2001 Volume 19 Issue 2 Pages 241-251
The mechanism of porosity formation in CO2, laser welding of thin steel sheets was investigated. Laser melt-runs were made in ultra-low-carbon steel sheets of 0.7 mm in thickness under a wide range of shielding gas conditions. In addition to the normal melt-run condition with Ar coaxial shielding gas and Air atmosphere at the backside, the nitrogen gas content was varied in coaxial shielding gas as well as in backside atmosphere.
The number of pores in weld metal was measured by radiographic tests, and the nitrogen content in weld metal was analyzed. The composition of gas in the pores was also quantitatively analyzed. These experimental results lead to an understanding that nitrogen in the backside atmosphere dissolves into molten weld metal and during solidification the dissolved nitrogen forms porosity due to an abrupt decrease in the nitrogen solubility. It is considered that nitrogen molecules are dissociated in the laser induced plasma at the backside, enhancing the dissolution of nitrogen into weld metal.