35 巻 (2017) 4 号 p. 160-170
Application of laser-arc hybrid welding on fabricating large steel structures such as ships and bridges are being extended. Selecting proper welding conditions is important based on joint types and plate thicknesses. There are many factors in hybrid welding conditions, for example, heat inputs of laser and arc, welding speed, gap between two welded plates, angles of laser head and arc torch, laser spot diameter and defocus, flow of shield gas etc. From the viewpoint of mechanics, heat input of each heat source and welding speed largely affect the strain, stress and deformation of joint. On the other hand, it is still unknown how the welding conditions affect cold cracking caused by the degree of restraint of joint. It is important to identify the welding conditions by which the cold cracking occur for extending the application of hybrid welding on fabrication of large steel structures.
In this study, a mechanical measure for occurrence of cold cracking in hybrid welding was investigated through a series of experiment and numerical simulation. The hybrid welding experiment was performed on slit joints by SM490A steel with thickness of 24mm. The welding conditions such as heat input from each heat source and welding speed were variously changed. The relationship between cold cracking occurrence and the mechanical state around the welded part was examined by the 3D thermal elastic plastic analysis. Based on the results, it was proposed that the average restraint strain obtained by the analysis was effective for evaluating occurrence of cold cracking in the hybrid welding. Among the welding conditions adopted in this study, the cold cracking occurred when the average restraint strain was over 5%.