| Rims are important components of automobile driving system, affecting both the stability and safety during operation process. Flash butt welding(FBW) method, characterized by high efficiency and good welding quality, has become the mainstream method of rim production. In order to achieve the development goal of lightweight, high strength steels have been used increasingly in rim manufacturing process. However, as steel strength grades continue to increase, the following welding problems appear, micro-cracks occurred completely or partly in/near the interface zone, which seriously affect the products quality and performance. In this paper, The RS590 CL low alloy high strength steel was employed as the base metal, the weld thermal cycle, microstructures characteristics of FBW joints, and effects of welding parameters on microstructures and mechanical properties of welded joints were systematically investigated, the method of quadratic orthogonal regression was used to optimize welding parameters, which improved the mechnical properties of FBW joints greatly.The research results showed that weld thermal cycle of FBW joints characterized by the high peak temperature, and larger heating and cooling rates. The peak temperature reached the highest value at the weld interface and reduced sharply with distance increasing from the interface. FBW joints can be divided into the interface zone(IZ), overheated zone(OZ), recrystallized zone(RZ) and partially recrystallized zone(PZ). The IZ consisted of bainite, ferrite and a small amount of pearlite, and contained α-Fe equiaxed grains and Fe3 C phase distributed along grains boundaries as well as a large amount of tangled dislocations based on TEM analyses of this zone. The basic microstructure in OZ was coarsen bainite, and the closer to IZ, the coarser the grains became. The RZ was characterized by grains refined obviously and consisted of fine ferrite and a small amount of pearlite, and finer carbide particles NbC as well as dislocations were also found in this zone. The basic microstructures in PZ were ferrite in different sizes and a small amount of pearlite.The welding paremeters had significant effects on microstructures and mechanical properties of FBW joints. As flash allowance and upset current time increased, the widths and grains sizes of overheated zone and welding zone increased, which were related to the increased heat inputs. With the upset pressure and upset allowance increasing, the overheated zone widths and welding zone widths decreased and the ferrite amount in interface zone increased. These can be explained by the larger plastic deformation experienced by the joints promoting the occurrence of the dynamic recrystallization and the recrystallization process. The results of joints tensile tests showed that all samples fractured at base metal and the tensile strength of joints was within the range of 584.13MPa~611.02 MPa, which can match the strength of base metal well. The joints had good bending performance and the maximum bending crack length was 1.38 mm under the experimental conditions, which can meet the relevant requirements(<3mm). The microhardness distributed unevenly across the joints. The hardness values in interface zone and overheated zone were much higher than that of recrystallized zone and base metal, and the highest hardness value appeared in the overheated zone near the interface zone mainly due to the upper bainite microstructures existed in overheated zone.The method of orthogonal quadratic regression of three factors and three levels was used to optimize welding parameters. Optimal welding parameters were as follows: flash allowance of 16 mm, upset pressure of 50 MPa, upset allowance of 3.5mm, upset current time of 0.4s. Under the optimal welding parameters, the interface zone of FBW joints formed well without welding defects such as cracks and joints had good mechanical peoperties. The impact energy reached 90.48 J/cm2, the tensile strength was higher than 590 MPa, the ductility was higher than 24% and no bending cracks occurred during the 180° bending tests. These all illustrated that the welding quality and the mechanical properties of FBW joints were improved under optimal welding parameters. The joint impact fractures had been analyzed and the conclusions can be made. The decreased joint impact energy mainly associated with the coarsen bainite, retained oxides and joining defects in the weld interface. |
