The oscillation laser beam is considered to be effective as a heat source of the narrow gap multi-layer welding, because the oscillation laser welding can control the penetration shape and prevent the lack of fusion. In this study, in order to establish a narrow gap welding process by oscillation laser beam, butt welding experiments of 50mm thickness carbon steel plate were performed. By the appropriate control of the heat input area using the in-process sensor for recognizing the groove shape, narrow gap welding of thick plate with groove which was cut by gas cutting was achieved. Properties of welded joint had been confirmed by nondestructive testing, tensile test, and side bend test. A two-dimensional numerical calculation model for welding deformation was developed. This calculation model was used for investigation of the optimal groove angle. The results of calculations were in quantitative agreement with the experimental results. Microstructure of the weld zone had multiple thermal histories. According to the hardness test results, maximum hardness of the heat affected zone of the upper layer has been lowered than that of the lower layer.
