Characteristics of deep penetration welding have been investigated using 20 kW CO_2 laser facility. Formation mechanism of the porosity has also been discussed. The porosity forms most significantly for upper focussing conditions, under which the low power density of the laser beam tends to incline the keyhole front wall at the root. The laser beam reflected at the inclined keyhole front superheats the molten metal, resulting in forming the porosity at the root. The porosity is not observed in N2 shield welding, although large N-plasma is formed during welding. The suppression might be caused by periodical change in the heat input due to periodical plasma formation and dissolution of the shielding gas in the molten pool. Pulse welding of low frequency is effective in suppressing the porosity due to promoting the periodical molten metal flow.
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