Turbine component repair and recoat
New advancements make repair worth considering
By Greg Napert
With increasing costs of engine overhauls over the years, many engine operators are continuously evaluating their options with regard to refurbishing and overhauling turbine engine components. The business of overhauling turbine components is growing, and overhaul facilities are constantly searching for more options to offer their customers, including the option of high temperature protective coatings. Additionally, the quest to improve turbine component life and performance of engines with higher thrust, operating at higher temperatures, has also created a demand for improved coatings.
Unlike other aftermarket overhaul operations in aviation, it's fairly common for turbine component refurbishment companies to seek manufacturer approvals for all of their repair processes. Due to a combination of the complexity of turbine component's geometry, access to engineering data, test facilities, and exotic superalloys; it is extremely advantageous for these companies to work closely with the original equipment manufacturer (OEM's) and to provide complementary services instead of competitive repair services.
The relationship works both ways. The OEM's can also benefit from these specialized services by providing the OEM customers with low cost options for keeping their engines running longer and take advantage of faster turnaround times.
One company that offers coating and refurbishment options for turbine components is Walbar Metals in Peabody, Massachusetts.
Walbar provides only "OEM approved repair and refurbishing processes" and offers numerous coating options for hot section components. Walbar, a division of Coltec Industries, originated in the early Ô70s, and offered metallurgical services. Since then, it has developed component repairs for operators and engine manufacturers, with a concentration on hot end components for Pratt & Whitney and GE engines, the preponderance of which are vanes, nozzles, and blades.
Jim Palatine, a plant manager for Walbar, says that the company is not limited to repairs that have already been developed. It takes pride in offering engineering service to solve problems that may exist with particular components or as a result of unusual operating conditions. For instance, if an airline wants to develop a repair scheme for a particular component, Walbar engineers will work with the airline's engineering department and in conjunction with the engine manufacturer, to find a mutually agreeable solution.
"We like to refer to these repairs as extended repairs," Palatine says, "and we offer them on a case by case basis."
Most of Walbar's repairs, however, involve "bringing the part back to the actual form, fit, and function of the original part. Any change, even in the coating applied, affects the part to a degree that it might require significant testing and a part number change" explains Palatine.
Types of coatings
Two key processes needed to complete the overhaul and repair process for turbine components are the removal or "stripping" of the old and re-application of a new coating.
Stripping involves a controlled and precise chemical etching process that takes a great deal of experience to perfect. According to Palatine, "If there is any one thing that distinguishes one company from another, it's expertise in performing the stripping process, and the ability of the company to strip components without significantly affecting the parent metal."
A company that doesn't have a well controlled stripping process will not be able to salvage as many components, and the result will be a higher scrap rate, thus, defeating the purpose of refurbishing.
Advances in stripping and coating processes
There have been significant advances in coatings over the years, but most of the important advancements related to coatings have been in the application processes. New application methods allow for more even distribution of coatings around the profile of the part and better control of the overall coating process. One of the advancements in the past 15 years has been the development of the "vapor" coating process.
Paul MacGregor, Sr. Coating Engineer for Walbar Metals explains that a key advantage to the the vapor coating process is that the coatings are not diffused into the base metals as deep as the pack coatings, in order to achieve the same results. This means, that when parts are eventually stripped for re-coat, there is a reduced loss of base metal during the stripping process.
According to Palatine, A high percentage of our repaired components are "older generation" components that are still coated with "pack cementation" coatings involving traditional technology to deposit an aluminum coating on the base metal. We are slowly implementing the vapor deposited aluminide coatings onto refurbished components where feasible. The vapor coatings are more commonly specified as OEM "new part" requirements.
Aluminide coatings can be modified with the addition of platinum to enhance the oxidation and hot corrosion protection of the base metal from the different atmospheric conditions the engine performs in. The "aluminide" portion of the process can be performed by the conventional pack cementation or the vapor processes.
The coating must protect the part from the environmental conditions that the component will be exposed to during operation. However, the cost for that must also be considered. Economically speaking, it doesn''t always make sense to use a coating that lasts for 3,000 hours, but the base metal may only last 1,000 hours; it would be more economically feasible to use a coating that has a life expectancy consistent with the base metal."
Palatine explains, "The key is to pick a coating that is compatible with the expected life of the component. The coating applied to a part is designed to be "sacrificial" versus deteriorating the properties of the base alloys. The coating specified for a given part is usually designed to meet the engine and environmental conditions that the part will be exposed to. Base metal properties will be inspected during and after engine service to ensure component integrity (for example, creep properties, erosion, sulfidation etc.). These inspection checks will ensure the serviceability of the part and the effectiveness of the coating.
A case in point is that Walbar applied a modified aluminide (RB505) to a LP blade. The turbine blade base metal fatigued before the coating failed. A different coating was specified for future hardware which was more compatible with the operating conditions of the part. Palatine says, "There are some instances where the selection of modified and improved coatings really make sense, however. The RB505, for example, is an aluminide coating that is offered as an option (it is approved on the Pratt & Whitney PT6) to CPW333, which is defined as a simple aluminide coating. Another option that is available is our Aluminum-Silicon coating, a modified aluminide through the addition of silicon." Silicon can be co-deposited with the aluminum during the coating thermal cycle. Silicon is added to improve hot corrosion and sulfidation protection of alloy without the added cost of platinum.
Although noble metals such as platinum have been introduced to improve turbine hot section coatings, many of the same existing variations of aluminides continue to be the most popular in the aviation industry.
According to MacGregor, "Walbar has been using pack aluminides and chrome coatings for the last 25-plus years. These coatings continue to be very popular on many turbine engines."
Aluminide coatings can be applied using many different types of applications. The aluminide layer can be diffused into the base alloy through the use of slurries, paints, powders, or vapors, provided that the alloy is receptive to the deposition of the coating element.
A significant amount of these aluminides are still applied with a process known as Pack cementation. This is done by surrounding the component with an aluminum powder source, and subjecting it to a thermal cycle in an inert atmosphere.
Another popular type of coating is known as chromides. Chromides provide good corrosion protection in lower temperatures portions of the turbine hot section.
A more advanced coating system is the application of metallic and thermal barrier coatings. The metallic coatings are known as MCrAlY's and can be applied through various manufacturing methods such as HVOF, Low Pressure Plasma Systems, and electron beam physical vapor deposition. The manufacturing equipment is relatively expensive in relation to the equipment for aluminide coatings. The coating has significantly different properties than the aluminides but in many cases will out-perform the properties of the aluminide coatings. These metallic coatings are generally more expensive than aluminide coatings. Metallic coatings are used as a "stand alone" coating or as a bond coat for thermal barrier coatings. Thermal barrier coatings are typically ceramic coatings used to "insulate" the base alloy and dissipate the heat more uniformly from the part.
Palatine explains that with all the various types of coatings available to the turbine market, you need to understand the purpose and engine application for each before you make a decision. The alloy protection properties and the cost will vary.
Are your turbine components worth refurbishing?
According to Palatine, it is the customer's job to make certain determinations about whether or not to recoat their turbine components.
Turbine blades offer a particular challenge. For instance, with PT6 blades, it's the customer's job to measure the overall diameter of the blades and disc when they are installed in the wheel.
"We cannot measure this dimension for the customer, we can only measure certain dimensions on the blades, but not the installed dimensions. In the case of compressor turbine blades, there are some guidelines as to how much of the blade can be left, but the customer needs to determine whether or not there is enough material to build another wheel that will provide enough life and performance. It's a stack-up issue; it's something that the customer has to determine as to whether or not they want to do it. The engine manual provides the criteria for helping them do this. It provides OD dimensions that gives them a range that the blades must be within, and below that range, they are not repairable," Palatine says.
"We sometimes notice that the blades are worn on the low side and will alert the customer to this fact. It's up to them if they still want to proceed with the rebuild process. Remember that the shortest blade in the set is what you have to grind the wheel to when you reassemble the wheel — you need to take this into consideration."
Palatine adds, "Additionally, we perform a metallographic evaluation, which involves destructive testing of at least one blade from a set. We evaluate the base metal condition for over temperature operation, inter-granular corrosion, stretch, creep, and a number of other conditions. Blade retipping is a practice in the industry that is becoming more and more common."
"There are some tip repairs for different types of blades, particularly in the cold section," says MacGregor, "yet, for turbine blades that are worn beyond limits, Walbar chooses not to restore tips on these blades."
For other hot section components such as vanes and vane rings, the determination of whether or not to overhaul should really be left up to the overhaul and recoating facilities. There have been many repairs developed over the years that allow the overhauler to replace large sections of the vane and to weld and braze areas that, to the operator, would not appear to be salvageable. It's a good idea to call first to make sure that the overhaul facility does indeed repair that particular component.
"There are certain components that we don't overhaul anymore," says Palatine. "One of those components is the Cobalt vane ring used in some of the PT6 engine models. This is simply a business decision. We stick with what we're good at."
Related Article: Compressor Airfoilsand Fan BladesCompressor Airfoils and Fan Blades
New repairs and reduced turn-times
A major source of compressor efficiency loss in gas turbine operation is due to imperfections in airfoil shape. These imperfections exist for new airfoils and, as an engine deteriorates in service, the airfoil shapes become further distorted due to erosion. These distortions, along with degraded airfoil surface finish and increased tip clearance, penalize aerodynamic performance. This penalty is evidenced by decreased engine capability, increased fuel consumption and higher exhaust gas temperature.
This is why it's especially timely to take an overview of recent progress in the art and science of airfoil refurbishment for the cold sections of jet engines. Over the past five years progress, though evolutionary, has been substantial and mostly in the compressor section. The net benefit to commercial aircraft operators is fourfold:
• Higher repair yields of compressor airfoils, leading to lower refurbishment costs.
• Performance of restored airfoils that closely matches that of new airfoils
• Lower engine life-cycle cost and longer service life
• Much shorter turn times for airfoil refurbishment.
Today at our four global airfoil repair centers, the average incoming rejection rate for a set of cold-section airfoils is literally half of what it was in 1993. Usually, depending on engine type, 80 to 90 percent of the airfoils can be refurbished, up from 60 to 70 percent. In addition, with the proper treatment, refurbished parts give away nothing to new airfoils in operating performance. Turnaround times have been halved as well, especially at our new Singapore facility that opened in February 1998. The results have not only reduced airfoil replacement costs for operators of commercial aircraft, but also safely reduced their parts inventory levels.
The benefits related to airfoil refurbishment stem basically from five key advances. These include repair of newer-generation, 3-D design airfoils with end-bends; improved surface finish on refurbished airfoils; total automation of the welding process; chord, as well as tip restoration; and quicker turnaround. Let's look at each development separately.
Restoring "end-bend" airfoils
Over the past two years, ATI has pioneered the restoration of end-bend airfoils to like-new condition on a high volume basis. Examples include airfoils from the GE CF6-80C2 (Phase 2 Aero Design), and Rolls Royce RB211-535E4 engines. So-called 3-D Aero or end-bend airfoils are found in a number of new engines, and certainly represent the wave of the future in compressor airfoil design. Because of the complexity of their geometry, however, they were regarded for years as extremely difficult to refurbish. For one thing, their trailing and leading edges are not coplanar. It's the airfoil "twisted-closed" geometry near the endwall region (tip) that makes the machining difficult, as you must maintain the profile to recover the airfoil to near-design shape.
From a repair standpoint, the main problem lies in the machining that follows the welding process, not the welding itself. The finished airfoil geometry, and therefore the required toolpath, is simply beyond the capabilities of manual or 3-axis machining.
ATI's solution was to employ specifically designed and constructed 5 and 6-axis CNC machining centers, which represent substantial capital investment in both equipment and software development. But given the contours to be reproduced, there was no real alternative. Today, our protocol for refurbishing end-bend compressor airfoils involves a number of major steps.
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.
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.
Faster turn times
Obviously, the more rapidly a set of cold section airfoils can be repaired, the sooner the engine can be installed back on-wing, lowering the spares inventory requirements. Quite simply, days saved mean dollars saved.
Five years ago, the industry standard turnaround time for refurbishing a set of such airfoils was 3 to 5 weeks. Today, ATI has reduced that time to 7 to 10 days under normal conditions. As a result, the need for exchange services or maintenance of a large pool of inventory has diminished accordingly.
The main reasons for this improvement have been our commitment to reduce turntime and the resulting developments in automation. Today's high yields in compressor airfoil restoration simply would not be achievable with manual processing. Further, it is the orchestrating of these technologies that has led to today's shorter turn times. The craft and skills are there. However, they are embedded in artificial intelligence software rather than the individual craftsman controlling the various processes.
One lesson learned in developing the superfinishing processes for compressor blades is the importance of a smooth surface finish on airfoils throughout the engine. The relationship between airfoil shape, surface finish and performance is direct. Therefore, a good case could be made for using those three conditions rather than tip wear or damage as the trigger point for airfoil refurbishment. Once the airfoil surface finish is degraded, the leading edge blunt and the chord reduced by particulate matter, the compressor efficiency drops, and overall engine performance deteriorates. While the blade is still structurally sound and functional, it will waste valuable fuel dollars and reduce engine time on-wing. Often the saving in fuel will more than justify the cost of initiating airfoil refurbishment earlier.
The other principal lesson is that today you can and should expect airfoil refurbishment to produce blades that are not just structurally and geometrically sound, but perform literally like new parts. When this level of performance is achieved, you get airfoils that extend the cost-effective life of the engine plus fuel savings associated with having near-design shaped airfoils installed.
Fan blade refurbishment
All of the compressor blade maintenance strategies discussed above apply in much increased measure to the fan blades. In a high-bypass turbofan engine, the fan alone is responsible for consuming nearly one-half of the total engine fuel burn. Therefore, the impact of advanced fan maintenance scenarios can be significantly greater than for any other cold section component. Presently, ATI is in the process of implementing the RD305 process and a superpolish (down to 15AA micro-inches) surface finish for fan blades. This is in addition to the industry standard fan repairs that consist of patching, welding, blending and creep forming. To prove that the advanced refurbishment techniques of RD305 and superpolish for fan blades can have a significant impact on overall engine performance, a back-to-back test was conducted on the fan blades of a revenue service engine from a major airline.
The fan and engine were first overhauled per normal airline practice and cell-tested to establish that the engine performance met flight acceptance standards. This test, which the engine passed, also established a performance baseline for the overhauled engine. At this point, the RD305 process and superpolish had not been applied. After the cell test, the fan was removed from the engine and sent to ATI where the RD305 and superpolish process was done. Upon completion of this advance rework, the fan was mated to the waiting baseline engine, and the engine was retested in the same cell. Results indicated a drop of 0.7 percent in TSFC relative to the baseline engine after the enhanced fan overhaul. In addition, a comparison of the engine exhaust gas temperature over the range of thrust levels indicated a 3.5 C drop in EGT on average. The airline estimated that this performance improvement would translate to a fuel burn savings of $15,000 per year per engine and additional time on-wing of 1000 hours.
(This advanced maintenance development was reported by Dr. William Roberts in the ASME Transactions, Journal of Turbomachinery, Vol. 117, pages 666-667, October 1995). It is clear from this study that advanced fan blade refurbishment can quickly repay the investment. This is true at the time of major engine overhaul and also while the engine is in service. Once the fan surface finish is degraded, the leading edge blunted and the chord reduced by particulate matter, the engine efficiency drops, and overall engine performance deteriorates. While the blade is structurally sound and functional, it will waste valuable fuel dollars and reduce time on wing. Because it is relatively easy to change out fan blades on-wing, especially during scheduled aircraft maintenance checks, the savings in fuel and extended engine time on-wing will more than justify the cost of initiating fan blade refurbishment while the engine is in service.
Airfoil Technologies International is a joint venture between Sermatech International and GE Aircraft Engines. The company is headquartered near Cleveland, OH, and operates service centers elsewhere in the US, the United Kingdom and Singapore.