Effect of laser surface melting on microstructure and hardness of Mg–4Y–2Zn–1Zr-0.6Ca alloy

Abstract

To improve the performance of magnesium and enhance its mechanical properties and microstructure, a laboratory-scale Mg alloy with the composition Mg–4Y–2Zn–1Zr-0.6Ca was developed. Subsequently, the alloy was subjected to homogenization and extrusion processes to reduce casting defects and the microstructure refinement. Additionally, surface modification was performed using a pulsed Nd:YAG laser. The findings of the experiment indicate that by controlling the laser processing parameters, it was possible to attain a microstructure devoid of any defects. Moreover, an examination of the microstructure and secondary phases was conducted through the utilization of characterization techniques such as the Optical Microscope (OM), Field Emission Scanning Electron Microscope (FESEM) equipped with Energy Dispersive Spectroscopy (EDS), and X-ray Diffraction (XRD). Furthermore, the micro hardness of the samples was evaluated using the Vickers micro hardness testing method. Consequently, the microstructures of all specimens exhibited equiaxed grains, predominantly composed of (α-Mg), (Mg₂₄Y₅+α-Zr), and the W-phase (Mg₃Y₂Zn₃). In addition, the laser-melted surface revealed a noteworthy refinement of the grain structure, with an evolution in grain morphology from the top to the bottom of the melted area. Equiaxed grains were present in the centerline, whereas columnar grains were observed in the fusion line. Additionally, as the scanning speed increased, a finer microstructure was observed. Due to the rapid heating and cooling involved in the laser surface melting (LSM) process, the secondary phase (Mg₃Y₂Zn₃) dissolved in the microstructure, and the (Mg₂₄Y₅+α-Zr) phase was uniformly distributed throughout the LSMed area. Ultimately, the micro hardness showed a significant increase due to the dual influence of grain refinement and the homogeneous dispersion of the secondary phase.