Study of the mechanism of grain refinement of aluminum after additions of Ti- and B-containing master alloys
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I.
INTRODUCTION
IN spite o f its commercial importance and the numerous scientific studies in this a r e a , the grain refinement o f aluminum is still a controversial subject. It appears that more than one mechanism is responsible for grain refinement, depending on the nucleant (or m a s t e r alloy) used, the alloy cast, and the processing conditions employed. To sort out some o f these complexities, the authors decided to embark on a new study in which: (1) nucleants would be added to very high-purity aluminum; and (2) a newly developed and highly sensitive thermal analysis technique would be used. The use o f high-purity aluminum removes all o f the variables and complications associated with the presence o f impurities, so that only the effect o f the master alloy addition is studied. The use o f new computer-automated thermal analytical techniques, developed by Backerud and co-workers, t1'2"3~ allows researchers to establish and accurately characterize the nucleation and growth processes during solidification, o r to look "inside" at the details o f the grain-refinement process. II.
EXPERIMENTAL T E C H N I Q U E
Approximately 500 g o f high-purity (99.995 pct) A1 was melted in a resistance-heated salamander crucible MATS JOHNSSON, Graduate Student, and LENNART BACKERUD, Professor, are with the Department of Structural Chemistry, Arrhenius Laboratory, Stockholm University, S-106 91 Stockholm, Sweden. GEOFFREY K. SIGWORTH, Principal Engineer, is with Metalworking Technology Inc., Johnstown, P A 15904. Manuscript submitted February 1 0 , 1992. METALLURGICAL TRANSACTIONS A
(ceramically bonded graphite material), w h i c h , during our earlier experiments, has shown to be equally inert to the melt as graphite crucibles. Unless stated otherwise, the melt in this study was held at 775 °C f o r the duration o f the experiment. A master alloy addition was made once this temperature was established, and samples were taken at various times a f t e r addition (this is called "contact" time). To prevent the influence o f sedimentation o f particles, all grain-refining samples were taken by first stirring to homogenize the melt and then pouring a sample into a small preheated graphite mold. The dimensions o f the mold and placement o f the thermocouples therein are described in Reference 1. The cooling rate just p r i o r to solidification in the experiments was approximately 1 ° C / s . This is a reasonable cooling rate; one w h i c h would be found in many l a r g e r industrial castings. Commercially available master alloys with Ti/B ratios equal to 4 . 8 / 1 , 5.3/0.1, and 6.2/0.03 were used in this study. Their chemical compositions are given in Table I. Regarding the impurities, there is no question that with the largest additions, a certain amount o f Fe and Si will be added to the high-purity melt. For the case o f 1 pct T i addition (as A1-6 pct Ti), the impurity l e v e l will be about 0.04 pct Fe and 0.01 pct Si. As we may surmise from the phase diagrams, the effect o f these impurities will be to l o w e
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