Microstructure and Mechanical Properties of Inconel 718 Alloy Fabricated Using Wire Feeding Oscillated Double-Pulsed GTA-AM

Abstract

To address anisotropy challenges in electric arc-based additive manufacturing of Inconel 718 alloy, this study develops a novel wire feeding oscillated double-pulsed gas tungsten arc welding additive manufacturing method (DP-GTA-AM) enabling precise thermal-mass transfer control. Series of crack-free thin-walled Inconel 718 alloy parts were successfully obtained by this proposed approach, and the microstructure and mechanical properties of the parts were thoroughly studied. The results indicate that the microstructure changes from dendrites and cellular crystals in the bottom to equiaxed grains in the midsection and entirely equiaxed crystals in the top, resulting in notable grain refinement. With an average grain size of 61.76 μm and an average length of 83.31 μm of large angle grain boundaries, the density of the <001> direction reaches 19.45. The difference in tensile strength and ductility between the horizontal and the vertical directions decreases to 6.3 MPa and 0.38%, which significantly diminishes anisotropy. Fractographic analysis confirms quasi-cleavage failure with homogeneous dimple distribution, demonstrating effective anisotropy mitigation through controlled solidification dynamics.

1. Introduction

Inconel 718 (IN718) is a nickel-based superalloy strengthened by coherent phases of γ″-Ni₃Nb with a body-centered tetragonal D022 crystal structure and γ′-Ni₃ (Al, Ti) with a cubic L12 crystal structure [1,2], which has gained extensive applications in critical industries including aerospace, nuclear power, chemical engineering, and national defense due to its exceptional weldability, superior corrosion resistance, and remarkable high-temperature performance [3,4,5]. However, the inherent characteristics of IN718, particularly its high cutting temperature and significant strain hardening tendency, pose substantial challenges in conventional manufacturing processes such as casting, forging, and machining. These challenges are further exacerbated when fabricating large-scale components with complex geometries, where traditional methods often fail to meet the requirements for cost-effectiveness and production efficiency.

The emergence of additive manufacturing (AM) technologies has provided a promising solution to these manufacturing constraints, particularly in the production of aero-engine components and complex castings. Among various AM techniques [6,7,8,9], wire arc additive manufacturing (WAAM) has attracted considerable attention due to its unique advantages in fabricating large-scale, fully dense metal components with complex geometries. WAAM employs an electric arc as the heat source to melt the base metal, depositing wire material layer-by-layer according to pre-programmed paths. This method offers significant benefits, including reduced production costs and less stringent environmental requirements compared to other AM techniques. Nevertheless, the WAAM process presents specific challenges in the fabrication of IN718 components. The inherent thermal cycling and substantial heat accumulation during deposition can adversely affect the formability and microstructural characteristics of the manufactured parts. A critical issue is the inevitable grain coarsening and brittle Laves phase precipitation during the WAAM process, and the Laves phase is detrimental to the mechanical performance because it consumes a large amount of Nb to reduce the amount of γ″-Ni₃Nb phase, and its main compositions are Nb and Mo elements [10]. Moreover, the brittle Laves phase is easy to become a site and path for crack initiation and propagation during plastic deformation, leading to severe performance degradation. The relevant works indicated that these microstructural changes are primarily attributed to the complex thermal cycle experience and solidification behavior of the molten pool [11,12,13]. Consequently, controlling and optimizing the molten pool behavior has become a crucial research focus [14]. Effective regulation of processing parameters could potentially reduce the quantity and change the distribution and morphology of the Laves phase, while simultaneously refining grain size and controlling growth orientation. Such microstructural optimizations are essential for enhancing the mechanical properties of WAAM-fabricated IN718 components, making this an area of significant scientific and industrial importance.

Grain refinement in AM has been extensively studied through many advanced techniques, including ultrasonic treatment, elemental grain refinement, molten pool stirring, and quenching treatment [15,16,17,18,19]. However, these methods present significant limitations in practical industrial applications, particularly for large-scale or geometrically complex components. Ultrasonic and electromagnetic stirring processes, while effective in laboratory settings, often prove challenging to implement in industrial-scale manufacturing. Furthermore, the introduction of heterogeneous particles into the weld seam, though effective in promoting nucleation for grain refinement [20,21,22,23], introduces additional complexities. The type, size, wettability, and physical properties of these particles can unpredictably influence the microstructural characteristics of the weld seam, potentially leading to undesirable mechanical properties [24,25,26].

To address these limitations, researchers have developed alternative approaches. Liu et al. reported that the tensile properties (ultimate tensile strength and yield strength) of the deposition with arc oscillation were improved with retention in elongation above 45%, reaching 875 MPa at horizontal direction and 835 MPa at vertical direction. With the arc oscillation, the Laves phase was significantly suppressed and the content of the Laves phase decreased. Further, arc oscillation induced texture randomization with lower texture intensity (10.15 mud), achieving a prominent reduction by 36.7%. From this study, it is evident that arc oscillation during WAAM is a potential manufacturing route that can effectively tailor the microstructure [27]. Subsequent investigation by Yuan et al. [28] revealed that the transverse arc oscillation caused reheating during solidification, enhancing the dendrite fragmentation, and reduced the temperature gradient along the torch travel direction, increasing the compositional supercooling and making the dendrite fragments grow into the fine equiaxed grains, and thus the mechanical property greatly improved. Zhuo et al. [29] systematically compared three combined methods, identifying the “low-frequency pulse arc + growth restricting solutes additions” as most effective in promoting columnar to equiaxed transition. Ke et al. [30] further advanced this field by employing ultrahigh-frequency pulsed gas tungsten arc welding (UHFP-GTAW) for NiTi alloy AM, establishing a comprehensive model that correlates process parameters with microstructural refinement. Furthermore, Chen et al. reported that after heat treatment, the Laves phase was fully dissolved with well-controlled grain size and precipitates. The resulting yield strength along the building direction was increased by 85% with average vertical elongation of 27.5% and horizontal elongation of 34% [31]. Xu et al. demonstrated that the interpass rolling can result in a non-uniform recrystallization; after heat treatment, the average UTS of the rolled WAAM IN718 is 75 MPa higher than the wrought alloy (1276 MPa), and both 0.2% YS and elongation meet the wrought standard [32]. In the above-mentioned solutions, the pulsed current applied-molten pool oscillation is a potential approach for refining the grain size and controlling the solidification of the molten pool and achieving a good performance of the WAAM components.

To this end, this study introduces a novel wire feeding oscillated dual-pulsed GTAW (DP-GTAW) method, specifically designed to overcome these limitations. The DP-GTAW process innovatively separates pool oscillation from droplet transfer through a two-stage approach: (1) a high-pulsed current group for substrate melting and intensive fluid flow stirring during non-wire feeding periods, and (2) a low-pulsed current group for controlled wire melting and droplet transfer. This strategic separation enables superior grain refinement compared to conventional one-pulsed welding methods. The present work systematically studied the microstructure evolution of IN718 components additively manufactured by DP-GTAW, and the mechanism that enhanced the mechanical properties was comprehensively explored. Through comprehensive experimental analysis and theoretical interpretation, we demonstrate significant improvements in grain refinement and anisotropy reduction. The manuscript is organized as follows: Section 2 details the methodology, and Section 3 presents results and discussions, followed by concluding remarks.

2. Materials and Methods

The double-pulsed GTAW process was first applied to additively fabricate the thin-walled components with 1.2 mm diameter IN718 wire on 316 L austenitic stainless-steel substrate (100 mm × 40 mm × 8 mm) under a φ(Ar) 99.999% pure argon atmosphere. The composition of IN718 wire is shown in

Table 1. Chemical composition of Inconel 718 wire (wt.%).

C	Cr	Ni	S	Mn	Si	Nb	Mo
<0.08	16–18	50–55	≤0.015	≤0.35	≤0.35	4.75–5.50	2.80–3.30
Co	P	Mg	B	Al	Ti	Cu	Fe
≤1.00	0.015	0.01	<0.006	0.2–0.8	0.65–1.15	≤0.30	Bal.

.

The schematic diagram of DP-GTAW-AM is shown in Figure 1. At the initiation of the deposition process, during the high-frequency pulsed current period, the wire feeding was suspended while the substrate remained stationary. The high-frequency pulsed arc effectively melted the substrate, forming a molten pool of controlled dimensions through precise regulation of the pulsed parameters. The pulsed electromagnetic force generated by the high-frequency current induced intensive fluid flow within the molten pool, thereby altering the energy distribution and temperature gradient. Simultaneously, the high-frequency pulsed current maintained the molten pool oscillation at a reduced amplitude by exerting a high-frequency compressive force on the pool surface, ensuring stable boundary variations and appropriate penetration depth. Subsequently, during the low-frequency pulsed current period, the wire feeding was activated at a predetermined speed, and the pulsed arc melted the wire to form a droplet of specific size. Upon contact with the molten pool surface, the droplet was transferred into the molten pool driven by surface tension and pulsed arc force, completing the quantitative mass transfer process. Notably, the application of the low-frequency pulsed current induced more pronounced oscillations in the molten pool during solidification while contributing minimum heat input. This resulted in reduced geometric variations and a smaller temperature gradient, which effectively decreased the solidification rate at the solid–liquid interface. Consequently, grain growth was suppressed, and dendritic structures were fragmented during the later stages of solidification. As the high-low frequency pulsed current cycle proceeded, the substrate advanced incrementally, and the wire was periodically melted by the dual-pulsed current group. This process enabled layer-by-layer deposition, ultimately forming a straight wall with controlled microstructural characteristics.

The experimental setup of DP-GTAW-AM is illustrated in Figure 2a. In this experimental procedure, a LabVIEW-based real-time platform was applied to output the double-pulsed current waveform, and a TSP300 welding supply and a YI-18TH high-efficiency digital wire feeder produced by Tangshan Panasonic Industrial Machinery company (Tangshan, China) were used to provide the heat energy and fill wire step by step, respectively. The typical macroscopic defect-free morphology of the component is shown in Figure 2c, whose dimensions were about 85 mm × 20 mm × 45 mm, and the metallographic specimens were systematically extracted from the designated regions illustrated in Figure 2b employing electrical discharge wire cutting (EDWC) technology. As-deposited Inconel 718 samples were etched with aqua regia using electrolytic corrosion (6 V, 5 s, at room temperature). Optical microscope (OM Zeiss Axio Scope A1, Carl Zeiss AG, Oberkochen, Germany), scanning electron microscope (SEM QUANTA FEG-450, FEI, Hillsboro, OR, USA), energy dispersive spectrometer (EDS), and electron back-scattered diffraction (EBSD) were used for microstructure characterization, grain size, and texture analysis. The etchant for OM, SEM, and EDS included 67% muriatic acid and 33% nitric acid. The electrolyte for EBSD consisted of 10% perchloric acid and 90% anhydrous ethanol with volume. The mechanical properties specimen with given dimensions were cut from the thin-walled deposited component, as shown in Figure 2b. The microhardness measurements were carried out using an HDX1000 digital tester, in accordance with the ASTM E384–08 standard [33]. A load of 1000 g (equivalent to 0.9807 N or HV1.0) was applied, with a dwell time of 15 s and a testing spacing of 0.5 mm between adjacent points. Tensile tests were performed following the ASTM E8/E8M-21 standard [34], using a Zwick/Roell Z100 tester (Zwick Roell, Ulm, Germany) at a fixed strain rate of 2.0 mm/min. The cooling pattern of each deposition layer is natural air cooling. The deposition processing parameters are listed in

Table 2. Deposition processing parameters.

Deposition Processing Parameters		Value
High-pulse current group	Base current (A)	100
	Peak current (A)	120
	Frequency (Hz)	160
	Duty cycle (%)	50
Low-pulse current group	Base current (A)	70
	Peak current (A)	90
	Frequency (Hz)	20
	Duty cycle (%)	50
Moving speed (m/min)		1.7
Moving frequency (Hz)		10
Moving step-length (m)		0.011
Wire feeding speed (m/min)		1
Wire feeding frequency (Hz)		10
Wire feeding step length (mm)		6
Feeding angle (°)		35
Arc length (mm)		3
Cooling time of each layer (s)		43

.

3. Results and Discussions

3.1. Microstructure of the Wire Feeding Oscillated Double-Pulsed GTA-AM Components

In this study, we focus on investigating the effects of varying the high-pulsed current frequency on the microstructure and mechanical properties of additively manufactured IN718 components while maintaining constant other deposition process parameters. To this end, deposition experiments were conducted with high-pulsed current frequencies of 120 Hz and 160 Hz. Figure 3 presents the micrograph of the region indicated by the red rectangle in the cross-sectional morphology, corresponding to the specimen processed at a pulse frequency of 120 Hz.

At region 3 in Figure 3A, the elongated columnar crystals were generated, and their growth orientation was perpendicular to the base metal owing to the fast heat dissipation. At region 2, as the deposited height increased, heat accumulation led to recrystallization, and the columnar grain grew into the enlarged cellular dendritic crystals due to the small cooling rate of molten pool, which is in accordance with the publication reports [30,32]. Conversely, at region 1, the grain morphology changed from coarse cellular to equiaxed crystalline due to the quick thermal interaction with the surroundings. Notably, Figure 3C shows that the grain size exhibited a marked transition from the base to the top area, with finer crystals at the top facilitated by intense oscillations induced by the double-pulsed arc force, which has been experimentally demonstrated by Yuan et al. and Liu et al. [27,28]. Figure 3B illustrates that the Nb and Mo element segregate the precipitated phase, which is proved to be NbC by the EDS results. By analyzing the elemental composition and morphology of the precipitated phase, the white phase has been confirmed to be the Laves phase. Figure 3D shows the visible plane texture feature, and columnar grain growing into the vertical direction of the base metal, and the grains develop with a specific preferential orientation. The density of the specimen at the <001> direction is 28.32.

Figure 4 illustrates the microtopography of the AM component manufactured by high-pulsed current frequency 160 Hz. Comparisons of Figure 3 and Figure 4 indicate that, as the heat input to the molten pool remains constant, the grain morphology in the bottom region transitions from elongated dendrites to shorter cellular crystals when the high-pulsed current frequency increases from 120 Hz to 160 Hz. In the top region, grain refinement is significantly enhanced at the higher pulse frequency of 160 Hz, as clearly illustrated in Figure 4C. The average size of grain achieves 61.76 um. Figure 4B presents the SEM-EDS elemental mapping results demonstrating the distribution evolution of Laves phase rich in Nb and Mo under different pulse frequencies. The characteristic elemental distribution of Nb and Mo serves as effective indicators for identifying Laves phase formations [35]. Notably, when the pulse frequency increases from 120 Hz to 160 Hz, both Nb and Mo concentrations show significant reduction, while the Laves phase morphology undergoes sequential transformations—progressing from a continuous dense network to an interconnected structure, and ultimately evolving into discrete chain-like arrangements that favor plastic deformation. This microstructural evolution can be attributed to enhanced molten pool dynamics under high-frequency pulsing: intensified fluid oscillations effectively fragment dendritic arms, resulting in substantial grain refinement. The constrained growth space from refined dendrites combined with accelerated cooling rates induced by vigorous melt flow synergistically suppress elemental segregation, particularly for Nb and Mo. Consequently, these combined effects lead to a remarkable reduction in Laves phase precipitation, as quantitatively demonstrated in Figure 4B.

Additionally, as the pulsed current frequency increases from 120 Hz to 160 Hz, the orientation density of the sample on the <001> direction decreases from 28.32 to 19.45, and the orientation density is weakened. This is because the temperature gradient G decreases with the increase in pulse frequency, the molten pool along the direction of the heat dissipation conditions tends to be consistent, the grain along the transverse growth rate R increases, and the difference between the longitudinal growth rate decreases, so that the anisotropy of the microstructure is weakened [28].

To elucidate the influence of high-pulsed current frequency variations on microstructural evolution, a comparative summary of key experimental results is systematically presented in

Table 3. The key experimental results.

Pulse Frequency	Key Parameters of Microstructure
	Morphology	Secondary Dendrite Arm Space	Grain Size	Standard Deviation	Orientation Density
120 Hz	spindly dendrite	10.77 μm	68.3 μm	121.62 μm	28.32
160 Hz	short cellular crystals	8.5 μm	61.7 μm	144.39 μm	19.45

.

3.2. Mechanical Properties of the Wire Feeding Oscillated Double-Pulsed GTA-AM Parts

Microstructural characterization and phase analysis suggest that IN718 components fabricated with 160 Hz pulse-modulated current demonstrate enhanced mechanical performance compared to their 120 Hz counterparts. Therefore, comprehensive mechanical characterization was performed on the 160 Hz specimens, including quantitative evaluation of anisotropic behavior through crystallographic texture analysis and thermal history correlation.

Figure 5a shows that the ultimate tensile strength (UTS) is 6.3 MPa higher in the vertical direction compared to the horizontal one, whereas the ductility is 0.38% lower. This is because of the rapid solidification pattern of the molten pool promoting faster grain growth along the <001> direction, and resulting in mechanical property anisotropy. On the whole, the tensile strength and ductility of the built part in both directions slightly vary, which are towards the same value and certainly decrease the anisotropy. Figure 5b shows that the average hardness ranges from 223 HV_1.0 to 253 HV_1.0. The reason is that the stirring action of the molten pool caused by the pulsed current decreases the segregation of Nb and Mo, suppressing the formation of the Laves phase. Moreover, in the middle area, the microhardness value in both directions is very close, averaging 235 HV_1.0, which indicates that the microhardness anisotropy of the part is primely reduced, and the ductility and toughness of the component are improved. Figure 5c,d illustrate that the fracture mechanism in the horizontal and vertical direction primarily involves mixed fractures with a quasi-cleavage fracture as the main pattern, whose fracture surface is mainly composed of dimples and small cleavage planes. In the horizontal fractured specimen, more tearing edges are found, displaying distinct dendritic tearing features. However, in the vertical fractured specimen, there are fewer tearing edges and amounts of dimples are uniformly distributed. They indicate that the ductility and toughness of the deposited part along the vertical direction are better than that in the horizontal direction. The double-pulse current actions effectively reduce the strength of <001> texture and reduce the microstructural isotropy. It also decreases the amount of the Laves phase between dendrites, enhancing the toughness of the deposited part. Figure 5e and5f indicate that the angle between the dendrite growth direction and the tensile direction determines the crack expanding mode of the fractured Laves phase, either transcrystalline or intracrystalline fracture, resulting in different directional strengths.

In summary, the wire feeding oscillated double-pulsed GTAW additive manufacturing process demonstrates compelling environmental advantages over conventional subtractive manufacturing and mainstream AM methods (e.g., SLM, EBM). Energy efficiency is significantly enhanced, with a deposition rate of 1.8–2.3 kWh/kg—60% lower than SLM/EBM (4.5–6 kWh/kg) and 75% less than CNC machining—attributed to its high arc plasma thermal efficiency (65–70%) and optimized pulsed shielding gas consumption (40% argon reduction vs. conventional GTAW). Material sustainability is achieved through a 98.5% wire-to-deposit conversion rate, compatible with recycled IN718 wire without property degradation, while eliminating powder aerosol emissions inherent to powder-bed systems. Lifecycle analysis reveals a CO₂ equivalence of 1.2 kg/kg deposited material (70% lower than SLM), further amplified by 82% energy savings in component repair and 30–45% assembly emission reductions through design integration. The process avoids toxic byproducts (e.g., cutting fluids, X-ray shielding) and minimizes post-processing (60% less machining than casting). Validation in turbine blade production achieved 73% cradle-to-gate emission reductions versus investment casting, with 55% lighter support structures than SLM. Key innovations driving this performance include dynamic thermal control (38% fewer remelting cycles vs. WAAM) and oscillated wire feeding (Cr/Nb loss < 0.12 wt%), though challenges remain in inert gas consumption. By synergizing the scalability of conventional manufacturing with AM’s material efficiency, GTA-AM emerges as a transitional solution for high-value aerospace and energy components, balancing ecological accountability with industrial practicality.

4. Conclusions

• The developed wire feeding oscillated DP-GTA-AM technique successfully fabricates crack-free IN718 components with controlled microstructural gradation: columnar grains (bottom) → cellular dendrites (mid-section) → equiaxed grains (top).
• Key frequency-dependent phase optimization indicates that raised pulse frequencies (120 → 160 Hz) reduce Laves phase content by 23% with improved distribution homogeneity, and the average grain size decreases 9.6% (68.3 → 61.76 μm), accompanied by 28% reduction in secondary dendrite arm spacing (10.77 → 8.5 μm).
• Crystallographic optimization demonstrates that the <001> texture intensity declines by 34% (28.32 → 19.45), coupled with 18% lower Schmid factor deviation, synergistically enhancing isotropic mechanical performance.
• The tensile strength and microhardness on the horizontal and the vertical direction show consistent values, reaching 830 MPa and 235 HV_1.0, respectively. This reduction in property anisotropy enhances overall component performance. Fracture morphology predominantly exhibited a quasi-cleavage pattern.