An ultra high vacuum diode laser welding(UHVDLW) system in which a diode laser(DL) welding can be performed in the ISS orbital pressure 10-5Pa was developed for investigating the effect of an environmental pressure on a DL welding phenomenon. The DL welding experiments with power density around 100kW/cm2 were conducted on 304 stainless steel in pressure levels between 105Pa and 10-5Pa. Although a laser-induced plasma plume is observed during welding in the environmental pressure 105Pa and 103Pa, no laser-induced plasma plume is found in the pressure ranging 10Pa to 10-5Pa. 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 105Pa and 103Pa. But, a melting process in the pressure lower than 10Pa changes to a heat conduction melting type. The penetration depth showed a decrease with the environmental pressure down to 103Pa and then appears to no change in a pressure lower than 10Pa. Establishment of space DL welding technology needs to develop a suppression technology of a metal vapor deposition to an optical device.
