Microstructural Evolution of A356 During NRC Processing

An experimental apparatus using a cooling slope has been developed to mimic the UBE New RheoCasting (NRC) process. The microstructural characteristics of A356 aluminium alloy processed by this route were characterised with this apparatus. The final microstructure is very sensitive to the superheat of the melt. Low pouring temperatures promote spheroidal morphology of the α-Al phase and longer holding times in the semisolid condition improve the spheroidicity of these particles. The microstructure is not completely homogeneous if the cooling slope product is cast in a cold chill mould. However, a great improvement in homogenisation and spheroidization of the α−Al phase is obtained if the mould is preheated and the duration of the holding time in the semisolid condition (at 580oC) is increased. Quenching from different temperatures and holding times in the semisolid state allowed the characterization of the microstructural evolution of α-Al particles. Using optical microscopy, regions of quenched solid were easily distinguished from the surrounding quenched liquid. The results reveal that the cooling slope is an effective source of crystal seeds and, provided there is sufficient holding time at the semisolid temperature, the process gives successful production of raw material suitable for thixoforming.