| In order to deal with the important challenges of environmental pollution and energy crisis caused by the rapid development of the automobile industry,vehicle lightweight has become a pressing matter of the moment.The 7000 series aluminum alloy has a high specific strength,which is the first choice for lightweight materials.However,7000 series aluminum alloy has poor formability at room temperature.Although the warm forming can improve the forming performance,it will cause the strength of the material to decrease.The hot stamping of aluminum alloy can enhance the formability and ensure the strength of the material through aging treatment.However,this process requires a short time to complete the forming,resulting in a narrow process window.A complicated cooling system is required to maintain the low temperature of die during the production cycle.In order to solve the above problems,the hot stamping process with warm-forming die for 7075 aluminum alloy was put forward,and the influence of die temperatures on the mechanical properties,forming accuracy and microstructure of aluminum alloy was investigated in this paper.The interface heat transfer coefficient?IHTC?under different die temperatures,contact pressures and forming gaps were calculated by the finite element inverse optimization method.A finite element model of 7075 aluminum alloy U-shaped part in hot stamping process with warm-forming die was established.The influence of die temperature change of the part during the forming process was analyzed.The main conclusions are as follows:?1?For 7075 aluminum alloy,high strength and good formability could be obtained as the same time when the die temperature was 200oC.When the die temperature was 25oC,the tensile strength of the aluminum alloy specimen was 560MPa,the hardness was 190 HV.When the die temperature was increased from 200oC to 350?,the tensile strength of the specimen was reduced from 554 MPa to 394 MPa,the hardness was reduced from 180 HV to 93 HV.When the die temperature was increased from 25oC to 200oC,the maximum springback angle of the aluminum alloy part was reduced from 1.5o to 0.5o.As the die temperature further increased,the springback angle did not decrease significantly.?2?The gradual increase of the die temperature caused the coarsening of the sample grains and the decrease of the strengthening phase volume fraction after aging,which eventually lead to the gradual decrease of the mechanical properties of the aluminum alloy style.When the die temperature was 200oC,the grain size in the sample was evenly distributed,the grains in the range of 6-27?m account for 49%,and the grains larger than 100?m account for 6%;When the die temperature was350oC,the proportion of crystal grains in the range of 6-27?m dropped to 35%,the number of large-size crystal grains increased,and the proportion of crystal grains larger than 100?m increased to 18%.With the increase of die temperature,the cooling rate of the aluminum alloy blank decreased gradually,and the solute elements precipitated in the quenching process increased gradually,which made the content of the fine and dispersed strengthening phase in the aluminum matrix decreased gradually after aging,thereby affecting the mechanical properties of the aluminum alloy sample.?3?Under different die temperatures,pressures and forming gaps,the cooling rate of Aluminum alloy blank was the highest in 0-4 s,decreased in 4-6 s,and the lowest in 6-10 s.IHTC had a negative correlation with die temperatures and forming gaps and a positive correlation with pressures.When the die temperature was increased from 25oC to 350oC,the temperature difference between blank and die decreased gradually,which weakened the heat transfer between blank and die,and the cooling rate of blank decreased gradually,and IHTC decreased from 6436 W/m2oC to2436 W/m2oC;When the pressure was increased from 0 MPa to 24 MPa,the contact area between the die and the blank increased,and the heat transfer hindrance decreased gradually,and IHTC increased from 870 W/m2oC to 6436 W/m2oC;When the forming gap was increased from 0 mm to 3.13 mm,the blank and die were no longer in contact,the heat transfer between them was weakened gradually,the heat flow density was reduced gradually,the cooling rate of blank was reduced gradually,and IHTC decreased from 6436 W/m2oC to 100 W/m2oC.?4?The IHTC obtained by reverse calculation was introduced into the finite element model of the U-shaped part to simulate the stamping process respectively when the pressure was 24 MPa and the die temperature was 25oC,200oC and 350oC.The results showed that when the die was closed,the pressure on the bottom and flange of the U-shaped part was larger,the heat transfer speed was faster,and the cooling rate was larger;the pressure on the side wall was smaller,and the cooling rate was smaller.The temperature change curve of the temperature acquisition point at the bottom of the U-shaped part had a high degree of fit with the experimental temperature curve,the overall temperature difference was controlled withiną4?. |
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