| The continuous development of the automobile industry brought serious environmental and safety problems, automotive lightening has become people’s consensus. The laser tailor-welded sheets both can reduce vehicle weight and can guarantee strength, meet the requirements of lightweight. With good strength and toughness, the dual phase steel was used widely in advanced high strength steel. And the usage of TRIP steel was also increasing gradually. The application of advanced high strength steels will promote the development of laser welding technology. It can combine the steel sheets freely, improve the material utilization, and expend the application range and potential of advanced high strength steels.This paper presents the laser welding the same DP steel and the dissimilar DP & TRIP steel respectively. Study the effects of welding line energy on microstructure and properties of welded joints, and the corrosion resistance of TWB by test of electrochemical performance. Analysis the forming mechanism of weld interfaces deeply, and research on orientation relationships between the solid state phase transformation grains and the original grains.Firstly, the effect of welding line energy on microstructure and property of laser welding joints of DP590 steel with the thick of lmm was studied. Reach a range of welding line energy was 280 J/cm-560 J/cm. There was no obvious effect on mechanical properties of the welding line energy. The tensile failure always existed at base area and the elongation decrease obviously. The average hardness of weld zone was more than 1.6 times that of the base metal, the weld width was increased with the increase of weld line energy, and the soft zone was existed all the time. The microstructure of weld zone was mainly lath martensite, and it will became much coarsened because of the austenite growing in welding of high temperature with the increase of welding line energy. The microstructure of heat-affected zone was martensite and ferrite. The martensite volume fraction increased when the welding line energy rose. The effect of welding line energy on soft zone in HAZ was mainly tempered martensite increased and ferrite grew up obviously. The corrosion resistance in surface of weld zone was worse and the corrosion rate was higher than base metal. Anode parts of polarization curve all occurred reaction platform, the main reason was that there was a passivation film produced in surface. The corrosion resistance of joints was influenced by the microstructure. The lath martensite of weld had large dislocation density and the carbon gathered in grain boundary easy to let the chloride ions absorbed on metal surface increasing the corrosion tendency. The microstructure in base metal of DP steel has uniform refined grains and low-defect, decrease the corrosion tendency of material surface.Secondly, the effect of welding line energy on microstructure and property of laser tailor-welded sheet joints of DP590 and TRIP590 steels with different thicknesses was studied. Reach a range of welding line energy was 280 J/cm-560 J/cm. The tensile test fracture in DP steel side, the average hardness of weld zone was 1.6 times more than that of base metal. Weld width was largest on the welding line energy of 420J/cm. The weld zone presented a typical columnar crystal, and the microstructure was obvious bath martenite. The lath martensite in TRIP steel side was longer than the DP steel side, the higher the welding line property, the coarser the bath martensite. The microstructure on both sides of the HAZ was martensite and ferrite. The grains became coarser and the martensite content increased with the increasing of welding line property. The electrochemical performance test results showed that the corrosion resistance of TRIP steel was best and that of weld was the worst. Also the anode parts of polarization curve even occurred reaction platform.Finally, the formation mechanism of the weld interface microstructure was further explored from analysis the joints of dissimilar steels by EBSD. The result showed that the grains of two kind steels had no significant preferred orientation. The grains of weld had obvious preferred orientation, and the grain texture mainly was<001>. The grains in both sides of HAZ were equiaxed grains principally. And there was no obvious preferred orientation. The dislocations strengthening of martensite bath in weld zone had played a major part for the lower of plastic. The high hardness of HAZ has relation to the pinning effect of grain boundary. Lattice distortion of high-angle and low-angle boundary prevents the plastic deformation of metal, so the base metal showed good strength and toughness on macro.In conclusion, it can gain optimum properties when the welding line energy was 280J/cm to 560J/cm for DP590 steel with the thick of lmm. And to DP590 and TRIP590 steels with different thicknesses, it can gain optimum properties when the welding line energy was 280J/cm to 560J/cm. The results showed that the mechanical properties and corrosion resistance of tailor-welded sheets had no obvious distinction with two kinds of base metal. So the tailor-welded sheets can almost working in all the environment of the base metal application. The research can lay a good experimental and theoretical foundation for the industrial application of tailor-welded sheets. |
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