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Metal Casting Technologies : June 2008
TECHNICAL FEATURE CDC Process: Low cost method for producing near-net shape bi-metallic castings Stephen P. Midson, The Midson Group, Inc., Denver, Colorado, USA David J. Browne, Engineering & Materials Science Centre, University College Dublin, Belfield, Dublin 4 Ireland I n a recent issue of Metal Casting Technologies, Dr. Maity of CAPARO PTU School of Manufacturing and Materials Technology outlined potential advantages of various bi-metallic components(1). This paper describes a low cost, bi-metallic casting process for producing near-net shape components from two different alloys or metals. Developed at University College Dublin (Ireland) and called the CDC Process (Cast-Decant-Cast), the process allows an engineer to tailor materials properties at different locations within a component (2,3,4). Utilizing standard foundry equipment, the process is carried out in a single multi-step casting operation. The problem addressed by the CDC Process is that when designing a component, an engineer often needs different material properties at different locations within the component. For example, a component might require excellent wear resistance at one location and excellent machinability at another location. Usually the engineer must compromise and use a single alloy having only moderate machinability and moderate wear resistance. One approach to having different material properties at different regions of a component is to use an insert approach. Often inserted components are produced using casting techniques where a solid metallic piece is inserted in the mold, and a second alloy is cast around it. A disadvantage of the insert approach is that there is no chemical bonding between the two alloys. Maity(1) points out that soundness and integrity of the interfacial region is essential. With inserts, a sharp interface is present, producing a step-wise change in chemical composition, microstructure and properties. Delamination or other problems with the interface can often occur during use. An alternate approach is to produce a component from two alloys having a smooth gradient in concentration, microstructure and properties between the two alloys. Such a material is called a functionally gradient material (FGM). Figure 1 shows an example of a functionally gradient material, showing a photomicrograph of the gradient region between 34 www.metals.rala.com.au Figure 1: Microstructure of a FGM produced using the CDC Process the two alloys. Instead of the sharp interface found with inserts, the FGM exhibits a gradual change from hypereutectic aluminum alloy 390 (Al-17%Si-4.5%Cu) in the bottom left of the photograph to hypoeutectic aluminum alloy A356 (Al- 7%Si-0.3%Mg) in the upper right of the photomicrograph. A smooth gradient in composition, microstructure and properties exists between the two alloys. In this microstructure, the transition zone between the two alloys is approximately 150 µm wide, but as shown later in this article, the thickness of the transition zone can be widely varied. FUNCTIONALLY GRADIENT MATERIALS The initial application of functionally gradient materials occurred in the 1980s and involved the preparation of thermal barrier materials(2). However, bulk FGM’s have been produced using techniques such as powder metallurgy and centrifugal casting(5,6), but both of these techniques have severe limitations. FGM’s produced by powder metallurgy are normally limited in size, with large scale parts not being possible. Centrifugal casting can be used for the production of larger parts, but the process is limited in that only basis shapes (e.g., cylinders) can be produced, and the centrifugal forces often result in microstructural segregation within the castings. In contrast, the Cast-Decant-Cast (CDC) process is a low cost alternative that produces FGMs using a simple casting approach, and there is essentially no upper limit on the size of components that can be produced.