Abstract
Abstract This work describes the design, construction and testing of a continuous casting device coupled to an electromagnetic stirring system for the production of thixoforming feedstock using Al-Si alloys. The following processing and construction parameters were tested: pouring temperature (660-800 °C), mould cavity roughness (Ra = 0.17-1.06 µm), casting speed (2-4 mm/s), use of a combined (primary and secondary) cooling system and the intensity and radial penetration of electromagnetic stirring (0, 1500 and 3000 W stator power). The macro-appearance of the ingots and the longitudinal and radial microstructural parameters were then evaluated. The results indicate that the surface finish of the mould cavity was an important construction parameter as it had a strong influence on heat transfer and, consequently, feed speed; use of a secondary cooling system for the mould allowed greater grain refinement and microstructural homogeneity to be achieved for all the conditions tested; optimal electromagnetic stirring power was 1500 W (EMFmax = 22 Gauss); this resulted in a grain size-to-dendritic arm spacing ratio of close to 1, which is suitable for subsequent SSM processing.
Highlights
Semisolid metals (SSM) processing involves mechanically forming metal alloys in the semisolid state and can be performed by cooling liquid alloy to the semisolid state (0.2 to 0.5 solid fraction, fS) in a range of operations called rheocasting[1,2] or by heating solid alloys to the semisolid condition (0.4 to 0.8 liquid fraction, fl) in a range of operations known as thixoforming[3]
The dendrite arm spacing (DAS) can be better visualized in these images than in the colour micrographs and can be compared with the results shown in Figure 8 and 9
The development of a continuous casting system that would provide an efficient route for producing Al-Si alloys with a refined microstructure for use as SSM feedstock resulted in the following conclusions: a) ingots produced with combined cooling had the best microstructural homogeneity in all the directions analyzed; b) optimal electromagnetic stirring (EMS) was achieved with a power level of 1500 W (EMFmax = 22 G), which resulted in the most effective grain refinement of all the conditions tested
Summary
Semisolid metals (SSM) processing involves mechanically forming metal alloys in the semisolid state and can be performed by cooling liquid alloy to the semisolid state (0.2 to 0.5 solid fraction, fS) in a range of operations called rheocasting[1,2] or by heating solid alloys to the semisolid condition (0.4 to 0.8 liquid fraction, fl) in a range of operations known as thixoforming[3]. Both processes can be superior alternatives to conventional metal forming and casting operations[4,5]. It is crucial for the process that the remaining solid particles in the liquid have a refined, near-spheroidal microstructure so that the semisolid
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