Research On Hydroforming Of Aluminum Alloy Thin-wall Curved Parts Based On Upper Layer Sheet

Large curved thin-walled parts are easy to appear wrinkles,cracks and other defects in the suspending area through the ordinary hydroforming process.The introduction of the double-layer sheet hydroforming process can significantly inhibit the generation of these defects,but the parts still have defects such as local wall thickness thinning and fittability accuracy not up to the standard.In this paper,the thin-walled semi-ellipsoid parts of 6061-T6 aluminum alloy with the axial length ratio of 1.06 are taken as the research object,and the forming mechanism of the doublelayer sheets is further analyzed by combining numerical simulation and experiment.In order to improve the simulation accuracy,the coefficient fitting method of the Fields-Backofen constitutive equation at high temperature is further optimized,so that the fitting range of each coefficient is extended to different temperatures and different strain rates.In this way,the problems of uneven temperature distribution and inconsistent deformation speed in the process of simulating sheet deformation can be considered.By calculating the correlation coefficient between the constitutive equation and the original data at different temperatures,it is found that with the increase of temperature and the decrease of strain rate,the fitting accuracy gradually increases,and the maximum value can reach 0.9966,which reflects the effect of dynamic softening of the modified constitutive equation.In this paper,the minimum wall thickness value and the fittability accuracy are taken as the evaluation criteria.Firstly,the influence of the upper sheet with different strength and the loading path of the liquid chamber on the formability of the parts is analyzed.The upper sheet material is determined to be 1Cr18Ni9 Ti stainless steel,and the maximum liquid chamber pressure is 25 MPa.At the same time,in order to slow down the wall thickness reduction rate of the punch contact area,the Pre-bulging process is introduced to improve it.Finally,6 MPa is selected as the Pre-bulging pressure of sheet metal forming.Through the response surface method,the interaction between the circumferential pressure of the double-layer sheet,the friction coefficient between the sheets,and the thickness of the upper sheet is studied.The optimal process parameters of the part forming are predicted by the existing data,and the prediction accuracy is verified by ABAQUS software.The final simulation results are basically consistent with the predicted results.On the basis of the optimal process parameters,the warm forming technology is introduced to further improve the forming quality of the parts,and it is found that the parts have good forming performance at 150 °C.The forming test is carried out by using this process,and the results are basically consistent with the simulation.