Enhanced resistance to liquation cracking during fusion welding of cast magnesium alloys: Microstructure tailoring via friction stir processing pre-weld treatment

Abstract

Magnesium alloys provide critical opportunities for energy and lightweight-materials challenges. The alloy design approach based on utilizing the second phase strengthening mechanism plays an essential role in producing low-density high strength Mg-based alloys. However, the second phase in the microstructure (e.g., Mg17Al12 phase in Mg–Al–Zn alloys) can deleteriously affect the weldability of the metallic materials by promoting the liquation and liquation cracking during fusion welding. The lack of sufficient weldability of Mg-based alloys is a crucial barrier to their potential use in safety-critical applications. In this paper, it is shown that the severe plastic deformation induced by friction stir processing, as an effective pathway to microstructure refinement, before fusion welding enables tailoring the initial microstructure of an Mg–Al–Zn cast alloy for enhancing its resistance to liquation cracking. The formation of ultra-fine sub-micron Mg17Al12 particles in friction stir treated base metal translates the liquation mechanism from sub-solvus constitutional liquation to super-solvus melting alloy, reducing the size of susceptible partial liquation zone to cracking. Moreover, the higher grain boundary density induced by dynamic recrystallization phenomena during friction stir processing can enhance the resistance to liquation cracking of the alloy. It is shown that this pre-welding strategy enables a significant enhancement in the weld mechanical properties compared to the conventionally produced welds. The joint efficiency, defined as the ratio of the joint tensile strength to the as-cast material tensile strength, was changed from 0.74 in case of welds made on cast base metal to 1.4 in case of welds made on friction stir processed base metal. This simple modification to the fusion welding process can be used as a practical pathway to enhance the weldability of liquation-sensitive second phase-strengthened alloys.