Studies on addition of inclusions to molten aluminum using a novel technique
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INTRODUCTION
INCLUSIONS find their way into metallic melts from various sources. In aluminum alloys, a wide variety of inclusions is found. Some of them occur naturally in the system, e.g., A1203, MgO, spinel, etc., and some are intentionally added as grain refiners, e.g., TiC and TiB2. Many of these inclusions occur simultaneously in a melt, complicating the systematic study of their behavior. For example, in the case of grain refinement by the use of A1-Ti-B master alloys, the crystallites that are commonly believed to nucleate grains are TiB2. u~ However, in addition to the introduction of TiB2, TiA13 and dissolved Ti also enter into the melt and complicate the detailed analysis of grain refinement. Over the last 4 decades, several investigations t2J have been carried out to identify the nucleating crystals in this complex system and the mechanism by which they operate. These attempts have met with little success for several reasons. First, the presence of dissolved Ti and TiA13 may alter the behavior of TiB2 significantly, t31 Second, control of particle size is very difficult from the salt reaction, and subsequent metallographic identification of the particles is uncertain. In the context of enhanced porosity in strontium modified alloys, it is often mentioned that the presence of SrO and Sr(OH)2 inclusions may nucleate pores, t41 There is however no experimental evidence showing if such inclusions can exist in the aluminum melt in the first place. An ideal solution to the difficulties involved with the systematic study of inclusion behavior during solidification of metals would be to develop a suitable technique for the controlled addition of known types of inclusions into the melt while preserving their characteristics and to avoid incorporation of those inclusions P.S. MOHANTY, Postdoctoral Fellow, and J.E. GRUZLESKI, Gerald Hatch Professor, are with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2A7. F.H. SAMUEL, Professor, is with the Department of Applied Science, University of Quebec at Chicoutimi, Chicoutimi, PQ G7H2B1, Canada. Manuscript submitted July 22, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS B
which are naturally formed during the processing stage, such as oxide films and gas bubbles. Efforts to introduce solid particles into molten metal in a controlled way are not new. From time to time, various attempts have been made in this direction, with different degrees of success or failure. They can be classified into two broad categories: (a) production of solid particles in the melt through chemical reaction and (b) mechanical introduction of preprocessed particles into the melt. The principles on which the first category is based have been widely used for the production of grain refiners tS-9j and production of in situ TiC-reinforced composites. I~~ Although considerable success has been achieved in creating solid particles in the melt through chemical reaction, the control of the chemistry of the products and particle size is difficul
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