Microstructure evolution mechanism of dual-phase Mg-Li alloy in laser surface melting for hardness and wear resistance enhancement

Abstract

Mg-Li alloy exhibits great application potential in fields with an urgent need for lightweighting, such as electronics, automobiles, and aerospace, owing to its advantage of low density. However, its characteristics of low hardness and poor wear resistance have become key bottlenecks hindering its widespread application. Thus, this study applies Laser Surface Melting (LSM) to enhance the hardness and wear resistance of the dual phase Mg-Li alloy. The influence of LSM parameters on the microstructure evolution, microhardness, and wear resistance of the dual phase Mg-Li alloy and the microstructure evolution mechanism for hardness and wear resistance enhancement were revealed. The experimental results show that the grains in the melting layer are significantly refined compared with substrate layer. With the increase of depth in the melting layer, the grains gradually change from equiaxed grains to columnar grains, and the grains near the surface are refined to nanometer level. As the off-focus amount increases, the scanning speed rises, and the overlap ratio decreases, the laser energy density shows a downward trend. This change makes the melting layer thickness of the sample thinner and the grain size smaller, which in turn promotes the improvement of the hardness and wear resistance of the sample. The maximum hardness of the melting layer is increased by 95 % compared with that of the original sample, the average friction coefficient is reduced from 0.311 to 0.235, the quality loss is a decrease of 53.1 % compared with the original sample, and the width and depth of the wear scars are also significantly reduced. From the observation of microstructure, it is found that besides grain refinement, α-Mg and β-Li phases form supersaturated solid solution, and the hardness of both phases is higher than that of the substrate. Moreover and the hardness of β-phase is higher and its proportion is increased. The hardness and wear resistance of dual-phase Mg-Li alloy are significantly enhanced by the synergistic effect of grain refinement strengthening, solid solution strengthening and increasing the proportion of β-Li phase. This study provides crucial theoretical foundations and technical routes for expanding the application boundaries of Mg - Li alloy.