| With the gradual improvement of people's environmental protection awareness and the national clean energy policy,automobile lightweight has become the main direction of the development of the automobile industry.Using high-performance lightweight materials to replace steel materials can effectively reduce the weight of the car body and reduce fuel consumption.7000 series aluminum alloy has high specific strength and good corrosion resistance,making it an ideal lightweight material for automobiles.However,the 7000 series aluminum alloy has poor formability at room temperature.Although increasing the forming temperature can improve the forming performance of the material,the high-temperature forming will cause the material strength to decrease.Based on this,this paper proposes a 7075 aluminum alloy pre-cooling and hot stamping process,which effectively solves the defect of poor forming performance of aluminum alloy at room temperature,and ensures the strength and hardness of the aluminum alloy.In this paper,the following researches are carried out on the influence of the critical cooling rate and forming temperature on forming performance and structure in the pre-cooling hot stamping process:1.The quenching sensitivity of 7075 aluminum alloy was studied by using the graded quenching method,and the hardness-isothermal time relationship curve of the material after aging was obtained,and the TTP curves of 90%,95%,and 99.5% hardness in the T6 state were drawn.The cooling curve and hardness change of 7075 aluminum alloy under three different pre-cooling paths of air cooling,mold cooling,and water cooling were measured,and the influence of different cooling rates on the hardness of the material was analyzed in combination with the TTP curve.Obtained 7075 aluminum alloy in the quenching sensitive temperature range,reaching the critical cooling rate above 95% T6 hardness.The quenching factor method was used to predict the performance of 7075 aluminum alloy under three different pre-cooling paths: air cooling,mold cooling,and water cooling,and the quenching factor time step value range under different cooling rates were analyzed.2.Conduct uniaxial hot tensile experiments of 7075 aluminum alloy with different tensile temperatures and strain rates,and study the relationship between the true stress-strain curve and different tensile temperatures and strain rates.The strain hardening index at different temperatures and strain rates is obtained by fitting,and the relationship between the strain hardening index and the forming temperature and strain rate is analyzed.A comprehensive evaluation of the effects of strain hardening index,tensile strength,and elongation at the break on the forming performance of the material,and obtaining the forming temperature that guarantees the optimal forming performance of 7075 aluminum alloy.3.The plate pre-cooling experiment was carried out by developing a pre-cooling device with a controllable cooling rate,and the corresponding pre-cooling rate under different airflow rates were obtained.Combining the cooling curve and TTP curve of the solid solution-pre-cooling-plate mold quenching experiment of the sheet metal,the pre-cooling rate and the forming temperature of the sheet material reaching 90% and 95% T6 hardness are obtained.The pre-cooling rate is 7.3?/s and 12.5?/s before and after aging of the plate material to observe the microstructure,analyze and compare the influence of different pre-cooling rates on the microstructure and properties of the material.4.The pre-cooling and hot stamping experiments were carried out through the self-developed U-shaped mold,and the influence of the two pre-cooling paths of 12.5?/s and 97.2?/s on the forming performance and hardness of the U-shaped part was studied.By measuring the different areas of the U-shaped part The thickness and hardness changes after aging,and the microstructure of the top surface of U-shaped parts with different pre-cooling rates at the same forming temperature is observed.Through comprehensive analysis,the optimal U-shaped part pre-cooling and hot stamping process are obtained.5.A material model of 7075 aluminum alloy was established.Based on the thermal-mechanical coupling finite element analysis model of the U-shaped part pre-cooling and hot stamping process,the temperature change of the U-shaped part in the pre-cooling and hot stamping process was gradually analyzed through numerical simulation.The temperature-time curve of the top surface,sidewall,and flange area after solid solution,pre-cooling,forming,and holding pressure is combined with the aluminum alloy TTP diagram,which explains the U-shaped parts after aging in different areas in the pre-cooling and hot stamping experiment The reason for the difference in thickness.By comparing the equivalent stress,equivalent strain,and thickness evolution results of U-shaped parts at different forming temperatures,appropriate process parameters were determined,and the forming temperature for 7075 aluminum alloy with optimal forming performance was verified. |
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