Introduction to Gamma Ti for EBM

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History

Titanium aluminides are titanium alloys that are based on the formation of two intermetallic phases or compounds. These alloys include, for example, the α-2 alloys that are based on the Ti3Al intermetallic phase and a variant of the Ti2AlNb phase, and the gamma titanium aluminide alloys (TiAl). The family of TiAl alloys was developed a few decades ago and it was the subject of extensive evaluation for use in aerospace applications in the 1990s [1].

Mechanical Properties and Relation to Microstructure

In general, three types of microstructures develop in TiAl alloys. These consist of fully or near lamellar (α2/γ) microstructure, the equiaxed (γ) microstructure, and the duplex microstructure in which colonies of fully α2/γ and pure γ exist. For aerospace applications, a balance must be struck of microstructures to achieve a good balance of mechanical properties; whereas the duplex microstructure typically imparts better ductility and increased performance for low cycle fatigue (LCF) applications, the fully or near lamellar microstructures result in increased creep resistance and fracture toughness. An image of the duplex microstructure is shown reproduced from [2] (Figure 1).

Figure 1. Duplex microstructure in TiAl alloy. Reproduced from [b].

Uses of the Material

When comparing versus nickel-based superalloys and high-temperature titanium alloys, three major benefits can be obtained from the use of TiAl alloys. These include 1) up to 50% higher specific elastic stiffness which helps reduce vibrations for structural components such as compressors or turbine blades, 2) higher specific tensile strengths and creep resistance, and 3) the high fire resistance of γTiAl alloys [1].

Improved oxidation resistance of titanium aluminides arises due to an increased volume fraction of alumina (Al2O3) in the oxide layer that develops when exposed to ambient conditions. This oxide layer or scale is continuous for the gamma (γ-TiAl) alloys compared to the α-2 alloys thus providing better resistance of the former, which can make these class of materials suitable for applications requiring continuous exposure to temperatures in excess of 600°C. Upon processing, the alloys provide a low-density alternative to nickel base alloys with good strength, creep, and oxidation resistance to temperatures up to 750°C [1].

A comparison of titanium aluminides with superalloys (nickel and iron-based) and to other titanium alloys is presented in the table below, reproduced from [3]. When compared to materials such as Inconel 625, although TiAl alloys have lower ductility, and stiffness, at lower operating temperatures, their use is justified due to a substantial decrease in weight of over a half for γTiAl [4].

Table 1. Properties of titanium aluminides compared to super alloys and titanium alloys. Reproduced from [3].

The EBM Advantage

Manufacturing of TiAl through conventional processes have been used with different degrees of success. Nevertheless, the intermetallic nature of the material limits its workability, such as by milling, even under high-temperature conditions. Another issue is the lack of control of the chemical homogeneity for large ingots, requiring refinement through double or even triple melting using either plasma arc or induction melting. Micro and macrosegregation due to the solidification conditions of the alloy is also a problem in the production of TiAl ingots [4]. Although progress has been achieved in the processing of TiAl through castings, such as the introduction of Ti borides as grain refiners, insufficient control on microstructure, and residual porosity, are still issues that must be addressed [4]. Current fabrication techniques employed for processing of TiAl are not viable as they result in high scrap rates [5].

The processing of TiAl with electron beam melting (EBM) has been demonstrated recently [6]. The use of EBM technology for this high-temperature material makes perfect sense for various reasons. For instance, the ability to recycle and reuse un-melted powders without drastically modifying the chemistry of the precursor powder material is an important point in favor as it makes this processing technique more economically viable. For the aerospace industry, in particular, the technology can help to reduce development time and the times to reach market. At the same time, scrap rates are virtually eliminated with the use of EBM [5].

[1] Clemens, H., & Mayer, S. (2016). Intermetallic titanium aluminides in aerospace applications – processing, microstructure, and properties. Materials at High Temperatures, 33(4–5), 560–570. https://doi.org/10.1080/09603409.2016.1163792

[2] Bewlay, B. P., Nag, S., Suzuki, A., & Weimer, M. J. (2016). TiAl alloys in commercial aircraft engines. Materials at High Temperatures, 33(4–5), 549–559. https://doi.org/10.1080/09603409.2016.1183068

[3] Chestnutt, J. C. (1992). Titanium Aluminides for Aerospace Applications. Superalloys. The Minerals, Metals & Materials Society.

[4] Appel, F., & Oehring, M. (2005). γ-Titanium Aluminide Alloys: Alloy Design and Properties. In Titanium and Titanium Alloys (pp. 89–152). Weinheim, FRG: Wiley-VCH Verlag GmbH & Co. KGaA. https://doi.org/10.1002/3527602119.ch4

[5] Porter J., Wooten J., Harrysson O., & Knowlson K. (2011). Digital Manufacturing of Gamma-TiAl by Electron Beam Melting.

[6] Sara, B., A, P., U, A., S, S., O, T., & P, F. (2011). Electron beam melting of Ti–48Al–2Cr–2Nb alloy: Microstructure and mechanical properties investigation. Intermetallics, 19(6), 776–781. https://doi.org/10.1016/J.INTERMET.2010.11.017

 

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