Semi-Solid Casting of Aluminium and Magnesium Alloys: Slurry Production and Rheocasting using the Rapid Slurry Forming Technology

Sammanfattning: Semi-solid casting was developed in the early 1970’s and involves the use of a partially solidified metal slurry. This method of casting has become a popular alternative for production of automobile components. Semi-solid casting is mainly directed towards the light metals aluminium and magnesium as an alternative to the high pressure die casting process. The semi-solid casting route mainly considered in this thesis is rheocasting. The new Rapid Slurry Forming (RSF) technology, a vital part of the RheoMetalTM process, has been used exclusively throughout the experiments reported in this thesis. The new technology is based on an enthalpy exchange between two alloy systems to produce the partially solidified metal slurry.

In this thesis a range of slurry process parameters have been studied with the aim of revealing their microstructural consequences on slurry formation and the mechanisms thereof. Parameters were established for (1) production of commercial components and (2) for production of billets used for horizontal reheating purposes. Experimental work showed that slurry process parameters had a significant influence on slurry formation for the aluminium and magnesium alloys tested. The most strongly affected microstructure feature was the final slurry grain size. Slurry formation with the RSF technology occurs by local massive nucleation due to rapid cooling of the melt during processing whilst uniform temperatures are provided by forced convection. Components produced by rheocasting contained significantly reduced porosity as compared to those components made by high pressure die casting. The microstructures obtained in the reheated RSF billets for thixocasting were found to be more favourable than those obtained in commercially available billets. Methods used for evaluation include mechanical testing, microstructure analysis, cooling curve analysis, chemical analysis and X-ray analysis.

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