A molten pool and keyhole dynamics study in near IR-blue hybrid laser welding of AZ31B magnesium alloy: A dual-mode synergy strategy for defect suppression in reflective low-melting materials

Abstract

Magnesium alloy with excellent strength-to-weight ratio and good formability has been primarily applied in automotive and aerospace applications. However, the laser welding of magnesium alloys is often challenged by low absorption efficiency, tendency to oxidize, thermal sensitivity and susceptibility to evaporation because of low boiling point, leading to instability of molten pool and keyhole. In light of the current lack of sufficient research, the effects of blue and hybrid laser welding were clarified, and the mechanisms behind the observed phenomenons were analyzed in this study. The research found that near-infrared (NIR) laser welding caused porosity defects due to keyhole collapse and turbulent molten pools. Blue laser improved hardness (76.7 Hv) and strength (305.44 MPa) but induced discontinuous solidification from gravity-driven fluid accumulation. Hybrid laser welding stabilized the molten pool by reducing temperature gradients and stress concentrations, enhancing recoil pressure to counteract gravity. This achieved superior hardness (78 Hv) and maintained strength (287.86 MPa), with surface roughness reduced by 23.1 % compared to NIR and 3.5 % compared to blue laser, demonstrating synergistic benefits for magnesium alloy welding performance through optimized energy and fluid dynamics. This research not only employs an energy-morphology-defect correlation analysis to investigate the welding process of magnesium alloys which bridging critical knowledge gaps in blue and hybrid laser welding mechanisms but also introduces a interesting hybrid laser welding technique that overcomes the longstanding challenges in magnesium alloy welding, offering a substantial methodological innovation with potential applications in high-performance manufacturing industries.