在產業界冷鍛模具設計普遍地依賴實務法則, 主要以高度減縮比來定義胚料成形極限, 做為判斷鍛件是否產生表面破裂的依據, 然而這些實務法則並無法有效地完成模具設計評估工作。因此如何有效評估胚料的冷鍛可成形性與準確地預測其成形極限是有效地進行冷鍛模具設計之關鍵。
大多數文獻係應用線性應變路徑所建構之成形極限圖(Forming Limit Diagram, FLD) 來進行冷鍛成形極限的評估, 而無法有效預估非線性應變路徑多道次冷鍛之成形極限。本文提出「比能量消耗」新觀念, 採用圓柱鍛粗模擬與實驗, 結合有限單元法模擬分析, 以獲得適用性最佳之延性破壞準則, 並獲知胚料成形極限受到胚料尺寸比與定剪摩擦因子之影響。採用四種不同型式幾何設計胚料之鍛壓模擬與實驗, 以了解冷鍛可成形性受胚料幾何之效應。
由於極限應變無法建立模具設計最佳化所需要之品質目標函數, 本文以具有過程函數特性之「比能量消耗」做為設計參數最佳化所需要之品質特性, 在胚料最低比能量消耗條件下, 採用田口設計法(Taguchi method), 分析設計參數之影響權重, 得到設計參數之最佳組合方案, 以建立多道次冷打頭模具幾何與道次設計的最佳化, 並以實驗驗證其合理性。

In industry, the forging die design depends mainly on the experiences. The height-reduction of the billets is often adopted to define the forming limit and mainly taken as the basis for the forging design. These technical approaches can not evaluate die design effectively and efficiently. Therefore, to evaluate the billet forgeability and to predict the forming limits accurately are the indispensable tasks for the die design in cold forging.
The forming limit diagrams (FLD), which are constructed by the linear strain paths and often used to study for forgeability evaluation in most literatures, can not afford to predict accurately the forming limits of the multi-stage nonlinear forging.
In this paper, the notion of specific energy consumption is proposed to obtain the effects of the forming limits influenced by the specimen aspect ratio and constant shear friction factor. Using the cylinder upsetting, the FEM simulations and tests are performed to find the most appropriate ductile energy criterion. Four types of geometry design of billet are adopted to study the effects of the forgeability on the process parameters and to develop the testing model and the measuring method for predict the forming limit of the billets with various forgeability.
Since the limit strains can not be constructed as the objective function needed by the die design optimization, the specific energy consumption, having the path function property, is taken as the quality characteristics needed for the design parameter optimization. Using the Taguchi method and assuming the minimum specific energy consumption, the most appropriate design parameters combination of die design is successfully obtained, verified by the experimental test, to accomplish the die design optimization of multi-stage cold heading.

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