Development of a novel optical measurement technique to investigate the hot cracking susceptibility during laser beam welding

The weldability of materials is still for many years a highly contentious issue, particularly regarding the causes of the hot crack formation. Because of the process-related temperature and emissions, direct measurement for the arising strain in the close vicinity of the welding process is challenged. Therefore, the externally loaded hot cracking tests remain for decades the only way to determine the critical straining conditions for solidification cracking. In this study, a novel 2D in situ observation technique has been developed to analyze the strain evaluation during the welding process in the moment of crack formation. For the first time, the employed technique enabled the in situ measurement of the transient strain field at the surface of the workpiece directed to the laser beam in the critical range, where the solidification cracking normally occurs. Thus, the critical threshold strain values at high temperatures characterizing transition from crack-free to crack-concomitant welding process could be deduced. The influence of the global straining conditions on the direct local measured strain and strain rate for the stainless steel 316L has been analyzed and discussed.