Effect of Laser Energy Density on the Microstructure and Mechanical Properties of Al₂O₃/Inconel 718 Nanocomposites Fabricated by SLM

Abstract

Metal-matrix nanocomposites (MMNCs) with high performance have broad application prospects. Selective laser melting (SLM) was employed to fabricate Al₂O₃-reinforced Inconel 718 nanocomposites. The influence of laser energy density (E) on the microstructure and properties of the materials was thereafter investigated. The results show that the microstructure and mechanical properties of the composite can be significantly improved by optimizing E. When E increased from 219 J/mm³ to 288 J/mm³, the size of the Al₂O₃ reinforcement reduced, and the average grain diameter of the matrix was found to decrease from 1.09 μm to 0.22 μm. Additionally, the relative density improved from 89.82% to 97.04%. When the laser energy density is 288 J/mm³, the sample exhibits favorable hardness and wear resistance. The average microhardness of samples with 288 J/mm³ reaches 379.32 HV_0.5 Compared with 219 J/mm³ sample, the increase is 15.01%. The average friction coefficient and wear rate decreased to 0.24 and 3.75 × 10⁻⁴ mm³/N·m, respectively. Notably, compared with the samples with E of 219 J/mm³, these values reduced significantly by 60.65% and 60.15%, respectively. The study results can provide technical support for the production of MMNCs with high performance by SLM in industry.

1. Introduction

Nickel-based superalloys are key structural materials, which are widely used in the manufacture of key components in special environments due to their excellent high temperature performance and processability. Inconel 718 is a precipitation-hardened, high-temperature nickel-based alloy that possesses outstanding strength, creep resistance, and corrosion resistance at high temperatures (700 °C). As a result, it has gained a lot of attention in the production of equipment for aviation engines and industrial gas turbines [1,2].

However, due to advancements in aviation and industry, Inconel 718 is facing challenges to meet the performance demands for hot-end components. Recently, various ceramic particle reinforcing phases have been added to Inconel 718 superalloys to improve the mechanical properties of metal-matrix nanocomposites [3,4]. Raghavendra, C.R. et al. [5] employed alumina particles with a 40 nm particle size as the reinforcement phase to be added to the nickel-based alloy matrix, and prepared alumina particle-reinforced nanocomposite coatings using the electrodeposition method. The study mainly investigated the impact of temperature and particle loading rate on microhardness and wear rate. Hafiz et al. [6] mixed Al₂O₃ with pure Ni powder and prepared alumina–nickel composite material by spark plasma sintering. The research discovered that reducing the matrix particle size in the alumina/nickel nanocomposites from micrometers to nanometers was found to improve the thermo-mechanical properties. However, the continuous growth of the aerospace sector has led to a pressing need for high-quality, precise processing of complex components within short cycles. Traditional metallurgical technology used for the preparation of MMCs often results in the clustering of enhanced phase particles and poor wettability distribution [7] in the metal substrate. Such defects, including micro-cracks, can negatively impact part performance. In the production of complex parts, there are also problems such as high time cost and complex processing, and some parts cannot even be manufactured by conventional methods [8,9,10].

Additive manufacturing (AM) is a novel manufacturing technology that fundamentally differs from traditional methods like casting, forging, and machining. By directly printing the finished workpiece layer by layer from a 3D model in accordance with the requirements of the design, the production cycle is reduced significantly while product design freedom is enhanced, making it especially advantageous for small-batch production. The unique manufacturing method makes it develop rapidly in various manufacturing fields [11,12]. SLM is one of the most widely used additive manufacturing technologies in the manufacturing industry; it is a typical metal powder additive manufacturing process. Components made by SLM exhibit a density exceeding 99% with no post-processing, and their mechanical strength is equivalent to forged parts [13,14]. While there are promises, defects unique to SLM remain significant concerns. These include rough surface, porosity, and residual stress, etc. At the same time, diversified additive manufacturing powders are also key to the development of SLM [15]. Therefore, a thorough understanding of both the powder materials and SLM processing parameters is necessary.

In recent years, numerous studies have been conducted regarding the production of MMNCs using the SLM method. Han Q et al. [16] researched TiC particle-reinforced nickel-based superalloys produced through the SLM method, and discovered that the reinforced particles can eliminate SLM hot cracking whilst improving tensile strength, as well as promoting grain refinement. T Rong et al. [17] conducted a study on the impact of varying laser scanning speeds in SLM machining of Inconel 718 reinforced with WC particles on the performance of composite material parts. They discovered that the gradient at the interface between the reinforcing particles and the matrix improved the tribology performance of the parts. Numerous studies have demonstrated the significance of SLM process parameters on the quality of formed workpieces. Specifically, laser energy density has been proven to have a significant impact on the microstructure and mechanical properties [18,19,20]. It is crucial to consider these parameters when aiming to achieve optimal forming quality. Cooper, D.E. et al. [21] evaluated the performance of alloy materials with microscale ceramic powders and proposed the prospect of performance improvement for MMNCs with sizes less than 500 nm at high E values. At the same time, the particle size of ceramic particles added with composite powders in other studies is mostly micron, and there are few studies on nanocomposites. This study investigates the microstructure evolution and mechanism for improving mechanical properties of Al₂O₃/Inconel 718 nanocomposites during SLM processing with varying parameters. The samples were prepared using SLM technology at various E values, and the correlation between laser energy and surface morphology, phase composition, microstructure, and mechanical properties was analyzed.

2. Materials and Methods

2.1. Materials

The spherical Inconel 718 powder used in this study was produced by the plasma rotating electrode process (PREP). Provided by Shandong Runsi New Material Technology Co., LTD., Jinan, China. The purity of the powder is 99.8%, and the particle size ranges from 15 to 53 μm. The chemical composition is shown in

Table 1. Chemical compositions of Inconel 718 alloy.

Element	Ni	Cr	Si	Mo	Nb	Co	Mn	Al	Ti	C	Fe
wt%	53.10	18.20	0.07	3.10	4.99	0.08	0.09	0.45	0.92	0.06	Bal.

, and the morphology is shown in Figure 1a. The enhanced phase was an irregular Al₂O₃ powder with a purity of 99.70% and a particle size range of 50–250 nm. The particle size distributions of the two powders are shown in Figure 1i,j.

When the composite powder is mechanically mixed, the fluidity of the powder will be poor. After many mechanical mixing experiments, it is found that the fluidity of composite powder meets the requirements of SLM when the amount of alumina powder is less than 0.5%. The two powders were thoroughly mixed through a mediumless, ball-free V-mixer (speedmaxer dac 600, Hauschild, Hamm, Germany) to prepare a composite powder containing 0.5 wt% Al₂O₃, as shown in Figure 1b–d. It was thought that nano-alumina would adhere to the mixer when mixing the powder. Therefore, we first performed the powder mixing. Second, we weighed the alumina attached to the powder mixer, and finally, the weight wase subtracted when calculating the proportion of Al₂O₃ added to the alloy. At the same time, the powder was mixed 5 times, and the same measures were taken to ensure that the content of oxidized aluminum alloy was stable at 0.5%. At the same time, four points in Figure 1c,d were taken for component detection; their EDS images are shown in Figure 1e–h.

2.2. SLM Processing

SLM processing was performed using a FARSOON FS121M(Hunan Farsoon High-Tech Co., Ltd., Changsha, China) device equipped with a fiber laser with a maximum laser power of 200 W and a spot size of 80 μm. To prevent oxidation, sample fabrication was carried out in an argon atmosphere with less than 0.1% oxygen. The 67° rotating checkerboard scanning strategy was adopted, which can ensure a relatively long laser scanning path, provide a longer heat dissipation time, and have a small temperature gradient, which can ensure better forming performance and lower surface roughness [22].

The laser melting process is affected by a number of process parameters. The laser power, scanning speed, hatch spacing, and layer thickness determine the laser energy that the powder can absorb per unit time. Therefore, Laser Energy Density (E) represents the laser energy acting on the powder per unit time.

$$E={\displaystyle \frac{P}{vdh}}$$

(1)

where E (J/mm³) is the E, P (W) is the laser power, v (mm/s) is the scanning speed, d (mm) is the hatch spacing, and h (μm) is the layer thickness. Increasing the laser power or reducing the scanning speed, hatch spacing and layer thickness will increase the E.

The research group previously studied the SLM processing parameters of Inconel 718, and at the same time, the SLM parameters of pure metal were slightly adjusted because the laser absorption rate of alumina was slightly higher than that of Inconel 718. The aim is to find a rule between E and sample performance. The forming parameters are shown in

Table 2. Forming parameters of Al₂O₃/Inconel 718.

Molding Parameters	Value
Laser power (P)	145 W, 160 W, 175 W, 190 W
Scanning speed (v)	200 mm/s
Hatch spacing (d)	110 μm
Layer thickness (h)	30 μm
Laser spot size	80 μm
Construct chamber oxygen content	<0.1%
Scanning mode	67° rotary scan

. Four different E values were used: 219 J/mm³, 242 J/mm³,composites 265 J/mm³ and 288 J/mm³, and the unit energy density was calculated by the formula: E = P/vdh, the calculated power parameters were 145, 160, 175 and 190 W, respectively. The effects of different E values on the microstructure and properties of Al₂O₃/Inconel 718 fabricated by SLM were studied. The samples prepared by the SLM process test are shown in Figure 2.

2.3. Microstructure Characterization and Mechanical Properties Tests

The density of SLM samples was measured by the Archimedes principle. During the testing process, 6 groups of random samples were selected for experiment for each parameter, and the average value was finally taken as the final result. The samples were ground and polished according to the standards of metallographic examination, and corroded with a solution containing HCl and H₂O₂ (volume ratio 1:1) for 15 s. Optical microscopy (OM, BX53MRF, Olympus, Tokyo, Japan) and thermal field emission scanning electron microscopy (SEM, Gemini SEM 500, ZEISS, Oberkochen, Germany) were used to characterize and evaluate the microstructure and defects of the prepared samples by observing the XY and XZ faces in Figure 2b. The chemical composition was detected by energy dispersion spectrometer (EDS). The surface hardness of the samples processed by different laser parameters was measured by a micro-Vickers hardness tester(Shanghai Optical Instrument No.5 Factory Co., Ltd., Shanghai, China). The samples were divided into four groups according to E. There were three samples in each group. Five test points were taken for each sample on a surface that had been sanded. The hardness of the samples was tested. The load was 500 gf, and the test force retention time was 15 s. The test results were statistically analyzed. The tribological tests were performed on a pin-disc friction testing machine (MPX-3X, Hengxu Testing Machine Manufacturing Co., Ltd., Jinan, China) at a room temperature of 25 °C. Bearing steel GCr15 balls with a ball diameter of 3 mm and an average hardness of HRC60 were polished with diamond gypsum, cleaned with acetone, and examined after hot air drying. The test load was 7 N, the friction element was rotated at 50 rpm for 350 min, and the rotation diameter was Φ12 mm. Under the same test parameters, the dry sliding test was carried out on different samples to study the effect of E on the wear performance of Al₂O₃/Inconel 718 composite parts, and the wear rate was calculated. During the testing process, five groups of random samples were selected for experiment for each parameter, and the average value was finally taken as the final result. Noticeably, all the test errors in this study are represented in the figure in the form of error bars.

2.4. Statistical Analyses

In order to validate the statistical significance of the results obtained, statistical tests were carried out. Analysis of variance (ANOVA) was used to evaluate whether the difference in density of samples produced with different E values was significant. The p-values of the F-test (ANOVA) were used as indicators, and where the p-value was smaller than 0.05, the difference between the means of the samples at the 5% significance level was considered to be statistically significant.

3. Results and Discussion

3.1. Effects of Laser Energy Density on the Sample Compactness

Some researchers have found that the laser energy density is positively correlated with the relative density of the sample [17,20]. The relative densities of the four samples prepared by different E values are shown in Figure 3. In order to verify the statistical significance of the results, a statistical test was conducted. The p-value is reported in

Table 3. ANOVA of relative density with respect to E values.

Laser Energy Density (J/mm3)	219	242	265	288	p-Value
Relative Density (%)	89.82 ± 0.06	93.76 ± 0.03	96.14 ± 0.02	97.04 ± 0.03	0.0305

, and the p-value is less than 0.05. For the change in E value, the density difference of the four groups of samples is significant. For all samples, the higher the E value, the higher the sample density. Therefore, the difference in printing density corresponding to different E values is statistically significant.

The laser energy density has an effect on the compactness of the Al₂O₃/Inconel 718 composite. When E is 219 J/mm³, a large number of lack of fusion pores appear in the sample profile, with an average size greater than 40 μm. Meanwhile, there are unmelted powders inside and around the lack of fusion pores, as shown in Figure 4a. It is believed that E is too low, which leads to the low temperature of the laser scanning powder layer and insufficient powder fusion. A large amount of unmelted powder leads to poor fluidity of the molten pool [23]. The void between the unmelted powder is fixed on the molten pool boundary by molten metal, and the relative density is 89.82%. When E increases to 242 J/mm³, there are still a large number of lack of fusion pores in the section, but the size is significantly reduced, with the average size being 20 μm; the unmelted powder is also greatly reduced, and the relative density is increased to 93.76%, as shown in Figure 4b. When E is 265 J/mm³, the lack of fusion pore size continues to decrease, and the relative density of the sample is 96.14%. However, there are still some air pores and large sized pores in the sample, as shown in Figure 4c. When E was further increased to 288 J/mm³, the lack of fusion pores and unmelted powder disappeared, there were no obvious defects, only a large number of small sized pores, and the relative density reached 97.04%; this indicates that the powder is fully melted and has good structural density, as shown in Figure 4d [24]. The results show that when E is 265 J/mm³, the laser provides sufficient energy to fully melt the powder layer, the temperature gradient in the molten pool is large, and the Marangoni convection is enhanced, so that the gas between the powders and the hollow powders is rapidly discharged from the molten pool, the lack of fusion pores and the pores are greatly reduced, and good wettability is produced between the solid and liquid interfaces. Finally, it enhances the compactness.

3.2. Effect of Laser Energy Density on the Microstructure of Sample

Figure 5 shows the morphology of the molten pool on the XY plane at different E values. Among them, the figures in the figure are preliminary measurements of the size of the molten pool, making the data more intuitive. According to the E value, the samples were divided into four groups. Six samples were taken from each group for the size statistics of the molten pool. A statistical test was conducted to verify the statistical significance of the results. The p value is shown in

Table 4. ANOVA of the molten pool size on XY plane of the sample with respect to E.

Laser Energy Density (J/mm3)	219	242	265	288	p-Value
Molten pool depth (μm)	32.21 ± 5.23	41.69 ± 4.65	55.84 ± 4.89	69.21 ± 6.38	0.0212
Molten pool width (μm)	82.23 ± 8.65	82.23 ± 8.65	106.65 ± 7.35	126.02 ± 7.98	0.0234

, and the p value is less than 0.05. For the change in E value, the change in molten pool size in the four groups of samples is significant. For all samples, the higher the E value, the larger the molten pool size of the sample. Therefore, the molten pool sizes of the samples corresponding to different E values are statistically significant. Figure 6 shows the size variation diagram of the molten pool in the XY plane of the sample. It is obvious that the average width of the molten pool increases with the increase in E. When E is 219 J/mm³, the average width and depth of the molten pool are 82.23 μm and 32.21 μm, respectively. When the E value is increased to 242 J/mm³, the average width and depth of the molten pool are 98.56 μm and 41.69 μm, respectively. When it was further increased to 265 J/mm³, the average width and depth of the molten pool were 106.65 μm and 55.84 μm, respectively. When the E value increases to 288 J/mm³, the average width of the molten pool reaches 126.02 μm and the depth reaches 69.21 μm. It can be seen that the molten pool size will increase with the increase in E.

Figure 7 shows the SEM morphology of the XY surface of the sample after corrosion under different E values, as well as the EDS energy spectrum at special points. When E is 219 J/mm³, a large number of white broken particles are aggregated on the edge of the grain boundary, and the particle aggregate size is large, forming a network structure, as shown in Figure 7a. Figure 7e,f shows that the white particles are the Al₂O₃-reinforced phase, and the darker part is the Inconel 718 matrix. The analysis shows that a lower E in the SLM process makes the molten pool temperature lower than the melting point of reinforced phase particles, and the reinforced phase is distributed in the matrix in its original form. When E increased to 242 J/mm³, the width of the network structure composed of reinforcement phases decreased significantly, but it was still clearly visible at the edge of the grain boundary, and some spheroidal reinforcement phases appeared and distributed around the network structure, as shown in Figure 7b. With the increase in E, the temperature of the molten pool increases; the edge part of the broken reinforced phase absorbs more heat, reaches the melting point first, and some of the edge tip melts, making the joint of the network structure become thinner. When E is 265 J/mm³, the mesh structure disappears and a large number of spheroidal reinforcing particles appear. Within a roughness range of 0.1–1 μm, the surface of the reinforcing particles becomes obviously smooth and dispersed in the matrix, as shown in Figure 7c. With the further increase in E, the heat absorption of the angular tip makes the mesh structure disappear, and the single reinforced phase particle changes from broken to spherical, and the surface melting makes the reinforced phase surface smooth [20]. When E is further increased to 288 J/mm³, the sphericity of the reinforced phase particles is further improved, and excessive E further melts the reinforced phase, significantly reduces its size, and also disperses in the matrix, as shown in Figure 7d.

It can be found in Figure 7a–d that the size of the columnar grains gradually decreases with the increase in E, and Figure 8 shows the change in average diameter of columnar crystal cross-sections in the XY plane of the sample under E. Each specimen was measured from at least 100 particles. A statistical test was conducted to verify the statistical significance of the results. The p value is shown in

Table 5. ANOVA of the average grain diameter of the sample with respect to E.

Laser Energy Density (J/mm3)	219	242	265	288	p-Value
Size (μm)	1.09 ± 0.062	0.78 ± 0.051	0.35 ± 0.035	0.22 ± 0.028	0.0172

, and the p value is less than 0.05. Therefore, the average grain diameter to different E values has statistical significance.

Grain size (Zv) is the number of grains per unit volume; the larger the Zv, the smaller the grain size. With a small grain size, material strength, hardness, shaping, and toughness can be improved, and grain size can be expressed as:

$$Z_v=0.9{\left({\displaystyle \frac{N}{V_g}}\right)}^{{\displaystyle \frac{3}{4}}}$$

(2)

where N is the nucleation rate and V_g is the grain growth rate.

As can be seen from the formula, the Zv of the sample after solidification depends on N and V_g during solidification; Zv increases with N and decreases with V_g

The grain growth rate V_g mainly depends on the crystal growth mode and undercooling. When the crystal grows in a continuous growth mode, the relationship between the average growth rate of the crystal and undercooling can be expressed as follows.

$$V_g={\displaystyle \frac{V_1}{{∆T}_k}}$$

(3)

where V₁ is the material proportionality constant and $∆$T_k is undercooling.

In the SLM process of pure Inconel718, a large E will bring high energy to the molten pool; $∆$T_k decreases, which reduces the solidify speed and is conducive to the growth of columnar crystals; and V_g increases with the increase in E [25,26].

However, due to the unique processing mode of SLM, the processing process has a high thermal gradient and a fast cooling rate, so the effect of V_g on Z_v is much smaller than N [27]. Some researchers have found that the enhancement particles have the effect of increasing N, and the smaller the size of the enhancement particles, the better the effect of increasing N [28]. During the SLM process, the dispersed nano-reinforcing particles Al₂O₃ act as nucleation sites to increase N and eventually Z_v.

According to the melting law of the enhanced phase mentioned above, when the value of E is close to the melting point of the enhanced phase, the particle size of the enhanced phase will decrease with the increase in E, and the smaller size of the enhanced phase particles will play a better role in refining the grain. This analysis is consistent with the hardness test results of the sample, which explains its rationality.

3.3. Influence of Laser Energy Density on Hardness and Wear Performance of Sample Parts

The hardness values of the four samples prepared by different E values are shown in Figure 9. When E increased from 219 J/mm³ to 265 J/mm³, the average hardness of the sample increased from 322.36 HV_0.5 to the maximum 379.32 HV_0.5. The statistical test was conducted to verify the statistical significance of the results. The p value is shown in

Table 6. ANOVA of the hardness values of the sample with respect to E.

Laser Energy Density (J/mm3)	219	242	265	288	p-Value
Average hardness (HV0.5)	322.36 ± 9.24	349.82 ± 11.82	359.32 ± 14.31	379.32 ± 14.12	0.0362

, and the p value is less than 0.05.

The analysis shows that with the increase in E, the laser energy input increases, the sample density increases gradually in the SLM process, and the hardness increases accordingly. In addition, grain refinement is also one of the main reasons to improve the microhardness of the sample, which has been analyzed in detail in the previous article and will not be repeated here. The hardness measurement results are consistent with the OM images and relative density values of the sample, which can explain the mechanism of hardness change.

Figure 10 shows the friction coefficient and wear rate curves of Al₂O₃/Inconel 718 composite samples under different E values. It can be seen that E has an important impact on the wear performance of SLM processed samples. As E increases from 219 J/mm³ to 242 J/mm³, the average friction coefficient decreases from 0.61 to 0.49, and the wear rate decreases from 9.41 × 10⁻⁴ mm³/N·m to 6.82 × 10⁻⁴ mm³/N·m. By increasing E further to 265 J/mm³, the average friction coefficient continues to decrease to 0.32, and the wear rate is 5.54 × 10⁻⁴ mm³/N·m. When E increases to 288 J/mm³, the average friction coefficient decreases to 0.24 and the wear rate reaches 3.75 × 10⁻⁴ mm³/N·m. In summary, the sample processed under E of 288 J/mm³ has good anti-wear properties. The average friction coefficient and wear rate decreased to 0.24 and 3.75 × 10⁻⁴ mm³/N·m, respectively. Notably, compared with the samples with E of 219 J/mm³, these values reduced significantly by 60.65% and 60.15%, respectively.

Figure 11 is the SEM image of the wear morphology of the sample after the friction test under different E treatments. The workpiece wear widths are 1183.2 μm, 1096.3 μm, 996.5 μm, and 898.6 μm, respectively, as shown in Figure 11a,d,g,j. The decreasing trend of wear marks is consistent with the decreasing trend of the friction coefficient and wear rate.

When the E value is 219 J/mm³, serious plastic deformation occurs on the surface of the sample, as shown in Figure 11b. It can be observed from Figure 11c that the abrasive particles are granular with a small amount of lumps. The surface material is torn into lumpy abrasive particles, which are then repeatedly extruded to form agglomeration particles after spalling off. The friction coefficient curve fluctuates seriously. In addition, there are a large number of dispersed white particles on the wear surface of the matrix. According to Figure 11c and the local EDS data (Figure 12), apart from the components of the matrix and reinforcement phase, the oxygen content is relatively high, and the white wear particles are assumed to be oxides of Ni, Fe, and Cr. The loose wear surface is oxidized, but the oxide film generated is thin and not dense. Under the action of alternating stress, it is easy to fall off and become abrasive chips [29], which is also one of the reasons for poor wear performance. Therefore, there is serious plastic deformation, abrasive wear, and micro-oxidation wear on the surface of the sample at this time.

When E increases to 242 J/mm³, there is obvious delamination on the surface of the sample, which is a typical adhesive wear feature, as shown in Figure 11e,f. These results are consistent with the findings of Rong et al. [30], who reported that the laser scanning speed reduced from 650 mm/s to 350 mm/s, and the wear mechanism continuously changed from severe abrasive wear to adhesive wear. It is worth noting that as the scanning speed decreases, the E in this article increases.

When E is 265 J/mm³, the stratification phenomenon disappears, a large number of particles are dispersed on the surface, and there are some slight grooving scratches on the wear surface, as shown in Figure 11h,i. When E is 288 J/mm³, the surface becomes flat as a whole, a small amount of particles are dispersed on the surface, and some other particles are embedded in the matrix, as shown in Figure 11k,l. The analysis shows that the particles are the agglomeration of the reinforced phase, some of the reinforced phase is peeled off by the grinding ball, and the surface is micro-cut, resulting in the grooving scratches.

When E is large enough, the sample density is high, and the enhanced aggregates are tightly embedded in the matrix. The higher hardness improves the wear resistance of the surface. In summary, the sample processed at 288 J/ mm3e has good anti-wear properties. In summary, the sample processed at 288 J/mm³ E has good anti-wear properties.

As shown in Figure 13 (The yellow arrows represent the direction of molten metal flow in the molten pool.), the analysis shows that the following factors affect the friction and wear performance of the sample.

First of all, when E is too low, the temperature of the laser scanning powder layer is too low, the Marangoni convection is weak, and the powder fusion is not sufficient. There will be a large number of unfused holes in the sample in the SLM process, the gas between the powder and the hollow powder cannot be discharged from the molten pool, the reinforced phase is not firmly combined with the matrix, and the falling reinforced phase intensifies the wear of the sample. These results are consistent with the findings of King et al. [24], who revealed porosity formed during keyhole-mode laser melting during metallurgical investigation. When E is large enough, sufficient energy input makes the reinforcement phase closely combine with the matrix, and the microhardness of the reinforcement phase is high, which can effectively inhibit microcutting and deformation in the wear process, thereby reducing friction damage and improving its wear resistance.

Secondly, some researchers have found that enhanced phase particles have the effect of refining grains, and the smaller the size of the enhanced particles, the better the refining effect [31]. Combined with the melting law of the reinforcing phase mentioned above, the analysis shows that when the value of E is close to the melting point of the reinforcing phase, the particle size of the reinforcing phase will decrease with the increase in E, and the smaller size of the reinforcing phase particles will play a better role in refining the grain, which will improve the friction and wear performance of the sample.

Third, the Orowan effect is a strengthening mechanism by which reinforcing phase particles in a metal matrix impede moving dislocations. The above experiment found that the size of the enhanced phase decreases with the increase in E. Combined with the particle size analysis of the enhanced phase, it can be seen that at a higher E, the particle size of some enhanced phases is less than 10 nm, and the Orowan effect is expected to be significantly enhanced. These results are consistent with those of Cooper et al. [21], who suggested that reducing the particle size to 10 nm would significantly improve the strengthening effect. As the dislocation moves around the grain, the dislocation line grows. The stress causing dislocation movement increases, and the smaller the particle, the larger the curvature radius at bypass. The greater the increase in stress required. Thus, the strengthening effect is produced and the friction and wear performance of the sample is effectively improved. These results are consistent with those of Zhang et al. [32], who found that when the size of the enhanced phase is too large and the interval is too wide, the dislocation movement cannot be prevented and the Orowan effect is weak.

4. Conclusions

The Al₂O₃/Inconel 718 nanocomposite samples were successfully prepared by the SLM method by mixing the two powders mechanically. The results are as follows:

(1)

Laser energy density has a great effect on the compactness of Al₂O₃/Inconel 718 nanocomposites processed by SLM. When E increases from 219 J/mm³ to 288 J/mm³, the relative density increases from 89.82% to 97.04%.

(2)

With the increase in laser energy density, the overall size of the molten pool increases. With the input of high thermal energy, the particle size of the enhanced phase decreases with the increase in E, and it is diffusely distributed in the matrix when E is 288 J/mm³. The average grain diameter of the columnar grains decreased from 1.09 μm to 0.22 μm.

(3)

The laser energy density affects the hardness and wear properties of Al₂O₃/Inconel 718 nanocomposites processed by SLM. When the laser energy density is high, the sample exhibits favorable hardness and wear resistance. The average microhardness of samples with 288 J/mm³ reaches 379.32 HV_0.5 Compared with 219 J/mm³ sample, the increase is 15.01%. The average friction coefficient and wear rate decreased to 0.24 and 3.75 × 10⁻⁴ mm³/N·m, respectively. Notably, compared with the samples with E of 219 J/mm³, these values reduced significantly by 60.65% and 60.15%, respectively.

In such a view, the composites reinforced with nano-sized particles have great potential to improve mechanical properties such as strength, hardness, and wear resistance. The study results can provide technical support for the production of MMNCs with high performance by SLM in industry. Although the above laws were found, however, experiments with higher laser energy densities were not performed due to the limitations of the equipment. Performance comparison of pure alloys under the same experimental conditions has not been carried out. In the course of the subsequent experiment, a focus study will be carried out.