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Effect of cooling rate on solidification characteristics of aluminum alloys A356 and AA5182 Thompson, Stephanie

Abstract

Microstructural reactions during solidification of aluminum alloys A356 (unmodified and strontium modified) and AA5182 were analyzed using cooling curve analysis to determine the evolution in fraction solid. The experiments involved recording the temperature/time data under different cooling rates so that the various phase changes associated with solidification could be identified. Once collected, the cooling curve data was processed to calculate the first derivative together with a baseline cooling curve (cooling curve in the absence of any transformations). Comparison of the baseline cooling curve to the first derivative data allowed accurate identification of start temperatures of the various liquid to solid transformations and characterization of fraction solid formed during the cooling process. In A356, strontium modification altered the morphology, decreased the solidification start temperature of new phases (2°C to 11°C) and increased solid fraction of the beginning of eutectic solidification and Mg2Si precipitation (0.01 to 0.05). For the conditions studied, cooling rate appeared to show no effect on fraction solid evolution but an increase in cooling rate reduced solidus temperature (48°C to 75°C), resulting in a larger freezing range. In AA5182 the main eutectic reaction and subsequent reactions, including Mg2Si formation, occurred at much lower temperatures and higher fraction solids than previously published. The primary eutectic was found to begin solidifying between 575- 588°C, which corresponds to a solid fraction between 0.87-0.91 and Mg2Si was found to precipitate between 551-560°C, which corresponds to a fraction solid between 0.96-0.97. Increasing cooling rate from 0.5°C/s to 2°C/s was observed to result in a slight increase in fraction solid for primary eutectic transformation from 0.87-0.88 at the low cooling rate to 0.89-0.91 at the intermediate cooling rate and 0.91 at the highest cooling rate. The highest cooling rate also resulted in a drop in solidus temperature to 461°C from 500- 510°C at the low and intermediate cooling rates. This led to an increase in the solidification interval from 123°C to 151°C.

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