離心加壓力及熔液處理對於A390過共晶鋁矽合金之顯微組織及耐磨耗性之影響

Effects of Centrifuging Force and Melt Treatment on the Microstructure and Wear Resistance Property of A390 hyper-eutectic Al-Si Alloy

賴易辰(Yi-Chen Lai)

指導教授 ： 潘永寧

國立臺灣大學/工學院/機械工程學研究所/碩士(2016年)

https://doi.org/10.6342/NTU201600842

摘要

A390過共晶鋁矽合金在重力鑄造時，由於矽的比重(2.33g/cm3)較鋁 (2.74 g/cm3)低，因此在凝固過程中容易產生初晶矽(primary silicon)上浮的現象，而使得同一鑄件在不同位置的機械性質不一致。本研究係利用矽與鋁比重之差異，以一自行設計之旋轉裝置進行A390鑄造，使得A390之凝固過程在旋轉狀態下受到一穩定離心力作用，促使比重較低、硬度較高的初晶矽產生顯著的徑向偏析，往旋轉軸中心側聚集，預期鑄件在該局部位置能得到較高比例的初晶矽分佈，進而達到提升耐磨耗性之效果。本研究在固定轉速150 rpm(重力倍數約10 G)之離心鑄造條件下，探討P之細化處理(針對初晶矽)、Sr之調質處理(針對共晶矽)以及P+Cr複合細化處理(針對初晶矽)等三種不同熔液處理對於A390過共晶鋁矽合金之顯微組織及耐磨耗性之影響，最後利用迴歸分析以獲致各參數與耐磨耗性之間的關係。
實驗結果顯示，所有經離心鑄造之A390鑄件其耐磨耗性皆以內側(接近旋轉軸中心)為最佳，符合本研究之預期結果(以離心鑄造提升A390初晶矽集中處之耐磨耗性)。對於細化A390之初晶矽而言，以0.02%P之細化劑、持溫5分鐘，其細化效果最佳，可使初晶矽平均尺寸由96µm降低至43µm，且單獨添加含P之細化劑其細化效果優於P + Cr之複合細化劑；在調質共晶矽方面，添加0.02%Sr調質劑後，須持溫10分鐘以上方能完全發揮其調質效果，共晶矽平均調質級數可達AFS 4~5級。所有爐次中以添加0.02%P之細化劑、持溫5分鐘之細化處理所獲離心鑄件內側之耐磨耗性為最佳，其耐磨耗性較未施以任何處理、Sr調質處理以及P+Cr複合細化處理等均佳，且較未處理之A390重力鑄件上層提升約2倍。另外，對於影響耐磨耗性之各參數作綜合迴歸分析，由迴歸分析結果可知：當初晶矽面積率愈高、初晶矽顆粒尺寸愈小、共晶矽調質效果愈佳及孔洞面積率愈低，則耐磨耗性愈佳。

關鍵字

A390 ； 離心加壓力 ； 初晶矽偏析 ； 細化處裡 ； 共晶矽調質處理 ； 耐磨耗性

並列摘要

While casting the A390 hypereutectic alloy, primary Si particles tend to float to the upper part of the casting due to the difference in specific weight between primary Si particles and the aluminum bulk liquid, the so-called gravity segregation. As a result, the microstructures and the mechanical properties in different parts of the casting vary. This research utilized the above-mentioned phenomenon of the primary Si gravity segregation, and further to enhance this segregation nature by employing centrifuging casting process to achieve better wear resistance in specific area of the castings. A centrifuging casting apparatus was constructed which allows the casting to be solidified under the centrifugal force. Under the centrifuging casting condition, the primary Si particles which have low specific gravity and high hardness tend to move toward the rotational axis and aggregate at the inner portion of the castings, and thus to enhance the wear resistance of the A390 alloy. In addition, P refinement (for primary Si), Sr modification (for eutectic Si) and P + Cr refinement (for primary Si) of A390 alloy were performed to examine the effects of these different melt treatments on the microstructures and wear resistance property of the centrifuging castings at the rotational speed of 150 rpm (~10G). Further, the relationship between the microstructure features obtained and the wear resistance property was evaluated.
The microstructure of the A390 alloy without any treatments exhibits somewhat large primary Si particles with an average size of around 96μm, and acicular type eutectic Si of class 1. When the alloy was refined with an appropriate amount of P, i.e., 0.02%P, the size of the primary Si was substantially reduced to around 43μm, while the eutectic Si remained more or less unchanged. In addition, when the alloy was refined with various combinations of P and Cr, no beneficial effect in reducing the size of the primary Si particle was found. Instead, a slight increase in primary Si size was obtained, with the average size being increased to around 65μm. On the other hand, when the alloy was modified by 0.02%Sr, even though the eutectic Si was improved to class 4-5, the primary Si was coarsened significantly to around 250μm. Furthermore, the effects of the microstructure features on the wear resistance property of A390 alloy was studied, and regression analyses were performed by looking into the correlations between the four main dependent variables, namely, the area ratio of the primary Si (X, %), the average primary Si size (Y, μm), degree of eutectic Si modification (Z, class), and the % porosity (C, %), and the wear rate (W, mm3/m). The obtained regression equation is: W = 0.029245 – 0.00098X + 0.000045Y – 0.00347Z + 0.001448C. The analysis results indicate that the wear rate can be reduced by increasing area ratio of the primary Si, decreasing primary Si size, increasing degree (class) of eutectic Si modification, and reducing % porosity.

並列關鍵字

A390 ； Centrifuging force ； Primary Si segregation ； Refinement ； Eutectic Si modification ； Wear resistance property

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