Abstract
High power laser is a promising tool to weld heavy section plate members. One of the problems, in this case, is formation of some weld defects such as porosity and hot cracking. In the present paper, formation mechanism of the porosity has been investigated in the deep penetration laser welding. 20 kW CO2 laser was used to attain the deep penetration in excess of 20 mm. Dynamic keyhole behaviour was observed during welding using a micro-focused x-ray transmission imaging system. Significant fluctuation in the keyhole depth of about 40% was observed even in the CW welding. The bubble was formed at the keyhole tip during abrupt decrease in the keyhole depth. The keyhole was closed with surrounding molten metal just above the tip, resulting in bubble formation. This is caused by capillary instability of the cylindrical keyhole. Most of the bubbles were combined with each other in the molten pool and remain after solidification. It causes the porosity. The bubble formation is promoted by fluctuation of the keyhole in the radial direction. Increasing the power density in the keyhole is effective in suppressing the porosity by stabilizing the keyhole. However, optimization of the welding variables was not enough to prevent the porosity.
