Influencing mechanism of Zn interlayer addition on hook defects of friction stir spot welded Mg-Al-Zn alloy joints

2.4 mm thick Mg-Al-Zn alloy sheets were friction stir spot welded (FSSW) without and with the addition of 0.1 mm thick Zn interlayer. The influence of interlayer addition on the microstructural features and mechanical properties of FSSW joints was investigated by optical microscope, scanning electron microscope, transmission electron microscope, X-ray diffraction and tensile testing. The results show that the addition of Zn interlayer resulted in complex alloying reactions between Mg substrate and Zn interlayer, forming a bonded zone composed of alpha-Mg, (alpha-Mg + MgZn) eutectoid structure and a mixture of Mg4Zn7 and unreacted Zn, thereby increasing the area of bonded zone and reducing the hook defects. This results in a significant increase in tensile-shear load from 2.4 kN to about 4 kN. (C) 2015 Elsevier Ltd. All rights reserved.