Stabilization mechanism of arc behavior and droplet transition in laser-arc hybrid welding of NS70 magnesium alloy

The welding stability for magnesium alloy presents a significant challenge owing to the inherent softness of the welding wire and the propensity for high vaporization rates. The arc behavior and droplet transition, serving as pivotal indicators of welding stability, are mainly concerned. This study conducted laser-arc hybrid welding experiments on NS70 magnesium alloy, a novel lightweight material . The welding process was monitored using multi-source information from infrared temperature measurement, spectroscopy, and high-speed camera imaging. A comparison was made between the droplet transition behaviors during laser- metal inert gas (MIG) and laser-cold metal transfer (CMT) welding processes. The variations in arc plasma morphology, molten pool temperature, and ionization intensity were synchronously detected and quantitatively analyzed. The study investigated the effects of welding current and laser power on arc characteristics and droplet transition behavior in the CMT arc mode. Optimal process parameters were determined based on the weld seam surface quality and porosity rate. The results indicate that the increase of welding current enlarges the arc area, but disturbs plasma morphology. At a current of I = 65 A, the droplet transition frequency increased by 60.4 % compared to that under I = 35 A, and the fish scale density increased by 99.2 %. Analysis of the droplet transfer behavior during the wire feeding and withdrawal phases reveals that a welding current of 35 A and a laser power of 1900 W mitigate the intense droplet impact on the molten pool and the issue of incomplete droplet transfer. These findings provide a reliable basis for industrial welding production of NS70 magnesium alloy. • The droplet growth volume is mainly determined by the competition between welding current and droplet transfer frequency. • Welding current primarily influences phase I (wire preheating) and phase II (droplet formation), indirectly determining the fish scale pattern density and weld toe region formation. • Laser power predominantly affects phase III (droplet transition to the molten pool) and phase IV (complete wire withdrawal), ultimately governing the extent of droplet spattering.