顯微組織對壓鑄式鋁合金A356-T6疲勞性能影響之研究

Effect of Microstructure on the Fatigue Properties of Die Casting A356-T6 Al-Alloy

徐宇彤(Yu-Tung Hsu)

指導教授 ： 徐永富 ； 林新智

國立臺北科技大學/工程學院/材料及資源工程系研究所/碩士(2013年)

摘要

本研究針對商用A356-T6鋁合金汽車輪圈實體取樣後進行顯微組織觀察、成分分析、相鑑定、硬度測試、拉伸試驗及疲勞試驗，疲勞測試應力由50 MPa至150 MPa，紀錄試片破斷的循環次數，製作應力-循環曲線(S-N Curve)。並且針對破斷試片的破斷面進行觀察，與原始材料顯微組織進行比對，以了解疲勞破壞的機制，進而可以評估材料的鑄造缺陷對於疲勞性質的影響。
由實驗結果顯示，本實驗使用之三種鋁合金成分皆符合ASTM B108/B108M-12e1之規範，但其中A356-S之鎂含量偏低導致其Mg2Si析出物量較少，進而影響硬度測試及拉伸試驗結果。由顯微組織觀察發現，三種合金均可發現明顯的α-Al樹狀晶結構以及α-Al與Si的共晶組成，但因為冷卻速率的差異，發現A356-S的鑄造組織最為細小。在三種合金當中，最大差異為鑄造缺陷-縮孔的尺寸，其中A356-R1的縮孔尺寸最大可達500 μm以上。疲勞測試結果顯示，A356-S之疲勞強度為102.98 MPa，A356-R1為65.21 MPa，A356-R2為73.21 MPa。由疲勞破斷面發現，縮孔尺寸及分部為影響此合金材料疲勞性能的主要原因。

關鍵字

鋁合金 ； A356 ； 壓鑄 ； 疲勞 ； 顯微組織

並列摘要

In this study, the commercial A356-T6 aluminum alloy car wheel were properly sampled for microstructure observation, composition analysis, phase identification, hardness test, tensile test and fatigue test. The loading of fatigue test is from 50 MPa to 150 MPa, and the fracture cycle life was recorded for the fitting of stress - cycle curve (S-N Curve). Then the fracture surface of the tested specimen was observed, with the comparison of original microstructure of raw material, we could truly understand the mechanism of fatigue failure.
The experimental results show that all three kinds of aluminum alloy’s chemical composition compliance with the standard specification ASTM B108/B108M-12e1, but due to the lower magnesium content in A356-S, it reduced the amount of Mg2Si precipitation, therefore affecting the hardness and tensile strength. By the microstructure observation, we can see α-Al dendritic structure and the eutectic of α-Al and pure Si in all three Al-alloy, but because of the differences in cooling rate, we found that A356-S has a most fine cast structure. The biggest difference in these three alloy is the size of casting defect¬-shrinkage, where the shrinkages in A356-R1 sizes up to 500 μm or more. The fatigue test results show that, A356-S has a fatigue strength of 102.98 MPa, A356-R1 is 65.21 MPa, and A356-R2 is 73.21 MPa. By observing the fatigue fracture surface, the dimension and distribution of shrinkages is the main reason affects the fatigue property of these alloys.

並列關鍵字

Aluminum alloy ； A356 ； Die-casting ； Fatigue ； Microstructure

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