Laser welding of sheet metals is an important application of high power lasers, and has many advantages over conventional welding techniques. Laser welding has a great potential to replace other welding technique in the car-body manufacturing because of high laser weld quality and relatively low manufacturing cost associated with the laser technique. However, a few problems related to the laser welding of sheet metals limit its applications in industries. To have a better understanding of the welding process, laser welding experimental studies and theoretical analysis are necessary.
Temperature-dependent absorptivities of various metals are obtained theoretically for CO2, COIL (Chemical Oxygen-Iodine Laser) and Nd:YAG lasers. It is found that the absorptivities for COIL and Nd:YAG lasers are 2.84 and 3.16 times higher than for the CO2 laser, and the absorptivity increases with increasing temperature of the metals. Surface roughness and oxide films can enhance the absorption significantly. The reflectivity of as-received steel sheets decreases from 65-80% to 30-40% with surface oxide films for CO2 lasers. Laser welding experiments show that the tensile strengths of the weld metals are higher than the base metals. For samples with surface oxide films, the oxygen concentration in the weld metals is found to be higher than in the specimens without oxidation, and the toughness of the weld metals is degraded. This new technique improves the utilization of laser energy with minimal reduction in the weld toughness. When steel powders are added to bridge the gap between two sheets, the oxygen content in the weld metals decreases and the toughness increases. Another contribution of this experiment is a technique for the tensile test of small weldments by introducing notches at the edges of weldments.
A mathematical model is developed for the melt depth due to a stationary laser beam. The model results show that the melt depth increases rapidly with time at the beginning of laser irradiation and then increases slowly. Also, the melt depth is found to increase rapidly with laser intensities and then increases slowly for higher intensity. The average rate of melting and the times to reach the melting and boiling temperatures at the substrate surface are obtained by using the model. Another mathematical model is developed for the weld width and weld pool shape due to a moving Gaussian laser beam. The model predictions compare well with experimental results.
Doctor of Philosophy (Ph.D.)
College of Engineering
Mechanical, Materials, and Aerospace Engineering
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Length of Campus-only Access
Doctoral Dissertation (Open Access)
Dissertations, Academic -- Engineering; Engineering -- Dissertations, Academic
Xie, Jian, "Laser welding of sheet metals" (1998). Retrospective Theses and Dissertations. 2589.