改良式SIMA法製備鎂合金半固態成形胚料之研究

Abstract

半固態製程是一種在半固態溫度區間下進行金屬加工的近淨形製程，其相較於固態下的鍛造加工具有較低的變形抵抗，而相較於液態下的壓鑄加工，成品會具有較佳的機械性質。目前要將半固態製程應用在鎂合金上，最大的問題就在於如何製備具有球狀晶粒而非枝狀的胚料。回顧過去的文獻，應變導引熔漿活化法(SIMA)是一種適用於高固相率且製程成本較低的胚料製備方法，但傳統的SIMA製程是利用壓縮的方法來給予材料應變，鎂合金因為在室溫下的滑移系統限制，當壓縮率超過15%時，材料即會產生裂開的現象，而無法施加更大的應變量。 為了克服鎂合金應用在SIMA時的困難，本研究提出一種改良式的SIMA法，將傳統應用在晶粒細化上的大量塑性變形法(SPD)，應用在SIMA法的冷加工段上。本研究中所選用的大量塑性變形法為等徑轉角擠製(ECAE)，原理是利用兩個交角為90°的等徑模具，利用沖頭下壓將材料擠過90°轉角，藉此施予材料應變。此方法的優點在於擠製前後材料的截面積不變，故可重覆多次擠製，透過控制擠製道次可調整材料的顯微結構及機械性質。 本研究以AZ61、AZ80鎂合金為對象，配合改良式SIMA法，利用ECAE施予材料大量應變，再重新升溫至半固態溫度區間，觀察其顯微組織。實驗結果顯示此法相較於傳統SIMA法，兩種材料在室溫下均可得到較細化的晶粒，但在半固態溫度下的顯微結構顯示，AZ61可得到較佳的球狀晶粒結構，AZ80則效果不彰。根據這樣的結果，可證明改良式SIMA法應用在AZ61鎂合金半固態胚料製備上，的確具有實用上的價值。

Semi-solid processing is one of the most widely used methods for forming magnesium alloy components because of its low resistance to deform comparing to solid alloy forging. Semi-solid process produces components with high mechanical properties of formed comparing to liquid alloy die casting. Semi-solid processing is defined as a forming process in which semi-solid billets with a spheroidal microstructure are formed to produce a near net shaped product at the semi-solid temperature. For semi-solid processing of magnesium alloy, the most important thing is to prepare semi-solid billets, which have spheroidal microstructure. Comparing with other methods for preparing semi-solid billets, strain-induced and melt-activated (SIMA) is simple and cheap, especially for magnesium with high solid fraction. In "traditional SIMA method", the billet is first upsetted then is isothermal treated in semi-solid temperature in order to obtain spheroidal grains. However, for upsetting process of magnesium alloy, plastic deformation is limited due to its limited slip system. If the deformation percentage of magnesium alloy is above 20%, crack will occur.

In order to overcome the difficulties of plastic deformation of magnesium alloys in die upset, a modified method for obtaining severe plastic deformation (SPD) was adopted. Equal channel angular extrusion (ECAE) is a promising technique for obtaining ultra-fine grain materials with increased strength and ductility. During ECAE, a billet is pressed through a die that consists of two channels with equal cross-section, intersecting at an angle. Since the cross-sectional shape of billet remains nearly the same, ECAE processing can be repeated for many passes to control the microstructure and properties of the material.

In this study, is to represent an attempt to prepare semi-solid billets of AZ61 and AZ80 magnesium alloys were selected as the billets. Experiment results indicate that modified SIMA method is very effective for preparing spheroidal grain AZ61, however, for AZ80 the effect was not so obvious. Based on the results, we can conclude that when SIMA method was incorporated with ECAE for preparing AZ61 semi-solid billet a fine dispersed spheroidal grain micro-structure can be obtained.