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Unidirectional columnar microstructure and its effect on the enhanced creep resistance of selective electron beam melted Inconel 718
Journal Pre-proof Unidirectional columnar microstructure and its effect on the enhanced creep resistance of selective electron beam melted Inconel 718 So-Young Im, Sun-Young Jun, Ji-Won Lee, Je-Hyun Lee, Byoung-Soo Lee, Hae-Jin Lee, Hyun-Uk Hong PII:
S0925-8388(19)34566-9
DOI:
https://doi.org/10.1016/j.jallcom.2019.153320
Reference:
JALCOM 153320
To appear in:
Journal of Alloys and Compounds
Received Date: 31 July 2019 Revised Date:
5 December 2019
Accepted Date: 6 December 2019
Please cite this article as: S.-Y. Im, S.-Y. Jun, J.-W. Lee, J.-H. Lee, B.-S. Lee, H.-J. Lee, H.-U. Hong, Unidirectional columnar microstructure and its effect on the enhanced creep resistance of selective electron beam melted Inconel 718, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2019.153320. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contributions Section The authorship has been appropriately made based on their following contribution on this manuscript, and all authors have materially participated in the research. All authors have approved and respect the authorship as its present form.
Unidirectional columnar microstructure and its effect on the enhanced creep resistance of selective electron beam melted Inconel 718 So-Young Ima, Sun-Young Juna, Ji-Won Leea, Je-Hyun Leea, Byoung-Soo Leeb,*, Hae-Jin Leeb Hyun-Uk Honga,* a
Department of Materials Science and Engineering, Changwon National University 20 Changwondaehak-ro, Changwon, Gyeongnam 51140, Republic of Korea b Additive Manufacturing Process R&D Group, Korea Institute of Industrial Technology Gwahakdanji-ro 134-41, Gangneung, Gangwon-do 25440, Republic of Korea
Abstract In this study, the selective electron beam melting (SEBM) process was used to fabricate Inconel 718 superalloy, and their microstructures and creep resistances were investigated. The proper SEBM condition ensuring a relative density higher than 99.9% was found in electron beam current range of 24 to 30 mA at a scanning speed of 4,500 mm/s and a preheat temperature of 1025 ˚C, from which the SEBMed Inconel 718 exhibited an unidirectional columnar grain microstructure with a strong <100> texture along building direction. It was suggested that the local thermal condition during solidification after SEBM facilitates a continuation of epitaxial growth of crystals on layer beneath. The γ″ precipitates formed in the as-built condition regardless of focus offset, and their size increased with increasing focus offset from 1 to 12 mA. The creep rupture life of the as-built sample fabricated with focus offset of 12 mA showed 2 times longer than that of conventional forged Inconel 718 subjected to full heat treatment under creep condition of 650 ˚C/700 MPa. EBSD analysis on the crept samples indicated that strain accumulation was significant at the porosities and the high-angle grain boundaries. Additionally, the grain boundaries inclined to loading axis were more damaged during creep. The γ″ particles were occasionally sheared by matrix dislocations with Burger vector of a/2<110>, without leaving stacking faults behind. The enhanced creep resistance of SEBMed Inconel 718 superalloy can be attributed to the columnar microstructure with a strong <100> texture and the high density of dislocations resulted from strong interaction with the γ″ particles. Keywords: Ni-based superalloys; Selective electron beam melting; Microstructure; Creep *
Corresponding author. E-mail address: [email protected] (Hyun-Uk Hong), [email protected] (Byoung-Soo Lee)
1. Introduction
The aerospace and gas turbine-based power generation industries have become increasingly interested in the potential use of Additive Layer Manufacturing (ALM) methods for the beneficial production of high-temperature components. ALM techniques are powerful tools to fabricate components with a high degree of freedom of design and tailored microstructures that cannot be obtained by using the conventional cast and wrought processes. Ni-based superalloys are being widely used as the small batches of complex components in the above-mentioned industries. Over the past several years, new superalloy processing and fabrication routes have been explored by ALM utilizing laser and electron beam melting technologies [1−4]. Recently, many studies have been reported about ALMed Inconel 718 superalloy due to its high potential of application in terms of price competitiveness as well as excellent high-temperature performances. The Ni-Fe-based superalloy Inconel 718 containing 5% Nb (in weight percent) is a precipitation-hardenable alloy that was originally developed for aerospace applications in the medium temperature range of up to 650 ˚C to provide a combination of high strength and good weldability [4,5]. The alloy is strengthened by a primary strengthener, γ'' (Ni3Nb, DO22: ordered bct crystal structure), and a secondary strengthener, γ' [Ni3(Al,Ti), L12: ordered fcc crystal structure] precipitates [5,6]. Inconel 718 has a good weldability, assuring high resistance to strain-age cracking due to the sluggish precipitation of γ'' particles, which makes this alloy suitable for ALM processes [7,8]. The ALM deposition includes non-equilibrium solidification and phase transformation resulted from rapid heating/cooling, unidirectional heat flow into building plate/substrate and repeated heat dissipation. Accordingly, the unique microstructures with anisotropy are developed, which are quite different from those of wrought Inconel 718. Previous studies
have characterized the microstructure of Inconel 718 fabricated by selective laser melting (SLM) process [9−23] and selective electron beam melting (SEBM) process [24−31]. Fine columnar grain structures are generally developed in ALMed Inconel 718 due to the formation of grains aligned parallel to the high temperature gradient. Laves phase and Nbrich MC carbides are commonly observed at the interdendritic regions due to Nb microsegregation which results from rapid cooling during ALM processing. The tensile properties of ALMed Inconel 718 after appropriate heat treatments have been demonstrated to be comparable or even superior, compared to the wrought Inconel 718 [9,11,16−19,21,24,25,31]. However, the creep resistance of ALMed Inconel 718 at elevated temperatures has been rarely reported with a consideration of the processing conditions. In the present study, we aim to obtain unidirectional columnar grains, which is analogous to those by directional solidification, by manipulating SEBM process parameters promoting epitaxial growth of crystals over previous layer. Additionally, this study addressed the characterization of creep behavior of SEBMed Inconel 718 without any post heat treatment.
2. Experimental procedures
2.1. Material and SEBM process
Plasma-atomized powder of Inconel 718, which was manufactured by Advanced Powders and Coatings Inc., Canada, was used for the SEBM process. The chemical composition of the powder was 52.8Ni–19.2Cr–18.2Fe–5.01Nb–3.03Mo–1.09Ti–0.59Al–0.1Cu–0.013O–0.05C which was determined by X-ray fluorescence analysis (XRF) and combustion analysis. The
particles have a spherical shape and their diameters were in the range of 45 µm (d10) ~ 107 µm (d90). All samples were built via SEBM using an Arcam EBM-A2X device (Arcam AB, Mölndal, Sweden). The samples were constructed in a cylindrical shape with dimensions of 15 mm (diameter) × 80 mm (height). The electron-beam current was varied in the range of 2−44 mA, with an acceleration voltage of 60 kV, a scanning speed of 500−6,000 mm/s, a layer thickness of 75 µm, and a line-offset of 0.125 mm. In addition, the preheating temperature of the building plate and vacuum pressure were 1025 ˚C and 5 × 10−3 mbar, respectively. In order to control the shape of the melt pool, the focus offset varied in the range of 1−40 mA. In addition to preheating, the shape of the melt pool affects directly the thermal gradient (G) and the solidification rate (R) so that the focus offset is the critical parameter for controlling the solidification microstructure [32]. The scanning direction of the electron beam was rotated by 90˚ on each successive layer to homogenize the temperature in the building region. The details can be found elsewhere [31].
2.2. Mechanical testing and microstructure characterization
All the mechanical properties in the present study were obtained from the as-built Inconel 718 without post heat treatment. Microhardness tests were conducted at room temperature using Vickers hardness tester (MITUTOYO HM-122). The hardness of each sample was measured using 10−15 points with a 1 kgf load and 15 s dwell, from which the average value of hardness was used. Tensile tests were conducted using specimens with a gauge length of 12.6 mm and a gauge diameter of 5 mm. The tensile specimens were machined such that the loading axis was parallel to the building direction. Tensile tests were conducted at 650 ˚C with a strain rate of 1.0 × 10−3 s−1 using a mechanically-driven servoelectric dynamic testing
machine (INSTRON 8862) with a capacity of ±100 kN. A cylindrical specimen of 6 mm gauge diameter and 25 mm gauge length was employed for creep tests. The creep specimens were machined to have building direction parallel to loading axis. Constant load creep tests were conducted in the temperature/stress conditions of 650 °C/700 MPa in air. Creep strain was continuously monitored using capacitive transducers connected to extensometers clamped to the humps at the end of specimen gauge section. The measurement of creep strain was initiated prior to loading of the creep specimen. The relative densities were measured by the Archimedes method for the quantitative analysis of voids in the as-built samples. Scanning electron microscopy (SEM) was performed on a JEOL JSM-5800 microscope with a tungsten filament operating at 20 keV for microstructural observation. The samples for SEM observation were mounted and polished. They were then etched in Kalling’s No. 2 reagent (5 g CuCl2 + 100 mL HCl + 100 mL C2H5OH). The deformed microstructures after creep tests were investigated by a FEI Helios D440 field emission scanning electron microscope (FE-SEM) equipped with an electron backscattered diffraction (EBSD) analyzer. The EBSD-based crystal orientation maps were analyzed using an Orientation Imaging Microscope (OIM) equipped with an EBSD measurement system (TexSEM Laboratories Inc., TSL). Thin foils for transmission electron microscope (TEM) examination were mechanically ground down to about 70 µm in thickness using a SiC and diamond embedded polishing media. Disks of 3-mm-diameter were punched out of the thin sheets, and electropolished to perforation with a 60% ethanol, 10% perchloric acid and 30% n-butanol electrolyte solution at −25 ˚C and 20 V, using a twin-jet electropolisher (STRUERS TenuPol-5). Special foils were machined by a focused ion beam (FIB) system (FEI Helios Nanolab 650i) to to track the <100> building direction. Special care was taken to avoid the introduction of artificial dislocations or defects during FIB machining. The TEM characterization was performed on a field emission type JEOL JEM-2100F
operating at 200 keV. The quantitative measurement of γ'' particles was made via manual outlining on the TEM micrographs at various magnification and image analysis (MEDIA CYBERNETICS Image ProPlus 5.0 software).
3. Results and discussion
3.1. Macro- and micro-structures of as-SEBMed Inconel 718
Fig. 1 shows the variation of relative density of as-SEBMed Inconel 718 samples, depending on the combination of electron beam current and scanning speed. It was found that the proper SEBM condition ensuring a relative density higher than 99.9% was found in electron beam current range of 24 to 30 mA at a scanning speed of 4,500 mm/s. It is notable that the relative density did not change sensitively with electron beam current, at the scanning speed of 2,000, 3,000 and 6,000 mm/s, and their maximum value of the relative density was below 99.6%. From the above SEBM process optimization, we have selected the best combination of electron beam current of 24 mA and scanning speed of 4,500 mm/s, while the focus offset was varied in the range of 1−40 mA in order to control the microstructures of as-SEBMed Inconel 718 superalloy. The relative density of as-built samples was maintained around 99.9% until the focus offset changed from 1 to 12 mA. The relative density then decreased when the focus offset changed from 15 to 40 mA. It should be also noted that the focus offset is a key process variable that determines the surface quality of SEBMed parts. The rough surface of the SEBMed part interferes with the uniform bedding of powders in the bedding process of
the next layer and results in the formation of internal pores in the build part. In our previous study [31], the effect of focus offset on the microstructures and mechanical properties of Inconel 718 was investigated. The surface roughness was not changed significantly when the focus offset was 10 mA to 24 mA. The surface roughness was increased to two times higher when the focus-offset was higher than 28 mA. In this study, the optimal focus offset was determined as 12 mA with a consideration of the relative density, the surface smoothness and the development of unidirectional columnar microstructure addressed in the following paragraph. Fig. 2 shows the macro- and micro-structures of as-SEBMed Inconel 718 sample fabricated with the focus offset of 12 mA. As shown in Fig. 2(a), the EB scanning paths, which is expected to be seen as chessboard, cannot be observed on xy plane. Additionally, the melt pool morphology resulted from the Gauss energy distribution of electron beam, was not detectable. This observation is quite different from the results from the SLMed Inconel 718 [33]. It is interesting that an unidirectional columnar grain microstructure was developed with a strong <100> texture along building direction. EBSD-IPF map analysis revealed that the <100> columnar microstructure was observed with a consistency from bottom to top with the building direction (Fig. 2(b)). This unique microstructure is analogous to those by directional solidification, except that the SEBMed shows a finer columnar width. SEM image on xz plane is presented in Fig. 2(c), indicating a full of columnar grains by which low- and highangle grain boundaries are parallel to the building direction. The elongated direction of those cellular microstructure almost corresponds to <001>, as determined by the locally created IPFmaps. Along the columnar boundaries, where final solidification occurred, Nb-rich phases were seen as the white particles (Fig. 2(d)). The three phases could be identified through SEM-EDS analysis. Laves [(Ni,Fe,Cr)2(Mo,Ti,Si)] and MC [(Nb,Ti)(C,B)], which are known to be the final freezing phases in Inconel 718 by an eutectic reaction with γ matrix [5,6,34,35],
were observed (Fig. 2(d)). The irregular-shaped Laves formation with size larger than 10 µm, implies that the cooling rate would not be so fast. The needle-like δ-Ni3(Nb,Ti) was also found due to multiple thermal cycles during SEBM process. In order to produce the unidirectional columnar microstructure of the as-SEBMed sample with a strong <100> texture, as shown in the present study, several prerequisites can be theoretically proposed. First of all, the mode of solidification, which is governed by the ratio of temperature gradient (G) to solidification rate (R), should not be in the regime of equiaxed dendrites. Accordingly, G/R should be as much as high to make sure that the mode of solidification would be in the columnar regime and Columnar to Equiaxed Transition (CET) during solidification should not occur. Dehoff et al. [36] conducted the analytical simulation on EBM process of Inconel 718 superalloy, by using dynamic models for fusion welding compiled by Grong [37]. Their EBM processing condition was quite similar with the present study, except that they used focus offset of 3 mA and 1,000 ˚C preheat temperature. Hence, we can approximately expect that the G value may be within the same range as that Dehoff et al. [36] calculated, i.e. 1.2 × 106 ~ 1.8 × 107 K/m. Meanwhile, R can be correlated with primary dendrite arm spacing (PDAS). Tian et al. [38] correlated the PDAS as a function of solidification front velocity during solidification of SLM process for Inconel 718. They solved the multicomponent Kurz-Giovanolva and Trivedi (KGT) model [39,40] coupled with the dendrite trunk spacing model of Kurz and Fisher [41]. In the present study, the PDAS was measured to be around 10.2 ± 0.18 µm, which corresponded to be approximately R value of 5 × 10−4 m/s. Now, we have the G and R values from which the solidification mode can be predicted. Nastac et al. [42] predicted a solidification map for Inconel 718, by developing a methodological approach for estimation of the solidification parameters. According to their solidification map for Inconel 718, we can predict the solidification mode of SEBMed Inconel 718 by overlaying our estimated G and R values (Fig. 3). It is clear that the local
solidification condition during SEBM process in this study, guarantees a fully columnar structure. Secondly, the columnar grains have to epitaxially grow from the previously deposited layer along the build direction which is perpendicular to the bottom of the melt pool. Epitaxial growth from the previously deposited layers will not occur if they are insufficiently remelted. Accordingly, a continuous melt pool should be produced [43]. Discontinuous melt pool (balling) cannot induce the epitaxial growth due to multi-directional heat flow as well as insufficient remelting. The columnar grains grow from the boundary towards the center of the melt pool along location dependent directions during solidification [3]. And, the grain growth direction is parallel to the maximum heat flow direction which is normal to the solidifying surface of the melt pool. In this sense, Thijs et al. [44] suggested that eptiaxial growth occurs only if the heat flow is directed favorably toward <001> direction. From the above consideration, it can be suggested that the shape of melt pool during SEBM process is also a major factor to obtain an unidirectional columnar microstructure. As DebRoy et al. [3] addressed, the melt pool should be long and shallow so that the columnar grains grow epitaxially from the previously deposited layer, along the build direction which is perpendicular to the bottom of the melt pool (Fig. 4(a)). In contrast, the short and deep melt pool, which has an obvious curvature at its trailing edge, will produce columnar grains in the central longitudinal plane of the melt pool (Fig. 4(b)). In Figs. 4(b) and 4(c), the curves marked with I, II and III represent the melt pool boundaries at three different locations along the scanning direction. The shape of melt pool during SEBM process could be adjusted by controlling the focus offset, in addition to the scanning speed and the layer thickness. As shown in Fig. 4(c), the fine tune of the focus offset (12 mA in the present study) is believed to contribute crucially to the development of the unidirectional microstructure, by facilitating to form the long and shallow melt pool. Further study is necessary to rationalize the shape of
melt pool as a function of the focus offset, by in-situ observation of melting pool during SEBM process. Meanwhile, it should be also noted that the scan strategy [45] and the energy density [46] were reported to be the crucial factors to accomplish the development of singlecrystalline texture in the SLM process.
3.2. Precipitation of γ'' and its relation with hardness
It should be noted that the γ″ precipitates were observed in the as-built condition, regardless of focus offset. The multiple thermal cycles during SEBM process and thermal exposure due to a preheat temperature of 1025 ˚C, may provide a preferential circumstance to promote the precipitation of the γ″ particles. Fig. 5 shows one example of TEM micrographs indicating that the γ″ precipitates formed in the as-built condition (focus offset of 12 mA). With the aid of a focused ion beam (FIB) milling system, the special foils were prepared to track the <100> building direction. As shown in Fig. 5, both γ″ and γ' particles were observed. The dark-field image of the γ″ phase shows their disc-shaped morphology (Fig. 5(b)), while the γ' phase is seen as spherical shape (Fig. 5(c)). It is evident that the γ'' ellipsoidal precipitates were coincident and coherent with the γ-fcc matrix {001} and {010} planes for [200] direction (Fig. 5(a)). The size of the γ ״precipitate was quantitatively measured from the dark-field images, and the statistical size distribution of the γ ״precipitate was obtained based on the measurement of more than 15 dark-field images. Fig. 6 shows the influence of of the focus offset on the size distribution of the γ″ precipitate. Depending on the focus offset, the average size (the value in the parentheses) of the γ″ precipitate varies between 2.5 nm and 5.0 nm. It should be mentioned that the average γ″ size becomes larger with increasing focus offset from 1 mA to 30 mA. This implies that a higher focus offset provides a longer exposure time at the
temperature range of γ″ precipitation: the exposure time could be increased by a slower cooling rate, and a more repeated thermal cycles due to more overlapped heat-affected zone (HAZ). Fig. 7 shows the influence of the focus offset on the γ″ average size and the microhardness. The value in the graph corresponds to the relative density for each condition. The green background indicates the focus offset range in which the as-built samples were produced with relative density higher than 99.5%. It is clear that the microhardness is strongly related with the γ″ average size within the focus offset range (1~12 mA). As the γ″ average size increases, the microhardness increases in the high density range. This relationship between the γ″ average size and the microhardness reflects that the cutting mechanism is primarily operative. The microhardness exhibits a peak at the focus offset of 12 mA and decreases as the focus offset increases from 15 to 30 mA. In the focus offset range of 15~30 mA, the microhardness is more closely related with the relative density rather than γ″ average size. This result indicates that the mechanical properties of the as-built Inconel 718 samples can be appropriately estimated only if the relative density is higher than 99.5%.
3.3. Creep properties of the as-SEBMed samples and its deformation characteristics
Fig. 8 shows the comparison of creep curves of as-SEBMed Inconel 718 samples fabricated with different focus offsets. The creep was tested at 650 ˚C/700 MPa. For comparison, the creep curve of fully heat-treated wrought Inconel 718 superalloy was added, which was cited from [47]. The creep rupture lives of all the as-SEBMed samples were longer than that of fully heat-treated wrought one, regardless of focus offset. The sample fabricated with focus offset of 12 mA showed the best creep resistance. The sample also
showed the highest hardness value since it had the largest γ″ average size (Fig. 7). It should be noted that the creep rupture life of the as-built sample fabricated with focus offset of 12 mA showed 2 times longer than that of conventional forged Inconel 718 subjected to full heat treatment. EBSD grain boundary and Kernel Average Misorientation (KAM) maps of the crept asSEBMed samples are presented in Fig. 9. The longitudinal cross-sections were taken near fractured surface. The crept sample fabricated with focus offset of 1 mA (tr=298 h) corresponds to Figs. 9(a) and 9(b) while the one fabricated with focus offset of 12 mA (tr=418 h) corresponds to Figs. 9(c) and 9(d). In the EBSD maps, white arrow and red arrow indicate Low-Angle Grain Boundary (LAGB delineated by green line, misorientaion angle of 5~15˚) and High-Angle Grain Boundary (HAGB delineated by blue line, misorientaion angle of 15~180˚), respectively. The porosity is indicated by yellow arrow. EBSD analysis on the crept samples indicated that strain accumulation was significant at the porosities and the HAGBs, while creep damage was negligible along the LAGBs. It is evident that the sample fabricated with focus offset of 12 mA is highly resistant to creep deformation. Fig. 10 shows dislocation substructures after creep fracture. In the case of the as-SEBMed samples fabricated with focus offset of 1 mA, The γ'' particles were majorly sheared by partial dislocations accompanied by forming stacking faults (SF). The SF was isolated within the γ'' particles, and did not extend into γ matrix. The density of matrix dislocations was very low. On the other hand, in the case of the as-SEBMed samples fabricated with focus offset of 12 mA, lots of matrix dislocations were tangled and stored at the γ'' particles. The γ'' particles were occasionally sheared by matrix perfect dislocations (see the yellow arrows) with Burgers vector of a/2<110>, without leaving stacking faults behind. The high density of dislocations resulted from strong interaction with the γ'' particles, reflects better creep properties. Pröbstle et al. [20] found superior creep strength of SLMed Inconel 718, which
can be attributed to a higher volume of uniformly-dispersed fine γ'' precipitates compared to the conventional wrought alloy. In contrast, Kuo et al. [48] reported that the creep rupture lives and ductilities of SLMed Inconel 718 were lower than those of conventionally wrought material. They claimed that the high-density dislocation and the interdendritic δ-phase with incoherent interfaces were primary causes to decrease the creep life of the SLMed Inconel 718. In the present study, the enhanced creep resistance of SEBMed Inconel 718 superalloy can be attributed to the columnar microstructure with a strong <100> texture and the high density of dislocations resulted from strong interaction with the γ″ particles.
4. Conclusions
The present study aimed to obtain unidirectional columnar grains by manipulating SEBM process parameters promoting epitaxial growth of crystals over previous layer. Superior creep resistance of as-built Inconel 718 superalloy was achieved, and its mechanism was discussed in terms of the process-structure-property relationship. The following conclusions could be drawn: 1. The proper SEBM condition facilitating a continuation of epitaxial growth of crystals on layer beneath, was found. The SEBMed Inconel 718 exhibited an unidirectional columnar grain microstructure with a strong <100> texture along building direction. 2. The γ″ precipitates were observed in the as-built condition: the γ'' ellipsoidal precipitates were coincident and coherent with the γ-fcc matrix {001} and {010} planes for [200] direction. As the average γ″ size became larger with increasing focus
offset from 1 mA to 12 mA, the microhardness was increased. 3. The creep rupture life of the as-built sample fabricated with focus offset of 12 mA showed 2 times longer than that of conventional forged Inconel 718 subjected to full heat treatment under creep condition of 650 ˚C/700 MPa. 4. The columnar boundaries parallel with loading axis, were found to be highly resistant to strain accumulation during creep. Furthermore, a high density of dislocations resulted from strong interaction with the fine γ'' particles, reflects superior creep resistance.
Acknowledgment
The authors acknowledge the financial support of the Global Expert Technology Development Program grant funded by the Korean government (MOTIE, 10076876) and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP, NRF-2017R1A1A1A05000754 and NRF-2018R1A5A6075959).
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Tables and Figures captions
Fig. 1. The variation of the relative density of as-SEBMed Inconel 718 superalloy is shown in (a) with regard to the combination of electron beam current and scanning speed. Optical micrographs showing the voids in the as-built samples fabricated at a scanning speed of 4,500 mm/s and with electron beam current of (b) 10 mA, (c) 16 mA, (d) 22 mA and (e) 28 mA. Fig. 2. Macro- and micro-structures of as-SEBMed Inconel 718 fabricated with the focus offset of 12 mA: (a) 3D optical metallographic image showing the details of columnar grains, (b) 3D Inverse Pole Figure (IPF) maps showing the building direction (BD) parallel with <001> crystallographic orientation from bottom to top. SEM image on xz plane is presented in (c) showing a full of columnar grains, and the high magnification of columnar boundary was shown in (d). Fig. 3. A solidification map for Inconel 718 [42], from which a fully columnar microstructure can be predicted in the present SEBM process. Fig. 4. Schematic diagrams illustrating the growth directions of the columnar grains in the longitudinal and the transverse planes of the melt pool, with regard to beam geometry associated with focus offset: (a) epitaxial columnar growth from the long and shallow melt pool, (b) inclined grain orientation from the short and deep melt pool. In the transverse plane, blue and red arrows indicate columnar grain growth parallel with BD and inclined to BD, respectively. Dotted line corresponds to columnar grain boundary, and solid line indicates cell/dendrite growing along <100> direction. (c) Beam geometry depending on the focus offset. Fig. 5. TEM micrographs showing both γ" and γ' precipitates in the as-built sample fabricated with a focus offset of 12 mA: (a) bright-field image and dark-field images of (b) γ" precipitates and (c) γ" + γ' precipitates. Selective area diffraction pattern (SAPD) is presented
in (d) when the zone axis of the γ matrix is [100]. Fig. 6. Statistical size distribution of γ" precipitates in the as-built Inconel 718 fabricated with (a) focus offset ranged from 1 mA to 12 mA, and (b) focus offset ranged from 15 mA to 30 mA. Fig. 7. Relationship among focus offset, γ" precipitates size, microhardness and relative density. The value in the graph corresponds to the relative density for each condition. The green background indicates the focus offset range in which the as-built samples were produced with relative density higher than 99.4%. Fig. 8. Comparison of creep curves of as-SEBMed Inconel 718 samples fabricated with different focus offset. Note the creep curve of fully heat-treated wrought Inconel 718 superalloy cited from [47], and the minimum requirement of creep rupture life recommended by ASTM. Fig. 9. EBSD grain boundary and Kernel Average Misorientation (KAM) maps of the crept as-SEBMed samples: (a), (b) sample fabricated with focus offset of 1 mA (tr=298 h) and (c), (d) sample fabricated with focus offset of 12 mA (tr=418 h). Fig. 10. TEM micrographs showing deformed microstructures of the as-SEBMed samples after creep fracture: (a), (b) sample fabricated with focus offset of 1 mA (tr=298 h) and (c), (d) sample fabricated with focus offset of 12 mA (tr=418 h).
Research highlights
1. Unidirectional columnar grains were successfully obtained by adjusting EBM condition. 2. Prerequisites for epitaxial growth of crystals on layer beneath, were theoretically proposed. 3. The creep life of as-built sample showed 2 times longer than that of wrought sample. 4. Superior creep resistance is due to directional grain boundaries and arrangement of γ".
Declaration of Interest Statement The authors declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. The funding body has been added with their institution and number in acknowledgment.
S0925-8388(19)34566-9
DOI:
https://doi.org/10.1016/j.jallcom.2019.153320
Reference:
JALCOM 153320
To appear in:
Journal of Alloys and Compounds
Received Date: 31 July 2019 Revised Date:
5 December 2019
Accepted Date: 6 December 2019
Please cite this article as: S.-Y. Im, S.-Y. Jun, J.-W. Lee, J.-H. Lee, B.-S. Lee, H.-J. Lee, H.-U. Hong, Unidirectional columnar microstructure and its effect on the enhanced creep resistance of selective electron beam melted Inconel 718, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2019.153320. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contributions Section The authorship has been appropriately made based on their following contribution on this manuscript, and all authors have materially participated in the research. All authors have approved and respect the authorship as its present form.
Unidirectional columnar microstructure and its effect on the enhanced creep resistance of selective electron beam melted Inconel 718 So-Young Ima, Sun-Young Juna, Ji-Won Leea, Je-Hyun Leea, Byoung-Soo Leeb,*, Hae-Jin Leeb Hyun-Uk Honga,* a
Department of Materials Science and Engineering, Changwon National University 20 Changwondaehak-ro, Changwon, Gyeongnam 51140, Republic of Korea b Additive Manufacturing Process R&D Group, Korea Institute of Industrial Technology Gwahakdanji-ro 134-41, Gangneung, Gangwon-do 25440, Republic of Korea
Abstract In this study, the selective electron beam melting (SEBM) process was used to fabricate Inconel 718 superalloy, and their microstructures and creep resistances were investigated. The proper SEBM condition ensuring a relative density higher than 99.9% was found in electron beam current range of 24 to 30 mA at a scanning speed of 4,500 mm/s and a preheat temperature of 1025 ˚C, from which the SEBMed Inconel 718 exhibited an unidirectional columnar grain microstructure with a strong <100> texture along building direction. It was suggested that the local thermal condition during solidification after SEBM facilitates a continuation of epitaxial growth of crystals on layer beneath. The γ″ precipitates formed in the as-built condition regardless of focus offset, and their size increased with increasing focus offset from 1 to 12 mA. The creep rupture life of the as-built sample fabricated with focus offset of 12 mA showed 2 times longer than that of conventional forged Inconel 718 subjected to full heat treatment under creep condition of 650 ˚C/700 MPa. EBSD analysis on the crept samples indicated that strain accumulation was significant at the porosities and the high-angle grain boundaries. Additionally, the grain boundaries inclined to loading axis were more damaged during creep. The γ″ particles were occasionally sheared by matrix dislocations with Burger vector of a/2<110>, without leaving stacking faults behind. The enhanced creep resistance of SEBMed Inconel 718 superalloy can be attributed to the columnar microstructure with a strong <100> texture and the high density of dislocations resulted from strong interaction with the γ″ particles. Keywords: Ni-based superalloys; Selective electron beam melting; Microstructure; Creep *
Corresponding author. E-mail address: [email protected] (Hyun-Uk Hong), [email protected] (Byoung-Soo Lee)
1. Introduction
The aerospace and gas turbine-based power generation industries have become increasingly interested in the potential use of Additive Layer Manufacturing (ALM) methods for the beneficial production of high-temperature components. ALM techniques are powerful tools to fabricate components with a high degree of freedom of design and tailored microstructures that cannot be obtained by using the conventional cast and wrought processes. Ni-based superalloys are being widely used as the small batches of complex components in the above-mentioned industries. Over the past several years, new superalloy processing and fabrication routes have been explored by ALM utilizing laser and electron beam melting technologies [1−4]. Recently, many studies have been reported about ALMed Inconel 718 superalloy due to its high potential of application in terms of price competitiveness as well as excellent high-temperature performances. The Ni-Fe-based superalloy Inconel 718 containing 5% Nb (in weight percent) is a precipitation-hardenable alloy that was originally developed for aerospace applications in the medium temperature range of up to 650 ˚C to provide a combination of high strength and good weldability [4,5]. The alloy is strengthened by a primary strengthener, γ'' (Ni3Nb, DO22: ordered bct crystal structure), and a secondary strengthener, γ' [Ni3(Al,Ti), L12: ordered fcc crystal structure] precipitates [5,6]. Inconel 718 has a good weldability, assuring high resistance to strain-age cracking due to the sluggish precipitation of γ'' particles, which makes this alloy suitable for ALM processes [7,8]. The ALM deposition includes non-equilibrium solidification and phase transformation resulted from rapid heating/cooling, unidirectional heat flow into building plate/substrate and repeated heat dissipation. Accordingly, the unique microstructures with anisotropy are developed, which are quite different from those of wrought Inconel 718. Previous studies
have characterized the microstructure of Inconel 718 fabricated by selective laser melting (SLM) process [9−23] and selective electron beam melting (SEBM) process [24−31]. Fine columnar grain structures are generally developed in ALMed Inconel 718 due to the formation of grains aligned parallel to the high temperature gradient. Laves phase and Nbrich MC carbides are commonly observed at the interdendritic regions due to Nb microsegregation which results from rapid cooling during ALM processing. The tensile properties of ALMed Inconel 718 after appropriate heat treatments have been demonstrated to be comparable or even superior, compared to the wrought Inconel 718 [9,11,16−19,21,24,25,31]. However, the creep resistance of ALMed Inconel 718 at elevated temperatures has been rarely reported with a consideration of the processing conditions. In the present study, we aim to obtain unidirectional columnar grains, which is analogous to those by directional solidification, by manipulating SEBM process parameters promoting epitaxial growth of crystals over previous layer. Additionally, this study addressed the characterization of creep behavior of SEBMed Inconel 718 without any post heat treatment.
2. Experimental procedures
2.1. Material and SEBM process
Plasma-atomized powder of Inconel 718, which was manufactured by Advanced Powders and Coatings Inc., Canada, was used for the SEBM process. The chemical composition of the powder was 52.8Ni–19.2Cr–18.2Fe–5.01Nb–3.03Mo–1.09Ti–0.59Al–0.1Cu–0.013O–0.05C which was determined by X-ray fluorescence analysis (XRF) and combustion analysis. The
particles have a spherical shape and their diameters were in the range of 45 µm (d10) ~ 107 µm (d90). All samples were built via SEBM using an Arcam EBM-A2X device (Arcam AB, Mölndal, Sweden). The samples were constructed in a cylindrical shape with dimensions of 15 mm (diameter) × 80 mm (height). The electron-beam current was varied in the range of 2−44 mA, with an acceleration voltage of 60 kV, a scanning speed of 500−6,000 mm/s, a layer thickness of 75 µm, and a line-offset of 0.125 mm. In addition, the preheating temperature of the building plate and vacuum pressure were 1025 ˚C and 5 × 10−3 mbar, respectively. In order to control the shape of the melt pool, the focus offset varied in the range of 1−40 mA. In addition to preheating, the shape of the melt pool affects directly the thermal gradient (G) and the solidification rate (R) so that the focus offset is the critical parameter for controlling the solidification microstructure [32]. The scanning direction of the electron beam was rotated by 90˚ on each successive layer to homogenize the temperature in the building region. The details can be found elsewhere [31].
2.2. Mechanical testing and microstructure characterization
All the mechanical properties in the present study were obtained from the as-built Inconel 718 without post heat treatment. Microhardness tests were conducted at room temperature using Vickers hardness tester (MITUTOYO HM-122). The hardness of each sample was measured using 10−15 points with a 1 kgf load and 15 s dwell, from which the average value of hardness was used. Tensile tests were conducted using specimens with a gauge length of 12.6 mm and a gauge diameter of 5 mm. The tensile specimens were machined such that the loading axis was parallel to the building direction. Tensile tests were conducted at 650 ˚C with a strain rate of 1.0 × 10−3 s−1 using a mechanically-driven servoelectric dynamic testing
machine (INSTRON 8862) with a capacity of ±100 kN. A cylindrical specimen of 6 mm gauge diameter and 25 mm gauge length was employed for creep tests. The creep specimens were machined to have building direction parallel to loading axis. Constant load creep tests were conducted in the temperature/stress conditions of 650 °C/700 MPa in air. Creep strain was continuously monitored using capacitive transducers connected to extensometers clamped to the humps at the end of specimen gauge section. The measurement of creep strain was initiated prior to loading of the creep specimen. The relative densities were measured by the Archimedes method for the quantitative analysis of voids in the as-built samples. Scanning electron microscopy (SEM) was performed on a JEOL JSM-5800 microscope with a tungsten filament operating at 20 keV for microstructural observation. The samples for SEM observation were mounted and polished. They were then etched in Kalling’s No. 2 reagent (5 g CuCl2 + 100 mL HCl + 100 mL C2H5OH). The deformed microstructures after creep tests were investigated by a FEI Helios D440 field emission scanning electron microscope (FE-SEM) equipped with an electron backscattered diffraction (EBSD) analyzer. The EBSD-based crystal orientation maps were analyzed using an Orientation Imaging Microscope (OIM) equipped with an EBSD measurement system (TexSEM Laboratories Inc., TSL). Thin foils for transmission electron microscope (TEM) examination were mechanically ground down to about 70 µm in thickness using a SiC and diamond embedded polishing media. Disks of 3-mm-diameter were punched out of the thin sheets, and electropolished to perforation with a 60% ethanol, 10% perchloric acid and 30% n-butanol electrolyte solution at −25 ˚C and 20 V, using a twin-jet electropolisher (STRUERS TenuPol-5). Special foils were machined by a focused ion beam (FIB) system (FEI Helios Nanolab 650i) to to track the <100> building direction. Special care was taken to avoid the introduction of artificial dislocations or defects during FIB machining. The TEM characterization was performed on a field emission type JEOL JEM-2100F
operating at 200 keV. The quantitative measurement of γ'' particles was made via manual outlining on the TEM micrographs at various magnification and image analysis (MEDIA CYBERNETICS Image ProPlus 5.0 software).
3. Results and discussion
3.1. Macro- and micro-structures of as-SEBMed Inconel 718
Fig. 1 shows the variation of relative density of as-SEBMed Inconel 718 samples, depending on the combination of electron beam current and scanning speed. It was found that the proper SEBM condition ensuring a relative density higher than 99.9% was found in electron beam current range of 24 to 30 mA at a scanning speed of 4,500 mm/s. It is notable that the relative density did not change sensitively with electron beam current, at the scanning speed of 2,000, 3,000 and 6,000 mm/s, and their maximum value of the relative density was below 99.6%. From the above SEBM process optimization, we have selected the best combination of electron beam current of 24 mA and scanning speed of 4,500 mm/s, while the focus offset was varied in the range of 1−40 mA in order to control the microstructures of as-SEBMed Inconel 718 superalloy. The relative density of as-built samples was maintained around 99.9% until the focus offset changed from 1 to 12 mA. The relative density then decreased when the focus offset changed from 15 to 40 mA. It should be also noted that the focus offset is a key process variable that determines the surface quality of SEBMed parts. The rough surface of the SEBMed part interferes with the uniform bedding of powders in the bedding process of
the next layer and results in the formation of internal pores in the build part. In our previous study [31], the effect of focus offset on the microstructures and mechanical properties of Inconel 718 was investigated. The surface roughness was not changed significantly when the focus offset was 10 mA to 24 mA. The surface roughness was increased to two times higher when the focus-offset was higher than 28 mA. In this study, the optimal focus offset was determined as 12 mA with a consideration of the relative density, the surface smoothness and the development of unidirectional columnar microstructure addressed in the following paragraph. Fig. 2 shows the macro- and micro-structures of as-SEBMed Inconel 718 sample fabricated with the focus offset of 12 mA. As shown in Fig. 2(a), the EB scanning paths, which is expected to be seen as chessboard, cannot be observed on xy plane. Additionally, the melt pool morphology resulted from the Gauss energy distribution of electron beam, was not detectable. This observation is quite different from the results from the SLMed Inconel 718 [33]. It is interesting that an unidirectional columnar grain microstructure was developed with a strong <100> texture along building direction. EBSD-IPF map analysis revealed that the <100> columnar microstructure was observed with a consistency from bottom to top with the building direction (Fig. 2(b)). This unique microstructure is analogous to those by directional solidification, except that the SEBMed shows a finer columnar width. SEM image on xz plane is presented in Fig. 2(c), indicating a full of columnar grains by which low- and highangle grain boundaries are parallel to the building direction. The elongated direction of those cellular microstructure almost corresponds to <001>, as determined by the locally created IPFmaps. Along the columnar boundaries, where final solidification occurred, Nb-rich phases were seen as the white particles (Fig. 2(d)). The three phases could be identified through SEM-EDS analysis. Laves [(Ni,Fe,Cr)2(Mo,Ti,Si)] and MC [(Nb,Ti)(C,B)], which are known to be the final freezing phases in Inconel 718 by an eutectic reaction with γ matrix [5,6,34,35],
were observed (Fig. 2(d)). The irregular-shaped Laves formation with size larger than 10 µm, implies that the cooling rate would not be so fast. The needle-like δ-Ni3(Nb,Ti) was also found due to multiple thermal cycles during SEBM process. In order to produce the unidirectional columnar microstructure of the as-SEBMed sample with a strong <100> texture, as shown in the present study, several prerequisites can be theoretically proposed. First of all, the mode of solidification, which is governed by the ratio of temperature gradient (G) to solidification rate (R), should not be in the regime of equiaxed dendrites. Accordingly, G/R should be as much as high to make sure that the mode of solidification would be in the columnar regime and Columnar to Equiaxed Transition (CET) during solidification should not occur. Dehoff et al. [36] conducted the analytical simulation on EBM process of Inconel 718 superalloy, by using dynamic models for fusion welding compiled by Grong [37]. Their EBM processing condition was quite similar with the present study, except that they used focus offset of 3 mA and 1,000 ˚C preheat temperature. Hence, we can approximately expect that the G value may be within the same range as that Dehoff et al. [36] calculated, i.e. 1.2 × 106 ~ 1.8 × 107 K/m. Meanwhile, R can be correlated with primary dendrite arm spacing (PDAS). Tian et al. [38] correlated the PDAS as a function of solidification front velocity during solidification of SLM process for Inconel 718. They solved the multicomponent Kurz-Giovanolva and Trivedi (KGT) model [39,40] coupled with the dendrite trunk spacing model of Kurz and Fisher [41]. In the present study, the PDAS was measured to be around 10.2 ± 0.18 µm, which corresponded to be approximately R value of 5 × 10−4 m/s. Now, we have the G and R values from which the solidification mode can be predicted. Nastac et al. [42] predicted a solidification map for Inconel 718, by developing a methodological approach for estimation of the solidification parameters. According to their solidification map for Inconel 718, we can predict the solidification mode of SEBMed Inconel 718 by overlaying our estimated G and R values (Fig. 3). It is clear that the local
solidification condition during SEBM process in this study, guarantees a fully columnar structure. Secondly, the columnar grains have to epitaxially grow from the previously deposited layer along the build direction which is perpendicular to the bottom of the melt pool. Epitaxial growth from the previously deposited layers will not occur if they are insufficiently remelted. Accordingly, a continuous melt pool should be produced [43]. Discontinuous melt pool (balling) cannot induce the epitaxial growth due to multi-directional heat flow as well as insufficient remelting. The columnar grains grow from the boundary towards the center of the melt pool along location dependent directions during solidification [3]. And, the grain growth direction is parallel to the maximum heat flow direction which is normal to the solidifying surface of the melt pool. In this sense, Thijs et al. [44] suggested that eptiaxial growth occurs only if the heat flow is directed favorably toward <001> direction. From the above consideration, it can be suggested that the shape of melt pool during SEBM process is also a major factor to obtain an unidirectional columnar microstructure. As DebRoy et al. [3] addressed, the melt pool should be long and shallow so that the columnar grains grow epitaxially from the previously deposited layer, along the build direction which is perpendicular to the bottom of the melt pool (Fig. 4(a)). In contrast, the short and deep melt pool, which has an obvious curvature at its trailing edge, will produce columnar grains in the central longitudinal plane of the melt pool (Fig. 4(b)). In Figs. 4(b) and 4(c), the curves marked with I, II and III represent the melt pool boundaries at three different locations along the scanning direction. The shape of melt pool during SEBM process could be adjusted by controlling the focus offset, in addition to the scanning speed and the layer thickness. As shown in Fig. 4(c), the fine tune of the focus offset (12 mA in the present study) is believed to contribute crucially to the development of the unidirectional microstructure, by facilitating to form the long and shallow melt pool. Further study is necessary to rationalize the shape of
melt pool as a function of the focus offset, by in-situ observation of melting pool during SEBM process. Meanwhile, it should be also noted that the scan strategy [45] and the energy density [46] were reported to be the crucial factors to accomplish the development of singlecrystalline texture in the SLM process.
3.2. Precipitation of γ'' and its relation with hardness
It should be noted that the γ″ precipitates were observed in the as-built condition, regardless of focus offset. The multiple thermal cycles during SEBM process and thermal exposure due to a preheat temperature of 1025 ˚C, may provide a preferential circumstance to promote the precipitation of the γ″ particles. Fig. 5 shows one example of TEM micrographs indicating that the γ″ precipitates formed in the as-built condition (focus offset of 12 mA). With the aid of a focused ion beam (FIB) milling system, the special foils were prepared to track the <100> building direction. As shown in Fig. 5, both γ″ and γ' particles were observed. The dark-field image of the γ″ phase shows their disc-shaped morphology (Fig. 5(b)), while the γ' phase is seen as spherical shape (Fig. 5(c)). It is evident that the γ'' ellipsoidal precipitates were coincident and coherent with the γ-fcc matrix {001} and {010} planes for [200] direction (Fig. 5(a)). The size of the γ ״precipitate was quantitatively measured from the dark-field images, and the statistical size distribution of the γ ״precipitate was obtained based on the measurement of more than 15 dark-field images. Fig. 6 shows the influence of of the focus offset on the size distribution of the γ″ precipitate. Depending on the focus offset, the average size (the value in the parentheses) of the γ″ precipitate varies between 2.5 nm and 5.0 nm. It should be mentioned that the average γ″ size becomes larger with increasing focus offset from 1 mA to 30 mA. This implies that a higher focus offset provides a longer exposure time at the
temperature range of γ″ precipitation: the exposure time could be increased by a slower cooling rate, and a more repeated thermal cycles due to more overlapped heat-affected zone (HAZ). Fig. 7 shows the influence of the focus offset on the γ″ average size and the microhardness. The value in the graph corresponds to the relative density for each condition. The green background indicates the focus offset range in which the as-built samples were produced with relative density higher than 99.5%. It is clear that the microhardness is strongly related with the γ″ average size within the focus offset range (1~12 mA). As the γ″ average size increases, the microhardness increases in the high density range. This relationship between the γ″ average size and the microhardness reflects that the cutting mechanism is primarily operative. The microhardness exhibits a peak at the focus offset of 12 mA and decreases as the focus offset increases from 15 to 30 mA. In the focus offset range of 15~30 mA, the microhardness is more closely related with the relative density rather than γ″ average size. This result indicates that the mechanical properties of the as-built Inconel 718 samples can be appropriately estimated only if the relative density is higher than 99.5%.
3.3. Creep properties of the as-SEBMed samples and its deformation characteristics
Fig. 8 shows the comparison of creep curves of as-SEBMed Inconel 718 samples fabricated with different focus offsets. The creep was tested at 650 ˚C/700 MPa. For comparison, the creep curve of fully heat-treated wrought Inconel 718 superalloy was added, which was cited from [47]. The creep rupture lives of all the as-SEBMed samples were longer than that of fully heat-treated wrought one, regardless of focus offset. The sample fabricated with focus offset of 12 mA showed the best creep resistance. The sample also
showed the highest hardness value since it had the largest γ″ average size (Fig. 7). It should be noted that the creep rupture life of the as-built sample fabricated with focus offset of 12 mA showed 2 times longer than that of conventional forged Inconel 718 subjected to full heat treatment. EBSD grain boundary and Kernel Average Misorientation (KAM) maps of the crept asSEBMed samples are presented in Fig. 9. The longitudinal cross-sections were taken near fractured surface. The crept sample fabricated with focus offset of 1 mA (tr=298 h) corresponds to Figs. 9(a) and 9(b) while the one fabricated with focus offset of 12 mA (tr=418 h) corresponds to Figs. 9(c) and 9(d). In the EBSD maps, white arrow and red arrow indicate Low-Angle Grain Boundary (LAGB delineated by green line, misorientaion angle of 5~15˚) and High-Angle Grain Boundary (HAGB delineated by blue line, misorientaion angle of 15~180˚), respectively. The porosity is indicated by yellow arrow. EBSD analysis on the crept samples indicated that strain accumulation was significant at the porosities and the HAGBs, while creep damage was negligible along the LAGBs. It is evident that the sample fabricated with focus offset of 12 mA is highly resistant to creep deformation. Fig. 10 shows dislocation substructures after creep fracture. In the case of the as-SEBMed samples fabricated with focus offset of 1 mA, The γ'' particles were majorly sheared by partial dislocations accompanied by forming stacking faults (SF). The SF was isolated within the γ'' particles, and did not extend into γ matrix. The density of matrix dislocations was very low. On the other hand, in the case of the as-SEBMed samples fabricated with focus offset of 12 mA, lots of matrix dislocations were tangled and stored at the γ'' particles. The γ'' particles were occasionally sheared by matrix perfect dislocations (see the yellow arrows) with Burgers vector of a/2<110>, without leaving stacking faults behind. The high density of dislocations resulted from strong interaction with the γ'' particles, reflects better creep properties. Pröbstle et al. [20] found superior creep strength of SLMed Inconel 718, which
can be attributed to a higher volume of uniformly-dispersed fine γ'' precipitates compared to the conventional wrought alloy. In contrast, Kuo et al. [48] reported that the creep rupture lives and ductilities of SLMed Inconel 718 were lower than those of conventionally wrought material. They claimed that the high-density dislocation and the interdendritic δ-phase with incoherent interfaces were primary causes to decrease the creep life of the SLMed Inconel 718. In the present study, the enhanced creep resistance of SEBMed Inconel 718 superalloy can be attributed to the columnar microstructure with a strong <100> texture and the high density of dislocations resulted from strong interaction with the γ″ particles.
4. Conclusions
The present study aimed to obtain unidirectional columnar grains by manipulating SEBM process parameters promoting epitaxial growth of crystals over previous layer. Superior creep resistance of as-built Inconel 718 superalloy was achieved, and its mechanism was discussed in terms of the process-structure-property relationship. The following conclusions could be drawn: 1. The proper SEBM condition facilitating a continuation of epitaxial growth of crystals on layer beneath, was found. The SEBMed Inconel 718 exhibited an unidirectional columnar grain microstructure with a strong <100> texture along building direction. 2. The γ″ precipitates were observed in the as-built condition: the γ'' ellipsoidal precipitates were coincident and coherent with the γ-fcc matrix {001} and {010} planes for [200] direction. As the average γ″ size became larger with increasing focus
offset from 1 mA to 12 mA, the microhardness was increased. 3. The creep rupture life of the as-built sample fabricated with focus offset of 12 mA showed 2 times longer than that of conventional forged Inconel 718 subjected to full heat treatment under creep condition of 650 ˚C/700 MPa. 4. The columnar boundaries parallel with loading axis, were found to be highly resistant to strain accumulation during creep. Furthermore, a high density of dislocations resulted from strong interaction with the fine γ'' particles, reflects superior creep resistance.
Acknowledgment
The authors acknowledge the financial support of the Global Expert Technology Development Program grant funded by the Korean government (MOTIE, 10076876) and the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP, NRF-2017R1A1A1A05000754 and NRF-2018R1A5A6075959).
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Tables and Figures captions
Fig. 1. The variation of the relative density of as-SEBMed Inconel 718 superalloy is shown in (a) with regard to the combination of electron beam current and scanning speed. Optical micrographs showing the voids in the as-built samples fabricated at a scanning speed of 4,500 mm/s and with electron beam current of (b) 10 mA, (c) 16 mA, (d) 22 mA and (e) 28 mA. Fig. 2. Macro- and micro-structures of as-SEBMed Inconel 718 fabricated with the focus offset of 12 mA: (a) 3D optical metallographic image showing the details of columnar grains, (b) 3D Inverse Pole Figure (IPF) maps showing the building direction (BD) parallel with <001> crystallographic orientation from bottom to top. SEM image on xz plane is presented in (c) showing a full of columnar grains, and the high magnification of columnar boundary was shown in (d). Fig. 3. A solidification map for Inconel 718 [42], from which a fully columnar microstructure can be predicted in the present SEBM process. Fig. 4. Schematic diagrams illustrating the growth directions of the columnar grains in the longitudinal and the transverse planes of the melt pool, with regard to beam geometry associated with focus offset: (a) epitaxial columnar growth from the long and shallow melt pool, (b) inclined grain orientation from the short and deep melt pool. In the transverse plane, blue and red arrows indicate columnar grain growth parallel with BD and inclined to BD, respectively. Dotted line corresponds to columnar grain boundary, and solid line indicates cell/dendrite growing along <100> direction. (c) Beam geometry depending on the focus offset. Fig. 5. TEM micrographs showing both γ" and γ' precipitates in the as-built sample fabricated with a focus offset of 12 mA: (a) bright-field image and dark-field images of (b) γ" precipitates and (c) γ" + γ' precipitates. Selective area diffraction pattern (SAPD) is presented
in (d) when the zone axis of the γ matrix is [100]. Fig. 6. Statistical size distribution of γ" precipitates in the as-built Inconel 718 fabricated with (a) focus offset ranged from 1 mA to 12 mA, and (b) focus offset ranged from 15 mA to 30 mA. Fig. 7. Relationship among focus offset, γ" precipitates size, microhardness and relative density. The value in the graph corresponds to the relative density for each condition. The green background indicates the focus offset range in which the as-built samples were produced with relative density higher than 99.4%. Fig. 8. Comparison of creep curves of as-SEBMed Inconel 718 samples fabricated with different focus offset. Note the creep curve of fully heat-treated wrought Inconel 718 superalloy cited from [47], and the minimum requirement of creep rupture life recommended by ASTM. Fig. 9. EBSD grain boundary and Kernel Average Misorientation (KAM) maps of the crept as-SEBMed samples: (a), (b) sample fabricated with focus offset of 1 mA (tr=298 h) and (c), (d) sample fabricated with focus offset of 12 mA (tr=418 h). Fig. 10. TEM micrographs showing deformed microstructures of the as-SEBMed samples after creep fracture: (a), (b) sample fabricated with focus offset of 1 mA (tr=298 h) and (c), (d) sample fabricated with focus offset of 12 mA (tr=418 h).
Research highlights
1. Unidirectional columnar grains were successfully obtained by adjusting EBM condition. 2. Prerequisites for epitaxial growth of crystals on layer beneath, were theoretically proposed. 3. The creep life of as-built sample showed 2 times longer than that of wrought sample. 4. Superior creep resistance is due to directional grain boundaries and arrangement of γ".
Declaration of Interest Statement The authors declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. The funding body has been added with their institution and number in acknowledgment.