Study On The Evolution Of Elevated Temperature Damage To 7075 Aluminum Alloy Using The GTN Model

Energy and environmental protection are the themes of today's world development.Light-weight of automobiles not only has important value for the development of new energy vehicles,but also has a very positive effect on reducing pollution and protecting the environment.There are two main ways to achieve light-weight in automobiles:one is to adopt a unique optimized design concept to simplify the structure and thus reduce the weight of the vehicle;the other is to use high specific strength light-weight materials.Aluminum alloy is a high-quality choice to replace steel materials in automobiles and achieve light-weight because of its characteristics such as high specific strength,high specific stiffness and good corrosion resistance.However,the room temperature formability of aluminum alloy is poor and the forming process is prone to defects such as rupture.To solve this problem,the commonly used method now is to increase the forming temperature so that the material is formed in a high temperature environment.And in the high temperature forming process,damage is an important factor that affects the forming properties of the material.The material is affected by external load and temperature,which causes changes in its own microstructure,and then generates micro cracks,micro voids and other fine damage inside the material,and with the accumulation of these damages,it eventually leads to the failure and fracture of the material.Therefore,the study of high-temperature damage behavior of aluminum alloy is beneficial to understand and improve its high-temperature forming performance,which is conducive to the better application of aluminum alloy materials in automotive light-weight.Among many damage theoretical models,the Gurson-Tvergaard-Needleman(GTN)model is the one with more research applications and unique advantages in predicting the damage behavior of aluminum alloy forming.The GTN model is a void damage model,first proposed by Gurson and refined by Tvergaad and Needleman,that has been widely used to predict the forming behavior of aluminum alloys and other metals.However,in previous work,researchers have applied the GTN model more to normal temperature forming or applied the normal temperature evolution law directly to high temperature environments.In order to make the GTN model more suitable for high temperature forming,it is necessary to investigate the high temperature damage behavior of materials and the high temperature evolution of damage model parameters.In this paper,damage parameters during high temperature tensile deformation of7075 aluminum alloy sheet were first calibrated using both the inverse problem optimization method and experimental measurements.Using these two methods,the damage parameters of the materials were determined for different temperatures and strain rates,respectively.Based on the experimental results,the effects of temperature as well as strain rate on the evolution pattern of high-temperature damage of aluminum alloy materials were analyzed.By comparing the results,the differences between the two methods and the possible reasons for the differences were analyzed.Also,the effect of a single damage variable on the high-temperature rheological properties of the material is investigated using the characteristics of the inverse problem optimization method.Finally,the effect of temperature and strain rate on the void evolution is added to the conventional GTN model using a secondary development.By comparing the results of hot stamping and forming of aluminum alloy sheet with the finite simulation results,the rationality of the parameter calibration method and the accuracy of the modified GTN model in predicting the hot stamping and forming properties of aluminum alloy sheet are verified.The main conclusions obtained in this paper are as follows.(1)The initial void volume fraction f₀ has a small effect on the material properties.The differences in f₀ values between the two methods measured for each condition are not significant.(2)The void nucleation volume fraction f _N has a large influence on the material properties.Also,the difference in the trend of f _N determined by the two methods.The reason for this is that the experimental method characterizes f N in terms of the content of the second phase inside the undeformed material,and its data do not vary much under different conditions.The inverse method,on the other hand,ignores this condition and calibrates its numerical magnitude only from the point of view of numerical fitting.(3)The critical void volume fraction f _C and the fracture void volume fraction f _Fincrease with increasing temperature and decrease with increasing strain rate below400°C.(4)All four damage parameters related to the void volume fraction have a large change at 450°C.The reason may be that many low melting points second phase precipitates in the 7075 aluminum alloy are re-fused in the matrix at this temperature,resulting in a large change in material properties.(5)The inverse problem optimization method can derive more accurate results from experimental data alone,using software calculations,but the results may deviate from the actual situation.In contrast,the experimental measurement method considers the physical significance of each parameter from the beginning and can produce results that are both realistic and more accurate.However,the use of this method requires more complex instrumentation and tedious test operations.