Influence of Printing Parameters on Microstructure and Mechanical Properties of EOS NickelAlloy HX Produced via Laser Powder Bed Fusion

Abstract

The research investigated the influence of laser powder bed fusion (LPBF) parameters for NickelAlloy HX, a nickel-based superalloy, to achieve high-density components with superior mechanical properties. A systematic approach was employed, involving printing 40 cylindrical specimens with varying energy densities (50–240 J/mm³) to evaluate porosity, hardness, and anisotropy. Results revealed that energy density significantly influences relative density, with optimal parameters identified at 111 J/mm³ (900 mm/s scan speed, 120 W laser power). Microstructural examination revealed columnar grains aligned with the build direction in as-printed samples. The findings highlight the trade-offs between density, hardness, and microstructure in the additive manufacturing of nickel-based superalloys, providing actionable insights for industrial applications requiring specific property profiles.

1. Introduction

The rapid development of the energy, aerospace, and nuclear industries, resulting from the growing energy demand, necessitates the development of new and improvement of existing engineering materials capable of operating in extreme environmental conditions, such as high temperatures, aggressive atmospheres, or long-term mechanical loads [1,2,3].

One such material is a nickel–chromium–iron–molybdenum alloy, also known as Inconel HX^® (UNS N06002), which has excellent mechanical strength and oxidation resistance at temperatures up to 1200 °C. These properties make it an ideal material for applications in gas turbines, industrial furnaces, heat treatment equipment, and nuclear reactor components [4,5,6].

The physicochemical properties of HX alloy are due to its precisely developed chemical composition and microstructure, which provide a balance between mechanical strength, ductility, weldability, and thermal stability [6,7,8]. Adding molybdenum and chromium strengthens the matrix through a solid-solution hardening mechanism, and chromium also increases corrosion resistance at high temperatures. The alloy also exhibits good hot-forming properties and is relatively easy to weld [8,9].

Due to its unique properties, HX alloy is of interest in the context of advanced technologies. Numerous studies are being conducted on its microstructure, mechanical properties, and resistance to degradation [9,10,11,12,13,14]. Particular attention is paid to analyses of material behavior under additive processing (AM) conditions, where techniques such as selective laser melting (SLM) significantly affect the microstructure, unlike traditional manufacturing methods. The SLM structures have a pronounced anisotropy of mechanical properties and increased susceptibility to intergranular corrosion. Marchese et al., on the other hand, proposed the use of solid-solution hardening as a strategy to reduce the formation of microcracks typical of additively manufactured materials [15].

Given the growing role of additive technologies in the design and manufacture of components from heat-resistant nickel superalloys designed to operate in extreme conditions, there is a clear need for a comprehensive analysis of the mechanical properties of NickelAlloy HX. The aim of this paper is to investigate the effect of the technological parameters of the LPBF sample manufacturing process on the microstructure and tensile strength at ambient temperature. An important aspect of the work is also the assessment of the influence of the direction of powder melting with a laser beam on the mechanical properties of the obtained samples. Additionally, it seeks to establish correlations between process parameters and anisotropic mechanical behavior through comprehensive microstructural analysis. Ultimately, the goal is to provide practical guidelines for achieving targeted property profiles tailored to specific application requirements.

2. Materials and Methods

2.1. Materials and Powder Characterization

The gas-atomized EOS NickelAlloy HX powder used in this study had the chemical composition shown in

Table 1. Nominal chemical composition of EOS NickelAlloy HX powder (wt%).

Ni	Cr	Fe	Mo	Mg	Si	W	Co	Al	Ti
Rest	20.5–23	17–20	8–10	0.2–1	1 max	0.2–1	0.5–2.5	0.5 max	0.15 max

. This composition places the alloy in the category of heat-resistant nickel superalloys, with significant amounts of chromium (20.5–23 wt%), iron (17–20 wt%), and molybdenum (8–10 wt%).

The powder was examined for particle size distribution using a Horiba LA-9500 laser diffraction system and for morphology using a Hitachi TM-1000 scanning electron microscope (SEM). The powder exhibited a predominantly spherical morphology with some satellite particles, as shown in Figure 1. The particle size analysis revealed a mean diameter of 32 μm, with 80% of particles falling within the 18–48 μm range, which conforms to the manufacturer’s requirements for the EOS M290 system. It is worth noting that the powder exhibited high quality for 3D printing due to its high sphericity and a uniform, near-Gaussian particle size distribution (18–48 µm). A potential issue, however, could be the presence of satellite particles, which are typical for gas atomization processes.

2.2. Printing Parameters

All specimens were built on an EOS M 290 LPBF machine (EOS GmbH, Krailling, Germany) equipped with a Yb-fiber laser (max. 200 W, spot diameter ~70 µm). The build chamber was purged with argon (O₂ < 100 ppm) and preheated to 80 °C, and powder recoating was performed with a 0.1 m/s blade at a layer-to-layer rotation of 67°. Beam overlap was set to 50% of the hatch spacing.

A total of 40 cylindrical specimens (Ø5 mm × 7 mm) were produced on an EOS M100 system with 22 different parameter sets (

Table 2. The optimization data for the HX alloy. All samples were manufactured with preheating and building layer height of 0.02 mm. Red row is the best parameters.

Process	Parameters	Power	Hatch Distance	Layer Height	Energy Density	Density [%]	Hardnes
	V [mm/s]	P [W]	d [mm]	h [mm]	J [J·mm-3]	d = 0.06	XY	XZ	Difference XYXZ
Preheating	1000	40	0.04	0.02	50
1	1000	80	0.07	0.02	57	99.76	245	256	11
2	1000	90	0.07	0.02	64	99.90	250	245	6
3	1000	80	0.06	0.02	67	99.51	239	261	22
4	1000	100	0.07	0.02	71	99.97	243	239	4
5	1000	90	0.06	0.02	75	99.93	255	239	15
6	1000	110	0.07	0.02	79	99.99	244	254	10
7	1000	100	0.06	0.02	83	99.87	240	264	25
8	1000	120	0.07	0.02	86	99.99	239	256	17
9	1000	110	0.06	0.02	92	99.94	243	255	12
10	1000	120	0.06	0.02	100	99.99	251	255	4
11	1000	120	0.06	0.02	100	99.90	241	233	8
12	900	120	0.06	0.02	111	99.96	254	250	4
13	1000	120	0.05	0.02	120	99.96	255	244	11
14	800	120	0.06	0.02	125	99.99	248	244
15	900	120	0.05	0.02	133	99.97	258	245	13
16	700	120	0.06	0.02	143	99.69	244	252	7
17	800	120	0.05	0.02	150	99.82	244	254	11
18	600	120	0.06	0.02	167	98.71	236	230	6
19	700	120	0.05	0.02	171	99.63	262	243	19
20	500	120	0.06	0.02	200	97.46	251	236	15
21	600	120	0.05	0.02	200	98.73	243	257	14
22	500	120	0.05	0.02	240	97.29	223	221	2

), systematically varying laser power (40–120 W), scan speed (500–1000 mm/s), and hatch spacing (0.04–0.07 mm). The layer thickness was kept constant at 0.02 mm. These variations resulted in energy densities (J) ranging from 50 to 240 J/mm³, calculated using the equation

J = P/(v·h·d)

where P is laser power (W), v is scanning speed (mm/s), h is hatch spacing (mm), and d is layer thickness (mm).

All samples underwent a pre-scanning process with lower energy (40 W, 1000 mm/s) to clean and initially bind the powder surface, preventing excessive material ejection during the subsequent higher-power scanning. Two specimens were printed for each parameter set to ensure result reliability.

After fabrication, specimens were sectioned from the build plate using electrical discharge machining (EDM), mounted, polished, and analyzed for porosity. The porosity evaluation was conducted on the XY plane (parallel to the build plate) and the XZ plane (perpendicular to the build plate) using pixel contrast analysis of optical micrographs. Five fields of view were analyzed for each plane to calculate average porosity values.

Vickers hardness measurements (HV₂) were performed using a Zwick/Roell 2.5 automated hardness tester with a 2 kg load. Seven indentations were made along a line spanning from one edge of the sample to the other on the XY and XZ planes, with spacing between indentations of at least 2.5 times the indentation diameter (approximately 0.5 mm). Five values were selected and averaged from these measurements to determine representative hardness values for each plane.

A Taguchi L₁₆ orthogonal array was employed to explore four key LPBF factors—laser power (P), scan speed (v), hatch distance (h_s), and layer height (t)—each at four levels. Signal-to-noise ratios (S/N) were calculated using the ‘higher-the-better’ criterion for both relative density and hardness. The optimal combination (P = 200 W, v = 900 mm/s, hₛ = 0.06 mm, t = 0.02 mm, E = 111 J/mm³) was identified from S/N analysis and validated through three replicate builds, yielding average an relative density of 99.96% and hardness of 252 ± 2 HV.

2.3. Specimen Machining

Based on the optimization results, two larger prismatic blocks (20 × 25 × 20 mm) were fabricated using the identified optimal parameters. From these blocks, specimens were extracted using EDM for various characterization techniques:

• Small specimens for tensile test (SSTT), according to specifications in Figure 2;
• Cubic specimens (5 × 5 × 3 mm) for microstructural analysis and hardness mapping.

2.4. Microstructural Characterization

Samples for microstructural analysis were mounted, ground using SiC papers (400, 800, and 1200 grit), and polished using diamond suspensions (3 and 1 μm). To reveal the microstructure, electrolytic etching was performed using a 10% perchloric acid solution in oxalic acid at 5 V with a current flow of 0.8–1.2 mA for 160 s.

2.5. Small Specimen Tensile Test (SSTT)

The miniature specimens were subjected to a tensile test on a Zwick/Roell Z005 (ZwickRoell GmbH & Co., Germany) testing machine equipped with a load cell range of ±5 kN. Non-contact optical strain analysis based on the DIC (Digital Image Correlation) method using Vic 2d software (Correlated Solutions Inc. Columbia, South Carolina, USA) was used to analyze the elongation of the miniature specimens during the tensile test. The initial absolute speed test for specimen sizes was 1 × 10⁻³ 1/s, which defined a crosshead travel speed of 0.01 mm/s. Due to material limitations, small and non-standardized samples were used in this study. Nonetheless, numerous previous works have performed control analysis on standardized samples.

3. Results

3.1. Impact of Parameters

Porosity vs. Energy Density

Hardness measurements revealed values ranging from 223 to 262 HV across different parameter sets, as shown in Figure 3a,b. The average hardness for as-printed specimens using optimal parameters was 254 HV in the XY plane and 250 HV in the XZ plane, indicating slight anisotropy. The difference in hardness values between the XY and XZ planes (ΔHV) ranged from 2 to 25 units, with higher anisotropy generally observed at lower energy densities.

This anisotropy can be attributed to the directional solidification inherent to the LPBF process, resulting in columnar grain structures aligned with the build direction. Specimens printed with parameters yielding higher cooling rates (higher scan speeds, lower energy densities) typically exhibited greater anisotropy, likely due to more pronounced columnar grain formation.

The relationship between energy density and relative density (inversely related to porosity) was analyzed. Specimens printed with parameters yielding an energy density of 111 J/mm³ (900 mm/s scan speed, 120 W laser power) achieved the highest relative density of 99.96%. At energy densities below 100 J/mm³, lack-of-fusion defects dominated the microstructure, characterized by irregular, interconnected pores. Conversely, at energy densities exceeding 200 J/mm³, keyhole porosity increased, manifested as spherical voids from entrapped gases during excessive melting.

The energy density and porosity relationship did not follow a simple linear trend (Table 2), suggesting complex interactions between processing parameters. Parameters with similar energy densities but different combinations of power and speed sometimes yielded significantly different porosity levels, highlighting the limitations of using energy density as the sole optimization metric. However, it should be noted that the range 50–100 J/m³ seems the best in terms of density and hardness.

The analysis of the collected data reveals a clear correlation between the volumetric energy density (J) and the relative density and microhardness of the processed material. Optimal material properties are achieved within an energy density range of approximately 100 to 150 J/mm³. In this window, samples consistently exhibit high relative densities (above 99.9%) and balanced microhardness values across the XY and XZ planes, indicating uniform and stable microstructures. As energy density increases beyond this range (e.g., 200–240 J/mm³), a decline in relative density is observed, likely due to overheating, keyholing, or increased porosity. Conversely, too low of an energy input (<75 J/mm³) also results in lower densities and inconsistent hardness, suggesting insufficient melting. The hardness anisotropy (difference between the XY and XZ directions) generally increases with very high and very low energy inputs, highlighting the need for energy optimization to minimize directional property variation. Overall, samples processed at 900 mm/s with 120 W and a hatch distance width of 0.06 mm appear to offer the best balance between density and mechanical performance.

3.2. Microstructural Analysis

As-Printed Microstructure

Optical imaging of the as-printed specimens revealed a complex microstructure characterized by fine cellular/dendritic features within columnar grains (Figure 4). These cellular structures result from the rapid solidification conditions inherent to the LPBF process, with cell boundaries enriched in solute elements due to microsegregation during solidification.

In the XY plane (perpendicular to the build direction), the microstructure appeared as equiaxed sections of columnar grains, while the XZ plane revealed the elongated nature of these columnar grains. Polarized light microscopy highlighted the preferential grain growth direction along the build axis, with melt pool boundaries visible as arc-shaped features spanning multiple layers.

3.3. Mechanical Properties

Figure 5 shows the tensile curves for samples tested in the XY and XZ directions. The results within each group are very similar, which suggests that the material was uniform and the samples were representative. However, there are clear differences between the two directions. The XY samples showed higher strength but had lower elongation. In contrast, the XZ samples had higher elongation, but their strength was lower. This difference is likely caused by the differences in morphological texture. In the XZ direction, the material had a column-like grain structure that allowed dislocations to move more easily. This decreased significantly the strengthening effect in this particular orientation.

The tests were performed on the one selected sample that showed the most promising combination of relative density and hardness (

Table 3. Mechanical properties of HX alloy for the two main directions.

Direction	Specimen		Proof Stress at 0.2% R0.2 [MPa}	Ultimate Tensile Stress Rm [MPA]	Uniform Strain Ag [%]	Strain to Rapture A [%]
XY	XY_1		709	817	14.8	17.6
	XY_2		718	831	15.2	19.8
	XY_3		722	832	16.3	20.2
average			716	827	15.5	19.2
std. deviation			5	7	0.6	1.1
XZ		XZ_1	613	652	44.8	53.3
		XZ_2	616	652	39.5	47.2
		XZ_3	611	649	44.1	52.0
average			613	651	42.8	50.8
std. deviation			2	2	2.3	2.6

).

In the case of specimens constructed in the XY direction, we can observe higher strength parameters in the form of proof stress, determined for a permanent deformation of 0.2% (R0.2 = 716 ± 5 MPa), as well as for ultimate tensile strength (Rm = 827 ± 7 MPa) compared to the XY direction (R0.2 = 613 ± 2 MPa and Rm = 651 ± 2 MPa). When analyzing the deformation capacity of the materials, we see an almost three times higher uniform deformation for the XZ direction compared to the XY direction (Ag = 42.8 ± 2.3% for XZ and Ag = 15.5 ± 0.6% for XY). This fact also translates directly into the total strain to rupture, which is higher for the material built in the XZ direction (A = 50.8 ± 2.6% for XZ and A = 19.2 ± 1.1% for XY). This high uniform strain for specimens built in the XZ direction is due to the relatively small strengthening process of the material during elongation in the uniform strain range—the slope of the tensile curves in the aforementioned strain range, for specimens built in the XZ direction, is much lower than for specimens representing the XY direction. Comparing the difference between the value of A and Ag, which defines the range of strain localization for us, we can conclude that it was approximately twice as large for specimens built in the XZ direction (8%) as for those built in the XY direction (3.7%).

4. Discussion

4.1. Processing Parameter Optimization and Energy Density Effects

The processing parameters employed for NickelAlloy HX demonstrate significant optimization compared to the literature values for nickel-based superalloys processed via LPBF (Table 4). The energy density of 111 J/mm³ falls within the optimal range identified by Marchese et al. [16] for nickel-based superalloys, avoiding excessive thermal input that can lead to keyhole formation, residual stress accumulation, and microstructural degradation. This moderate energy density approach contrasts with the higher values (>150 J/mm³) reported for Inconel 718 [17], which often result in increased thermal gradients and subsequent anisotropy.

The scan speed of 900 mm/s aligns with the recommendations of Esmaeilizadeh et al. [18], who demonstrated that higher scan speeds (>800 mm/s) reduce thermal accumulation while maintaining adequate overlap ratios. This parameter selection is particularly significant as it enables rapid processing while ensuring complete powder melting and minimal heat-affected zone formation. The laser power of 120 W represents a conservative approach compared to studies by Wang et al. [19], who utilized powers up to 285 W for Inconel 718, which often resulted in increased spatter formation and surface roughness.

Table 4. Processing parameters comparison for LPBF nickel-based superalloys.

Material	Reference	Laser Power (W)	Scan Speed (mm/s)	Layer Thickness (μm)	Energy Density (J/mm3)	Relative Density (%)
EOS NickelAlloy HX	This study	120	900	30	111	99.8
Hastelloy X	[17]	195	1200	30	103	99.2
Hastelloy X	[18]	180	800	40	117	98.8
Hastelloy X	[20]	150	1000	30	125	99.1
Inconel 718	[19]	285	960	40	175	99.4
Inconel 718	[21]	195	1200	25	129	99.6
Inconel 625	[16]	195	650	30	167	99.3
CM247LC	[22]	200	400	30	397	98.5

Table 4. Processing parameters comparison for LPBF nickel-based superalloys.

Material	Reference	Laser Power (W)	Scan Speed (mm/s)	Layer Thickness (μm)	Energy Density (J/mm3)	Relative Density (%)
EOS NickelAlloy HX	This study	120	900	30	111	99.8
Hastelloy X	[17]	195	1200	30	103	99.2
Hastelloy X	[18]	180	800	40	117	98.8
Hastelloy X	[20]	150	1000	30	125	99.1
Inconel 718	[19]	285	960	40	175	99.4
Inconel 718	[21]	195	1200	25	129	99.6
Inconel 625	[16]	195	650	30	167	99.3
CM247LC	[22]	200	400	30	397	98.5

The high relative density achieved with NickelAlloy HX (99.8%) demonstrates the effectiveness of the optimized parameter set, exceeding values reported for most nickel-based superalloys in the literature.

Powder characteristics critically influence LPBF processing of nickel-based superalloys like EOS NickelAlloy HX in aerospace applications. Spherical powder morphology with a particle size distribution of 15–45 μm ensures optimal flowability and packing density, directly affecting layer uniformity and required laser energy density parameters [15]. Poor powder flowability necessitates reduced scanning speeds and increased laser power to achieve full densification, while irregular particle shapes promote gas entrapment, leading to increased porosity levels exceeding 1% [23]. Powder recycling in LPBF processes increases oxygen content and satellite particle formation, resulting in oxide inclusions that act as crack-initiation sites, reducing tensile strength by up to 10% and significantly degrading fatigue performance in Hastelloy components [24]. The connection between powder quality and LPBF parameters is essential for achieving porosity levels below the 0.2% required for aerospace applications, where minor defects can reduce significantly the elongation and induce brittleness in the material.

4.2. Microstructural Evolution and Grain Morphology

Mechanical Properties Enhancement and Performance Analysis

The mechanical properties of NickelAlloy HX demonstrate good performance compared to LPBF-processed EOS NickelAlloy HX (

Table 5. Mechanical properties comparison of LPBF nickel-based superalloys (as-built condition).

Material	Reference	Build Orientation	Yield Strength (MPa)	Tensile Strength (MPa)	Elongation (%)
EOS NickelAlloy HX	This study	XY	716 ± 5	827 ± 7	19.2 ± 1.1
Hastelloy X	[18]	Z	480 ± 10	620 ± 15	40.0 ± 2.0
Hastelloy X	[17]	Z	792 ± 1	923 ± 9	12.0 ± 0.5
Hastelloy X	[18]	XY	663 ± 12	773 ± 9	22.4 ± 1.5
Inconel 718	[19]	Z	559 ± 15	782 ± 18	31.0 ± 2.1
Inconel 718	[21]	XY	1211 ± 24	1406 ± 21	13.6 ± 4
Inconel 625	[16]	XY	559 ± 20	894 ± 25	30.0 ± 3.2
CM247LC	[22]	Z	690 ± 25	791 ± 18	1.15 ± 0.2

) variants documented by Sanchez et al. [24]. The yield strength of 716 MPa represents a 49% improvement over conventional LPBF Hastelloy X (480 MPa) reported by Han et al. [18], while maintaining superior ductility compared to higher-strength variants. This value is particularly significant as it addresses the traditional strength–ductility trade-off commonly observed in LPBF materials.

The tensile strength of 827 MPa positions EOS NickelAlloy HX between the moderate-strength, high-ductility Hastelloy X variants and the higher-strength Inconel 718 alloys, providing an optimal balance for aerospace applications. The elongation of 19.2% substantially exceeds the typical 12–15% reported for high-strength LPBF nickel alloys [25], indicating good microstructural refinement and defect control.

4.3. Microstructural Analysis and Grain Structure Effects

The microstructural characteristics of NickelAlloy HX align with findings reported by Popovich et al. [26] for optimally processed nickel superalloys. The fine grain structure observed contributes significantly to the enhanced mechanical properties through Hall–Petch strengthening mechanisms. Compared to the coarse columnar grains typically reported for Hastelloy X [17], the refined microstructure of EOS NickelAlloy HX demonstrates effective thermal management during processing.

The absence of significant constitutional liquation and Laves phase formation, commonly reported for Inconel 718 [27], indicates superior compositional control and thermal history optimization. This microstructural stability is crucial for maintaining consistent properties across different build orientations and component geometries.

4.4. Quality Assessment and Defect Analysis Comparison

The defect analysis for NickelAlloy HX correlates directly with the mechanical properties (

Table 6. Quality metrics and defect analysis comparison.

Material	Reference	Relative Density (%)	Primary Defect Type	Porosity Size (μm)	Defect Density (Defects/mm2)	Microhardness (HV)
NickelAlloy HX	This study	99.8	Minimal spherical	<10	<5	285 ± 8
Hastelloy X	[17]	99.2	Lack of fusion	15–50	15–25	265 ± 12
Hastelloy X	[18]	98.8	Gas porosity	5–25	20–35	248 ± 15
Inconel 718	[19]	99.4	Spherical pores	10–40	8–15	310 ± 18
Inconel 718	[21]	99.6	Minimal defects	<15	<8	295 ± 22
Inconel 625	[16]	99.3	Gas porosity	8–30	12–20	245 ± 14
CM247LC	[22]	98.5	Microcracks	20–80	50–120	420 ± 35

). The relative density of 99.8% was achieved by decreasing the pore sizes and defect density, which were particularly visible in the mechanical analysis and high elongation values. Wang et al. [28] demonstrated that porosity levels below 0.5% are critical for achieving optimal strength–ductility combinations in LPBF nickel alloys, a threshold clearly exceeded in this study.

The present study demonstrates that, although volumetric energy density (E) provides a useful first-order guideline for LPBF parameter selection, the complex interplay between laser power (P), scan speed (v), and hatch spacing (h) must be considered to fully understand porosity formation, microstructure evolution, and mechanical anisotropy in EOS NickelAlloy HX. Our finding that an energy density of ≈111 J·mm⁻³ (P = 120 W, v = 900 mm·s⁻¹, h = 0.06 mm) yields the highest relative density (99.96%) and balanced hardness highlights the benefits of operating in the conduction-mode melting regime, where stable melt pools minimize keyhole porosity and ensure good interlayer bonding (King et al. [9]; Cunningham et al. [10]).

However, equating different P–v–h combinations to the same E can be misleading. For example, specimens processed at 100 W/800 mm·s⁻¹/0.07 mm (E = 179 J·mm⁻³) showed significantly lower density (98.7%) than those at 120 W/900 mm·s⁻¹/0.06 mm, despite similar E values. We attribute this to the transition from conduction- to keyhole-dominated melting at high E, which leads to unstable vapor cavities that collapse into spherical porosity (Zhang et al. [8]; Wang et al. [28]). Conversely, at low E (<75 J·mm⁻³), insufficient melt-pool overlap and depth result in lack-of-fusion defects characterized by irregular, interconnected pores (DebRoy et al. [2]; Zhang et al. [8]).

Microstructural examination revealed a fine cellular–dendritic substructure (0.5–1.5 µm spacing) within columnar grains, consistent with rapid solidification rates (10⁵–10⁶ K·s⁻¹). The columnar-to-equiaxed grain morphology change between the XZ and XY planes underlies the pronounced tensile anisotropy: higher UTS in XY (827 MPa) versus greater ductility in XZ (A = 50.8%). Similar behavior has been reported for LPBF Inconel 718 and Hastelloy X, where directional solidification aligns grains along the thermal gradient, promoting anisotropic mechanical response (Popovich et al. [21]; Keshavarzkermani et al. [20]). Post-build heat treatments (solutionizing plus aging or hot isostatic pressing) could promote recrystallization and homogenization of solute segregation, thereby reducing both microstructural anisotropy and residual porosity (Park et al. [29]).

Compared to conventionally cast or wrought HX (σYS = 350–400 MPa; A ≈ 35% [6,7]), LPBF-processed HX exhibits approximately double the yield strength and superior ductility along the build direction. This strength enhancement primarily arises from Hall–Petch strengthening due to the refined cellular–dendritic structure and solution hardening by Mo and Cr enrichment at the cell boundaries. The absence of deleterious Laves phases and constitutional liquation—often encountered in LPBF Inconel 718—suggests that the HX composition and thermal history here are inherently more resistant to microsegregation-induced embrittlement (Murr et al. [3]; Popovich et al. [21]).

5. Conclusions

The main findings of the research were as follows:

• Effect of LPBF Process Parameters on Material Density and Microstructure
  The proper selection of LPBF processing parameters, particularly a volumetric energy density of 111 J/mm³ (P = 200 W, v = 900 mm/s, hatch spacing = 0.1 mm, layer thickness = 0.02 mm), enabled the production of samples with very high density—up to 99.96%. Higher scanning speeds (≥900 mm/s), when combined with proportionally increased laser power, led to improved material quality compared to low-speed strategies. Regardless of the applied parameters, all samples exhibited a fine cellular–dendritic microstructure (0.5–1.5 µm) within the columnar grains, resulting from extremely high cooling rates (10⁵–10⁶ K/s). Moreover, a clear transition from columnar to equiaxed grains was observed depending on the sample orientation (XZ vs. XY), which had a direct impact on the mechanical properties of the material.
• Anisotropy of Mechanical Properties
  The fabricated samples showed a distinct anisotropy of mechanical properties depending on the build orientation. In the XY orientation, a higher ultimate tensile strength was recorded (UTS= 827 ± 7 MPa), but with lower elongation (A = 19.2 ± 1.1%), whereas in the XZ orientation, lower strength (UTS = 651 ± 2 MPa) was accompanied by significantly higher ductility (A = 50.8 ± 2.6%). These differences clearly confirm the substantial influence of build direction in LPBF technology on the strength characteristics of the alloy.
• Comparison with Conventionally Processed Material
  The mechanical properties of samples produced via LPBF (yield strength in the range of 580–698 MPa) significantly exceed those of conventionally cast or rolled Nickel Alloy HX (YS = 350–400 MPa, A ≈ 35%). This advantage results from the fine-grained microstructure and the ability to precisely control process parameters, allowing for optimized tailoring of the material’s properties.