Modeling of transport phenomena during laser welding process.

摘要

This dissertation, consisting of three papers, presents the results of research investigations on laser welding of zinc-coated steel sheets and three-dimensional laser keyhole welding by numerical analysis. In the first paper, the mathematical model and numerical techniques are developed to simulate the keyhole formation, zinc vapor formation and zinc vapor escaping processes. In the study, the keyhole is used as an effective way to vent the high-pressure zinc vapor generated at the interface between the two zinc-coated metal sheets. The interaction between the weld pool and the escaping zinc vapor is investigated. It is the first comprehensive mathematical model for laser welding of zinc-coated steel sheets.; In the second paper, the aforementioned model is employed to study the defect formation mechanisms in laser welding of zinc-coated steel sheets. Welding defects, such as voids at the root of welded metal, the undercut on the top surface and the bubbles trapped in the weld pool are simulated. The reasons causing these welding defects are given through the studies of the zinc vapor-weld pool interaction and the fluid flow and heat transfer during the keyhole collapse and metal solidification processes.; In the third paper, the aforementioned stationary 2-D model is extended to modeling a 3-D moving laser keyhole welding process. The heat transfer and fluid flow in the welding pool around the keyhole are calculated. Also, the laser induced plasma inside the keyhole due to the Inverse Bremsstrahlung absorption is considered to calculate the temperature distribution inside the keyhole. Energy distribution inside the keyhole is computed by considering the Fresnel absorption and multiple reflections phenomena.