Microstructure evolution and joint homogenization mechanism of SiCₚ/2A14 aluminum matrix composites under uniform thermo-mechanical action during stationary shoulder friction stir welding

Particle-reinforced aluminum matrix composites (AMCs) suffer from poor weldability. Although conventional friction stir welding (FSW) can effectively achieve the welding of such materials, it presents issues such as uneven heat input and agglomeration of reinforcing particles, which severely deteriorates the uniformity of welded joints. To address this critical issue, stationary shoulder friction stir welding (SSFSW) was adopted to join 15%SiC/2A14 AMCs, with a particular focus on the weld formation, microstructure, and mechanical properties. Results demonstrate that SSFSW achieves uniform heat input along the plate thickness, which significantly improves the homogeneity of SiC particle distribution in the nugget zone (NZ) with a coefficient of homogeneity dispersion (i.e., Chd) of 7.90%, effectively avoiding the upper-layer segregation of SiC particles observed in conventional FSW. Under the uniform thermo-mechanical coupling effect of SSFSW, the evolution of the microstructure of the joint is mainly dominated by continuous dynamic recrystallization (CDRX), supplemented by particle-stimulated nucleation (PSN). The uniform dispersion of SiC particles and the refined grains result in excellent microstructure uniformity in the NZ. The maximum single tensile strength of the joint reaches 89% of the tensile strength of the base metal. This study verifies the superiority of SSFSW in improving the joint homogeneity of AMCs, provides new ideas for achieving uniform particle distribution in particle-reinforced AMCs, and provides theoretical and experimental support for the application of SSFSW in high-uniformity, high-performance AMCs welding.