Advancement in fusion welded aluminium alloy joints by friction stir processing technique: a critical review

Fusion-based welding techniques for joining aluminium or its alloys are widely used in industries. However, their effectiveness is restricted due to the high thermal conductivity, low melting temperature, and strong affinity for oxygen of these materials. These inherent characteristics lead to solidification cracking, porosity, oxide entrapment, elemental segregation, excessive heat-affected zone softening, precipitate coarsening, and high residual tensile stresses, thereby degrading joint integrity and mechanical performance, particularly in heat-treatable alloys. To mitigate these issues, welding parameters optimization and post-weld treatment are employed. In the context of post weld treatment, heat treatment and thermomechanical processing have been in the focus, in which friction stir processing (FSP) has drawn an immense attention of researchers in recent years because of its solid-state and thermo-mechanical characteristics. FSP facilitates severe plastic deformation, dynamic recrystallization, grain refinement, homogeneous microstructure evolution and homogeneous phase distribution in the materials. Hence, when applied to fusion-welded aluminium joints, FSP effectively mitigates solidification-related defects since there is no melting, refines coarse dendritic microstructures due to severe plastic deformation into fine equiaxed grains, redistributes strengthening precipitates, reduces residual stresses, and narrows the effective heat-affected zone. This review paper deals with evaluating the role of FSP as a post-weld treatment technique, emphasizing microstructural evolution and its influence on mechanical and corrosion performance of fusion welds of aluminium alloy.