Dissolution of reactive strontium-containing alloys in liquid aluminum and A356 melts
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I.
INTRODUCTION
ALTHOUGH strontium and various strontiumcontaining master alloys have been used for several years as modifiers for aluminum-silicon foundry alloys, the dissolution of the various master alloys has not been investigated. Thus, the optimum conditions for strontium addition to foundry melts are far from clear. In an earlier work ~] on the determination of the A1-Sr phase diagram, it was observed that the various binary aluminumstrontium alloys exhibit different chemical and physical properties depending on their strontium content. Alloys containing more than 63 wt pct strontium were observed to be highly reactive in air and to form thick surface oxides. Dilute strontium alloys, on the other hand, proved to be stable in air. During a detailed study ]2] on the dissolution behavior, it was found that these differences in chemical reactivity translate into marked differences in dissolution behavior. In the present paper, the dissolution of high-strontium commercial alloys will be examined. These alloys dissolve reactively by forming chemical compounds with the melt. Although dissolution phenomena in metallic systems, in which compound formation is involved, have, on occasion, been reported, detailed studies have been found on only two occasions. ~3,4~Hence, the findings presented here are important not only for the utilization of high-strontium master alloys, but also in increasing our understanding of the reactive dissolution process.
M.O. PEKGULERYUZ, Manufacturing Engineer, is with IBM, Bromont, PQ JOE IL0, Canada. J.E. GRUZLESKI, Gerald Hatch Professor, is with the Department of Mining and Metallurgical Engineering, McGilt University, Montreal, PQ H3A 2A7, Canada. Manuscript submitted December 14, 1987. METALLURGICAL TRANSACTIONS B
II.
EXPERIMENTAL PROCEDURE
A. Materials The two commercial strontium additives studied were (1) commercial purity strontium and (2) 90/10 alloy (90 pct Sr-10 pct A1). Both alloys were supplied by Timminco, Ltd., Toronto, ON, Canada as extruded bars which were packed under vacuum. The chemical analyses of these alloys and of the liquid melts to which they were added (pure aluminum and A356) are listed in Tables I and II, respectively. Due to their high strontium concentration, the alloys proved to be reactive in air and to form thick surface scales which were determined by X-ray analysis to contain Sr(OH)2, SrO2, and (Ca, Sr)NO3 in the case of commercial purity strontium and Sr(OH)2 and A12Sr3(OH)12 in the case of the 90/10 alloy. The sample surfaces were cleaned mechanically to remove the scale before the experiment.
B. Experimental Procedure Two different experimental methods were used for dissolution studies. One method involved the immersion of cylindrical samples of the strontium alloys in 10 kg melts of either pure A1 or A356 alloy. The melts were prepared in a clay graphite crucible and melting was done in a gas-fired furnace. The cylindrical alloy samples were 0.015 to 0.08 m in diameter and 0.020 to 0.030 m in length. They were held in the melt using a simple
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