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Metal Casting Technologies : June 2008
TECHNOLOGY FEATURE Figure 3: FGM produced from aluminum alloy 390 (Al-17%Si-4.5%Cu) and aluminum alloy A356 (Al-7%Si-0.3%Mg), showing a wider transition zone MICROSTRUCTURES AND APPLICATIONS Much of the initial development work on the CDC Process focused on using Al-Si alloys, producing components having a surface of wear-resistant 390 (Al-17%Si-4.5%Cu) and a tough and machinable interior of A356 (Al-7%Si-0.3%Mg). Figure 1 shows these two alloys where the thickness of the gradient region was about 150 µm. However, modification of the processing conditions can drastically change the thickness of the gradient region. Figure 3 shows a functional gradient material from the same two alloys produced using the CDC Process where the transition zone between the two alloys is much wider, approximately 3-4 mm wide. Recent development work on the CDC Process has focused on alternate alloy systems. Figure 4 shows the microstructure of the gradient region between monolithic aluminum alloy A356 (Al-7%Si-0.3%Mg) and an aluminum metal matrix composite (Al-20% SiC). The transition zone between the two materials is about 1.5 mm wide. Figure 4: FGM produced from an aluminum alloy A356 (Al-7%Si-0.3%Mg) and an aluminum metal matrix composite (F3S20S) Finally the authors have shown that the CDC Process can be used to develop near-net shape parts with a gradient from one metal to another. The CDC Process can be considered as a method for 3D joining of dissimilar metals within a component. Figure 5 shows the microstructure at the gradient region between aluminum alloy A356 (Al-7%Si-0.3%Mg) to zinc alloy ZA-27 (Zn-27%Al-2.25%Cu). In this case, the transition zone was about 1 mm wide. Potential applications for this combination of alloys focus on reducing the weight of zinc components, and include lightweight bearings and lightweight non-sparking tools. Initial trials using the CDC Process to combine aluminum and magnesium alloys have also shown promise. A material with a gradient from aluminum alloy 357 (Al-7%-0.5%Mg) to magnesium alloy AZ91D (Mg-9%Al-0.7%Zn-0.2%Mn) has been achieved. Such Mg+Al FGMs have huge potential in the transportation section, e.g. in the manufacture of lightweight automotive blocks with built-in aluminum cylinders. The CDC Process should also be capable of making castings Figure 5: FGM produced from aluminum alloy A356 (Al-7%Si-0.3%Mg) and zinc alloy ZA-27 (Zn-27%Al-2.25%Cu) Surface Property Wear Resistance Excellent Anodized Appearance Electrical / Thermal Conductivity Metal Matrix Composite Bearing Surface Non Sparking Corrosion Resistance Bio Compatibility Table 1: Potential applications of the CDC Process 36 www.metals.rala.com.au from a range of dissimilar metals or alloys (aluminum alloys, magnesium alloys, zinc alloys, copper alloys, steels, superalloys). Table 1 lists a number of potential applications for functionally gradient materials produced using the CDC Process. Bulk Property Machinability / Toughness / Castability Castability Strength Monolithic Alloy Light Weight Lightweight Strength Strength / Fatigue Resistance