A390過共晶鋁矽合金之離心加壓鑄造製程研發

Abstract

A390過共晶鋁矽合金(Al-17Si-4.5Cu-0.5Mg)在重力鑄造時，由於矽的比重(2.33g/cm3)較鋁(2.74g/cm3)低，因此在凝固過程中容易產生初晶矽上浮的現象，而使得同一鑄件在不同位置的機械性質有所不同。本研究係利用矽與鋁比重之差異，在凝固過程中對鑄件施加離心力，促使初晶矽的偏析現象更加顯著，預期鑄件在局部位置能達到提升耐磨耗性之目的。本研究以一自行設計之旋轉裝置進行離心鑄造，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(以下稱內側)集中。藉由分析各種不同澆鑄溫度(780℃、820℃)和轉速(400 rpm、450 rpm、…、650 rpm)下鑄件在不同位置(內側、中央、外側)之顯微組織。實驗結果顯示，當鑄造溫度較高時，偏析現象較為明顯，且初晶矽之偏析程度會隨旋轉速度之增加而提高，當轉速達到550rpm時，偏析程度達到飽和，而不再隨旋轉速度增加而提高。故，本研究之離心鑄造實驗，其溫度及轉速分別設定為820℃及500 rpm。 又，本研究在固定轉速500rpm之離心鑄造條件下，探討P之細化處理(針對初晶矽)與Sr調質處理(針對共晶矽)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，經離心鑄造之A390鑄件其耐磨耗性皆以內側(接近旋轉軸)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)。在細化初晶矽方面，進行細化處理後可使初晶矽顆粒平均尺寸由98μm降低至61μm；在調質共晶矽方面，進行調質處理後可使共晶矽平均調質級數達到AFS 4~5級，但會使初晶矽顆粒平均尺寸增加至208μm。所有爐次中以添加0.02%P之細化劑、持溫10分鐘之細化處理所獲之離心鑄件內側之耐磨耗性為最佳，以數據比較來表示(選擇一比較基準，計算磨耗率之降低百分比)。最後，進行磨耗率與相關參數之綜合迴歸分析，分析結果得知：當初晶矽面積率愈大、初晶矽顆粒尺寸愈小、共晶矽調質級數愈大、硬度愈高，則磨耗率愈小，亦即耐磨耗性愈佳。

While casting the A390 hypereutectic aluminum alloy, primary Si particles tend to float to the upper part of the casting due to the difference in specific weight between the primary Si particles and the aluminum bulk liquid, the so-called gravity segregation. As a result, the mechanical properties in different parts of the casting vary. Utilizing the above-mentioned phenomenon of the primary silicon gravity segregation, the present study further enhanced this segregation nature by employing centrifuging casting process to achieve better wear resistance in specific areas 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 silicon 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. By analyzing the microstructures of different parts of the casting under different pouring temperatures (780, 820℃) and rotational speeds (400 rpm, 450 rpm, …, 650 rpm), we concluded that the degree of primary silicon segregation toward the inner portion of the casting increases with increasing both pouring temperature and rotational speed, and reaches a plateau at 500 rpm. Therefore, a pouring temperature of 820℃and a rotational speed of 500rpm were chosen in this study. In addition, P refinement (for primary Si) and Sr modification (for eutectic Si) were performed to examine the effects of either P refinement or Sr modification on the microstructures of the centrifuging castings at 500 rpm. The microstructure of the A390 alloy without any treatments exhibits somewhat large primary Si particles with an average size of some 98μm, and acicular type eutectic Si of class 1. When the alloy was refined with 0.02%P, the size of the primary Si was substantially reduced to about 61μm, while the eutectic Si remained more or less unchanged. On the other hand, when the alloy was modified by 0.02%Sr, the eutectic Si was improved to class 4-5, while the primary Si was coarsened significantly to about 208μm. Furthermore, the effects of the microstructure features on the wear resistance property of A390 alloy was studied, and regression analyses were performed by correlating 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 hardness (H, HRC) with the wear rate (W, mm3/m). The obtained regression equation is: W = 0.249367 – 0.00225X + 0.000393Y – 0.02212Z – 0.00382H. The regressing equation indicates 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 enhancing harness.