In the rheometal process when stirring of the liquid using EEM at different stirring speed is carried out. During the stirring the slurry formation takes place within 30 seconds and water quecnhing is also carried out. Due to the variation of stirring speed the refinement in the morphology of eutectic Si takes place. If slurry comprised solid fraction iabout 30% then eutectic forms during stirring or at the time of water quenching.
Eutectic formation is only dependent on composition and temperature. If I assume you are handling homogenious liquid then eutectic will form only below the critical temperature; then it has nothing to do with stirring speed. Important thing that is missing from your question is about the alloy system !!!
The alloy used in the process was A356 alloy. I searched some paper on the above-cited problem and found that investigations with same problem in the other rheocasting process were carried out in the past. From their observations it was found that if the eutectic temprature decreased then modification in the morphology of Si occurred. In connection to this DSC analysis of A356 alloy was carried out and was compared with the conventional cast A356 alloy. The results also showed similar kind of observation as observed by the different investigators. So due to such findings I think the modification in the morphology of eutectic Si takes place.
Dear Robin Gupta,
I cannot pretend to be an expert in this field. Further, being retired I have no access to facilities like a library. I was puzzled by your issue. From your description I got the idea that crystal segregation may play a part. The extent of crystal segregation depends on the cooling conditions and firstly influences the primary alpha phase. Later on, depending on the cooling rate, eutectic Si can be allowed to form. The resulting morphology of eutectic Si may hence depend on the extent of the crystal segregation, whereas the undercooling reached for the eutectic reaction and the possible presence of grain refiners may also play their parts. Hopefully, my suggestions are helpful.
Thank you sir for your kind reply. It may be one of the reason for the modification in the morphology of eutectic Si. Since I have never worked on the crystal segregation so I have no idea about that. But I will search for this and it may help to think in new directions.
Structure of the slurry for Rheohrocess is a-grain + eutectic (a+Si)/, The main problem in Rheo is uniform nucleation of a-grains and formation grains with good shape - factor and with similar grains size. Eutectic (a+Si)( should be uniformly distributed around grains. Morphology of Si eutectic depends of stiring time, temperature, refinement components etc/
In rheometal process the eutectic SI was uniformly distributed around the matrix of primary alpha Al phase. In this process stirring time and temperature for slurry formation was same for all the stirring speed. Grain refiner was not added in the process but due to different stirring speed the grain refinement of primary alpha Al phase takes place.The refinement and modification in the morphology of eutectic Si was also observed.
Nucleation of the alpha phase grains provided by special condition of the melt pouring ( see NRC (New Rheocasting Process)
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