鎂合金胚料顯微結構對半固態觸變加工影響之研究

Abstract

鎂合金具有高強度、電磁遮蔽、散熱性佳、耐衝擊性、耐高溫性、耐腐蝕性等優良的材料特性，但因鎂合金的結晶結構屬六方最密堆積，所以其塑性變形能力差，不易以塑性成形方式進行材料的加工，所以目前工業界對鎂合金材常使用的方法大多是壓鑄法中的冷室壓鑄法、熱室壓鑄法，以及半溶融加工方式中的觸變射出成型。利用鑄造方式雖具有成本低、生產率高、品質穩定、適合大量且快速生產等優點，但壓鑄法仍然有捲氣、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。若想利用鎂合金製造需具高強度之車用結構部品，必須採用別種加工方式方可符合強度上的需求。因此本研究室近年研究之最主要目的即為開發一種可用於半固態成形的鎂合金胚材，希望能夠開創出鎂合金新的應用領域。 本研究室過去運用半固態觸變緞造加工技術結合大量塑性變形法來製作所需胚料。並自行設計開發等徑轉角反覆擠製製程（Equal Channel Angular Repetitive Extrusion, ECARE），可快速製備具超細晶粒之半固態胚材，此細晶胚材亦被證實可具備更佳之成形性質與機械強度。但在研究過程中發現成形時的球狀晶顯微結構對成形負荷及成品機械性質影響甚巨，持溫時間長雖可增進球狀晶組織的球狀化效果但亦會造成晶粒大幅成長，若能針對持溫時間與半固態顯微組織變化之關係進行探討，應可找出一最佳之製程參數組合。此外過去在閱讀文獻時發現半固態成形與粉末冶金加壓成形在微觀上晶粒移動方式是相似的，文獻中指出粉末冶金進行加壓成形的過程中，粉末粒度若是太小且粒度分佈窄，反而會因顆粒間摩擦力過高導致粉體流動性差，理想的情況下應有一最佳之粉末粒度分佈範圍。半固態下固態晶粒與液態組織是否會產生此一現象，亦是我們在想盡辦法降低平均晶粒大小之餘，需探討的一項課題。 本計劃預計分兩年來完成： (1) 第一年預計利用可調整功率之感應加熱設備進行鎂合金半固態成形試驗，探討不同昇溫速度及持溫時間、溫度等加熱參數對球狀晶顯微組織之影響，希望能建立不同製程參數下的完整對照資料庫，並實際透過半固態成形加工驗證晶粒大小與圓球化效果對成形負荷與成品機械性質之影響。 (2) 第二年預計使用有限元素軟體針對等徑轉角擠製(ECAE)的各種路徑模擬出各種擠製參數下累積應變量的分佈，再利用前一計劃開發之等徑轉角反覆擠製(ECARE)設備，改變不同擠製路徑與道次，以得到具不同晶粒尺寸分佈之鎂合金胚料，觀察與比較經ECAE擠製後試片之半固態顯微組織，決定出適當的試片參數與位置，再將試片進行半固態鍛造，觀察其成形負荷與成形性，進一步探討晶粒尺寸分佈均勻程度對半固態下成形加工之影響。 綜合這兩年的研究成果，希望能建立出不同製程參數下的顯微組織對照資料庫，找出最適合半固態觸變鍛造的胚料顯微組織型態，並且提出一套可行之數值模擬模型，讓開發人員能夠簡化分析製造半固態胚料的流程，最後提出半固態觸變鍛造應用於鎂合金的相關製程條件，以期能對未來將此製程之實用化有所貢獻。

Recently, growing demands for light-weight products with high strength products in automobile and aircraft industries have push the application of magnesium alloys into a unprecedented high level in these fields. Magnesium has the lowest density among the metals. 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, conventional 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 higher mechanical strength are required. In the past few years, our research team developed a new process to prepare the billet for thixo-forging which called Equal Channel Angular Repetitive Extrusion (ECARE). This new process could continuously extrude the magnesium bar to obtain ultra-fine grain microstructure which in turn will bring better mechanical properties in the final products. However, from our precious work, the difference of billet's microstructure has a major influence during forming. Longer holding time can promotes the shape of the grain t become more spherical, but it also brings the disadvantage of grain growth. This is one of the reason why we like to find the most appropriate parameter for semi-solid thixo-forging. In addition, the relationship between the grain size distribution and the forming load will also be discussed in this study. With the aid of finite element method software, strain accumulated by ECAE process can be estimated. Compare the strain distribution obtained from FEM simulation with the microstructure of specimen observed under semi-solid temperature, the best semi-solid forging billet type could be chosen. The study is composed of the following stages: (1) Perform semi-solid thixo-forging process with high frequency induction heating equipment, investigating the influence of heating process. Establish data base between microstructure and forming load in semi-solid temperature zone. (2) Finite element simulation of the ECARE process. Apply different extrusion parameters to obtain distinct type of accumulated strain distribution for billets. Discuss the relationship between the uniformity of grain size distribution and the forming load.