An ultra high vacuum diode laser welding (UHVDLW) system in which a diode laser (DL) welding can be performed in the ISS orbital pressure 10 −5 Pa was developed for investigating the effect of an environmental pressure on a DL welding phenomenon. The DL welding experiments with power density around 100 kW/cm 2 were conducted on 304 stainless steel in pressure levels between 10 5 Pa and 10 −5 Pa. Although a laser-induced plasma plume is observed during welding in the environmental pressure 10 5 Pa and 10 3 Pa, no laser-induced plasma plume is found in the pressure ranging 10 Pa to 10 −5 Pa. A formation of melting process (weld pool shape) with this experimental condition is a keyhole melting type or a transition melting type in the environmental pressure 10 5 Pa and 10 3 Pa. But, a melting process in the pressure lower than 10 Pa changes to a heat conduction melting type. The penetration depth showed a decrease with the environmental pressure down to 10 3 Pa and then appears to no change in a pressure lower than 10 Pa. Establishment of space DL welding technology needs to develop a suppression technology of a metal vapor deposition to an optical device.

An ultra-high vacuum diode laser welding system capable of performing a diode laser (DL) welding experiment at the International Space Station orbital pressure of 10 À5 Pa was developed for investigating the effect of environmental pressure on DL welding phenomena. A DL welding experiment with a power density of about 100 kW/cm 2 was conducted on 304 stainless steel at pressure levels between 10 5 Pa and 10 À5 Pa. Although a laser-induced plasma plume was observed during welding at environmental pressures, 105 Pa and 10 3 Pa, no laser-induced plasma plume was found in the pressure range from 10 Pa to 10 À5 Pa. The mode of the melting process (weld pool shape) under these experimental conditions was a keyhole-type melting mode or a transition-type melting mode at environmental pressures of 10 5 Pa and 10 3 Pa. However, the melting mode at pressures lower than 10 Pa changed to a heat-conduction-type melting mode. Although the penetration depth decreased when the environmental pressure was dropped to 10 3 Pa, it did not change at pressures lower than 10 Pa. We also determined that the prevention technology of metal vapor deposition on optical devices needs to be developed for DL welding technologies used in space.