Coupling effect on rheology and microstructure of semi-solid SCN-5H₂O model alloy

The most important characteristic of semi-solid processing is that the solidification microstructure changes markedly from dendritic growth under traditional conditions to non-dendritic or globular growth, which results in apparent viscosity greatly reduction. In another hand, semi-solid slurry shows admirable fluidity compared with traditional processing slurry. With the intensive development of semi-solid meta1 processing technology, the interaction between melt flow and solidification microstructure becomes gradually one of the important fundamental research fields in materials science. In the previous work, it is very difficult to measure apparent viscosity and observe microstructure of semi-solid simultaneity using opaque metal materials. Conclusions of rheological behavior and microstructure evolution in semi-solid processing can be drawn qualitatively. Because of the metal materials not transparency and the researches on microstructure only after quenching, it is difficult to measure rheological behavior and observe dynamic microstructure evolution simultaneously. Therefore, it is also hard to grasp the coupling effect on rheology and microstructure of semi-solid alloy, which has affected the understanding and controlling accurately on solid forming process. In present paper, the coupling effect of shear rate and the processing (continuous cooling and remelted) processes on the rheology and microstructure of semi-solid SCN-5H₂O (molar fraction, %) transparent model alloys were investigated using rotating viscometer. It is found that the increase of shear rate will lead the apparent viscosity of the semi-solid alloy to present a transient shear-thickening and steady shear- thinning behavior, correspondingly, the size of non-dendritic microstructure decreases and the distribution of aggregate becomes more disperse with increasing the shear rate, while, the shape factor of the non-dendritic microstructure changes little; a larger particle size and slightly lower shape factor for non-dendritic microstructure were obtained in the semi-solid alloy slurry made by re-melted process on comparison with that by continuous cooling process, meanwhile, the aggregation of the aggregates becomes stronger, which also results in that the apparent viscosity of the semi-solid alloy slurry made by re-melted process is larger than that by continuous cooling process.