鋁合金飛機蒙皮拉伸成形缺陷分析與製程優化設計

Abstract

鋁合金飛機蒙皮拉伸成形製程常見的缺陷有破裂、回彈、皺褶、橘皮等，為了改善成形製程缺陷，本論文採用有限元素模擬做為研究工具，分析使用2024鋁合金成形不同造型產品中所產生之缺陷的成形機制，以及所對應之模面設計，藉以分析缺陷產生之原因，並進而探討改善缺陷之製程與模具優化設計。首先優化現有拉伸成形有限元素模擬模型，分別針對夾持端之幾何造型以及板材邊界條件進行優化，並透過現有多道次製程以及更換材料為T料之單道次製程進行實際成形試驗，以驗證模擬模型正確性，結果顯示優化後的模擬模型可重現實際成形中的多種缺陷，後續即採用此優化模擬模型針對單曲率截面非對稱機翼前緣、單曲率截面對稱前翼縫條，以及雙曲率馬鞍等三種不同蒙皮造型之拉伸成形製程，進行缺陷成形機制分析與缺陷改善之模具優化設計。 由於機翼前緣本身非對稱之造型再加上使用材料之強度較高，因此回彈除外開回彈以外還出現扭曲，經由分析得知外開回彈為頂部圓角應力差所導致，而扭曲則為板材延壁面方向應力釋放所造成。本研究發現透過增加模具位移或夾爪作動速度可改善外開回彈，而將製程順序更換為階段式則可減少頂塊對板材之摩擦力，從而改善扭曲；最後透過田口實驗法(Taguchi Method)對製程參數進行最佳化設計，可將扭曲由9度控制到1度左右。 前翼縫條模具由於本身之對稱造型，因此只會出現外開回彈，但同樣因為材料強度較高，因此使用現有製程會出現破裂缺陷以及回彈現象。藉由缺陷分析與製程優化設計，發現該破裂缺陷可透過增加模具負載階段之位移量加以改善，而透過成形階段夾爪位移之調整則可減少回彈量的產生，本研究在進行最佳製程參數設計以及模面補償優化後，可將各截面回彈量控制於0.7mm以內。 馬鞍模具由於雙曲率關係因此容易出現皺褶以及破裂缺陷，透過將成形分為多個階段並對各階段製程參數進行優化設計，可藉由右側夾爪階段位移量控制皺褶發生之位置，再透過製程參數設計可於板材破裂前完成貼模，防止皺褶缺陷之產生。 本論文優化現有模擬模型並進行相關驗證，使模擬模型更加完整，之後透過製程以及模面補償改善三種不同外型模具成形時之缺陷，本論文研究結果可提供未來拉伸成形遭遇相似缺陷時製程設計之參考。

Common defects in stretch forming include crack, springback, wrinkles, and orange peel. In order to improve the defects, this thesis applied the finite element simulation to analyze the deformarion mechanism of defects occurred in the stretch forming of 2024 aluminum alloy with different shapes of dies, and the process design and die compensation approach were investigated. This thesis first optimized the existing stretch forming simulation model which is based on the finite element method, including the geometry and the boundary conditions of the gripper zone. Comparing the simulation results with the production parts formed by the existing multi-pass process and the T-material single-pass process, the capability of the optimized simulation model for reproducing various defects was verified. This simulation model was then utilized to analyze the forming defects with two single curvature dies, termed as leading edge die and slat die, respectively, and a double curvature die, named as saddle die. Due to the asymmetric shape of the leading edge die and a highe-strength aluminum alloy used , springback defects include not only side-wall opening but also distortion. Analysis results show that the side-wall opening is caused by the stress difference of the top corner, and can be reduced by increasing the displacement of die or the speed of jaw. While the distortion is caused by the stress relaxation in the direction along the side-wall, and can be improved by dividing the forming process into multiple stages because of the lower friction between the top block and blank. Optimizing the process parameters using the Taguchi method can reduce distortion from 9 degrees to 1 degree. Due to the symmetrical shape of the slat die, springback defect only involves side-wall opening. Furthermore, because of the high strength material, applying current process will lead to both crack and springback, where crack can be improved by increasing the die displacement during loading stage, and side-wall opening springback can be improved by jaw displacement during forming stage. By optimizing the process parameters and die compensation, the springback of each section can be restricted within 0.7mm. Due to the double-curvature geometry of the saddle die, wrinkle and crack defects can barely be avoided during the forming process. By dividing the forming process into multiple stages, the wrinkle can be improved by the displacement of the right jaw. Moreover, through the process parameters design, wrinkle defects can be eliminated while crack is prevented as well. This thesis optimized the existing simulation model and implemented experiment to make the simulation model more comprehensive. By process design and die compensation, various defects were improved in the forming process with three different shapes of die. The achievement of this thesis could provide a reference for the stretch forming process design and optimization.