**1. Introduction**

Generally, the AA7075-T651 and AA2014-T6 are aerospace alloys, which are relatively high in strength compared to AA6082-T6 structural grade alloy. These heat-treatable grades AA2xxx, 6xxx, and 7xxx alloys have more difficulties with fusion welding. Mainly Kah et al. [1] stated that the low melting eutectic at the grain boundary of the heat-affected zone for precipitation-hardenable AA7075 or AA2014 alloys continued to crack during welding. Under extreme conditions, the cracks might be formed along with the fusion zone interface also. So, it is identified that the lower strength in HAZ leads to hot cracking or liquation crack due to the redistribution of intermetallic in fusion welding.

Thomas [2] showed that a sudden decrease in hardness in the HAZ when compared with the fusion zone and the unaffected base metal zone during welding of Al6061-T6 alloy. This HAZ degradation is due to the transformation of the intermetallic precipitates to the non-strengthening or coarsened precipitates at the temperature range between 290 and 400°C, while the whole weld was experienced the same temperature range during welding. This increases when heat input increases.

Beiranvand et al. [3] experimented that the percentage of the added alloying elements in the alloy was responsible for the solidification crack sensitivity. The AA6082 alloy is crack sensitive because the alloys contain approximately 1.0% magnesium silicide (Mg2Si), which gives greater chances of solidification crack sensitivity. The Mg2Si content invariably produces cracking during welding.

Ogbonna et al. [4] highlighted that the key cause of porosity in aluminum is hydrogen, which has a very high solubility in molten aluminum but very low solubility in solid. However, in MIG welding, minimizing the risk of porosity was accomplished by changing the cooling rate to allow the gas to escape, raising the welding current, and/or lowering the travel speed to increase the heat input. Thus, the fusion welding shows as liquation cracking, porosity, solidification cracks, poor dead profile, etc. As a result, friction stir welding is generally used to combine the alloys mentioned above.

This friction stir welding utilizes a tool shoulder-pin primarily to join the metals, which requires heat and pressure, similar to other solid-state welding processes [5]. It is therefore advantageous to combine similar/dissimilar materials without the addition of an auxiliary arrangement using a non-consumable stirring tool. This could be useful for joining many structural metals and their alloys such as aluminum, copper, lead, and steel plates [6–10]. The process is ecologically beneficial and nature-friendly in light of the fact that no toxic gases are produced during this process.

Mishra and Ma [5] investigated the FSW of AA7075 with process parameters of 350 rpm, 152 mm/min for 6.32 mm thick plate for cylindrical tool pin. Kumar et al. [11] experimented on the combination of dissimilar friction stir welding of AA7075-T651 and AA6061-T651 alloys to resolve fusion welding difficulties for similar alloy applications. Trials were carries out on the parameters such as the rotational speed of 800–1000 rpm, welding speed of 90–110 mm/min with various tool profiled tapered cylindrical threaded, simply square and taper square threaded tools. On considering this, here the comparative study on the dissimilar FSW on 6 mm AA2014-T6, AA6082-T6, and AA7075-T651 were studied using constant optimal parameters such as tilt angle (2°), tool rotation speed (900 rpm), and transverse feed of (80 mm/min) with the conical tapered pin tool.

The microstructural grain size and grain orientation relationships that control the strength properties and performance of the crystalline material could be characterized and analyzed by EBSD nowadays. These diffraction patterns helped to understand the plastic deformation across the weld interface [12]. The geometry-based rotations in the friction stir weld for the local shear during pin rotation changed the textures. So, it caused the low and/or high boundaries between the grains as (LAB)/ (HAB) in the weld center [13, 14]. The dynamic variations across grain size can be clearly explained using electron back scattered diffraction (EBSD) analysis [15, 16].

The main objective of this study is a comparative study of various dissimilar AA6082/ AA7075, AA6082/AA2014, and AA2014/AA7075 welds, which has been detailed here.
