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Microstructure and mechanical properties of additively manufactured IN718 after different heat treatments

Thursday (27.09.2018)
11:45 - 12:00 S1/01 - A3
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The superalloy IN718 is often used for components in aircraft turbines, due to its excellent high temperature strength. In the last few years, additive manufacturing was developed in this field of application. Due to the good weldability of the alloy IN718, components with high strength can be produced by selective laser melting (SLM). To reach a maximum of strength, different heat treatments can be carried out to adjust the microstructure. The standard heat treatment for cast and wrought material (C&W) consists of a solution heat treatment (SHT) at temperatures between 930 and 1000 °C, typically at the lower end of this range to form the δ-phase. This plate-like, niobium rich phase is preferably formed at grain boundaries, where it should prevent grain coarsening in C&W components. In a subsequent two-step ageing heat treatment, γ′ and γ′′ precipitates are formed which contribute enormously to the strength of the alloy. This work examines the effect of three different heat-treatments on the microstructure and its mechanical properties of additively manufactured samples. Contrary to C&W processing, SEM examinations showed, that there is no need for the δ-phase in additively manufactured material to stabilize the (sub)grains against coarsening. Thus, with the higher amount of dissolved niobium more γ′-and γ′′-phase can be precipitated. This was shown for a SHT at 1000 °C and an additional aging heat treatment, which exhibits higher strength compared to that with a 930 °C SHT or a direct aged state. All results are supported by TEM analysis and creep tests. In addition, atom probe tomography has shown the highest volume fraction of γ′′ for the SHT1000 state compared to three different states.

Benedikt Diepold
Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Additional Authors:
  • Dr. Steffen Neumeier
    Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
  • Prof. Mathias Göken
    Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
  • Martin Pröbstle
    Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)