Effect of Stacking Sequence and Process Parameters on the Thermomechanical Response and Joint Characteristics of Multilayer Friction Stir Welds of AA2024/AA6061/AA7075

Friction stir welding (FSW) is an increasingly common solid-state joining process for aluminum alloys, including dissimilar alloy combinations. This study presents a detailed analysis of joining three plates made of different aluminum alloys, AA2024, AA6061, and AA7075, commonly used in the aeronautical industry, with a thickness of 2 mm, stacked on top of each other. The research was carried out in two stages. In the first stage, three different alloy overlap methods were analyzed while keeping the process parameters unchanged. In the second stage, a single overlap configuration was defined, and the two process parameters, welding speed and tool rotational speed, were varied. For the three first-stage and six second-stage experiments, the temperature and axial force developed during the process, surface roughness, macrostructure, microstructure, and microhardness distribution were evaluated. The results showed that both the stacking sequence and the process parameters influenced the thermomechanical response and the resulting joint characteristics. Among the investigated stacking sequences, AA7075–AA2024–AA6061 exhibited the fewest observed defects and was selected for the second experimental stage. For this configuration, 650 rpm and 100 mm/min produced the most favorable macrostructural outcome within the investigated parameter range.