Abstract
This work aims to evaluate different routes of raw material production to be used in thixoforming process. The semisolid materials exhibit particular structural characteristics, such the characteristic non dendritic structure. The studies for the production of near-net-shape parts have progressed together with the studies of material weight reduction on the automobile industries production lines. In general the advantages are energy saving, manufacturing steps reduction, productivity and quality increasing. Five processing routes have been investigated (conventional casting, electromagnetic stirring, grain refinement technique and different cooling rates) through microstructural characterizations, conventional metallographic and colour metallography. The material used in this study was the A356 alloy (commercial) having a hypoeutectic structure and grain size of about 750 mm and primary dendrite arm spacing size of around 80 mm, the combination of electromagnetic stirring + grain refining + high cooling rate provided the best route processing to obtain a refined structure, with grain sizes of around 240 mm, being feasible at the thixoforming process. Note also that using conventional metallography (black and white) all routes tested showed almost the same primary dendrite arm spacing size values ie had identical characteristics. Importantly, only with the characterization via polarized colour metallography is that was achieved in fact the best processing route definition. Keywords: A356 alloy, casting, electromagnetic stirring, grain refinement, thixoforming.
Highlights
Thixoforming is a technological process that involves the metal alloys forming in the semisolid state (SSM)
The material used in this study was the A356 alloy having a hypoeutectic structure and grain size of about 750 m and primary dendrite arm spacing size of around 80 m, the combination of electromagnetic stirring + grain refining + high cooling rate provided the best route processing to obtain a refined structure, with grain sizes of around 240 m, being feasible at the thixoforming process
The use of the electromagnetic stirring technique has been widely used in the last years for the thixoforming processes raw material production due to its numerous advantages, among them we can mention the absence of contact between the liquid metal and the stirring environment, fact that doesnt happen in the mechanical agitation, allows the continuous and direct casting of billets with varied forms, has low electric energy consumption, among others
Summary
Thixoforming is a technological process that involves the metal alloys forming in the semisolid state (SSM). Among the various possibilities of thixoformable materials, Al-Si alloys are the most frequently used, A357 and A356 alloys present favourable thermodynamic characteristics to thixoforming: at the eutectic temperature, liquid fraction is around 50% and the 50% of solid is the primary alpha phase In this condition, the material thixoformability depends on the morphology and the solid crystals size present in the semisolid: small dimensions and globular morphology are required [7]. The use of the electromagnetic stirring technique has been widely used in the last years for the thixoforming processes raw material production due to its numerous advantages, among them we can mention the absence of contact between the liquid metal and the stirring environment, fact that doesnt happen in the mechanical agitation, allows the continuous and direct casting of billets with varied forms, has low electric energy consumption, among others. This work aims to evaluate different processing routes, using A356 alloy as base material the in terms of microstructural characterization by optical microscopy, using conventional metallographic (black and white) and polarized metallography (colour)
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