PNNL

Abstract

This study reports the first use of in-situ synchrotron X-ray diffraction (SXRD) to study the effects of ultrasonic melt processing (USMP) on phase and grain size evolution during solidification in a commercial Al-Si-Mg casting alloy. USMP is a technique that, when applied to aluminum as it solidifies, can be used to refine the local microstructure of large-scale castings. The ultrasonically refined, solidifying microstructure has a more powder-like 2D SXRD pattern, with smaller and more numerous primary a-Al diffraction spots. Analysis of these in-situ data to estimate the average grain size of primary a-Al grains during USMP demonstrates that USMP slows the growth rate of a-Al crystallites and reduces grain size by 36%. There is also evidence that USMP causes the primary a-Al crystallites to move relative to the X-ray beam; such motion increases the probability of primary a-Al grains colliding and fragmenting. This movement becomes constrained at the onset of the Al-Si binary eutectic, suggesting that USMP ceases to effectively refine the microstructure once the Al-Si binary eutectic begins to form. Complementary laboratory-scale X-ray diffraction (XRD) data were used to correlate the lattice parameters of the a-Al and Si (D-A4) phases with temperature in order to estimate cooling rate during solidification. Thus, this study can guide the design of novel castings with spatially distributed fine-grained regions produced using local ultrasonic processing.