Comparative study of microstructure and mechanical properties of laser welded–brazed Mg/steel joints with four different coating surfaces

Abstract

This paper presents a comparative study of laser welding–brazing Mg to steel with four different coating surfaces, including (Zn+pre-existing Fe–Al phase), pure Zn coating, pre-existing Fe–Al phase and fresh steel without any coating. The presence of Zn coating was found to significantly improve the wettability of liquid filler on steel. However, Mg–Zn products enriching at the seam head tended to cause cracking. The weak bonding of Mg–Zn products and Fe–Al layer was mainly responsible for the decreased tensile strength and interfacial failure that occurred in joints with the first two coatings. For joints with the latter two coatings, the thickness of newly formed Fe–Al layer determined the mechanical properties. The reaction layer formed at the Mg/fresh steel was thin, inducing interfacial failure, whereas the joint with pre-existing Fe–Al phase fractured at the seam, indicating that the pre-existing Fe–Al phase was beneficial to formation and growth of the Fe–Al phase.

Keywords:

• AZ31
• Laser welding-brazing
• Microstructure
• Zn coating

One of the authors (C. W. Tan) is grateful for the financial support by the China Scholarship Council for study at the University of Waterloo. Professor Y. Zhou at the University of Waterloo is gratefully acknowledged for his help in discussing and revising this manuscript.