| 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. |
PDF Full Text Request