Refinement of dendrite arm spacings in aluminum ingots through heat flow control
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important influence of segregate spacings, i . e . , dendrite a r m spacings, on mechanical properties of cast alloys and of wrought material produced from cast ingots is well documented.~'~ It is known that a) the d e g r e e of homogeneity which can be obtained in a cast alloy with commercially practical homogenization treatments depends on these segregate spacings, and b) the mechanical properties of the heat-treated casting depend on the homogeneity obtained. In wrought alloys, microsegregation from the original ingot is c a r r i e d to some extent into the final wrought product and affects mechanical properties in s i m i l a r fashion.7-~° Macrosegregation is at least as troublesome industrially as microsegregation. Generally, increased macrosegregation, hot tearing and shrinkage defects are expected as a result of slow cooling r a t e s coupled with l a r g e liquid-solid "mushy" regions in castings and ingots.~1-12 Experimental evidence available for a variety of cast alloys indicates that segregate spacing in a given alloy is influenced only by cooling rate or " l o c a l solidification t i m e " * . Generally, segregate spacing *"Local solidificationtime" is defined as time at a givenlocation in a casting or ingot between initiation and completion (or near completion) of solidification. It is inverselyproportional to average cooling rate at that location.
is found to be inversely proportional t o cooling rate t o some exponent (or directly proportional t o l o c a l solidification time to the same exponent).13-'8 Therefore, the only method of significantly reducing segregate spacings in short r a n g e segregation (microsegregation) and the overall magnitude of maT. W. CALDWELL and A. J. CAMPAGNA, formerly Graduate Students in the Department of Materials Science and Engineering, M.I.T., Cambridge, MA, are now Manager, Engineering, Bearings Division, NL Industries, Toledo, OH, and Independent Consultant in Boston, MA, respectively. M. C. FLEMINGS is F o r d Professor of Engineering, Department of Materials Science and Engineering, M.I.T., and R. MEHRABIAN is Associate Professor, Department of Metallurgy and Mining Engineering and Department of Mechanical and Industrial Engineering, University of Illinois, Urbana, IL. Manuscript submitted June 14, 1976. METALLURGICAL
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crosegregation in castings and ingots is t o a l t e r heat flow conditions within the solidifying metal. Applicable methods are those through which l o c a l cooling rate can be increased (local solidification time decreased), and size of the liquid-solid " m u s h y " region decreased, resulting in reductions in segregate spacing, i . e . , dendrite arm spacings, and macrosegregation. Much attention has been given recently t o studies a i m e d at seeking ways to accelerate rate of heat extraction, thereby increasing local cooling rate and reducing dendrite arm spacing. In this work, heat flow is studied in a solidifying " m u s h y " alloy, A1-4.5 pct Cu. The m a j o r aim, following successful computer modeling o
