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

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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.