| In order to cope with the environmental pollution and energy crisis brought about by China's growing automotive industry,the lightweighting of cars can effectively reduce fuel consumption and CO2 emissions while reducing body weight.At present,in the selection of materials for automotive parts,the research and application of aluminium instead of steel is more widespread.The 7075 aluminium alloy has high strength and hardness,and its low density and light weight can reduce fuel consumption,so it is becoming the main material used for automotive components.In order to obtain parts with high forming accuracy and complex shapes,7075 aluminium alloy sheets can be formed using hot forming technology to effectively overcome the disadvantages of poor room temperature forming performance.However,in the high-temperature forming process,the poor frictional behaviour will make the sheet adhere to the mould after softening at high temperatures,which will more easily cause defects such as part breakage under higher frictional conditions,which not only reduces the forming quality of the part,but also increases the wear and tear of the mould and production costs.Translated with www.Deep L.com/Translator(free version)In order to investigate and solve the influence of friction on the forming quality during the high temperature forming process of aluminum alloy,we designed the force-displacement data collection device for the forming process of sheet material,and conducted the hot stamping and forming experiments for the U-shaped parts without lubrication and coated with boron nitride,graphite and molybdenum disulfide lubricants respectively,and analyzed the influence of different lubricants on the force-displacement curve,surface quality,thickness thinning and inflow of sheet material of the formed parts.The pattern of different lubricants on the force-displacement curve,surface quality,thickness thinning and inflow of sheet was analyzed.A finite element model of hot stamping of 7075 aluminum alloy U-shaped parts was established,and the inverse method of friction coefficient was proposed,which was obtained by fitting and optimizing the friction coefficient through finite element simulation combined with experimental data,and the accuracy of the inverse friction coefficient was verified by experimental results.Finally,the influence of the friction coefficient on the forming quality is analyzed from the material forming temperature field,stress field and strain field.The main research findings are as follows.(1)The maximum force during the forming process of unlubricated parts is 66.9KN,the maximum forming force during the forming process of parts coated with boron nitride and graphite lubricant is similar,30.5KN and 30.12 KN respectively,while the maximum forming force of parts coated with molybdenum disulfide lubricant is 26.1KN.From the point of view of the surface quality of the formed parts,the unlubricated parts have serious hair pulling phenomenon,which will produce a large number The lubricated parts have good forming quality and fewer scratches,with the boron nitride and graphite slip parts having fewer hairline defects and the molybdenum disulphide lubricated parts having the best surface quality.The thickness distribution of the formed parts shows that the main areas of reduction are at the corners and sidewalls,with the greatest reduction in thickness for unlubricated parts and a significant reduction in thickness for lubricated parts,with the smallest reduction in thickness for molybdenum disulphide lubricants.The results for the maximum forming force,surface quality,thickness reduction and inflow of sheet material show that molybdenum disulphide has better lubrication properties than graphite lubricants,followed by boron nitride.lubricant,with the unlubricated formed parts having the worst quality.(2)The real stress-strain curves of 7075 aluminum alloy material were obtained by hot stretching experiments,and the eight anisotropic parameters required for the YLD2000-2D yield model were calculated according to the thick anisotropy and yield stress in different rolling directions,and the intrinsic structure model of the material was established.(3)The inverse problem was solved optimally to obtain the coefficient of friction for different lubricants,with the largest coefficient of friction for the unlubricated condition(0.450 for static friction and 0.290 for dynamic friction)and the smallest coefficient of friction for molybdenum disulphide(0.144 for static friction and 0.138 for dynamic friction),and a smaller coefficient of friction for graphite than for boron nitride.In order to demonstrate the accuracy of the inverse friction coefficient,the average error in fitting the force-displacement curve was calculated for both experiments and simulations,and the results were within 10%.(4)A comparison of the finite element simulation results for molybdenum disulphide and unlubricated parts,which are the best for inverse optimization,shows that the larger the friction coefficient,the higher the frictional heat generation and the higher the temperature distribution of the part,when the yield stress of the material decreases.As can be seen from the forming strain field,the higher the friction coefficient,the higher the strain will be as the unlubricated part is formed,making the sheet susceptible to higher strains during the forming process.The stress distribution is also greater in the larger strain areas.From the distribution of the stress field it can be seen that unlubricated parts have a greater stress distribution in the main strain areas compared to molybdenum disulphide lubricated parts. |
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