| Dual-phase steel has been widely used in automobile manufacturing industry for its advantages of high strength,thin thickness,high processing hardening rate and low yield ratio.With the development of automobile lightweight,the thickness of automobile steel has been gradually reduced,which also puts forward higher requirements for welding method.The common resistance spot welding,laser welding and common gas shielded welding in automobile manufacturing can not fully meet the development requirements of automobile manufacturing.Cold metal transfer(CMT)is a new type of gas shielded arc welding with very low heat input and no splash during welding,especially suitable for welding of sheet metal.Therefore,it has important theory and application value to study the influence of CMT welding technology on microstructure and properties of dual-phase steel welded joint.In this paper,the influence of welding parameters on the forming,microst ructure and mechanical properties of the welded joints in two kinds of modes,CMT and CMT+P,was summarized by the diameter of 1.0mm ER120-SG we lding wire used to weld the DP980 dual-phase steel with 1.2mm thickness by CMT welding and optical microscope,SEM,EDS,microhardness tester and ten sile tester were employed to test and analyze the welded joints.The welding s peed selects 350mm/min-600mm/min;the wire feed speed selects 3m/min-6m/m in;the arc length correction factor select-25%-+25%.The experimental results show that the welded joints obtained in CMT and CMT+P modes are aesthetically pleasing in the suitable welding parameters and have no obvious defects.With the increase of wire feed speed,the weld depth and weld width increase,and the oxidation area around the weld becomes larger.As the welding speed increases,the weld depth and weld width decrease,and the oxidation area around the weld decreases.With the increase of arc length correction factor,there is no obvious change in the weld depth,and the weld width becomes larger.The microstructure of the weld is martensite and a little bainite.The microstructure of the coarse grain area is lath martensite,bainite and a little ferrite.The microstructure of the fine grain region is martensite and bainite,in which the bainite ratio is higher.Themartensite in the incomplete phase change area is reduced,and the microstructure is more ferrite.The hardness distribution of the welded joints is uneven,and the highest hardness is found in the fine grain area of the heat affected zone(HAZ).In CMT mode,the hardness is about 425 HV,CMT+P mode with a hardness of about 430 HV.The minimum hardness appears in the incomplete phase change area.In CMT mode,the hardness is about 260 HV,CMT+P mode with a hardness of about 250 HV.With the increase of wire feed speed or the decrease of welding speed,that is,the increase of welding heat input,the area of HAZ and grain size will increase.The area of HAZ and the grain size are larger in CMT+P mode.In the tensile test,the welded joints of each welding process are broken in the incomplete phase change zone under two modes.The maximum load is about 13 kN,slightly lower than that of the base material 13.47 kN,which fully meets the requirements of the use.By analyzing the fracture morphology of tensile test,it is found that there are a lot of dimples in the welding joints,which belongs to ductile fracture,but the toughness is lower than the base material.In conclusion,CMT welding experiment of 1.2mm DP980 dual-phase steel can get good appearance and mechanical properties of welded joints.Finally,the optimum welding parameters suitable for the process of this paper are obtained,that is in CMT mode,the welding speed is 500mm/min,the wire feed speed is 3m/min and the arc length correction factor is 0. |
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