The Role of Assist Gas in CO₂ Laser Welding

Laser Welding of Metal (Rept.3)

Abstract

Optimum gas-assisting parameters have been determined to suppress plasma formation, and the role of the assist gas has been analized in CO₂ laser welding. Results obtained are summarized as follows:
(1) When the assist-gas pressure p is higher than p_p, which is slightly higher than vapor pressure or surface tension pressure the plasma formation is suppressed by forcing the vapor stream going up along the laser beam axis to eject along the rear wall of the cavity, providing increased penetration depth without weld defects. The value p_p decreases with increasing laser power and decreasing welding speed. The fluctuation of the penetration depth tends to increase with increasing p, since the cavity diameter is enlarged by the vapor pressure which exeeds the value for the surface tension pressure to keep the cavity stable.
(2) When p is too high, a lot of molten metal flowed out of the cavity closes the cavity entrance on re-entering into the cavity due to its large mass motion, providing humping bead containing large porosities.
(3) The plasma formation is also suppressed effectively at vacuum pressure range 10-20 torr, providing deeper penetration depth without fluctuation of the penetration.