Influence of Zn interlayer addition on microstructure and mechanical properties of friction stir welded AZ31 Mg alloy

2.4-mm-thick AZ31 Mg alloy sheets were friction stir spot welded without and with the addition of Zn interlayers ranging from 0.04 to 0.16 mm in thickness. For the joints without Zn interlayers, although the loads of the joints could be increased by changing the end surface geometry and size of the shoulders, the small bonded area and hook defects limited further increase of joint loads. For the joints with Zn interlayers, the Zn interlayer reacted with the Mg substrate, forming a Mg-Zn brazed zone composed of complex Mg-Zn intermetallics and a thin strip of (alpha-Mg + MgZn) eutectoid structure, thereby increasing the bonded area and reducing the hook defects of joints at the same time. As a result, the maximum joint load increased from 2.7 to 5.2 kN using a 10-mm-diameter concave shoulder with a 0.12-mm-thick Zn interlayer. A thicker interlayer resulted in a significant increase in the thickness of the thin strip in the hook region, and a thinner interlayer led to the formation of more defects due to intense diffusion reactions, thereby reducing the joint loads.