Effect of vacuum on penetration and defects in laser welding

The effect of vacuum on weld penetration and porosity formation was investi gated in high-power cw CO2 and YAG laser welding. It was consequently confi rmed in welding with both lasers that the penetration was slightly deeper i n aluminum alloys and austenitic stainless steel with a decrease in the amb ient pressure. It was also revealed that no porosity was present in the mat erials welded at lower pressures. The reason for no porosity formation in v acuum was examined by observing keyhole behavior, bubble and porosity forma tion situation, and liquid flow in the molten pool during high-power YAG la ser welding under various conditions through the microfocused x-ray real-ti me observation system. It was confirmed in the coaxial Ar or He shielding g as that a lot of bubbles were generated near the bottom part of the molten pool from the tip of a fluctuated keyhole and resulted in large pores. On t he other hand, under the vacuum conditions, no bubbles were formed in the m elt pool from the keyhole, although the middle and bottom parts of the keyh ole swelled up in the molten pool probably because the evaporation of metal s was so intense. Moreover, opposite liquid flows as well as different molt en pool geometry were observed between the normal atmospheric pressure weld ing with a coaxial shielding gas and the vacuum welding. Namely, during nor mal welding in a gas, the circulation of a strong molten flow was observed downward along the keyhole, from the keyhole tip along the bottom molten po ol to the upper rear, and subsequently below the surface to the forward key hole. On the other hand, in vacuum welding, the liquid flowed upwards along the rear keyhole wall, but there was no such strong flow near the bottom a nd the rear parts of the molten pool. It is considered in vacuum welding th at the liquid flow into the bottom part of the molten pool from the keyhole tip does not occur because of the direction of evaporated metals toward th e upper keyhole outlet. This may exert a beneficial effect on the reduction or prevention of pores or porosity. (C) 2001 Laser Institute of America.