Microstructure of Al-Cu Alloy in Rapid Solidification by CO_2 Laser

新的合金组织可望在快速凝固时，当熔体凝固时其生长速度接近绝对稳定极限。如果生长速率接近溶质的扩散速度，则达到该极限生长速率。本文以低溶质含量Al-Cu合金为研究对象，研究了Al-Cu合金快速凝固过程中凝固条件与组织的关系。由于低的溶质浓度会导致合金的微观组织变大，因此，它使合金的绝对稳定条件易于实现，也使合金的微观组织易于观察。高功率CO_2激光器（最大）6 KW）快速熔化凝固，实现每秒数米的快速凝固。用MARC数值分析方法研究了熔体中的温度分布，得到了固液界面处的温度梯度和生长速率，在Al-0.5mol%Cu合金中发现了胞状组织，其生长速率为1.7m/s。该生长速率与微扰理论预测的绝对稳定生长速率2.4m/s非常接近。而溶质浓度为1.0和4.5mol%Cu的合金则不呈胞状组织，而是呈枝晶状组织。初生茎直径随生长速率的增加以-1/2次方递减。

New microstructure of alloy is expected to be in rapid solidification, when the melt solidify in its growth rate close to the the absolute stability limit. This limiting growth rate is reached if the growth rate approaches to the speed of diffusion of the solute. The purpose of this work is to study the relation between the microstructure and solidification condition in rapid solidification of Al-Cu alloy.Low solute Al-Cu alloy is used in this experiment. It makes easy to realize the absolute stability condition and it also makes easy to cbserve the microstructure, because low colute concentration will result larger microstructure of alloy. High power CO_2 laser (Max. 6KW) is used to melt and solidify rapidly, and rapid solidification with a few meter per second is realized. The temperature distribution in the melt is studied by numerical analysis MARC.In terms of this distribution the temperature gradient and the growth rate at the solid-liquid interface is obtained.Cellular microstructure is found in Al-0.5 mol%Cu alloy, which is solidified with growth rate 1.7 m/sec. This growth rate is very close to the absolutely stable growth rate of this alloy, which is predicted to be 2.4m/sec by the pertubative theory. Other alloys with thjeir solute concentration 1.0 and 4.5mol%Cu alloy do not show cellular structure but dendritic microstructure. The diameter of the primary stem decreases with growth rate by power-1/2.