Abstract
Penetration mechanisms of welding by continuous wave (CW) YAG laser beam for mild steel were discussed. Bead-on-plate tests were carried out varing laser beam power and welding speed, and the cross-sectional shapes of the welds were measured. The fundamental welding characteristics for mild steel by YAG laser beam were estimated in comparison with those by CO2 laser beam. The results obtaine are as follows. (1) The penetration mechanism of key-hole welding in a steady state by YAG laser beam agrees with the heat conduction theory for a wedge type volume heat source model, like that by CO2 laser beam. (2) YAG laser beam has lower threshold power to enter fusion of mild steel and its absorption efficiency is less dependent upon the cavity depth of molten pool than CO2 laser beam. Then, input power can be controlled to adjust the irradiating beam power, that is necessary to slow down penetration depth at the end of weld.
