The dissimilar friction stir lap welding of 1A99 Al to pure Cu using Zn as filler metal with “pinless” tool configuration

•The multi-layer microstructures and diffusion in the interlayer is investigated.•The thermal histories under different parameters are successfully recorded.•The middle-layer evolution of typical parameter is investigated in detail.•The mechanical properties of the pinless FSW joints are tested.

Friction stir welding (FSW) with pinless tool provides many advantages over traditional FSW, the most significant of which are avoiding the wear of the tool and no retained keyholes of fabrication. This research is focused on understanding the interlayer of the dissimilar FSW of 2 mm copper plate and 2 mm aluminum plate using 0.2 mm zinc foil as filler metal with pinless tool. The macro and microstructure, heat history of the interlayer under different parameters and the weld properties have been investigated. The experimental observations showed that the thickness of the interlayer decreased with the increasing of rotating speed and the decreasing of traverse speed, but the rotating speed played a more important role in the process, which could also be proved by the thermo histories tested and recorded by embedding K-type thermocouples. The investigation of typical interlayer has also been carried out in order to study the evolution behavior of the interlayer. The mechanical properties have shown considerable relations with the interlayer, including the microstructure and the extent of diffusion and extrusion.