The columnar-to-equiaxed transition in Al 3 Pct Cu
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I.
INTRODUCTION
AN
investigation tl~ of the columnar-to-equiaxed transition (CET) in SnPb alloys solidified directionally against a chill showed the transition occurred when the temperature gradient ahead of the advancing dendrite tips reached a critical value. Critical values of 1.0, 1.1, and 1.3 ~ cm were observed for alloy additions to Sn of 5, 10, and 15 pct Pb, respectively. The transition could not be directly related to the dendrite tip velocity. The present investigation was undertaken to determine if the CET in A1 3 pct Cu occurs at a critical melt gradient, similar to that observed for SnPb alloys. One of the variables associated with the CET is the number of nucleating sites. This can be examined in A1 alloys by adding controlled amounts of the nucleating agent TiB2. Hunt and Burden [2,3] have proposed a model for the CET in which a critical variable is the amount of supercooling which occurs at the columnar dendrite tips. They have measured the supercooling in A1Cu alloys, t3~ which allows a reasonable comparison of measurements of the CET in A1Cu to be made with their model. " II.
PROCEDURE
The procedure used in this investigation follows that used for SnPb, t~] in which the alloy is solidified vertically from a water-cooled copper block. To reduce the solidification rate, 0.95-mm-thick discs of stainless steel are placed between the crucible and copper block, reducing the heat transfer. The melt was prepared from 99.5 pct A1 and Cu and held in a graphite crucible of 3.7-cm ID and 120-cm height in an argon atmosphere. In a typical test, the melt is heated to the required temperature, the furnace turned off, and water introduced into the copper chill. Temperature measurements were made during solidification with four chromel/alumel thermocouples of 3 - m m OD positioned 2, 4, 6 and 8 cm from the bottom of the crucible. A heat-transfer mathematical model was developed similar to that for SnPb tl~ and fitted to the temperature I. ZIV, Group Leader, Metal Casting Technology, is with the RAFAEL Ministry of Defence, P.O. Box 2250, Haifa, Israel. F. WEINBERG, Professor, is with the Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC V6T 1W5, Canada. Manuscript submitted June 24, 1988. METALLURGICAL TRANSACTIONS B
measurements by adjusting the heat-transfer coefficient (HTC) between the crucible and the chill. Inoculant was added to the melt just prior to solidification by adding weighed amounts of commercial hardener containing 5.3 pct Ti and 1.1 pct B2. The position of the CET was observed by sectioning the ingots along a vertical central plane, and polishing and etching the sectioned surface.
III.
OBSERVATIONS
The A1 3 pct Cu alloys exhibited a C E T similar to that observed for SnPb. Four examples are shown in Figure 1 for different H T C ' s . The CET is observed to occur relatively abruptly, on a near horizontal plane, and further from the chill, with increasing heat transfer between the crucible and chill. Results for ten tests on AI 3 pct Cu alloys, in
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