Turbine Component Repair and Recoat

Turbine component repair and recoat New advancements make repair worth considering By Greg Napert June 1999 With increasing costs of engine overhauls over the years, many engine operators are continuously evaluating their options with...

First comes an incoming inspection for each individual airfoil to assess suitability for restoration. This includes cleaning and non-destructive testing (NDT) utilizing the latest technologies and processes available. Only following acceptance to the incoming inspection criteria, can parts enter the repair cycle. Next, welding of the airfoil tip and/or leading edge, depending on engine type, by automated Micro Plasma Arc Welding (PAW) to replace material removed in service by rubbing and/or erosion. CNC machining then restores original shape, and a surface finishing process restores original surface finish — a key step. Finally, each blade is individually inspected for conformance to OEM engine manual specifications. The final inspection is akin to the familiar RD305 industry standard for airfoils.

To date, our experience with refurbishing these difficult end-bend compressor airfoils with a high level of automation has seen a yield of 70 to 80 percent, a five figure savings versus conventional methods. This has resulted in significant cost savings to the airlines. Thousands of such blades have been restored to like-new integrity and aerodynamic efficiency. End-bend airfoils are cost effective to refurbish! If you have unserviceable end-bend airfoils, you don't have to scrap them. ATI has a proven cost effective solution that exists; one that costs a fraction of a new part.

Improving surface finish
For years, cold-section airfoil repair focused only on integrity and geometry of the part and overlooked surface finish. The thinking at the time was that as long as the refurbished part worked, met engine manual specifications and cost less than a new part, it was good enough. Operators were satisfied to trade off engine efficiency in exchange for a lower overhaul cost. It is now known that airfoil surface finish in the cold section affects Thrust Specific Fuel Consumption (TSFC), Exhaust Gas Temperature (EGT), and therefore, fuel costs and time on-wing.

To improve this aspect of airfoil performance, ATI has recently developed tumbling and polishing techniques that improve the surface finish of refurbished compressor airfoils down to 15AA micro-inches. The extra step is making a measurable difference in operating costs. It is fast becoming standard practice among operators to specify this extra step. Remember even a 1% gain in TSFC can cut fuel costs by $100,000 per year per aircraft. Years of experience have shown that surface finish of the airfoil surfaces, especially in the fan and compressor sections, exerts a great deal of influence over TSFC.

Welding automation
Automation of the welding process and use of Micro PAW has dramatically improved consistency, yield, throughput and process economics of airfoil repair. In the decades preceding 1993, much of the development work in refurbishment centered on manual Gas Tungsten Arc Welding (GTAW), the metallurgy of the refurbished part, and welding techniques to maintain it. The welding process was done largely by hand using skilled operators. The parts came out fine, but slowly and expensively. And because of the expense, many airfoils were labeled non-repairable, or at least not worth repairing. Now, however, the main thrust has been to automate these technologies for improved economics, consistency and turnaround — and ultimately yield. In fact, automation of the welding process significantly contributes to today's consistent metallurgical quality, higher yields and faster turnaround times. Patterns of material loss in service have been recognized with experience, and weld programs to address them have been developed and refined.

Cleaning and preparation of the weld area have likewise been automated — in some cases through the use of robots — further improving consistency, yields and turn times.

Chord restoration of CFM56 airfoils
In the early Ô90s, restoration of the workhorse CFM56 airfoils was limited to replacing lost material at the tips. The result was a serviceable airfoil that extended engine life, but at reduced operating efficiency. The other result was that too many airfoils were scrapped because the chord reduction was excessive and deemed non-repairable.

Since mid-1996, however, ATI has been restoring the chords as well as the tips of CFM56 compressor airfoils. This complements the familiar RD305 process offered by ATI, which re-establishes the eroded leading edge radius (LER) to near-design shape. When blade chord is reduced below the RD305 process limit, the chord is restored to original shape by welding, followed by a proprietary coining and machining operation. As a result, the percentage of "non-repairable" CFM56 compressor airfoils has dropped by fully 50%, saving about $8,000 on a typical CFM56 compressor refurbishment cost.

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