應用等徑轉角反覆擠製於鎂合金半固態胚料製備之研究

Abstract

鎂合金作為密度最低的結構用金屬材料，相當適合應用在要求輕量化的消費電子產品及運輸載具，但因其結晶結構屬六方最密堆積，塑性加工性差，所以目前工業界對鎂合金材常使用的加工方式以壓鑄法為主流。但壓鑄法仍具有捲氣、廢料過多、危險易燃、無法製造太薄的物件、鑄件強度低等缺點，造成成品設計上多有受限制。為了擴展鎂合金的應用範圍，開發適用之塑性加工製程將為必要之課題。 本研究針對半固態觸變成形加工技術進行研究開發。此製程目前的技術關鍵在於能在半固態溫度區間具有良好球狀晶的鎂合金胚料製備，雖然傳統應變導引熔漿活化法製備鎂合金半固態胚料的製程簡單，但材料內之應變量分佈不均及無法連續進行的缺點，使其至今仍無法有工業上的實際應用。本研究提出等徑轉角反覆擠製法此一創新設計，能快速且連續給予鎂合金胚材大量且均勻的應變，使其能具有奈米等級的超細晶粒，並開發出自動化的製程設備，在八道次製程下，相較傳統製程能大幅縮短十倍以上的時間，快速製程所製備的胚料還可提高約50%的機械強度，此外在將胚材昇溫到半固態溫度區間時，可得到較以往製程更細微的球狀晶，此結構不但能降低成形時的負荷，還可改善材料的流動性質，進而提高成形性。 本研究探討路徑、道次、溫度、背壓、製程時間等參數對鎂合金顯微結構之影響，找出最佳半固態胚料製備條件為200℃路徑Bc八道次。再進一步透過半固態成形加工實驗，證明經等徑轉角反覆擠製後，確實可降低成形負荷改善成形性，建立一由胚料至成品的完整半固態觸變成形製程，成功提高此製程的工業量產可行性。

Recently, the growing demands for high strength light-weight products in automobile and aircraft industries have pushed the application of magnesium alloys into a unprecedented high level. Magnesium has the lowest density among all the structural metallic materials. Its strength-to-weight ratio is the highest of all the structural metals. Because of the ease of recycling, magnesium alloys attract the attention even in the consumer electronic industry. Fabrication of magnesium alloys are often processed by die casting, casting and sometimes thixo-forming since they have limited ductility at room temperature. All of these manufacturing methods have disadvantages especially when a very thin section or better mechanical properties are required. In this study, semi-solid forming technique which has the advantage of lower forming load and suitable for materials that have little ductility will be adopted for the processing of magnesium alloys. In order to improve the homogeneity of the grain size in the microstructure of the magnesium alloys billets, the alloys are intended to subject severe plastic deformation named ECARE (Equal Channel Angular Repetitive Extrusion), in room temperature. Then with proper heat treatment, the alloys are expected to develop a nano-crystalline structure which in turn will bring better mechanical properties. Finally we hope that we can develop a new commercial process of semi-solid thixo-forming.