**1. Introduction**

Friction stir welding (FSW) is classified as one of the welding techniques that joins materials through heat generated during friction occurring between the tool shoulder and the workpieces [1]. The invention of this welding technique was based on the materials which were classified as un-weldable materials through conventional methods. Those classified materials included certain classes of aluminium alloys. Aluminium alloys are mostly used in many industries like aviation, shipbuilding, and automotive because of their light weight. The focus towards FSW has expanded such that it includes testing the capability of the technique in welding other materials that are outside the aluminium class. This includes the welding of copper and its alloys, titanium and magnesium and its alloys [2]. When the aluminium alloys are welded with the conventional technique, they are likely to have weld splitting on the joint line. The fusion welding of aluminium alloys is more difficult than the welding of steel due to their low melting point, softness, and so forth. The aluminium alloys sometimes bend and shrink when welded using conventional welding and those effects are caused by residual thermal stress [3].

In some materials, FSW possesses good metallurgical properties when compared to fusion welding, and this is caused by the microstructural modification that occurs during welding. Mishra and Ma [4] reported that to have a great weld and to avoid defects on the weld, it is important to take welding parameters, material flow, and heat generation into consideration during the welding process.

There are mainly four crucial steps that are involved during the performance of FSW, i.e., plunging, dwelling, welding, and pulling. The rotating tool is inserted into the butt joint slowly until the shoulder

touches the surface of the workpieces (plunging). The plunged tool remains in one location for a certain period with the purpose of building up input heat (dwelling). The plunged tool moves along the plates being joined at a specified speed (welding). The rotating tool gets removed vertically from the welded plates soon after reaching the ends of the plates (pulling), and this normally leaves a hole that indicates the end of the weld [5].

There are several studies focusing on FSW of aluminium alloys [6]. Typical examples include the production of Al/NiTi composites by FSW assisted by electrical current, analysis of welding properties in FSW of AA6351 plates added with silicon carbide particles, and dynamics of rotational flow in FSW of aluminium alloys [7–9]. It has been reported that for the production of the good weld, the tool geometry and welding parameters play a very important role. This includes the rotation speed, traverse speed, tool tilt angle, and plunged depth. Plunge depth has been found to be a critical parameter in the heat generation and for proper consolidation of material without defects. The plunge depth was also identified as one of the parameters that plays an important role towards the microstructural arrangement of the joint [10].

Welding dissimilar materials is quite challenging when compared with similar welding materials due to the difference in mechanical properties and chemical composition of the base materials. To acquire better weld mechanical properties, the harder material must be placed on the retreating and softer material placed in the advancing side [11,12]. The tool geometry plays a very important role in welding dissimilar materials. Welding dissimilar alloys requires the use of different tool profiles such as threaded, squared, and triangular profiles to transfer the material from top of the joint to bottom and vice versa by stirring movement [13]. Kundu and Singh [14] reported that tool pin profile geometry plays an important role in weld quality, while the surface quality of the weld joint depends upon the tool tilt angle. The increase in tool tilt angle affects the flow and fill up of material during welding.

In most cases, the welding of dissimilar materials involves the welding of aluminium alloys, which are not far from each other in terms of mechanical properties, e.g., 5xxx will be welded together with 6xxx, 6xxx welded with 7xxx, etc. Recently, there are attempts that have been made in trying to weld the aluminium alloys that are mechanically far apart from each other, i.e., FSW of AA2024 to AA6061 [15]. This investigation used a single- and a dual-pin tool. The defect-free joint was obtained in all selected parameters except the welding speed beyond 90 mm/min. The onion rings were visible on the nugget region for joints produced using the dual-pin tool but absent on the single-pin tool. The highest ultimate tensile strength (UTS) was obtained with the dual-pin tool at a welding speed of 150 mm/min, whereas the single-pin produced the UTS at a welding speed of 90 mm/min. However, the UTS provided by single-pin was always less than the one produced by the dual-pin tool.

There are various aspects that have been studied through the use of dissimilar materials. This involves the analysis of the strain hardening behaviour on the friction stir welded dissimilar alloys, which are mechanically far apart from each other, i.e., 2024-T351 and 5083-H112, 2024-T351 and 7075-T651 [16]. This analysis was performed on two types of joints, i.e., 2024-T351 and 5083-H112, with 2024-T351 on the advancing side and 5083-H112 on the retreating side. The second joint was 2024-T351 and 7075-T651, with 2024-T351 on the retreating side and 7075-T651 on the advancing side. It was discovered that the strain-hardening rate of the AA7075/AA2024 joint was higher than that of the parent material, while the strain-hardening rate of the AA2024/AA5083 joint lay between those of the parent material. It was also found that the tensile properties of both joints were lower than those of the parent material. Xia-Wei et al. [17] did the microstructural analysis correlatively with mechanical properties of the FSW joint using dissimilar alloys. The lamellar structure in the bottom of the nugget zone was found to be more homogeneous and finer than other regions. The hardness on the copper side of the nugget was higher than that on the aluminium side. The UTS of the joint was found to be relatively lower than that of the base metal. The tensile morphology revealed ductile-brittle fracture mode.

Kumbhar and Bhanumurthy [18] did a comparative study on friction stir welding of similar to dissimilar aluminium alloys, i.e., AA5052 to AA6061, and AA6061 to AA6061. The similar and dissimilar joints were produced at various combinations of tool rotation speeds and tool traverse speeds. The microstructural analysis revealed that there was no rigorous mixing in the nugget region for both materials. The tensile properties of dissimilar materials (AA5052-AA6061) were much better compared to the properties of similar materials (AA6061-AA6061). Welding dissimilar aluminium alloys that are mechanically far apart has gained much attention and interest from researchers [19]. This includes the analysis of friction stir welding of dissimilar AA2017A-T451 and AA7075-T651 plates at a different tool rotation speed. The results revealed that the best tensile properties were achieved when AA2017A-T451 was on the retreating side. It was also established that the material that is located on the retreating side dominates the weld centre, and this is consistent with the results reported by other researchers [11,12,16]. Ranjith and Kumar [20] analysed the impact of joining two dissimilar aluminium alloys AA2014 T651 and AA6063 T651 by friction stir welding. They discovered that the tensile strength was better when the tool was offset towards AA2014 (advancing side). When it was offset towards AA6063 (retreating side), it resulted in insufficient heat generation on the advancing side, which then resulted in an incomplete fusion of AA2014. Sarsılmaz and Caydas [21] conducted a study on statistical analysis on mechanical properties of friction-stir-welded AA1050-H14/AA5083-H321 couples. The study investigated the effect of friction stir welding parameters focusing on rotational speed, traverse speed, and stirrer geometry. In their investigation, they discovered that traverse speed has a significant effect on UTS and nugget hardness. Their analysis also included the optimized welding parameters to be used in welding the said aluminium alloys.

The analysis of the effect of material positioning during FSW has gone outside the aluminium family. This includes the study which analyzed the effect of location variation in FSW of steel with different carbon content. It was discovered that the placement of the stronger material on the advancing side reduced the weld nugget size and increased the amount of martensite formation. The location of the strongest material on the advancing side led to higher temperature and stress due to the highest temperature on the advancing side [22]. It is evident from the literature that the FSW that involved materials that are mechanically apart involved mainly 2xxx as the weaker material. There are very few studies which utilized AA1xxx [21].

This paper reports on the mechanical properties of the weld produced by the friction stir welding technique using AA1050-H14 and AA5083-H111. The AA1050-H14 is mostly used in the chemical industry, automotive industry, reflectors, heat exchangers, and food industry [23]. The AA5083-H111 is widely used in the prevention of corrosion, hence used in shipbuilding. This alloy is also used in the automotive industry as well [24]. It was then crucial to analyze different aspects related to the joint formed from these two distinct materials. This was done so as to prepare the future application of these two materials in producing products and components. It then became crucial to analyze comparatively the mechanical behaviour of the joint in different locations. This type of analysis will give information regarding the best location for sampling the welds produced from the materials with unique properties and composition.
