High speed imaging analysis of laser welding

Abstract: Laser welding is often considered a new and exotic manufacturing method even though it has been used in industrial applications for nearly fifty years. In the early years only a few special applications justified the high investment cost involved, but as the price of the laser sources has reduced, industrial interest in laser welding has increased. As different weld situations have appeared, involving new materials etc. there is an increasing need to understand the weld process on a fundamental level, especially for the newer, high power and high quality 1μm laser sources (Disk laser, Fiber laser). Laser welding sometimes involves production limitations that are caused by the process itself, not the laser source. Weld defects such as humping or severe spattering can make the weld quality unacceptable and more knowledge of the physics involved in defect generation is needed. In this thesis high speed imaging is used as a method of acquiring fundamental knowledge about laser welding. Modern digital high speed cameras in combination with powerful laser illumination provide a clear and detailed view of the actual weld process. The information in these high speed videos provides a possibility to see how the process behaves. Just as a slow motion goal camera helps the referees to rule accurately in an athletic event, the high speed cameras can help laser welding researchers to improve their fundamental understanding. This thesis is composed of seven publications in scientific journals which are thematically linked by their focus on high speed imaging analysis of laser welding. In two shorter letters, new measurement methods are presented. In the first case a streak image method is utilized to measure the fluid flow velocity on the keyhole front, and in the second a pulsed digital holography method was employed to measure deformation during laser spot welding. The streak image method is further developed in two subsequent papers to confirm and quantify the downward flow on the keyhole front during high speed welding. In the three additional papers both new and previously known laser welding phenomena are analyzed by high speed imaging. The first of these papers discusses the correlation between the size of the vapor plume above the keyhole and the signal acquired by a commercial “laser weld monitoring” system. The next paper gives practical guidelines on how to choose parameters in a laser hybrid welding system, and the final paper discusses conditions under which surface tension effects can produce a self-sustaining hole in the melt pool that might produce defects in the weld.