Turbine Technology: Brazing Repair

CF6-80 series engines are high bypass turbo-fan engines with two shafts, one stage of fan, three stages of LP compressor and five stages of LP turbine installed at LP shaft, 14 stages of HP compressor and two stages of HP turbine connected by HP shaft. The first stage of HP turbine is located behind the combustion chamber. HP turbine shroud support is used for supporting and maintaining high-pressure turbine shroud in place in hot gas path and operates in a high temperature condition. Front surface of HPT shroud support contacts turbine nozzle support to sustain load from turbine nozzle. Location of HPT shroud support is shown in Figure 1. HPT shroud support is made from Rene 41 or Rene 77 Nickel superalloy.

HPT shroud support stage 1 exhibits erosion, scratches, nicks, wear, dents, and cracks on cast area of parts after removal from gas turbine engine during engine overhaul. See Figure 2. It is normal for hot section component in high time operation. Engine repair manual has a welding repair for cracks, erosion, and other defects on surface of HPT shroud support. However, superalloy Rene 41 or Rene 77 is sensitive to cracking during post welding heat treatment, resulting in excessive welding cycles and thus causing significant dimensional distortion to the part.

Repair proposal and procedures
In this repair design brazing process is implemented to avoid cracks resulting from excessive welding and heat treatment cycles and thus ensures that defected part is salvaged efficiently without causing distortion or other discrepancies. The following are procedures which are used in this repair.

  • Grit blast surface of HPT shroud support using aluminum oxide grit and pressurized air.
  • Remove air baffle from shroud support in accordance with engine repair manual.
  • Remove plasma coating or hard coat from forward face lip in accordance with engine repair manual.
  • Polish or blend to clean repair surface and remove abrasive contaminants from repaired surface. Blend to remove cracks for brazing preparation.
  • Fluorescent penetration inspection for machined surface for cracks.
  • Cap weld seal slot break out with Inconel 718 welding filler.
  • Set up and put shroud support into furnace for vacuum cleaning. Evacuate furnace to vacuum and run furnace per setting temperature profile.
  • Prepare braze filler and braze alloy at certain ratio and mechanically mix powder to ensure alloy powder is mixed thoroughly. Add binder into mixture and completely mix blended powder with binder.
  • Put several beads of alloy mixture over the worn or cracked area to get a buildup. Apply braze stop-off to the area adjacent to the repair alloy, seal slot, and holes to prevent hole plugging of non-reworked areas during alloy flow.
  • Set up and put shroud support into furnace. Evacuate furnace to vacuum and run furnace per setting temperature profile to braze and diffuse shroud support.
  • Gas fan cool and remove shroud from furnace to perform metallographic inspection. Put test sample in mount and metallographic prepare it. Section and examine samples for porosity, void, delineation, and cracks using magnifier. Ensure there are 100 percent bonding between braze alloy and substrate material.
  • Machine repaired area per dimensions specified in engine repair manual.
  • Blend excessive braze alloy flush with parental material.
  • EDM machine seal slot forward end per engine manual.
  • FPI inspection for HPT shroud support for crack after EDM machining.
  • Install air baffle and do age heat treatment per engine repair manual.
  • Apply thermal plasma coating on front surface of lip.

Table 1

Major Repair Substantiation Sheet
Yes
No
1. Does the action have significant effect on the product?    
Weight
X
Balance
X
Structural Strength
X
Performance
X
Power Plant Operation
X
Flight Characteristics
X
Other Qualities Affecting Airworthiness
X
2. Must the action be accomplished by methods other than accepted practices or elementary operation?
X
3. Will action cause an increase in functional failure rate?
X
4. Will the consequences of a failure be more significant?
X
5. Will the action have a major effect on product’s TC?
X
6. Will the action change approved specified material?
X
If all above items are checked ‘NO”, the action may be considered MINOR.
If any above item is checked “YES”, the action is considered MAJOR and FAA-DER approval must be received prior to proceeding with the action.


Table 2: EASA Classification

Major Repair Substantiation Sheet Yes No
1. Structural performance    
Static Strength   X
Fatigue   X
Damage Tolerance   X
Flutter   X
Stiffness Characteristics   X
2. Weight and Balance    
Weight Change   X
C.G. Location   X
Control Surface Stiffness   X
Control Surface Mass Distribution   X
Control Surface Profile   X
Flutter and Controllability   X
3. System    
Effect on Complete System   X
Effect on System Redundancy   X
Effect on adjacent or remote systems parts   X
4. Operational Characteristics    
Stall Characteristics   X
Handling   X
Performance and Drag   X
Vibration   X
5. Other Characteristics    
Changes to Load Path and Load Sharing   X
Change to Noise and Emissions   X
Fire Protection / Resistance   X
6. Continued Airworthiness    
Additional Inspection   X
Life Limited or Critical Parts   X
Changes to Aircraft Flight Manual   X
7. Extensive static, fatigue and damage tolerance strength justification and/or testing   X
8. Unusual methods, techniques or practices (unusual material selection, heat treatment, material processes, jigging diagrams, etc)   X
IIf all above items are checked ‘NO”, the action may be considered MINOR.
If any above item is checked “YES”, the action is considered MAJOR.


Table 3

FAR Sec. 33.4 Instructions for Continued Airworthiness
The applicant must prepare Instructions for Continued Airworthiness in accordance with Appendix A to this part that are acceptable to the Administrator.

Appendix A. The Instructions for Continued Airworthiness for each engine must include the Instructions for Continued Airworthiness for all engine parts. If Instructions for Continued Airworthiness are not supplied by the engine part manufacturer for an engine part, the Instructions for Continued Airworthiness for the engine must include the information essential to the continued airworthiness of the engine. The Instructions for Continued Airworthiness must contain the following ma nuals or sections, as appropriate, and information: (a) Engine Maintenance Manual or Section. (b) Engine Overhaul Manual or Section. The Instructions for Continued Airworthiness must contain a section titled Airworthiness Limitations that is segregated and clearly distinguishable from the rest of the document. This section must set forth each mandatory replacement time, inspection interval, and related procedure required for type certification.

Continued airworthiness requirements for OEM part as published in Engine Overhaul Manual will apply to the repaired part without changes. All inspection and repair procedures applicable to OEM part may also be performed on the repaired part. The part is inaccessible during engine maintenance thus it is not necessary to add supplements to Engine Maintenance Manual. Another MRO facility can use current engine maintenance manual to perform inspection, repair, and overhaul for the repaired part. It is concluded that Engine Maintenance Manual and Engine Overhaul Manual is not affected by the repair scheme.
FAR Sec. 33.15 Materials.
The suitability and durability of materials used in the engine must:
(a) Be established on the basis of experience or tests; and
(b) Conform to approved specifications (such as industry or military specifications) that ensure they have the strength and other properties assumed in the design data
All materials are as specified for CF6 HPT shroud support by TC holder. This repair is very similar to a brazing repair in OEM manual which is currently being applied on the same part. There is no change for function and operating condition except repair location. Previous experience indicates that no repaired parts have been returned to our facility during normal service therefore it is anticipated that the component repaired by the repair process will react similar to the stated approved repaired component through intended use. So suitability and durability of used materials conform to approved specifications that ensure they have the strength and other properties assumed in the design data.
FAR Sec. 33.19 Durability.
(a) Engine design and construction must minimize the development of an unsafe condition of the engine between overhaul periods.
The durability of CF6 HPT shroud support stage 1 repaired by the process will not have adverse effect on engine durability. This repair is very similar to a brazing repair in OEM manual which is currently being applied on the same part to restore similar defects. The methods, techniques and practices utilized during the repair process are as stated by TC holder and therefore it is anticipated that the repaired part will react similarly to TC holder approved repaired part through its intended use. Therefore the repair techniques minimize the development of an unsafe condition between overhaul periods.
FAR Sec. 33.75 Safety Analysis.
(a) (1) The applicant must analyze the engine, including the control system, to assess the likely consequences of all failures that can reasonably be expected to occur.
Safety is not affected because part is returned to original condition.


Table 4: EASA Compliance Review

CE-E 25 Instructions for Continued Airworthiness
(a) Manual(s) shall be provided containing instructions which are acceptable to the Agency for continued airworthiness of the Engine. They must be updated as necessary according to changes to existing instructions or changes in Engine definition.

Continued airworthiness requirements for OEM part as published in Engine Overhaul Manual will apply to the repaired part without changes. All inspection and repair procedures applicable to OEM part may also be performed on the repaired part. The part is inaccessible during engine maintenance thus it is not necessary to add supplements to Engine Maintenance Manual. Another MRO facility can use current engine maintenance manual to perform inspection, repair and overhaul for the repaired part.
It is concluded that Engine Maintenance Manual and Engine Overhaul Manual is not affected by the repair scheme.
CS-E 70 Materials and Manufacturing Methods
(a) The suitability and durability of materials used in the Engine shall be established on the basis of
experience or tests. The assumed design values of properties of materials shall be suitably related to the
minimum properties stated in the material specification.
(b) Manufacturing methods and processes shall be such as to produce sound structure and mechanisms
which retain the original mechanical properties under reasonable service conditions.
(c) Material specifications, manufacturing methods and processes shall be identified and referenced in the
Engine type design.
Al materials are as specified for CF6 HPT shroud support by TC holder. This repair is very similar to a brazing repair in OEM manual which is currently being applied on the same part. There is no change for function and operating condition except repair location. Previous experience indicates that no repaired parts have been returned to our facility during normal service therefore it is anticipated that the component repaired by the repair process will react similar to the stated approved repaired component through intended use. So suitability and durability of used materials conform to approved specifications that ensure their having the strength and other properties assumed in the design data.
CS-E 100 Strength
(a) The maximum stresses developed in the Engine shall not exceed values conforming to those established by satisfactory practice for the material involved, due account being taken of the particular form of construction and the most severe operating conditions. Where a new type of material is involved, evidence shall be available to substantiate the assumed material characteristics. For Turbine Engines, due consideration shall be given to the effects of any residual stresses in Engine Critical Parts.
(b) The Engine components which form part of the Engine mounting and any other parts of the Engine liable to be critically affected shall, when the Engine is properly supported by a suitable Engine Certification Specifications for Engines mounting structure, have sufficient strength to withstand the flight and ground loads for the aircraft as a
whole in combination with the local loads arising from the operation of the Engine.
(c) Each Engine must be designed and constructed to function throughout its declared flight envelope and
operating range of rotational speeds and power/thrust, without inducing excessive stress in any Engine
part because of vibration and without imparting excessive vibration forces to the aircraft structure.
The durability of CF6 HPT shroud support stage 1 repaired by the process will not have adverse effect on engine durability. This repair is very similar to a brazing repair in OEM manual which is currently being applied on the same part to restore similar defects. The methods, techniques and practices utilized during the repair process are as sated by TC holder and therefore it is anticipated that the repaired part will react similarly to TC holder approved repaired part through its intended use. Therefore the repair techniques minimize the development of an unsafe condition between overhaul periods.
Environmental Protection
(a), Ensure that products parts and appliances comply with the applicable requirements for environmental protection.
(b), Issue the appropriate environmental certificates i.e. the type-certificate data sheet for noise.
Environmental and operational design requirements for engine are not affected as the part is returned to original condition.

Classification of major and minor repair

  • FAA classification
    According to definition of major/minor repair at 14 CFR Part 1.1 and other sections in CFR, a table is created for the classification of major/minor repair. See Table 1. In this repair, brazing and heat treatment processes is neither elementary operation nor included in accepted practices, thus the repair is considered major repair and FAA DER approval is required.

    Although the FAA and EASA have similar definitions for what constitutes major and minor repairs, the requirement for acceptable or approved data is quite different.

    For a major repair, FAA requires that an operator use approved data from the FAA or FAA designee: designated engineering representative (DER) or authorized representative (AR). EASA requires that technical data must be approved by EASA or EASA design organization approval (DOA).

    For a minor repair, FAA requires that an operator use acceptable data from the operator or type certificate (TC)/supplemental type certificate (STC) holder such as OEM drawings and specifications. In the United States, all operators have authority to use acceptable repair data for minor repairs without additional FAA approval. EASA regulations require approved data for both minor and major classifications of airplane repairs. This policy is in contrast to the FAA system that requires approved data for major repairs only. Approved data herein include approved data by EASA or EASA DOA or acceptable data from the TC/STC holder or third party.

  • FAA compliance review
    The repair design shall be reviewed to ensure that it complies with FAA Federal Aviation Regulation Part 33 Airworthiness Standards: Aircraft Engines. See FAA regulation compliance review in Table 3.

Results
Brazing repair for CF6-80 gas turbine engine HPT shroud support is approved by both FAA and EASA and is being used in repair shops. Customers benefit from this cost-effective repair project. AMT

William Hui Ma is a senior repair engineer working in Canada.

References: CF6-80C2 Engine Shop Manual, revision 67, Dec. 1, 2007

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