THE INFLUENCE OF BUOYANCY ON EQUIAXED GRAIN NUCLEATION AND GROWTH CHARACTERISTICS AS OBSERVED VIA IN SITU X-RADIOGRAPHY OF AL-CU ALLOYS

摘要

Laboratory-based near-isothermal solidification experiments were performed on a number of Al-Cu alloys utilising in situ real-time X-radiography. These experiments were carried out in preparation for an upcoming sub-orbital microgravity solidification experiment on board the European Space Agency Sounding Rocket, MASER 13, with a view to quantifying the buoyancy effects on solidification dynamics within the sample cell. A number of thin (200 urn) Al-Cu samples, of varying copper concentrations and Al-5Ti-lB (wt%) grain refiner addition levels, were solidified near-isothermally at constant cooling rates in the range 0.025 to 0.5 K/s, thereby promoting equiaxed solidification within the X-ray field-of-view (FOV). To control the effect of gravity-induced equiaxed grain buoyancy, the X-ray FOV was oriented both perpendicular and parallel to the gravity vector, with the latter case exhibiting solidification dynamics similar to those observed under normal casting conditions. In the former case, however, with gravity acting along the thin dimension of the sample, macro-scale equiaxed grain buoyancy was largely eliminated from the FOV, providing solidification conditions similar to those observed under microgravity conditions. It was observed that, using a combination of low cooling rates and inoculant addition levels, equiaxed grain buoyancy caused a significant reduction in the final grain size due to early grain coherency resulting in a cessation of primary growth. Higher inoculant addition levels caused an increase in the grain density, resulting in a lower sensitivity to cooling rate, along with a morphological change from dendritic to cellular-equiaxed.