Microstructural stability and mechanical performance of friction stir and electron beam welded Al-Mg-Si joints subjected to post-welding heat treatment

This study reveals how fundamentally different weld thermal histories influence precipitate evolution, mechanical properties, and strain localization during post-welding heat treatment (PWHT). For this purpose, two welding techniques, solid-state friction stir welding (FSW) and the fusion joining technique, electron beam welding (EBW) were employed on a precipitate-hardened Al-Mg-Si alloy. Particular attention is given to the distinct responses of solid-state and fusion welding processes to subsequent thermal treatments and their implications for joint performance. Both welding techniques generated tensile residual stresses exceeding 130 MPa in the as-welded condition. However, the weld microstructures exhibited markedly different thermal stability during PWHT. In EBW joints, the fusion zone retained its coarse dendritic morphology (average size ∼34 μm), while the base material consisted of grains with an average size of ∼15 μm. In contrast, FSW produced a dynamically recrystallized stir zone with a fine-grained structure (∼2 μm), which remained stable during aging but experienced pronounced grain coarsening to approximately 300 μm after solution treatment and aging. This abnormal grain growth altered the fracture behavior, promoting grain-boundary fracture, whereas all other conditions exhibited ductile failure characterized by a dimpled morphology. The microstructural differences were directly reflected in the mechanical response of the joints. Following full PWHT, EBW specimens achieved the highest tensile performance, reaching a joint efficiency of 108% relative to the T6 base material.