本研究針對 A206(Al-Cu-Mg)及 A356(Al-Si-Mg)鋁合金在重力鑄造過程中之熱裂與孔隙缺陷進行模擬與實驗分析。模具設計採用中心澆口搭配六根不同長度之細長鑄件，並於其尾端配置小冒口；這些冒口之方向故意設計與重力方向相反，以避免有效補縮，使鑄件於凝固時段產生應力累積。評估不同冒口尺寸（Ø21.2mm 與 Ø25.7 mm）下的斷面模數對最大主應力與熱裂風險之影響。再配合MAGMA 模擬用以比對熱裂嚴重之區域的應力值大小及溫度變化。A206 合金在冒口尺寸為 Ø21.2 mm 之下顯示高熱裂敏感性，其臨界主應力分布可分為四區域，最高可達 21 MPa，熱裂發生與澆口與鑄件模數比 S/C 具高度關聯。A356 則在相同模具尺寸(Ø21.2 mm)下並未觀察到熱裂，顯示其具優異抗裂能力，但發現加大冒口尺寸能夠讓模數比 S/C 的應力降幅增大，進一步降低熱裂風險。進一步透過冷卻速率與孔隙率回歸模型建構，推得 A206 合金具有較低孔隙生成傾向，尤其在 Ø21.2 mm 冒口設計下。模擬亦指出提高模溫可有效降低主應力並抑制熱裂生成，惟 HTS 指標未能完全反映熱裂發生位置與應力交互關係。綜合而言，本研究建立一套整合模數比、應力分布與孔隙生成預測模型，對鋁合金鑄件設計與製程優化提供具體依據。

This study investigates hot tearing and porosity defects in A206 (Al-Cu-Mg) and
A356 (Al-Si-Mg) aluminum alloys during the gravity casting process through both simulation and experimental analysis. The mold design incorporates a central gate feeding
six elongated castings of varying lengths, each equipped with a small riser at its end.
These risers are deliberately oriented against the direction of gravity to inhibit effective
feeding, thereby promoting stress accumulation during solidification. The effect of riser
diameter (Ø21.2 mm and Ø25.7 mm) on the section modulus and its influence on maximum principal stress and hot tearing susceptibility was evaluated. MAGMA simulation
was employed to correlate the magnitude of stress and thermal evolution in regions
exhibiting severe hot tearing.
The A206 alloy demonstrated high hot tearing sensitivity under the Ø21.2 mm
riser condition. Its critical principal stress distribution could be divided into four zones,
with a peak value reaching 21 MPa. The occurrence of hot tearing was strongly correlated with the modulus ratio between the sprue and casting (S/C). In contrast, no hot
tearing was observed in the A356 alloy under the same mold configuration (Ø21.2 mm),
indicating its superior resistance to cracking. Furthermore, increasing riser size resulted
in a greater reduction in stress associated with the S/C modulus ratio, thereby lowering
the risk of hot tearing.
A regression model constructed using cooling rate and porosity data revealed that
the A206 alloy exhibits a lower tendency for porosity formation, particularly under the
Ø21.2 mm riser design. Simulations also indicated that elevating the mold temperature
effectively reduced principal stress and suppressed hot tear formation. However, the
HTS (Hot Tearing Susceptibility) index did not fully capture the spatial correlation between stress concentration and actual tear initiation.
In summary, this study establishes an integrated predictive framework combining
modulus ratio, stress distribution, and porosity formation to provide a quantitative foundation for optimizing aluminum alloy casting design and process parameters.

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