Study On Hot Self-Piercing Riveting Technology And Mechanical Properties Of Riveted High-Strength Steel/Aluminium Joint For Car Bodies

As an important solution to energy saving and emission reduction in the automotive industry,light weighting has always been one of the focuses of automotive engineering science and technology.Using high-strength steel and aluminium alloy car bodies can not only ensure the passive safety for cars,but also effectively reduce the weight of car bodies.However,the most important problem restricting the mixed application of the two materials is the connection technology between them.This is mainly because the high-strength steel has high strength but poor plasticity,and the conventional welding or riveting techniques cannot achieve high strength connection between the two materials.According to the phase transition theory that the yield strength of the high-strength steel will be greatly reduced in the high-temperature austenitic state and the plasticity will also be significantly increased at the same time,which provides the feasibility for hot self-piercing riveting technology between high-strength steel and aluminum.The forming high-strength steel/aluminium joint can be reduced to the martensite structure through rapid cooling and keep its mechanical properties.The hot self-piercing riveting technology of high strength steel/aluminium alloy for car bodies was presented in this paper.The 22MnB5 high-strength steel plate rivetingzone was heated to be austenitized firstly,and then it was riveted with the 7075 aluminium alloy plate,and at last the forming joint would be rapidly cooled to normal temperature.The influences of forming temperature,plate riveting sequence and rivets of different materials and heat treatment processes on hot self-piercing riveting joint cross section macroscopic morphology were analyzed through a large number of experiments.The results showed that the effective riveting connection between 2mm 800℃ high-strength steel and 2mm room temperature aluminum plate could be realized when using 42 CrMo rivet(the heat treatment process was normalizing at 850℃ for 30 minutes and water quenching)and the riveting sequence was first aluminum and later high-strength steel.The hot deformation damage constitutive model of the high-strength steel plate,aluminum plate and rivet were established in this paper to simulate the hot self-piercing riveting process and formulate the optimized process plan.Based on this constitutive model,the finite element simulations of the hot self-piercing riveting process and mechanical properties of the hot self-piercing riveting joints were carried out,and the accuracy of the simulation models were verified by comparing with the actual experiment results.By using the verified finite element models,the punching pressure and dynamic changing process of sheet metal stress and strain during hot self-piercing riveting process were studied.And the influence trends of die size,rivet size and other main process parameters on the static tensile mechanical properties and hot self-piercing riveting joints cross section size(self-locking value,bottom thickness minimum residual thickness)were analyzed and the corresponding optimization interval of these parameters were determined.According to the optimal size of the selected process parameters,the orthogonal experimental design(DOE)was carried out,and the hot self-piercing riveting cross section size and the tensile load under two static tensile conditions were made as the evaluation criteria.Finally,through finite element simulation the optimal hot self-piercing riveting process scheme was analyzed and formulated.In addition,the tensile fatigue properties of the hot riveted joints optimized by the DOE method were tested in both radial and axial directions,and the F-N curves in both directions were fitted.The fracture mode of the radial and axial tension fatigue samples were studied and it was found that the fracture of the hot self-piercing riveting joints were all presented on the aluminium plates,and the fracture had a great relationship with the stress concentration generated during the riveting processing.In a large number of fatigue experiments,it was shown that when the stamping speed was reduced to 40 mm/s,the stress concentration would be reduced and the fatigue ultimate loads on the radial and axial directions of the joints could be increased 8.68% and 17.19%,respectively.The fatigue test showed that the fatigue source of all hot self-piercing riveting joints did not directly originate on the rivet forming area,and the fatigue performance was good.Finally,the temperature changes of the hot self-piercing riveting joint and heat affected zon of the plates during hot self-piercing riveting process were simulated and analyzed by ProCAST software,and the joints’ cross section microstructure were detected by SEM.The SEM results were consistent with the microstructure transformation simulated by temperature field,which verified the accuracy of the simulation.It was found that the microstructure of high-strength steel at the hot self-piercing riveting joints was still martensite structure after heating and cooling within the central radius of 10 mm.And there was a continuous microstructure change within the radius of 10-32 mm,from the near to far sequence was incomplete quenching,high tenperture tempering,mediumtemperature,low temperature tempering and no effect.The rivet was only partial annealing at the pin foot in the whole process,and the other parts did not change.The aluminum plate stayed the same and did not be affected by the heat condution.From the fracture mode of hot self-piercing riveting joints mechanical property tests,it could be seen that heating and cooling had little influence on the mechanical properties of hot self-piercing riveting joints,the hot self-piercing riveting process could achieve the expected connection effect.