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

新合金的显微组织应处于快速凝固状态，当熔体凝固时其生长速度接近于绝对稳定极限。当生长速度接近溶质的扩散速度时，达到这个极限生长速度。研究了Al-Cu合金快速凝固过程中组织与凝固条件的关系。本实验采用低溶质铝铜合金。合金的绝对稳定条件容易实现，显微组织也容易观察到，因为较低的溶质浓度会使合金的显微组织变大。采用大功率CO_2激光（最大6KW）快速熔化和凝固，实现了每秒几米的快速凝固。采用MARC数值分析方法对熔体温度分布进行了研究。根据这种分布，得到了固液界面处的温度梯度和生长速率。Al-0.5 mol%Cu合金中存在胞状组织，凝固时生长速度为1.7 m/sec。这一生长速度非常接近该合金的绝对稳定生长速度，即由微扰理论预测的2.4m/sec。当Cu的溶质浓度为1.0和4.5mol%时，合金不表现为细胞组织，而表现为枝晶组织。主茎直径随生长速率的增大而减小1/2次幂。

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.