A356鑄鋁合金為一Al-Si亞共晶型之合金，其結構係由初晶鋁、共晶矽及富鐵相組成。A356合金常藉由添加微量元素來改善其微結構，像是共晶矽型態改變、樹枝狀初晶鋁之細化程度以及合金鑄件之完整性等，進而對其機械性質產生優化。因此本實驗目的在於探討微量Sc元素添加對於A356合金機械性質之影響。 本研究設計三組合金A、B及C來表示A356合金添加三種不同Sc含量(0、0.1、0.25wt%)，經由金相觀察、影像分析來評估樹枝狀間距(Dendrite Arm Spacing, DAS)之細化程度；並以Tatur試驗分析合金鑄件完整性以及氣孔分布，最後由拉伸性質、破斷面分析以及品質指標判斷合金機械性質的改變。 實驗結果指出：合金A、B及C之微結構經影像分析後，合金B相對於未添加Sc之合金A，DAS細化程度為30.7%；而合金C相對於合金A之DAS細化效果則為32.8%。表示添加Sc元素有細化DAS之效果，使脆性之共晶區域面積均勻分布基地中，造成拉伸時微裂縫成長不連續，進而增加合金之延伸率，品質指標亦同時上升；在鑄件完整性(Soundness)方面，經由Tatur試驗及數據的計算後，發現合金B、C之視密度(Apparent density)提升、微縮孔體積低於合金A，表示添加Sc元素會改善合金微縮孔含量，增加鑄件完整性，進而提高合金延性；機械性質方面，合金B、C之延伸率分別為13.1%及12.3%，相較於合金A延伸率7.5%皆有顯著增加，品質指標則分別增加為456.9MPa及457.8MPa，亦高於合金A之品質指標407.6MPa。由此可見Sc元素添加於A356鑄鋁合金中，能大幅提升傳統A356合金之機械性質，並增加其應用範圍。 The A356 alloy which contains primary aluminum, eutectic silicon and Fe-bearing phase is one of the hypoeutectic aluminum-silicon alloys. The characteristic of eutectic region influences the mechanical properties of A356 alloy. Engineers often change the morphology of eutectic silicon by the addition of certain elements. In this study, the effects of trace scandium (Sc) on microstructure and mechanical properties of A356 alloy were investigated. In this study, the A356 alloy with 0wt%, 0.1wt% and 0.25wt% Sc additions were named after alloy A, alloy B and alloy C, respectively The microstructural features and the mechanical properties were elucidated by optical microscopy (OM), scanning electron microscopy (SEM), image analysis, tensile test and Tatur test. The addition of Sc refined dendrite arm spacing (DAS) and caused the brittle eutectic regions more homogeneously distributed in contrast to the absence of Sc. The amount of microporosity in A356 casting decreased with Sc addition. The apparent density and the soundness of A356 casting increased due to the reduction of microporosity. The change of microstructure enhanced the mechanical properties. The homogeneous distribution of eutectic region postponed the crack growing and increased the ductility in contrast to the random distribution. The improved ductility accompanied the increase of tensile strength. And the enhancement of tensile strength and ductility produced outstanding quality index of A356 alloy.
