21 Apr 2015
Heriot-Watt method allows integrated sensors for intelligent structures.
Among other possibilities, it opens up the prospect of incorporating valuable internal features into structural components during their manufacture - features which would be very difficult to include during a monolithic production operation.
A team at Heriot-Watt University has demonstrated a potentially significant example of this capability, by embedding fiber-optic sensors directly into 316-grade stainless steel structures during manufacture, using a selective laser melting (SLM) technique.
The presence of the sensors could be a valuable means to build intelligent materials and structures, able to monitor their environment and report accordingly.
Working in collaboration with European partners as part of the Oxigen Project under the European Commission's FP7 research umbrella, the team successfully embedded nickel-coated fiber-optic sensors into stainless steel coupons. The latest results were discussed during the recent Industrial Laser Applications Symposium organized by AILU
Although the principle of laying a fiber-optic sensor in an appropriate position while the SLM operation solidifies metal powder around it is relatively straightforward, the practical difficulties of accurate positioning and avoiding damage to the sensor are considerable.
"Other groups have shown in the past that fiber-optic sensors can be incorporated into stainless steel by different means," commented Dirk Havermann of Heriot-Watt. "Previous work included incorporating fibers with very large nickel coatings of greater than 5 mm outer diameter, and using a laser metal deposition (LMD) process with several kilowatts of output power. LMD is capable of manufacturing free-form 3D structures, but its ability to make hollow or complex internal structures is very limited."
SLM improves matters through the use of high-brightness and small-spot-size fiber lasers, allowing close control of the energy being delivered both to the fiber-optic sensors and to their immediate vicinity.
"The very small spot size leads to smaller melt pool dimensions and less heat conduction into the surrounding material, which allowed us to reduce the amount of coating needed to protect the fiber by an order of magnitude," Havermann noted. "The fibers we were able to embed into stainless steel have a outer diameter of only 300 to 350 microns, including the metallic protective jacket."
Since the fiber's nickel coating sits between the optical cores and the steel surroundings, the coating's integrity and durability was a key concern during the development process.
"In the SLM operation the laser melts both nickel and steel, and fuses them solidly, Havermann said. "The coating provides thermal protection for the fiber during the SLM process; but the bonding between the fiber and the nickel exhibits limited shear strength, which is our main concern at present in terms of long-term stability or operations in harsh environments."
The project's initial focus was on understanding exactly how this fusion of nickel and steel was affected by the laser parameters and ensuring that no damage was done to the optical fiber. As the process was developed further, emphasis shifted towards incrementally reducing the coating thickness, in order to mitigate the influence of the sensor on the physical properties of the SLM-fabricated component.
"SLM only applies thin layers - less than 100 microns - whereas the optical fiber is at least 125 microns in diameter, so the SLM operation has to be adapted to accommodate the coated fiber without interfering with the powder deposition process," commented Havermann. "We soon realized that embedding the fiber in pre-fabricated grooves would mitigate any movement of the fiber due to thermal expansion and solve most of the issues the large fiber causes in respect to the powder delivery."
Mind the gaps
The nature of the SLM process means that the laser energy is only be applied from above, potentially leading to the fiber being only properly attached to the structure from the top side, with gaps occurring underneath.
However, Havermann noted that from the point of view of structural integrity, the embedded fiber is a foreign inclusion, and if considered as an element of zero strength in a stress analysis then should not necessarily pose a structural problem. "When considering the fiber as a non-load-bearing part, we can accept the occurrence of gaps," he said. "We are, however, investigating methods and process parameters aimed at minimizing them."
The team are also working on ways to transfer the process onto a commercial system. There should be no significant obstacles to using it in established SLM processes, although the need to apply pre-tensioning and clamping of the fiber inside the powder bed will need to be considered. But once all the process parameters are defined, Havermann is confident that translation of the process over to commercial SLM platforms will be viable.
The Oxigen project is specifically concerned with developing materials and processes suitable for structures exposed to very high operational temperatures - hence the value of embedded sensors able to monitor the temperature as it approaches optimum values, and the importance of "smart" components in such environments.
To date the Oxigen team has embedded fiber Bragg gratings to monitor temperature and strain, but is also embedding fiber-optic temperature-sensing elements based on Fabry-Perot cavities, through a design in which the sensing component is encapsulated into a micro-capillary in order to isolate the sensor from strain in its environment.
"Smart materials have become standard in composite materials and are widely used in wind turbine blades or in airframes, for example," Havermann commented. "However, the operating environments of polymer-based composite materials top out at 300 degrees centigrade, above which only metals will suffice. Embedded sensors will help extend the capabilities and benefits found in smart composites to these higher temperatures."
About the Author
Tim Hayes is a contributor to Optics.org.