| The automotive industry has been making progress in the direction of lightweight as a result of the energy conservation and environmental protection.It can no longer meet the purpose of vehicle weight reduction in the use of traditional single steel products,and the application of a variety of materials is the trend of the future development of lightweight vehicles.Aluminum alloys are widely used in automotive sheet metal parts because of their excellent material properties,but resistance spot welding presents obvious disadvantages in the application of high strength lightweight alloys.As an alternative connection technology of the spot welding,self-piercing riveting?SPR?can effectively achieve the connection of metal plates such as aluminum alloys and heterogeneous plates.However,fractures often occur in the riveting process of the 7075-T6 Aluminum alloys sheet as a result of its high strength and poor formability at normal temperature,which becomes a difficulty in the manufacture of the vehicle body.In present paper,a thermal self-piercing riveting?T-SPR?process has been proposed,which can effectively solve the cracking problem of 7075-T6 aluminum alloy in the riveting process.Based on the T-SPR process,the formability of the T-SPR joint riveted at different temperatures of 7075 aluminum alloy has been studied in this paper,as well as the experimental and simulation studies for mechanical behavior of T-SPR joints under various loading conditions.Firstly,the thermal tensile tests were conducted for 7075-T6 at a strain rate of 0.001 s-1based on the thermal tensile test platform to obtain the rheological behavior at different temperatures,and the temperature range is 20 to 450?.The parameters of the Johnson-Cook model were fitted according to the experimental data,and the numerical model was established to verify the accuracy of the model.In addition,the mechanical parameters of rivets were determined by experiments and simulations.The results show that the strength of7075-T6 aluminum alloy decreases with the increase of temperature,and the elongation after fracture increases first and then decreases with the increase of temperature.The rheological behavior at different temperatures of 7075-T6 aluminum alloy could be characterized well by Johnson-Cook.Subsequently,a thermal self-piercing riveting platform was set up to carry out the riveting process under different temperatures.The section parameters of T-SPR joints riveted at different temperatures were measured,as well as the hardness tests.In addition,the lap-shear tests of T-SPR joints riveted at different temperatures were conducted to obtain the effect of temperature on its mechanical properties.The results show that the formability and mechanical properties of T-SPR joints can be significantly improved with the increase of riveting temperature,and the ideal T-SPR riveting temperature is determined at 400°C.Finally,an improved Arcan experimental device was used to perform the composite loading experiments of the T-SPR joints riveted at the ideal temperature.The effects of loading angle and loading rate on the failure modes and mechanical behavior of T-SPR joints were investigated.In addition,the finite element model was established and compared with the experiment.The results show that the peak load of the T-SPR joint increases with the increase of loading angle when the loading rate is constant,but the corresponding experimental displacement decreases.And there are four failure modes of T-SPR joints with increasing of loading angle.The peak load decreases with the increase of loading speed when the loading angle is constant,but the corresponding displacement increases.In addition,it has been proved that the finite element model can accurately represent the failure mode and mechanical properties of T-SPR joints. |
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