Research On Resistance Spot Welding Process And The Microstructure And Properties Of TRIP980 AHSS Joints

With the development of China automobile industry,energy consumption,safety and environmental problems have been attracted people's attention gradually.Advanced high strength steel is the material that can ensure lightweight and improve automobile safety performance.The production and application of AHSS prospect have a wide prospect in our country's automobile industry.Resistance spot welding is widely used in the field of automobile industry.Usually,a regular passenger car has about 4000 welding joints,and the strength of these joints in the quasi-static,impacting and fatigue loading conditions directly affects the service life and safe performance of the automobile.As an important material of lightweight,welding performance of TRIP980 is an obstacle to its wide application.It is advantage to study the welding process and microstructure properties of the steel to promote the application of this material.(1)This paper optimized the process of resistance spot welding of TRIP980 steel,and got its welding lobe,through single pulse spot welding experiment on TRIP980 steel.The optimum welding parameters were:welding current was 7kA,welding time was 24cyc,and electrode force was 4kN.(2)The effects of welding parameters on micro-hardness of nugget zone were studied.It was found the lath martensitic got coarser and micro-hardness got lower in the nugget zone with increasing amount of heat input.When the heat input was very large,there would be splash defect,which could change the amount of heat input to increase the micro-hardness of nugget zone.The micro-hardness of the nugget zone was the lowest when the welding current was 7kA,the welding time was 24cyc,and the electrode force was 4kN.(3)The effect of single pulse currents on the impacting shear performance of TRIP980 steel spot welding joint was studied.Impacting peak force and impacting absorbing energy were the highest when the welding current was 7kA,the welding time was 24cyc,and the electrode force was 4kN.(4)The tempering process had an effect on the shear impacting performance of TRIP980 steel resistance spot welding joint.It was found impacting peak force and impacting absorbing energy were the highest when tempering current is 5kA-40cyc in the experiment,which increased over original process by 20.76%and 34.25%.Through analysis of the impacting fracture,it was found that the specimen with the tempering process 5kA-40cyc was ductile fracture with many dimples,and the specimen with the tempering process 4kA-40cyc 3kA-40cyc were mixed fracture with a few dimples,and the specimen with the tempering process 6kA-40cyc and 9kA-3cyc were brittle fracture.(5)The tempering process had an effect on the peeling impacting performance of TRIP980 steel resistance spot welding joint.It was found impacting peak force and impacting absorbing energy were the highest when tempering current was 5kA-40cyc in the experiment,which increased over original process by 23%and 73%.The specimen with the tempering process 4kA-40cyc had poor performance in the tensile shear experiment,but it had good performance in the peeling impacting experiment,which meant that the mechanical properties of the joints could be affected by different conditions of loading.