Evaporation characteristics in laser welding ‐ fundamental study on theoretical modelling of laser welding (Report 2)

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Abstract
Evaporation characteristics in CO2 laser welding stainless steel were experimentally analysed at power densities in a range from 5 × 104 to 107 W/cm2 with M2 of 1.4 and 5.6. Welding mode changes from a thermal conduction type to a deep penetration type at a critical power density around 4–5 × 105 W/cm2. Penetration depth tends to increase with decreasing M2 with accompanying decreased evaporation loss when the power density at the work surface is the same. The latent heat of evaporation is negligibly small, at most, around 1 % of laser power. In thermal conduction welding, where the power density is less than 4–5 × 105 W/cm2, the evaporation rate per unit area was found to be equivalent to that of deep penetration welding, indicating that the recoil force of the evaporation is at least as high as that of keyhole welding, producing a shallow hemispherical depression by the recoil force in the molten pool. The evaporation rate in deep penetration welding decreases with increasing aspect ratio of the weld bead due to the condensation of the vapour at the side wall of the keyhole.

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