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/cm
2 were conducted on 304 stainless steel in pressure levels between 10
5Pa and 10
-5Pa. Although a laser-induced plasma plume is observed during welding in the environmental pressure 10
5Pa and 10
3Pa, 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 10
5Pa and 10
3Pa. 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 10
3Pa 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.
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