Performance evaluation of friction stir welded 7075-T6 aluminium alloy joints under bi-directional loading using the disc pressure test

Aluminium alloy 7075-T6 sheets were butt-welded by friction stir welding (FSW) using a conventional tool. The cross section of the joint was analysed macroscopically, and the results highlight the characteristic zones of FSW joints in aluminium alloys: the nugget zone, the thermomechanically affected zone, and the heat-affected zone (HAZ). Optical microscopy shows grain refinement closer to the centre of the joint, particularly in the nugget zone. In terms of zone distribution, the microhardness profile across the joint follows a typical W-shape, with maximum hardness in the centre of the joint. Tensile testing under digital image correlation showed that the largest strain fields were located in the weld zone, with failure between ZAT and ZATM. Evaluation of the mechanical performance of the FSW joint under bidirectional loading using a disc pressure test (DPT) shows a failure pressure 33% lower than that of the base materials (BM). The fracture form shows a low ductile fracture mode in both the welded joint and the BM, but the BM has greater deformability and ductility than the welded joint.