Repairing what is arguably the most complex single part of an engine calls for staying on top of technology in your workshop. Engine manufacturers and MRO specialists thus keep investing in new techniques to restore high-pressure turbine (HPT) blades to working order. These moving parts can be barely 3 in. long on a CFM56, but the business case for repairing versus replacing is very strong.
An HPT blade’s shape is designed for aerodynamic efficiency—and it also uses several layers of coating to resist erosion and extremely high temperature, around 1,500C (2,732F). HPT blades also have an internal cooling system made of tiny, labyrinthine channels ending on the airfoil’s surface.
Despite these protections, a blade endures a lot. “The tip often rubs against the shroud, creating abrasive tip wear, erosion and small tip cracks. The thermal barrier coating suffers from thermal cycling, extreme temperature, erosion and may undergo spalling. Sand particles may block cooling holes. They melt and create a deposit on the surface of the airfoil, causing a chemical attack to the thermal barrier coating known as CMAS attack—calcium, magnesia, alumina, silica,” says Bernd Kriegl, MTU Maintenance’s technical program manager for commercial repair.
The hottest blades—those closest to the combustion process—can be subject to oxidation that can accelerate wear, General Electric consulting engineer John Cupito adds. Billy Power, SR Technics’ country manager for Cork, Ireland-based Airfoils Services, also mentions creep and impact damage.
The complexity of manufacturing such a sophisticated blade explains the business case for repairing it. Repairing is about 70% cheaper than replacing, Kriegl estimates. The list price of an HPT blade is $6,000-12,000, and there are 50-70 blades on a disk.
However, Pratt & Whitney’s aftermarket repair director, Jayne Nye, asserts that “repair-or-replace” decisions are complex, involving trade-offs between maintenance costs, time on wing and turnaround time. New blades offer the longest life, Cupito concurs.
An HPT airfoil typically has to be repaired every 3-5 years. It usually can be repaired 2-3 times over its lifetime. This depends on the scope of the repair and the blade’s type, design and base material. Removal of diffusion coatings typically reduces wall thickness, which limits successive repairs.
Advanced technologies are key to making new repairs possible, improving quality and thus extending part life and cutting costs. The bottom line is finding a competitive edge. For Snecma, recent progress relies on automation, such as five-axis electro-discharge machining and blending robotization. This results in a faster, more reliable repair and better repeatability, Philippe Alassoeur, component repair general manager, notes.
Among the latest technologies MTU is using is induction heating. It enables a local high-temperature brazing process without putting the whole component into a furnace. There is no negative effect on base material properties due to high brazing temperatures in other critical areas of the component, such as the foot, Kriegl says.
A new coating, used under the platforms, better protects against corrosion and sulfidation (when the fuel’s sulphur attacks the single crystal material). The coating, now free from chromium-6, is more environmentally friendly.
MTU is developing new inspection techniques such as computer tomography, which enables better internal inspection of a cooling system, before and after a repair, than X-rays.
Also, MTU is looking at where additive layer manufacturing (ALM) could be used. The company uses it now to manufacture parts on the Pratt & Whitney family of geared turbofans. ALM is highly suitable to repair a complex geometry, as you can build up a section of a blade, Kriegl says. Laser-powder cladding, another additive process, is used for blade-tip repair. A laser beam welds a powdery filler material onto the component.
GE is looking for new technology to improve blade life. For example, new anti-corrosion coatings for lower-temperature areas and better-performing weld materials offer higher resistance to oxidation. “We also are focusing on automating inspections and repair processes, which can yield a more consistent and repeatable result,” GE’s Cupito says.
Pratt & Whitney is factoring in the environment, too. “Some of the technologies we are developing are focused on achieving our sustainability goals for 2015,” Nye says. Among others, he mentions next-generation cleaning technologies, enhanced durability materials and real-time radiography inspection.
At SR Technics’ Cork facility, new methods of visual and dimensional inspection are being devised.
Making repairs less expensive is as much a concern as improving quality. “To cut costs, automation is being introduced in inspection, stripping and cleaning for faster operation,” MTU’s Kriegl says. Snecma is implementing lean manufacturing methods and trying to improve the diagnosis phase to limit the extent of a repair.
“It is important to also consider durability and to ensure that the repaired part functions as expected within the engine system. An inexpensive repair doesn’t do an operator much good if it doesn’t last and drives down time on wing and/or residual value,” Cupito points out.
Last but not least, a repair is also an opportunity to introduce an upgrade. “During repair, service bulletins can be incorporated,” Power notes. Thermal barriers, for example, evolve as they do in newly built engines. “We apply new repair and coating on a part designed 20 years ago—this is part of the ‘MTU-plus’ repair concept,” Kriegl says.
A version of this article appears in the October 6 issue of Aviation Week & Space Technology.