Effects of hydrogen and microstructure on tensile property and failure of base metal and friction stir processing region in aluminum alloy friction stir welding joint

In this study, the tensile properties and hydrogen embrittlement failure of the base metal and friction stir processing region in 2219 aluminum alloy friction stir welding joints used for rocket fuel tanks were investigated, with a particular focus on hydrogen and microstructural effects. A series of tensile tests were conducted on the base metal and friction stir processing region, with and without hydrogen charging, and the associated deformation and failure processes were analyzed. Hydrogen reduced the yield and ultimate tensile strengths of both metals via a hydrogen-enhanced local plasticity mechanism, with more pronounced effects in the base metal. The high density of θ' precipitates in the virgin base metal served as preferential sites for hydrogen atom segregation and dislocation pile-ups, which synergistically promoted crack initiation and caused a severe reduction in strength. Conversely, only a few θ precipitates were present in the friction stir processing region prior to deformation, whereas a high density of θ′ precipitates formed during tensile loading. This can be attributed to large-scale dynamic aging in the α-Al matrix of friction stir processing region, which mitigates hydrogen accumulation and dislocation pile-ups around precipitates, thereby suppressing strength degradation owing to hydrogen. In addition, dynamic θ′ precipitation was responsible for the observed plastic instability during deformation, as well as the serrated flow in the stress–strain curves of the friction stir processing region.