By Dirk Havermann, William N. Macpherson, Robert R. J. Maier and Duncan P. Hand
Laser Additive Manufacturing provides novel and exciting possibilities when constructing 3-dimensional free form components in polymers and metals. The process of building 3D components from the inside-out opens up the possibility of embedding sensors into the heart of a component. Fiber Bragg gratings are an ideal sensor for smart composite materials and are already used for delivering in-situ measurements of polymer components. Recently developed high temperature compatible fiber Bragg gratings, suitable for continuous use at temperatures of up to 1000 °C, provide the opportunity of extending sensing capabilities to metallic components. In this cross-disciplinary project, fiber optic sensing and laser additive manufacturing are merged to encapsulate fiber optic sensors into stainless steel components.
We have developed an approach to successfully embed optical fibers with Bragg gratings into stainless steel (SS 316). The first step is to remove any polymeric jacket from the fiber and replace this with a metal coating. Such a metal coating allows the fibers to bond to the steel environment as well as protecting them during the embedding process. Therefore, thin electrically conductive layers of Chromium are applied using vacuum sputtering followed by electroplating Nickel onto the fiber with thicknesses of up to half a millimetre. Then, in a two step process, these metal coated fibers are laser welded onto a stainless steel substrate and encapsulated using a laser based powder sintering process. This sintering process is similar to the commonly used powder bed processes. Both processes, coating and encapsulating, are tailored with respect to the delicate optical fiber with Bragg grating. In particular, the sintering process is carefully adjusted to achieve the best possible bonding between the optical fiber and the metal environment as well as to minimize the energy input to prevail the fibers structural integrity.
Using this approach, it has proved possible to embed fiber Bragg gratings into stainless steel components. The gratings optical properties are maintained during the embedding process, enabling in-situ measurements of strain and temperature changes. Preliminary results indicate that repeatable temperature measurements are possible from within the component for temperatures of up to 400°C.
In future, we are aiming to expose these components to even higher temperatures of up to 1000°C. Ultimately, we are aiming to encapsulate metal jacketed fiber Bragg gratings into steel components which are capable of measuring strain changes in hostile environments where temperatures exceed 700°C. Possible applications for such sensors include monitoring components in saturated steam processes of power plants and turbine components in the aerospace or energy industries.