Global pressure to deploy more flights, using larger fleets comprised of fuel-efficient, lightweight, aerodynamic, and durable aircraft presents a potential problem when it comes to keeping these aircraft in the air. Higher temperature aircraft engines reduce consumption and operating costs but are more prone to cracking, eroding, and corroding. With the cost of having an aircraft out of operation critically high in terms of cost and inconvenience, alternative maintenance, repair, and overhaul (MRO) methods could provide a solution to this awkward balancing act.

Materials manufacturers are in a quandary. The aerospace sector is increasingly demanding materials and components that can deliver enhanced performance against extreme temperatures and corrosive gases. However, it also places components under extreme operating temperatures and stress, causing them to degrade, eventually leading to a need to quickly access replacement parts.

Super alloys improve component durability under these harsh environments, but they are not wear-resistant through time. It also can be challenging to reproduce replacement parts from super alloy materials at the rate at which aircraft manufacturers are demanding them. Casting and machining individual components can be time consuming and costly in terms of additional aircraft downtime. Pre-sintered preforms (PSPs), a concept introduced to the aerospace market in the early 1990s, can alleviate pressures for MRO companies while allowing for improved dimensional controlling, minimizing the need for post-braze grinding and machining.

The case for PSPs
Low-pressure compressor stator and flange, seal slot PSPs.

A blend of super alloy and braze powders, PSPs are increasingly being used for crack repair and dimensional restoration of gas turbine engine components. They can be customized to fit the individual shapes of components and later, tack-welded into place and brazed. The brazing process allows complete components to be heated in a vacuum furnace, reducing distortions and increasing consistency, resulting in a fast, high-quality repair process.

This makes PSPs suitable for restoration projects where various compositions and shapes are required, including curved, tapered, and cylindrical. Traditional welding processes can require a skilled welder to work from five minutes to an hour per each component. Alternatively, when times are shorter, multiple welders are used at considerable cost. In contrast, PSPs enable MRO engineers to create a material with the same thickness as a part that has been welded, and this can be applied in seconds. As many as 200 parts can then be placed in a furnace together, enabling capacity gains with higher-quality finished products.

PSP flanges, slots for crack repair and component restoration.
Alternative MRO solutions

Electron-beam welding, a high-energy-density process, is usually performed in a vacuum enclosure by striking the surface of a material with fast moving electrons – transforming the kinetic energy of each individual electron in the beam into thermal energy in the component. This transformation is suitable for a high percentage of metals for a range of component restorations, but it can only be performed by highly trained and skilled engineers, which, together with the capital costs of the equipment required, can make it expensive. Manual labor used to complete repairs means projects take longer to complete and can be prone to human error, particularly considering electron-beam welding must be completed layer-by-layer.

For components with complex geometries, pre-mixed pastes are often used instead, particularly when filling fatigue cracks. Pre-mixed pastes use a binder to give them a slurry-like consistency. Binders burn off at lower temperatures, resulting in uncontrolled shrinkage and soiling of brazing furnaces and parts. In many cases, additional paste applications and braze cycles are required to fully restore the crack. This creates a longer lead time for projects when compared with using PSPs, and when considering the average shelf life of pre-mixed pastes is six months, this does not offer the same resistance and reliability as many other solutions.

Though widely used, these solutions aren’t always the most successful when dealing with complex geometries, something that makes welding more difficult to achieve. Conversely, PSPs can be used in paste and paint format, making them adaptable to a range of shapes and surfaces, convex and concave. PSPs also eliminate heat affected zone (HAZ) issues often associated with welding, which can result in less distortion and therefore, potential weak spots prone to cracks and damage. After welding, a heat treatment is completed to relieve the stresses that occur with welding. With the PSP process, the furnace cycle can do both the bonding of the PSP and the stress relieving in the same cycle.

With the emergence and growth of PSPs in aircraft MRO, engineers can meet increasing demand for the quick yet robust repair of aero-engine components, to time and costs that component replacement and other methods of repair simply cannot.

Morgan Advanced Materials

www.morganbrazealloys.com

About the author: Adam E. Ebert is a business development engineer at Morgan Advanced Materials, Wesgo Metals. He can be reached at 574-400-3075 or adam.ebert@morganplc.com.