Investigation On Optimization Of Pre-bulging And Non-isothermal Hydroforming Of High Performance Aluminum Alloy Parts With Special-shaped Bottom

For aluminum alloy components with complex shapes,thin walls,and local convex-concave small rounded corners,forming accuracy is one of the important performance indicators to characterize forming quality.At present,more researches on special-shaped bottom parts focus on improving defects and improving forming limit,and there are few researches on forming accuracy.However,complex aluminum alloy special-shaped bottom parts have poor formability and low forming accuracy,which is difficult to meet the needs of industrial production.Therefore,this paper proposes a pre-bulging and non-isothermal hydroforming precision forming method.Taking 6061-T6 aluminum alloy cylindrical parts with inner concave bottom as the object,the pre-bulging process was optimized,the non-isothermal forming process was studied.The interaction of forming conditions was explored through response surface method.Finally,a special-shaped bottom part with good formability and high precision was obtained.In order to solve the problem of poor fittability to the bottom of inner concave bottom cylindrical part in pre-bulging process,different pre-bulging transition shapes were designed.Research shows that the stepped transition shape can improve stress concentration problem and the bottom strain distribution is more uniform,which is beneficial to prevent wall thickness from thinning.The influence of pre-bulging pressure and circumferential pressure on the bottom formability of pre-bulging part is studied.It is a difficult problem that the complex formed bottom is easy to be pulled apart and cannot keep the acquired shape due to the large meridional tensile stress caused by the downflow of the punch in deep drawing.A method of independent circumferential pressure around the flange in the later stage of pre-bulging is proposed to maintain the acquired shape of the bottom.The numerical simulation of non-isothermal hydroforming is carried out,and the drawing depth and forming quality of parts under different temperature combinations of the punch,die and blank holder are studied.The results show that the maximum drawing depth of part is 78 mm,the height-to-diameter ratio is 1.3 and forming quality is better under the combined conditions of the punch temperature of50°C,die temperature and blank holder temperature of 180-210?.Using the response surface method,the minimum wall thickness value and fittability value were taken as response values,and the pre-bulging pressure,liquid chamber pressure and circumferential pressure were taken as the response factors.The parameters of objective function were optimized by Box-Behnken model,and the optimum forming conditions of special-shaped bottom part were obtained: prebulging pressure is 25 MPa,liquid chamber pressure is 35 MPa and circumferential pressure is 35 MPa,the minimum wall thickness of obtained part is 0.8013 mm,and the fittability is 94.2%.In order to verify the reliability of numerical simulation and the practicability of the pre-bulging and non-isothermal hydroforming precision forming method,the experiment was carried out to verify the inner concave bottom cylindrical parts.The experimental conditions are consistent with the numerical simulation conditions.The drawing limit,wall thickness distribution and forming quality of parts are studied.The results show that the forming parameters of test parts and numerical simulation parts are basically consistent,which indicates that the simulation can effectively predict the forming condition of part,and provides scientific guidance for the precision forming of high performance aluminum alloy parts with special-shaped bottom.