FRETTING CORROSION EFFECT ON FATIGUE RESISTANCE OF GAS TURBINE ENGINE PART MATERIAL

The paper presents the results of a study on the fretting corrosion effect on strength as per the diagrams of material fatigue of low-cycle and high-cycle loading. An experimental study of the effect of fretting corrosion on fatigue of a titanium alloy VT3-1 showed that, when choosing a particular technological method to increase fatigue resistance of gas turbine engine parts operating under fretting corrosion conditions, it is necessary to take into account the fact that fretting itself is intensive strengthening and at the same time a softening factor affecting the surface layers of the material. Technological methods of surface plastic deformation treatment result in a significant change in the parameters of metal surface layer state, which depends on both the type and duration of treatment. If the power and time parameters of processing are too high, the surface may be re-cold worked and the surface layer of the material almost completely exhausts the reserve of plasticity, as a result dangerous microcracks occur, and the formation of particles of flaking metal emerge. Possible ways using technological methods have been suggested by the authors to improve fretting strength of part surfaces operating under fretting corrosion conditions. Słowa kluczowe: korozja cierna, zmęczenie materiału, wysoka częstotliwość, niska częstotliwość, zwiększanie odporności na zmęczenie, metody odkształcania plastycznego powierzchni. Streszczenie: W artykule przedstawiono wyniki badań wpływu korozji ciernej na wytrzymałość zmęczeniową przy niskiej i wysokiej częstotliwości obciążania. Eksperymentalne badania wpływu korozji ciernej na zmęczenie stopu tytanu VT3-1 wykazały, że wybierając metodę technologicznego zwiększania odporności zmęczeniowej części silników turbogazowych pracujących w warunkach korozji ciernej, należy wziąć pod uwagę fakt, że sam fretting jest czynnikiem wzmacniającym i jednocześnie osłabiającym warstwy wierzchnie materiału. Technologiczne metody powierzchniowej obróbki plastycznej powodują istotną zmianę parametrów stanu warstwy metalu w zależności zarówno od rodzaju, jak i czasu trwania obróbki. Gdy parametry obróbki (moc i czas) są zbyt duże, powierzchnia zostaje poddana obróbce na zimno, warstwa wierzchnia materiału nie jest podatna na odkształcenie plastyczne, wobec czego powstają niebezpieczne mikropęknięcia oraz tworzą się odpryski. Wskazano potencjalne zabiegi technologiczne rekomendowane do zwiększenia odporności elementów maszyn pracujących w warunkach występowania korozji ciernej. * ORCID: 0000-0002-7287-2495. P.A. Solovyov Rybinsk State Aviation Technical University (152934, Rybinsk, Yaroslavl region), Pushkin Street 53, Russia, e-mail: technology@rsatu.ru. ** ORCID: 0000-0002-1049-7853. Łukasiewicz Research Network – Institute for Sustainable Technologies, Pułaskiego 6/10 Street, 26-600 Radom, Poland. *** ORCID: 0000-0003-4480-7220. P.A. Solovyov Rybinsk State Aviation Technical University (152934, Rybinsk, Yaroslavl region), Pushkin Street 53, Russia. **** PJSC “UEC-Saturn”, Rybinsk, Russia. INTRODUCTION The intensity of the effect of metal fatigue strength due to fretting corrosion depends on many factors. However, the number of fretting corrosion cycles becomes the determining factor for aircraft engine parts, since the other factors are regulated in one way or another by the design and operating conditions of real gas turbine engine designs. 8 ISSN 0208-7774 T R I B O L O G I A 2/2020 Analysis of operating conditions and causes of the destruction of disks of GTE low-pressure compressor second stage selected as the object research has shown that this gas turbine engine unit requires scientific substantiated approach to the choice of design and technological measures to ensure the required cyclic durability. Despite the fact that the destruction of disks occurs as per the mechanism of low-cycle fatigue, in this work, an attempt was made to obtain more detailed information about the effect of fretting corrosion on both the low-cycle and high-cycle fatigue of the structural and technological model of the selected object. EXPERIMENTAL STUDY OF EFFECT OF FrETTInG DAMAGES On HIGH-cyclE FATIGuE OF TITAnIuM AllOy VT3-1 When working on the experiment methodology, the following main points were implemented: 1. Ambient temperature θ = 288 К. 2. Displacement amplitude А = 100 μm, frequency f = 30 Hz, contact pressure рн = 5 MPa. 3. The study was carried out on the special specimens made of VT3-1 and L70 alloys. 4. The parameters of the state of the specimen surface layer prior to the experiment corresponded to the technical conditions for disk manufacture applying special finishing operations. 5. The study was accomplished on the single-factor experiment principle using the combined method. Special specimens that were preliminarily damaged by fretting corrosion with different running hours were tested for fatigue prior to destruction of each specimen. 6. The duration of the continuous run for fretting wear of specimens was determined based on a previously obtained fretting wear diagram of this pair of materials for reasons of the reliable achievement of each of the three main stages of the process (adhesion interaction, run-in, steady wear). 7. The influence of the structural and technological stress concentrator in the form of a chamfered circular hole on fatigue of VT3-1 alloy under the simultaneous influence of fretting corrosion was studied. The design of the special specimens made of VT3-1 alloy for conducting comparative tests for high-cycle fatigue is shown in Fig. 1. The design of L70 alloy counter-pieces used to create fretting corrosion damage in laboratory conditions is shown in Fig. 2. The method of conducting the experiment was as follows. Special specimens (Fig. 1) totalling 20 pieces were divided into four structural and technological groups: specimens without a hole and fretting-corrosion damage (the initial state); specimens with a ∅ 3 mm hole without fretting corrosion damages; specimens without a ∅ 3 mm hole with fretting corrosion damages of different run; specimens with a ∅ 3 mm hole, and fretting corrosion damages of a different run. Prior to the experiment all specimens were subjected to recrystallization annealing in the protective atmosphere (Тann = 650 С) and were instrumented with strain gages with a gage length of l = 10 mm. In order to determine the necessary continuous run of specimen fretting wear to reliably achieve each of the 3 main stages of the fretting process, the fretting wear diagram for the specimen pair VT3-1-L70 was taken at the load parameters set above. It is established that the end of the adhesion interaction stage corresponds to N = 5 ⋅ 104 cycles, and the end of the run-in stage and the onset of the stage of steady wear corresponds to N ≈ 105 cycles of interaction. Results of comparative tests for fatigue based on N0 = 5 ⋅ 106 cycles are presented in Table 1. Fig. 1. Outline drawing of the specimen for conducting comparative tests for high-cycle fatigue (Split image A – A is enlarged 5-fold) Rys. 1. Szkic próbki do przeprowadzenia testów porównawczych zmęczenia w wysokiej częstotliwości (fragment A–A jest powiększony 5-krotnie) Fig. 2. Outline drawing of the counter-piece to produce fretting corrosion damages Rys. 2. Szkic przeciwpróbki do badania zużycia w wyniku korozji ciernej Table 1 indicates that fretting-corrosion damages of the surface, including those in conjunction with the structural and technological stress concentrator in the form of a circular chamfered hole, practically do not affect the high-cycle fatigue of the titanium alloy VT3-1. Indeed, comparing the obtained values of σ–1 cond in Groups 1 and 3, it is easy to see that decrease in endurance limit of specimens with fretting wear damage makes 9 ISSN 0208-7774 T R I B O L O G I A 2/2020 Table 1. Specimen fatigue test results Tabela 1. Wyniki testu zmęczeniowego próbki Group No. Specimen No. Conventional endurance limit: σ–1 cond, MPa Average value σ–1 cond, MPa Specimen rupture life Nrupt, tsd cycles