Embedding optical sensors within additive manufactured high melting point metals for condition measurement within harsh environments

Abstract

The use of optical fibre sensors, such as fibre Bragg gratings (FBG), provides opportunity for condition health monitoring of a structure or process. FBGs allow for in-situ measurement of temperature and strain. The measurement of these parameters is important for advanced and complex structures where these sensors can provide real-time information to support lifetime condition monitoring. In general, sensor reliability drops as the temperature, moisture content and environmental corrosiveness increases. These limitations can be overcome by embedding sensors in high melting point metals to extend their operation for use at elevated temperatures and within harsh environments. A fibre embedding process chain has been devised to facilitate embedding of optical fibres within additive layer manufactured (ALM) stainless steel (SS) components. The ALM process provides access to any point of the component during manufacture, which is beneficial for the embedding of sensors inside a functional component. Furthermore, parts with complex geometries and internal features, unattainable with other forms of manufacture, become feasible using ALM technologies, but pose additional challenges for accurate modelling and external monitoring. Embedding sensors directly into such structures is a possible solution to this challenge. Selective laser melting is a powder bed fusion ALM technique where a laser selectively melts metallic powder in accordance with the geometrical data of the build layer, defined by a 3D model. The small spot sizes achievable by selective laser melting (SLM) systems are well suited for fibre embedding as the localised melt pools limits interaction through conduction with the surrounding material. A system has been set up for use with SS-316 powder, that is capable of embedding fibres in test structures. The high nickel content within SS-316 allows for intermixing of the host material with the fibre protective nickel jacket thereby bonding the fibre to the surrounding material. The work of this thesis defines process parameters suitable for repeatable and reliable embedding of FBG sensors inside SS-316 coupons. 5µm thick Cr layers are deposited onto lengths of stripped optical fibres containing FBGs using the RF sputter deposition system (RF power 100W, deposition time 30mins, processing pressure 410 −4mBar). This is followed by a ≥300 µm Ni coating using the developed Ni plating system (current density 2-7 A/dm2 , voltage 30V max, plating time ≥16hrs). The SLM build parameters for building SS-316 coupons (100W Laser power, 300mm/s scanning velocity, 60μm hatch spacing, 200μs pulse duration, 1kHz modulation frequency and scan orientation rotated by 90° between layers) and the modified build parameters for the embedding process (90W laser power, 300mm/s scan velocity, 100μm hatch spacing, 200μs pulse duration, 1kHz modulation frequency and a scan orientation parallel to fibre profile) are defined with respect to optimisation of the available equipment for fibre embedding procedures.