Very-High-Cycle Fatigue Behavior of Inconel 718 Alloy Fabricated by Selective Laser Melting at Elevated Temperature

Abstract

This study investigates the very-high-cycle fatigue (VHCF) behavior at elevated temperature (650 °C) of the Inconel 718 alloy fabricated by selective laser melting (SLM). The results are compared with those of the wrought alloy. Large columnar grain with a cellular structure in the grain interior and Laves/δ phases precipitated along the grain boundaries were exhibited in the SLM alloy, while fine equiaxed grains were present in the wrought alloy. The elevated temperature had a minor effect on the fatigue resistance in the regime below 108 cycles for the SLM alloy but significantly reduced the fatigue strength in the VHCF regime above 108 cycles. Both the SLM and wrought specimens exhibited similar fatigue resistance in the fatigue life regime of fewer than 107–108 cycles at elevated temperature, and the surface initiation mechanism was dominant in both alloys. In a VHCF regime above 107–108 cycles at elevated temperature, the wrought material exhibited slightly better fatigue resistance than the SLM alloy. All fatigue cracks are initiated from the internal defects or the microstructure discontinuities. The precipitation of Laves and δ phases is examined after fatigue tests at high temperatures, and the effect of microstructure on the formation and the propagation of the microstructural small cracks is also discussed.

The Hardness Evolution of Cast and the High-Cycle Fatigue Life Change of Wrought Ni-Base Superalloys after Additional Heat Treatment

“…The obtained fatigue life values were compared with the results of the fatigue test at an ambient temperature of about 22 °C ± 5°C, but the loading frequency, in this case, was f = 20,000 kHz. Fatigue failure and fatigue life are affected by several factors such as the formation of oxides, which form at high temperatures at the surface and affect the mechanism of fatigue crack closure by excluding phases such as BCT DO 22 Ni 3 Nb γ″-phase and orthorhombic D0 a Ni 3 Nb δ-phase, as demonstrated in Ling et al [ 64 ]; the frequency of loading and the number of cycles to fracture, as reported by Song et al [ 32 ]; or the heat treatment method, as reported by Zhao et al [ 65 ].…”

“…This load method is more complex and combines bending and tensile load, which corresponds to significantly lower values of fatigue life compared to the classic push–pull load. The obtained results are unique in that most authors [ 32 , 64 , 65 ] have performed fatigue tests either only at ambient temperature and with a low load frequency or with a different parameter of the cycle asymmetry R. The data on increased δ-phase volume fraction and its influence on fatigue life have not been studied up until now. For this reason, we considered the results of high-temperature fatigue and its comparison with the fatigue life obtained at ambient temperature to be unique, especially if they were made on materials with the same grain size and the same processing method (i.e., the effect of changing the grain size), so a different γ′/γ″ phase ratio was eliminated.…”

“…Their research showed that an alloy with a columnar structure showed a significant anisotropy of LCF properties, while samples with a parallel structure did not. High-cycle and high-frequency (HCHF) fatigue of IN 718 alloy are discussed in [ 31 , 32 ]. Zhong et al [ 31 ] describe the high-cycle fatigue (HCF) properties of the alloy IN718 in the following states: (i) solid solution state and (ii) after precipitation hardening at ambient temperature with a cycle asymmetry parameter R = −1 and a load frequency of about 130 Hz.…”

“…Their findings showed that there was a minimal difference in HCF life between the state after solution annealing and after precipitation hardening (only 6.3%). Song et al [ 32 ] described the very high cycle fatigue properties of SLM alloy IN718 at the test temperature of 650 °C, which is the critical temperature of use for the alloy IN718 due to the transformation of metastable γ″-phase to stable δ-phase, and compared the results with classical wrought alloy. They stated that the test temperature of 650 °C had an almost negligible effect on the fatigue life of the SLM alloy at the fracture number of cycles up to 10 8 , but with an increasing number of cycles, above 10 8 , the fatigue life decreased significantly.…”

Fourier Transform (FT) Analysis of the Stress as a Tool to Follow the Fatigue Behavior of Metals

“…The visual analysis of the fracture surface (fractography) shows for the six tests presented in Figures 6-8 a smooth and a rough surface, which are separated by red lines as a guide for the eye. Cracks are generated and grow slowly in the smooth surface area [33], before fast crack growth occurs when the remaining cross-section area can no longer withstand the applied deformation. Therefore, it can be concluded that crack growth occurs, which can be followed in the mechanical stress response by the I 2/1 intensity.…”