An innovative process for designing advanced turbine blades: the spark plasma sintering

Wednesday, December 07, 2016

Physicists from the Centre d'élaboration de matériaux et d'études structurales of Toulouse (CEMES-CNRS) and from the ONERA have designed a new process to produce aircraft turbine blades in a light and performing intermetallic alloy. These results open a route to introduce TiAl alloys in more constrained stages of these motors, and thus to significant performance improvement and to reduction of air pollution. These works have resulted in 2 patents, and have just be published in Metallurgical and Materials Transactions.

For aircraft motors, improvement of efficiency and sustainability as well as gains in terms of air and noise pollution require introducing light and performing materials in the various stages of the turbo-reactors. The intermetallic alloys of the TiAl family provide today the best solution thanks to their low density (half of the nickel-based superalloys which are currently employed) and to their high mechanical strength. However, their industrialization is still limited because of the difficulty to set-up a production process of these blades in a performing alloy. A group of French researchers of the CEMES and of the ONERA has demonstrated how producing turbine blades in a TiAl alloy exhibiting high mechanical properties by using the spark plasma sintering (SPS) process. This technique consists in elaborating a material by densifying a powder under pressure and in presence of a high intensity electric current.

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Mechanical properties and microstructure of the Ti-Al48-W2-B0,08 alloy densified by SPS. (a) Creep curve at 700°C/300MPa, showing a creep life exceeding 4000 h. The minimum creep rate corresponding to the slope of the curve is of 3.7 10-9 s-1. (b) Microstructure of the alloy observed by scanning electron microscopy. Note the presence of borders at the periphery of the lamellar grains.

The alloy developed of Ti-Al48-W2-B0,08 composition presents an exceptional mechanical strength at high temperature with for example a creep life of more than 4000 h at 700°C under 300 MPa (Fig. 1a). This means a gain of more than one order of magnitude of mechanical strength at high temperature with respect to the currently employed alloys. Moreover, it exhibits a room temperature ductility higher than 1.5 %, beyond the industrial specifications for the application. These exceptional results have been obtained by the mastering of the metallurgical mechanisms activated during the densification by SPS. These impressive properties result for the formation of single phased borders of controlled size and W composition, at the boundaries of the grains of lamellar microstructure (Fig. 1b). These borders are deformable and hence provide ductility, but remain resistant thanks to their limited size and their enrichment in W.

The other innovation of this work is the direct production of blades preforms by SPS (Fig. 2). The challenge was in obtaining a part presenting of complex geometry in an intrinsically fragile material. The method developed consists in giving its shape to the piece with the graphite system encapsulating the powder. The part presented in Fig. 2a has been obtained in a single step. Its microstructure is controlled, without necessity to perform additional heat treatments. The realization of this kind of part by SPS is a première in literature. This opens the route to other applications and other materials.

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Turbine blade preform in TiAl produced in a single SPS step. (a) Part realized. (b) Numerical model of the corresponding blade. (c) Thermoelectric modeling of the process.

Reference

Mechanical properties of the TiAl IRIS alloy
Thomas Voisin, Jean-Philippe Monchoux, Marc Thomas, Christophe Deshayes et Alain Couret
Mettalurgical and Materials Transactions A (2016), doi:10.1007/s11661-016-3801-3

Contact

Alain Couret - Directeur de recherches CNRS, directeur adjoint du CEMES
Jean-Philippe Monchoux, Thomas Voisin et Lise Durand, CEMES
Marc Thomas, ONERA.

 

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