Heat treatment optimization and mechanical characterization of a Nickel-based superalloy obtained via hot isostatic pressing (HIPping)

The next generation of aeronautic engines aims at continuously improving performances and efficiency, reducing at the same time emissions. However, increasing this temperature severely stresses the turbine constitutive materials, which must be improved. This thesis work was developed in the framework of a Clean Sky European Project coordinated by INSTM-Local Research Unit (LRU) of Politecnico di Torino (I) with the final purpose of producing two turbine casings via Near Net Shape Hot Isostatic Pressing (NNSHIP) using Astroloy as constitutive alloy. Further to coordination the Politecnico di Torino Local Research Unit was heavily involved in the research on the HIP structure analysis, the set up of the heat treatment and the study of the interface generated between the capsule and the consolidated Astroloy. Another LRU of INSTM, based at Politecnico di Milano (I), was involved in the project for their unique expertise on Fstigue Crack Growth tests and analysis. The Aubert & Duval (Fr) company was the other major partner of the project and provided the HIP process, methods and optimization as well as the industrial facilities for Heat Treatment and the production of the two big size casing demonstrators produced within the project. Finally AVIO AERO (I) supervised the project, having the role of Topic Manager and provided the industrial requirements and assessment of the whole project. Current casing constitutive materials are designed to work at ca. 650° C and superalloys like Inconel 718 or Waspaloy are typically used. Different aeronautic propulsion producers have various strategies to increase the efficiency of turbines, but they are all trying to find materials and manufacturing routes capable to provide casings which can readily operate at 700 to 800°C. Besides the requirement on maximum operating temperature casing constitutive materials are also requested to have high stiffness, so as to guarantee the maintenance of accurate clearances between the rotatory and stationary parts of turbines. Materials capable to provide such characteristics are indeed available on the market, but with major problems in terms of workability. Astroloy, the material studied within this thesis work, is characterized by an higher amount of Ti + Al alloying elements with respect to Waspaloy and the increased content of these elements gives the possibility to develop a higher fraction of gamma prime precipitates which are responsible for the improved properties at high temperature. At the same time increased Ti and Al additions provide poor forgeability which can highly limit the feasibility of a casing production cycle with such material.Therefore, although appealing service performances can be achieved by designing with Astroloy, there is of course a strong industrial interest to apply alternative near net shape processing routes (such as Hot Isostatic Pressing) to form such an alloy, so as to increase cost efficiency of manufacturing and reduce the environmental impact of the overall component processing. The aim of this thesis work was manifold: 1) a deep study of the structure achieved in Astroloy Ni superalloy when fabricated via Hot Isostatic Pressing (HIPping); 2) the optimization of the heat treatment of HIPped Astroloy, so as to maximize especially its creep and high temperature tensile properties; 3) the evaluation of microstructural and mechanical properties evolution after high temperature over-aging; 4) the study of the HIP capsule-consolidated Astroloy interface, with a consequent definition of the actual overstock to be considered in the fabrication via HIP of Astroloy components. HIPping parameters have already been optimized thus, great efforts were exerted to improve heat treatment; sub- and super- solvus recipes were investigated paying attention also to the cooling rate to be used. The aim was to obtain the best precipitates morphology and distribution preserving serrated grain boundaries and sufficiently small grains. Also the ageing treatment was optimized in order to enhance mechanical properties, avoiding overageing and indicating the best combination of time and temperature to be used to maximize γ’ precipitation. Three optimized recipes were obtained which were verified through tensile and creep tests giving very similar properties one to each other. According to all the microstructural features belonging to these recipes, and studying their thermal stability after a high temperature overageing test, one of these recipes was selected as the most promising heat treatment and thus was further investigated with a more extended campaign of experiments. From tensile experiments, it was observed a ductile behavior which is, indeed, altered in some extent, by certain brittle microstructural features occurring only at 760°C, thus indicating the possible presence of a well-known phenomenon for superalloy, called intermediate temperature brittleness. Actually, 760°C is indicated as a threshold temperature, above which creep mechanism is contemporary present. From Creep tests, it was derived that, also in Astroloy, the damage is preferentially located at grain boundaries and then it extends to the entire fracture surface. In this case, it was also evidenced how carbides located at grain boundaries are negative features, since they prevent dislocation motion, causing brittleness and easier grains detachment. From fatigue tests, it was observed how surface defects were less tolerable in respect to those located below the surface. Actually samples with surface defects exhibited a failure in less than 106 cycles, while those with sub-superficial defects provided a failure after higher cycles numbers. Fatigue Crack growth test were performed in order to verify the Astroloy capability to withstand the presence of defects, such as PPBs. Results are in good agreement with literature and in some cases (as for higher temperature) even higher. At the end of the work, also a technological issue was tackled. Actually, once the mechanical properties of HIPped Astroloy were demonstrated to be comparable or even higher than those of concurrent materials fabricated via traditional manufacturing route, it was necessary to deal with real components fabrication issues. In particular the topic of the minimum allowable overstock material was analysed and discussed. To achieve HIPping a steel capsule is filled with Astroloy powders and subjected to high temperature and pressure regime. HIP capsule in these operating conditions isotropically collapses for creep effects and transmits the pressure to the powder mass, which as a consequence of pressure and temperature is sintered to a zero level of porosity. It is not surprising that a number of transformation and reaction take place at the interface between the capsule and the consolidated powder mass. As a consequence altered superficial and sub-superficial Astroloy layers are formed close to this interface. These layers have to be removed via acid leaching or machining. Therefore to achieve the optimal raw material usage and to guarantee reliable service performances of HIPped components it is vital to exactly predict the amount of minimum allowable overstock to be provided in part design. To this purpose, a complete analysis of the interface between the capsule and Astroloy was performed and then, a procedure to dissolve the HIPping mold was prepared. First of all, the pickling bath composition was optimized in such a way that corrosion rate was very high but, at the same time, superalloy underwent only little chemical corrosion. Then a series of tests were performed in order to deeply understand the amount of material to be removed from the mold-Astroloy interface to obtain the optimal mechanical properties. In the end, it was established that a typical 500 μm overstock has to be prescribed. This overstock level makes HIPping a real Net-shape process.