Research Of Pre Aging-Warm Forming(PAWF) Process For 7075 Aluminium Alloy

In recent years,the rapid development of the automotive industry has aggravated the consumption of petroleum energy and global greenhouse effect.Energy conservation and emission reduction have become significant measures for the development of many countries,and the lightweight of automobiles plays a dominate role in it.Due to its high specific strength,better plasticity and corrosion resistance,the aluminum alloy plays an important role in the fields of automobiles,military industry,aerospace and so on,which gradually replaces traditional steel materials.However,the aluminum alloy has poor formability at room temperature,and it is difficult to process parts with complex shapes,which greatly limits its application.Hot and warm forming technology of aluminum alloy have emerged as a novel technology for sheet processing.It greatly improves the plasticity of materials and facilitates the processing of complex parts.However,the heating up will lead to the dissolution and coarsening of the precipitated phase inside the alloy and reduce the strength of the parts.Therefore,an additional heat treatment process is required to make up for the loss of strength,which prolongs the production rhythm and wastes resources.After forming and assembling the body parts,the second phase dissolved during the warm forming process will precipitate again by baking process,which can compensate the influence of the warm forming process on the strength reduction of the parts.If the material is under-aged before forming,the over-aging of the material can be effectively avoided after forming at warm temperature and baking,and the strength close to the T6 state can be obtained.Based on this,Pre Aging-Warm Forming process（PAWF）of 7075 aluminum alloy was proposed in the paper.The effects of pre-aging,warm forming and baking process parameters on the formability and mechanical properties of 7075 aluminum alloy were studied.Finite element analysis method was used to analyze the forming process.The relevant research content is as follows:（1）By exploring the technological parameters of 7075 aluminum alloy during preaging-warm forming,the effects of different forming temperature and holding time on the properties of the material after forming and baking were studied,and the influence of pre-aging time on the properties and microstructure of the materials was analyzed.The results shows that the hardness recovery effect was obvious after regression and re-aging treatment,and the under-aged aluminum alloy could be close to T6 condition after heating at 200℃ and baking.（2）Warm tensile experiments were carried out at 180-220℃、0.01-0.1s^-1 to explore the rheological behavior of under-aged 7075 aluminum alloy at different forming temperature and strain rates.The variation of tensile strength,yield strength,strain hardening rate,elongation,etc.under different forming temperature and strain rates were studied.Finally,the forming temperature corresponding to the optimum formability of under-aged 7075 aluminum alloy was obtained.When the pre-aged7075 aluminum alloy heated up to 200℃,the flow stress decreased compared with the room temperature and the deformation resistance decreased;while the strain hardening rate is basically the same with the room temperature,and the total elongation and uniform elongation increased significantly.（3）In order to explore the stability of the material properties after pre-aging,the hardness changes of the under-aged 7075 aluminum alloy were measured at room temperature for 1 to 21 days after solid solution and quenching,and the inhibition effect of pre-aging on natural aging was studied.The pre-aged aluminum alloy can significantly inhibit the natural aging.（4）The warm forming experiments of U-shaped parts were carried out.The temperature of the punch and the die were set to be isothermal with the sheet,the punch was at room temperature,the die was isothermal with the sheet,and the punch and the die were all at room temperature.The influence of three cooling paths on hardness and section thickness of parts were studied.The optimum forming conditions were obtained by synthesizing the formability and post-forming properties of materials.A 3D scanner was used to scan the U-shaped parts,and the springback analysis of the U-shaped parts formed under different cooling paths was carried out.The dissolution and precipitation of the second phase of 7075 aluminum alloy were analyzed by differential scanning calorimetry at different pre-aging time,after forming at warm temperature and baking.Compared with isothermal forming and in-die quenching,non-isothermal forming can obtain better comprehensive mechanical properties between formability and strength.（5）An modified FB model was used to fit the rheological behavior of under-aged aluminum alloy under the warm forming condition.The thermo-mechanical coupling finite element simulation of non-isothermal forming,isothermal forming and in-die quenching were carried out to study the variation law of temperature field,thickness,effective stress and effective strain of sheet metal during forming and holding pressure.Compared with isothermal forming and in-die quenching,non-isothermal formed parts receive less stress value and more uniform thickness distribution.