Study of top sheet thinning during friction stir lap welding of AZ31 magnesium alloy

Friction stir lap welding was applied to AZ31 magnesium alloy sheets to investigate the effect of tool designs and welding variables on top sheet thinning. Three tools with different shoulder designs were used. Sheet thinning aroused by the hooking or cold lap feature was quantified, and the lap shear fracture load of the joints was evaluated and correlated with the effective top sheet thickness. The tool geometry has a significant effect on the morphology and extent of both hooking and cold lap features, as well as welding variables such as welding pitch and plunge depth. The morphology and extent of these features influenced the effective top sheet thickness, which exhibited a linear relationship with the unit width fracture load via the welded material strength. High fracture loads can be achieved by suppressing the hooking and cold lap features as well as by strengthening the material in the nugget.