Pilot-Scale Investigation of Liquid Aluminum Filtration through Ceramic Foam Filters: Comparison between Coulter Counter
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aluminum alloys adversely affects the quality and properties of aluminum products in many different ways. This is detrimental to the fatigue strength of aluminum alloys used specifically in the automotive and aeronautic industry. The presence of inclusions also results in defects that occur during processing operations:[1–3] stringers on rolled products (for high-hardness inclusions), decohesion on forged products, and tearoff during forming with high plastic deformation (extrusion, rolling, and beverage-can-drawing processes). In the can industry, the constraints imposed with respect to the inclusion content are drastic: the inclusion concentration must be less than 104 parts per million (weight), to meet can producer requirements; this typically corresponds to much less than 20 defects per million cans produced. A case of HERVE´ DUVAL, Associate Professor, and JEAN-BERNARD GUILLOT, Professor and Director, are with the Chemical and Material Engineering Laboratory, E´cole Centrale Paris, ChaˆtenayMalabry 92290, France. Contact e-mail: [email protected] CARLOS RIVIE`RE, formerly Doctoral Student, Rio Tinto Alcan Voreppe R&D Center, is Development Engineer, La Metro–Communaute´ d’agglome´ration de Grenoble, Grenoble 38000, France. E´MILIE LAE´, Research Engineer, and PIERRE LE BRUN, R&D Senior Engineer, are with the Rio Tinto Alcan Voreppe R&D Center, Voreppe 38340, France. Manuscript submitted April 8, 2008. Article published online February 5, 2009. METALLURGICAL AND MATERIALS TRANSACTIONS B
tearoff due to a fibrous refractory shot is shown in Figure 1. The elimination of inclusions is particularly critical in the case of liquid aluminum. Although efforts have been undertaken to reduce the inclusion concentration, the high reactivity of liquid aluminum and the presence of refractory material necessarily result in new inclusions being formed in liquid metal, from the melt-processing stage through the casting into semifinished products. Inclusions are classified as endogenous or exogenous, according to their origin. Endogenous inclusions are formed in situ as a result of the chemical reactions of the liquid metal (e.g., the formation of surface oxides). Exogenous inclusions originate from the external medium and penetrate the liquid metal. Such inclusions may consist of pieces of refractory material from furnaces or troughs. In numerous industrial applications, liquid aluminum filtration supplements other processes aimed at eliminating impurities (the settling of inclusions in a holding furnace and flotation in degassing ladles). One distinguishes between two modes of liquid metal filtration: cake-type filtration, in which suspended particles greater in size than that of the filter pores, deposit at the filter inlet and form a cake of growing thickness, which constitutes the actual filtering medium, and depth filtration, in which suspended particles are smaller in size than the filter pores and, therefore, enter the filter and deposit within the filter (thus causing gradual clogging of the filter). The filters used in today’s VOLUME 40B
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