Joining mechanism in pinless FSSW of aluminum-steel with or without Zn and brass interlayers

In this paper, a pinless tungsten tool is used to friction stir spot welding of AA6061 aluminum alloy and DP600 dual-phase steel from the steel side. Three different joint interfaces were formed by inserting Zn foil interlayer, brass coating of steel, and without any interlayer at the AA6061/DP600 lapped interface. Maximum lap-shear tensile load is related to joint without any interlayer at double pass welding condition. The failure load in this condition is 6.2 kN. Three different joint mechanisms recognized and studied: (i) Fe/Al intermetallic layer in joints without an interlayer, (ii) intermetallic + Al/Zn dissolved zone in joints with Zn foil, and (ii) remained brass interlayer + Al/Cu/Zn dissolved zone in joints with brass coated steel. Microstructure features, fracture surface morphology, and phases were characterized with optical microscopy, scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (EDS) analysis system, and X-ray diffraction methods.