不銹鋼薄板圓筒精微引伸成形分析

Abstract

隨著各種產品的微小化，微型零件的需求量也越來越大，因此精微成形製程之研究亦日趨重要。而在精微圓筒引伸成形方面，其應用範圍相當廣泛，如手機震動馬達外殼或是微小電池外殼。目前傳統圓筒引伸已有相當成熟之技術，然而當材料尺寸微小化後，其材料性質將會產生尺寸效應（Size effect）而有所改變。由於尺寸效應之存在，使得傳統的成形理論不能直接應用到精微成形技術中。 本研究以不銹鋼304做為研究材料，首先探討在不同厚度下，不同晶粒尺寸對於材料應力應變曲線、異向性指數與加工硬化指數之影響。實驗結果顯示隨著晶粒尺寸增加，應力應變曲線與平均異向性指數將會下降，而加工硬化指數將會上升。且增加晶粒尺寸亦會導致材料各方向拉伸曲線差異越來越大。本研究並以一套手機震動馬達外殼多道次成形工法驗證CAE分析技術在精微圓筒引伸成形之準確性。其結果顯示PAM-STAMP與DEFORM兩種不同之成形分析軟體皆具有相當之準確度。另外以CAE分析開發另一套手機震動馬達外殼多道次成形工法，並再一次實驗驗證，其結果亦顯示模擬結果具有一定之準確度。 由於改變晶粒尺寸將會改變材料之機械性質，進而改變引伸成形之結果。故最後本研究以不同厚度與晶粒尺寸之材料參數進行模擬，探討改變晶粒尺寸後對於單道次與二道次引伸成形之影響。模擬結果顯示隨著晶粒尺寸的增加引伸後之最薄厚度將會降低。

With the ongoing development of product process, there is a growing demand on micro products. Though the macro-drawing process has been well-developed, the design concepts may not be directly applicable to the micro-drawing due to the size effect occurred in the micro-forming processes. In the present study, experiments were conducted first to establish the stress-strain curves, r-values and work hardening exponents of 304 stainless steel sheets with different grain sizes. The experiment results reveal that the stress-strain and r-value become smaller and the work hardening exponent increases for larger grain sizes. The difference between stress-strain curves in various directions of 0°, 45° and 90°, respectively, is significant when the grain size increases. The stamping of a vibration motor shell of cell phone, which bears a circular cylindrical shape, was also examined in the present study. The finite element simulations were performed to evaluate the formability of the multi-stage drawing process with initial die design. The forming characteristics were identified and an optimum die design was then developed with the use of the finite element analysis. The stamping process with multi-stage tooling design based on the finite element analysis was implemented and the actual stamping experiments were conducted to verify finite element analysis. The experimental results confirm the validity of the modified tooling design and the efficiency of the finite element analysis. In order to investigate the size effect on the micro-drawing process of a circular cup, the first and second operations of the multi-stage forming processes were simulated with sheets of different grain sizes. The simulation results show that the minimum thickness decreases when the grain size increases.