| Lightweight is an effective way to realize energy conservation and emission reduction for automobiles.Compared to traditional steels,high strength aluminum alloys exhibit many advantageous properties,such as low density,high strength-to-weight ratio,good recoverability,etc.,which are a kind of perfect lightweight materials to manufacture structural components to resist impact by replacing traditional steels.With the development of hot/warm forming technologies,the poor formability of high strength aluminum alloys at room temperature is improved significantly,which greatly extends their application in automotive lightweight.The process parameters of hot/warm forming are determined mainly based on the thermal formability of aluminum alloys.7075 alloy is a typical high strength 7000 series aluminum alloy,which has gained much attention in automotive manufacture.Research on the thermal formability of 7075 alloy is representative.Focusing on the thermal formability of 7075 alloy,experimental,theoretical and numerical research was conducted in this paper.Experiments were performed to obtain the thermo-mechanical properties of 7075 alloy,and then accurate constitutive model and damage evolution model were developed,which were further applied in numerical simulation to predict the thermal deformation behavior and fracture behavior of 7075 alloy.The main research works are summarized as follows:(1)The true stress-true strain curves of 7075 alloy in uniaxial tension were obtained in the temperature range from room temperature to 450°C.The results show that the steady flow stress of 7075 alloy decreases with the increase of temperature and the decrease of strain rate,and the elongation extends totally with the increase of temperature and strain rate;besides,the flow stress of 7075 alloy in plastic deformation stage almost keeps balance without obvious stress peak as temperature exceeds 300°C;however,the microstructure coarsens when temperature reaches to 450°C,which degenerates the mechanical performance of 7075 alloy and results in the decrease of strength and elongation.Elliptic dimples were observed on the fracture section of each tested specimen.The size and depth of dimples increase gradually with the increase of temperature and strain rate,which indicates the plasticity and fracture ductility of 7075 alloy are enhanced at higher temperature and strain rate.(2)A unified visco-plastic constitutive model was developed by integrating a visco-plastic flow model,a dislocation density evolution model and an isotropic hardening model.The effect of strain rate on initial yield stress is considered in this constitutive model,which improves the flexibility of the visco-plastic flow model and extends its applicable strain rate range.Furthermore,a damage evolution model was developed based on the basic theories of continuum damage mechanics.The damage model was then combined with the unified viscoplastic constitutive model.The thermal flow behavior and damage evolution behavior of 7075 alloy in tensile tension were accurately predicted by the developed damage-coupled constitutive model.(3)Uniaxial tension tests and plane-strain tension tests in different directions,disk compression tests and earing in cup drawing tests were performed to research the thermal anisotropic behavior of 7075 alloy in the temperature range of 300°C～400°C.The results show that,with tension direction changing from 0° to 90° at different temperatures,normalized stresses in uniaxial tension and plane-strain tension increase and uniaxial r-values increase rapidly and then decrease gradually;besides,equal-biaxial r-value increases with the increase of temperature;in addition,four ears are generated in hot cup drawing process at 45° and the symmetrical directions with respect to 0° and 90°.Based on the experimental data,the anisotropic yield criterion with 18 coefficients proposed by Barlat et al.(Yld2004-18 p yield criterion)was calibrated,which was further applied to predict the thermal yield loci of 7075 alloy.(4)Thermal forming limit diagrams(FLDs)of 7075 alloy were obtained in the temperature range of 300°C～400°C.The influence of temperature,speed and anisotropy was researched.It is observed that the forming limit curve(FLC)is elevated with the increase of temperature and speed;in addition,the FLC obtained from the specimens with length direction in rolling direction is totally higher than the FLCs in other directions.The above observations indicate that the formability of 7075 alloy is improved at higher temperature and speed with the direction of major strain in rolling direction.On basis of the uniaxial constitutive model,a damage correction formula,which contains two parameters: the ratio of minor strain to major strain and the angle between rolling direction and major strain direction,was introduced,and then a multiaxial unified visco-plastic damage constitutive model was established by combining the Yld2004-18 p yield criterion.The thermal damage evolution behavior and forming limit of 7075 alloy in different strain paths were theoretically predited by this multi-axial constitutive model.(5)The material subroutine of the multi-axial constitutive model was coded and then implemented in finite element(FE)model to numerically analyze the thermal formability of7075 alloy.Firstly,earing process was analyzed through hot cup drawing simulation,and earing positions and final earing profiles at different temperatures were predicted.Whereafter,limit strains of 7075 alloy were predicted through hot forming limit simulation,and punch loaddisplacement,major strain distribution,fracture position,damage distribution and evolution in bulging process were further analyzed in detail.Simulation results are overall in great agreement with experimental results,which proves the thermal forming process and final fracture of 7075 alloy can be effectively predicted by implementing the multi-axial constitutive model in FE simulation.(6)Hot stamping simulation with coupled thermal-mechanical-damage fields was conducted upon a car B-pillar of 7075 alloy.Forming result and fracture with different forming processes were numerically predicted.The simulation results were verified by real B-pillar stamping tests.It is indicated that higher temperature and stamping speed,with proper holding force,contribute to better forming quality and higher strength of the formed part via artificial aging after forming process.The research contents and achievements in this thesis can provide scientific reference and guidance for the design and optimization of hot/warm forming process parameters of high strength aluminum alloys represented by 7075 alloy,and promote the applications of high strength aluminum alloys in automotive lightweight.Meanwhile,the research contents and achievements also introduce scientific research methods,theoretical bases and technical supports for systematical research on the thermal formability of other metallic materials. |
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