1. Introduction
Alloy 718 is a precipitation-hardened nickel-based superalloy that is widely used in high-performance industries such as aerospace, power generation, oil and gas, and cryogenics. Its combination of excellent high- temperature strength (up to approximately 650 °C), corrosion resistance, fatigue resistance, and weldability has made it one of the most commercially successful superalloys to date1-5). In particular, its stable mechanical properties under demanding conditions have led to its application in critical aerospace components, including turbine blades, disks, and combustor parts6-8). The mechanical strength of Alloy 718 primarily results from the precipitation of γ′ (Ni3(Al,Ti)) and γ″ (Ni3Nb) phases during heat treatment, which significantly enhance both creep resistance and fatigue performance9-11). Chemically, the alloy consists of a Ni-based matrix containing approximately 50-55 wt.% Ni, 17-21 wt.% Cr, and a balance of Fe, along with 4.75-5.5 wt.% Nb, 2.8-3.3 wt.% Mo, and 0.65-1.15 wt.% Ti9). Elements such as Nb, Al, and Ti contribute to precipitation hardening via the formation of γ’’ and γ’ phases. Mo and Cr improve phase stability and corrosion resistance, whereas Fe and Ni constitute the base matrix, affecting both thermal and mechanical properties of the alloy.
Conventionally, Alloy 718 has been processed via casting or forging, followed by standardized heat treatment cycles defined under Aerospace Material Specifications (AMS)12). These include homogenization, solution treatment, and multi-step aging, optimized for controlling phase distribution and improving mechanical integrity. Specifically, homogenization and solution treatment are intended to dissolve or control the distribution of inter-dendritic phases such as Laves and δ, thereby promoting a more uniform γ matrix10,11). This microstructural refinement facilitates the controlled precipitation of strengthening phases γ′ (Ni3(Al,Ti)) and γ″ (Ni3Nb) during subsequent aging treatments, which are critical to achieving the desired mechanical properties in Alloy 718. However, conventional casting and forging routes often involve long lead times, high material waste, and limited design flexibility, making them less suitable for modern high-performance and customized applications. In response, the growing demand for more efficient, flexible, and cost-effective manufacturing-particularly in the aerospace and energy sectors-has led to increasing industrial interest in Additive Manufacturing (AM) as a promising alternative13-15). AM enables the layer-by-layer fabrication of complex components directly from digital models, offering design flexibility, material savings, and potential for rapid production13,15-17).
In metal AM, feedstock selection is a critical factor influencing process characteristics. Powder Bed Fusion (PBF) and laser-based Directed Energy Deposition (L-DED) typically utilize powder feedstock, while arc- based DED processes, such as Wire Arc Additive Manufacturing (WAAM), employ metal wires18,19). Although all are categorized under AM, these methods impose different thermal histories and solidification conditions compared to conventional processing. Laser- based processes such as PBF and L-DED offer high dimensional accuracy20-21), whereas arc-based WAAM is advantageous in terms of deposition rate and overall productivity22). In particular, WAAM introduces repeated thermal cycling and high heat input, leading to microstructural challenges such as elemental segregation, coarse dendritic structures, high residual stresses, and the formation of Nb-rich Laves phases23-26). These defects can severely degrade mechanical performance unless adequately controlled through post-processing.
Despite increasing industrial interest in AM 718 components, there is currently no established AMS-level heat treatment standard specifically tailored to additively manufactured products. In most cases, heat treatments originally designed for cast or wrought materials are applied without validation, or conditions are selected empirically. This lack of standardization contributes to quality variability and limits the broader adoption of AM in safety-critical applications27). To address this gap, the present study investigates the microstructural evolution and heat treatment response of WAAM-produced Alloy 718 under various homogenization conditions. By analyzing the effect of homogenization on phase dissolution and elemental redistribution, the study aims to derive insights that may contribute to the development of a dedicated post-heat treatment standard applicable to wire-fed AM components such as WAAM 718.
2. Conventional processing routes and AMS heat treatment standards
The primary processing routes for Alloy 718 have conventionally included casting, forging, and rolling. These methods are preferred due to their ability to produce components with refined microstructures, excellent mechanical properties, and well-established process control protocols. Such capabilities are particularly important in critical applications, such as aerospace and space systems, where material consistency and reliability are essential under extreme service conditions and stringent safety requirements. To ensure traceability and quality control, materials in these sectors are processed in accordance with standardized specifications established by the Society of Automotive Engineers (SAE), most notably the Aerospace Material Specifications (AMS). Each AMS code consists of a four-digit number assigned according to an internal classification system. Although the digits themselves do not directly indicate material class or product form, each AMS document defines a specific material composition, product type, heat treatment schedule, and application scope. Table 1 summarizes representative AMS standards applied to Alloy 718, categorized by material form, including castings, bars/forgings/rings, seamless tubes, and sheet/strip/plate products. For each category, the corresponding AMS code and its associated heat treatment procedures-such as solution annealing temperatures, aging steps, and cooling methods-are also listed. These heat treatment conditions are specifically defined for each product form to optimize microstructure and mechanical performance in accordance with aerospace application requirements. Among the AMS standards applied to Alloy 718, AMS 5383 and AMS 5662 represent the primary heat treatment specifications for cast and forged forms, respectively. These specifications are designed in consideration of the distinct microstructural features associated with each manufacturing route. Fig. 1 serves as a representative example, presenting SEM micrographs that compare the microstructures of Alloy 718 under three different processing conditions: (a) as-cast, (b) as-forged, and (c) solution heat treated at 1005 °C for 3 hours. In the as-cast condition, a dendritic microstructure with significant elemental segregation is observed, along with the formation of Nb-rich Laves phases, MX-type carbides, and δ-phase precipitates along grain boundaries. The as-forged specimen exhibits a refined and recrystallized grain structure with a noticeable reduction in Laves phase content, although some MX-type carbides remain. Following solution heat treatment, partial dissolution of the Laves phase occurs and the microstructure becomes more homogenized, with δ- phase and carbides showing decreased size and more uniform distribution. These microstructural transformations are expected to influence the mechanical properties of the alloy, particularly through their impact on the precipitation behavior of the γ′ and γ″ phases.
Table 1
AMS Alloy 718 - heat treatment standard condition
Fig. 1
Microstructural comparison of Alloy 718 in different processing states, (a) As-cast, (b) As-forged, and (c) after solution heat treatment at 1005 °C for 3 hours
AMS 538328), which governs investment-cast forms of Alloy 718, a specific subset within casting processes, addresses the challenges associated with the slow solidification rates characteristic of this method. These include elemental segregation, coarse dendritic structures, and the formation of Nb-rich Laves phases. To mitigate such issues, AMS 5383 prescribes a three-step heat treatment sequence consisting of homogenization, solution treatment, and double aging. Homogenization is conducted at 1093 °C for 1-2 hours followed by air cooling, enabling elemental diffusion, Laves phase dissolution, and redistribution of alloying elements. This is followed by solution treatment at 954-982 °C for 1 hour, designed to stabilize the γ matrix and suppress δ phase formation. Aging is carried out in two steps: 8 hours at 718 °C, furnace cooling to 621 °C, and holding for a total aging time not exceeding 18 hours, followed by air cooling. This sequence facilitates refined precipitation of γ′ and γ″ phases, enhancing tensile strength, creep resistance, and fatigue performance while restoring structural integrity to the cast material.
Specifically, Hyzak and Pickens reported that AMS 5383 heat treatment applied to cast Alloy 718 effectively reduced the Laves phase and promoted δ-phase precipitation along grain boundaries, refining the grain structure to ASTM 6.5 and resulting in improved mechanical properties with a yield strength of 1018 MPa, ultimate tensile strength of 1201 MPa, and 13% elongation29). In contrast, Zhang et al. found that when AMS 5383 was applied to conventionally cast Alloy 718, the microstructure remained coarse and dendritic, with residual irregular Laves phase, acicular δ precipitates, and grain boundary carbides present in the inter-dendritic zones30).
In contrast, AMS 566231), which applies to forgings, bars, and rings, specifies a simplified thermal cycle tailored to the relatively uniform microstructure of wrought products. Solution treatment is performed at 941-1010 °C, with holding time adjusted based on cross-sectional thickness, followed by air cooling. This step aims to dissolve residual δ phase and homogenize the matrix. The subsequent double aging consists of 8 hours at 718-760 °C, furnace cooling to 621-649 °C, and holding for an additional 8 hours before final air cooling. This sequence supports controlled sequential precipitation of γ″ and γ′ phases, achieving an optimal balance of strength and durability.
For wrought Alloy 718, homogenization at 1066 °C for 1 hour leads to the dissolution of most secondary phases except NbC, while furnace cooling results in the re-precipitation of strengthening phases (γ′, γ″), increasing hardness; in contrast, iced brine quenching suppresses precipitation, decreases hardness, and promotes NbC segregation at grain boundaries32). Also according to Schirra et al.33), wrought Inconel 718 heat- treated under AMS 5662 conditions with fast cooling (Hard Cycle) exhibits a yield strength of 1170 MPa, ultimate tensile strength of 1340 MPa, elongation of 17.5%, and hardness of HRC 43.4, whereas slow cooling (Soft Cycle) reduces the yield strength to 1080 MPa and hardness to HRC 41.2.
The differences in these heat treatment strategies reflect the microstructural demands imposed by each processing method. While cast products require prior homogenization to correct segregation and remove casting-induced defects, forged products benefit from thermo-mechanical refinement and therefore omit the homogenization step. Together, AMS 5383 and 5662 exemplify how thermal treatments are purposefully engineered to meet the distinct structural and performance requirements of Alloy 718 in conventional manufacturing routes. However, despite this level of standardization for conventionally processed Alloy 718, no equivalent specification currently exists for additively manufactured (AM) components. AM processes-such as PBF, L-DED, and WAAM-introduce vastly different solidification conditions and thermal histories, leading to unique microstructural features like severe elemental segregation, residual stresses, and Laves phase formation. These factors challenge the direct application of existing AMS standards and necessitate a re-evaluation of post-processing strategies tailored specifically to AM- produced materials.
3. Proposed Heat Treatment Strategy for Addi- tively Manufactured Alloy 718
Additively manufactured Alloy 718 exhibits significantly different thermal histories and microstructural characteristics compared to its conventionally processed counterparts. Due to rapid solidification and repeated thermal cycling, AM 718 often shows elemental segregation, coarse dendritic structures, and the formation of Nb-rich Laves phases in the inter-dendritic regions. These features severely impair mechanical properties and hinder subsequent precipitation strengthening. Therefore, homogenization and solution treatments play a critical role in alleviating segregation, dissolving detrimental phases, and enabling uniform precipitation of γ′/γ″ during aging. In particular, homogenization is essential for minimizing elemental segregation, promoting the formation of a compositionally uniform γ matrix, redistributing alloying elements, and dissolving brittle Laves phases. These microstructural modifications are key prerequisites for achieving consistent strengthening behavior and mechanical performance after aging.
In contrast to wrought and cast Alloy 718, for which AMS heat treatment standards such as AMS 5383 and 5662 are well-established, AM-fabricated Alloy 718 lacks dedicated thermal processing guidelines tailored to its unique solidification characteristics. This chapter focuses on post-heat treatment strategies for AM Alloy 718, with particular emphasis on the homogenization step, aiming to inform the development of standardized protocols applicable to wire-fed AM processes. AM processes are commonly classified by the type of feedstock used. In this context, PBF and L-DED are considered powder-fed methods, whereas WAAM represents a wire-fed approach.
In powder-based AM systems, extensive studies have proposed optimized thermal treatments to address the fine cellular microstructures and complex thermal histories. For instance, Ghaemifar and Mirzadeh34) reported near-complete Laves phase dissolution in PBF- fabricated Alloy 718 after just 120 s at 1050 °C, attributing this to reduced segregation and a relatively low activation energy (~160 kJ/mol. Zhang et al.35) further emphasized that, despite reduced elemental segregation in Selective Laser Melting(SLM), a higher Laves phase fraction can still form, and the dissolution mechanism shifts from diffusion- to interface-controlled above 1150 °C. Cao et al.36) demonstrated that the Laves phase was effectively dissolved under a heat treatment condition comprising homogenization at 1080 °C, solution treatment at 980 °C, and standard aging. Consequently, the specimen exhibited superior mechanical performance, achieving the highest tensile strength and longest creep rupture life at 650 °C among the tested conditions.
In L-DED systems, several studies have examined the dissolution behavior of Laves phases and their influence on mechanical performance. Sui et al.37) reported that a short solution treatment at 1150 °C for 5 minutes nearly eliminated the Laves phases, with the dominant dissolution mechanism shifting due to reduced Nb segregation. In a subsequent study, the same group showed that converting elongated Laves networks into fine granular particles at 1050 °C significantly enhanced high-temperature properties, achieving a stress rupture life of 65 h at 650 °C under 725 MPa-outperforming even forged counterparts38). Similarly, Costello et al.39) demonstrated that homogenization heat treatment effectively dissolved detrimental Laves phases in WAAM-fabricated Alloy 718, thereby improving tensile strength and yielding more uniform mechanical properties compared to the as-deposited condition.
While these powder-based AM studies have contributed valuable insights into heat treatment design, the thermal response and microstructural evolution of wire-fed processes such as WAAM remain less explored. WAAM, in particular, has gained industrial attention due to its high deposition rate, scalability, and material efficiency. However, the severe elemental segregation and coarse microstructures unique to WAAM call for a distinct approach to thermal processing. To address this gap, the present study investigates the effect of homogenization heat treatment on WAAM-fabricated Alloy 718. Focusing on Laves phase dissolution behavior, this work aims to provide a microstructure- driven foundation for post-processing strategies tailored to wire-based AM systems.
Although the heat treatment of laser-based AM processes such as L-DED and PBF has been extensively studied, WAAM, which employs arc plasma with significantly higher heat input, exhibits much slower cooling rates, leading to pronounced elemental segregation. While these powder-based AM studies have contributed valuable insights into heat treatment design, the thermal response and microstructural evolution of wire-fed processes such as WAAM remain less explored. In particular, WAAM has gained industrial attention due to its high deposition rate, scalability, and material efficiency. However, the severe elemental segregation and coarse microstructures unique to WAAM call for a distinct approach to thermal processing. To address this gap, the present study investigates the effect of homogenization heat treatment on WAAM-fabricated Alloy 718. Focusing on Laves phase dissolution behavior, this work aims to provide a microstructure- driven foundation for post-processing strategies tailored to wire-based AM systems.
In this study, single-pass WAAM specimens were fabricated using the GMAW-CMT process, as shown in Fig. 2(a), employing a TPS500i CMT system (Fronius, Austria) and a 6-axis robotic arm (ABB, Switzerland). Deposition was carried out using a 1.2 mm-diameter solid KW-M718 wire (KISWEL, Republic of Korea), which conforms to AWS A5.14-2011. The chemical composition of the wire feedstock was 52.5 wt.% Ni, 18.5 wt.% Cr, 18.5 wt.% Fe, 5.13 wt.% Nb, 2.95 wt.% Mo, 0.92 wt.% Ti, 0.49 wt.% Al, and 0.08 wt.% C. The process was conducted at an arc voltage of 15.8 V and a current of 200 A. The torch travel speed and wire feed rate were maintained at 5 mm/s and 5.7 m/min, respectively. Argon shielding gas was supplied at a flow rate of 15 L/min to protect the molten pool from atmospheric oxidation. During deposition, as illustrated in Fig. 2(b), a thermocouple was used to precisely monitor the inter-pass temperature, ensuring consistent thermal conditions throughout the build. Fig. 2(c) shows the fabricated thin-walled specimen, which was sectioned according to build height for subsequent heat treatment.
Fig. 2
Schematic illustration of WAAM process, (a) Experimental setup, (b) Interlayer temperature measurement positions and method, and (c) Thin-walled specimen sectioned by build height for heat treatment
Heat treatment was conducted as illustrated in Fig. 3, following only the homogenization step of the conventional Alloy 718 multi-step process. A box-type furnace was used at 1000, 1050, 1100, and 1185 °C for 12 hours under ambient air atmosphere. Metallographic preparation involved sectioning, hot mounting, grinding, polishing, and chemical etching. Microstructural characterization was carried out using field-emission SEM (Scanning Electron Microscopy; JSM-7001F, Jeol, Japan) equipped with EDS (X-Max 80, Oxford Instruments, UK) and EBSD (Electron Backscatter Diffraction; NordlysNano, Oxford Instruments, UK). EBSD scans were performed with a 70° tilt and a 200 nm step size in both x and y directions.
To determine appropriate homogenization temperatures for Laves phase dissolution, thermodynamic simulations were performed based on the equilibrium phase diagram and Scheil solidification model. Fig. 4 was calculated based on the chemical composition of the wire used in this study. Although minor differences in temperature and phase volume fraction may exist depending on the specific composition, the overall trend of the calculated phase diagram remains consistent within the typical compositional range of conventional Alloy 718 specifications. As shown in Fig. 4(a), the equilibrium diagram indicated the solvus temperature of the Laves phase to be approximately 1150 °C. Fig. 4(b) shows that niobium segregates heavily in the final stages of solidification, promoting the formation of Nb-rich Laves phase in inter-dendritic regions-consistent with the cast-like dendritic microstructure typical of WAAM-processed Alloy 718. Based on these predictions, homogenization heat treatments were conducted at 950 °C, 1100 °C, and 1170 °C for 12 hours.
Fig. 4
(a) Equilibrium phase diagram, (b) Scheil cooling curve of Alloy 718 calculated using Thermo-Calc with TCNI11 database
Fig. 5 presents the FE-SEM micrographs of the as-built and heat-treated samples, illustrating the morphological evolution of the Laves phase and matrix uniformity as a function of temperature. In the as-built condition, a dendritic structure with abundant Laves phase was observed in the inter-dendritic regions.
Fig. 5
SEM images and EDX spectrum of a Laves phase precipitate in the as-built condition of WAAM-fabricated Alloy 718
As shown in Fig. 6, at 950 °C, the Laves phase remained in the microstructure but appeared more spheroidized, likely due to interfacial energy minimization. The δ phase was also observed to precipitate preferentially around the Laves phase, which can be attributed to localized Nb enrichment. At 1100 °C, partial dissolution of the Laves phase occurred, with some residual particles and MX-type carbides observed. At 1170 °C, the Laves phase was completely dissolved, and only MX-type carbides were observed in the microstructure. In addition, no significant microstructural differences were observed among the top, middle, and bottom regions of the specimens in both the as-built and heat-treated conditions (950 °C, 1100 °C, and 1170 °C). However, it should be noted that in WAAM-processed Alloy 718, there are various heat input conditions beyond those employed in this study that could influence microstructural evolution. Therefore, further investigations are needed to validate these findings under a broader range of thermal and processing parameters.
Fig. 6
FE-SEM images showing microstructural evolution across different layer positions under heat treatment conditions ranging from 950 °C to 1170 °C for 12 hours
Complementary EBSD analysis (Fig. 7) revealed corresponding texture evolution. The as-built specimen showed a strong <001> texture aligned with the build direction. At 1100 °C, partial recrystallization occurred, with the emergence of some equiaxed grains, although the <001> orientation still remained dominant. At 1170 °C, complete recrystallization was observed, accompanied by a transformation of the texture toward a <101>-oriented grain structure and significant grain coarsening. These results collectively indicate that, for WAAM-fabricated Alloy 718, homogenization at temperatures above 1100 °C is required to fully dissolve the Laves phase and improve matrix uniformity. Furthermore, treatment at 1170 °C promotes complete recrystallization and texture transformation, leading to a more isotropic grain structure.
Fig. 7
Inverse Pole Figure (IPF) maps of WAAM-fabricated Alloy 718, (a) as-built, (b) homogenized at 950 °C for 12 h, (c) homogenized at 1100 °C for 12 h, and (d) homogenized at 1170 °C for 12 h
In Alloy 718, mechanical performance is primarily governed by precipitation strengthening from γ′ and γ″ phases, whose effectiveness is significantly influenced by grain structure, elemental segregation, and the presence of δ and Laves phases. Therefore, understanding how microstructural evolution affects mechanical response is essential for optimizing thermal processing strategies and ensuring consistent performance in additively manufactured components. In this study, Vickers hardness was measured as a quantitative indicator under three homogenization conditions. The hardness values progressively decreased with increasing treatment temperature: 220.75 HV0.3 in the as-built condition, 185.9 HV0.3 at 950 °C, 173.02 HV0.3 at 1100 °C, and 167.18 HV0.3 at 1170 °C, respectively. This trend reflects the progression of recrystallization and grain growth during heat treatment, which reduces dislocation density and internal stresses.
Overall, these findings demonstrate that optimizing homogenization heat treatment is essential for achieving a more uniform microstructure and improving mechanical properties in WAAM-processed Alloy 718. Moreover, the results highlight the need to establish dedicated heat treatment standards that are distinct from conventional AMS standards for wrought and cast forms. Such standardization will be critical for ensuring structural reliability and enabling the broader industrial adoption of WAAM components.
4. Conclusions
This study demonstrates that homogenization at 1170 °C is effective in dissolving Nb-rich Laves phases in WAAM-fabricated Alloy 718, which are otherwise persistent at lower temperatures such as 950 °C and only partially dissolved at 1100 °C. Complete dissolution at 1170 °C was accompanied by recrystallization, texture evolution from <001> to <101>, grain coarsening, and a reduction in hardness, indicating the trade-off between phase stability and mechanical performance. These findings highlight the necessity of temperature- optimized post-processing routes tailored to WAAM, distinct from powder-based additive methods. Overall, the results contribute to establishing a microstructure- informed foundation for designing WAAM-specific heat treatment protocols and emphasize the importance of further investigation under varied deposition and thermal conditions to support future standardization in wire-based additive manufacturing.
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