Processing and microstructures of fiber bragg grating sensors embedded in stainless steel
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RODUCTION
LAYERED manufacturing provides capabilities not easily achieved with conventional manufacturing processes. Examples include the ability to create complex threedimensional shapes with internal cooling channels and production of parts with continuously varying material properties or parts with embedded mechanical and electrical components.[1,2] As a layered manufacturing technique, shape deposition manufacturing (SDM)[3] may be used to embed sensors into metallic structures during manufacturing. Shape deposition manufacturing uses sequential steps of material deposition and removal to form three-dimensional structures. A laser-assisted SDM process can be used for the fabrication of metallic parts. The material in powder form is deposited using an intense laser beam focused onto the substrate where it creates a melt pool. An alternative method of supplying new material is to use a predeposited powder bed on the substrate. By moving the feed nozzle over the substrate according to the deposition path generated by the process planner, one layer can be deposited in near net shape. To obtain the final net shape, the layer is machined using a five-axis mill or an electric-discharge machine before the deposition of the next layer. Building parts in incremental layers allows complete access to the internal geometry of components. Thus, sensors can be placed close to points of interest before their enclosure. Obvious challenges for sensor embedding arise from XIAOCHUN LI, Assistant Professor, is with the Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706. Contact e-mail: [email protected] JILL JOHNSON, Research Assistant, and JOANNA GROZA, Professor, are with the Department of Chemical Engineering and Materials Science, University of California-Davis, Davis, CA 95616. FRITZ PRINZ, Professor, is with the Department of Mechanical Engineering, Stanford University, Stanford, CA 94305-3030. Manuscript submitted November 27, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the fact that most tools and components in the manufacturing, automotive, power, and oil industries are metallic and layered fabrication deposits at high temperatures. Past research has demonstrated the feasibility of embedding thin-film sensors into metallic structures.[4] Ideally, the sensors should be small and rugged inside the metal matrix. The addition of one or more layers at high temperature results in highquality interlayer bonding either by interdiffusion or by partial remelting of the substrate surface. However, sensor materials have to be protected during the high-temperature deposition steps. In addition, the insulation layer in the thinfilm embedded sensor is prone to cracks induced by stresses developed during the high-temperature material processing. Therefore, a nonobtrusive embedding process is necessary to maintain the integrity of the functional metallic structures. Recently, efforts have been devoted to the integration of fiber optic sensors and communication links into structural materials.[5]
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