Single-Pass Full Penetration Laser Welding of 10-mm-Thick EH40 Using External Magnetic Field

Single-pass full penetration laser welding of thick plate generally suffered from the hump and drop-out defects, which would seriously affect mechanical properties of welded joints. In order to efficiently suppress these defects of thick plates and obtain thick plate welded joints with better performance, this paper proposed a method of the high-strength steel thick plate welding by coupling external magnetic field and single-pass full penetration laser welding. The effect of static magnetic field on the microstructure and mechanical properties of welded joint was investigated. The suppression mechanism of periodic root humps in thick plate was clarified by the dynamic balance equation and the schematic diagram of the magnetic field stirring in the molten pool. The results showed that compared with sample without magnetic field, it was obvious that the periodic root humps and drop-out of thick plate welded joints with 30 mT external magnetic field were suppressed, the top width, middle width of fusion zone and the width of heat-affected zone of joint were broadened, respectively, while the bottom width of fusion zone was narrowed. Formation of polygonal ferrites in fusion zone was prevented and the ferrite side plate was produced in heat-affected zone. The tensile strength of the EH40 thick plate welded joint was increased by 14.5%, the ductility and microhardness of heat-affected zone and fusion zone were improved. It indicated that the optimal joint of thick plate with excellent welding appearance and mechanical properties could be obtained by choosing the appropriate magnetic field.