鎂合金晶粒尺寸分佈對半固態成形負荷之影響

Abstract

鎂合金擁有優良的材料特性，但因其結晶結構屬六方最密堆積，在常溫下不易以塑性成形方式加工。半固態加工係將材料加熱至固、液相並存的加工方法，此時材料的流動較容易，是一種適合鎂合金的加工技術。但是要進行半固態成形需要先製備具有球狀化晶粒且固液相混合均勻之胚料，使得此成形方式成本大幅提高。 大量塑性變形法中的等徑轉角擠製法能夠給予鎂合金胚材大量的應變，使胚料產生超細晶粒，之後再加熱至半固態溫度區間時，即可得到球狀晶粒顯微結構。此方法的優點在於擠製前後材料的截面積與幾何形狀沒有太大的改變，故可重覆多次擠製，透過控制擠製溫度、路徑與道次可調整材料的顯微結構及機械性質。 本研究藉由等徑轉角擠製法擠製不同路徑的試片，再配合有限元素軟體，進行模擬分析試片經由等徑轉角擠製產生的累積應變量，觀察胚料在半固態溫度下之金相顯微組織，以尋找最佳半固態胚料之製備條件及取樣位置。將此胚料進行半固態閉模鍛造，觀察半固態鍛造成形時所需負荷和成形性。 實驗結果顯示晶粒尺寸分佈越均勻的試片，半固態鍛造成形負荷越小，而且在定荷重下有較好的填充性。

Magnesium alloys have good material properties but poor ductility at room temperature, one of the processes that solves the disadvantage is semi-solid forming. Semi-solid forming requires raw materials with spheroidal grains and homogeneous solid-liquid distribution. However, preparing billets for it cost much. Equal-Channel Angular Extrusion (ECAE) is a promising technique for obtaining ultra-fine grain materials with increased strength and ductility through severe plastic deformation. After ECAE, the specimen is then isothermal treated in semi-solid temperature in order to obtain spheroidal grains. One of the advantages of ECAE is that the cross-sectional area and geometry shape of the specimen won’t change much after extrusion; therefore it can be extruded repeatedly to increase internal strain. By controlling the temperature, the route of extrusion and the number of pressings, material microstructure and mechanical properties can be adjusted. In this study, magnesium alloy AZ80 was selected as the raw material. Equal-Channel Angular Repetitive Extrusion (ECARE) is applied to extrude specimens in different routes. With the aid of finite element method software, DEFORMTM-3D, strain accumulated by ECAE process can be simulated. 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 final forming load curve proved that homogeneous strain distribution resulted better formability. Semi-solid closed-die forging results indicated that the distribution of grain size was one of the main factors in relation with the forging load. The more even grain size distribution, the smaller forming load and the better the filling property will be.