Microstructure Evolution-Induced Mechanical Response in Welded Joints of 7075-T6 Aluminium Alloy Thin Sheets Subjected to Different Friction Stir Paths

As a solid-state joining technology, friction stir welding (FSW) exhibits significant advantages for aluminium alloys, including low heat input and minimal intermetallic compounds formation, thereby enhancing joint quality and mitigating deformation. This study investigates the single-sided and double-sided FSW processes of 3-mm-thick 7075-T6 aluminium alloy sheets, focusing on characterizing the microstructure and mechanical properties of the joints. Experimental results show that under 1500 rpm rotation speed and 80 mm/min welding speed, the double-sided co-directional FSW joint achieves a tensile strength of 388 MPa and an elongation of 7.09%, significantly outperforming the other two welding paths. In the weld nugget zone (WNZ), continuous dynamic recrystallization (CDRX) occurs, generating uniformly refined equiaxed grains (average size: 1.10 μm) and facilitating the transformation of low-angle grain boundaries (LAGBs) to high-angle grain boundaries (HAGBs). Meanwhile the strong Rotated Cube texture is remarkably weakened into random recrystallized Brass textures with the lowest kernel average misorientation (KAM) value in the WNZ. In contrast, the thermo-mechanically affected zone (TMAZ) accumulates high-density LAGBs due to welding-induced plastic deformation. Microhardness testing reveals a typical "W"-shaped distribution: WNZ hardness is relatively high but slightly lower than that of the base metal (BM), and the minimum hardness in the advancing side (AS) heat-affected zone (HAZ) is higher than that on the retreating side (RS). This study confirms that double-sided co-directional FSW crucially regulates microstructural evolution and improves mechanical properties of 7075-T6 joints, providing a viable process optimization strategy for high-quality welding of thin-gauge sheets.