Springback Research On Multi-Point Forming-Milling Manufacturing Method Of The Aluminum Alloy Integral Panel

The aluminum alloy integral panel flat workpiece is milled from a huge thick plate,whose stiffeners and skin are an integral structure.It has many advantages such as high strength,light weight and long service life.And it has been widely used in the manufacture of aircraft,rocket and other aircraft.Due to the complex stiffener structure of the integral panel,it is difficult to form.Especially for the panels with large size,high stiffers and complex curvature,there are many defects by using the traditional forming method.For example,roll bending and incremental bending are easy to destroy the structure of the integral panel,resulting in the instability and fracture of the stiffeners.Creep aging forming requires large-scale tank pressure equipment,and has a high production cost.Shot peening forming is not suitable for forming the panel with large curvature and high stiffeners because of its small loading force.Therefore,an efficient manufacturing method for this kind of integral panel is needed.In this paper,an alternative manufacturing process is proposed based on Multi-point forming(MPF)technology.First,the aluminum alloy thick plate is formed into the designed shape by MPF,and then milled into the final stiffened panel using a high-speed milling machine.The multi-point bending-milling process can effectively avoid the instability of the stiffeners in the forming proces of the integral panels with high stiffeners and complex curvature.However,the manufacturing process also has some problems on the springback prediction and control,such as the springback in MPF process,residual stress distribution,the clamping issues of plate during milling process,the springback of in milling process and so on,which affect the final forming accuracy.In this paper,a thorough study was made on these problems.The residual stress distribution of the plate after bending was analyzed.And the springback prediction models of multi-point bending process and milling process were established,respectively.Moreover,the influence of forming factors on springbackwas also discussed.To reduce the force exerted on the fixture,the milling path was optimized to control the release of residual stress.The main research contents are as follows:1.Based on the power-exponential hardening material model,the formulas for calculating distribution of the stress in thickness direction of the plate in cylindrical bending were derived.The loading force of a single punch was calculated by the stress in thickness direction,and the bending moment was obtained according to the arrangement of multi-point punches.Through elastic-plastic bending theory,the variation of the curvature and the residual stress distribution of plate after springback were obtained.The springback results under different bending radii were calculated theoretically,and were verified by finite element simulation and multi-point bending experiment.The results show that the springback prediction model proposed can effectively predict the springback and residual stress distribution.2.A method for calculating the distribution of the stress in thickness direction in double-curved bending process was presented.To make the results more accurate,the plate was divided into different areas according to the arrangement of the multi-point punches.The curvature variation of each area after springback was calculated,and the coordinates of the characteristic points in each area were also obtained through geometric approach.By utilizing the Bezier surface blending method,the profile after springback was obtained.The comparison with the results of finite element simulation and double-curved multi-point bending experiment show that the error of the analytical results decreases with the increase of bending radius.To improve the calculating accuracy,the prediction model can be used in a certain range of forming radius according to the actual situation.3.According to the residual stress distribution of the plate after bending,the springback prediction model in milling process was established.And the redistribution of stress in the remaining plate was also considered.The effects of milling layer thickness,bending radius and milling depth on the springback results were discussed.The results of finite element simulation and experimental show that the analytical result can reflect thevariation of springback under different milling conditions with a small error.4.The milling path was optimized by the finite element method.The optimum milling depth is determined,and different milling paths were also designed for comparison.Finally,a reasonable milling scheme was planned.The simulation results show that the optimized milling path can effectively control the release of residual stress in milling process and reduce the force of plate on the fixture.5.Through the finite element model and analytical model,the influence of the factors,such as thickness of plate,punch size and structure of the panel,on the forming results in the multi-point bending-milling process were analyzed.It is pointed out that smaller punch size can make the distribution of residual stress more uniform.The change of plate thickness has no obvious influence on the springback results during bending process,while the springback in milling process is greatly affected.And the structure of the panel has an important influence on the springback results.The analytical model established can be applied to the panels of complex structure.