Friction stir welding of ductile iron and low carbon steel

Abstract

Friction stir welding (FSW) is a low distortion, high quality solid welding. There is no melting during the welding process, which results in improved welding quality. Ductile iron has the advantages of being low cost, of excellent castability, and of being good mechanically. Therefore, it is generally used in many structural engineering parts. In this study, ferritic ductile iron and low carbon steel were used to explore the qualities of dissimilar metal welding under different conditions. The FSW process, changes in the microstructure of the welding area and the mechanical properties of joints were explored. According to the research, we found that when dissimilar metal welding is conducted at 982 rev min-1 with a travelling speed of 72 mm min-1, flawless welding quality can be obtained if the stir rod rotates counterclockwise with carbon steel fixed in the advancing side and with ductile iron in the retreating side. FSW successfully provided defect free welds. However, fine pearlite and martensite structures appear in the stir zone, which result in mechanical property degradation of weldments. The stir zone in the weldments is very hard due to martensitic transformation. After heat treatment, the tensile strength improves, and the fracture site appears in the base metal of the carbon steel. However, the welding nugget is not completely filled when the stir rod directs ferritic ductile iron to the advancing side in the clockwise direction and carbon steel in the retreating side, which results in defects and lower welding quality.