Synchrotron experiment and simulation studies of magnesium-steel interface manufactured by impact welding

Abstract

The effective weight reduction in the automotive industry by the wide adoption of lightweight magnesium (Mg) alloys demands high-quality joint between magnesium alloys and massively-used steels in order to wring the excess weight with strength and safety assurance. However, Mg-steel joint is difficult to achieve because there is no mutual solubility between magnesium and steel and huge disparity in physical properties. An impact-based welding method recently showed successful Mg-steel joining. In this work, the characteristics of Mg-steel interface joined by the impact welding method were investigated. Synchrotron high-energy X-ray computed tomography and diffraction were applied to characterize the microstructure across Mg-steel interface. Results revealed a deposit layer formed at the joint interface where Fe-rich particles spread deep into the Mg matrix. High-resolution 3D morphology of Mg-steel interface demonstrated the trapped pores and cracks inside the deposit layer. The formation of the deposit layer and the void/cracking evolution were analyzed by using finite element models. These findings provide insights into the immiscible Mg-steel joining process.

Introduction

Weight reduction by using lightweight alloys to mitigate the commonly-used heavy steel parts plays a vital role in the modern automotive industry for the long-standing pursuit of environmentally-friendly vehicles with fuel and cost efficiencies [[1], [2], [3], [4], [5]]. Magnesium (Mg) and aluminum alloys are gradually applied by engineers to wring the excess weight, because these alloys have much higher specific strength as compared to steel that is pervasively adopted in the automotive applications owing to the economic and massive productions. Mg alloys are 33% lighter than aluminum alloys while keeping a similar strength and have greater potential for wide adoption in the vehicle industry. Joining Mg to steel has been increasingly desired for high-strength and low-weight products [[6], [7], [8]]. However, direct joining between Mg and steel is challenging because it is difficult to form a bonded Mg-Steel interface [9,10]. As the Mg and iron (Fe) binary phase diagram shows, Mg and Fe are almost immiscible even up to a high temperature (1900K) and no intermetallic phases form [7]; furthermore, there is a huge difference in melting temperature between Fe (1812 K) and Mg (922 K). Thus, the immiscible nature in liquid state and mismatch of thermal expansion between Fe and Mg make direct Mg-steel joints unfeasible and unsustainable for conventional fusion welding processes.

Interlayer of mutual diffusers between Mg and steel base alloys has been deployed to facilitate the effective Mg-steel joining interface. Thin layer of Zn [11,12], Al [6], Cu [13,14], Ni [15,16], and Sn [17,18] was typically sandwiched between Mg and steel base alloys, melted to liquid regime during the joining process, and had reaction with Mg alloy or solid solution in steel alloy. Intermetallic compound phases, especially from reaction with Mg alloy due to the nature of low melting temperature, were usually found in the interlayer or coating layer and contributed to the formation of a bonding interface. In the ultrasonic spot welding of Mg to Zn-coated steel, a thin layer of Mg–Zn intermetallic compounds was present at the Mg side [12] while a 450-nm-thick layer of Fe₃Al intermetallic compound was found on the steel surface [11]. Intermetallic compound phases of Mg₂X (X = Cu, Ni, Sn) were also reported in Mg-steel welds by using arc spot welding, ultrasonic welding and laser tungsten inert gas welding methods. However, the formation of the intermetallic compound phase in the interfacial reaction process during joining is difficult to control and tune precisely, but significantly affects the maximum joint strength. Because most intermetallic compound phases are brittle and lead to a weak joint [19]. Hence, joining Mg to steel without interlayer or coating needs to be investigated.

A direct joint between Mg alloy AZ31 to uncoated high-strength steel DP590 has been obtained recently by using vaporizing foil actuator welding (VFAW) method [20]. VFAW is an impact-based method for joining dissimilar materials with disparate properties, which utilized the rapidly expanding plasma gas generated from vaporizing an aluminum foil to accelerate the flyer sheet and cause a high-velocity impact towards the target. After the VFAW processing, bonding Mg-steel interface is successfully achieved and no Al- or Zn- based intermetallic layer at the AZ31-bare steel VFAW joint is observed by energy-dispersive X-ray spectroscopy (EDS) measurement. Further volumetric characterization of microstructure across the Mg-steel interface needs to be addressed thoroughly.

Therefore, in this paper, high-resolution synchrotron X-ray computed tomography and high-energy diffraction methods, in combination with finite element (FE) analysis, were applied to analyze the direct bonding formation mechanism between the Mg alloy and bare steel in the VFAW process. Interestingly, a deposit layer at the Mg-steel interface was revealed as active transition layer to join the disparate layers of Mg side and steel side. Pores and cracks are prevailingly found in the deposit layer. And the phase structure of the deposit layer is mixed with Mg and Fe phases. Finite element analysis elucidated that the localized melting of thin Mg layer on the steel side due to severe plastic deformation facilitates the formation of pores and cracks. The formation of Mg-steel joining interface was discussed based on the synchrotron experiment and FE simulation results.