Numerical modeling on formation of periodic chain-like pores in high power laser welding of thick steel plate

Abstract

The porosity with chain-like shape is found in high power laser welding of thick steel plate, which are periodically distributed at the lower and middle parts of the weld and parallel to bottom fusion line. A 3D numerical model is developed to reveal the effect of laser energy absorption and transfer, keyhole fluctuation, molten pool flow and solidification behavior on the formation process of chain-like pores. It is found that the chain-like pores are mainly determined by the unique profile of molten pool concaved inward at middle part on its rear wall. The periodic distribution of chain-like pores is resulted of the periodic variation of molten pool profile influenced by simultaneous periodic change of keyhole collapse frequency, heat accumulation at lower part, heat transfer through clockwise vortex as well as solidification latent heat accumulation at middle part. With the increasing of weld speed, the distribution of pores changes to form in the same height from the bottom fusion line due to the unchanged molten pool profile, which is resulted from insufficient solidification latent heat accumulation at middle part and less heat transferred by weaken upward flow in clockwise vortex. While the bubbles tend to reach a relatively higher position or escape from molten pool for more heat input, slower solidification speed and violent upward flow in clockwise vortex when weld speed decreases.