The Laser Butt Welding Simulation of the Thin Sheet Metal

Abstract

For highly accurate laser welding, it is necessary to understand the distortion caused by thermal processing with laser. First of all, as a standard distortion caused by the laser heat source, bead-on-plate was performed on a thin metal plate by moving the heat source with continuous wave oscillations. With this experiment, the amount of distortion for a butt welding in the ideal state was measured, assuming that there is no gap. At the same time, calculation by computer simulation was done under the same processing conditions to confirm the experimental result and both were compared and examined. With this comparative work, the form of distortion and the thermal stress distribution were clarified for a straight “bead-on-plate”, moving the laser heat source in the middle of a regular tetragon plate. Distortion was observed along the welding line as well as at both edges of the plate parallel to that line. The amount of distortion varies depending on the pate thickness, size and power of laser. The faster the welding speed gets, the smaller the distortion amount becomes. The amount also depends on the duration that a jig holds the material while welding. However, there exists a gap at the welding surface in an actual butt welding by all means. In butt welding with a gap, the boundary surface of welding is mathematically discontinuous; therefore, it is difficult to establish the heat conduction theory and the theoretical calculation of the distortion. The computer simulation is the only technological method to resolve this problem. The simulation to solve this problem is not established up to now. Therefore, there is little research paper of this field. This paper tries to obtain a solution through a computer simulation by setting a new calculable simulation model. Based on the established calculation model, it was examined how gap size influences the welding distortion and the shape of the cross section of the butt welding with a gap at the welding surface. In a butt welding with a gap, energy distribution varies depending on gap size and relative position between the material surface and the lens. Not only the simple thermal stress but also the thermal expansion and oxidization as well as the constriction due to cooling are deeply involved in the distortion in butt welding, especially at the molten layer where welding characteristics are largely linked with the distortion. As a result of considering those factors together, various cases of distortion behavior caused by laser welding have been clarified.