Effect of Laser Power on the Microstructure, Wear and Corrosion Resistance of Laser-Clad Ni45 Nickel-Based Alloy Coatings

Abstract

This study utilized oscillating laser cladding technology to fabricate nickel-based composite coatings, systematically investigating the influence of varying laser powers on their morphology, microstructure, and properties. The results indicate that as laser power increases from 800 W to 1400 W, the dilution rate of the coating exhibits a non-monotonic change, reaching a maximum at an intermediate laser power due to the competing effects of enhanced substrate melting and melt-pool instability. The microstructure of the coatings is primarily composed of dendritic and equiaxed crystals. Elemental analysis revealed that Ni is predominantly enriched within the dendritic regions, whereas Cr segregates toward the grain boundary areas. Furthermore, the microhardness of the coating, as well as its anti-wear and anti-corrosion properties, are positively correlated with the laser power. When the power reaches the maximum value of 1400 W studied, the performance of the coating significantly improves. The average hardness is 482 HV, and the relative wear resistance is approximately 1.8 times that of the coating when the power is 800 W. The corrosion current density is 9.04 × 10⁻⁷ A/cm².

1. Introduction

The integrity of critical industrial components is frequently compromised by surface degradation mechanisms, such as wear and corrosion, which significantly curtail their operational longevity. Consequently, the application of surface modification strategies has become indispensable for optimizing interfacial properties. However, conventional methodologies—including thermal spraying [1], electroplating [2], and vapor deposition [3]—increasingly struggle to satisfy the rigorous requirements of modern engineering or the demands of extreme service environments. Coatings synthesized via these traditional routes often exhibit deleterious characteristics, such as microstructural coarsening, inadequate interfacial adhesion, and adverse environmental impacts. Therefore, there is a pressing necessity to pioneer innovative surface engineering solutions capable of delivering superior functional performance.

Laser cladding (LC) has increasingly been employed to fabricate metallic coatings for critical industrial components in demanding service environments. Compared to conventional surface modification techniques, LC coatings offer distinct advantages, including lower dilution rates [4], denser microstructures [5], and superior metallurgical bonding with the substrate [6]. For instance, Liu et al. [7] developed a laser-clad composite coating on a Ti-6Al-4V alloy, achieving a mean microhardness double that of the substrate and a 10%–30% reduction in wear loss. Similarly, Fesharaki et al. [8] demonstrated that the metallurgical bonding of laser-clad Inconel 625 coatings is superior to that produced via tungsten inert gas cladding. Awasthi et al. [9] deposited a Ni-Mo-Cr-Si hardfacing alloy onto SS316L stainless steel, resulting in a clad layer with a microhardness exceeding three times that of the substrate and significantly enhanced wear resistance. Furthermore, Bourahima et al. [10] utilized a 4 kW continuous wave laser to successfully clad Ni-based powders onto Cu-Ni-Al substrates. Investigating process parameters, Tanigawa et al. [11] applied Ni-Cr-Si-B alloy layers to C45 carbon steel using varying powder particle sizes (30, 40, and 55 µm); their findings indicated that smaller particles decrease the required heat input, thereby narrowing the heat-affected zone.

Currently, the most common powders utilized for laser cladding include cobalt-based, iron-based, and nickel-based alloys [12,13,14,15]. Among these, nickel-based alloys are extensively employed due to their exceptional mechanical properties, as well as their superior resistance to wear and corrosion [16,17,18,19]. Sun et al. [20] investigated the influence of NbC on the tribological performance of Ni45 coatings, demonstrating that NbC addition significantly enhances both microhardness and wear resistance. Similarly, Chang et al. [21] characterized the microstructure and mechanical properties of laser-clad Ni-Cr-Si-B-Fe composite coatings. Their findings revealed that the coatings primarily consist of an amorphous phase and a γ-(Fe, Ni) solid solution; the presence of the amorphous phase contributes to the coating’s excellent wear resistance. Furthermore, Moskal et al. [22] deposited NiCrAlY coatings onto Inconel 625 and 316L stainless steel substrates, achieving high-quality bonding with the base materials. Notably, strong epitaxial growth was observed exclusively in the upper regions of the coating deposited on the 316L substrate.

The traditional laser cladding technology, as an efficient surface modification method, has a concentrated heat input and a fast-cooling rate, which often results in a short duration of the molten pool and makes it difficult for bubbles to escape fully. This leads to defects such as pores and cracks, which seriously affect the formation quality and service performance of the cladding layer. To overcome these limitations, the oscillating laser cladding technology has emerged as a new process. This technology uses an oscillating mirror to control the laser beam to move along a predetermined trajectory at a high frequency and in a periodic manner. It generates a strong stirring effect on the molten pool, effectively promoting the escape of bubbles and significantly reducing pores and crack defects. At the same time, the oscillating movement of the laser beam disperses the energy distribution, avoids excessive local energy concentration, and reduces the overall thermal input of the substrate, which helps to inhibit material overheating and deformation. At the microscopic level, the shear force generated by oscillation can break dendrites, increase heterogeneous nucleation points, promote grain refinement, and promote the transformation of columnar crystals to equiaxed crystals, thereby improving the performance of the cladding layer [23,24]. For instance, Li et al. [25] investigated the influence of circular beam oscillation on the microstructure, tensile properties, and electrochemical corrosion resistance of Inconel 625. Their findings revealed that circular oscillation promotes equiaxed grain growth and reduces the Laves phase content by 50.66%, significantly enhancing both ultimate tensile strength and yield strength. Furthermore, the refined grain size increased the density of nucleation sites for corrosion while reducing the corrosion current density by 62.3%, thereby effectively suppressing the overall corrosion intensity.

Laser power is one of the most crucial parameters in laser processing, as it directly determines the amount of energy input to the material surface. In laser cladding, the power density must reach a specific threshold to form a stable molten pool, enabling metallurgical bonding between the coating and the substrate. If the power is too low, the energy is insufficient to fully melt the powder or the surface coating of the substrate, resulting in poor bonding or interruption of the coating continuity; if the power is too high, the substrate may be excessively melted, causing a significant increase in dilution rate and even leading to defects such as composition segregation, pores, and cracks. Therefore, laser power is the primary variable that controls the feasibility and stability of the cladding process [25].

Q345R steel is widely used in the fabrication of wind turbine gearboxes. However, such components operate under harsh service conditions and are therefore prone to degradation caused by wear and corrosion. To address this issue, an oscillating laser cladding technique was employed in this study to fabricate a nickel-based alloy coating on the surface of Q345R steel. The key feature of this technique is the active control of molten pool flow, heat transfer, and solidification behavior by introducing laser beam oscillation along predefined trajectories. This effectively suppresses the formation of coarse columnar grains, promotes the development of fine and uniform equiaxed microstructures, and reduces elemental segregation, thereby enhancing the overall performance of the coating. Since laser power is the most critical process parameter governing energy input and thermal cycling behavior, it fundamentally influences molten pool morphology, solidification kinetics, and final microstructural evolution. Accordingly, this study systematically investigated the effects of different laser power levels on the performance of oscillating laser-cladded coatings. The microstructure, microhardness, wear resistance, and corrosion resistance of the fabricated coatings were comprehensively characterized and analyzed. The results are expected to not only deepen the understanding of the process–structure–performance relationship in oscillating laser cladding but also to provide theoretical insights and practical guidance for enhancing surface durability and extending the service life of key components such as wind turbine gearboxes.

2. Materials and Methods

Q345R alloy steel was selected as the substrate in this study, and its chemical composition is summarized in

Table 1. Main chemical composition of Q345R alloy steel (wt.%) [26].

Grade	Q345R Alloy Steel
Element	C	Si	Mn	P	S	Cu	Ni	Cr	Nb
	≤0.20	≤0.55	1.20–1.70	≤0.025	≤0.010	≤0.30	≤0.30	≤0.30	≤0.050

. Commercial Ni45 nickel-based alloy powder (supplied by Ngxindun Alloy Welding Material Spray Co., Ltd., Nan Gong, China.) served as the cladding material, as specified in

Table 2. Chemical composition of nickel-based powder (wt.%).

Grade	Chemical Composition
	C	Cr	Si	Fe	Mo	B	Ni	Cu
Nickel-based	0.35	13.0	3.50	≤5.0	2.50	2.50	Bal	2.50

. The particle size of the powder ranges from 150 to 320 mesh. The powder is spherical in shape, ensuring good fluidity during the powder feeding process. Prior to laser processing, the powder was thermally treated at 150 °C for 2 h to reduce moisture-related porosity and promote stable powder delivery.

Cladding was performed using an oscillating laser system based on a YLS-3000 fiber laser (wavelength: 1.07 µm, beam quality: Beam Parameter Product < 3) equipped with a platform galvanometer scanning head (Shenzhen Hanwei Laser Equipment Co., Ltd., Shenzhen, China). The laser beam operated in a sinusoidal oscillation mode with a spot diameter of 1.8 mm and a focal length of 15 mm. Given the significant impact of laser power on the macroscopic morphology and coating properties, four power levels were investigated: 800, 1000, 1200, and 1400 W. Other process parameters, optimized based on prior research, were held constant: a scanning speed of 5 mm/s, an oscillation frequency of 300 Hz, an oscillation amplitude of 0.5 mm, and a powder feeding rate of 8 g/min. High-purity argon was employed as both the shielding gas (15 L/min) and the carrier gas (6 L/min).

Following the laser cladding experiments, electrochemical corrosion samples measuring 10 mm × 10 mm × 8 mm and wear samples measuring 20 mm × 15 mm were sectioned from the coatings using wire electrical discharge machining. To prepare for the electrochemical tests, the peripheral surfaces of the specimens were lightly polished. The bottom and front surfaces underwent fine grinding before being cold-mounted in a 1:1 resin solution. This process ensured that only the designated test surface and one end of the copper wire lead remained exposed for subsequent analysis.

The microstructure samples were cut into 10 mm × 10 mm × 10 mm using the wire electrical discharge machining. The samples were sequentially ground using 800, 1000, 1200, 2000, and 3000 mesh silicon carbide papers and then polished to a 0.25 µm finish using diamond spray compounds. To reveal the microstructure, the specimens were etched with aqua regia (HCl:HNO₃ = 3:1 (V/V)), after which the cross-sectional morphology of the coatings was examined via optical microscopy. Subsequent to this initial observation, a JSM-7900F scanning electron microscope (JEOL Ltd., Akishima, Tokyo) was employed to characterize the coating morphology in greater detail, while energy-dispersive X-ray spectroscopy (EDS) was utilized to analyze the elemental distribution, and the working voltage is set at 15 KV. Furthermore, the three-dimensional morphology and cross-sectional area of the coatings were determined using AutoCAD (2018) software [27].

At room temperature, the wear resistance of the coatings was evaluated using an MDW-05 high-frequency reciprocating friction and wear tester (Jinan Yihua Frictional Testing Technology Co., Ltd., Jinan, China). The experimental parameters were set to a load of 30 N and a frequency of 2 Hz for a total duration of 30 min. Each coating was measured three times. To determine the wear loss, the mass of each sample was measured three times before and after the test, with the average value recorded as the final result. Friction coefficient curves were subsequently generated and analyzed using plotting software (Origin 2022). Following the wear tests, the worn surface morphology was characterized using a JSM-7900F scanning electron microscope (JEOL Ltd., Akishima, Tokyo), and the elemental composition of the worn areas was analyzed via EDS [28].

Before mechanical characterization, the specimens were sequentially ground with silicon carbide papers ranging from 200 to 2000 mesh, followed by polishing to obtain a smooth surface suitable for indentation testing. The microhardness of the coatings was evaluated using a digital Vickers hardness tester (HVST-1000Z, Guangzhou Junda Instrumentation & Metering Co., Ltd., Guangzhou, China) under a load of 0.2 kg with a dwell time of 15 s. Indentations were performed at a step interval of 0.06 mm. To ensure statistical reliability, a total of 20 points were measured across three separate groups per sample, and the average value was recorded as the final result [29].

At room temperature, the potentiodynamic polarization curves and electrochemical impedance spectra of the samples were measured using a three-electrode system on an electrochemical workstation (CHI 760E, Shanghai Chenhua Instrument Co., Ltd., Shanghai, China). The cladding layer is the working electrode (WE), a saturated calomel electrode (SCE) is the reference, and a platinum sheet is the counter electrode (CE). A 3.5% NaCl aqueous solution served as the corrosive medium. The electrochemical parameters were configured with an initial potential of −1 V and a final potential of 1.5 V, utilizing a scanning rate of 1 mv/s and a sampling frequency of 100 Hz. Each specimen was immersed in the solution for 10 min prior to testing to ensure that the open circuit potential reached a stable state before measurements commenced [30].

3. Results

3.1. Microstructure

Dilution behavior plays a critical role in determining the geometric integrity and metallurgical quality of coatings [31]. Figure 1 shows the geometric characteristics of the cross-section of the coatings. The geometric dilution rate and the aspect ratio are calculated as shown in Equations (1) and (2) [32].

$$\eta ={\displaystyle \frac{h}{H+h}}\times 100\%$$

(1)

$$\sigma ={\displaystyle \frac{W}{H}}$$

(2)

In the above formula, $\eta$ represents the dilution rate of the coatings, %; $h$ represents the depth of the coatings, mm; $H$ represents the height of the coatings, mm; $W$ represents the width of the coatings, mm; and $\sigma$ represents the aspect ratio of the coatings. Figure 2 presents the macroscopic morphologies of the cladding layers produced under different laser power conditions, while the corresponding geometric dimensions and dilution rates are summarized in Figure 3 and

Table 3. The height ($H$), depth ($h$), width ($W$), dilution rate ($\eta$), and aspect ratio ($\sigma$) of the cladding layers at different powers.

Power	$\mathit{h}$ (mm)	$\mathit{H}$ (mm)	$\mathit{W}$ (mm)	$\mathit{\eta }$ (%)	$\mathit{\sigma }$
800 W	0.36	0.56	2.06	39.13	3.68
1000 W	0.56	0.58	2.24	49.56	3.86
1200 W	0.78	0.57	2.45	57.04	4.30
1400 W	0.78	1.01	2.57	43.82	2.57

.

To further investigate the morphological characteristics of the coatings, the geometric parameters were extracted and are illustrated in Figure 3. The results demonstrate that laser power significantly influences both the formation parameters and the dilution rate. Increasing the laser power from 800 W to 1200 W enhanced substrate melting and increased the volume of melted powder, leading to an expansion of the molten pool area and a corresponding increase in coating width [33]. Furthermore, the elevated energy absorption per unit area and time increased the penetration depth, resulting in a rising trend in the dilution rate. Interestingly, as the power was further increased from 1200 W to 1400 W, the depth, dilution rate, and aspect ratio of the coatings decreased. This suggests that the increase in penetration depth through laser absorption is subject to a threshold. When the power reaches a higher level, as shown in Figure 2d, the thickness of the cladding layer increases, and more powder is melted, indicating that a higher power may improve the capture efficiency of the powder, allowing more energy to be used for melting the powder rather than penetrating the substrate, thereby reducing the dilution rate [34,35].

Figure 4 illustrates the microstructure of the coatings at varying laser powers. The left column of Figure 4 displays the interface between the coatings and the substrate, while the right column provides magnified views of middle regions. A bright white band, identified as a planar crystal zone, is visible at the interface, confirming effective metallurgical bonding between the deposited coating and the substrate. As shown in Figure 4a,b, at a laser power of 800 W, the coating exhibits an inhomogeneous structure without distinct dendrite formation. Conversely, at 1000 W and 1200 W, as shown in Figure 4c,e, the bottom of the coating is mainly composed of cellular crystals. In the middle regions, the microstructure transitions into a combination of cellular crystals and columnar crystals growing toward the surface. This phenomenon occurs because the high cooling rate near the substrate surface initially promotes cellular growth under conditions where the solute diffusion and crystallization processes are more complete. As the distance from the substrate increases, the temperature gradient further decreases. Because heat is primarily conducted from the top downward, the cooling rate in the vertical direction exceeds that in the horizontal direction. This directional cooling preference promotes epitaxial growth, causing the crystals to gradually transform into columnar structures oriented toward the top of the coating. Furthermore, the microstructure of the cladding layer produced at low laser power is relatively coarse, whereas that obtained at higher laser power is finer and denser. This difference can be primarily attributed to the higher cooling rate associated with high-power laser cladding, which limits dendrite growth by promoting rapid solidification. As a result, a fine and dense microstructural morphology is formed.

Figure 5 presents the EDS elemental maps of the nickel-based coatings. As illustrated, Fe, Si, Cu, C, and Mo are uniformly distributed throughout the coating without discernible segregation. This uniformity is attributed to their relatively low diffusion rates and the fact that their solidification temperatures are comparable to those of the matrix and cladding materials, preventing significant phase separation during the cooling process. In contrast, Ni is primarily enriched within the dendritic regions, while Cr is concentrated along the grain boundaries. This distribution occurs because, although Ni and Cr are mutually soluble, their different cooling rates during solidification cause Ni to preferentially occupy the dendrite cores, while Cr segregates into the residual liquid between dendrites, eventually enriching the grain boundary regions. Furthermore, although the initial Fe content in the nickel-based powder is less than 5.0 wt.%, the final coating contains a higher concentration. This is due to the dilution effect from the Fe-based Q345R substrate; under the thermal action of the laser, Fe atoms diffuse significantly from the matrix into the cladding layer.

Figure 6 illustrates the microstructure of the nickel-based coatings. To characterize the microstructural features, EDS analysis was performed on distinct regions, where points 1–3 represent the intragranular regions and points 4–6 correspond to the grain boundaries. The elemental composition of each measurement point is summarized in

Table 4. Elemental analysis results of six points in the selected area (wt.%).

	C	Si	Cr	Fe	Ni	Cu	Mo
Point 1	2.46	1.56	5.13	43.12	35.34	0.59	0.63
Point 2	3.37	1.30	4.85	46.38	33.61	0.70	0.10
Point 3	3.48	1.43	2.47	39.27	36.62	0.51	0.64
Point 4	6.92	0.64	17.42	46.19	14.43	1.25	1.46
Point 5	4.39	0.61	9.68	41.37	23.38	0.25	4.36
Point 6	8.36	0.73	14.70	44.33	16.61	0.73	0.38

. The data reveal that C and Cr are primarily concentrated at points 4–6, which promotes the formation of M₇C₃-type carbides (where M represents Fe and Cr) along the grain boundaries [36].

The laser cladding process also significantly influences the substrate microstructure, as illustrated in Figure 7. The original substrate constituent consisted of a combination of ferrite and pearlite. Within the heat-affected zone (HAZ), this microstructure transformed into refined acicular martensite as a result of the rapid thermal cycles inherent to the laser cladding process. Consequently, the laser treatment not only refined the HAZ microstructure but also successfully induced a martensitic transformation.

3.2. Hardness

Figure 8 displays the microhardness distribution of the samples produced under varying laser powers. Across all experimental conditions, the hardness of the coatings significantly exceeds that of the substrate. The hardness profile, measured from the coating surface toward the substrate, exhibits an initial increase followed by a gradual decrease before abruptly dropping to the base material level. This reduction in hardness primarily occurs within the bonding and HAZ. Within the bonding zone, the decrease is attributed to the dilution of coating elements by the substrate. Conversely, in the HAZ, the high laser energy input induces a partial martensitic transformation, which results in hardness values superior to those of the unaffected substrate. Furthermore, the average coating hardness correlates positively with laser power, rising from 304 HV at 800 W to 436 HV at 1000 W, 439 HV at 1200 W, and reaching a maximum of 482 HV at 1400 W.

The indentation morphologies within the coating cross-sections were recorded using a microscope integrated with the Vickers hardness tester. As illustrated in Figure 9, at a consistent depth from the coating surface, the indentation size in specimens produced with higher laser power is notably smaller than that processed at lower power levels. Since the applied load and dwell time remained constant, these smaller indentation dimensions directly indicate the superior hardness achieved through elevated laser power. Furthermore, the indentation corners in the high-power coatings remained smooth and well-defined without the formation of microcracks, suggesting that the coatings possess excellent crack resistance [37,38].

3.3. Wear Behavior

Wear performance of the coatings was assessed through analysis of their frictional response during sliding tests. Figure 10 presents the evolution of the friction coefficient for coatings fabricated under different laser power conditions. For all specimens, the friction coefficient exhibits moderate fluctuations within a stable range throughout the testing duration. With increasing laser power from 800 W to 1400 W, a gradual reduction in the average friction coefficient is observed for the Ni45 nickel-based alloy coatings. This trend suggests that higher laser power promotes improved wear resistance, which can be attributed to the enhanced microstructural uniformity and bonding quality of the coatings [39].

To further evaluate the wear resistance of the Ni45 nickel-based alloy coatings deposited at various laser powers, the specimens were weighed before and after the tests using an electronic balance with a precision of 0.1 mg. The wear loss, denoted as Δm, was determined according to Equation (3). Figure 11 presents the wear mass loss and the calculated relative wear resistance for the cladding layers. The measured mass losses for the specimens processed at different laser powers were 1.1, 0.9, 0.8, and 0.6 mg, respectively. The wear resistance of the cladding layers produced at different laser powers was further evaluated using the relative wear resistance. The coating fabricated at 800 W was taken as the reference, and its wear resistance was defined as 1. The relative wear resistance of other coatings was calculated as the ratio of the wear loss of the 800 W coating to that of the corresponding coating. The results indicate that the relative wear resistance increases gradually with increasing laser power. The coating produced at 1400 W exhibits the highest relative wear resistance, which is approximately 1.8 times that of the coating produced at 800 W [38,40].

$$∆\mathrm{m}={\mathrm{m}}_1-{\mathrm{m}}_2$$

(3)

In the above formula, $∆\mathrm{m}$ represents the wear amount, ${\mathrm{m}}_1$ represents the sample mass before wear, and ${\mathrm{m}}_2$ represents the sample mass after wear.

The wear resistance of the cladding layer exhibits a progressive improvement as laser power increases. This trend correlates directly with the aforementioned microhardness results, which demonstrate that coating hardness scales positively with the applied laser power. In accordance with established tribological principles, higher material hardness typically corresponds to enhanced resistance to abrasive and adhesive wear. Consequently, the elevation of laser power facilitates a corresponding enhancement in the overall durability and wear resistance of the resulting Ni45 nickel-based alloy coatings.

To clarify the dominant wear mechanisms, the worn surface of the laser-clad nickel-based coating fabricated at a laser power of 1400 W was examined by scanning electron microscope (SEM). As shown in Figure 12a. From the wear morphology, it can be seen that a large number of deep furrows and debris appeared on the wear surface of the coatings. This is because during the friction process, the adhesion and deformation between the coatings and the grinding ball caused the adhesion area to tear and peel off under the action of shear force, thereby generating furrows and debris [41,42,43]. Therefore, it can be concluded that the main wear mechanisms of the coatings are abrasive wear and adhesive wear. In addition, Figure 12b shows that oxygen was present in the areas where white matter existed on the wear surface, indicating that during the wear process, the surface of the coatings reacted with air under the heat of repeated friction to generate a white and brittle oxide film, resulting in oxidation wear. In summary, the wear mechanism of the coatings is mainly composed of abrasive wear, adhesive wear, and oxidation wear [44,45].

3.4. Corrosion Resistance

Figure 13 shows the Tafel polarization curves obtained for the coatings fabricated under different laser powers in NaCl solution. The corrosion potential (Ecorr) and corrosion current density (Icorr) were determined through the linear extrapolation of the Tafel curves. The results are shown in

Table 5. Electrochemical parameters of coatings at different laser powers.

Sample Number	Laser Power	${\mathbf{E}}_{\mathbf{c}\mathbf{o}\mathbf{r}\mathbf{r}}$ (V)	${\mathbf{I}}_{\mathbf{c}\mathbf{o}\mathbf{r}\mathbf{r}} ({\mathbf{A}/\mathbf{c}\mathbf{m}}^2$)
B-1	800 W	−0.554 (±0.025)	2.45 × 10−6 (±0.089 × 10−6)
B-2	1000 W	−0.499 (±0.012)	1.82 × 10−6 (±0.096 × 10−6)
B-3	1200 W	−0.566 (±0.016)	1.50 × 10−6 (±0.085 × 10−6)
B-4	1400 W	−0.473 (±0.009)	9.04 × 10−7 (±0.077 × 10−7)

, an increase in laser power leads to a shift in the corrosion potential toward more noble values, indicating a reduced thermodynamic tendency for corrosion. Concurrently, the corrosion current density decreases with rising laser power, signifying a reduction in the overall corrosion rate. The Ni45 nickel-based alloy coatings fabricated at higher laser power exhibit significantly higher corrosion potentials than those produced at lower powers, while the corrosion current density demonstrates the opposite trend. Generally, the corrosion potential serves as a reliable indicator of the electrochemical corrosion susceptibility; higher values correspond to a lower corrosion tendency. Meanwhile, the corrosion current density reflects the average kinetic rate of corrosion, where lower values signify enhanced protective performance. Consequently, the coating deposited at a laser power of 1400 W exhibits the best corrosion resistance. The comparison of the Tafel curves between the cladding layers prepared in this study and those from other studies is shown in

Table 6. The results of the Tafel curves of the cladding layer prepared in this study were compared with those of other studies.

	${\mathbf{E}}_{\mathbf{c}\mathbf{o}\mathbf{r}\mathbf{r}}$ (V)	${\mathbf{I}}_{\mathbf{c}\mathbf{o}\mathbf{r}\mathbf{r}} ({\mathbf{A}/\mathbf{c}\mathbf{m}}^2$)
This work	−0.473	9.04 × 10−7
Liu et al. [46]	−0.127	3.865 × 10−6
Li et al. [47]	−0.916	6.448 × 10−6
Qiao et al. [48]	−0.441	3.74 × 10−6

. The results indicate that the corrosion current density of the cladding layer prepared in this study is significantly lower, which means that its corrosion resistance is better and can provide more effective protection for the substrate.

As illustrated in Figure 14a, the Nyquist plots of the Ni45 nickel-based alloy coatings prepared under various laser powers exhibit incomplete capacitive loops. This morphology indicates that the corrosive medium has interacted with the coating and participated in the electrochemical reactions. It can be observed that the impedance of the coating is positively correlated with the laser power; specifically, as the laser power increases, the radius of the capacitive loop expands progressively. Since the capacitive loop radius is a critical parameter for evaluating the corrosion resistance of alloy coatings—where a larger radius signifies a lower corrosion rate and superior protective performance—the results confirm that corrosion resistance improves with higher power. By comparing the loop radii in Figure 14, it is evident that the nickel-based coating fabricated at 1400 W exhibits the highest impedance. This trend indirectly indicates that the increase in laser power facilitates the formation of a stable passive film. Furthermore, these impedance analysis results are in excellent agreement with the findings from the polarization curve analysis discussed previously. These two analytical methods complement each other, collectively demonstrating that increasing laser power significantly enhances the corrosion resistance of Ni45 nickel-based alloy coatings [49,50]. Subsequently, the equivalent circuit shown in Figure 14b was employed to fit the Nyquist plots. In the circuit, Rs represents the solution resistance between the working electrode and the reference electrode in a 3.5 wt.% NaCl solution, while Rct and CPE denote the charge transfer resistance and the interfacial double-layer capacitance, respectively [24]. The fitting results are summarized in

Table 7. Fitting results.

Laser Power	Rs (Ω cm2)	CPE (S Secn cm−2)	n	Rct (Ω cm2)
800 W	254.8	4.07 × 10−5	0.796	3.182 × 104
1000 W	278.2	2.753 × 10−5	0.707	4.045 × 104
1200 W	285.5	2.216 × 10−5	0.689	9.493 × 104
1400 W	288.1	9.125 × 10−6	0.767	1.283 × 105

, where the parameter n corresponds to the exponent of the CPE. As the laser power increases, the Rct value of the cladding layer gradually increases, indicating a progressive enhancement in corrosion resistance with increasing laser power. With increasing laser power, the grain size of the cladding layer becomes finer, leading to a significant increase in grain boundary density. Since Cr tends to segregate at grain boundaries in this system, these high-energy regions act as favorable sites for the rapid and uniform formation of a Cr₂O₃-rich passive film. The formation of this protective film effectively suppresses pitting and intergranular corrosion by hindering the penetration of corrosive species, thereby resulting in an overall improvement in corrosion resistance.

4. Conclusions

In this study, Ni45 nickel-based alloy coatings were successfully fabricated on Q345R alloy steel via oscillating laser cladding, with a systematic investigation into the influence of laser power. The principal conclusions are as follows:

(1) The width and height of the coating gradually increase with the increase in laser power, showing a positive correlation trend, while the dilution rate exhibits a non-monotonic change, rising first and then decreasing. At lower laser power, the dilution rate increases with the increase in power; when the power reaches a higher level, more energy is used to melt the powder rather than penetrate the substrate, so the dilution degree actually decreases. The microstructure of the coating is mainly composed of dendrites and equiaxed crystals, and under the optimized power setting, it presents fine-grained characteristics.

(2) Both the average microhardness and wear resistance of the coatings improve significantly with increasing laser power. The wear mechanism is characterized by a synergistic combination of abrasive, adhesive, and oxidative wear.

(3) Higher laser power effectively enhances the corrosion resistance of the coatings, as evidenced by a shift toward more noble corrosion potentials, a reduction in corrosion current densities, and an expansion of the capacitive loop radii in the Nyquist plots. Specifically, the coating deposited at 1400 W demonstrates the optimal corrosion resistance, providing the most effective protection for the substrate.

This study only investigated laser power under fixed oscillation parameters and did not further clarify the important interaction between the oscillation parameters (oscillation frequency and oscillation amplitude). A systematic study of the coupling mechanism between the two will be the core direction of our future research. Additionally, this study did not calculate the ratio of the temperature gradient (G) to the solidification rate (R) (G/R) to quantitatively describe the transformation from columnar crystals to equiaxed crystals (CET).