Research On The Milling Deformation Of Stiffened Curved Parts Of Aluminum Alloy

Aluminum alloy integral panel used in modern aircraft fuselage have high stiffener and complex structure,so it was prone to produce wrinkle,fracture and other defects when forming by traditional method.A new forming method has been put forward by some researchers.Firstly,the curved surface of aluminum alloy thick plate was formed by multi-point bending,then the redundant material on the thick plate was removed by CNC,and the needed stiffener structure was obtained.In the process of removal,milling deformation was difficult to predict and control.Therefore,it is of great practical application value to study the prediction and control of the machining deformation of the whole panel.In this paper,based on the distribution of residual stress inside in sheet after bending,the redistribution of residual stress after material removal under the assumption condition was studied,and a prediction method of springback was proposed.The results showed that for a certain thickness of the sheet,the concave side was milled and removed,and the neutral layer moved to the convex side.As the milling depth increased,the stresses on both sides of the neutral layer of the sheet changed separately,and the springback caused by the stress release would also be affected by the change of bending moment.When the milling removed more than the original neutral layer,the springback reached its maximum value.The finite element simulation and milling experiments were used to verify the calculated results,and it was proved that the springback prediction model established could predict the residual stress distribution and springback effectively.The influence of geometric parameters and milling parameters on the deformation of workpiece were discussed.The springback results of the curved parts with different parameters showed that the double-curvature specimens with the same bending radius in both directions had more uniform springback;the smaller the initial thickness of the sheet,the greater the springback of the curved specimen after milling;the greater thethickness of the stiffener,the smaller the springback of the curved specimen.The relation between milling parameters and surface stress of the workpiece after milling is as follows: with the increase of spindle speed,the surface stress changed from compressive stress to tensile stress;the greater the depth of cut,the greater the surface compressive stress;as the cutting speed increased,the surface compressive stress gradually decreased.Redistribution of non-equilibrium stress caused deformation of the sheet: the larger the milling depth,the smaller the milling deformation;the milling deformation increased when the milling spindle speed and milling speed increased.The interaction of milling stress and initial stress had a certain influence on the deformation effect of workpiece,which mainly depended on the properties of residual stress at the position to be milled.According to the experimental part of this paper,three milling projects were put forward,the milling process of each project was compared and analyzed,and the " back" shape path from the outside to the inside was determined to be the best path.Milling project and parameters were as follows: in the first step,according to the order of stress in the "sub-concentration area-non-concentration area-concentration area" in the sheet,and the smaller spindle speed and milling speed were selected to remove 6 mm;in the second step,the path and parameters were the same as the first step,milling 2mm in stress concentration area,and 6 mm in the other areas;in the third step,the removal order of "concentrated area-secondary concentration area" was selected,kept the smaller spindle speed and milling speed to remove 2 mm;then increased the spindle speed and milling speed,removed the 2 mm material again in the same path until the milling of the specimen was completed.The optimized path and parameters could ensure that the deformation of the workpiece at each step was minimal.