Research on properties of ductile iron/mild steel joints welded by laser

Due to the presence of spheroidal graphite in ductile iron, the mechanical properties of welding joints between ductile iron and mild steel are poor. Therefore, a welding method combined graphite-removed processing of ductile iron surface and laser welding is proposed. Compare with the direct welding method of dissimilar metals, a surface remelting method of removing graphite from ductile iron before welding was adopted to study the characteristics of the graphite-removed zone of ductile iron and its influence on the formation, microstructure, and mechanical properties of welded joints. The results show that after the laser surface remelting treatment of ductile iron, the spheroidal graphite on the surface of ductile iron is basically eliminated, the carbon content is greatly reduced compared with the base metal, the surface quality of laser-welded joints is better, and no porosity and micro-crack defects appear inside the weld. The tensile fracture of unprocessed welding joints (welded by laser directly) arises at the welding heat affected zone of the ductile iron side, and its maximum tensile strength is 400 MPa. The tensile fracture of graphite-removed welding joints(welded by laser after laser remelting treatment)arises at the transition zone of Graphite-removed Ductile Iron, and its average tensile strength reaches 483 MPa, which is 98 % of the base metal of QT450. The processing combined laser remelting and laser welding significantly improves the mechanical properties of laser-welded joints, which realizes the high strength and no defects connection between ductile iron and mild steel.