Methodology of SSM Characterization
The microstructure of the SSM alloy billets plays an important role during fabrication of finished engineering component and its performance in service. As a result, it is critically important to characterize the microstructure to ensure high-quality feed
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Methodology of SSM Characterization
Abstract The microstructure of the SSM alloy billets plays an important role during fabrication of finished engineering component and its performance in service. As a result, it is critically important to characterize the microstructure to ensure high-quality feed stock. This chapter provides a detailed account of the characterization techniques available to SSM researcher to confidently examine the quality of as-cast billets through thermal analysis, rheological characterization, and quantitative metallography.
Conventional solidification of hypoeutectic Al–Si foundry alloys takes place with dendritic formation of primary α-Al phase within the liquid. The alloy composition, temperature gradient within the melt, thermo-fluid convection, and rate of heat extraction and the resulting constitutional supercooling are the most effective parameters on the morphology of the primary α-Al phase. Variation of any of these factors during solidification would alter the as-cast structure. For instance, introduction of agitation (forced convection) into the solidifying melt changes the distribution of alloying elements and localized chemical composition, could remove constitutional supercooling, and promote dendrite-to-equiaxed transition, i.e., break down and globularization of the α-Al phase. Degeneration of the α-Al phase results in some opportunities which are of interest from commercial viewpoints. The advantages of the SSM processing along with different available technologies as discussed in Chap. 2 are enormous. However, the lack of industrial interest in the 80s and 90s stemmed mainly from the high cost of billet preparation, the issue of recycling the returned and scraped parts, and to some extent the lack of proper characterization of the both semifinished billets and finished engineering components. Recently, the cost issue is resolved by introducing novel cost-effective rheocasting techniques and development of new alloying systems. In order to generate a semi-solid structure, the alloy system plays the key role where the coexistence of liquid and solid within a temperature range is the prerequisite for the slurry preparation. The mechanics and mechanisms of the primary particles’ evolution, dendrite to equiaxed transformation, is the next concern since the formation of globular morphology is expected to enhance die filling and improve mechanical properties of as-cast parts. The ideal microstructure for SSM slurry is fine spherical solid particles uniformly distributed within a liquid matrix. © Springer International Publishing Switzerland 2016 S. Nafisi, R. Ghomashchi, Semi-Solid Processing of Aluminum Alloys, DOI 10.1007/978-3-319-40335-9_4
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4 Methodology of SSM Characterization
The solid fraction should be considered carefully, since low fraction solid may lead to SSM slurry handling and mold filling difficulties due to insufficient viscosity and turbulence while high fraction solid adversely affects the die filling and requires more powerful machinery and thus incr
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