Deep penetration welding with high power laser under vacuum

High power and high brightness disk lasers have been developed, and it is revealed that a deeper weld bead can be produced with an increase in the power density. It is furthermore expected that sound deeper penetration welds can be produced under vacuum, although those can hardly be made in a gas shielding at 1 atm. Thus, a vacuum chamber using a high beam quality laser was developed to produce sound ultra deep penetration welds. Therefore, this study was undertaken on Type 304 stainless steel and A5052 aluminum alloy under low vacuum produced by rotary pumps. The observation technique of welding phenomena was utilized to obtain a fundamental knowledge of laser weldability under low vacuum. As a result, the welds of both metals under low vacuum were over twice deeper than those under the atmospheric pressure. And it was found that the effect of ambient pressure on the penetration depth became significantly at low welding speed. Especially, over 0.6 m/min, the penetration depth with A5052 began to decrease below the pressure which produced the deepest penetration weld. In order to clarify welding phenomena under low vacuum, the laser-induced plume and the keyhole behavior in the molten pool were observed by a high-speed video camera. It was found that under low vacuum the plume was stable and the molten pool became narrower and steadier compared with those under atmospheric pressure. Especially, in A5052 the plume shifted back and forth, and the keyhole expanded and contracted backward to the laser beam spot.