Laser Re-melting of Mg–Zn–Zr Alloys: Microstructure Evolution and Solidification Defects

Laser re-melting has demonstrated significant potential for optimizing the microstructure and enhancing the mechanical properties of magnesium alloys, while also providing solidification conditions analogous to those in laser metal deposition. This enables a scientifically sound evaluation of grain morphology evolution from traditional casting to additive manufacturing of magnesium alloys. However, its practical implementation is often limited by the formation of solidification defects, including porosity and hot cracking. This study systematically investigates the microstructural evolution and mechanisms of defect formation during laser re-melting using ultrafast in-situ X-ray imaging and analytical electron microscopy. The results show that intense melt flow during laser re-melting facilitates the movement of bubbles and unmelted particles, which are entrapped at the solid–liquid interface. These bubbles and particles ultimately evolve into porosities and coarse particles, respectively, within the fusion zone (FZ) and near the fusion line. Additionally, fluid-driven heat transfer promotes extensive static recrystallization and precipitation in the heat-affected zone, resulting in a bimodal grain structure, contrasting with the fine equiaxed grains in the FZ. For high Zn content alloys (~6 wt pct), the extended solidification range increases susceptibility to liquation cracking. These findings provide critical insights into the microstructural and defect evolution during laser re-melting and offer valuable guidance for advancing the effectiveness and reliability of laser processing techniques for magnesium alloys.