The Morphology of Laser Arc Hybrid Welds

Sammanfattning: This thesis is about formation of surface imperfections formed in welding when using the manufacturing methods of laser welding and laser arc hybrid welding. In hybrid welding a traditional arc welding source and a laser share the same melt pool, making the process even more complex. Laser welding is often considered as a non-traditional but highly advanced manufacturing technique in industry. As more is getting known about these advanced welding techniques, coupled with reduced prices of laser sources, the interest in industry gradually increases. In welding, control over the quality is essential, particularly to suppress imperfections and unfavourable surface geometries, like undercuts. The mechanical behaviour of a product in service, in particular fatigue life, can suffer from these small, sometimes hardly visible weld imperfections. The final weld quality results from a very complex process, involving non-linear multi-physics. The documentation of parameters, process conditions and the resulting quality is also complex, difficult and so far unsatisfactory. Therefore, the survey manuscript Paper i address the mechanisms and challenges for the documentation of knowledge in laser welding. In addition to the survey manuscript, six journal publications utilize the study of macrographs, surface scanning and High Speed Imaging as methods for capturing and identifying why weld surface imperfection formation can take place. Paper I studies the surface geometry of welds resulting from fibre laser welding with various parameters, also applying the new documentation method called Matrix Flow Chart, MFC. In Paper II, III, IV and VI the formation and shape of undercuts under various circumstances are compared, mapped, analysed and explained. When causes for undercuts are known, counter measures are suggested. Paper V is about accepting undercut formation and instead “repairing” the welded surface by re-melting it with a defocussed laser, effectively eliminating previous surface imperfections. Finally, Paper VI also provides a survey on undercuts in welding and describes the different parameter causes and physical mechanisms.