A390過共晶鋁矽合金之初晶矽偏析行為及耐磨耗性研究

Abstract

A390（Al-17Si-4.5Cu-0.5Mg）過共晶鋁矽合金在鑄造時，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現像，使得同一鑄件在不同位置的機械性質不同。本研究利用此比重之差異，在凝固過程中對鑄件施加離心力，促使初晶矽偏析的現象更加顯著，而預期所生產之鑄件在局部位置能達到提升耐磨耗性之目的。本研究設計一旋轉裝置，A390鑄件之凝固過程在旋轉狀態下進行，而使初晶矽在鋁液內藉由離心力會往旋轉中心側（以下稱內側）集中。由分析各種不同轉速(50rpm、100rpm、150rpm與200rpm)下鑄件不同位置(內側、中央、外側)之顯微組織可以得知，初晶矽之偏析程度隨旋轉速度之增加而增高，且當轉速達到150rpm時，偏析程度達到飽和，而不再隨旋轉速度之增加而增高。故，本研究之離心鑄造實驗，其轉速均設定為150rpm。 又，本研究在固定旋轉速度150rpm之離心鑄造條件下，探討P之細化處理(針對初晶矽)與Sr之調質處理(針對共晶矽)對於鑄件之內側、中央及外側顯微組織的影響，並進一步探討上述之合金處理對於耐磨耗性之影響。結果顯示，A390+0.02%Sr鑄件內側之耐磨耗性最佳，其磨耗率較A390離心鑄造之鑄件降低約51%。另外，對於影響耐磨耗性之各參數對耐磨耗性做綜合迴歸分析，可以得知初晶矽的面積率越高、孔洞面積率越低、初晶矽顆粒尺寸越小及共晶矽調質效果越佳(範圍Class 2~ Class 5)，則耐磨耗性越佳。

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 mechanical properties in different parts of the casting vary. This research takes the advantage of the above-mentioned phenomenon of the primary silicon gravity segregation, and 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 tend to move toward the rotational axis and aggregate at the inner portion of the castings. The experimental results indicate that the degree of primary silicon segregation toward the inner portion of the casting increases with increasing rotational speed, and reaches a plateau at 150 rpm. On the other hand, the P refinement (for primary Si) and Sr modification (for eutectic Si) of A390 alloy were performed to examine the effects of either P refinement or Sr modification on the microstructures of the centrifuging castings at 150 rpm. Further, the relationship between the microstructure features obtained and wear resistance property was evaluated. The results indicate that the inner section of the A390+0.02%Sr centrifuging casting exhibits the best wear resistance performance, which is 51% better in wear rate while compared with the A390 centrifuging castings without any treatments. In addition, segregation analysis were performed and the findings conclude that the higher the % primary Si, the lower the % porosity, the smaller the primary Si particles size, and the higher the modification degree (form class 2 to class 5), the better the wear resistance property of the A390 alloy.