High-Magnification In Situ Observation of Welding Solidification

In welding solidification, the morphology of residual liquid in the solid–liquid coexistence region affects the susceptibility to solidification cracking because this cracking is due to localized shrinkage strain in the residual liquid. Therefore, it is important to observe the residual liquid state during solidification in detail to elucidate the occurrence of solidification cracking. In this study, a high-magnification in situ observation system was developed by combining an optical microscope and a high-speed camera. This system enables continuous, high-magnification, and high-resolution observation of welding solidification, because the objective lens of a microscope is attached to a high-speed camera. Laser welding solidification of stainless steel sheets was observed using this system and the morphology of residual liquid was visualized with higher magnification and higher definition than previous observation methods. Compared with a high-magnification image and quenched solidification microstructure, the residual state of the liquid phase during solidification could be observed in detail and dynamically. Additionally, the difference in solidification between two types of stainless sheets could be observed with high magnification in situ at one point. Finally, the combination of the observation results from this system and a high-temperature ductility curve revealed the relationship between the morphology of residual liquid and solidification cracking susceptibility.