In-situ observation of microstructurally small fatigue crack initiation and growth behaviors of additively-manufactured alloy 718
Introduction
Additive-Manufactured (AM) products are increasingly being utilized in a broadening range of industries. For example, the National Aeronautics and Space Administration (NASA) have been developing, and have successfully hot-fire tested, a rocket engine in which 3D-printed metallic parts are used [1]. General Electric (GE) have recently received approval from the US Air Force for additively-manufactured critical jet engine parts [2]. Knowledge of the structural reliability of AM materials, including fatigue fracture behavior, is therefore of growing importance.
Nickel-based type 718 alloy, which has superior mechanical properties and heat resistance, is a common alloy that is suitable for AM in the aerospace field. Several AM processes, such as powder bed electron beam melting (EBM), selective laser melting (SLM) and wire-fed electron beam additive manufacturing (EBAM) are currently employed to manufacture type 718 alloy.
The fatigue behavior of AM 718 alloy has been widely investigated for its suitability for components intended for long-term use. According to these previous investigations, defects [[3], [4], [5], [6], [7], [8], [9]], surface conditions [10,11] and peculiar microstructures [[12], [13], [14], [15]] can degrade its fatigue property.
Balachandramurthi et al. investigated the effect of defects and surface roughness on the fatigue life of AM 718 alloy built by EBM and SLM [4]. They reported that SLM gives a longer fatigue life than EBM, and that these fatigue lives are improved by hot isostatic pressing (HIP) which eliminates defects. Yamashita et al. [3] and Yang et al. [8] have reported the applicability of Murakami's √area model [16] to predict the effects of pores on the fatigue of 718 alloy manufactured using SLM.
As described above, most previous AM research on fatigue property has centered on the effects of defects. Although it might be possible to eliminate microporosity by HIP, it is usually difficult to control and eliminate the effects of microstructural features such as texture and grain size. Hence, understanding the microstructural effect of AM on fatigue properties is important for fatigue evaluation of AM substances. For example, Kirka et al. investigated the effect of texture on the fatigue life of AM 718 alloy that had a columnar and equiaxed microstructure built by EBM and subjected to HIP [13]. They demonstrated that the fatigue life of a columnar microstructure with a robust texture shows strong fatigue life anisotropy. However, the mechanism of these microstructural effects has not been elucidated.
Investigation of microscopic fatigue crack initiation and propagation behavior is helpful in understanding the influence of microstructures on fatigue property, since small fatigue cracks are sensitive to microstructure. However, the microscopic fatigue behavior of AM 718 alloy has not hitherto been investigated. This lack of microscopic investigation is partly due to the near-impossibility of observing small fatigue cracks. We have, however, recently developed an automatic small fatigue crack observation system [17] and applied it to several materials [[18], [19], [20]]. This panoramic microscope observation system is useful for tracking the behavior of small fatigue cracks.
In this study, to reveal the effect of microstructures on the fatigue properties of AM 718 alloy, we employed our automatic microscope system to successively observe microscopic fatigue crack initiation and growth behaviors. To prepare the different microstructures for comparison, fatigue specimens were produced using three AM methods: powder bed EBM and SLM, and wire-fed EBAM. The microstructural features were also investigated based on electron back scatter diffraction (EBSD) and fracture surface observations using a scanning electron microscope (SEM).
Section snippets
Material and specimen preparation
The material used in this study was Nickel base 718 alloy, additively manufactured using SLM, EBM, and EBAM, and commercially-prepared comparable hot-rolled plate with 25 mm thickness. The gas-atomized powders and alloy 718 wire with the chemical compositions shown in Table 1 were used for SLM, EBM and EBAM respectively. Table 2 shows the building conditions, all of which were determined to be stable by prior examination. Building conditions with two different melting states were chosen for
Microstructural characterization
Fig. 2 shows microstructure images of AM 718 alloy. The loading direction (LD) and building directions (BD) of AM materials are consistent with the vertical direction of the images. The transverse direction (TD) and normal direction (ND) of SLM and EBM are equivalent, according to the scanning path during building. The direction of deposition of EBAM was parallel to ND. The fatigue observation surface of the specimen was consistent with the LD - ND surface. The TD of the rolled plate was the
Discussion
Our experimental investigations revealed the fatigue life of EBM and EBAM specimens, which have a strong (001) texture, to be shorter than those of the rolled plate and SLM specimens (Fig. 5). Continuous crack observation revealed that fatigue cracks in the rolled-plate, SLM and EBAM-1 specimens were initiated along a slip plane (Fig. 6, Fig. 7, Fig. 8) while those in EBM and EBAM-2 were initiated from microscopic defects and hot cracks (Fig. 9, Fig. 10, Fig. 11). The FCGRs of EBAM and EBM were
Conclusions
In this study, the fatigue crack initiation and growth behavior of rolled plate, SLM, EBM and EBAM of 718 alloy were investigated to reveal the effects on fatigue property of the microstructural features characteristic of AM. It was experimentally clarified that the (001) texture decreases fatigue life by accelerating early-stage FCGR. Notable experimental results are represented as follows.
- 1)
The fatigue strength and life of the EBM and EBAM specimens, which have a strong (001) texture, showed a
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time due to technical or time limitations.
CRediT authorship contribution statement
Hideaki Nishikawa: Conceptualization, Investigation, Data curation, Writing – original draft. Yoshiyuki Furuya: Writing – review & editing. Houichi Kitano: Writing – review & editing. Terumi Nakamura: Writing – review & editing. Kosuke Kuwabara: Project administration, Funding acquisition, Resources, Writing – review & editing. Yoshiharu Kanegae: Resources, Writing – review & editing. Dong-Soo Kang: Investigation, Resources. Kinya Aota: Investigation, Resources, Writing – review & editing.
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: This study was funded by Hitachi Metals, LTD.
Acknowledgement
The X-ray analysis of this study were performed by Dr. T. Hiroto of Materials Analysis Station at NIMS.
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2023, Theoretical and Applied Fracture MechanicsCitation Excerpt :Crack initiation research should be performed in order to resolve this issue. Cyclic loading affects material microstructure [13] and cause slip bands, leading to the formation of micro-cracks [14–17]. This stage is the crack initiation stage.