Mechanical evaluation on cold cracking of laser-arc hybrid welding by restraint strain

Application of laser-arc hybrid welding on fabricating large steel structures such as ships and bridges are being extended. It is important to select proper welding conditions based on joint types and plate thickness. There are many factors in hybrid welding conditions, for example, heat inputs of laser and arc, welding speed, groove gap in butt joint, angles of laser head and arc torch, laser spot diameter and defocus, flow of shield gas etc. Especially, the heat input and welding speed affect the degree of restraint of joint which causes cold cracking. It is necessary to identify the welding conditions by which prevent the occurrence of cold cracking for extending the application of hybrid welding on fabrication of large steel structures. In this study, the occurrence of cold cracking in hybrid welding was mechanically evaluated by using a concept of restraint strain. The hybrid welding experiment was performed on different lengths of slit joints by SM490A steel with thickness of 24 mm. 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 average restraint strain calculated by a 3D thermal elastic plastic analysis. Among the welding conditions adopted in this study, the criterion of cold crack occurrence might exist between 1.7 % and 6.4 % of the average restraint strain. Furthermore, an estimation method for average restraint strain based on the welding conditions was proposed. It might be useful for simply evaluating the occurrence of cold crack without performing the precise thermal elastic plastic analysis. © 2019 Japan Welding Society. All rights reserved.