| As problems of the energy crisis and environmental pollution continue to intensify,the automotive industry is facing more and more serious challenges.The results show that vehicle quality reduces by 10%,fuel consumption decreases by 6% to 8%,emission falls 4%.Therefore,the automotive lightweight is an effective way to save energy,reduce emission,and improve fuel efficiency.Ultra-high strength steel and magnesium alloy are major lightweight materials,and increasing use of ultra-high strength steel and magnesium alloy is an effective way to achieve automotive lightweight.However,due to the significant differences between steel and magnesium in chemical composition and physical property,the weldability of steel and magnesium is extremely poor,which deteriorates the mechanical property of the joint seriously.It has become one of the key scientific and technical problems that restrict the development of lightweight technology.Hence,the research of this paper has importantly theoretical and practical values.In this paper,the microstructure and mechanical behavior of MIG welded joints of SYG960 E ultra-high strength steel and AZ31 B Mg alloy are studied.And the effects of welding parameters and alloy elements on microstructure and mechanical property of MIG welded joint are investigated.The results show that the MIG welded joint with AZ31 B Mg alloy welding wire has welding-brazing characteristics,and the joint mainly includes weld zone,interface zone and bond zone.The weld zone is mainly composed of ?-Mg and ?-Mg+?-Mg17Al12 eutectic.The interface zone has obvious two-layer structure characteristic: the interface layer neared to the steel mainly consisted of AlFe3,closed to the magnesium weld is the layer of Mg(Fe,Al)O4+Mg3.1Al0.9.And the growth mechanism of the interface layer is discussed.The ?-Mg grains in the bond zone are coarsened.The microhardness distribution of the MIG welded joint of steel and magnesium is uneven,and the lowest microhardness occurs in the bond zone because of the coarsening of ?-Mg grains.The microhardness of the interface zone is highest(440.0HV),mainly due to the presence of brittle intermetallic compounds and metal oxides.Under the action of tensile stress,the joint fracture mainly occurs in the interface zone;consequently,the interface zone is the weakest area of the MIG welded joint of ultra-high strength steel and magnesium alloy.The influence of welding parameters on microstructure and mechanical property of the MIG welded joint is investigated.It is helpful to improve the MIG weld formation by selecting proper groove angle and welding wire position.With the increase of welding current,?-Mg grain coarsens in the weld zone,the thickness of interfacial layer increases,incomplete joining defects reduce and crack sensitivity increases.With the increase of welding speed,?-Mg grains are refined obviously in the weld zone,the thickness of interfacial layer decreases,local incomplete joining defects increase.The effects of welding current and welding speed on the mechanical property of the MIG welded joints are mainly related to the changes of the welding heat input and the microstructure of the interface zone.When the welding heat input is 1987-2100J/cm,the tensile strength(171.0MPa-174.0MPa)of the MIG welded joint of ultra-high strength steel and magnesium alloy can be obviously improved.The alloy elements have a significant effect on the microstructure and mechanical property of the MIG welded joint of ultra-high strength steel and magnesium alloy.The increase of Al content(6.24%)in the magnesium alloy weld by AZ61 welding wire is helpful to improve the wetting and spreading of liquid magnesium alloy,promote the diffusion and interfacial reaction of Al,reduce the incomplete joining defects in the interface zone and improve the mechanical performance.The joint tensile strength increased to 201.0MPa,which is 83.8% of the AZ31 B Mg alloy base metal.Thus,the proper increase of Al content is beneficial to improve the mechanical property of the MIG welded joint of ultra-high strength steel and magnesium alloy.The strength of the MIG welded joint with Zn addition is obviously reduced,which is mainly related to the increase of MgZn2 intermetallic compound in interface zone.Cu is able to reduce the melting temperature of Mg alloy,promote the wetting,spreading and interfacial reaction of liquid weld metal,reduce the incomplete joining defects;therefore,the mechanical property of joints is improved obviously.The tensile strength of joint added the Cu interlayer with the thickness of 100?m(194.8MPa)is 81.2% of AZ31 B Mg alloy base metal.Consequently,the addition of appropriate amount of Cu is beneficial to improve the mechanical property of MIG welded joints of ultra-high strength steel and magnesium alloy. |
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