升溫速率與持溫時間對超細晶粒AZ80鎂合金半固態顯微結構之影響

Abstract

鎂合金具有質輕、比強度高等優點，但在室溫下其成形性不佳，以致它在應用上不如鋁合金廣泛。半固態成形是在固液相共存時進行加工的成形技術，比起傳統鑄造法，半固態成形可製作出機械性質良好的工件，適合應用於鎂合金。 半固態鑄造╱鍛造是半固態成形技術中成本相對較低的成形方法，此方法需要先準備具有球狀晶的胚料，再把胚料升溫到半固態溫度進行加工成形；而大量塑性變形法中的等徑轉角擠製法能產生超細晶粒，此超細晶粒材料在升溫到半固態後會得到尺寸較小之球狀晶，適合作為半固態成形之胚料。 本研究主要探討半固態加工溫度、升溫速率與持溫時間對超細晶粒鎂合金半固態顯微組織與機械性質之影響。利用高週波感應加熱實現對材料快速升溫，觀察材料在加熱與半固態持溫過程中顯微組織變化；並且利用壓縮試驗，比較經過等徑轉角擠製的超細晶粒材料與原素材在成形性上的差異。 實驗結果顯示半固態加工溫度對晶粒尺寸有重大影響；較快的升溫速率、較小的晶粒尺寸以及較好的圓球化效果都有助於半固態成形；而經過等徑轉角擠製的材料，可以縮短半固態成形的持溫時間。

Magnesium alloy has many advantages, such as light weight, high specific strength, but its formability is limited under room temperature, so that it is not used widely as aluminum alloy. Semi-solid forging and casting is a low cost process that can be used in magnesium forming, it can make good products, this process need the billets that have spherical grains while they are heated to semi-solid temperature for forming. In this study, the microstructure evolution influenced by holding temperature, heating rate and holding time were studied, ultra-fine grain AZ80 produced by ECAE (Equal Channel Angular Extrusion) was compare with As Extrude AZ80 in semi-solid state. In this study, we used the high frequency induction heating to provide a fast heating rate. And observe the microstructure evolution in the heating and temperature holding process after quenching. The result showed that holding temperature had great effect on grain size in semi-solid state; smaller grain size and more circular spherical grain shape were benefit for semi-solid forming. ECAE AZ80 was a better choice for semi-solid forming than As Extrude AZ80 as billet material.