| Twin induced plasticity(TWIP) steel get widespread concern of the materials research community because it’s excellent combination of strong plastic performance, more than 10 years of research shows that TWIP steel has broad application prospects in the automotive industry as a structural material, but its widespread application is inseparable from advanced welding technology. Laser welding because of its high energy density, small deformation, green processing and other advantages to become one of the advanced welding technology in the 21 st century. The study about the microstructure evolution and mechanical behavior of TWIP welded joints will have important theoretical significance and application value. But so far the research about the microstructure and properties of TWIP welding joint relative to the material itself is very limited. This paper made comparison and analysis of laser welding and TIG welding method of TWIP steel performance, and studied about the tensile load, fatigue load, impact load and stress corrosion environment of organization evolution and change in performance of TWIP steel laser welded joint under different strain rate. The main contents and conclusions are as follows:TIG welding and laser welding method were used for TWIP steel to analyze the microstructure and mechanical properties of the joint, the results showed that: the grain of the laser welding head welds is fine, coarse grain area don’t formed, TIG welding head weld area consists of coarse columnar crystals, which formed a significant HAZ. Since vaporization in laser welding process leads to Mn content of the weld decreased by almost 7%, two welds are the austenite phase detected by XRD. Tensile tests showed that the incidence of fracture laser weldments in the base material, TIG welding head cracking along the fusion line; Relative to the parent metal, the tensile strength and elongation of the joint decreases, but the intensity and toughness of the laser welding head were significantly better than that of TIG welding head. After deformation, deformation twin were observed in two kinds of joint in the weld, showed that the weld deformation mechanism is for sliding and twin interactions, but because no orientation, the quantity of deformation twin is far less than the parent metal after deformation, the deformation twin formed more in the laser weld; parent metal and joint are both the microporous joint ductile fracture.The influence of tensile speed for the mechanical properties and the effect of deformation and fracture behavior in laser welded joint of TWIP steel was studied. Results show that the TWIP steel material and laser welding joint have the lower yield strength, high tensile strength and good elongation; With the increase of strain rate, the yield strength significantly increased of the parent metal and laser welded joint, the change of tensile strength was not significant, elongation was on the decline; Deformation twin formation were observed of the parent metal and weld under all the deformation rate, and the amount of deformation twin increases with the decreasing of strain rate; Weld deformation twin in both quantity and size is smaller than the parent metal, the grain density of deformation twin near solidification grain boundary is larger; TWIP steel parent metal and weld fracture are microporous connection type of ductile fracture, fracture along longitudinal welding occurred in the welds, weld at fracture under normal stress and shear stress at the same time.Test about impact performance of TWIP steel material and laser welded joint from-196 ℃ to room temperature was studied, deformation and fracture behavior of parent metal and joint were analyzed under impact load. Results show that under all the shock temperature, TWIP steel all showed good impact toughness, fracture consisted by crater shape big toughening nest and small toughening distribute in the torn edge of big toughening nest; Shock absorption function of TWIP steel, deformation in the process of deformation twin both quantity and fracture morphology change with temperature impact are not obvious. The impact crack of the TWIP steel laser joint in the extension process by deflection to the parent metal, the morphology of welding seam weld zone impact fracture is smaller close toughening, as the impact of temperature drop, toughening nest becomes shallow depth; TWIP steel laser welding head in-117 ℃ to room temperature have better impact toughness, small number of deformation twin formed in the process of deformation; Under the impact in-196 ℃, a large amount of deformation twin formed in grain, but fracture of the weld zone of appear intergranular fracture morphology caused by brittle phase from(Fe, Mn)3C, lead to the toughness of joint impact is far lower than the other material and temperature of the weld.The fatigue performance and fatigue crack initiation and propagation characteristics of TWIP steel material and laser welded joint were studied. Results show that the fatigue strength of TWIP steel is 252 MPa, the laser joint fatigue strength of 211 MPa, which is 83.7% of the parent metal; Joint material and fatigue fracture are including the source area of crack, crack spreading area and transient breaking zone, and the fatigue crack propagation are in line with the Paris formula; The fatigue crack of the parent metal begins in the face of the near surface defects, and weld crack source are in the discontinuous points of weld toe; The base material and interface extensions and final off areas are appeared typical fatigue strips and tear dimple; Due to lower weld toe elastic modulus and hardness, fatigue crack from the weld joint to weld toe and go along the weld toe extension.As for the stress corrosion behavior of parent metal of TWIP steel material and laser deposition thing in 3.5 % NaCl and pH=5.5 H2SO4 solution. The results show that the parent metal and deposited material appear sensitivity to the stress corrosion of the NaCl solution than H2SO4 solution, the stress corrosion index of the deposition thing in two kinds of corrosive medium is lower than the parent metal; TWIP steel in both etching solution are anodic dissolution type corrosion, but corrosion processes are different: NaCl solution firstly occurs serious pitting in surface direct anodic dissolution; TWIP steel in H2SO4 solution firstly occurs general corrosion, stress-induced material microporous in the surface connected forms cracking, then appears anodic dissolution. Both corrosive media, the degree of corrosion of the weld is more serious than the base metal, due to the presence of residual stress of welding, stress corrosion cracks are formed and extended in the joints; laser welding process causes Mn, C occurs at grain boundaries in the weld branches, which increased the corrosion of extent joints. |
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