Peculiar roles of nickel diffusion in intermetallic compound formation at the dissimilar metal interface of magnesium to steel spot welds

Abstract

There is an increased need to join magnesium alloys to high-strength steels to create multi-material lightweight body structures for fuel-efficient vehicles. Direct joining of these two metals is difficult, primarily because pure Mg and pure Fe are immiscible. Although a variety of interlayer materials have been used to facilitate the dissimilar joining in the literature, it is not well understood how the interlayer material affects the joint properties. In this study, a newly developed process, ultrasonic interlayered resistance spot welding (Ulti-RSW), was used to join an Mg alloy to a dual phase high-strength steel with two different interlayer materials: austenitic stainless steel and pure nickel. The IMCs formed with the two interlayers were drastically different, and computational thermodynamics and diffusion simulations revealed that such different IMCs resulted from the peculiar difference in nickel dissolution governed by local chemical potential gradients and diffusion kinetics. The joint with austenitic stainless steel interlayer had an under one micron thick Al-Fe IMC layer and a joint efficiency superior than that with the nickel interlayer as well as other joining processes in the literature.