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Article

Effect of Electric Pulse Treatment on Microstructure and Mechanical Property of Laser Powder Bed Fused IN718

by
Hongmei Zhang
1,*,
Jie Liu
2,* and
Zhanfeng Wang
2,*
1
School of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, China
2
School of Mechanical and Electrical Engineering, Suqian University, Suqian 223800, China
*
Authors to whom correspondence should be addressed.
Metals 2024, 14(7), 751; https://doi.org/10.3390/met14070751
Submission received: 29 May 2024 / Revised: 16 June 2024 / Accepted: 23 June 2024 / Published: 25 June 2024
(This article belongs to the Section Additive Manufacturing)
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Versions Notes

Abstract

This study investigated the impact of electric pulse treatment (EPT) on the microstructure and mechanical properties of laser powder bed fused Inconel 718 (IN718). Through a comprehensive experimental characterization, we found that EPT induced significant improvements in the microstructure of IN718. In the YOZ plane of EPT-700, the molten pool diminished and replaced by a grain boundary with granular Ni3Nb precipitates, and the dislocations increased while the irregular porosity decreased. Concurrently, enhanced mechanical properties of EPT-700 were obtained, including a hardness of 354.7 HV, an ultimate tensile strength of 930.21 MPa, and an elongation of 34.35%. Fractographic analysis revealed a transition in fracture mechanisms, highlighting the intricate relationship between microstructural modifications induced by EPT and mechanical response under load. These findings underscore the potential of EPT as a promising post-processing technique for optimizing the microstructure and mechanical properties of IN718 components fabricated via laser powder bed fusion additive manufacturing. This study contributes to the advancement of knowledge in the field of additive manufacturing and provides valuable insights for the development of high-performance metallic components.

1. Introduction

Inconel 718, a nickel–chromium–iron-based superalloy, has found widespread application in industries such as nuclear power, aerospace, and petrochemicals, owing to its exceptional high-temperature mechanical properties, weldability, and resistance to corrosion [1,2,3]. Its stability at temperatures below 650 °C renders it indispensable for critical structures like aircraft turbine engine compressor discs, thrust reversers, blades, and fasteners [4,5,6]. Comprising multiple components and phases, Inconel 718 primarily consists of γ, γ′, γ″, and δ phases, where the γ phase serves as the matrix, γ″ as the primary strengthening phase, and γ′ as the secondary strengthening phase [7,8,9]. However, the presence of excessive δ particles can lead to stress concentration, microcrack formation, and brittle fracture [10,11,12]. Current challenges include the difficulty of controlling the quantity and distribution of these phases within Inconel 718 [13,14].
Heat treatment represents a critical step in controlling the microstructure and phases of Inconel 718, thereby influencing its mechanical properties. For instance, Qin et al. reported a decrease in the mechanical strength of Inconel 718 with decreasing γ″ phase size [15]. Among various heat treatment techniques, the application of electric current through conductive materials emerges as an efficient and straightforward method [16,17,18]. Pan et al. demonstrated grain refinement and improved mechanical properties in Al-Mg-Si alloys through pulse electric current treatment [19]. Similarly, Zhang et al. observed the promotion of second-phase precipitation in nickel-based alloys during tensile tests under pulse electric current [20]. However, a comprehensive understanding of the influence of a high electric current on the microstructure and phase evolution of Inconel 718 remains elusive [21].
Additive manufacturing (AM) techniques, such as laser powder bed fusion (LPBF), offer rapid and flexible fabrication of complex metal components. LPBF, a widely adopted AM technology, selectively melts metal powder layers to produce near-net-shaped parts [22]. However, LPBF parts typically exhibit non-equilibrium microstructures characterized by non-uniform grain growth, strong texture, residual stresses, and chemical segregation [23,24,25]. Post-processing techniques, such as heat treatment, are commonly employed to alleviate these effects [26,27]. Nevertheless, the geometric deformation associated with additional heat treatment during high-temperature exposure renders it impractical. Thus, the exploration of alternative post-processing methods, such as electric pulse treatment, to rapidly alter the microstructure of LPBF parts becomes imperative.
Electric pulse treatment (EPT) involves the passage of electric current through a material and has been applied in various metalworking processes since Troitskii’s pioneering work in the 1960s [28,29]. This technique has been shown to accelerate processes such as recrystallization, grain refinement, and phase transformation compared to conventional heat treatment methods [30,31]. However, the underlying physical mechanisms governing microstructural changes during electric pulse treatment remain poorly understood [32]. Therefore, there is a need to investigate the conditions required to achieve the desired microstructure through electric pulse treatment, particularly for materials produced using AM processes such as LPBF.
This study aims to examine the effect of high electric current on the microstructure and phase evolution of laser powder bed fused Inconel 718. Through material characterization and tensile tests, we seek to understand the changes in microstructure and mechanical properties induced by electric pulse treatment. By exploring the potential of electric pulse treatment as an alternative post-processing technique, this research contributes to the optimization of microstructural properties in LPBF-produced components.

2. Materials and Methods

Rectangular samples of IN718, produced via laser powder bed fusion (LPBF), underwent thorough examination in this study. The sample dimensions measured 70 mm × 10 mm × 5 mm, and the LPBF process parameters were meticulously set: laser power (P) at 350 W, scanning speed (V) at 1000 mm/s, and a layer thickness of 50 μm. Notably, the scanning direction was rotated by 90° between adjacent layers, ensuring comprehensive coverage. An in-house-developed electric pulse treatment (EPT) apparatus with an output voltage of 0–24 V and an output current of 0–1500 A was employed, with the temperature monitored using an infrared thermal imager to assess Joule heating during the process shown in Figure 1. The following two distinct sets of conditions were tested: (1) at 700 °C, with a voltage of 5.7 V and a maximum current of 3038 A (121.5 A/mm2 and 1250 Hz), and (2) at 960 °C, with a voltage of 7 V and a maximum current of 3731 A (149.2 A/mm2 and 1250 Hz).
Following EPT treatment, the surfaces of the Inconel 718 rods underwent sequential polishing steps, starting with mechanical polishing and followed by chemical polishing for 10 min using a solution comprising 100 mL HCl, 100 mL C2H5OH, and 5 g CuCl2. The microstructure of the polished samples was meticulously characterized using both optical microscopy (OM) and scanning electron microscopy (SEM) (JSM 5600, JEOL, Tokyo, Japan), as well as energy-dispersive X-ray spectroscopy (EDS) and electron backscatter diffraction (EBSD). Subsequently, samples with a diameter of 3 mm, extracted near the center of the electric current-treated Inconel 718 rods, underwent further preparation steps. They were mechanically ground to a thickness below 80 μm and subjected to double-jet electrochemical polishing in a 10% HClO4 ethanol solution at 0 °C and 22 V. The microstructure of these electrochemically polished samples was further analyzed using transmission electron microscopy (TEM) (JEM 2010, JEOL, Tokyo, Japan). The grain size and the distribution of grain orientation was ascertained by EBSD. Vickers hardness measurements of the central portion of the Inconel 718 rods were conducted using a WILSON VH 1102 hardness tester, applying a normal load of 0.5 kg with a dwell time of 10 s. Subsequent tensile tests were performed on the Inconel 718 rods after current treatment using a WDW-100KN universal testing machine with a strain rate of 0.5 mm/min. Finally, the fractured surface of the tested rods was meticulously analyzed using scanning electron microscopy (JSM 5600), with precise measurements of the cross-sectional area of the fractured rods carried out.

3. Results

3.1. Microstructure

The optical microscopy (OM) images showcased in Figure 2 offer a detailed insight into the microstructural evolution of Inconel 718 samples under different processing conditions, including the as-built state and subsequent EPT conducted at 700 °C and 960 °C, respectively. In the as-built configuration (Figure 2a), the microstructure exhibits conspicuous porosity and voids dispersed throughout, particularly evident at interlayer boundaries and within individual melt pools. These voids manifest as darker regions against the metallic matrix, indicative of incomplete fusion or the entrapment of gases during the LPBF process. Upon subjecting the material to EPT treatment at 700 °C (Figure 2b), a discernible reduction in porosity becomes apparent in comparison to the as-built counterpart. The diminished presence of pores, both in number and size, implies an enhanced material densification. This mitigation of porosity is related to the localized heating facilitated by electrical pulses, facilitating the expulsion of entrapped gases and impurities from the material. Similarly, under the EPT treatment at 960 °C (Figure 2c), the degree of porosity maintains a relatively low level, akin to that observed at 700 °C. The microstructure presents a clean appearance, with minimal indications of voids or defects. The consistent reduction in pore content between the 700 °C and 960 °C treatments underscore the robust efficacy of the EPT process in pore elimination, indicating a marginal sensitivity to slight variations in treatment temperature within this range.
In Figure 3, the SEM images offer a comprehensive depiction of the surface morphology and microstructural attributes of distinct samples, complemented by energy-dispersive X-ray spectroscopy (EDS) analyses. Within Figure 3a, the SEM images unveil pronounced alterations in surface morphology subsequent to EPT. Specifically, a cellular dendritic structure emerges, accompanied by elongated grains compared to the as-built state in the XOY plane. Notably, the boundaries delineating melt pools exhibit diminished clarity in YOZ plane, suggestive of a degree of microstructural homogenization induced by the EPT process. Figure 3b supplements this observation with EDS analysis at a representative matrix point, elucidating the elemental composition comprising nickel (Ni), chromium (Cr), iron (Fe), and niobium (Nb), consistent with Inconel 718′s alloy constituents.
Deeper insights into elemental distribution within the material are furnished by subsequent EDS analyses at designated microstructural points, depicted as Points 1 and 2 in Figure 3c,d, respectively. At Point 1, corresponding to the EPT-700 sample, the analysis unveils a relatively uniform dispersion of niobium-rich phases within the matrix. This underscores the propensity of the EPT process at 700 °C to facilitate niobium dissolution into the matrix, thereby engendering a more homogeneous distribution of niobium-rich phases. Conversely, at Point 2, reflective of the EPT-960 sample, EDS scrutiny reveals the presence of clustered niobium-rich phases within the matrix. This clustering phenomenon intimates the accelerated kinetics of phase separation or precipitation at higher EPT temperatures, such as 960 °C, culminating in the aggregation of niobium-rich phases within the microstructure. The observed alterations in surface morphology, grain morphology, and elemental distribution are attributed to the thermal and kinetic ramifications instigated by the EPT process. The localized heating and rapid solidification resulting from electrical pulse application during EPT influence the nucleation and growth kinetics of microstructural entities. Additionally, the thermal cycling intrinsic to the EPT regimen fosters diffusion-driven phenomena, including phase transformation and segregation, further modulating the material’s microstructural landscape.
In Figure 4, TEM imagery unveils intricate microstructural nuances inherent to the EPT-960 sample, complemented by selected area electron diffraction (SAED) patterns offering elucidation on the crystalline characteristics of discerned precipitates and matrix phases. Figure 4a,c unveil the presence of precipitates nestled within the microcosm of the EPT-960 specimen. Notably, these precipitates manifest a distinctive morphology, discerned as Ni3Nb intermetallic phases, delineated by their characteristic shape and elemental composition. Their emergence signifies the occurrence of phase separation and precipitation amid the EPT regimen at 960 °C, wherein niobium atoms segregate, amalgamating into Ni3Nb intermetallic compounds within the matrix. Figure 4b supplements this observation with a SAED pattern emanating from the precipitate zone in Figure 4a, furnishing invaluable insights into the crystallographic attributes of the Ni3Nb intermetallic phases. The diffraction profile unveils discrete diffraction spots corresponding to crystalline planes inherent to Ni3Nb, thereby corroborating the crystalline disposition of the precipitates and their specific orientation within the microstructure.
Similarly, Figure 4d showcases a SAED pattern sourced from the matrix locale depicted in Figure 4c, unraveling the coexistence of γ’ and γ″ phases ensconced within the EPT-960 sample’s matrix. These phases epitomize a coherent interrelationship with the matrix, engendering a superlattice structure congruent with the progenitor phase. The coherent emergence of γ’ and γ″ phases within the matrix alludes to the manifestation of age hardening or precipitation hardening phenomena amid the EPT regime, wherein minute precipitates fortify the material by impeding dislocation mobility, thereby augmenting the overall alloy strength. The unveiled microstructural attributes, spanning from the presence of Ni3Nb precipitates to heightened dislocation density and coherent precipitation of γ’ and γ″ phases, are ascribed to the thermomechanical milieu characterizing the EPT process at 960 °C. The amalgam of elevated temperatures and precipitous solidification rates catalyzes the inception, expansion, and dispersion of precipitates within the matrix, precipitating the observed microstructural metamorphosis. The conventional solution and double aging treatment also resulted in a high density of γ″ phases, showing barriers to the dislocation motion with high dislocation density [33]. Thus, EPT with a short processing time shows similar effect in microstructure evolution of LPBF-processed Inconel 718.
In Figure 5, inverse pole figures (IPFs) and kernel average misorientation (KAM) maps afford elucidation on the crystallographic orientation and grain morphology of as-built Inconel 718 specimens compared with those subjected to EPT at 700 °C and 960 °C. Figure 5a–c unveil conspicuous alterations in grain morphology consequential to EPT processing. In the as-built state (Figure 5a), a heterogeneous grain structure is evident, characterized by a juxtaposition of elongated and equiaxed grains. However, subsequent EPT treatment at 700 °C (Figure 5b) evinces a discernible shift towards grain equiaxialization, manifesting in a more uniform dissemination of grain orientations. This observation intimates that EPT processing at 700 °C fosters grain homogenization, engendering a more isotropic grain configuration. Conversely, as depicted in Figure 5c, corresponding to the EPT-960 sample, a substantial amplification in grain size is discerned compared with both the as-built and EPT-700 conditions. The grains exhibit an elongated morphology, indicative of grain enlargement amidst the EPT regimen at 960 °C. This phenomenon is ascribed to the elevated temperature and protracted exposure to thermal cycling, facilitating grain coalescence and Ostwald ripening mechanisms, precipitating the observed augmentation in grain dimensions.
Figure 5d–f proffer insights into the localized misorientation distribution within the microstructural domain. In the as-built configuration (Figure 5d), zones of elevated misorientation, signifying grain boundary aberrations and dislocation configurations, pervade the microstructure. Upon subjecting the material to EPT at 700 °C (Figure 5e), the KAM map discerns a diminution in the prevalence of high misorientation domains, indicative of microstructural refinement and a commensurate reduction in dislocation density. In contrast, Figure 5f, representative of the EPT-960 sample, delineates an escalation in the density of high misorientation regions compared with the EPT-700 condition. This phenomenon implies a surge in dislocation density and the manifestation of more extensive deformation structures within the microcosm, ostensibly attributable to heightened thermal energy and prolonged exposure to thermal cycling during EPT processing at 960 °C.
In Figure 6, grain boundary maps and distributions of grain size and aspect ratio offer nuanced insights into the microstructural attributes of as-built Inconel 718 specimens, juxtaposed with those subjected to EPT at 700 °C and 960 °C. Figure 6a–c conspicuously delineate disparities in grain boundary density and distribution across the specimens. In the as-built state (Figure 6a), grain boundaries manifest as irregular and haphazardly dispersed throughout the microstructure. However, subsequent EPT treatment at 700 °C (Figure 6b) evinces a palpable surge in grain boundary density, indicative of grain refinement and a more uniform dissemination of grain boundaries. This phenomenon suggests that EPT processing at 700 °C fosters the genesis of finer grains imbued with augmented grain boundary area, thus augmenting mechanical properties and fortifying resistance to deformation. Conversely, Figure 6c, representative of the EPT-960 sample, reveals a further escalation in grain boundary density compared with both the as-built and EPT-700 conditions. This upsurge is suggestive of grain fragmentation and refinement, ostensibly precipitated by heightened thermal energy and protracted exposure to thermal cycling during EPT processing at 960 °C. Moreover, the grain boundaries exhibit a more contiguous and interconnected disposition, indicative of an elevated degree of grain boundary network establishment.
In Figure 6d–f, grain size, aspect ratio, and grain boundary distributions offer corroborative insights into the microstructural characteristics of the specimens. Figure 6d portrays grain size distribution histograms, depicting a shift towards diminutive grain sizes in the EPT-700 and EPT-960 samples compared with the as-built condition, affirming the grain refinement efficacy of EPT processing. Similarly, Figure 6e portrays aspect ratio distributions, underscoring a proclivity towards more equiaxed grains in the EPT-700 and EPT-960 samples, thereby corroborating grain equiaxialization and refinement. Lastly, Figure 6f elucidates grain boundary distributions, spotlighting an augmented density of grain boundaries in the EPT-700 and EPT-960 samples compared with the as-built configuration. This observation mirrors the findings of grain boundary maps, affirming the impact of EPT processing on grain boundary density and dispersion.

3.2. Mechanical Properties

In Figure 7, a comprehensive examination of mechanical properties and fracture behavior is facilitated through microhardness measurements, engineering stress–strain curves, ultimate tensile strength (UTS), elongation data, and fracture morphology images, elucidating the impact of EPT at 700 °C and 960 °C on as-built Inconel 718 samples. The microhardness in XOY plane of all samples is higher than that in YOZ plane owing to the cellular structure (Figure 7a). The as-built condition manifests relatively low microhardness (XOY: 323.1 HV and YOZ: 312.5 HV), indicative of inherent defects and a heterogeneous microstructure. Conversely, the EPT treatment at both temperatures engenders a discernible augmentation in microhardness, where the EPT-700 shows the maximum hardness (XOY: 354.7 HV and YOZ: 338.4 HV).
The engineering stress–strain curves (Figure 7b) delineate the mechanical response under tensile loading, revealing distinct enhancements post-EPT treatment. In the as-built state, the stress–strain curve evinces comparatively modest strength and ductility, characterized by premature plastic deformation onset and limited strain hardening. However, the ultimate tensile strength (UTS) and elongation of EPT-700 improves from 880.14 MPa to 930.21 MPa, and 19.62% to 34.35%, respectively, showing a combined enhancement of strength and ductility. The strength of EPT-960 is similar to the as-built sample, while the elongation (32.13%) is also increased significantly (Figure 7c). Both UTS and elongation witness significant increments, indicative of profound material strength and ductility augmentation compared with the as-built condition. The improvement is attributed to the γ″ and granular Ni3Nb precipitates, which could produce large long-range strain fields and, thus, increase the combined mechanical and thermal work required to overcome a barrier. Besides, the enhanced low angle grain boundary of EPT samples could impede the crack propagation, ultimately improving the mechanical properties.
The fracture morphology images unveil distinctive fracture characteristics of different samples. In the as-built state, the fracture surface is rough and irregular with uneven columnar structures (Figure 7d). However, following EPT treatment at 700 °C and 960 °C, the fracture surfaces become smooth with longer and deeper dimples. Besides, some cleavage surface with corrugated structures (Figure 7d–f) is also observed, corresponding to ductile failure with higher elongation.

4. Conclusions

In conclusion, our investigation into the effect of electric pulse treatment (EPT) on the microstructure and mechanical properties of laser powder bed fused Inconel 718 (IN718) presents compelling evidence of the efficacy of this post-processing technique. Through a comprehensive analysis encompassing microscopy and mechanical testing, we have demonstrated that EPT induces significant improvements in both the microstructural characteristics and mechanical performance of IN718. Specifically, EPT leads to uniform microstructures with reduced porosity and enhanced mechanical properties, including increased hardness, tensile strength, and ductility. Moreover, the observed changes in fracture mechanisms highlight the intimate relationship between microstructural modifications induced by EPT and mechanical behavior. These findings underscore the potential of EPT as a valuable tool for enhancing the quality and performance of IN718 components fabricated via laser powder bed fusion additive manufacturing. Moving forward, further research efforts should focus on elucidating the underlying mechanisms governing the microstructural evolution during EPT and optimizing processing parameters to tailor material properties for specific applications in aerospace, automotive, and other high-performance industries. Overall, our study contributes to advancing the understanding of post-processing techniques in additive manufacturing and underscores their significance in optimizing material properties for advanced engineering applications.

Author Contributions

Conceptualization, H.Z. and Z.W.; methodology, H.Z. and J.L.; validation, J.L. and Z.W.; writing—original draft preparation, H.Z. and Z.W.; writing—review and editing, H.Z. and Z.W.; funding acquisition, H.Z., J.L. and Z.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the China Postdoctoral Science Foundation (No. 2022M711385), the Natural Science Foundation of Jiangsu Higher Education Institutions of China (No. 23KJB460033), Suqian Sci&Tech program (No. Z2021134), and Jiangsu Qinglan Project.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) The EPT treatment, (b) the maximum temperature history of LPBF-processed IN718, and (c) the dimension of the rectangle sample.
Figure 1. (a) The EPT treatment, (b) the maximum temperature history of LPBF-processed IN718, and (c) the dimension of the rectangle sample.
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Figure 2. The OM images of XOY and YOZ planes of (a) as-built, (b)EPT-700, and (c) EPT-960 samples.
Figure 2. The OM images of XOY and YOZ planes of (a) as-built, (b)EPT-700, and (c) EPT-960 samples.
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Figure 3. (a) The SEM images of XOY and YOZ planes of different samples, (b) EDS point of matrix, (c) EDS of point 1, and (d) EDS of point 2.
Figure 3. (a) The SEM images of XOY and YOZ planes of different samples, (b) EDS point of matrix, (c) EDS of point 1, and (d) EDS of point 2.
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Figure 4. (a,c) TEM images of the EPT-960, (b) SAED pattern of the precipitate in (a), (d) SAED of the matrix in (c).
Figure 4. (a,c) TEM images of the EPT-960, (b) SAED pattern of the precipitate in (a), (d) SAED of the matrix in (c).
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Figure 5. Inverse pole figures of (a) as-built, (b) EPT-700, and (c) EPT-960 samples. KAM maps of (d) as-built, (e) EPT-700, and (f) EPT-960 samples. IPF for (g) as-built, (h) EPT-700, and (i) EPT-960 samples.
Figure 5. Inverse pole figures of (a) as-built, (b) EPT-700, and (c) EPT-960 samples. KAM maps of (d) as-built, (e) EPT-700, and (f) EPT-960 samples. IPF for (g) as-built, (h) EPT-700, and (i) EPT-960 samples.
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Figure 6. Grain boundary map of (a) as-built, (b) EPT-700, and (c) EPT-960 samples. (d) Grain size distribution, (e) grain aspect ratio distribution, and (f) grain boundary distribution of different samples.
Figure 6. Grain boundary map of (a) as-built, (b) EPT-700, and (c) EPT-960 samples. (d) Grain size distribution, (e) grain aspect ratio distribution, and (f) grain boundary distribution of different samples.
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Figure 7. (a) Microhardness, (b) engineering stress–strain curves, (c) ultimate tensile strength and elongation of different samples. The fracture morphology of (d) as-built, (e) EPT-700, and (f) EPT-960 samples.
Figure 7. (a) Microhardness, (b) engineering stress–strain curves, (c) ultimate tensile strength and elongation of different samples. The fracture morphology of (d) as-built, (e) EPT-700, and (f) EPT-960 samples.
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Zhang, H.; Liu, J.; Wang, Z. Effect of Electric Pulse Treatment on Microstructure and Mechanical Property of Laser Powder Bed Fused IN718. Metals 2024, 14, 751. https://doi.org/10.3390/met14070751

AMA Style

Zhang H, Liu J, Wang Z. Effect of Electric Pulse Treatment on Microstructure and Mechanical Property of Laser Powder Bed Fused IN718. Metals. 2024; 14(7):751. https://doi.org/10.3390/met14070751

Chicago/Turabian Style

Zhang, Hongmei, Jie Liu, and Zhanfeng Wang. 2024. "Effect of Electric Pulse Treatment on Microstructure and Mechanical Property of Laser Powder Bed Fused IN718" Metals 14, no. 7: 751. https://doi.org/10.3390/met14070751

APA Style

Zhang, H., Liu, J., & Wang, Z. (2024). Effect of Electric Pulse Treatment on Microstructure and Mechanical Property of Laser Powder Bed Fused IN718. Metals, 14(7), 751. https://doi.org/10.3390/met14070751

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