Processing of Gas Turbine Engine Blades with Intense Pulsed Electron Beams

Processing of Gas Turbine Engine Blades with Intense Pulsed Electron Beams

Vyacheslav A. Shulov^a, Vladimir I. Engelko^b, Georg Mueller^c, Alexander G. Paikin^d

^aMoscow Aviation Institute, 125993, Moscow, Russia ^bEfremov Institute of Electro-physical Apparatus, 196641, St. Petersburg, Russia ^cForschungszentrum Karlsruhe, Institut für Hochleistungsimpuls- und Mikrowellentechnik, Postfach 3640, D–76021 Karlsruhe, Germany ^dChernyshev Machine Building Enterprise, 125362, Moscow, Russia

Abstract. The objective of the present research is the analysis of test results, dedicated to the effect of electron beam irradiation conditions upon the fatigue strength, oxidation resistance, erosion resistance, and salt corrosion resistance of metallic materials. The shaped specimens, simulating the gas turbine engine blade operation and directly the blades, produced of titanium and nickel alloys as well as refractory steels by machining method are used as the study objects. The irradiation of targets by the intense pulsed electron beam was realized by means of GESA-1 and GESA-2 accelerators. The test results showed that the endurance limit, oxidation, erosion and salt corrosion resistances of samples and blades subjected to the electron beam irradiation with the post-process vacuum annealing at the optimum conditions, could be increased by 10–40 %, 200–300 %, 200–250 %, and 600-900 % correspondingly depending on the alloy type. Due to irradiation with high energy density in a pulse, the surface layer of operated blades with oxidized resistant coatings can be ablated. The usage of intense pulsed electron beams allowed one to achieve high values of the ablation rate (up to 4–7 mm per a pulse) and to carry out the repair of aircraft engine blades.

INTRODUCTION

The majority of manufactured in Russian gas turbine engines (GTE) malfunctions on the stage of sizing and at operation are due to fatigue failure, caused by defects. By frequency of defect occurring GTE main components can be arranged in the next order: compressor blades, turbine blades, compressor disks, bearings, combustion chamber casing, turbine disks and shafts. Thus, compressor and turbine blades are the most critical parts of GTE, that is stipulated by the simultaneous effect of constant and cyclic loads, corrosive environment at high temperatures, dust and fluid erosion et al. These parts are mode from titanium and nickel-iron alloys, high-temperature refractory steels (lately with coatings — TiC, ZrN, Cr23C6 and others) and nickel super-alloys with protective coatings (NiCrAlY, NiCoCrAlY, NiCrAlY+NiAlSiB et al.).

The development and introduction of new techniques for increasing reliability and life of GTE super-alloy parts is one of most important problems of aviation engine construction science. This is stipulated that just the super-alloy parts, being the most expensive components, determine life of the product and its reliability during operation. The objective of this paper is the review of data, being issued in periodic publications, which concern the problems of properties modification of super-alloy products by using and electron beam treatment as well as manufacturing method development of GTE part surface treatment on the base of conducted investigations and the equipment creation for realising these methods in production quantities.

Experimental

The gas turbine engine blades from VT6, VT8, VT9, EP866sh, GhS26NK steels the composition of which are given in [1], were used as the study and test objects. The determination of the surface layer physical and chemical state of these objects was carried out by electron Auger spectroscopy (chemical composition), X-ray analysis (phase composition and residual stresses), scanning electron microscopy (surface topography), exo-electron emission and optical metallography. Besides, such characteristics as the surface roughness (R_a) and microhardness (H_m) were also determined. The electron beam treatment was performed with the use of «GESA-1» and «GESA-2» accelerators [2] at the rotation of targets under the beam. The irradiation compositions were as follows: accelerating voltage of 100–120 kV, pulse duration of 15–40 ms, electron beam energy density of 20–80 J/cm², beam cross-section area of 40–55 cm² and pulse number of 1–15. After irradiation the part of targets was annealed under vacuum (10^–3 Pa) for 6 hours at the operating temperatures. Initial, irradiated and annealed samples and blades were tested for fatigue at the operating temperatures in air with high (3000 Hz) loading frequency. After the irradiation and vacuum annealing, the specimens and parts were subjected to tests for the oxidation resistance in air at 500–900^o C for 100–500 hours and the salt corrosion resistance at the presence of Cl^- ions under the thermal cycling condition from the operating temperature to the room temperature (cooling in sea water).

Results and discussion

It is well known, that the main technological parameter of the IPEB irradiation process is energy density (w) in a pulse, other parameters (energy and pulse duration) are not varied in the experiments. With a rise of the energy density the following phenomena proceed in a near — surface layer of refractory alloy targets during irradiation: evaporation of organic impurities, heating and melting of a surface material, crater and crack creation, evaporation and sublimation, plasma formation and ablation. These phenomena determine physical and chemical state of material in the surface layer of irradiating targets and result in modification of their properties [1–5]. Thus, the optimal level of operating properties of components modified with ICEBs was achieved only by variation of energy density values and following investigation of the surface state. The irradiation with a great value of the energy density (w>25–30 J/cm²) was carried out in [1–5] with the goal of strengthening the surface layers and improving the oxidation resistance both due to the creation of fine-dispersed conglomerates based on complex carbides (for example, (Me)₆C₂₃ or TiC), the decrease of surface roughness and the dislocation structure change during the rapid melting and solidification of the material in the surface layer with thickness of 20-30 mm. The minimum irradiation energy density (18–20 J/cm²) was selected with the account of the retention of strengthening phases, which were formed in the initial material at the production stage of a part (b-phase, c¢-phase, and carbides). In this case only the thin surface layer can be melt and craters does not be formed on the surface. The effect of irradiating conditions on the phase composition and structure of blade surface layers is illustrated in [1, 3–5] and Figure 1. The results presented in [1, 3–5] and here allow to conclude that the most perspective regimes of irradiating the refractory alloys and steels can be achieved at low values of the energy density when the crater creation doesn’t take place, but strengthening phases are conserved. Under these regimes the surface roughness decreases from 0.20–0.25 up to 0.10–0.12 mm (titanium alloys and steels) and from 1.6–2.1 up to 0.4–0.6 mm (GhS26NK alloy blades with vacuum-plasma NiCrAlY coating). Some fatigue and corrosion test results of turbine and compressor blades after irradiation under the optimal irradiating conditions are given in Table 1. The test results show that the fatigue strength, oxidation, erosion and corrosion resistances of blades, subjected to electron beam irradiation with the post-process vacuum annealing at the operating conditions, could be increased by 10–40 %, 200–300 %, 200–250 %, and 600–900 % correspondingly. It was shown in [5] that the increase of the energy density from 25 to 80 J/cm² leads to a rise of the material ablating rate from the surface of blades with resistant coating. The maximum rates of ablating the surface layer achieve 8–9 mm during a pulse for titanium alloy blades with ZrN coating when the irradiation is carried out with high values of the energy density (w=48–52 J/cm²) and 4–6 mm during a pulse for nickel alloy blades with NiCrAlY coating (Figure 2) when the irradiation is carried out with high values of the energy density (w>80 J/cm²). After complete removal of coating damaged during the operation the blade surface consists single microdefects (craters). Crater creation on the surface of refractory alloy blades leads to the catastrophic deterioration of target properties. In order to reduce the negative effect of crater creation on operating properties of gas turbine engine components subjected to ICEBs, it was proposed [5] to carry out the final electron-beam treatment with low values of the energy density (18–22 J/cm²). At present the blade party (55 compressor and turbine blades) has prepared for realization of operating tests in content of RD33 technological engine (Figure 3).

TABLE 1. Fatigue, oxidation, erosion and corrosion test results: test temperature — 450 ^oC (VT9), 550^oC (EP866sh), 900 ^oC (GS32NK); erosion tests — V=200 m/s, α=90^o, quartz sand (80–120 mm), sand load — 40 mg/mm²; oxidation tests — τ=500 hours