Enhanced Penetration Depth during Reduced Pressure Keyhole-Mode Laser Welding

Keyhole-mode laser welding under reduced ambient pressure is known to provide improved weld penetration, narrower width, and reduced incidences of defects, but the underlying mechanism for these benefits is not known. We sought to elucidate the mechanism by an experimental and theoretical program of investigation. Potential causative factors, such as the depression of the boiling point of alloys at reduced pressures and the changes in laser beam attenuation by metal vapors/plasma, were investigated using a well-tested heat transfer and fluid flow model of keyhole-mode laser welding for various ambient pressures. The model was tested with experimental data for the welding of four alloys — Structural Steel Q690, Aluminum Alloy A5083, commercially pure titanium, and Nickel 201 — that have very different thermophysical properties. The results showed the changes in the boiling point alone were unable to explain the enhanced depth of penetration at low ambient pressures. The experimental and calculated fusion zone geometries showed excellent agreement when both the boiling point depression and the beam attenuation by metal vapor were considered. The reduction of ambient pressure also affected the heat transfer pattern near the keyhole, owing to a decrease in the keyhole wall temperature and changes in the temperature gradient near the keyhole wall.