Identification of intermetallics in the laser-welded joint of rare-earth magnesium alloy and the corresponding strengthening mechanisms

Abstract

Morphology and distribution of intermetallic compounds in the heat-affected zone (HAZ) are easily altered during welding thermal cycles, contributing to the softening of the HAZ. Finding solutions to mitigate the softening and enhance the strength of the joint is currently at the forefront of Mg alloy processing and manufacturing. In this work, 0.7 mm thick Ce-containing Mg alloy sheets were successfully joined using fiber laser welding. The microstructure of the joint was observed employing scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Accordingly, the phase compositions in both HAZ and fusion zone (FZ) were determined. Subsequently, the mechanical properties of the joint were evaluated through tensile-shear and hardness tests. The results revealed that the HAZ experienced softening, retaining 76.6% of the strength of the base metal in the tensile-shear test. Interestingly, intermetallics composed of conventional alloying elements within the HAZ transformed into coral-like structures during the welding thermal cycles, whereas the morphology and distribution of intermetallics containing rare-earth (RE) elements barely changed. As a result, the joint exhibited cracks along the boundaries of the coral-like precipitates during tensile loading. In comparison, both intermetallics consisting of conventional elements and those containing RE elements were fragmented into micron-sized fine particles in the FZ, leading to an increase in the strength of the FZ. Ultimately, the mechanisms responsible for the softening of the HAZ and the strengthening of the FZ were elucidated, presenting a promising welding solution for Mg alloys.