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Technological Process Development of Repair and Property Recovery of Gas Turbine Engine Blades from GhS26NK Alloy with NiCrAlY

Resistant Coating with Intense Current Pulsed Electron Beams

A.S. Novikov, A.G. Paikin, V.A. Shulov*, O.A. Bytzenko, D.A. Teryaev*, A.D. Teryaev, V.I. Engelko**, and K.I. Tkachenko**

Chernyshev Machine Building Enterprise, 7, Vishnevaya str., A-80, GSP-3, Moscow, 123362, Russia Phone: +7(495) 491-49-88, Fax: +7(495) 491-56-52, E-mail: shulovva@mail.ru

*Moscow Aviation Institute, 4, Volokolamskoye shosse, A-80, GSP-3, Moscow, 125993, Russia Phone: +7(495) 158-44-24, Fax: +7(495) 158-29-77, E-mail: shulovva@mail.ru

**Efremov Institute of Electro-Physical Apparatus, 1, Sovietsky ave., Metallostroy, St. Petersburg, 189631, Russia

Phone: +7(812) 462-78-45, Fax: +7(812) 463-98-12, E-mail: engelko@niiefa.spb.ru

Abstract – The present paper reviews the experi-mental results dedicated to the effect of irradiating conditions with intense pulsed electron beams on ablation kinetics of the surface layer of gas turbine engine blades from GhS26NK with NiCrAlY resis-tant coating. It is shown that intense pulsed elec-tron beam of microsecond duration is high effec-tive instrument for repair of turbine blades from refractory nickel alloys with resistant coatings. Ap-plication of intense pulsed electron beam allows one to ablate per a pulse the surface layers fractured during operation with thickness of 5–10 μm, if the energy density is equal to 50–55 J/cm2.

1. Introduction

Quality and in time carried out repair work provides more than 50% increase of the product operation time, reduce consumption of expensive materials and ex-penses for manufacturing of new parts. At the same time research and fundamental investigations in area of development of new technological processes for parts repair are carried out very little and it leads to consi- derable lag of this link from the general development of the product manufacturing technology. It was pro-posed in [1, 2] to use intense pulsed electron beams (IPEBs) for removal of the coal deposit and oxidized and damaged surface layers of gas turbine engine compressor blades. The authors of these papers have developed the repair technological processes of high pressure compressor 3 and 7 stages blades from VT9 titanium alloy and EP866SH steel including the opera-tion of electron beam treatment. The objective of the present paper is study of technological basis of gas turbine engine nickel alloy blade repair and property recovery with the use of IPEB forming at the GESA-1 and GESA-2 accelerators.

2. Materials, equipment and research techniques

As an object of investigation there were used samples and turbine blades of ZhS26NK (Ni; 1.0 Ti; 5.6 Cr; 6.2 Al; 1.4 Mo; 10.0 Co; 1.2 V; 1.4 N; 12.5 W; 0.18 C; < 0.1 O, N; < 0.02 H; < 0.015 B) with coating

from NiCrAlY (SDP-2; Ni; 18–22 Cr; 11–13.5 Al; 0.3–0.6 Y). Composition of alloy and conditions of its heat treatment corresponded to the Engineering Speci-fications: ZhS26HK – annealing in vacuum at 1250 °C during 3 h, cooling at the rate of 50–60 deg./min, sta-bilizing annealing in vacuum at 1000 °C during 2 h. Some part of these blades before irradiation was cut with the use of electro-erosion machine and was investigated by the following methods: electron Auger-spectroscopy (EAS), scanning electron microscopy (SEM), exo-electron emission, X-ray structural analy-sis (XSA) and optical metallography (OM) in pola- rized light. In addition, microhardness (Hµ) and rough-ness (Ra) were measured. Blades treatment by intense pulsed electron beam was carried out at the “GESA-2” accelerator (electron energy – 115–150 keV; pulse duration – 30–60 µs; energy density in the beam – 40–88 J/cm2; beam cross section area – 30–80 cm2; density non-uniformity in beam cross section – 5%). Blades after irradiation were cut, and from the re-ceived samples-evidences cross section metallogra- phic specimens were made and as a result of it mate-rial specific mass loss was determined depending on energy density and pulse number. Besides, targets surfaces were investigated by EAS, SEM, XSA, and OM methods to define thickness of layers being re-mote from the pulse, recrystallized and modified areas of the target. Finally, comparative fatigue tests were done on series produced blades, as well as blades re-paired as per series production technology and with the use of intense pulsed electron beams.

3. Experimental data and discussion

Results of investigation and testing of original and radiation treated blades are given in Figs. 1–8 and in the Table. One can see that intensive removal of the SDP-2 coating from the surface of GhS26NK nickel alloy blades begin already to proceed during irradia-tion at the energy density (W) more than 60 J/cm2. The irradiation with high values of W (Fig. 2) leads to the total ablation of SDP-2 coating per 10 pulses. Because in this case on the surfaces of repaired blades

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some craters and wave micro-relief were formed, so for their removing and surface smoothing the irradia-tion with energy of low density in melting condition (W = 42–45 J/cm2) was performed.

Fig. 1. The blade after ablation of coating by IPEB

Fig. 2. The blade after smoothing by IPEB

Fig. 3. Microstructure of the surface layer after mask irradia- tion at W = 50 J/cm2 and n = 10 pulses

30 40 50 60 70 80 900

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10

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25 GESA-2

NiCrAlY (50 μm) ZrN (26 μm)

Thic

knes

s of r

emov

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yer, μm

Energy density, J/cm2

Fig. 4. The kinetics of SDP-2 coating ablation

As it is shown in [1–5] the main practical interest has the phenomenon of ablation (relaxation explosion-emission process of non-equilibrium vapour-plasma phase creation). Realization of this process opens wide possibilities for repair of gas turbine engine rich com-

20 µm Fig. 5. Microstructure of the surface layer after irradiation at W = 40 J/cm2 and n = 10 pulses

20 µm Fig. 6. Microstructure of the surface layer after irradiation at W = 50 J/cm2 and n = 10 pulses

10 µm Fig. 7. Microstructure of the surface layer after irradiation at W = 60 J/cm2 and n = 10 pulses

Effect of the energy density on the surface roughness, exo-electron emission, residual stresses and microhardness of NiCrAlY-coating deposited on the sample surface from GhS26NK

W, J/cm2 n, pulse

Ra, µm,±0.05

Ieee, pulse/s

σ, MPa ±50

Hµ, HV, units,

p = 2N – – 2.12 240 ± 60 –170 420–490

22–26 5 1.14 390 ± 90 +120 440–520 22–26 10 1.03 420 ± 40 +130 460–510 42–45 5 0.36 610 ± 30 –60 480–490 42–45 10 0.32 620 ± 20 –70 470–480 50–55 5 0.99 720 ± 80 –90 390–530 50–55 10 1.12 740 ± 70 –100 380–520

ponents and ensures intensive removal of the coal deposit and oxidized surface layers from the surface of blades after operation of gas turbine engine. Kine-

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tics of ablation from the surface of high pressure tur-bine 1 stage blades from GhS26NK nickel alloy with SDP-2 coating is presented in Fig. 4.

After irradiation vacuum stabilizing annealing was carried out at temperature 1000 °C during 2 h for removing residual tensile stresses and stabilization of physical and chemical state. For demonstration of electron beam repair effectiveness the results of abla-tion kinetics with the use of resistant mask and ap-pearance of blades after coating removal are presented in Figs. 1–4.

One of the most impotent questions of repair elec-tron beam technology realization is as follows: how is it possible to achieve non adequate removal of the material from the different areas of blade surface? Indeed, thicknesses of remaining coating and oxidized layers after operation of gas turbine engine are varied greatly on the different areas of blade surface, there-fore the use of irradiation with the same number of pulses leads to the partial removal of GhS26NK re-fractory nickel alloy from the blade areas with already ablated coating. This problem can be decided using the simple selection of the energy density in a pulse providing the effective removal of coating material, while the blade material does not ablate during irradia-tion at this energy density. It comes easily because refractory nickel alloy consists low quantity of pores and a great amount of refractory alloying elements in comparison with the SDP-2 coating material. In this case it is obviously that values of energy density pro-viding the effective removal of coating and alloy ma-terials will be differed to a considerable extent.

Figures 5–8 present the results of 60-µm-SDP-2 coating removal with the surface of monocrystalline samples from GhS26NK nickel alloy with the use of electron beam treatment. Energy of electrons was equal to 125 keV and pulse duration was varied at the in-crease of energy density from 30 (40 J/cm2) to 80 µs (70 J/cm2).

20 µm Fig. 8. Microstructure of the surface layer after irradiation at W = 55 J/cm2 and n = 10 pulses

Te developed technological process of repair was realized with the use of cylindrical monocrystalline fatigue samples made of GhS26NK alloy. The 60-µm-SDP-2 coating was deposited on the surface of their

samples be vacuum-plasma method. After deposition this coating was ablated with electron beam irradiation at 55 J/cm2 using 10 pulses and the surface was

smoothed by IPEB treatment at 42–45 J/cm2 using 4 pulses. These samples passed the fatigue tests at 975 °C and 3000 Hz frequency in air. The test results are presented in Fig. 9.

Fig. 9. The results of fatigue tests

4. Conclusion

It was shown that intense pulsed electron beam of micro second duration is a high efficiency means for turbine blades repair. Use of IPEB allows to remove during ten pulses coal deposit and damaged resistant coating from the surface of blades made of GhS26NK nickel alloy. The thickness of removed during 10 pul- ses at energy density of 50–55 J/cm2 surface layers of these parts is 60 µm.

Experiments have proved that after electron beam removing coal deposit and coating in service surface layers the main properties of blades became worse (surface roughness is increased, endurance limit is reduced, residual tensile stress is formed). In order to attain operational properties level of original blades it is necessary to carry out blades repair technological process in several operations: 005 – removing coal deposit and coating by IPEB; 010 – polishing microre-lief by IPEB; 015 – inspecting the surface condition; 020 – finish heat treatment to remove residual tensile stresses.

References

[1] A.G. Paikin, A.B. Belov, V.I. Engelko et al., Phys. and Chem. of Machining 2, 32 (2005).

[2] A.G. Paikin, A.B. Belov, and V.I. Engelko et al., Strengthening Technologies and Coatings 11, 9 (2005).

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