Investigation of solidification crack formation in laser beam welding of stainless steel with high-speed x-ray imaging

Laser beam welding, a contactless joining technique, is increasingly favored in automated industrial production due to its rapid processing and localized heat effects. However, a significant challenge associated with laser beam welding is the susceptibility of the material to solidification cracking. One potential strategy to mitigate solidification cracking involves manipulating the melt pool shape and flow dynamics through laser parameter adjustment. However, current approaches based on user observations and assumptions often lack a mechanistic foundation, leading to an empirical trial-and-error process for identifying suitable processing parameters, materials, or geometries. This necessitates extensive and time-consuming experimentation. To overcome this limitation and achieve significant advancements in laser beam welding, a quantitative understanding of solidification crack formation mechanisms and their correlation with process parameters is crucial. Although numerous research efforts, both experimental and simulative, have been dedicated to this topic, existing theories primarily rely on qualitative explanations focusing on metallurgical, strain, or stress-based phenomena. Unfortunately, these approaches have not yet yielded a clear and quantitative model description that can be readily implemented through experimentation. Experimental approaches are hampered by the poor visibility of the process zone. Although cracks can be identified postmortem, it is challenging to draw conclusions about the mechanisms of formation. This study addresses this gap by employing in situ x-ray high-speed imaging to investigate the dynamics of crack formation in laser beam welding. Experiments conducted at the German Electron-Synchrotron (DESY) at Petra III, beamline P07 compare parameter-dependent crack formation behavior in stainless steel AISI 304. The results suggest that the distribution of laser energy within the weld zone and its influence on melt pool behavior and microstructure play a critical role in solidification cracking.