Electro-superplastic Welding Process And Mechanism Of 1.6%C-UHCS/40Cr

The engineering application of ultrahigh carbon steel (UHCS) is restrained though it display excellent mechanical properties via hot-mechanical processing, because its weldability is poor resulted from the existence of large quantities of carbides in the steel. In order to achieve a sound joint of the materials with poor weldability, electro-superplastic welding, which combines the advantages of superplastic welding and electro-plastic deformation, was developed in this study.The materials used in this study were 1.6%C-UHCS steel and 40Cr steel, in which the 1.6%C-UHCS was spheroidal annealing treated and the 40Cr steel was heat treated through cyclic salt-bath quenching before electro-superplastic compression or welding. In this study, electro-superplastic welding conditions were selected, based on the influences of additional electric field on microstructure and properties of materials compressed under electric field. The microstructure and properties of electro-superplastic welding 1.6%C-UHCS/40Cr joint were studied, and then the relationship between welding process and performance was investigated. The mechanism of the electro-superplastic welding was clarified.The true stress-strain curves of electro-superplastic compressive deformation for 1.6%C-UHCS and 40Cr steel had superplastic characteristics. The steady flow stresses were lower during electro-superplastic compressive than that obtained under conventional superplastic compression conditions. Moreover, the strain rate sensitivity was larger, the superplastic flow activation energy was lower, the grains of 1.6%C-UHCS were more equiaxial, and the movable dislocation in grain increased, the grain boundary became arc. Namely, the superplasticity of the 1.6%C-UHCS was improved and the rate of material migration increased under compression deformation with electric field. The 1.6%C-UHCS/40Cr electro-superplastic welding could be achieved under an optimum welding parameters level, which electric field intensity +3kV/cm, welding temperature 780℃, welding time 20min and initial strain rate 1.5×10^-4s^-1. In the case of electric field intensity +3kV/cm, welding temperature 780℃, welding time 20min and initial strain rate 1.5×10^-4s^-1, the 545MPa tensile strength (σb) of 1.6%C-UHCS/40Cr electro-superplastic welding joint was obtained, which was larger than the 433MPa that obtained when without using electricity. And such a result reaches 83% intensity of 40Cr under the same thermodynamic circles; the welding rate amplifies from 73% to 82%. It is considered that the additional electric field improves the 1.6%C-UHCS steel's superplasticity.The electro-superplastic welding joint of 1.6%C-UHCS/40Cr could be divided into interface zone, transitional zone and the base zone. The original interface of 1.6% C-UHCS/40Cr electro-superplastic welding joints formed dozens to 300nm interface area. In the interface zone, a few big M₇C₃ and continuous globular or short rod like M₇C₃(20– 150nm) phases appeared, some discontinuous nano Fe-Cr particles and local dynamic recrystallization were found in the joint interlayer. Moreover, this zone included random distribution ferrite and ferrite (α/α), ferrite and carbide (K/α) and carbide and carbide (K/K). Nano Fe-Cr particles increased when additional electric field. The globular carbides in the 1.6%C-UHCS turns into layer flake and short rod ones in the joint transition zones because of the diffusion of Al and C,. There are no such phenomena in normal superplastic welding, however, less and bigger globular carbides are the products. Besides, microstructure changes with increasing of distance from the welding interface, and distributed little ball carbides appear in the near interlayer transition zones, however, flake pearlites grow in far interlayer. A few lower bainite were detected between the two microstructures. The diffusion zone was wider when the electricity was used, which declares that electrostatic field promoted element diffusion. The microhardness of the joint changes near the welding interface were corresponding with the microstructure. Compared with normal superplastic welding, cracks along the grains boundary were greatly reduced in electro-superplastic welding. Additional electric is beneficial to obtain sound joints, becucase the electric field can increase element diffusion and dislocation movement, encourage the diffusion of Al, and accelerate the formation of metallurgic region.