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Fatigue cracking in additively manufactured gamma-TiAl
Abstract
In the last decade, additive manufacturing technologies, remarkably those based on selective powder bed fusion, have proved to be enabling technologies in many industrial sectors, especially where geometrical complexity of the mechanical components or the technical limitations in the use of highly specialized materials do not allow the application of consolidated, conventional technologies. Additive manufacturing by Electron Beam Melting (EBM) may be used to effectively produce gamma titanium aluminide (TiAl) intermetallic alloys with mechanical properties suitable for structural components.
Despite the intrinsic limitations of TiAl intermetallics, due to their relatively limited strain at fracture under tension and fracture toughness, especially at ambient temperatures, TiAl intermetallic alloys offer the advantage of favorable specific strength, also at moderately high temperatures, and they are now considered as a potential substitute of currently employed alloys in some specific applications in the energy, aerospace, and automotive industry.
Within this framework, fatigue properties of different variants of gamma-TiAl intermetallics, with slightly different chemical compositions and microstructures, produced by (selective) Electron Beam Melting (EBM) are presented. After EBM, the TiAl alloys undergo hot isostatic pressing and subsequent heat treatment; depending on the heat treatment temperatures, fully lamellar or duplex microstructures may be obtained. The fatigue properties of an enhanced Ti-48Al-2Nb-2Cr alloy, a high Nb containing and a Mo-containing TiAl alloys are compared with already established alloys, all produced by additive manufacturing by selective Electron Beam Melting (EBM), with the aim of highlighting, when possible, the effect of the microstructure on the fatigue properties. High-cycle fatigue experiments have been conducted in the fatigue crack growth threshold region for examining how the local microstructure influences local damage accumulation processes.
Additionally, specific monotonic and cyclic loading experiments with sub-size samples have been recently conducted for investigating the influence of the microstructure in the strain accumulation process by fatigue loading by ex-situ high-resolution Digital Image Correlation (DIC). The DIC analysis provides insightful information on the role of the intermetallic phases on the fatigue behaviour of gamma-TiAl alloys and, by allowing a comparison between different heat treatments and resulting lamellar colonies size, permits to highlight the influence of the position of grain boundaries and the orientation of the lamellae for the onset of fatigue cracking. Finally, a handful of fatigue crack growth experiments with sub-size specimens have been conducted with the aim of highlighting the behaviour of fatigue cracks in the threshold region, with special focus to the contribution of shielding mechanisms in the near threshold regime. The analysis and comparisons of different microstructures and alloy formulations is aimed at providing information for the selection of the microstructures suitable for designing against fatigue with gamma-TiAl intermetallics, in view of the application of TiAl alloys to structural components.
Short biography
Mauro Filippini is associate professor at the Department of Mechanical Engineering at Politecnico di Milano, Italy. His main interest is structural integrity of metallic materials and high temperature fatigue of materials, more recently with special interest in the fatigue properties of titanium aluminide intermetallic alloys. In the last few years, Mauro Filippini have participated to the activities of the EU-funded research projects in the field of structural durability of materials and components employed in aero-engines (MANHIRP, PreMeCCy, E-BREAK, H2020 Clean Sky NEWTEAM). Since 2016, Mauro Filippini has been successfully managing international exchange programs for Mechanical Engineering students of Politecnico di Milano, promoting the exchange of talents at international level through challenging study programs and international networks (Alliance4Tech, double degree programs, student mobility in top ranking universities).