Gravity segregation of complex intermetallic compounds in liquid aluminum-silicon alloys
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I.
INTRODUCTION
IRON,manganese, and chromium in aluminum-silicon alloys form high melting point, hard, intermetallic compounds that cause excessive machine tool wear. The compounds (sludge) form when these elements are present in sufficient amounts (the total percentage of more than 0.9) and when melting and metal treatment conditions are not controlled efficiently. Compound formation is particularly significant when the melt is held for long times at temperatures just above the melting point of the alloy. Under these conditions, the intermetallics represent a primary phase that grows in the melt and eventually settles to the bottom due to the relatively high density of the iron, manganese, and chromium phases, tl] When the level of iron is high, intermetallics are normally present in a platelet or needlelike morphology, which is detrimental to fluidity and feeding. Accordingly, their presence increases shrinkage and shrinkage crack tendencies. 12] Manganese is widely used as an alloy addition to neutralize the effect of iron and to modify the platelet morphology to a more globular shape. If the manganese-to-iron ratio is low (e.g., 1:4 or less), the platelet (needle) phase forms, and when this ratio increases, globular phases such as Chinese-script and starlike compounds form, which have an increased segregation tendency. For this reason, it is common practice to observe a 1:2 manganese-to-iron ratio for many alloys, t2'3] Several empirical formulas have been developed to predict whether a given composition is likely to cause intermetallic compounds to be formed and to segregate. Based on the elements present, usually iron, manganese, and chromium, a segregation factor is calculated that is matched to a suggested minimum holding temperature S.G. SHABESTARI, Ph.D. Student, and J.E. GRUZLESKI, Gerald Hatch Professor, are with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2A7. Manuscript submitted January 31, 1994. METALLURGICAL AND MATERIALS TRANSACTIONS A
above which sludging would be minimized,t4"5"rJSludge will form in melts held above the sludge formation temperature when scrap is added into the melt, since local cooling below this critical temperature occurs. Similarly, sludge can be formed near the cold refractory linings of a melting furnace. Gravity segregation of intermetallic compounds can be prevented by proper temperature control during holding and by thorough stirring of the molten bath. It is important to realize that even with perfect temperature control and no gravity segregation, sludge particles will still be present in a dispersed state, t7's] Segregation may occur either in the mold during casting, or in the furnace or ladle during melt processing. In the first case, slow cooling of the metal in the mold favors gravity segregation, since primary crystals grow larger and have more time to sink. Segregation is thus more likely to occur in large sand castings and slowly cooled ingots than in permanent mold or die castings. In previous work, [9
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