Abstract
Effects of Semi-Solid Rheocasting (SSR) process on the structure and mechanical properties of Al-7.1wt%Si alloy were studied. The results showed that the non-dendritic structure could be formed by a short stirring period below liquidus temperature provided that stirring was combined with localized rapid heat extraction from the melt. The effect was examined by calculation of the time needed for detachment of dendrite arms at the initial stages of solidification and was associated with a large particle density in the rheocast slurry. Samples processed in semisolid state had lower porosity, higher tensile strength and higher elongation than that processed in fully-liquid state. The SSR- processed sample had a lower tensile strength but a higher elongation than the sample cooled without localized cooling and stirring. It was suggested that the lower strength of SSR-processed alloy was related to the inability of the α-particles to restrain crack propagation through the brittle eutectic matrix.
Highlights
Researches on rheology of semisolid slurries suggest that the ideal slurry for semisolid metal (SSM) processing is one containing a suitable volume fraction of fine and spheroidal particles uniformly dispersed in a low melting liquid matrix [1]
The dendrites are surrounded by eutectic constituents which appear darker in the micrograph
The figures suggest that the formation of non-dendritic structures in rheocast alloys can be the result of a significantly greater particle density than that of the dendritic structure in conventional solidification
Summary
Researches on rheology of semisolid slurries suggest that the ideal slurry for semisolid metal (SSM) processing is one containing a suitable volume fraction of fine and spheroidal particles uniformly dispersed in a low melting liquid matrix [1]. In the second type of processes, formation of dendritic particles at the start of solidification is prevented by controlling the cooling conditions of the liquid These processes are more time and cost efficient than those based on the dendrite fragmentation mechanism. Several rheocasting processes based on the nucleation mechanism have been developed in recent years, some of which are being commercialized in the casting industry These include, for example, include the NRC (New RheoCasting) [3], the SEED (Swirled Enthalpy Equilibration Device) [4], Low superheat pouring with a shear field (LSPSF) [5], Gas induced semi-solid (GISS) [6], Cooling slope [7] SSR (SemiSolid Rheocasting) [8] and RSF (Rapid Slurry Forming) [9] processes. Attempt is made to explain the microstructural changes brought about by the SSR process based on its effect on the density of the primary particles formed during the early stages of solidification
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