Very High Cycle Fatigue Investigations on the Fatigue Strength of Additive Manufactured and Conventionally Wrought Inconel 718 at 873 K

Alexander Schmiedel,Christina Burkhardt,Horst Biermann,Sebastian Henkel,Anja Weidner

doi:10.3390/met11111682

Alexander Schmiedel, Christina Burkhardt + Show 3 more

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https://doi.org/10.3390/met11111682

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Journal: Metals	Publication Date: Oct 22, 2021
Citations: 11	License type: CC BY 4.0

Affiliation: TU Bergakademie Freiberg

Abstract

The fatigue lives of additively manufactured (AM) Inconel 718 (IN718) produced by selective electron beam melting and conventional wrought material as reference conditions were studied in the very high cycle fatigue regime under fully reversed loading (R = −1) at the elevated temperature of 873 K using an ultrasonic fatigue testing system. The fatigue lives of the AM material were significantly reduced compared to the wrought material, which is discussed in relation to the microstructure and a fractographical analysis. The additively manufactured material showed large columnar grains with a favoured orientation to the building direction and porosity, whereas the wrought material showed a fine-grained structure with no significant texture, but had Nb- and Ti-rich non-metallic inclusions. Crystallographic crack initiation as well as crack initiation from the surface or internal defects were observed for the AM and the wrought IN718, respectively.

Highlights

Ni-based superalloys such as Inconel 718 (IN718) consist of the fcc matrix phase γ, which is the solid solution of elements such as Fe, Co, or Cr in Ni
Three different types of defects were observed at the metallographic cross sections: (i) process-related shrinkage porosity (Figure 2d), (ii) gas porosity (Figure 2e), and (iii) carbides (NbC)
The SN data of the investigated IN718 will be discussed according to previous investigations on the low cycle fatigue (LCF), high cycle fatigue (HCF), and very high cycle fatigue (VHCF) range at room temperature (RT) and elevated temperatures

Summary

Introduction

Ni-based superalloys such as IN718 consist of the fcc matrix phase γ, which is the solid solution of elements such as Fe, Co, or Cr in Ni. It is well established that fatigue lives up to 109 cycles have to be studied at elevated temperatures in particular where no endurance limit exists, which is called very high cycle fatigue (VHCF). Numerous investigations on their high-temperature fatigue behaviour have been published [1,2,3,4,5,6,7,8,9,10]. The results showed a significantly lower fatigue strength of the coarse-grained IN718 compared to the fine-grained state. The aged state showed the highest fatigue strength at RT and a lower fatigue strength at elevated temperatures

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