Effect of solutionizing temperature on the microstructural evolution during double aging of powder bed fusion-additive manufactured IN718 alloy

https://doi.org/10.1016/j.matchar.2020.110868Get rights and content

Highlights

  • Effect of Nb segregation on precipitation in PBF-AM IN718

  • Quantitative analysis of γ″ and γ′ precipitation in AM IN718 using Small angle X-ray scattering

  • A comparative SAXS study of wrought IN718 and AM processed IN718

  • 3D-ATP analysis of as-build IN718 to understand the Laves phase formation

Abstract

Quantitative microstructural analysis was carried out to determine the size and volume fraction of strengthening phases: gamma double prime (γ″) and gamma prime (γ′); in additively manufactured (AM) Inconel 718 (IN718) with an emphasis on the utilization of Nb for γ″ formation during aging treatment. Powder bed fusion AM (PBF-AM) technique was used to generate coupons using indigenously synthesized IN718 alloy powder. Microstructural studies using transmission electron microscope (TEM) and small angle x-ray scattering (SAXS) were carried out on powder bed fusion-additive manufactured (PBF-AM) coupons in as-built condition (as processed) and in three different heat-treated conditions: solutionizing at three different temperatures (980, 1030 and 1080 °C) followed by double aging treatment. As-built microstructure consists of dendritic structure accompanied with Nb segregation along the interdendritic regions resulting in the formation of a Nb rich interdendritic phases dispersed within the interdendritic Nb-rich γ matrix. The inhomogeneous Nb distribution resulted in preferential precipitation of coarse δ during solutionization at 980 °C and further coarse γ″during direct aging and aging subsequent to solutionizing at 1030 °C. As-cast structure and related elemental segregation got modified during solutionization at 1080 °C and resulted homogeneous γ″ precipitation. Though the average size of γ″precipitates remained same in all aged samples, volume fraction increased with increasing solutionization temperature. SAXS studies were carried out on PBF-AM as well as conventionally produced wrought samples with the similar heat treatment conditions and confirmed that PBF-AM built IN718 aged samples solutionized at 1030 °C and 1080 °C resulted in γ″ precipitates with average precipitate size and volume fraction similar to that of the wrought-IN718 sample solutionized at 1080 °C.

Introduction

Metal additive manufacturing (AM), a revolutionary manufacturing process capable of building structural components with near net shapes directly from alloy powders, has unfolded the possibility to create components with complex designs that were never envisaged because of certain constraints in conventional manufacturing routes [1]. AM process offers a cost effective manufacturing solution with promising advantages such as a) very low ratio of weight of raw material utilized to that of the final component, b) simple supply chain, c) low inventories, and d) shorter lead times for the components that are expensive to make via familiar subtractive routes [2]. Considering these advantages, AM route was adopted by the aerospace industry to manufacture complex designs like turbine engine components [3]. Among the nickel based aerospace alloys, AM of Inconel 718 (IN718) superalloy components has gained significant interest not only due to its AM conducive nature because of low (Al+Ti) content, but also being the most applied alloy in terms of tonnage consumed [4,5]. Manufacturing of aerospace components based on IN718 through conventional routes involves casting or wrought methods followed by solutionizing and aging which was well established in the literature. However, the effect of similar heat treatment conditions on the final microstructure and aging behaviour is not very well understood.

Inconel 718 (IN718) is a precipitation-hardenable alloy, with metastable γ″ (gamma double prime) and γ′ (gamma prime) precipitates, dispersed uniformly in a face centered cubic (FCC) γ (gamma) matrix. The γ″ phase is disc-shaped precipitate having a stoichiometry of Ni3Nb with body centered tetragonal (BCT) structure whereas the γ′ phase is spherical shaped precipitate with L12 type ordered structure [6]. In addition to these metastable precipitates, NbC and δ-phase (which is a stable phase of γ″) can also be observed in this alloy [7]. γ″ is the prime strengthening phase that imparts superior high temperature mechanical properties to IN718. Coherent interface with large lattice misfit between γ″ and γ matrix aids in restricting dislocation movement and thereby brings down creep rate during high temperature deformation [8,9]. Controlled precipitation of the δ-phase along the grain boundary further promotes in arresting grain boundary sliding during high temperature deformation thereby reducing the creep rate during tertiary creep regime [10]. The presence of NbC precipitate at grain boundary retards grain growth during exposure to high temperatures [11].

Laser based PBF-AM (Powder bed fusion-additive manufacturing) is one of the prominent additive layer manufacturing techniques which constitutes creation of a component by integrating large number of layers. Each layer in turn is created by the integration of a large number of micro-castings, which are created by scanning a laser on a dense powder layer. During the process, whole of bulk undergoes non-equilibrium solidification leading to the formation of dendritic structure [12]. During the PBF process, major alloying elements of the IN718 alloy: Nb, Ti, Cr, Mo and Fe; tend to segregate at the interdendritic regions resulting in the formation of secondary phases: NbC and the Laves phases (Ni, Fe, Cr)2(Nb, Mo, Ti) [[13], [14], [15]]. A similar phenomenon occurs even during conventional solidification, where with the increasing cooling rate the dendrite size and the interdendritic features become finer [16]. Moreover, the variation in input energy and scan speed during laser assisted additive manufacturing leads to variation in size of the interdendritic phases, primarily due to the large difference in solidification rate [17]. Direct laser deposition (DLD) technique, involving slower solidification rate compared to PBF-AM, results in larger dendrites and coarser interdendritic phases compared to that of PBF-AM technique, where higher rate of solidification is involved [14,17]. Further, the extent of Nb present in the Laves phase also depends on the solidification rate. Xiao et al., applied two laser modes with varying input energy (which directly influences the cooling rate) while building IN718 alloy using direct laser deposition (DLD) technique and observed that the low input energy suppressed Nb segregation and facilitates finer Laves phase [18]. Consolidating the aforementioned information available in literature following observations can be made: a) segregation of alloying elements leads to the formation of NbC and the Laves phase at interdendritic regions, b) the size of the interdendritic features varies depending on the process parameter, and c) partitioning of Nb varies depending on the solidification rate.

Similar to the conventionally cast IN718, PBF-AM processed alloy requires post heat treatment, involving solutionization to bring Nb into the solid solution which is essential as Nb is the prime contributing element in the formation of the γ″ phase [19]. Post heat treatment routine for conventionally processed IN718 consists of solutionization treatment at 980 °C for a wrought IN718 and homogenization treatment at 1100 °C for cast IN718, followed by a double aging treatment which involves holding the sample at 720 °C for 8 h followed by furnace cooling to 620 °C and holding at the temperature for another 8 h [20] before air cooling to room temperature. It was observed that solutionization at 980 °C results in the formation of the δ-phase in AM processed IN718 [15,21,22]. It was also reported that both direct aging after processing and aging after solution treatment results in precipitation of the γ″ and the γ′ phases [15,23]. Nonetheless, Nb concentration and its distribution in the austenite matrix (γ) depends on the solutioning condition preceding the aging treatment and the extent of elemental Nb available and its distribution within the austenite matrix affects the volume fraction of γ″ its distribution within the matrix [15,23].

It is also clear from the earlier discussion that the process parameters and the solidification rate can influence the as-build microstructure and its evolution during post AM heat treatment. Being a precipitation hardened alloy, high temperature mechanical behaviour of IN718 depends on the size, spatial distribution and number density of γ″ after aging treatment [9]. However, quantitative information about γ″ and γ′ in terms of number density in the post aged microstructure of PBF-AM IN718 is not available in literature. In addition, comparative study of the influence of heat treatment conditions on the precipitation behaviour of PBF AM processed and conventionally processed wrought IN718 alloys are not well reported. Authors of the present work carried out detailed microstructural characterization to understand the influence of the inhomogeneous distribution of Nb during PBF-AM processing of IN718 on microstructural evolution during subsequent post heat treatment. Detailed microstructural studies were carried out to understand the response of powder bed fusion additive manufactured (PBF AM) IN718 alloy to different solutionizing temperatures followed by double aging. Quantitative microstructural analysis of the γ″ and the γ′ is performed using small angle X-ray scattering (SAXS) technique to understand the effect of solutionizing temperature on precipitation. Finally, SAXS studies were carried out on heat treated wrought IN718 and compared the response to the solutionizing temperature to that of PBFAM-IN718.

Section snippets

Experimental details

Horizontal builds of the IN718 alloy with build size of 60 × 20 × 10 mm were made using SLM280 PBF machine equipped with fibre laser capable of delivering maximum power of 400 W. Inconel 718 alloy powder synthesized using inert gas atomization technique having particle size distribution of D50~ 45 μm and composition: Ni-18.65Fe-18.15Cr-5.2Nb-3.0Mo-0.35Al-0.92Ti-0.009B-0.003C-0.035Si (wt%) (determined using ICP-OES) is used as feed stock. Test coupons were built with laser power at 200 W, scan

Results

Microstructure of the as-built IN718 alloy at lower magnifications obtained using optical bright field imaging and orientation imaging microscopy (OIM) is shown in Fig. 1. Macrostructure of as-the built IN718 from the three adjacent faces of the built coupon is taken using optical microscope and arranged to show 3D perspective (Fig. 1a). The structure reveals independent melt-pools extending along the laser scan path in the X-Y plane, whereas cross-sections of these melt-pools can be observed

Discussion

Building a bulk sample using PBF-AM process consists of layer by layer addition of solidified metal, formed directly from densely spread alloy powder. The process commences by spreading a uniform dense metal/alloy powder layer of finite thickness on a substrate plate. A laser beam with spot size, usually around 100 μm, with sufficient power to melt the powder throughout the layer thickness, is rastered in a predetermined fashion. Powder gets melted upon exposure to the laser beam and the

Conclusions

Indigenously synthesized IN718 alloy powder was processed using laser assisted powder bed technique. The hierarchical microstructure developed due to the specified process parameters applied during additive manufacturing has been studied and further the effect of solutionization treatment given prior to double aging on the microstructure evolution especially in the context of Nb availability for the precipitation of γ″ precipitates, which is a prime high temperature strengthening phase was also

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are grateful to Dr. K Satya Prasad for the insightful discussions and support provided with respect to microscopy investigations. The authors also would like to acknowledge Dr. S B Chandrasekhar for providing atomised IN718 alloy powder. Authors are thankful to National Facility for Atom probe tomography (NFAPT) IIT Madras, Chennai for carrying out the atom probe measurements.

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