A comprehensive review of microstructure and mechanical properties of friction stir processed magnesium alloys and composites

Friction stir processing (FSP) induces severe plastic deformation, generating intense strains and localized heating, which modifies the surface and enables the fabrication of magnesium (Mg)-based composites. This technique refines the microstructure of Mg alloys, enhancing mechanical properties—particularly ductility, a key limitation of these HCP alloys. This review addresses the underlying microstructural evolution during FSP of Mg alloys and Mg-matrix composites, including (i) grain refinement via continuous and discontinuous dynamic recrystallization (CDRX and DDRX), (ii) fragmentation and redistribution of secondary phases and intermetallics, (iii) transformation of low- and high-angle grain boundaries, and (iv) additional microstructural changes induced by external reinforcements. This review provides a comprehensive analysis of the strengthening mechanisms and their impact on the mechanical properties of FSP Mg alloys and Mg-matrix composites (MMCs). The paper examines the correlation between FSP processing parameters, microstructural evolution, and resulting mechanical properties. It critically highlights how the type of reinforcement and the dynamic recrystallization induced by friction stir processing influence grain boundary character and, consequently, the material’s strengthening response. It includes a comparative evaluation of yield stress, ultimate tensile strength, microhardness, elongation, and fractography for various FSP-treated Mg alloys and MMCs, including AZxx, WExx, ZExx, ZKxx, AMxx, AExx, and Mg-rare earth alloys. Additionally, the novelty of this review lies in its emphasis on connecting microstructural transformations to mechanical performance trends across different alloy systems and processing strategies, an aspect that has been underexplored in previous reviews. Recent advancements in FSP techniques and their implications for improving the performance of Mg-based materials are also discussed.