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

•Mg–Al–Zn sheets were friction stir spot welded (FSSW) with addition of Zn interlayer.•Complex alloying reactions occurred between Mg sheets and Zn interlayer during FSSW.•Alloying reactions increased the area of bonded zone and eliminated the hook defects.•Tensile–shear load of FSSW joints increased from 2.4 to 4 kN by adding Zn interlayer.

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 α-Mg, (α-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.