PSI - Issue 52

Tomáš Vražina et al. / Procedia Structural Integrity 52 (2024) 43 – 51 Tomáš Vražina et. al/ Structural Integrity Procedia 00 ( 2022) 000 – 000

44 2

assemblies of different high-temperature facilities. However, microstructural features (defects, unstable phases and residual stresses) introduced during fabrication conditions in the LPBF process could have unfavorable consequences for component fatigue performance. (Sanchez et al. 2021). Thus, it is necessary to investigate the fatigue behavior of LPBF materials prior to their service deployment. VDM 699 Alloy XA has recently been developed to ensure high resistance against metal dusting attack which occurs at highly aggressive corrosive environments i.e. in methanol plants, iron ore reduction and synthesis gas production (White et al. 2023), (Schlereth et al. 2022). It is in these applications that temperature gradients can lead to the development of elastic-plastic deformation resulting in fatigue damage that, in combination with the aggressive environment, will significantly affect component life. The two most important stages of fatigue life are crack initiation and crack propagation. The first stage, fatigue crack initiation, usually starts with the localization of cyclic straining into slip bands that can be present even after the first cycle and later become persistent slip bands (Babinský et al. 2021). Further cyclic straining leads to the evolution of intrusion and extrusion contained into persistent slip markings and, after considerable straining, cracks are formed from these features (Polák et al. 2017). Apart from this, various kinds of manufacturing defects like gas porosity, shrinkage cavities, keyholes, inclusions, etc. act as stress concentrators and can lead to a catastrophic premature failure of the structures (Zheng, Fu, and Zeng 2022), (Chlupová et al. 2023), (Šulák et al. 2022) The second stage (fatigue crack propagation) can be described by the Laird model (Laird and Smith 1962) and its kinetics by Paris-Erdogan Law (Paris and Erdogan 1963). Evaluation of kinetics thus crack growth rate against the range of stress intensity factor is provided from tests conducted on CT specimens with help of a high frame rate camera. Data obtained this way are then visualized in so-called v-k curves. However, for alumina alloys (Shyam and Lara-Curzio 2010) or Ni-based superalloy turbine components (Shanyavskiy 2013) a striation assessment approach for estimation of crack growth rate from fracture surface can be utilized and it is even desirable in cases where component or specimen shape is irregular and the camera is not able to spot the exact location of cracking. This paper pioneers the first comparable results of fatigue lifetime assessment of alloy 699 XA fabricated by LPBF and its conventionally produced counterpart. On top of this, an ex-situ approach described by (Nedbal et al. 2008) based on striation spacing estimation from fatigue surfaces is introduced to improve the understanding of 699 XA fatigue crack growth behavior.

Nomenclature a

Crack length

AM

Additive Manufacturing Fatigue hardening exponent

b

B. D

Building direction

c Fatigue softening exponent C Paris law material constant σ ƒ –”‡•• ƒ’Ž‹–—†‡ σ ˆ Ʋ ƒ–‹‰—‡ Šƒ”†‡‹‰ ‘‡ˆˆ‹ ‹‡– ε a Strain amplitude ε ap Plastic strain amplitude ε ˆ Ʋ ƒ–‹‰—‡ •‘ˆ–‡‹‰ ‘‡ˆˆ‹ ‹‡– EBSD Electron backscattered diffraction EDS Energy dispersive spectroscopy HR Hot rolling IPF Inverse pole figure ∆ K Stress intensity factor range ´ L.D Loading direction Low cycle fatigue LPBF Laser powder bed fusion m Paris law material constant LCF

Cyclic Stress-strain curve fitting constant