Novel metrology to determine the critical strain conditions required for solidification cracking during laser welding of thin sheets

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 testes remain for decades the only way to determine the critical straining conditions for solidification cracking. In this study, a novel optical two-dimensional in situ observation technique has been developed to allow the strain evaluation during the welding process in the moment of crack formation. Additionally, the Controlled Tensile Weldability (CTW) test was used to generate the hot crack under different global straining conditions. To record the welding process and the moment of the solidification crack initiation a HDR-CMOS camera was used together with an 808 nm diode laser as an illumination source, so that the melt pool and the re-solidifying metal could be visualized in a single image. In order to obtain good temporal resolution, the frame rate of the camera was set to 1100 frame per second. The contrast in images obtained using this unique setup allows to apply the optical flow technique based on Lucas-Kanade (LK) algorithm to follow the pixels in each image sequence and then to calculate the displacement field. The strains were calculated based on the estimated displacements. Using this technique, the local strains under different global strain rate conditions has been determined and analysed. Moreover, the described procedure of the optical measurement allows to determine the real martial dependent values of critical strain characterizing transition to the hot cracking during laser welding processes. The experiments as well as the measurement has been performed on the stainless steel 316L (1.4404)