Effect of post–weld heat treatment on multi-scale microstructures and mechanical properties of friction stir welded T-joints of Al–Mg–Si alloys

Defect-free T-joints of Al–Mg–Si plates were successively welded using a friction stir welding (FSW) process. Multi-scale microstructure analysis was performed to investigate the metallography of FSW T-joints and comprehend the clustering behavior of base metals (BM) and stir zones (SZ). The mechanical properties of T-joints were evaluated during post–weld heat treatment (PWHT) using tensile tests. BM with a 47.3 μm average grain size was transformed into approximately 2.6 μm fine equiaxed grains in the SZ. Simultaneously, dissolution of clusters and/or precipitates in the SZ was related to the decrement of the mechanical properties of the joints. Softening was confirmed in the SZ due to the peak-heat by friction stirring of the FSW tool. Thinning of the plate on the advancing side (AS), which was affected by the shoulder zone of the FSW tool, acts as a weak spot for crack initiation. There was a cup and cone fracture on heat affected zones (HAZ) of the plate in the AS. Nanocluster formation was quantitatively analyzed during PWHT using a three-dimensional atom probe (3DAP). Small nanoclusters with approximately 0.6 nm were dominantly formed in the SZ than the BM. Nanoclusters with higher Mg/(Mg + Si) were predominantly generated in the BM. The number densities of the clusters in the BM and SZ were 15.10 × 10²³ m⁻³ and 38.60 × 10²³ m⁻³, respectively. This study confirmed that FSW facilitates better the formation of refined nanoclusters and precipitates in the SZ with smaller size and higher number density compared to BM.