【作者】 刘勇；

【导师】 张宜生；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2018， 博士

【摘要】 随着人们对节能减排和汽车轻量化的日益重视,越来越多的铝合金被用于制造汽车车身零件。为了解决铝合金板料成形中的破裂、回弹及热处理过程中的热变形问题,可热处理强化铝合金板料的热冲压技术应用而生,且处于初步研究阶段。但固溶、时效时间长,零件表面易损伤等问题限制了该技术在工业化生产中的应用。为了解决上述问题,本文以高强铝合金6061和7075板料为研究对象,针对其高温流变行为及本构模型、热冲压生产时的高效固溶和时效工艺及流程控制、热冲压成形过程中的高温摩擦和润滑行为及摩擦系数测量、全尺寸复杂铝合金零件的热冲压成形工艺及破裂机理等方面展开了研究。基于热冲压成形性研究及数值模拟的需求,采用高温拉伸实验获得了6061-T6和7075-T6两种铝合金的应力-应变曲线,分别使用Cowper–Symonds和应变修正的Arrhenius本构模型对其高温流变行为进行了表征,并分析比较了两种本构模型对流变应力的预测能力。固溶温度和时间是铝合金热冲压中的重要参数,因此研究了不同固溶温度和时间对6061和7075铝合金力学性能的影响,确定了两种铝合金的固溶温度和时间。基于有限差分法对6061铝合金的固溶过程进行了数值模拟,结合理论分析和实验验证计算结果的合理性和可靠性,为铝合金固溶工艺优化提供参考和指导。在上述基础上提出了铝合金的高效固溶工艺,利用在铝合金表面喷涂BN等方法提升了铝合金表面的发射率(吸收率),使得铝合金固溶加热时间缩短到3～5min;并且喷涂的BN在成形过程中可以当做润滑剂使用,大大提升铝合金热冲压过程中的成形性。基于缩短时效时间和提升铝合金热冲压生产流程连续性的需求,研究了6061和7075两种铝合金的停放效应;结合热冲压工艺及汽车工业中的烤漆处理对6061和7075铝合金进行了热冲压时效优化研究;并在上述基础上研究了铝合金热冲压生产的流程控制。最终6061铝合金热冲压后的预时效处理缩短到5～10min;7075获得不低于95%的供货态T6强度时时效时间缩短到1～6h,缩短为原来的4.2%～25%;高效率热冲压工艺提升了6061和7075铝合金热冲压生产流程的连续性,降低了设备投入和能耗。板料与模具之间的摩擦磨损是铝合金热冲压生产中不可忽视的问题,直接影响产品质量和模具寿命。使用高温摩擦试验机UMT-Tribolab研究了6061铝合金在不同法向载荷和温度下的摩擦行为,测量了摩擦系数。在此基础上,为了更加真实模拟铝合金热冲压过程中板料与模具之间的摩擦磨损行为,利用课题组自行研发的高温摩擦试验机研究了6061铝合金在不同温度和润滑条件下的摩擦行为,测量了摩擦系数。综合两种实验装置的测试结果,阐明了铝合金热冲压过程中的摩擦磨损机理,获得了干摩擦和润滑条件下的摩擦系数,为铝合金热冲压的数值模拟和实际生产工艺提供了参考。为了研究铝合金热冲压生产中的成形性及润滑,采用6061-T6和7075-T6铝合金在工业生产线上热冲压成形了全尺寸的B柱,并结合热拉伸实验、摩擦实验和有限元数值模拟阐明了铝合金热冲压过程中零件破裂的原因,提出了避免铝合金在热冲压过程中破裂的措施,为工业生产提供指导。更多还原

【Abstract】 Due to the increasing attention to lightweight of automobile and energy-saving & emission reductions,more and more aluminum alloys are used to make car body parts.Hot stamping process has been developed for heat-treatable aluminum alloy forming,which can improve the formability of the alloy sheet,overcome the springback of the part and avoid thermal distortion of formed part during heat treatment,and this technology is in its infancy.However,longtime solution heat treatment(SHT)and aging of aluminum alloys and the poor surface quality of parts are still challenges,which have limited industrialization of aluminum alloys’ hot stamping.In order to solve these problems,in this thesis,based on the high strength aluminum alloy sheet 6061 and 7075,their flow behaviors and constitutive models at elevated temperatures;high efficient solution and aging in hot stamping production process and process control;friction and lubrication behaviors at elevated temperatures during hot stamping process and the measurements of friction coefficients;hot stamping process of full-size complex aluminum alloy part and its crack mechanism were studied.Due to the requirement of formability in hot stamping and numerical simulation,stress-strain curves of 6061-T6 and 7075-T6 aluminum alloys were obtained based on hot tensile tests,and further modified Arrhenius model and Cowper–Symonds model were developed to characterize the material flow behaviors of the two aluminum alloys at elevated temperatures.Finally,the flow stress predictive abilities of the two constitutive models were discussed.SHT temperature and time are important parameters in hot stamping of aluminum alloy,so the effects of SHT temperature and time on the mechanical properties of 6061 and 7075 aluminum alloys were investigated,and then the SHT time and temperature were determined.The numerical simulation of the dissolution process of the precipitate during SHT in 6061 aluminum alloy was carried out by finite difference method,and the rationality and reliability of the simulation results were verified by theoretical analysis and experiments;it can provide reference and guidance for the optimization of SHT process in hot stamping.Furthermore,the high efficient solution process,which can improve the surface emissivity(absorptivity)of aluminum alloy by spraying BN,was developed based on the above research,and thus the heating time of SHT was greatly shortened to 3～5 min;besides,the sprayed BN can be used as lubricants in forming process,greatly improving the formability of aluminum alloy in hot stamping process.Due to the demand of shortening aging time and improving the continuity of the hot stamping process,the keeping effect of 6061 and 7075 aluminum alloys were studied;combining with hot stamping process and paint baking process in automotive industry,pre-aging or aging treatment of 6061 and 7075 was optimized;furthermore,the process control of hot stamping production of aluminum alloy was studied.Finally,the pre-aging treatment of 6061 aluminum alloy after hot stamping was shortened to 5～10 min;the aging time of 7075 was shortened to 1～6 h when more than 95% of the as-received T6 condition was obtained,which was reduced to 4.2%～25% of the standard aging;thus the continuity of hot stamping process of the two aluminum alloys was improved due to the high efficient hot stamping process and the equipment investment and energy consumption was reduced.Friction and wear between the sheet and tools in hot stamping production of aluminum alloy cannot be ignored,which directly affects the product’s quality and the tools’ life.The friction behaviors of 6061 under different loading force and temperatures were studied by UMT-Tribo Lab high-temperature tribometer and the friction coefficients were also measured.In order to simulate the friction and wear behaviors between the sheet and tools in hot stamping more realistic,the friction and wear behaviors of 6061 under different temperatures and lubricated conditions were studied by a high-temperature tribometers developed by our research group,and the friction coefficients were measured at the same time.Combining the results tested by the two tribometers,the friction and wear mechanism during hot stamping process of aluminum alloy was illuminated,and the friction coefficients under dry friction and lubricated conditions were obtained,which can be used as references in hot stamping production process of aluminum alloy and its numerical simulation.In order to investigate the formability and lubrication during hot stamping of aluminum alloy,hot stamping experiments of B-pillars with full-size using 6061-T6 and 7075-T6 were carried out in a production line;further finite element simulations with the implementation of Cowper–Symonds constitutive equation and the tested COF values(as references)were used to investigate the crack mechanisms that arise in hot stamping;finally the measures which can avoid cracking during hot stamping of aluminum alloy were put forward to provide a guide for the forming of aluminum alloy parts with complex geometric shape by hot stamping process in industry.更多还原