Microstructural and mechanical properties of multi-pass rapid cooling friction-stir welded A1050 aluminum alloy

Conventional multi-pass friction stir welding (FSW) often causes thermal softening in pure aluminum joints. This leads to grain coarsening and a marked reduction in dislocation density, limiting the simultaneous achievement of grain refinement and work hardening. To address this issue, a multi-pass rapid cooling friction-stir welding (MPRC-FSW) process was developed using liquid CO 2 as a cooling medium. The objective was to achieve cumulative grain refinement while preserving deformation-induced substructures. 2-mm-thick A1050 pure aluminum plates were welded with up to five overlapping passes under same welding conditions. This approach progressively refines the grain structure while retaining a high dislocation density. After five passes, the average grain size in the stir zone (SZ) decreases from 2.7 μm (1 pass) to 1.3 μm. The kernel average misorientation value, which reflects dislocation density, increases from 0.07° to 0.31°. Microstructural evolution follows a two-stage mechanism. The initial stage is dominated by continuous dynamic recrystallization which refines grains and increases the fraction of high-angle grain boundaries. In the subsequent stage, further passes introduce new dislocation substructures within the ultrafine grains. The yield strength of the SZ reaches 88 MPa, comparable to that of the base material (91 MPa), while elongation exceeds that of the base material, demonstrating improved strength-ductility synergy. This hybrid process effectively overcomes thermal softening in conventional multi-pass FSW by enabling cumulative grain refinement and sustained dislocation strengthening.