Observation and understanding in laser welding of pure titanium at subatmospheric pressure

The effects of ambient pressure on the phenomena in laser welding of pure titanium were observed by X-ray transmission in situ apparatus and high speed camera. The penetration depth increased to 18 mm as the ambient pressure decreased to 0.1 kPa. The depth increment from 100 to 0.1 kPa is nearly 200%, far higher than that of mild steel (60%). Both the backward expansion at keyhole tip and the high-speed spatters could be suppressed by decreasing ambient pressure to 1 kPa or lower. The spatter number decreased at least 4 times as the ambient pressure decreased from 10 to 0.1 kPa. It could be deduced that the melt flow decelerated with decreasing ambient pressure. Relevant mechanisms were discussed by the metallic vapor ejection from keyhole and the melt flow types in molten pool.