The unidirectional solidification of Al-4 Wt pct Cu ingots in microgravity
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I.
INTRODUCTION
THE recent ability to perform experiments in the reduced gravity of space allows the unique opportunity to study solidification of metals and alloys in an environment where gravity-induced convective mixing is minimal. The fundamental knowledge gained from such experiments will ultimately provide the basis for commercial materials processing in space. At the same time, a much better understanding of the effects of buoyancy-induced convective mixing during solidification on the structure and properties of castings processed in Earth’s unit-gravity environment will be obtained.[1] Some of the published research on solidification of alloys in microgravity, concerning dendrite spacing and macrosegregation, has been done with low solidification rates, less than 1 3 1025 m/s,[2,3] which would be typical of very large castings. Smaller castings solidify at considerably higher rates of 1 3 1024 m/s or more. Published research concerning solidification in microgravity at these higher rates has included dendrite spacing experiments on NH4ClH2O mixtures, and Pb-Sn, Fe-C, MAR M246, and PWA 1480 alloys[4] but macrosegregation results were not included. Glicksman et al.[5] reported dendritic growth velocities for succinonitrile solutions in both unit gravity and microgravity over a range of supercoolings from 0.04 to 2 K. However, dendrite spacings were not explicitly included. J.R. CAHOON, Professor, and M.C. CHATURVEDI, NSERC Chair Professor for Aerospace Materials, are with the Materials Engineering Laboratory, Department of Mechanical and Industrial Engineering, University of Manitoba, Winnipeg, MB, Canada R3T 5V6. K.N. TANDON, formerly Associate Professor, Materials Engineering Laboratory, Department of Mechanical and Industrial Engineering, University of Manitoba, is with the Repair Development Group, Standard Aero Limited, Winnipeg, Manitoba, Canada R3H 1A1. Manuscript submitted April 18, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
Therefore, a solidification apparatus was designed and constructed with the objective of studying the effects of the minimization of gravity-induced convection on dendrite arm spacings and macrosegregation in ingots solidified at rates typical of smaller castings. Since solidification rates on the order of 1 3 1024 m/s were desired, the 5 to 9 minutes of microgravity afforded by a sounding rocket would be sufficient for a reasonably sized ingot. II.
EXPERIMENTAL
A. Selection of the Alloy An Al-4 wt pct Cu alloy was chosen for the study, since a considerable amount of solidification data for unit gravity is available in the literature. Further, this alloy composition has important commercial applications, since it is the base composition for the 2000 series of Al-Cu alloys. Also, the Al-4 pct Cu alloy melts at temperatures below 950 K, and, therefore, furnaces are fairly easy to construct. As indicated subsequently, an ingot measuring 10 mm in diameter and 25-mm long and weighing about 5.8 g was selected, since this size of ingot could be solidified in the microgravity
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