The aging characteristics of resistance rivet welded aluminum/steel joints

The aging characteristics of Al/UHSS resistance rivet welding (RRW) joints are investigated for the first time in this paper. Microstructure studies have shown that the aging treatment does not change the phase composition of the Al/steel mixed nugget still remaining the ferrite structure, while the precipitation, growth and dissolution occur for the microstructure of Al alloy varying with the distance from the rivet. Mechanical tests revealed that for interfacial fracture (IF) and button pullout fracture (BPF) joints, the maximum peak load increased by 13% and 26% with the prolonged aging time, respectively. The improvement in the joint strength after aging can be attributed to the relaxation of the residual stress for IF joint and the age hardening of Al sheet for BPF joint. In addition, interestingly, the accelerated artificial aging is discovered firstly in Al/steel RRW joint. The phenomenon can be attributed to the higher solid solubility of Mg and Si in softening zone of Al sheet, resulting in faster and more precipitation during the aging treatment. The precipitation kinetics indicated that the average Avrami exponent was 1.25, proving the one-dimensional linear growth of fresh nuclei of the hardening phase β''. The block shear model analysis suggested that the Canadian Standards Association, giving the calculating/test values ratio ranged from 0.88 to 1.08, could provide a more reasonable calculation, which showed that the shear failure was mainly determined by the yield of the gross shear plane, and the tension plane failed because of tensile rupture during the lap-shear test.