The penetration depth of magnesium alloys was observed to increase and then decrease with decreasing ambient pressure during laser welding process. This variation was different from that of aluminium, titanium, and steel. Therefore, the mechanism behind this phenomenon was investigated. The result showed that when ambient pressure decreased from 101 kPa to 30 kPa, enhanced penetration was observed due to the disappearance of plasma plume, increased laser energy absorption, and increased recoil pressure. At 5 kPa, the small difference between melting and boiling points of magnesium resulted in excessive recoil pressure, destabilising the keyhole and generating condensed particles which hindered laser transmission, and reducing penetration. This study further clarified the physical mechanisms of sub-atmospheric laser welding and broadened its applicability.
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