鎂合金胚料顯微結構對半固態成形負荷之影響

Abstract

鎂合金半固態的成形方式同時擁有鑄造與鍛造的優點，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和均勻胚料，使得此成形方式成本大幅提高，而製造半固態胚料的方法中，以應變導引熔漿活化法（Strain Induced Melt Activation，SIMA）最為簡單，也不需要昂貴設備，只需在室溫下進行一定量的冷加工，再加熱至半固態溫度區間時，即可得到球狀晶粒顯微結構。傳統的SIMA法利用壓縮應變給予冷加工量的方法，過程中很容易造成胚料裂開，變形量受到限制。因此本研究將傳統晶粒細化上使用的大量塑性變形法，應用在SIMA法中加大加工量。本研究中所選用的大量塑性變形法為等徑轉角擠製與循環壓縮。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，透過控制擠製溫度與道次可調整材料的顯微結構及機械性質。 本研究以AZ80鎂合金為對象，以改良式SIMA法製備胚料，觀察其半固態溫度下顯微組織，並進行半固態鍛造探討胚料顯微組織對成形負荷之影響。實驗結果顯示改良式SIMA法應用在AZ80鎂合金半固態胚料製備上，的確具有優良半固態成形胚料之性質，在進行半固態凸緣鍛造，胚料半固態溫度下的晶粒圓球化效果和平均晶粒大小為影響半固態鍛造成形負荷的主要因素；晶粒形狀越圓球化及平均晶粒大小越小的胚料，成形所需之負荷越小。

Semi-solid forming of magnesium alloy has the advantage of casting and forging. Semi-solid forming requires raw materials with the spheroidal grains, and homogeneous solid-liquid distribution. However, preparing billets of this kind increase costs. Strain Induced Melt Activation (SIMA) method is simple and does not require expensive equipment for preparing semi-solid billet. In SIMA method, the billet is first upset then is isothermal treated in semi-solid temperature in order to obtain spheroidal grains. However, traditional SIMA method uses upsetting process, which causes crack easily, the deformation is limited. In order to overcome the difficulties of the plastic deformation of magnesium alloys in die upset, a modified method for obtaining severe plastic deformation (SPD) was adopted. In SPD, Equal channel angular extrusion (ECAE) and channel die compression process (CCDC) are combined for obtaining ultra-fine grain. This method has the advantage of the same cross-sectional area and geometry before and after deformation. The process can be repeated for many passes. Through the control of temperature, extrusion passes, material microstructure can be modified and mechanical properties can be improved. A semi-solid forging, uses modified SIMA prepared billet was conducted, the forging loads were compared under various conditions. In this study, magnesium alloy AZ80 was selected as the raw material, modified SIMA prepared billet, were observed of its microstructure. And semi-solid forgings of modified SIMA prepared billet were conducted. Semi-solid flange forging results indicated that shapes and grain size were main factors in relation with the forging load. More spheroidal and smaller the average sizes of the billet, the smaller forming load.