Parameter optimization and microstructure evaluation of welding of structural steel 1020 and AA6062

AA6062 and ST1020 materials are used to perform friction stir spot welding (FSSW). In this experiment, the mechanical and microstructural characteristics of the welded joint were determined using a square-head tip tool. To enhance the life of the welded joint, three essential process parameters, namely tool speed, dwell time, and plunge depth were employed with four levels. The L16 orthogonal array was adapted for experimental work to develop a process parameter table. A tensile test was performed for a larger better value to predict the strength of the welded joint, and the value is 5.65 kN. The outcome shows that dwell time is a more prominent parameter than tool speed and plunge depth. The optimal combination of welding parameters is a dwell time of 24 s, a tool rotational speed of 1200 rpm, and a plunge depth of 0.8 mm; dwell time contributes 39.66%, followed by a plunge depth 35.7%, and at last, tool speed 6.171%. Furthermore, to determine the optimum level of parameters. The microstructure of this joint was observed with respect to heat dissipation and changes in grain size. Microstructure studies were carried out to see the area under stir zone for workpieces with high and low tensile strength. This work was validated using the response surface method (RSM). The RSM results are very close to the experimental results.


Introduction
Demand to join dissimilar metals with different processes is increasing in today's automotive, aviation, and aerospace sectors. The strength to weight plays a major role in describing an effect join where durability, reliability, and sustainability are much important properties that the join should withstand. Friction stir spot welding plays a major role in satisfying these properties as Resistance spot welding RSW has limitations regarding the electrode wearing out on multiple usages for joining of steels. To overcome this, spot friction can be used where the joining takes place at a constant place with a spot over a period of time using a tool. Many new tools are coming up to join the steel material. Aato et al [1] used a tool without a tip to join steel in a lap position 0.5 mm thickness with a 3.6 mm diameter tool, and the joint showed failure at 1.8 kN by rotating the tool at 18000 rpm. There was no significant plastic flow near the welding surface which indicated It was due to the heat of the friction and Friction force pressure via a mechanism Close to distributed welding. Baek et al [2] used FSSW for the joining of steel and found it to be very effective and obtained a strength for the lap joint near 3.40 kN and stated that the top sheet of the joint recrystallization and grain growth occurred while the bottom sheet only encountered thermal cycling during welding. Sun et al [3] have successfully joined low carbon steel in a lap position with a thickness of 1 mm and found that the penetration should be done perfectly, and it should reach the bottom sheet to a certain depth; then only the join will show challenging results with respect to microstructure. Tutar et al [4] found that plunge depth has a good effect on weld joints due to an increase in plunge depth; the heat-affected zone shows starts to expand. In this work, process parameters should be understood properly to execute the right experimentation. The investigation is carried out by using DOE to perform the joining, and tensile strength is found. Minitab is used for predicting the ranking of the parameters and deciding the parameter that is affecting much in the process. The microstructure is viewed after sectioning the joint and placed in the electronic microscope to obtain the details of the grain structure elongation in different zones. Bakkiyaraj et al [5] and Karthikeyan et al [6] deployed the response surface methodology (RSM) for the validation and optimization of process parameters of FSSW. Other notable works carried out by researchers in the FSSW can be found in [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].

Details of work and tool material
The tool is designed with respect to the size and shape critical for the effective implementation of the operation. Different FSSW work utilizes various device geometries depending on the welded material and the appropriate process performance. At present, tool design is a trend in the field of stir-spot welding. Figure 1 is graphical abstract of the entire process carried out. Workpieces utilized for performing the joint are Aluminum alloy 6062 (AA6062) with structural steel 1020 placed one above the other with aluminum alloy on top having a thickness of 0.5 mm, individual length is 150 mm, and overplay length is 50 mm. AA6062 chemical compositions are Si 0.40 to 0.8%, Fe 0.7%, Cu 0.15 to 0.40%, Mn 0.15%, Mg 0.8 to 1.2%, Cr 0.04 to 0.14%, Zn 0.25%, and Ti 0.15%.
Structural Steel (ST1020) chemical compositions are C 0.17%-0.23%, Fe 99.08%-99.53%, Mn 0.30 to 0.60%, P0.040%, and S0.050%. The compositions of the tool material H13 used are C 0.40%, Mn 0.40%, Si 1.00%, Cr 5.25%, Mo 1.35%, and V 1.00%. The diameter on the shoulder is 15 mm, 5 mm is the height structure of a square pyramid, and the diameter of the shank is 10 mm. Figure 2 illustrates the line diagram of workpieces in lap position and fabricated tool.

Design of experiment (DOE)
Taguchi model is adapted for optimizing the parameters of AA6062 and ST1020. This model saves time and material because of the reduced number of experiments with effective results. An orthogonal array is selected depending on the number of factors and levels of these factors. Four levels are considered for each of the three factors; levels are selected depending on the intensive background work. Depending on the degree of freedom, an orthogonal array is chosen, which may be determined by adding each parameter's specific DOF. This design has three factors and four levels. The total number of experiments will be 16. Here the levels are four so for three parameters total of 15 DOF is obtained [7,8]. Hence the L16 orthogonal array is selected to define the parameters with strength. Lager the better characteristic analysis for S/N ratio is used. Because of the higher value, the strength of the welded joint will be good. At last, ANOVA is used to determine the effect of parameters in the welding process. Table 1 shows the factors selected and the levels of these factors.
For testing the tensile strength computerized UTM machine is utilized, which has an 80-ton capacity, which shows the load varies deformation curves automatically. After the weld is performed electronic microscope 40X is used to see the detailed structure of the welded region. Work material is surface cleaned with etchants, and then the microstructure of the joint is visualized under it.

Experimentation and tensile test
The factors and levels are dropped in Mini tab software which in return gives the table of experimentation to be performed [8]. The obtained Taguchi orthogonal array for L 16 is shown in table 2. Experimentation is carried out on 400 kN (40T) capacity PC-based electronic and servo-controlled UTM machine Model TUF-40 which is make of Hi-tech engineers has been used. Figure 3 depicts the weld joint of  workspiece obtained with overlapping position and figure 4 is the typical tensile fractured speciment after tesing. Tensile strength has been obtained for each experiment by using the above machine.

Results and discussion
To assess the influence of each level of welding parameters, the mean S/N ratios of each level of welding parameters are computed with the use of Mini tab software and are given in table 3 with regard to the experiment number [9]. Table 4 elaborates on the main effect of means in which dwell time is ranked first, followed by the tool rotational speed, and later is the plunge depth. Table 5 shows the S/N ratio to tensile load larger-the-better so the optimal combination of welding parameters is a dwell time of 24 s, a plunge depth of 0.8 mm, and a tool rotational speed of 1200 rpm. But this combination is not available in the Orthogonal table, so to which another   experiment was performed using these levels, and test was performed on UTM, which has given a good tensile strength of 5.65 kN. The same style of pattern is shown in both means and S/N ratio plots with respect to process parameters (figure 5) herein as the dwell time increases, the strength of the joint shows high [10].
ANOVA is implemented to find the relative effect of parameters and table 6 presents their percentage contribution. By calculating the contributions, it is found that Dwell time contributes 39.66%, followed by plunge depth 35.70% and tool speed 6.171% rest of the percentage is error compensation [11,12].

Validation with regression equation
The results obtained by the experimental work are validated with regression. Values obtained by this verification are very close to experimentation values. The prediction of optimal parameters was found to be more similar by this method. The equation for finding the values is given in section 5.2. Table 7 shows the tensile strength obtained with respect to the regression process and the error found to be very minimum. The effects of parameters on for means and S/N ratio are depicted in figure 6. From the experimentation observations, it is clear that parameters rotational speed of the tool (level 1), time for dwell (level 3), and tool plunging depth (level 4) are optimum level parameters. The optimal combination of welding parameters is a dwell time of 24 s, a tool rotational speed of 1200 rpm, and a plunge depth of 0.8 mm. Table 8 and figures 7 and 8 illustrate the comparison of the RE and experimental results obtained for optimal level paramenters.  Figure 6. Plots for means and S/N ratio.

Microstructure evaluation
Due to dwell nature, the lap area has undergone a high heat effect. The area is divided into four zones stir zone (SZ), thermo mechanically affected zone (TMAZ), heat affected zone (HAZ), and base metal zone (BM); the sample piece is used to show the microstructure, which is the one welded with optimum level parameters. It is clearly visible in the structure shown in figures 9 and 10 that the whole material is affected by heat generated during the process. The stir zone represents the area where the two metals have joined, and it has been noted that there is no defect in this area. When in the stirring process, it is observed that tool's shoulder angle is restricting the material to spell out of the zone [13]. The tool is penetrated through the top and has reached bottom plate to form the joint, and no distortion has been noted. Elongation and deformation size of grain takes place at TMAZ with the flow of material. The size of the grain is very clear in TMAZ when compared to HAZ and SZ due to the interaction of the tool Square Pyramid nib at this particular zone which produces high heat. SZ has a good gain structure when compared to TMAZ because of the stirring effect and also the dynamic motion of the metal flow at that place. With the help of the tool square pyramid nib, the grain is equally distributed along the axis with fine grade [13]. It seems that the breaking starts at SZ because the crack propagated in this zone and elongated towards the outer edge. Whereas in HAZ, changes in micro disturbances occurred in structure, but the zone did not deform similarly to the BM [14].
The microstructure results show that the high tensile strength specimen does not undergo any kind of porosity and undisturbed grain structure due to equal temperature distribution during the stirring process. Figure 11 is the microstructure view of high and low tensile strength, sufficient amount of heat is generated at the  interaction of tool and workpiece to form a proper joint thereby, has given high tensile strength when tested on UTM. Whereas the other microstructure which has low tensile strength is because of insufficient heat generated at the interphase, due to which rough patches, long tunnels, and pin holes have been formed there by the strength has been decreased.
SEM was utilized to analyze the fracture surfaces and evaluate the failure mechanisms of tensile fracture specimens of high tensile and low tensile strength. Figure 12 shows the microstructure of the specimen of high tensile strength tested workpiece. Under loading, two sheets are prone to split at the bonding zone i.e. at the stir zone. As a result of this split, a circular crack occurred around the stir zone. But the joint has a bonding of grains between two materials and is tightly compacted, which resulted in high strength. The grains formed are almost of the same size; the stir zone displayed more smooth and acicular grains. On the other hand, figure 13 is the microstructure of the specimen with low tensile strength. The failure of this specimen has occurred due to large grain sizes which overlap on one another and intend to form poor bonding. The grains are distorted over the zone when compared to the high-tensile specimen. The main reason for the low tensile strength exhibition is due to the insufficient plunge depth, as both the test specimen has a common plunge depth value i.e., 0.6 mm.

Conclusions
For AA6062 and ST1020, larger-the-better is taken into consideration for determining strength in the lap joint. The calculated optimal value is compared to the Minitab program's output, and they are found to be identical. The conclusions drawn from this experiment are as below.
1. From the experimentation observations, it is clear that parameters rotational speed of the tool (level 1), time for dwell (level 3), and tool plunging depth (level 4) are optimum level parameters. The optimal combination of welding parameters is a dwell time of 24 s, a tool rotational speed of 1200 rpm, and a plunge depth of 0.8 mm.
2. The optimum level parameter values obtained are taken to perform a joining operation to validate the result with Minitab and found to be effective, and strength is 5.6 kN.
3. Dwell time contributes 39.66%, followed by plunge depth 35.70%, and at last, tool speed 6.171% rest of the percentage is error compensation.
4. To verify the tensile strength, regression analysis is performed, which has proven that obtained values are closer to the experimental values 5.6∼5.7 RE value.  5. Evaluation of microstructure is carried out from which it is clear that all four zones are affected by heat, but only two of them, namely SZ and TMAZ, will undergo deformation with clear grain structure stating that elongation forms at this place. 6. The structure which has given lower tensile strength shows clearly that the grains are harshly distributed over the body, and the structure which has given high tensile strength clearly shows that the grains are bonded very near and arraigned unidirectional.

Data availability statement
All data that support the findings of this study are included within the article (and any supplementary files).