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1 THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS 345 E. 47th St, New York, N.Y GT-427 The Society shall not be responsible for statements or opinions advanced in papers or discussion at meetings of the Society or of its Divisions or Sections, or printed In its publications. Discussion Is printed only if the paper is published M an ASME Journal. Authorization to photocopy material for internal or personal use under circumstance not fang vathin the fair use provisions of the Copyright Act is granted by ASME to librettos and other users registered with the Copyright Clearance Center (CCC) Transactional Reporting Service provided that the base fee of, $0.30 per page is paid directly to the CCC, 27 Congress Street Salem MA Requests tor special permission or bulk reproduction should be addressed to the ASME Tezhnical Publishing Department. Copyright by ASME All Rights Reserved Printed in U.S.A. HIGH STRENGTH DIFFUSION BRAZE REPAIR FOR GAS TURBINE COMPONENTS Roger D. Wustman Howmet - Operhall Research Center Whitehall, Michigan Jeffrey S. Smith Howmet - Operhall Research Center Whitehall, Michigan Leonard M. Hampson Howmet - Tulsa Refurbishment Center Tulsa, Oklahoma 740,17 Marc E. Suneson Howmet - Wichita Falls Refurbishment Center Wichita Falls, Texas ; ABSTRACT The goal of all repair processrs is to return the hardware to a serviceable condition. Diffusion braze repairs utilize metallurgical processes to achieve economical repairs of expensive gas turbine components, especially in the turbine section. Component repairs often require dimensional restoration and crack repair on the same part. To achieve this goal, a new diffusion braze repair alloy was developed that combines high strength crack repair and dimensional build up into one material. This new material has mechanical property strength approaching that of the base metal. The improved mechanical properties result from a homogenous gamma prime strengthened diffusion braze zone. As part of an FAA approved test plan, the Howmet ESR (Effective Structural Repair) diffusion braze material was evaluated by tensile and stress rupture testing at elevated temperature. The test results showed high tensile strengths and long stress rupture life. In addition, the effect of the diffusion braze thermal cycle was evaluated on the base metal. A comparison was made between the gamma prime size and shape of engine run JT8D LPT vane clusters before and after the thermal cycle. The thermal cycle was shown to have a beneficial effect on the gamma prime size and shape relative to overaged engine run nozzles. The low cycle fatigue (LCF) life of MarM247 was also shown to improve with the ESR thermal cycle relative to a typical LPT nozzle heat treatment. INTRODUCTION Gas turbine components experience significant distress during service (Anthony & Coward, 1988). The high cost of replacement hardware makes component repair an important activity to an airline or industrial customer. The goal of all repair processes is to return the part to service, where it has to operate in the same manner as a new part until the next overhaul. Diffusion braze repairs have become an important part of gas turbine component repair (Ellison, et al., 1993). Diffusion braze repairs are used for crack repairs, wide gap closure and dimensional restoration. The diffusion braze repair materials are available in tape and slurry form depending upon the specific application. Good repair practice limits the amount of braze material used in order to minimize excess work and the detrimental effects of excessive boron diffusion into the alloy. The most cost effective repairs can be applied again and again. In an ideal world, the component could be repaired indefinitely, however that is seldom the case. Howmet Refurbishment Inc. has developed a new material (ESR) for gas turbine component repair. Like other diffusion braze repair materials (Wein & Young, 1978; Ellison et al., 1993; and Tandon'& German, 1994), ESR is an alloy blend of a high melt and a low melt powder. The nominal composition of a mechanical blend is Ni-11.3W-10Co-8.6Cr-5.3A Ta Hf- 0.5Cb-0.3Mo-0.13C. The alloy mixture can be (and often is) tailored to specific applications. Wide gap repairs and build ups use a thicker (i.e. more high melt powder) blend, while crack repairs use a thinner (i.e. more low melt powder) blend. All test results presented in this paper were from mixtures. ESR first gained acceptance with repairs of small engine components, like Garrett stage 2 vane rings, Garrett stage 1 TFE33I nozzle segments, Garrett TFE731 HPT vane segments, and Pratt & Whitney - Canada PWI00 stage 2 LPT vane rings. This paper discusses the expansion of this technology by the FAA approval process, from small gas turbine repairs to large commercial engine repairs, specifically the lt8d-200 series low pressure turbine (LPT) vane clusters (Figure I). Technical data for repair sequencing, materials, process controls, and outcome controls for specific components can be approved by the FAA to permit repair development and inclusion of technical improvements. TO achieve FAA approvals, a disciplined approval prococs is followed. The process includes the preparation of a test plan which defines the important steps of the proposed repair, how it will be controlled and how it will be tested for compliance. The process also outlines how the results gained will meet the requirements for service. This may require substantial Presented at the International Gas Turbine and Aeroengine Congress & Exhibition Birmingham, UK June 10-13,1996 Downloaded From: on 05/12/2018 Terms of Use:

2 Table 1 Nominal Chemistry Comparison Alloy Ni Cr Al Ti Co W Mo Ta Cb Hf Si B C Zr MarM247 Bal IN'713C Bal ESR Bal testing, both destructive mechanical property testing and nondestructive inspection. Property testing, particularly for newly developed materials, would at least include elevated temperature tensile and stress rupture testing, ductility tests, oxidation behavior, parent metal effects, and metallographic analysis for process response and quality. Following acceptance by the engineering functions of the FAA, test lots of sample components are processed according to the approved plan. During and at the culmination of the experimental work, a report is compiled along with all the supporting data and presented to the FAA. In turn, the package is reviewed by cognizant engineering personnel familiar with and qualified to analyze the requirements, assure that the plan was followed and that the results are sound. If all the requirements are met, an approval is issued. EXPERIMENTAL PROCEDURE Elevated temperature tensile and stress rupture testing was used to evaluate the strength of the diffusion braze material. The test specimens were prepared in simple butt joint configurations. The tensile testing was performed at 1600 F (870 C), while the stress rupture testing was performed at 1800 F (980 C) and 2000 F (1090 The diffusion braze thermal cycle effect was evaluated by low cycle fatigue testing MarM247. The MarM247 was prorne.pd through a typical MarM247 heat treatment and an ESR thermal cycle. This testing was performed at 1600 F at 20 cycles per minute (CPM) with zero ID maximum strain cycling (A ratio I). The thermal cycle effects were also evaluated by metallographic comparisons of engine run MarM247 and 1N'713 JT8D LPT vane clusters as received from the engine and after the ESR thermal cycle. For comparison, the nominal chemical compositions of MarM247, IN'713 and ESR are given in Table 1. The goals of the testing were to demonstrate the mechanical properties of the diffusion braze repair material and to show that the thermal cycle had no adverse effect on the LCF properties of the parent metal (MarM247). TEST SPECIMEN PREPARATION The tensile test specimens were prepared from investment cast MarM247 blocks (Figure 2) that were brazed together and machined into inch (3.8 mm) diameter round bars (Figure 3). The tensile test specimen manufacturing procedure was as follows: I. As cast MarM247 Blocks (Figure 2) 2"x 1.5" x 0.3" (51mm x 38mm x 7.6mm) 2. Mate face grind. 3. Shim gap at inch (0.3 mm). 4. Tack weld. 5. Vacuum clean. 6. Apply alloy to both sides of joint 7. Vacuum braze at F (1204 C). 8. Diffusion Heat treat at F (1177 C). 9. Age heat treat 10. Machine 0.150" (3.8 mm) diameter test bar (Figure 3). The stress rupture test specimens were prepared from investment cast MarM247 sheets that were brazed together and machined into 0.060" (1.5mm) thick flat sheet specimens (Figure 4). The stress rupture test specimen manufacturing procedure was as follows: I. As cast MarM247 sheets 1.5" x 0.75" x 0.1" (38mm x 19mm x 2.5mm) 2. Mate face grind. 3. Shim gap at 0.005" (0.13 mm) and tack weld. 4. Apply alloy to both sides of the joint, using a slurry mix. 5. Furnace braze at F (1204 C). 6. Diffusion Heat treat at F (1177 C). 7. Age heat treat 8. Machine inch (1.5 mm) thick sheet specimens (Figure 4) The LCF test bars (Figure 5) were prepared from investment cast MarM247 round bars, that were prepared to the following procedure: I. As cast MarM247 round bars. 2. Age heat treat per typical MarM247 LPT vane F(1080 C) for 4 hours F (871 C) for 12 hours. 3. Machine into LCF bar (Figure 5). 4. Re-heat treat with the ESR thermal cycle and re-age. Furnace braze cycle at F (1204 C). Diffusion Heat treat at F (1177 C). Age heat treat F (1080 C) 4 hours F (871 C) 12 hours. RESULTS AND DISCUSSION OF MECHANICAL PROPERTY TESTING AND METALLURGICAL EVALUATION 1600 F (8700 Tensile Properties Eight (8) diffusion braze samples were tensile tested at 1600 F (870 C). The ultimate tensile strength was approximately the same as the 0.2% yield strength for most of the tests. This is believed to be due to the specimen configuration and the relatively low ductility of the diffusion braze joint. Similar results were observed by other authors (Wein & Young, 1978). Table 2 and Figure 6 compares the strength of the ESR to MarM247 and IN'713C base metal data found in the ASM Metals Handbook Ninth Edition vol. 3. The percent elongation was used as the ductility measure for both the MarM247 2 Downloaded From: on 05/12/2018 Terms of Use:

3 and the IN'713, while the reduction of area was used as the ductility measure for the ESR. The reduction in area (R of A) was used for the ESR ductility rather than the percent elongation, because it is a better estimate of braze joint ductility. The percent elongation is calculated using the total gage length of the test specimen and may contain additional parent metal elongation. Table 4- Strain Controlled 1600 F (870 C) LCF Pseudo Life Life Stress Cycles Cycle KS! (IvfPa) (Ni) (NO Baseline 60 (414) Table F (870 C) Tensile Strength Comparison Baseline 80.6 (556) Baseline 91.2 (629) Alloy UTS YS Ductility KSI (MPa) KSI (MPa) - % MarM (800) 97(670) 5.0 IN'713C 105(725) 72(495) 14 ESR 96.9 (666) 91.7 (632) 1.3 Stress Rupture Test Results The stress rupture testing was conducted at 1800 F (980 C) and 2000 F (1090 C). The ESR stress rupture test results were compared to other high strength diffusion braze repairs (Wein and Young, 1978 and Miglietti and Pennefather, 1996). Unfortunately the test temperatures and stress conditions were different A Larson-Miller parameter plot (Figure 7) was used to make a comparison. It showed that two other high strength diffusion braze materials and the ESR had strength comparable to that of IN'713. The ESR stress rupture life was 75% of IN'713 at 1800 F (980 C) and 90% of IN'713C at 2000 F (1090 C). The MarM247 stress rupture data was shown to be better. Table 3 Stress Rupture Comparison Alloy 1800 F / 20 KSI 2000 F / 5 KSI MarM hours 1119 hours* I14' hours 172 hours ESR on MarM hours hours" extrapolated from ASM Metals Handbook and Larson-Miller Plot average of 4 tests faarm247 Low Cycle Fatigue Test Results Long high temperature furnace cycles can be detrimental to the mechanical properties of the parent metal. Low cycle fatigue (LCF) is a common cracking mode of nozzles and vanes. LCF testing at 1600 F (870 C) was used to evaluate the mechanical properties of MarM247 processed through the ESR thermal cycle versus a typical new make LPT vane cluster heat treatment. The testing showed that ESR thermal cycle had a positive effect on the LCF life of MarM247 (Figure 8). The base line heat treatment in this case was that of a typical LPT vane segment, as cast and aged. The 2200 F (1204 C) high temperature diffusion braze cycle resulted in more gamma prime precipitation making the material stronger and more LCF resistant Baseline (834) Baseline l50(1035) ESR 70.7(488) ESR 79.6 (549) ESR 90.2 (622) ESR 99.6 (687) ESR (827) ESR (1027) A ratio = 1.0 zero to max. strain cycling 20 cycles per minute Microstructure; Characterization of ESR. MarM247 And IN 713C Alloys Metallographic evaluation of braze joints after the ESR thermal cycle revealed a homogenous gamma prime strengthened microstructure (Figure 9), which is believed to account for the high strength reported. There were no eutectic phases or obvious boride phases present in the braze joint, which would have weakened the braze joint. The narrow inch (0.13 mm) gap is a benefit in achieving a homogenous braze joint and is intended to simulate crack repair. Wider gaps would be expected to be less homogenous and would be expected to have slightly lower properties. For the purpose of evaluating the ESR thermal effect on the base metal, two scrap engine run TD3D stage 3 LPT vane clusters were returned for evaluation and metallurgical comparison. One of the vane clusters was investment cast from IN'713C, while the other was investment cast from MarM247. Several airfoil sections were removed from the mid airfoil spans of each segment. One section from each was mounted, polished and evaluated as received. Another section from each vane was processed through an ESR thermal cycle prior to being mounted, polished, and evaluated. The metallogmphic mounts were examined by optical metallography and SEM after being etched with molybdic acid to bring out the gamma prime microstructure. The engine run microstructures of the IN'713 vane and the MarM247 vane were significantly different (Figure 10), probably reflecting a difference in engine operating history rather than that of the gamma prime reaction kinetics. The exact engine operating condition for these two vane segments is not known, although they came from different engines. The engine run IN'7I3 gamma prime microstructure was overaged and agglomerated, while the engine run MarM247 gamma prime structure was more uniform. Gamma prime over aging occurs in nickel based alloys and would be expected after prolonged hot operation (McLean and Tipler, 1984). Over aging is a function of 3 Downloaded From: on 05/12/2018 Terms of Use:

4 both time and temperature of operation. The ESR thermal cycle was shown to refine the microstructure of the 1N'713 material. Because the MarM247 gamma prime microstructure was not over aged the diffusion braze thermal cycles had little affect on the microstructure. The ESR thermal cycle at 2200 F (1204 C) is above the gamma prime solvus for both alloys, and would be expected to solution the gamma prime, which would then re-precipitate during the subsequent age heat treatments. SUMMARY AND CONCLUSION Elevated temperature tensile and stress rupture properties were established for a new diffusion braze repair alloy. The tensile yield strength of the ESR material was greater than that of IN'713 and 95% of MarM247. The ESR stress rupture life was --75% of 114'713 at 1800 F (980 C) and 89% of IN'713 at 2000 F (1090 C). Miglietti, W., 1993, "Correlation Between Microstructure and mechanical properties of Diffusion Brazed Mar M 247", 93-GT-295 presented at ASME International Gas Turbine and Aeroengine Congress and Exposition, Cincinnati, Ohio May 24-27, Miglietti, W.M., Pennefather, LC., 1996, "The Microstructure, Mechanical Properties and Comability of Diffusion Brazed CMSX-4 Single Crystal", to be presented at ASME International Gas Turbine and Aeroengine Congress and Exposition, Birmingham, England June 10th, Ramirez, J.E. and Liu, S., 1992, "Diffusion Brazing in the Nickel-Boron System", Welding Research Sunnlement Oct., 1992 pp Tandon, R. and German, R.M., 1994, "Supersolidus-Transient Liquid Phase Sintering Using Superaftoy Powders", The Jnternational Journal of Powder Metallurgy Vol. 30, no 4, pp Wein, J.A. and Young, W.R., 1978, "Cost Effective Repair Techniques for Turbine Airfoils", AFML Wright Patterson AFB, July The high elevated temperature tensile and stress rupture lives were the result of a gamma prime strengthened diffusion braze zone. The diffusion braze repair thermal cycle was found to rejuvenate the gamma prime structure on overaged engine run LPT nozzles (IN'713). The LCF life of MarM247 was shown to improve slightly with the diffusion braze thermal cycle. A new high strength repair alloy was developed and approved for service using the FAA approval process. REFERENCES Anthony, ICC. and Coward, G.W., 1988, "Aircraft Gas Turbine Blade and Vane Repair", Superalloys 1988, Duhl. D. N., Maurer, G., Antolovich, S. and Lund, S., eds., The Metallurgical Society, pp ASM Metals Handbook Ninth Edition, vol. 3, pp Demo, W.A., Ferrigno, Si, 1992, "Brazing Method Helps Repair Aircraft Gas-Turbine Nozzles", Advanced Materials and Processes, Vol. 141, No. 3, pp D.S., Owczarslci, W.A., Paulonis, D.F., 1974, "TLP Bonding: a New Method for Joining Heat Resistant Alloys", Ilekingkamaj, April, 1974 pp Ellison, K.A., Lowden, P., Liburdi, J., Boone, D.H., 1993 "Repair Joints in Nickel-Based Superalloys with Improved Hot Corrosion Resistance.", ASME International Gas Turbine and Aeroengine Congress and Exposition, Cincinnati, Ohio May 24-27, McLean, M. and Tipler, H.R. "Assessment of Damage Accumulation and Property Rejuvenation by HIP and Heat Treatment of Laboratory and Service Exposed 1N'738LC", Sunerallovs1.213A PP Downloaded From: on 05/12/2018 Terms of Use:

5 O INCH I 1. 1 I T 1 H 0 CM 2 HOWMET CORPORATION 0 INCH I, ' 0 CM 2 HOWMET CORPORATION a) As Received b) Fully processed through ESR and coating Figure I. J78D Stage 2 LPT Vane Clusters (IN713)

6 Figure 2. SkematIc view of MarM247 tensile bar braze blocks PLACES 6.4 R 4 PLACES 4.7 Figure 4. Flat stress rupture bar drawing R X 45 CHAMFER, R Figure 3. Round tensile bar drawing. Figures. LCF test bar drawing. Note: All units In mm. 1mm = inches.

7 1 Strength Comparison Stress Rupture Comparison 100 ins vs UTS vs MS vs , , ,, to, --+ NIM247 - o -1N713 ESR o Wein & Young Miglielti 8 Pennetedher zo Figure 6. MM 241 IN 713 ESR 56 Figure?. se so 62 so se Larson Miller Parameter P = (T+460)*(25+log(tIme))/1000. Pseudo Stress (KSI) MM247 LCF F (070 C) LCF I. A ratio u 1.0 I It I 20 CPM Strain Controlled IIII Eall Basotho. Val I o -Baseline +ESR Igo PAM247 Banana IIII 'IL. lit Ill. I I ii mumill In II WI III 1111 II I nun '111: "I'. an Cycles NI Figure 8. Low Cycle Fatigue (LCF) comparison of MarM247 with and without the ESR thermal cycle.

8 *Je4 q a) 100X; ESR Diffusion Braze. 0000X 5.00pm 0257I 737A4 RM b) 2000x; SEM photograph of diffusion braze gamma prime structure. c) 2000x; SEM photograph of the MarM247 basemetal structure after ESA thermal cycle. Figure 9. Metellographic photographs of an ESR/MM247 test bar after diffusion brazing and heat treatment (Moldbic Etch).

9 gritsmispitiv4epzarwitteeli Itiailabgani*Viti wit C In*T -w NI:1441WeaN91 "AU Or. 7 :1414Ner 41141`0 a VI' 4101eiTto biblita4,41 mit; y.4,-t it, -. egfra ;01 a Ant Ana VI. ' st4p kat OSP te414/ k "i lbas... t..44a4 w a iitteki 'CAA Siktireen 10. " ifif Ar 4 set aim e a) As received 1N713. 2til1' SE 10KV 5000X 2.00pn St b) after ESB Thermal Cycle. c) As received MarM247. d) MarM247 after ESR Thermal Cycle. Figure 10. SEM photographs of JT8D Stage 2 LPT Vane Clusters before and after the ES118 Thermal cycle.