Laser-welding and heat treatment of high strength steels

Sammanfattning: Laser materials processing, including thermal treatment and laser welding has been undergoing continuous growth in the manufacturing industry for decades. A laser beam offers high precision and energy transfer, capable of various processing. For many cases a Gaussian beam is applied, but lately development of more complex beam shapes has been developed, where e.g. multiple beams (beam splitting) can be used for increased tolerances during welding.This thesis presents six papers (Papers A-F) on welding of high strength steels, laser pulse shaping, thermal treatments, and microstructural investigations. Different methods for obtaining a desirable weld were investigated through tailoring of the laser beam process. This affected the resulting temperature fields and thermal histories of the specimens. Experimental analysis was supported through various in-situ observation techniques and metallurgical studies.Papers A-C present thermal processing and chemical manipulation to obtain the desired microstructure, by introducing and applying the here introduced Snapshot method. Paper A explores tailoring a laser pulse to mimic a hybrid welding process, Paper B elaborates the simulation to a multi-cycle process, and Paper C explores dilution. The manuscripts utilize a specialized experimental setup, optical analysis methods, and standard thermal measuring techniques. Metallographic analysis showed that thermal process optimization and/or dilution rate control during welding improved weld zone characteristics.Improvements also include joint macrostructure characteristics, which are impacted by process stability, the theme of Papers D-F. Melt pool phenomena are studied in depth in Papers D and E. Paper D explores material ejections in a single beam welding scenario. Paper E investigates six beam shapes, from a single beam to a quad-beam arrangement. Paper F studies hybrid welding, a process that was simulated in Papers A-C but focused on the stability of the process instead of thermally guiding the microstructure. The studies complement each other in knowledge and methods. Welding of high strength steel is joining method-dependent, which imposes a unique thermal profile that affects the microstructures. The microstructure is also influenced by the chemical composition, an important point when multiple materials are used. The studies contribute an analysis of certain aspects of thermal and chemical effects of different laser-based processes to further optimize processing of specifically high strength steels, though the aspects can be generalized to other metals