One way for commercial airlines to reduce emissions is to adopt lighter, more aerodynamic jet engines. Such advanced jet engine technology already exists, but before these more fuel-efficient engines can be widely adopted by the industry, researchers must first find a method for repairing them in a cost-effective manner.
Leading the way on this front is a team of Canadian researchers headed by the National Research Council of Canada (NRC). “NRC researchers developed a vision and strategy for a new repair technology that can serve the global jet engine market,” says Dr. Priti Wanjara, a researcher from the NRC Aerospace portfolio. “And we put together a research team to move it forward.”
The team also includes experts from the University of British Columbia at Okanagan (UBC–Okanagan), and the Canadian Neutron Beam Centre (CNBC). StandardAero, a jet engine service and overhaul company, provided the team with technical information about industry practices to ensure that the results are as useful as possible to the industry as a whole. In step with this focus on real-world applications, the research is supported by an NSERC Engage Grant, which helps universities to collaborate with industry.
“The team’s work has resulted in practical industrial knowledge, and enabled further development of new technologies in the aerospace industry.”
The reason a new repair technology is needed lies in the difference between how traditional and more advanced jet engines are made. Jet engines use sets of turbine blades on rotating disks to compress and then expel air to create thrust. In traditional jet engines, these blades and rotating disks are made separately and then fastened together. In more advanced engines, such as the ones used in military jets, the blades and disks are made from a single forged piece called a ‘blisk.’
Blisks are more fuel efficient than their multi-piece counterparts not only because they can be more aerodynamic, but also because they require less material, thereby reducing engine weight. Furthermore, blisks can be more reliable because they have fewer locations where defects may develop while in normal use.
While the commercial aerospace industry sees the advantages of blisks, it is concerned that engines using them are difficult and costly to repair. That’s because, in a traditional jet engine, if one turbine blade is damaged, that blade can simply be switched out. But if a blade on a blisk is damaged, the entire engine must be removed and the whole blisk replaced, greatly adding to the operational expense. Not surprisingly, commercial airlines—which already spend about $15 billion per year on engine repairs globally—are keen to minimize repair costs and avoid revenue losses from prolonged airplane downtimes.
For that reason, the research team is looking into an advanced welding technology that might be the answer to more cost-effective blisk repair. “We are studying how linear friction welding could be used to repair blisks effectively and efficiently, thereby helping to provide the assurance that the industry needs before deploying blisks more widely,” says Professor Lukas Bichler from UBC–Okanagan.
Linear friction welding joins alloys together without melting them. Like all welding methods, linear friction welding introduces changes to the alloy’s properties, including its stress—which is a key indicator of how the component will perform during service. Therefore, in order to guarantee the reliability of an engine that’s been repaired with this method, the changes in the welded alloy need to be measured, understood, and if appropriate, mitigated.
To meet this industry need, Wanjara and her NRC Aerospace colleague Dr. Javad Gholipour replicated the key aspects of an engine repair in which a damaged blade might be cut out from a used blisk and a new blade welded into place using the proposed method. Following industry practices, they prepared a used turbine disk, which was taken from an actual jet engine, for refurbishment. The disk was made of INCONEL® 718, the alloy most commonly used for turbine blades and disks. Using the NRC’s linear friction welding facility, which is unique in Canada, they investigated welding protocols and applied the procedure that was most promising for this first-of-its-kind weld to join a new piece of INCONEL 718 to a section removed from the used disk.
Mathew Smith, a doctoral student supervised by Bichler and Wanjara, examined the combined block of new and used INCONEL 718 using a variety of techniques to determine how the weld affected the alloy’s properties. Importantly, these investigations are the first ever to examine the effects of linear friction welding on INCONEL 718 after it has been used in service or exposed to realistic service conditions.
Guided by Dr. Dimitry Sediako of the CNBC, Smith employed neutron beams at the CNBC to measure the stress inside the welded block non-destructively. The results of the stress measurements show no reason why linear friction welding could not be used to repair blisks made of INCONEL 718. The research team is now comparing these results to stress data obtained in collaboration with the Institut de recherche d’Hydro-Québec (IREQ), which used a destructive evaluation technique to obtain data on an identical sample. So far, the findings suggest that linear friction welding is a promising technology for refurbishing jet engines.
Notably, the research team’s investigations at the CNBC demonstrate the ability to measure stress at precise locations in the welded block using neutron beams. In fact, “Neutron beams were the only tool that could provide the stress data non-destructively in this case, which is complicated by how welding affects the microstructure deep inside the block,” says Sediako.
The research team plans to continue testing and characterizing the microscopic properties of repair-welded materials, with the broad research goal of optimizing the linear friction welding process to obtain the best mechanical performance possible. Moreover, the researchers intend to determine how in‑service conditions contribute to the loss of performance over time, and also to develop heat treatment protocols to restore that performance. Importantly, neutrons can provide vital measurements for these aims.
Commenting on the team’s work to date, Kim Olson, a Senior Vice President at StandardAero, says, “It has resulted in new fundamental scientific data, as well as practical industrial knowledge, and enabled further development of new technologies for component manufacture, repair, and maintenance in the aerospace industry.”
This research story was republished with the permission of the Canadian Institute for Neutron Scattering.