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Researches on Microstructure Formation and Rheology during Semi Solid Processing Through in Situ Observation Technique

Author: LiTao
Tutor: HuangWeiDong
School: Northwestern Polytechnical University
Course: Materials Processing Engineering
Keywords: semisolid spherical microstructures solidification morphology convection interface stability rheology powder
CLC: TG249
Type: PhD thesis
Year: 2003
Downloads: 574
Quote: 13
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Abstract


At present, the exact formation mechanism of globular structure during solidification under forced convection seems still unclear, but an intensive understanding of that mechanism is crucial to the successful development of SSM processing technologies. So the formation mechanism and the coupling relation between the formation mechanicsm and rheological characteristics of SSM slurries are research focuses and attracting increasing attention from scientists owing to the complexity of the solidification behaviors and rheology characters, and from engineers since it must be controlled carefully for successful forming operations. Considering all of these, a series of experiments on a succinonitrile-5at% water transparent model alloy and a Sn-15wt. %Pb alloy were carried out to investigate the primary phase morphology evolution behaviors, the formation mechanism of non-dendritic structures under the shear condition and the relationship between it and the slurry viscosity during semi-solid material processing. Three self-made apparatus were employed in the present experiments: transparent model alloy SSM observation apparatus, transparent model alloy SSM observation and measuring apparatus, metal alloy SSM slurry fabrication apparatus. The main results are as follows:The exact mechanism of the formation and evolution of globular primary microstructure under forced convection was determined as not dendrite fragmentation but growth controlled.The process of primary microstructure formation of succinonitrile-5at%water transparent model alloy and Sn-15wt. %Pb alloy were studied with the self-made apparatuses. The first observable primary microstructures were spherical structures with a shear beginning above the liquidius. The shear promotes the spherical structure formation at a higher temperature. This result indicates that the spherical structure can be direct formed form the melt with the stirring and a discontinuous shear rate can accelerate the formation of spherical structure.. Dendrites can be broken up if the stirring beginning under the liquidus, but the morphology of fragmentation is not global.Based on the morphological stability of a solid particle growing from a diffusion-controlled undercooled binary alloy melt, the particle movement in the melt under the stirringwas studied. Then a one-dimensional mathematical analysis was performed on the redistribution of solute of a particle growing from the convected melt. A criterion of spherical particle relative stability is established for the particle growth in convection-controlled undercooled binary alloy in view of the promotion of shear on the nucleation. It is can be seen that the shearing increase the stability of sphere interface and the sphere can stably grow to a critical radius with a shearing condition.A transparent model alloy SSM measurement and observation apparatus was invented, and imployed in researches on the coupling relation between primary microstructures and the rheology of SSM slurry.Under the continuous cooling condition, the experimental results show that the spherical structures at a low solid fraction have no important influence on the slurry viscosity. While the solid fraction exceeds one certain value, the apparent viscosity increases rapidly. There exists a temperature fluctuant point which may be the result of multi-nucleation. The experiments make sure that the viscosity depends on the degree of agglomeration rate of solid particles, which, is the result of a dynamic equilibrium between agglomeration and deagglomeration process under the shearing condition. A rheological model which combining the temperature, shear rate, particle radius with the agglomeration degree was developed and it shows good agreement with the experimental results.Based on the intensive studies on the evolution of primary spherical microstructure in the whole mush zone with different solidification conditions and the optimization of the controlling parameters, the semisolid coaxial-cylinders rotational dispersed power fabrication technolo

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