Effect of vacuum on penetration and defects in laser welding

Abstract

The effect of vacuum on weld penetration and porosity formation was investigated in high-power cw CO2 and YAG laser welding. It was consequently confirmed in welding with both lasers that the penetration was slightly deeper in aluminum alloys and austenitic stainless steel with a decrease in the ambient pressure. It was also revealed that no porosity was present in the materials welded at lower pressures. The reason for no porosity formation in vacuum was examined by observing keyhole behavior, bubble and porosity formation situation, and liquid flow in the molten pool during high-power YAG laser welding under various conditions through the microfocused x-ray real-time observation system. It was confirmed in the coaxial Ar or He shielding gas 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 the other hand, under the vacuum conditions, no bubbles were formed in the melt pool from the keyhole, although the middle and bottom parts of the keyhole swelled up in the molten pool probably because the evaporation of metals was so intense. Moreover, opposite liquid flows as well as different molten pool geometry were observed between the normal atmospheric pressure welding with a coaxial shielding gas and the vacuum welding. Namely, during normal 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 pool to the upper rear, and subsequently below the surface to the forward keyhole. 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 and the rear parts of the molten pool. It is considered in vacuum welding that 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 the upper keyhole outlet. This may exert a beneficial effect on the reduction or prevention of pores or porosity.