical behavior has been analyzed through stress strain curves as
shown in figure 6. The high strength joint (experiment no. 4) failed at the stainless steel
and weld interface. The necking phenomena can be observed before its fracture which
shows the prominent elongation and ductile nature. In contrast to this, the stress strain
curve of low strength joint (experiment 9) depicted that fracture occurred without
noticeable elongation (strain) and it happened at before yield point. The joint has been
fractured at the aluminum-weld zone interface.
160
ϭϯϴDWĂ
140
120
ϵϰDWĂ
Stress (MPa)
100
80
Experiment 4
60
40 Experiment 9
20
0
0 0.05 0.1 0.15 0.2
Strain
Figure 6. Stress and strain curves
5 Validation
The validation of the Taguchi method result has been performed through confirmatory
test. The results in Table 6 indicate a better degree of agreement between actual and
predicted tensile strength at optimal levels condition (80 A, 100 mm/min, 10 l/min).
Table 6. Confirmatory test result
Actual value Predicted value error Percentage error
137 140.2 3.2 2.33
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MATEC Web of Conferences 381, 02007 (2023) https://doi.org/10.1051/matecconf/202338102007
MTME-2023
6 Conclusion
This study examines the impact of GTAW parameters on dissimilar joints of aluminum
2024 and stainless steel 304 using the Taguchi method. The Cu-10%Ni filler wire has been
employed as a welding filler material. In light of the results, the following conclusions
have been drawn.
• Main effects plots displayed that a decrease in S (100 mm/min), a rise in G FR (10
l/min), and an increase in I (80 A) resulted in a rise in the joint tensile strength.
• The ANOVA results revealed that I contributed 75% to the tensile strength and the
most significant parameter, followed by S and GFR.
• Signal-to-noise ratios optimization results indicated that optimal levels have been
achieved as I of 80 A, S of 100 mm/min, and G FR of 10 l/min that yielded a tensile
strength of 140.2 MPa.
• Furthermore, EDS and microstructure analysis of better weld joint (experiment no.
4) revealed fine grain and CuAl, NiAl solid solutions in the weld zone whereas low
strength sample (experiment no. 9) depicted coarse grains with FeAl brittle phase.
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