Research On Pulsed Plasma Arc Welding Technology Of High Strength Steel For Vehicle

Lightweight is the trend of developing the car, it is an important measure to reduce emissions and to protect the green environment. The use of ultra high strength steel is an important support to ensure lightweight. Pulsed plasma arc welding has the advantages of high concentration of energy, precise control of heat input,low requirement of environmental variables, so it has good adaptability to the super high strength steel for the vehicle with a thin dimension. For this purpose, this paper takes the cold rolled dual phase steel DP780 and hot forming ultra high strength steel B1500HS as the research object, butt joints were used in the form of welding. The effects of different processing parameters on the micro structure and hardness distribution of B1500HS and DP780 steel butt joints and dissimilar steel butt joints were studied, the effect of different parameters on the tensile and flexural properties of the joints were studied too. For serious softening phenomenon of B1500HS, back water measure was taken to research the effect of accelerated cooling rate on microstructure and mechanical properties of welded joint.The study on butt weld joint of DP780 showed that the macro morphology of the weld is changed with the increase of the pulse frequency in a certain welding speed and the weld joint could not well formed with too low or too high frequency. The lowest value, tensile strength and bending strength of the joint are improved with the increase of the pulse frequency under different duty cycle; the failure position of the joint under tensile test and bending test were all occurred in the sub critical heat affected zone of HAZ. The best mechanical properties of the joint were obtained under the duty cycle was 30% and the pulse frequency was 7HZ, the tensile strength and bending strength of the joint were all above 800MPa. The fracture of the joint from ductile fracture to quasi cleavage fracture mode as the pulse frequency increased in a certain range of pulse frequency.The study on butt weld joint of B1500HS showed that the weld zone, the coarse grain and fine grain belonging to heat affected zone were mainly dominated by martensite, severe tempering softening occurred in the sub critical heat affected zone with the peak temperature of the thermal cycle below Acl, the lowest hardness value was 280HV, only about 60% of the base material. The failure position of the joint tensile test appeared in the softening zone of the heat affected zone under the condition of air cooling, the tensile strength of the joint appeared to decrease with the increase of duty cycle and basal pulse ratio. The maximum tensile strength value appeared in the sample with 30% duty cycle and a base pulse ratio of 0.23, the tensile strength is about 1000MPa. When the back water measure was taken under the HAZ zone, the failure position of the joint appeared in the weld zone and the tensile strength increased about 200MPa. In the bending test, joints were not cracking in the bending load under air cooling condition, but fracture failure occurred after bending of the joint to a smaller angle under the condition of water cooling. The bending strength of weld joint could reach to 1000MPa.On the whole, the appropriate process parameters under the condition of air cooling could obtain excellent comprehensive mechanical properties of welded joints.The study on dissimilar weld joint between DP780 and B1500HS showed that excellent dissimilar steel butt joint could be obtained with pulsed plasma arc welding and the micro structure of the weld zone was mainly composed of martensite and partial bainite.The hardness distribution was uneven and the heat affected zone on both sides of the weld is tempered and softened, besides, the B1500HS side had serious softening.The failure position of tensile and bending test appeared in the HAZ of DP780 under different duty cycle and the tensile and bending strength of the joint decreased with the increasing of the basal pulse ratio.The best results of the mechanical properties of the joint appeared under the parameter of the duty cycle was 30% and the base pulse ratio was 0.22. The tensile strength and bending strength were all above 800MPa.