Development of a New Ferrous Aluminosilicate Refractory Material for Investment Casting of Aluminum Alloys
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INTRODUCTION
THE investment casting technique has tremendous advantages for the production of quality cast components having the key benefits of accuracy, versatility and integrity. As a result, the process is one of the most economic methods of forming a wide range of metal castings. The manufacture of an investment casting requires an expendable pattern, which is coated with multi-component slurries, or ‘investments’, to form a ceramic mold.[1,2] The procedure is illustrated in Figure 1. First, multi-component slurries are prepared, composed of a fine mesh refractory filler system and a colloidal binder system. The wax component is then dipped into the primary slurry, sprinkled with coarse refractory stucco and dried. The purpose of the stucco is to minimise drying stresses and to facilitate a mechanical bond between the primary coating and the back up or secondary investment. When the primary coat has set, (air-dried until the binder gels) the assembly is systematically dipped into secondary slurry and stuccoed until the required thickness of shell is built up. The particle size of the stucco is increased as more coats are added to maintain maximum mold permeability and to provide sufficient shell build for strength. Thus, an investment casting mold consists of individual layers of fine refractory material and granular refractory material held together by a binder that has been set to a rigid gel. Flexibility exists in changing the composition of each CHEN YUAN and SAM JONES, Research Fellows, and STUART BLACKBURN, Professor, are with School of Metallurgy and Materials, University of Birmingham, Elms Road, Edgbaston, Birmingham B15 2TT, U.K. Contact e-mail: [email protected] Manuscript submitted February 14, 2007. Article published online August 16, 2012 5232—VOLUME 43A, DECEMBER 2012
layer. The composition of the primary is normally different with that of secondary slurry as it is direct in contact with metal. The pattern is then removed, leaving a hollow mold with an extremely smooth internal surface. Molds are fired and cast with molten metal. After cooling, the ceramic is removed by mechanical or chemical methods to obtain the metal parts for final finishing. The requirements of an investment casting mold include: (I) Sufficient green (unfired) strength to withstand wax removal without failure; (II) sufficient fired strength to withstand the weight of cast metal; (III) high thermal shock resistance to prevent cracking during metal pouring; (IV) high chemical stability to prevent the mold-metal interaction; (V) sufficient mold permeability to allow trapped air to permeate through the mold walls during metal pouring; (VI) sufficient thermal conductivity to maintain an adequate thermal transfer through the mold wall and hence allow the metal to cool and (VII) limited creep to prevent dimensional changes within the mold wall and ultimately the casting.[3,4] Various combinations of materials have been used to produce the ceramic mold. For casting aluminum based alloys, foundries commonly use zirconium silicate based refractories
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