In this paper, the research on control of intermetallic compound during aluminum/steel friction stir lap welding is reviewed. Several means that improve the properties and quality of the joints are presented by controlling the IMCs layer. The thickness of IMCs layer is controlled by the processing parameters, tool structure and aided processing, and the types of the IMCs are controlled by the processing parameters and the addition of the interlayer design. Advantages and disadvantages of the above methods are discussed. A new research direction with a large-lift angle structure of the tool is also proposed for the development of the aluminum/steel welding technology in this paper.

The friction stir lap welding (FSLW) of metal to polymer is a challenging work due to the unavoidable polymer overflowing. Facing this problem, a novel seal-flow multi-vortex friction stir lap welding (SM-FSLW) technology based on the subversively-designed multi-step pin was put forward. Choosing 7075 aluminum alloy and short glass fiber-reinforced polyether ether ketone (PEEK) as research subjects, the welding temperature, material flow, formation and tensile shear strength of dissimilar materials lap joint under the SM-FSLW were studied and compared with those under traditional FSLW based on the conical pin. The multi-step pin rather than the conical pin effectively hindered the polymer overflowing due to the formation of vortexes by the step, thereby attaining a joint with a smooth surface. Compared with traditional FSLW, the SM-FSLW obtained the higher welding temperature, the more violent material flow and the larger area with high flow velocity, thereby producing the macro-mechanical and micro-mechanical interlockings and then heightening the joint loading capacity. The tensile shear strength of lap joint under SM-FSLW was 27.8% higher than that under traditional FSLW. The SM-FSLW technology using the multi-step pin provides an effective way on obtaining a heterogeneous lap joint of metal to polymer with the excellent formation and high strength.

A novel friction stir double-riveting welding (FSDRW) technology was proposed in order to realize the high-quality joining of upper aluminum (Al) and lower copper (Cu) plates, and this technology employed a Cu column as a rivet and a specially designed welding tool with a large concave-angle shoulder. The formations, interfacial characteristics, mechanical properties and fracture features of Al/Cu FSDRW joints under different rotational velocities and dwell times were investigated. The results showed that the well-formed FSDRW joint was successfully obtained. The cylindrical Cu column was transformed into a double riveting heads structure with a Cu anchor at the top and an Al anchor at the bottom, thereby providing an excellent mechanical interlocking. The defect-free Cu/Cu interface was formed at the lap interface due to the sufficient metallurgical bonding between the Cu column and the Cu plate, thereby effectively inhibiting the propagation of crack from the intermetallic compound layer at the lap interface between the Al and Cu plates. The tensile shear load of joint was increased first and then decreased when the rotational velocity and dwell time of welding tool increased, and the maximum value was 5.52 kN. The FSDRW joint presented a mixed mode of ductile and brittle fractures.

In order to avoid the depth increasing of repaired hole and eliminate the super-fine grain band in stir zone by radial-additive friction stir repairing (R-AFSR), a solid-state repairing technique of active-passive radial-additive friction stir repairing (AP-RAFSR) assisted by the truncated cone-shaped filling material was proposed in this study. The mechanical hole out of dimension tolerance of AZ31 magnesium alloy was chosen as the repaired object. The results indicated that the AP-RAFSR process rather than the R-AFSR process avoided the kissing bond in the bottom of the repairing interface under the condition of the tool pin length equal to the height of the standard mechanical hole. The continuously-distributed and large-length super-fine grain bands were eliminated in the stir zone by AP-RAFSR. The maximum tensile and compressive-shear strengths of repaired hole by AP-RAFSR reached 190.6 MPa and 138.9 MPa at 1200 rpm respectively, which were equivalent to 97.7% and 89.6% of those of the standard mechanical hole. This AP-RAFSR process assisted by the truncated cone-shaped filling material provides a new technique to obtain a no-depth-increasing, defect-free and high-strength repaired mechanical hole.