Effect of initial butt surface on tensile properties and fracture behavior of friction stir welded Al–Zn–Mg–Cu alloy

A new experimental procedure—stir-in-plate welding was adopted to eliminate the initial butt surface of two plates to be joined and examine the effect of the initial butt surface on the formation of the zigzag line and the tensile properties of the welds. The comparison between the butt and stir-in-plate welds indicated that under as-welded condition the zigzag line did not show up in the welds, and two welds exhibited similar tensile properties and fracture characteristics. After post-weld T6-treatment, the zigzag line appeared on the butt weld as zigzag micro-crack at the root tip and discontinuously-distributed cavities of 50–200 μm throughout the weld, which were verified to be associated with the oxide particles. This resulted in the reduced tensile strength and significantly deteriorated ductility with the fracture initiating and propagating along the zigzag line. No zigzag line was discernible on the T6-treated stir-in-plate weld.