Effect of melt cleanliness on the properties
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INTRODUCTION
M E T A L quality or, more particularly, cleanliness of molten aluminum metal or alloy prior to casting is chiefly determined by the hydrogen and inclusion content of the melt; the deleterious effect of both hydrogen and inclusions on the quality of the cast product is amply witnessed in the literature. 1~23 While removal of hydrogen is accomplished by various melt procedures, such as degassing or fluxing, inclusions can be removed by the use of filtration t31 and properly designed gating systems. 141 Filtration allows for the removal of the nonmetallic and intermetallic inclusions whose presence in the melt can otherwise reduce the melt fluidity and mechanical properties and increase the internal porosity of the casting, giving rise to poor surface quality and machinability. In addition, they can also induce premature failure in the cast component, rSI With the present day demands on product integrity, filtering the metal prior to casting has become the norm on most cast shop floors. Apart from the well-known methods of deep-bed and cake filtration, 13r the introduction of ceramic foam filters 16j in recent years has provided a new and attractive means of filtering. A ceramic foam filter initially acts as a deep-bed filter and, later on, as a cake filter after many inclusions have been trapped on it. The interconnected network of voids comprising its structure, with a large surface area-to-volume ratio, aids in trapping the inclusions on and within the body of the filter, with the passage of the molten metal through it. 171These filters come in various grades of fineness, e . g . , 10 ppi, 20 ppi, 30 ppi, etc. ("fineness" being assessed as the number of pores per inch (ppi) of filter material). Among the different techniques available for detemaining the inclusion F.H. SAMUEL, Alcan-NSERC Industrial Research Chair Professor, H. LIU, Research Professor, and A.M. SAMUEL, Research Associate, are with the Department of Applied Sciences, Universit6 du Qu6bec Chicoutimi, Chicoutimi, PQ, Canada G7H 2BI. Manuscript submitted October 21, 1992. METALLURGICAL TRANSACTIONS A
content of a melt, the porous disc filtration apparatus (PoDFA) method is one of the most recent on-line methods developed for direct application on the shop floor. IS1 With respect to aluminum and A1 alloys, the advantages of their ceramic-reinforced composites have been widely quoted: they possess better tensile properties (in particular, strength and specific stiffness) and reduced coefficients of thermal expansion. 19I However, while several studies on the effect of inclusions and melt quality on the properties of Al alloys have been carried out, very little literature can be found for their composites. Overall, when one compares the large data bases that exist for aluminum or steel, for example, similar data for metal matrix composites are relatively recent and only just beginning to surface. As part of an on-going study on A1-Si-SiC(p) composites being carried out in our laboratory, it was thought appropriate to document the effec
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