| Automotive production and sales of our country have ranked first in the world for many years.However,the huge car ownership has brought tremendous pressure on the energy environment,so that energy conservation and emission reduction have become an inevitable requirement for the development of the automobile industry.At present,one of the most effective method to reduce the weight of the vehicle is the utilize of light-weighting material.7075 aluminum alloy belongs to ultrahigh-strength Al-Zn-Mg-Cu alloy.Compared with the widely-used 5000 series and 6000 series aluminum alloys,strengthening effect of heat treatment on 7075 alloy sheets is more remarkable,which has higher specific strength and impact resistance.7075 aluminum alloy has gradually become an important research object in the field of automotive lightweight and aerospace.However,it is difficult to form complex-shaped components through cold working methods,owing to the forming limit of age-hardenable 7075 aluminum alloy.Therefore,it is important and urgent to improve the formability of 7075 aluminum alloy sheets.In 2005,Professor Lin Jianguo from Imperial College London proposed a promising hot stamping technology,namely Heat-Forming-Quenching（hot forming-quenching,HFQ）,which can achieve both the improvement of the formability and strengthening effect of 7075 aluminum alloy.This process has important guiding significance for promoting the wide application of high-strength aluminum alloy.The research group preliminarily studied the optimal heat treatment process window based on HFQ process for 7075-T4 aluminum alloy supplied by Southwest Aluminum（Group）Co.,Ltd.（the preferred parameter of solution treatment was 510℃×30 min,aging treatment parameter was 120℃×24 h,the quenching method was water-cooled die quenching）.Based on this,the high-temperature mechanical properties and forming limit of 7075 aluminum alloy sheet were further studied to determine the optimal hot stamping process window and the material performance parameters of aluminum alloy.In this paper,the high-temperature mechanical properties and forming limit of 7075aluminum alloy sheet are studied,and the main research results are shown as follows.（1）The high-temperature tensile test of 7075 aluminum alloy sheet was carried out through simulating HFQ process in order to study the high-temperature flow behavior and fracture mechanism by MTS Electronic Universal Material Testing Machine.The results demonstrated that under the combined effect of strain rate and temperature,the elongation to fracture raise gradually with the increase of temperature,from 15%at the quenched state to 26.1%at 440℃,being 74%higher than the elongation to fracture at RT;When strain rate increased above 0.01 s-1,the tensile strength of 7075 grew up as strain rate increased,indicating a positive correlation with strain rate;However,when strain rates decreased below 0.01 s-1,the tensile strength of the metal sheet decreased with increasing strain rate and indicated a negative correlation with strain rate.When tensile direction is rolling direction,the tensile strength and elongation to fracture are greater than those of other tensile directions.In addition,it is concluded that the ductile shear-necking fracture transition of 7075 based on HFQ took place at 358℃and nucleation,growth and aggregation of cavities are responsible for the breakage in tensile tests by observing the fracture morphology.（2）Based on the uniaxial high temperature tensile test of 7075 aluminum alloy,Fields-Backofen constitutive equation based on material properties was constructed,which established the relationship between typical variables such as deformation temperature and strain rate,describing the hardening and softening characteristics of7075 aluminum alloy during high-temperature deformation.The prediction curves were analyzed by comparing the high-temperature flow stress curve obtained by the high temperature tensile test.It is found that the equation better described the high-temperature flow behavior of 7075 aluminum alloy at the strain of 0.01 s-1,the temperature from 25℃to 440℃and the strain of 0.1 s-1,the temperature of 400℃-440℃.（3）The forming limit diagrams（FLD）of 7075 aluminum alloy sheet were obtained by Nakazima hemispherical bulging test.As the forming speed increased,the longitudinal position of the forming limit curve raise significantly,indicating that when the punching speed was 10 mm/s,the material has a high forming limit under each strain path,and has better formability under a complicated strain path;As the time the sheet stayed on the blank holder was extended,the longitudinal position of the forming limit curve is significantly reduced.When the holding time was extended to 20 s,the fracture mode of the sample was converted from necking fracture to ductile shear fracture.（4）The thermo-mechanical coupling finite element simulation was used to simulate the formability and thinning rate of the specimen under the plane strain path in the hemispherical bulging test to verify the accuracy of the performance parameters of 7075.In the temperature field simulation,the highest temperature of the specimen appeared on the side wall of the hemisphere near the concave corner of the die,and the lowest temperature appeared in the top region of the hemispherical sample.When the holding time was 10 s,the initial forming temperature of the bulging area reduced to377℃-379℃and the fracture type was necking fracture.When the holding time was extended to 20 s,the initial temperature of the bulging area dropped to 348℃-354℃and the fracture type belonged to shear fracture,which was in line with the necking to shear transition temperature in high temperature tensile test.Metallographic analysis showed that the average grain size increased with increasing holding time.The thermo-mechanical coupling numerical simulation and mechanical performance measurement of the front reinforced panel of cross member were carried out later.The accuracy of the constitutive equation and the high-temperature forming limit diagram of 7075 aluminum alloy sheet was further determined.The strength of the parts after aging treatment was improved.The forming strength from the shallow position to the deep position was from 499 MPa to 528 MPa and the deviation between the parts and the digital model was within±1 mm,which satisfied the dimensional accuracy requirement of the formed parts.Therefore,the HFQ process enabled automotive load-bearing parts achieve both the mechanical performance and the dimensional accuracy. |
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