Study on the influence and control mechanism of solution treatment on the microstructure and properties of AZ31B magnesium alloy laser welded joints

This study systematically investigated the microstructural evolution and phase precipitation behavior in AZ31B magnesium alloy laser-welded joints subjected to various solution treatment using advanced characterization techniques. The underlying mechanisms governing microstructural transformations during solution treatments were revealed. Key findings demonstrate that the rapid cooling-induced segregation in the weld zone was effectively alleviated through solution treatments, resulting in enhanced microstructural homogeneity accompanied by moderate grain coarsening. The β-Mg 17 Al 12 secondary phase dissolved into the matrix under prolonged high-temperature exposure, with subsequent water quenching (80 °C) stabilizing the supersaturated solid solution state, thereby achieving significant solution strengthening effects. Additionally, solution treatments reduced dislocation density and improved structural uniformity, which facilitated elastic deformation and dislocation slip coordination, leading to enhanced plasticity and toughness. Representative treatment at 410 °C for 6 h resulted in a slight decrease in average microhardness and marginal improvement in ultimate tensile strength, while the elongation-to-failure notably increased to 11.0%. Fractographic analysis revealed a persistent ductile-brittle mixed fracture mode, suggesting incomplete elimination of localized stress concentration. Through response surface methodology optimization, the ideal solution treatments parameters were determined as 422 °C for 5.8 h, the predicted tensile strength and elongation at break were 251.3 MPa and 10.9%, respectively, while the measured values were 251.4 MPa and 10.7%.