Rheocasting: Low Pouring, SEED, and EMS Techniques
This chapter concentrates on understanding the mechanisms of nucleation and growth in three different rheocasting processes of low pouring, SEED, and electromagnetic stirring processes. The application of thermal analysis, parallel plate viscometry, and q
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Rheocasting: Low Pouring, SEED, and EMS Techniques
Abstract This chapter concentrates on understanding the mechanisms of nucleation and growth in three different rheocasting processes of low pouring, SEED, and electromagnetic stirring processes. The application of thermal analysis, parallel plate viscometry, and quantitative metallography verified the mechanisms responsible for microstructural evolution during the mentioned SSM processes.
5.1
Introduction
As discussed in Chap. 2, there is a range of SSM processes to produce billets having globular or near globular primary particles. If it is required to classify the entire SSM processes, it may be logical to use criteria such as temperature and fluid flow as the main parameters to induce globularity in resulted solidification structure through thermal or mechanical or thermomechanical disintegration of dendritic structure during solidification. In this regard three well-researched processes, which may be classified as mainly thermal (Low Pouring), and a combination of both mechanical and thermal (thermomechanical) of Electromagnetic Stirring, EMS and Swirled Enthalpy Equilibration Device, SEED have been selected to discuss in this chapter.
5.2
Low Pouring Temperature Technique
Pouring temperature directly affects the nucleation and growth of primary crystals such as α-Al in hypo-eutectic Al–Si alloys, where nucleation controls the quantity and size of the grains formed and growth determines the grain morphology and distribution of alloying elements within the matrix. The driving force for nucleation is the degree of undercooling created during solidification and growth is controlled by the temperature gradient and solute concentration in the liquid. However, both processes are affected by the rate of heat extraction. The final structure is therefore dependent on the nucleation density, growth morphology, fluid flow, and diffusion and transport of solutes. Therefore, close control of casting conditions, such as pouring temperature, cooling rate, nucleation sites, and temperature gradient, results in formation of the desired as-cast structure. © Springer International Publishing Switzerland 2016 S. Nafisi, R. Ghomashchi, Semi-Solid Processing of Aluminum Alloys, DOI 10.1007/978-3-319-40335-9_5
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5 Rheocasting: Low Pouring, SEED, and EMS Techniques
From semi-solid standpoint, microstructural variation resulted from pouring temperatures is turned into various viscosities during the application of the external force to the slurries. This is because the flow characteristics, as specified by the viscosity, are dependent on the metallurgical parameters including the fraction solid and its morphology (e.g., dendritic, rosette, or globular), solid particle size and distribution, chemical composition of the alloy, and pouring temperature [1–3]. The effect of solid phase characteristics on the viscosity and rheological behavior of SSM billets is also discussed in details in Chap. 4 of this book. In the current section, effect of pouring temperature is specifi
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