Thermal residual stresses in functionally graded and layered 6061 Al/SiC materials
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INTRODUCTION
UNLIKE conventional composites that are homogeneous on a macroscopic scale, functionally gradient materials (FGMs) are a relatively new class of composite materials that vary continuously in composition and/or microstructure. Consequently, FGMs have the potential to be utilized in a wide range of engineering applications, since the compositional gradient can be tailored to meet specific requirements. This versatility means that FGMs are not only of increasing interest in advanced engines and airframes, but also in other potential applications, such as armor, medical implants, and electric or dielectric devices,m However, for all of their potential uses, FGMs are primarily in the evaluation stage. Little is known about their actual properties or performance. This is due in part to a lack of clear direction in the design and processing of functionally gradient materials. The current fabrication methods of FGMs include chemical vapor deposition,t2] the plasma spray technique,t3] and various powder metallurgy techniques. I41 Among the techniques available, spray atomization and codepositiontS] are of considerable interest as an attractive route to fabricate FGMs for several reasons. First, an ability to expose the reinforcement to relatively low processing temperatures, thus minimizing any potential reactions between the matrix and the reinforcement. Second, because of the potential to synthesize difficult-to-form materials into near-net shapes, minimizing expensive joining and machining operations that are typically required after conventional forming. Third, the ability to minimize surface oxidation and other
deleterious surface reactions, because processing is performed under environmentally controlled conditions. Currently, using spray atomization and codeposition technology, FGMs consisting of aluminum alloy matrices, and reinforced with SiC and other reinforcements, are being successfully fabricated,t6j Dissimilar thermal expansion coefficients exist in FGMs, which can generate significant thermal residual stresses during fabrication. Their distribution often plays an important role in determining and understanding the mechanical behavior of the fabricated material when it is subjected to its intended service loads and environment. In some cases, such stresses may be unavoidable due to the nature of the processing conditions, and may be very detrimental. In other cases, stresses may be purposely introduced for beneficial effects. In any event, their effective control depends critically on investigators' ability to detect their presence and distribution accurately within FGMs. Therefore, by judiciously tailoring the microstructure of heterogeneous materials, thermal residual stresses can be dispersed and minimized during the fabrication process. In the present study, thermal residual stresses that develop in the graded and layered 6061 A1/SiC materials fabricated by the spray atomization and codeposition process are investigated using thermo-elastoplastic finite element analysis. The effect of S
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