The Effect of Cooling Rate on the Microstructure of High Pressure Die Casting Alloys
Abstract
The current research project explored the effect that heat extraction has on the micro-constituents of the A380 and Silafont 36 high pressure die casting (HPDC) alloys. Phase evolution and distribution, SDAS measurements, and the alpha and beta iron-bearing phases were all examined as a function of heat extraction. Literature was found to be limited on the quantification of the micro-constituents of these two alloys as a function of cooling rate. Different cooling rate apparatuses were used to manipulate the alloys via heat extraction. Magma simulations of the mold were run and Pandat thermodynamic calculations determined the solidus and liquidus of the alloys based on composition. Statistical testing was done on the SDAS measurements. The A380 alpha and beta phase were measured along with the SDAS to create quantitative correlation. Beginning with the A380 microstructure, the FCC-Al, beta/alpha phase, and the Al-Cu phases appeared in the slow and fast cooled sample confirmed by visual and EDS analysis. Cooling rate has the ability to refine microstructure and distribute phases more effectively at higher heat extraction rates but heat extraction rates cannot eliminate the type of phases formed and their specific morphology within the A380 alloy seen at lower cooling rates. The reason is due to the similar phases in fast and slow cooled samples. Higher heat extraction rates can however form unpredicted phase with chemical compositions not usually seen. The reason is due to unique phases with Cu/Zn/Mg found in the A380. The beta phase composition contains Al-Si-Fe and the alpha phase composition contained Al-Si-Fe-Mn. Manganese was also seen to substitute for the Fe to create the Mn-alpha phase with the A380 alloy. The Al-Cu phase appears to have used the iron-bearing phases as nucleation spot thus confirming its phase order to be after that of the FCC, Al-Si eutectic, and iron bearing phases. All confirmed by EDS and visual analysis. The Al-FCC, Alpha-Mn, Al-Si Eutectic, and Mg phase appeared in both fast and slow cooled samples for the Silafont 36. The overall cooling effects were the same as the A380. Higher cooling rates were seen to redistribute the Chinese-script alpha phase into more needle-like morphologies. The magnesium phase should be included in literature as a distinct phase of the alloy. The reason is due to its appearance for both slow and high heat extraction samples and lack of literature representation. The Silafont 36 alloy has a finer Eutectic structure than the A380 alloy at the slower cooling condition seen from the EDS mapping. The alpha phase within the Silafont 36 alloy has more Mn by weight\% than the same phase in the A380 confirmed by spot EDS analysis. The SDAS was statistically different from apparatus to apparatus within the alloys showing that each apparatus provided statistically different conditions. No conclusive relationship was drawn regarding the influence of the alloy content when the alloys SDAS measurement were compared between apparatuses. The SDAS for the A380 alloy and Silafont 36 alloy has an inverse relationship to cooling rate and a proportional relationship with solidification time. The relationship was seen by the shape of the curves comparing SDAS vs. cooling conditions. Both alloys showed to have comparable results to the Magma simulations. No trend was found regarding total \%alloying weight and SDAS. The alpha and beta phase within the A380 alloy are shown to have an inverse relationship with cooling rate the same type of relationship as the SDAS vs. cooling rate.
Degree
M.S.
Advisors
Han, Purdue University.
Subject Area
Medical Ethics|Mechanical engineering|Materials science
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