| Currently, environmental pollution and resource depletion have became more and more aggravate. Automobile is one of the main resources of pollution. Automobile energy saving and emission has became the top priority. Reducing the automobile weight was a effective way to achieve this target. But with the strength increasing, the welding property of the steel become worse. In order to promote the application of new steel, a research on the welding property of new steel is necessary and important.In this paper, 700 MPa grade C-Mn microalloyed steel with Nb and Ti was studied. The welding process finished with IPG 6.0 k W fiber laser and gas shield welding machine. For different thickness of plate, laser welding and laser-MIG hybrid welding were used in this study. We analysed the evolution of mechanical properties and microstructure under laser welding and laser-arc hybrid welding. Established the essential relationship between microstructure and properties. We solved the problem of strength and toughness decreased in arc welding, besides, we also provided the necessary theoretical date for the connection of high strength steel. The main work and results were as follow:(1) Microstructure evolution of laser welding, WS and CG-HAZ were mainly consisted of martensite. FG-HAZ and MG-HAZ were mainly consisted of ferrite with different morphology. With the increase of heat input, a little of bainite and ferrite were occurred in WS and CG-HAZ. But has little influence on the microstructure FG-HAZ and MG-HAZ.(2) The study of regulation of mechanical properties of laser welding showed that: the hardness and strength relationship of welded joint was that: weld>HAZ>BM of different heat input. The elastic modulus of the weld is higher than base material, which is mainly related to the precipitates of BM dissolution in laser welding, which leads to the decrease of the distance between the atoms. Weld joint has a great impact toughness which reach the level of base material. With the increase of heat input, the average hardness of weld decreased, the strength and toughness have little changes.(3) Microstructure evolution of laser-arc hybrid welding: The typical microstructure of WS was acicular ferrite; The typical microstructure of CG-HAZ was granular bainite and acicular ferrite; The typical microstructure of FG-HAZ and MG-HAZ were similar with laser welding. With the increase of laser power and welding current, the microstructure of WS transformed from acicular ferrite to acicular ferrite and granular bainite; the CG-HAZ microstructure transformed to granular bainite with acicular ferrite decreased. In WS, the precipitates of alumina and titanium oxide were existed, which benefited to the formation of acicular ferrite.(4) The study of regulation of mechanical properties of laser-arc hybrid welding showed that: the hardness relationship of typical welded joint was that: WS>HAZ>BM. There is no softening zone in HAZ; The tensile fracture position occurred in BM; The impact toughness of WS was equal to BM; while the HAZ toughness 30% lower than BM; The hardness of WS decreased with the increased of laser power and welding current, which has a great effect with the transformation of acicular ferrite to granular bainite.(5) In summary, for the tested steel and experimental equipment, the strength and toughness of weld joint were similar to BM with laser welding. Especially for thin plate such as 4.6mm steel plate in this study. But for thick plate, laser-arc hybrid welding can finished the connecting, such as 8mm steel plate in this paper. The penetration was improved greatly with the strength of weld joint has no decrease, and the impact toughness decreased 30% than BM. But both laser welding and laser-arc welding joint were better than traditional arc welding. |
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