An Apparatus For Observation & Measurement Of Semisolid And Study Of Morphological Evolution

The microstructures obtained during the semisolid metal processing make up of spheres, rosette like crystals but not the very fine dendritic microstructure. Due to the complex of the study, the fundamental understanding of the particle formation (nucleation, growth and clustering) and the relation between the morphology selection and the rheology is very poor. In order to get more information about the semisolid microstructure and the influence of the shear rate, the temperature gradient and the cooling rate on the formation of undendritic microstructure and final microstructure, and the rheological behaviors during the semisolid processing, it is very necessary to do simultaneously viscosimetric measurements and microscopic observations. On this purpose, we designed two apparatus to study the microstructure and rheology: one is the observation apparatus, the other is the measurement apparatus. The microstructure formation during semi-solid material processing using succinonitrile-5at%water as a transparent model alloy was experimentally studied through direct observation on the observation apparatus. Then we analyses the formation of the sphere micro structure and the agglomeration of the spheres, and the main results are as follows: 1. The designed observation apparatus can work well on the simultaneously microscopic observations and records of the semisolid transparent model alloy at different stages of solidification under a wide range of cooling rate and stirring speed.. 2. The designed measurement apparatus can finish both the simultaneously viscosimetric measurements and microscopic observations of the transparent model alloy on the semisolid processing. 3. It is the first time to show that the globular crystals can form from the direct nucleation of the liquid phase, instead of from fragments of broken dendrites or the coarsening of the dendrites into spherical particles using succinonitrile-5at%water as a transparent model alloy. 4. The shearing rate during stirring has an important influence on the globular formation, and there is a threshold value about the stirring rate on the formation of An Apparatus for Observation & Measurement of Semisolid and the Study of Morphological Evolution Li Tao the spheres. The sudden change of the stirring rate can accelerate the formation of spheres. When the solid fraction became steady, if stopped the stirring and held at the same temperature, the sphere size and morphology changed inconsiderably. 5. At the beginning of stirring, the dendrite and the globular crystal all can appear. If the cooling rate is slow and the shear rate is fast, it is easy to form the globular crystal, and if the cooling rate is fast and the shear rate is slow, it is easy to form the dendrite. 6. After the stirring, even the intial structure being dendrite, it also can become the globular crystal. 7. The analyses of the formation of the sphere microstructure show that the vigorous agitation changes the condition on the interface of crystal growth. It benefits to keep the steady growth of sphere. The calculation also provides it, and the agglomeration of the spheres is the result of the fluid force.