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Studies of Transport Phenomena and Their Effect on Weld Quality in Laser Beam Welding

机译：激光焊接传输现象及其对焊接质量的影响研究

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The objective of this project was to study complicated transport phenomena (laser-material interaction, heat transfer, and molten metal fluid dynamics) and their effect on weld quality in laser beam welding. The study investigated the formation of keyholes, keyhole dynamics, and possible weld defects caused by keyhole instability. A mathematical model based on continuum formulation (for handling metal flow during alloy melting/solidification) and volume-of-fluid (for handling free surfaces) was successfully developed to simulate the transient behavior of a keyhole in laser beam welding. The authors are now able to predict the following: (1) the formation and collapse of an axisymmetric keyhole, (2) the formation of porosity in the weld, (3) interaction between filler materials and weld pool, (4) zinc vapor escape in laser welding of galvanized steel, and (5) 3-D moving laser welding. Based on mathematical modeling, the authors proposed three methods to eliminate/reduce porosity in the weld. The predicted porosity formation compares favorably with published experimental reports. Simulation results show that the formation of porosity is caused by two competing mechanisms: one is the solidification rate of the molten metal surrounding the keyhole, and the other is the speed at which molten metal backfills the keyhole after laser energy is terminated. By controlling laser pulses or applying an electromagnetic force after laser energy has ceased, pores in the weld can be reduced or eliminated. The results from simulation illustrate that uniform composition of weld pool is difficult to achieve by filler metals due to the very rapid solidification of weld pool in laser welding. In laser welding of galvanized steel, keyhole can be utilized as the channel for releasing the zinc vapor and, as a result, pores caused by the trapped zinc vapor are avoided. Avoiding the interaction between zinc vapor and the welding pool can improve welding shape. (18 figures).

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