Interface stability and fracture mechanism of Al/Steel friction stir lap joints by novel designed tool

[Display omitted] •Hook feature was eliminated by enlarged pin, which ensured the interface stability.•The interface composed of laminated IMCs structure was beneficial to the mechanical properties.•Four different interface failure modes were firstly found and established during the shearing process. A novel welding tool, characterized by the enlarged pin design was developed to solve the hook feature or insufficient interface deformation for Al/steel friction stir lap welded joints. The welding tool enhanced the interface deformation effect and eliminated hook feature. The thickness of the intermetallic compound (IMC) layer at the interface decreased from 3.3 μm to 0.46 μm, when the welding speed increased from 30 mm/min to 300 mm/min. The laminated structure composed of IMCs and fine steel grains gradually disappeared, with the interface gradually changing from serrated to straight as welding speed increased. The nanohardness value of the microstructure reached 9.4 ± 0.3 GPa at a distance of 10 μm from the interface layer. Due to the larger metallurgical bonding area, the best line load 499.4 N/mm reached 52 % of the 3 mm 6082-T6 alloy, which was obtained at the rotational speed of 1200 r/min and the welding speed of 50 mm/min. Four different interface failure modes were found and established during the shearing process. The strain concentration phenomenon of the successive occurrence, development and transfer of interface failure presented obvious interval characteristics in time and space.