A.G. Murphy

Microgravity and Hypergravity Observations of Equiaxed Solidification of Al-Cu Alloys Using In Situ X-Radiography Recorded in Real-Time on Board a Parabolic Flight

During solidification of metallic alloys, thermosolutal natural convection plays a significant role in grain nucleation, subsequent growth and morphology, as well as the formation of casting defects. In this work, an Al-5wt%Ti-1wt%B inoculated Al-20wt%Cu alloy was solidified, near-isothermally, using a Bridgman-type gradient furnace, while being monitored in real-time via in-situ X-radiography as part of a parabolic flight microgravity campaign. Each parabola consisted of a transition through 24 seconds of hypergravity (1.8 g), followed by 22 seconds of microgravity, and a then a further 24 seconds of hypergravity. Solidification was controlled such that nucleation occurred coincident with the onset of microgravity. This allowed for the effects of microgravity on equiaxed nucleation and initial growth, followed by continuing solidification in hypergravity, to be observed, as well as the effect on the semi-coherent grain structure when transitioning between the two.

In situ X-ray observations of gas porosity interactions with dendritic microstructures during solidification of Al-based alloys

In situ X-radiography solidification experiments were performed on Al-based alloys, using both synchrotron and laboratory-based X-ray sources, in conjunction with a gradient furnace and a newly developed isothermal furnace, respectively. The effect of gas porosity nucleation and growth within the semi-solid mush during both columnar and equiaxed solidification was thereby observed. In all experimental cases examined, gas porosity was observed to nucleate and grow within the field-of-view (FOV) causing various levels of distortion to the semi-solid mush, and thereafter disappearing from the sample leaving no permanent voids within the solidified microstructure. During columnar growth, a single bubble caused severe remelting and destruction of primary trunks leading to secondary fragmentation and evidence of blocking of the columnar front. Equiaxed solidification was performed under microgravity-like conditions with restricted grain motion in the FOV. The degree to which the nucleated gas bubbles affected the surrounding grain structure increased with increasing solid fraction. However, bubble sphericity remained unaffected by apparent solid fraction or grain coherency.

In‐Situ X‐Ray Radiographic Observations of Eutectic Transformations in Al‐Cu Alloys

In-situ X-ray radiography is currently being used to study solidification processes in a wide range of binary and ternary alloy systems. In this study, thin samples of hypoeutectic Al-Cu alloys were mounted in a Bridgman furnace and solidified near-isothermally from above the liquidus to below the eutectic. The eutectic transformation was easily identified from a contrast difference between the semi-solid mush and the fully solid phase in the field of view (FOV). By virtue of real time in-situ X-ray radiography, and image analysis techniques, it was possible to directly observe and measure eutectic nucleation as well as transformation rate across the FOV. Post-solidified samples were then subjected to further microstructural analysis, whereby the lamellar eutectic spacings were measured at random locations across the FOV region. The lamellar spacings were correlated to the eutectic transformation rate and the results compared to theoretical predictions. Reasonable agreement was observed and some interesting observations are noted, e.g. liquid feeding induced grain motion and the possible precursor to hot tearing.