Implementation of lightweight low-ductility materials such as aluminum alloys, magnesium alloys and composite materials has become urgently needed for automotive manufacturers to improve the competitiveness of their products. However, hybrid use of these materials poses big challenges to traditional joining process. Self-piercing riveting (SPR) is currently the most popular technique for joining dissimilar materials and has been widely used in joining all-aluminum and multimaterial vehicle bodies. However, in riveting magnesium alloys, cracks always occur for its low ductility. In this paper, a hybrid joining process named friction self-piercing riveting (F-SPR), which combines mechanical joining mechanism of SPR with solid-state joining mechanism of friction stir spot welding (FSSW) by making rivet rotating at high speed in riveting process, was proposed aiming at joining the low-ductility materials. The effectiveness of the F-SPR process was validated via riveting 1 mm thick AA6061-T6 and 2 mm thick AZ31B. The results showed that the F-SPR process could significantly improve the rivetability of magnesium alloys, and greatly increase the joint strength, comparing with the traditional SPR process.

Magnetic fluid dynamic behaviors during resistance spot welding have important influences on weld nugget formation. In this paper, a coupled multiphysics finite element model, which realizes the coupling of mass conservation, momentum conservation, energy conservation, and the Maxwell system of equations, is proposed to systematically investigate the fluid flow and heat transfer behaviors in a spot weld nugget. It is found that the flow field in the spot weld nugget evolves through five consecutive stages: incubation, fluctuation, rapid growth, rapid attenuation, and termination. Molten metal in the nugget makes regular rotational flow in four symmetrical loops, which changes the temperature gradient in weld nugget during heating and holding, and also yields significant effects on the shape and thickness of weld nugget.