γ-TiAl alloys are a family of intermetallic compounds which, thanks to their excellent physical and mechanical properties, are arousing big interest in the aerospace and automotive industries. In particular, they are considered an attractive alternative to nickel-based superalloys due to a lower density (about 4 g/cm3 for γ-TiAl alloys and 8 g/cm3 for Ni-based superalloys) that makes their specific mechanical properties comparable to those of nickel-based superalloys. The low weight of these materials allows to reduce the overall weight of the aircraft engine component or automotive engine component. As a result, it is possible to enhance the components performances and reduce fuel consumption and emissions. The low weight of γ-TiAl alloys, it will also contribute to achieve the targets for fuel-burn and emission reduction proposed by the European Commission and NASA. The most applied conventional industrial scale processing routes for titanium aluminides include ingot casting, ingot forging, hot-rolling sheet production, investment and permanent mold casting and powder metallurgy processing. The processing of titanium aluminide via these conventional manufacturing methods can be complex due to the low ductility and fracture toughness of the material and casting process is an expensive solution and presents several problems such as the reactivity of the molten material with ceramics. The Electron Beam Melting (EBM) additive manufacturing technology is well known and considered for the processing of TiAl alloys, in particular for the aerospace application. This additive manufacturing technology uses an electron beam to generate parts by selectively melting the powder layer by layer according to CAD data. EBM technology allows to produce lighter and complex-shape components with a minimum material and energy waste. The goal of this thesis was to investigate and characterize both γ-TiAl specimens and components produced by EBM and the heat treatments set-up in order to optimize the material properties. Since the starting material for the EBM process is the pre-alloyed powder, the characterization and the optimization of the powders was a fundamental preliminary step in order to guarantee a successful production of the final parts. The experimental activities are related to four different TiAl alloys, three of which for aircraft engine application that are the 48-2-2 alloy (Ti-48Al-2Cr-2Nb (at.%)), the High-Niobium alloy (Ti-(45-47)Al-2Cr-8Nb (at.%)) and the TNM alloy (Ti-43.5Al-4Nb-1Mo-0.1B (at.%)) and one of automotive interest that is the so called RNT650 alloy (Ti-48Al-2Nb-0.7Cr-0.3Si (at.%)). The TiAl 48-2-2 powder reuse investigation has demonstrated the possibility to reuse the powder up to six EBM cycles without a significant pick-up of contaminants, modification of particle size distribution, flowability and apparent density during the subsequent EBM jobs. This achievement means that, by reusing the powders for several cycles it is possible to obtain a considerable advantage in terms of cost and material saving. However, it is important to specify, that it is possible to mix the recycled powder with new powder between different cycles in order to maintain the proper powder characteristics. The effect of the EBM processes parameters on the TiAl 48-2-2 material properties has been investigated by varying certain building parameters according to a Design Of Experiment (DOE) matrix. The study has shown that i) there is a parameters combination window in which the amount of process defects in the built material is very limited, ii) inside this window it is possible to perform a further fine parameters tuning in order to obtain an homogeneous microstructure and limit the evaporation of low-melting elements such as aluminum. In fact, this study has confirmed that the aluminum content and microstructure are very sensitive to the parameters variation. Regarding the 3rd generation TNM alloy for aircraft engine application, it has been demonstrated the possibility to process it by EBM obtaining fully densified parts and, after a proper heat treatment, it has been possible to obtain the desired microstructures in order to improve the mechanical properties of the material according to the application. The RNT650 alloy for the automotive application has been successfully processed by EBM and both massive specimens and lightweight hollow turbocharger wheels have been produced and characterized. Also in this case, a proper microstructure for the application has been obtained by means of a proper heat treatment. In addition, the characterization of a turbocharger wheel-shaft assembly prototype has been performed and it has shown a promising junction quality with a complete adhesion and without the presence of any defects at the interface. Regarding the aircraft engine application of γ-TiAl, big part of the research activity was done in collaboration with AvioAero, within some European and Regional research projects. In particular, the 48-2-2 alloy as well as new generation alloys such as High Niobium alloy and TNM alloy produced by Electron Beam Melting were investigated in the frame of the European project E-BREAK. Considering the automotive application, the part of the work on the RNT650 alloy was done within the European project TIALCHARGER.
|Titolo:||γ-Titanium Aluminide Alloys for Aircraft and Automotive Engine Components Applications Processed by Electron Beam Melting|
|Data di pubblicazione:||8-feb-2018|
|Appare nelle tipologie:||8.1 Doctoral thesis Polito|
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