Study On Welding Process And Numerical Simulation Of Ultra High Strength Steel QP980 CMT

With the development of automobile lightweighting,the requirements for steel in the automobile manufacturing industry are increasing day by day.The third generation of advanced high-strength steel QP980 has the advantages of high strength,high plastic toughness and good economic efficiency,which can meet the steel demand of automotive components.At present,cold metal transfer welding(CMT)with its excellent welding characteristics(low heat input,no spatter)has been popularly used in body manufacturing,however,there is a lack of information on the CMT welding process and numerical simulation of QP980,which to a certain extent limits the promotion and application of this steel in auto body manufacturing.Therefore,this paper takes GHS-90 high-strength steel welding wire as filler metal to study the CMT welding process,welding parameter optimization and numerical simulation of QP980 lap welding,which has practical significance and theoretical basis for actual component welding and acceleration of automobile lightweight process.Firstly,the specification interval of each welding parameter(wire feeding speed,welding speed,welding gun angle)was determined through the process test,and based on the response surface(RSM)design test,a mathematical relationship model was established between each welding parameter and the characteristic dimensions of the joint(weld width,weld height,depth of melt),the influence of process parameters alone and pairwise interactions on joint forming was analysed.The results show that the welding speed has a greater influence on the weld forming than other technological parameters,when the wire feeding speed and welding speed are properly matched,a better weld joint can be obtained by choosing a larger welding torch Angle.The applicability of the mathematical model was verified by welding tests with optimized process parameters,and the microstructure and mechanical properties of CMT welded joints were analyzed.The results show that the weld microstructure is mainly lamellar martensite and the hardness distribution is uniform,about 474 HV.The heat affected zone includes the coarse grain zone,the fine grain zone and the incomplete phase transition zone,and the hardness distribution is uneven.The highest value of the joint hardness appears in the fine grain zone of the heat affected zone(503.1HV),while the minimum value of the joint hardness appears in the incomplete phase transition zone(278.3HV);Under the optimized process parameters,the strength of the welded joints is good,the average maximum tensile load reaches more than 90% of the base metal,and all the fractures are in the heat-affected zone,and the fracture forms are mixed fracture of toughness and brittleness.The temperature field and stress field of QP980 CMT lap welding were simulated,it was found that near the heat source loading place,isotherms are symmetrical long elliptical distribution,with the change of time,the temperature field shows a constant rise and fall of alternating changes,in line with the actual CMT ‘hot-cold-hot' welding characteristics,the simulation results are more consistent with the measured thermal cycle curve,verifying the correctness of the model.The distribution law of temperature field with space and the influence law of welding process parameters on temperature field were studied.The results show that the temperature gradient changes greatly along the direction of vertical weld and thickness.The farther away from the weld center,the lower the peak temperature and the longer the time to reach the peak temperature.The lower the welding speed and the higher the wire feeding speed,the higher the welding energy and the higher the overall temperature field temperature.The simulation results of residual stress and deformation show that the residual stress is mainly generated in the cooling stage,and the longitudinal tensile stress is mainly distributed in the welding seam and its two sides,and the distribution law of compression center tension at both ends is presented,the maximum residual stress reaches 802MPa;The overall deformation after welding is small,which is a typical "wave deformation" of thin plate.The maximum deformation is in the middle of the edge of the lap upper plate,showing a concave trend.Compared with the actual postwelding deformation of sheet metal,the deformation trend is consistent,which verifies the correctness of the stress field model and can be used to predict the post-welding deformation and residual stress distribution.