Study on the mechanism of 6061-T6 Al and ZK61M Mg on FSLW joints: influence of overlapping order

Friction stir lap welding (FSLW) of Mg and Al sheets has great application potential in the industrial field. Overlapping order is an important factor affecting the quality of dissimilar joints. In this paper, two overlapping orders (Al/Mg: Al as top sheet or Mg/Al: Mg as top sheet) are studied to carry out FSLW experiments of dissimilar 6061-T6 Al and ZK61M Mg alloy sheets. Optical microscope, scanning electron microscope and energy dispersive spectrometer are carried out to explore the interface characteristics and microstructure evolution mechanism of the hybrid joint. The results indicate that the tensile shear strength of Mg/Al order is higher than that of Al/Mg order. In the Al/Mg order, Mg alloy with less plasticization hinders the mutual diffusion of the two alloys, leading to the deposition of substances at the lap interface. Meanwhile, it promotes the incomplete fusion of Mg and Al alloys and produces uneven and thick intermetallic compounds (IMCs) layer. In the Mg/Al order, more Mg alloys are plasticized and fully blended with Al alloy to form a ring structure, exhibiting better fluidity. At the same time, more Zn in Mg alloy enters the nugget zone to form solid solution with Al, which reduces the formation of IMCs. More uniform and thinner IMCs are formed, which is beneficial to micro-mechanical interlocking. Finally, the Mg/Al joint is achieved with a maximum tensile shear strength of 3987.5 N, exceeding the strength of the Al/Mg joint by 153% at the same parameter.