Change of microstructure and cyclic deformation behavior along the thickness in a friction-stir-welded aluminum alloy

The microstructure and cyclic deformation behavior of three slices of a thick plate of friction-stir-welded Al-Cu alloy were investigated. Friction stir welding resulted in a composite-like nugget zone consisting of fine particles uniformly dispersed in the recrystallized fine-grained matrix. The top slice had larger grains, smaller particles, higher hardness, stronger cyclic hardening, higher hysteresis energy and lower fatigue life than the bottom slice. Fatigue cracks initiated from near-surface particles in the low-hardness zone, and propagated via striation formation mechanisms along with particle cracking.