Issue 30

A. Carofalo et alii, Frattura ed Integrità Strutturale, 30 (2014) 349-359; DOI: 10.3221/IGF-ESIS.30.42 350 I NTRODUCTION esign of gas turbine engine components is influenced by repeated request of higher performances in terms of high power, efficiency improvement, reduction of weight and costs. Another important point is the reliability of the design model used, that determines the flight safety. These objectives are generally achieved following two approaches: firstly, the development of material that could increase the maximum working temperature and could have a direct impact on the performance of the gas turbine; secondly, the development of advanced modelling techniques that could lead to the reduction of the safety factors generally used in the design phase. Thermomechanical damage is the main limiting factor of the life in a Nickel-based superalloy mechanical component [1]. Complex loads often lead to severe non-linear material behaviour, such as creep deformation and fatigue, while high temperature increases the probability of localized material plasticization under the service loads. Therefore, design against fatigue failure utilizes fatigue behaviour obtained through strain control fatigue tests. Creep and fatigue damages are generally studied using different models, even if some attempts to unify the material behaviour can be reached in literature [2, 3]. Anyway, these models were developed and experimentally verified for high strain levels in case of fatigue and for high stress-temperature condition in case of creep. The extension of their prediction to moderate strain levels, where elastic behaviour is predominant, constitutes a serious limitation [4]. Nucleation of fatigue crack represents the critical phenomenon that lead to failure of Nickel-based components [5-7]. From a metallurgical point of view, Nickel-based superalloys at high temperature are characterized by a change of ductility and the formation of an oxidation film on the surface. These two phenomena are antithetical with respect to the fatigue damage: the softening of the metallic matrix causes an increase of the probability of a fatigue crack initiation, while crack nucleation and propagation is interfered by oxide layer on the surface [8]. The combination of these factors determines the behaviour of the considered material. The interaction of creep and fatigue phenomena is a further complication and it is a phenomenon not well understood [9]. In this work, the mechanical properties of base material and TIG welded Waspaloy have been compared carrying out static, low-cycle fatigue and creep test on specimen obtained by a laminated sheet having a nominal thickness of 3.1 mm. In most cases, fatigue tests have been carried out with applied strain range Δε ≤ 0.6%. This kind of low stress level is associated to limited damage process. In this condition, the damage models are generally inapplicable or lead to inconsistent prediction. On the other hand, material behaviour at this stress level is very useful for industrial application, since it is close to component working condition. Therefore, simple and quick indicators of the different behaviour performance for static, fatigue and creep properties have been chosen and calculated, both for base and TIG welded material. The reduction of performances originated by welding has been estimated by the percentage variation of these indicators. M ATERIAL AND E XPERIMENTAL PROCEDURE aspaloy is a conventional Nickel-based superalloy that is subjected to hardening precipitating heat-treating. Waspaloy is widely used in the aeronautical field due to its good strength to corrosion and to high temperature, in particular for the realization of turbine disk, blade and casing. Mechanical strength is comparable to Haynes R-41 and higher than Inconel 718 at temperature higher than 650-705°C [8]. Microstructure is characterized by a face centered cube matrix with dominant precipitates γ’. Ductility of Waspaloy has been studied in [10], showing a reduction starting from room temperature to about 300 °C, an irregular behaviour up to 600 °C and a quick reduction at higher temperature. Waspaloy can be welded using conventional or advanced welding techniques. In presence of a localized damage in a large high-cost component, a possible maintenance strategy is to repair it removing the damage zone and substituting it with welded material. The residual life of the component would be increased with evident economic advantages, on condition that the reliability and the safety of the component are not reduced. Weldability of Waspaloy from a technological and metallurgical point of view has been studied in [11, 12]. Generally speaking, the fusion and the following solidification determine at least grain growth and introduce defect like microcracks. From a macroscopic point of view, these phenomena produce a reduction in ductility, showed by lower elongation at failure and lower fatigue and creep strength [13-17]. Starting from a laminated plate having a nominal thickness of 3.1 mm, base material specimens for static, fatigue and creep tests have been realized. At the same time, a similar plate has been butt-welded using an optimized TIG process in D W