Experimental and finite element investigation of resistance spot welding of mild steel sheet covered aluminum alloy, AA 2017

Abstract Resistance spot welding (RSW) is one of the welding technologies that uses the force and heat generated by resistance to the flow of electricity to join metal surfaces. The goal of this research is to investigate the mechanical behavior of RSW,the welding parameters of sheet metal-covered aluminum spot-welded junctions, and finally to verify the micro-hardness of the weld structure. RSW process is a complicated operation that combines electrical, thermal, and mechanical processes. Moreover, serious complications are observed when the weld material is aluminum, because it is a very soft metal and difficult to weld as compared to other metals, Hence, we used mild steel as a cover on both sides and easily did the RSW. Aluminum (Al) thickness, cover sheet metal thickness, and overall welding time achieved relative impacts of 3.890%, 3.250%, and 84.390%, respectively. The percentage impacts of aluminum (Al) thickness, weld cover sheet metal thickness, and welding time in the deformation scenario are 1.171%, 8.731%, and 80.881%, respectively. The percentage impacts of aluminum thickness, cover sheet metal thickness, and welding time duration on temperature are 9.960%, 87.820%, and 1.660%, respectively. The thickness of the cover mild steel sheet is the second-most important factor, next to welding time. Validation results of the two critical weld constraints (welding temperature and welding time) agrees with the experimental results. In addition, the welding temperature response has a percent error of 10.620% due to materials characteristics and the impact of additional welding process constraints.


Introduction
Resistance spot welding (RSW) is a method of joining thin sheets of metals such steel, aluminum, magnesium, titanium, and their alloys.This method of welding incorporates Joule's law of heating, which uses the resistance present on the work surface to heat and melt the surface, resulting in the formation of a weld nugget.RSW is processes that produces the heat required for welding through what is known as joule heating (J = I 2 Rt) (Alden, 2017;Alizadeh Sh et al., 2015;Brien, 2000;Das, Rawal, et al., 2020;Zhigang et al., 2006).A resistance spot weld is a simple process in which a strong current is driven through the work sheets under pressure to create the joint.
Welding current, welding time length, and the acting electrode force are the primary input factors impacting the development of a spot weld.The most popular form of resistance welding is RSW.In this procedure, both electric current and mechanical force are used concurrently to form joints, as depicted in Figure 1.RSW can weld a variety of materials, such as stainless steel, high-strength low-alloy steel, advanced high-strength steel, and low-carbon steel (Alizadeh Sh et al., 2015;Zhigang et al., 2006).Despite the high current flow in RSW, there is no risk of electric shock because only low voltage is transmitted (Brien, 2000;Raut & Achwal, 2014).
Aluminum's (Al) high thermal and electrical conductivity requires 2-3 times higher current and less welding time, around 25.00% of which is required to spot weld steel.Because the welding temperature range is restricted, precision current and electrode force control and synchronization are necessary.The issues are increased when welding unalloyed aluminum (Das et al., 2021).
Aluminum is a soft and light metal that may be utilized in a variety of industrial applications, including the automobile industry, but welding aluminum is much more difficult than welding hard metals.As a consequence, in this study, we employed mild steel sheet as a cover for both sides of Al, which allows for easy RSW and characterization of weld strength and weld zone hardness.Lastly, model verification is done by comparing the results of a cost-effective finite element simulation, which agrees with the experimental results.

Dissimilar material
A proper selection of materials is vital to obtaining a good quality outcome of resistance spot welded joints due to its mechanical properties and material properties.Aluminum alloy AA 2017 is a series of the most frequently used structural material in the automotive, aerospace, and rail transportation industry.As a result, aluminum alloy 2017 and mild steel 4130 cover was chosen for this study with parameters shown in Tables 1 and 2 (Alden, 2017;Brien, 2000;Das & Paul, 2020;Das Rawal, et al., 2020;Raut & Achwal, 2014).

Welding parameter selections
For the welding parameter selections, iterative welding trials were used to determine the optimal welding parameters with the chosen electrode combination by using Minitab 2021 software package and ANNOVA statistical tool (Chen & Farson, 2006;Han et al., 2010).The following welding parameters were finalized after 36 iterative welding trials for spot welding of 0.7-mm, 1.0-mm, and 1.3-mm-thick aluminum alloy AA 2017, as well as 0.6-mm, 0.8-mm, and 1.0 mmthick mild steel cover.To achieve this, the following steps were carried out: • Electrode on both side: tapered flat the tip is prepared.
• Electrode force: asper the machine standard has fixed electrode force 1.6 kN.

Sample preparation
For the mechanical tests, the sample material with a thickness of 1.0 mm was cut using the dimensions specified by a shearing machine.We checked that the hydraulic lever was open by turning on the machine.We entered the length value that corresponds to the dimensions provided.The initiation button and machine is depicted in Figure 2.
The experiments were carried out at the federal technical vocational and educational training institute with welding thicknesses of 0.7, 1.0, and 1.3 mm on sheets of alloyed aluminum AA2017 on a hand-held resistance spot welding machine with the specifications listed in Table 3.

• Steps Followed for the FE Simulation
The procedure of how to use the ANSYS workbench working window is shown in Figure 3.
Step 1. Selecting the procedure used for resistance spot welding, forms the system analysis, as shown in Figure 4.
Step 2. Creating the Model, see Figure 5.
Step 3. Assigning materials/importing new materials if not available on the material library of the software (see Figure 6).
Step 5. Meshing (Discretization), is the process of selecting the best meshing technique for doing the welding.The tetrahedral meshing is used for the simulation, as shown in Figure 8.The sheet in this model has shell elements, whereas the spot weld uses 8-node solid components.The two element types are coupled (Das & Paul, 2021;Shim et al., 2002).

Step 6. Checking for Quality of Meshing
To reduce runtime when adjusting the models, loads, and boundaries, Figure 9 shows the quality of the elements in all the model regions.All around joints have a higher or equal to 0.70 element quality.The choice for element quality compromises a combined quality metric between 0 and 1 (Das & Paul, 2021;Löven Born, 2016;McCune et al., 2000;Moarref Zadeh, 2011;Shim et al., 2002).Step 7. Analysis setting: this comprises of inserting the time step size and loading step size, checking the time step on, large deformation on, and making the solver iterative rather than direct solver because there is no accurate solution by direct solver for complex geometry and other complex operations (see Figure 10).

Experimental results
The experimental results were achieved based on the following input parameters: thickness of aluminum sheet (0.7, 1, and 1.3) mm, cover sheet (0.6, 0.8, and 1) mm, welding current (3 kA), electrode force (1.6 kN), electrode tip diameter (4) mm, and welding time (5, 8, and 11) seconds.Nine (9) tests were performed by inserting three different test groups based on the aluminum weld sheet and weld cover sheet thickness.
Experimental tests on specimens 1, 2, and 3 are shown in Figure 11.For test 1, the plot of stress versus strain has a linear relationship up to the elastic limit; the maximum stress is above 60 MPa.For the experimental tension test of specimen 2, stress versus stain have a linear relationship up to the elastic limit, and the maximum stress is above 60 MPa.For test specimen 3, the stress strain curve shows a linear relationship up to the elastic limit and reaches its maximum above 55 MPa.Experimental tests on specimens 4, 5, and 6 are shown in Figure 12.For test-4, stress and strain have a linear relationship; they have an elastic limit, and then the maximum stress is above 40 MPa.For test specimen 5, stress and strain have a linear relationship; they have up to the elastic limit, and then the maximum stress is above 60 MPa, which is similar to the FEA result obtained at 76 MPa.This shows good validation of the FEA result obtained.Test 6 shows stress and strain have a linear relationship; they have an elastic limit, and then the maximum stress is above 40 MPa.Experimental tests on specimens 7, 8, and 9 are shown in Figure 13.For test 7, stress and strain have a linear relationship; they have an elastic limit, and then the maximum stress is above 40 MPa.For test specimen 8, stress and strain have a linear relationship; they have an elastic limit, and then the maximum stress is above 60 MPa.Lastly, for specimen 9, stress and strain have a linear relationship; they have an elastic limit, and then the maximum stress is above 50 MPa.

FEA results
This section presents the results of finite element analysis on the following experimentally validated specimen sizes taken as study parameters: aluminum alloy weld sheet thickness (0.7, 1, and 1.3) mm; mild steel weld cover thickness (0.6, 0.8, and 1) mm; and constant diameter of copper alloy electrode on boundary conditions such as electrode force 1600 N, welding current 3 kA, and welding time (5, 8, and 11) seconds.In the three specimens as shown in Figures 14 -16 From the three specimens as shown in Figures 17 -19, for specimens 4, 5, and 6, for the same aluminum sheet thickness, electrode force, and weld current, increment or decrement of cover sheet thickness and weld time has a significant effect on RSW joint response on von-Misses stress (81.79,76.83,and 79.26 MPa,respectively),electric voltage (114.31,126.11,and 144.30) mV, respectively, and weld temperature distribution around the weld zone (227.85, 265.45, and 326.35) °C, respectively.
From the three specimens as shown in Figures 20 -22, for specimens 7, 8, and 9, it shows an increment in both weld cover sheet temperature and electric voltage increment irrespective of the weld time, and von-Misses stress is strongly affected by welding time, similar reports presented by other researchers (American Society for Testing and Materials [ASTM], 2003;Andersson, 2013;Novakova et al., 2009;Rysul & Shawon, 2014;Saleem et al., 2012).To summarize the FEA simulation response of the RSW, nine (9) specimens were tested both experimentally and numerically in the laboratory to validate the research findings.The von Mises stress distribution in RSW is depicted in Figures 14-22.The largest stress has happened at the electrode's edge.The electrode was observed to have a deformable body rather than a rigid body, which explains these phenomena.The stress singularity that caused the  divergence issue occurred near the electrode's edge when the electrode assumed a rigid body, as shown in Table 4.

Validations
The model's estimated values should be compared to experimental data to determine the correctness of each FEM.From the welding parameters, welding temperature and welding time were significant as compared to other factors, so the validation was done for the above two most important factors, welding temperature versus weld time plot.Hence, similar trends in FEA results with experimental    work are obtained, which shows the validation of the finite element simulation, which has less cost and wastage of material as compared to experimental work (Den Uijl et al., 2012;Lee et al., 2005;Naimi et al., 2015;Sun & Khaleel, 2004;Zhao et al., 2019).
As shown in Figures 23 -25, for both the numerical and experimental plots shown, temperatures are increasing until the maximum weld temperature is reached, and due to the weld time limit (11, 5, and 8 sec., respectively), the temperature will decrease after weld.As a result, the FEA response for these two critical weld parameters, weld temperature and weld timeagrees with the experimental result, as shown in Table 5, with a maximum error of 10.620% that may be due to the nature of the material and the influence of additional welding constraints such as welding current, weld cover thickness, welding voltage, and weld thickness itself.

Results of hardness tests and microstructure examinations
To do a hardness test and microstructure examination, the initial optimal condition is determined to avoid extra wastage of materials and time, and three selected test samples are taken as shown in Table 6 (ASTM, 2003;Andersson, 2013;Novakova et al., 2009;Rysul & Shawon, 2014).The results show that it is the feature setting that offers the highest tensile strength and hardness at the maximum temperature values.
The 1 mm thickness of aluminum sheet with 0.8 mm of cover mild steel sheet and the 11 second welding time are the best optimum combinations of the parameters that yield the largest value of tensile strength and hardness of the welded area of the aluminum alloy.
To do this experiment, the above-mentioned three optimized samples are selected and tested; their microstructure examination and hardness tests are done.Following the microstructure test, analysis of the weld cross-section reveals that the microstructure of the base metal, HAZ, and NZ varies from region to region.
The microstructure of AA 2017 in NZ was found to be composed of very fine equated grains (lower size) and uniformly distributed fine precipitates, as shown in Figures 26 -28.
The most important factors in managing heat input for resistance spot welding are welding temperature and welding time.Spot weld settings cause the variation in microstructure.The degree of cooling influences the microstructure of all of these spot welds (Das, Rawal, et al., 2020;Gupta & Amitava, 1998;Li et al., 2010;Saleem et al., 2012;Shelly & Sahota, 2017;Tsai et al., 1992;Zhigang et al., 2007).

Conclusion
This paper presents an experimental and finite element based investigation on resistance spot welding of aluminum alloy 2017 covered by mild steel 4130 sheet.The following conclusions were obtained from the study: • Variations in weld mild steel 4130 thickness, cover sheet thickness, and welding time with constant welding current and electrode force have a strong influence on the weld joint response (welding strength, welding temperature, and weld deformation).
• The percentage impacts of aluminum thickness (AT), cover sheet metal thickness, and welding time are 3.890%, 3.250%, and 84.390% respectively.According to these findings, the importance of process factors in achieving weld joint tensile strength is highly dependent on welding time.
• The percentage impacts of AT, cover sheet metal thickness, and welding time duration on deformation are 1.171%, 8.731%, and 80.881%, respectively.As a result, weld time is the most important factor.
• The percentage impacts of AT, cover sheet metal thickness, and welding time duration on temperature are 9.960%, 87.820%, and 1.660%, respectively.The second most important factor is the thickness of the cover mild steel sheet.
• In NZ, the microstructure of AA 2017 consists of very finely equated grains (smaller size) and uniformly distributed fine precipitates.
• The most essential parameters in optimizing heat input for resistance spot welding are welding temperature and welding time.The variability in microstructure is caused by spot weld settings.
• The experimental test result of the tension test shows the maximum stress which ranges between 40 and 100 MPa, which is strongly consistent with the FEA result, in which the maximum result ranges between 60 and 100 MPa.
• For both experimental and FEA results, the maximum von Mises stress is found on the tip of the weld electrode and around the weld areas.
• Validated results of two selected most significant parameters (welding temperature and welding time) validation on the first three samples demonstrate good agreement between FEA and experimental works, with a percent error of 10.620%, which is good but not strong agreement.This could be due to the material's composition and the combined effect of other weld factors.

Figure 2 .
Figure 2. Prepared sample and hydraulic shearing machine.

Figure
Figure 6.Setting the material and importing new material.

Figure 8 .
Figure 8. Generated mesh and symmetry of the FE model.

Figure
Figure 10.Setting the analysis and solving the mathematical model.

Figure
Figure 26.Cross section of the 0.7 mm-thick aluminum-clad mild sheet: the thickness of the aluminum layer is 0.6 mm and the thickness of the steel layer is 0.8 mm.

Figure
Figure 27.Cross section of the 1 mm-thick aluminum-clad mild sheet: the thickness of the aluminum layer is 0.8 mm and the thickness of the steel layer is 0.8 mm.

Table 3 . Input parameters resistance spot welding parameters Test No. Welding Current (kA) Electrode Force (kN) Welding Time (s) Thickness of Al Alloy (mm) Thickness of cover mild steel (mm)
, for constant welding current, electrode force, and aluminum sheet thickness, weld joint responses in von Mises stress are68.86,73.88, and 76.89 MPa, respectively; electric voltages are 106.72,126.73,  and 141.43 mV, respectively; and weld temperatures are 206.66,241.30, and 306.14 °C,  respectively.