The Role of Cooling Rate on Microstructure in Sand-Cast Aluminum A205

Abstract

Modern aerospace castings consist of highly complex geometries that require solidification simulations to optimize fill pathways so to control the thermal evolution and thus microstructure during solidification. Accurate solidification simulations require knowledge of the melt physical properties, mold-filling sequence, mold material properties and subsequent validation by thermal and microstructural measurements. The present work describes the most significant challenges and detailed methodology to maximize a multi-thermocouple acquisition system used to measure transient temperatures during solidification of aluminum alloy A205, and to correlate the measured transients to microstructural observations for three complex full-scale castings and a control refractory cone geometry. The control geometry was used to directly correlate a continuous gradient in cooling rate to variations in the microstructural grain size. The resulting microstructures and their morphologies in the local regions surrounding the thermocouple tips were linked via an empirical relation to the calculated cooling rates. The empirical relation incorporates the TiB2 grain refiner level and an approximation of the A205 alloy-specific growth restriction factor, Q. Results indicated that there was a high correlation (R^2=0.99) to an inverse square root relationship in the measured grain sizes (d) with respect to the calculated initial cooling rates (T ̇) during solidification of the primary α-Al phase for material from all sampled molds as d(μm)≈A/[4.5]^(1⁄3) +B/(85∙T ̇^(1⁄2) ) (i) with the fitting parameters A=55.0±9.2 μm∙[wt.%]^(1⁄3) and B=1371±121 μm∙K^(3⁄2)∙s^((-1)⁄2). This relationship is important because it allows for the average as-cast grain size to be predicted in A205 alloy for initial cooling rates commonly observed during sand casting. The microstructural knowledge of A205 alloy was also extended into the 3rd dimension through micro-computed tomography, indicating that the cast microstructure is isotropic and granular in nature, even in the presence of large thermal gradients.