LZ91鎂合金筆記型電腦蓋板之沖壓模具設計

Abstract

摘 要 近年來，因要求輕薄短小的3C產業快速發展，鎂合金逐漸受到產業界的重視，加速了鎂合金的應用，目前用於沖壓製程之鎂板以AZ31為主，然因其原子結構屬於六方最密堆積(HCP)，在常溫的成形性甚差，必須加溫至200℃以上方具備良好之成形性。但藉由鋰元素之添加，增加了體心立方(BCC)之原子結構，由於BCC可供滑移的系統眾多，因此鎂鋰合金之常溫成形性較佳，可採用一般塑性加工方式成形。 筆記型電腦外殼因為有兩個幾何特徵比較複雜的形狀，所以在沖壓過程中更增加其困難度。其中一個幾何特徵複雜部分是因為底部圓角半徑太小所造成，因而使衝壓過程中產生皺褶及底部圓角破裂情形，而影響此一參數設計有壓料板之力量、胚料板之尺寸大小及壓料阻條之所在位置，然而這個破裂問題是比較容易解決，從漸進式改變衝頭圓角大小、胚料板尺寸之最佳化設計及增加壓料力方式來解決。此外，另一個幾何特徵複雜部分是因為鉸鍊凸緣之位置太過靠近邊緣造成圓角之幾何半徑變化過於劇烈所造成，因此在沖壓過程中會產生鉸鍊凸緣部分有破裂情況，而影響此一設計參數有鉸鍊凸緣與邊緣之距離位置、母模之圓角半徑、衝頭角隅半徑、模擦係數及潤滑劑，此一問題可藉由成形過程中切割局部胚料板方式來解決，至於所切割位置則需使用有限元素分析來模擬。 本研究將進行實際開設模具來進行沖壓為以驗證有限元素之分析結果，因此共開設三組模具，板材使用鎂合金LZ91薄板，模具設計製作參數完全採用有限元素分析結果，並經實際沖壓結果證實使用有限元素分析軟體來進行鎂合金LZ91薄板沖壓之模具設計之正確性。

Abstract Due to its lightweight and high specific strength, magnesium alloys have been widely used for structural components. However, because of the hexagonal closed-packed (HCP) crystal structures, magnesium alloys show low ductility at room temperature, and require thermal activation to increase their formability. It is well known that ductility of magnesium alloys can be improved with addition of lithium which develops the formation of body centered-cubic (BCC) crystal structures. The BCC crystal structure gives rise to high formability at room temperature. In the present study, the forming process was examined by the finite element simulations. According to finite element simulation analysis, two features in the notebook case were found to be relatively complex geometrically and were thus more difficult to form in the stamping forming. One of the complex features was caused by the small radius of the bottom, which would cause fracture and wrinkling during the stamping forming process. The process parameters resulting in the fracture defect were blank-holder force, blank size, and the location of draw-bead. However, this fracture defect could be easily removed by changing the punch comer radius, optimum blank size, and decreasing the blank-holder force. The other geometric complexity was caused by the dramatic change in the corner radius when the flange of hinge got too close to the edge of the notebook, which would easily cause the fracture defect around the flange of hinge during stamping forming process. Process parameters causing this defect includes the distance between the flange of hinge and the edge of the notebook, die corner radius, punch comer radius, friction coefficient, and lubricity. This fracture defect around the flange of hinge was eliminated through local trim of blank during the stamping forming process. As for the optimum position distance of local trim blank, it was determined based on the finite element simulation results. To validate the finite element simulation results, an actual stamping process for producing notebook cases was performed. Three sets of tooling were machined to the profiles according to the finite element simulation results. The LZ91 sheets were then stamped into the shapes of notebook cases with the process parameters determined above. The experiment results have validated the finite element analysis for the process design for stamping LZ91 magnesium alloy sheets.