Effects of Laser Remelting on Microstructure, Wear Resistance, and Impact Resistance of Laser-Clad Inconel625-Ni/WC Composite Coating on Cr12MoV Steel

Abstract

In this study, an Inconel625-Ni60-Ni60/25%WC (Inconel625-Ni/WC) composite coating was fabricated on Cr12MoV steel by first-stage laser cladding, followed by second-stage laser remelting with various laser powers, and the better laser energy density of 25.0 J/mm² for laser remelting test was obtained by macroscopic morphology and microhardness analysis. The effects of laser remelting on the microstructure, microhardness, wear resistance, and impact resistance of the composite coating was systematically investigated by combining various characterization methods. The results showed that laser remelting did not cause the composite coating to produce new phases. The microstructure of the Ni/WC layer in the remelted composite coating was denser and finer, and the average grain size of the surface layer was reduced by 11.69%. The impact depth of laser remelting was about 2.0 mm. The average microhardness of the Ni/WC layer in the remelted composite coating increased by 5.9%, and the average wear rate of the surface was reduced by 50.12% compared with that before laser remelting. The wear surface of remelted composite coating exhibited abrasive wear, and the wear resistance was significantly improved. In addition, the impact toughness value of the remelted composite coating reached 5.15 J/cm², which increased by 87.96% compared with that before laser remelting. The impact resistance of the composite coating was further improved.

1. Introduction

Cr12MoV steel, as a commonly used cold work tool steel [1], is subjected to high-intensity impact and high friction during service, which will gradually lead to failures such as wear and crack, affecting its service life. Therefore, surface engineering techniques should be considered to repair and strengthen damaged surfaces to extend their service life. Coating technology is a very useful method for surface repair and strengthening of metallic materials. Laser cladding (LC), as a promising laser coating technique [2,3], prepares coatings with advantages such as high bond strength [4], low dilution rate, and fine internal microstructure [5,6], and a lot of related research work has been carried out by many researchers [7,8,9]. For the study of LC coatings on Cr12MoV steel, most researchers focused on the wear resistance of Cr12MoV steel for LC, but not much research has been done on the impact resistance of the coating. Gao et al. [10] prepared Ni-based coating on Cr12MoV substrates by LC and investigated the characteristics and properties of the coatings by combining various characterization methods. The results showed that the hardness of the coating reached 745 HV, which was 3.5 times that of the substrate. The wear volume of the coating was 48% lower than that of the substrate. Fang et al. [11] prepared CrS/NbC Co-based self-lubricating composite coatings by LC on Cr12MoV steel and investigated their phase, microstructure, and wear resistance. The results showed that The coatings were mainly composed of γ-Co, CrS, NbC, Cr₂₃C₆, and CoCx. Due to the distribution of the relatively hard phase of NbC and the solid lubricating phase CrS, the coatings had better wear resistance.

Laser remelting (LR) is similar to the LC process, and the difference between the two is that LR only produces laser and does not add any additional material. LR [12,13] can not only improve the surface quality of the substrate or coating but also make certain changes in the macroscopic morphology and microstructural composition of the material surface layer [14,15] and reduce or eliminate defects such as cracks and porosity within the coating [16]. It effectively improves the performance of the original workpiece in service [17] and increases the lifetime. Liu et al. [18] investigated the influence of LR on the corrosion and wear resistance of Fe82Cr16SiB alloy coatings fabricated by high-speed LC. The result showed the flatness of the defect-free coatings was substantially improved. Moreover, the remelted coating presented greater corrosion resistance, higher hardness, and good wear resistance with a lower friction coefficient. Cai et al. [19] prepared Ni-Cr-Co-Ti-V high entropy alloy coating by LC and investigated the effect of LR on the wear resistance of the Ni-Cr-Co-Ti-V high entropy alloy coating. The results showed that the as-remelted high-entropy alloy coatings had high friction coefficient and low wear mass loss, indicating that the wear resistance of as-remelted coatings was improved. Zhou et al. [20] conducted LR treatment of Fe-based amorphous composite clad layers and investigated the effect of LR on the organization and properties of the composite coatings. The results showed that the cracking and porosity of the molten layer were reduced, and the surface quality was improved. The microstructure of the top of the coating was refined, and the corrosion resistance and microhardness were enhanced.

At present, most researchers have studied LR post-treatment of single-layer coatings to enhance wear resistance, while LR of multilayer composite coatings to improve wear resistance and impact resistance has not been reported. In this work, considering the complexity of mold steels subjected to high impact and high friction forces in service, an Inconel625-Ni60-Ni60/25%WC (Inconel625-Ni/WC) composite coating was fabricated on the surface of Cr12MoV steel by first-stage LC. Subsequently, second-stage LR was applied to improve the microstructure, wear resistance, and impact resistance of the composite coating. The results obtained in this study can provide a useful reference for surface laser repair and strengthening of components with high friction and impact performance requirements (e.g., Cr12MoV die parts).

2. Materials and Experimental Procedures

Cr12MoV steel was selected as the substrate material, and its chemical composition is shown in

Table 1. Chemical composition of Cr12MoV steel (wt.%).

C	Cr	Mo	V	Si	Mn	Ni	S	P	Fe
1.45–1.70	11.0–12.5	0.4–0.6	0.15–0.30	≦0.4	≦0.4	≦0.25	≤0.03	≤0.03	Balance

. The annealed Cr12MoV steel plate was first made into a rectangular plate of size 80 mm × 40 mm × 12 mm by wire cutting machine, and the microhardness was 45–48 HRC after quenching and tempering treatment. The surface was polished smooth by an angle grinder and sandpapers (180 #-1000 #) and dried after ultrasonic cleaning. The chemical composition of Inconel625 alloy powder (melting point 1290 °C) with particle size varying from 45 µm to 105 µm was selected as transitional layer material, with a chemical composition of 0.05 wt.% C, 21.30 wt.% Cr, 0.20 wt.% Si, 2.40 wt.% Fe, 8.50 wt.% Mo, 3.50 wt.% Nb, 0.05 wt.% Mn, 0.05 wt.% Ti, and balanced Ni. Inconel625 alloy has good ductility and weldability. Ni60 powder (melting point 1027 °C) and Ni60/25%WC composite powder with a particle size of 45 µm to 100 µm were selected as strengthening layer materials, where Ni60/25%WC powder was mixed with 75% Ni60 powder and 25% WC powder. The chemical composition of Ni60 alloy powder was 0.8 wt.% C, 15.5% wt.% Cr, 4.0 wt.% Si, 15.0 wt.% Fe, 3.5 wt.% B, and balanced Ni. WC powder (melting point 2870 °C) as hard ceramic particles can enhance the wear resistance of the coating. The SEM images of different powders are shown in Figure 1.

The equipment used for the first-stage LC experiment mainly consists of MFSC 6000 multimode continuous fiber laser, LC head, DPSF-3 turntable type powder feeder and coaxial type powder feeder, special L-shift machine, and modular control system. The MSFC 6000 multimode continuous fiber laser operates in a continuous mode with an output power of up to 6000 W, an output wavelength of 1070–1090 nm, water cooling, and an ambient temperature of 10–40 °C. For the LC experiment, the actual operating ambient temperature for the LC test was 30 °C. The L-shifter was controlled by programming the cladding path through the control system, thus controlling the movement of the laser head and powder feeder head, with the Z-axis lift being the thickness of the previous layer of coating, and two adjacent layers were deposited using a mutually perpendicular scan. After preliminary process exploration, a combination of process parameters for forming a good quality coating can be obtained as follows: laser power 1000 W, scanning speed 6 mm/s, spot diameter 3 mm, powder feeding rate 12 g/min, defocusing amount 0, protective gas argon, flow rate 10 L/min. The Z-axis lift was equal to the single-layer coating thickness when multi-layer multi-layers are fused, and the multi-layers lap rate was 40%. The schematic diagram of the LC test and scanning strategy is shown in Figure 2b.

The same MFSC 6000 multimode continuous fiber laser was used for the first-stage LR experiment, and the schematic diagram and scanning strategy of the second-stage LR test are shown in Figure 2c. The second-stage LR test was carried out 3 min after the first-stage LC experiment stopped. The laser scanning path of the second-stage LR was the same as the laser scanning path of the top layer in the composite coating. We found in a large number of pre-exploration process experiments that the experimental process of second-stage LR should have a larger out-of-focus amount and a slightly faster scanning speed compared to first-stage LC. Therefore, the scanning speed was fixed at 8 mm/s, the defocusing volume was set at 4 mm, other parameters were kept the same as first-stage LC, the lap rate was 40%, and the spot diameter was 3 mm. Only the laser power was varied to the second-stage LR of Inconel625-Ni/WC composite coating. The macroscopic morphology and microhardness of the coatings after the second-stage LR were analyzed to determine the better laser energy density for the second-stage LR test. The specific LR process parameters are shown in

Table 2. Process parameters of LR.

No.	Laser Power (W)	Scanning Speed (mm/s)	Laser Energy Density (J/mm2)
1	400	8	16.7
2	600	8	25.0
3	800	8	33.3
4	1000	8	41.7

.

The specimens were made into blocks with the test surface size of 10 mm × 10 mm by a wire cutting machine and polished with 180 #-1500 # sandpapers, and a Smartlab 9 kW high-resolution X-ray diffractometer (XRD) was selected to investigate the phase changes of the surface layer of the coating. Cu target was selected as the X-ray source of the experiment, the scanning angle range was set to 20°~90°, and the scanning speed was set to 5°/min.

The specimens of size 10 mm × 10 mm × 5 mm were cut to observe the microstructure of the coating, and the surface to be measured was ground, polished, and cleaned after mounting. Subsequently, electrolytic corrosion with a saturated oxalic acid solution was applied with a voltage setting of 6 V and an etching time of 8 s, followed by cleaning with alcohol and drying with a hair dryer. The microstructure of the coating cross-section was observed by TESCAN Mira 3 XH scanning electron microscope (SEM), and its elemental composition was analyzed by the self-contained energy disperse spectrometer (EDS).

The microhardness measurement of the coating surface and cross-section was performed using the HXD-1000TM/LCD type digital display microhardness tester with a test load of 9.8 N and a loading time of 15 s. When measuring the microhardness of the cross-section, the measurement was taken every 200 µm from the coating surface to the substrate. Three points were measured for each measurement depth with a lateral interval of 50 µm, and an average value was taken as the final microhardness value.

Specimens with dimensions of 10 mm × 10 mm × 8 mm were cut by a wire cutting machine for the friction and wear test, and the test surfaces were sanded in sequence with 180 #–1500 # sandpapers and subsequently polished. The surface of the substrate and coating specimens were subjected to a reciprocating dry sliding friction wear test at room temperature on a Bruker UMT-5 friction and wear tester. A silicon nitride (SiN) ceramic ball with a diameter of 4 mm was selected as the counter-abrasive ball. The test load was 20 N, the friction rate was 5 Hz, the friction stroke was 4 mm, and the friction time was 30 min. The schematic diagram of sliding friction and wear test is shown in Figure 3. The abrasion test was repeated three times under the same test conditions to obtain the average value. The specimens were then observed by three-dimensional confocal laser scanning microscopy and SEM to analyze the wear mechanism.

Standard Charpy V-notch specimens with dimensions of 55 mm × 10 mm × 10 mm were made from the substrate and different coating specimens using a wire cutting machine with a V-notch angle of 45° and a depth of 2 mm, where the V-notch of the coating specimen was opened on the substrate side. The SUNS PTM2200-D1 pendulum impact tester was selected to perform the impact test at room temperature, and the fracture morphology was observed by SEM. The schematic diagram of impact fracture test is presented in Figure 4.

3. Results and Discussion

3.1. Determination of LR Process Parameters

The surface morphology of Inconel625-Ni/WC composite coating after LR with different laser power is shown in Figure 5. It can be seen that after LR, each remelted surface is flatter and glossier than the coating before LR in Figure 5a, and the surface quality was obviously improved. When the laser power was 400 W, the coating surface still had the phenomenon of sticky powder, and the surface ripple degree was larger, which affected the surface roughness. This means that the laser power was low at this time, resulting in the laser energy density acting on the coating was not enough to improve the surface quality. When the laser power increased to 600 W, the surface quality of the coating was good, without obvious cracking defects. When the laser power was 800 W and 1000 W, the surface of the coating was also relatively flat, but obvious cracks could be seen, which was analyzed to be mainly caused by the thermal stress accumulated inside the coating exceeding the yield limit of the material due to the larger laser energy density.

To further verify the optimal laser power of LR, microhardness tests were performed on the cross-sections of the composite coating with different LR powers. The microhardness distribution of the Inconel625-Ni/WC composite coatings after LR with different laser powers is shown in Figure 6. It can be seen that even with different power LR, the microhardness distribution from the Inconel625 transition layer to the substrate part did not differ much from the Inconel625-Ni/WC composite coating and remained almost the same, while the uppermost Ni60/25%WC layer to the Ni60 layer part of the microhardness change was more obvious, combining these two layers as Ni/WC layer. It indicated that the influence depth of LR was limited to about 2.0 mm. The average microhardness of the Ni/WC layer in the composite coating before LR was 679.5 HV, and when the laser power was 400 W, 600 W, 800 W, and 1000 W, the average microhardness of the Ni/WC layer was 690.2 HV, 719.4 HV, 658.3 HV, and 667.8 HV, respectively. Among them, the most obvious improvement was observed at the laser power of 600 W, and the average microhardness of the Ni/WC layer increased by 5.9%. When the laser power was higher than 600 W, the average microhardness of the Ni/WC layer in the composite coating decreased, mainly because the laser energy density per unit time was higher due to the high laser power, the temperature gradient in the melt pool increased, and the solidification rate decreased, which caused the internal grains to increase. It was not conducive to the improvement of coating performance. Therefore, the better LR power should be 600 W. In summary, the optimal LR process parameters are a laser power of 600 W, a scanning speed of 8 mm/s, and an out-of-focus amount of 4 mm when the laser energy density was 25.0 J/mm². Inconel625-Ni/WC composite coating was obtained after LR, and the specimen was called LRed Inconel625-Ni/WC composite coating.

3.2. Phase

The surface XRD patterns of Inconel625-Ni/WC composite coatings before and after LR are presented in Figure 7. It can be seen that there was almost no difference in the surface phases of the two coatings, which were mainly γ-(Fe, Ni), FeNi₃, CrB, Cr₇C₃, Cr₂₃C₆, etc., indicating that LR did not change the phases of the composite coating. Among them, γ-(Fe, Ni) was the solid solution phase generated during the LC process, which could play the role of solid solution strengthening. The presence of γ-(Fe, Ni) solid solution and hard phases such as Cr₇C₃ could enhance the microhardness and wear resistance of the composite coating. In addition, although the appearance of the WC phase in the diffraction peaks cannot be fully identified, it suggested that there may be incompletely melted WC particles in the composite coating. In addition, it can be found that after LR, the peak of each diffraction peak of the coating slightly decreased and broadened, indicating that LR also had a facilitating effect on grain refinement. To further demonstrate this phenomenon, the average grain size was calculated for each major diffraction peak in the XRD patterns of different specimens by the Scherrer formula [21]. From the calculated results, the average grain size of the surface layer of Inconel625-Ni/WC composite coating was 24.81 nm, while after LR, the average grain size of the surface layer of LRed Inconel625-Ni/WC composite coating was reduced to 21.91 nm, which was 11.69% smaller than that before LR.

3.3. Microstructure

Figure 8 demonstrates the cross-sectional SEM image of the Inconel625-Ni/WC composite coating. It can be seen that the Inconel625 transition layer had good bonding with the substrate with no obvious defects. The white band area with a large number of cellular crystals distributed in the bonding area of the Inconel625 layer and the Ni60 layer could be clearly seen. A good metallurgical bond between the substrate and the coating and between each coating was exhibited. The Inconel625 layer was dominated by coarse columnar crystals inside. There was no obvious fusion boundary between the Ni60 layer and the Ni60/25%WC layer, and the analysis concluded that they were ideally combined due to their similar composition. Therefore, they can be combined into a Ni/WC layer. The Ni60 layer was composed of cellular crystals, small-sized columnar crystals, and equiaxed dendrites in order from the bottom to the top. Moreover, with decreasing temperature gradient and increasing solidification rate, the Ni60/25%WC layer was mainly fine equiaxed dendrites with no obvious growth direction [22], and diffusely distributed WC particles can be seen. Due to the fast melting and fast solidification during the interaction between the laser and the substance in the LC process, the composite coating had good overall structural growth characteristics.

In order to investigate the elemental content distribution inside the coating, EDS map scanning was performed on the partial enlargements of the middle region of the Ni60 layer and the top region of Ni60/25%WC layer in Figure 8e,i, and the energy spectra results of EDS map scanning are shown in Figure 9. The presence of Mo and Nb elements in addition to Ni, Fe, Cr, and C elements in the energy spectrum analysis results of Figure 9a indicated that Ni, Mo, and Nb elements in the Inconel625 transition layer entered the Ni/WC layer through the Marangoni convection effect [23] inside the melt pool. The content of W elements was significantly increased in Figure 9b, which was analyzed to be mainly due to the presence of WC particles in Figure 8i causing the corresponding elemental content changes. In addition, the distribution of the elements in the energy spectrum showed that the Fe and Ni elements were concentrated in the interdendritic structures, while the Cr, C, and W elements were concentrated in the dendrites. It indicated that the γ-(Fe, Ni) solid solution and FeNi₃ were mainly concentrated in the interdendritic structures, while the dendrites were mainly composed of the eutectic compounds of Cr₇C₃, Cr₂₃C₆.

The SEM images of the cross-sectional microstructure of LRed Inconel625-Ni/WC composite coating are presented in Figure 10a–i. Similar to the Inconel625-Ni/WC composite coating, the internal microstructure in the Inconel625 layer was mainly coarse columnar crystals, and a white band of the bonded area with a large number of cellular crystals growing perpendicular to the bonding interface can be seen between the Inconel625 layer and the Ni60 layer. The Ni/WC layer, which consisted of the Ni60 layer and Ni60/25%WC layer, was composed of cellular crystals, diffusely distributed small-sized columnar crystals, and disordered growth of fine equiaxed dendrites from the bottom to the top. Moreover, from Figure 10e–i, it can be seen that compared with the Inconel625-Ni/WC composite coating before LR, the microstructure of the Inconel625 layer in LRed Inconel625-Ni/WC composite coating did not change and remained as coarse columnar crystals, but the microstructure in the Ni/WC layer in the LRed coating was denser, and the grain size was reduced. The analysis suggested that this was mainly due to the fact that in the process of LR, the continuous laser beam irradiated the surface of the formed coating, and the coating was remelted at a certain depth to form a new melt pool. The internal microstructure will be remelted and solidified as well. The internal coarse dendrites were broken up to form smaller equiaxed dendrites. Furthermore, the reduced laser energy density in the LR process reduced the temperature gradient in the coating and increased the solidification rate, making the internal microstructure finer, which was consistent with the average grain size comparison between the two surface layers obtained by Scherrer formula in Section 3.2.

The energy spectra results of EDS map scanning of the middle region of the Ni60 layer and the top region of the Ni60/25%WC layer in the LRed Inconel625-Ni/WC composite coating are demonstrated in Figure 11. The elemental composition of the Ni60 and Ni60/25%WC layers in the LRed Inconel625-Ni/WC composite coating was similar to that before LR, and there was an elemental expansion between the layers, with Ni, Fe, Cr, C, Mo, and Nb as the main elements in the Ni60 layer. The main elements within the Ni60/25% WC layer are Ni, Fe, Cr, C, and W elements. However, compared with the Inconel625-Ni/WC composite coating, the Ni60 layer in the LRed Inconel625-Ni/WC composite coating had an increased elemental content of Ni, Fe, and C and a decreased elemental content of Cr, Mo, and Nb. The Ni60/25%WC layer had an increased elemental content of Ni and Fe and a decreased elemental content of Cr, C, and W. Moreover, the content of each element in the Ni60 layer and the Ni60/25%WC layer did not differ significantly, indicating that the LR made the element diffusion inside the coating more intense and the element distribution more uniform, which could avoid the adverse effects of elemental segregation on the coating. In addition, the distribution of the elements in the EDS energy spectrum of Figure 11 indicated that the main composition of the interdendritic structures was still γ-(Fe, Ni) solid solution and FeNi₃, while the dendrites were mainly composed of eutectic compounds such as Cr₇C₃, Cr₂₃C₆. The analysis concluded that LR made the element diffusion between the layers within the composite coating more intense, and the microstructure was re-broken and distributed more densely. Macroscopically, this was manifested as a change in the mechanical properties of the composite coating.

3.4. Microhardness

The microhardness distribution of the composite coating cross-section before and after LR is shown in Figure 12. It can be found that the curves of the microhardness of both coating cross-sections exist in four parts, which were the Ni60/25%WC layer, Ni60 layer, Inconel625 layer, and the substrate. It can be observed from Figure 12 that the microhardness distributions of the Inconel625 layer and substrate remained almost the same regardless of whether the composite coating was LR or not, and only the microhardness of the Ni/WC layer changed, indicating that the LR did not affect the Inconel625 layer and its following parts, and the influence depth of LR was about 2.0 mm. For the Inconel625-Ni/WC composite coating, the average microhardness of its Ni/WC layer was 679.5 HV, and the average microhardness of the Inconel625 layer was 260.9 HV. For the LRed Inconel625-Ni/WC composite coating, the average microhardness of the Ni/WC layer reached 719.4 HV, which was 5.9% higher than that of the Inconel625-Ni/WC composite coating and 62.16% higher than that of the substrate. The analysis suggested that the reason for the slight increase in microhardness of the Ni/WC layer of LRed Inconel625-Ni/WC composite coating, combined with the analysis in Section 3.3, was due to the fact that the internal structure of the Ni/WC layer in the composite coating was remelted and solidified during the LR process, which led to an increase in subcooling due to the increase in scanning speed [24], resulting in a finer microstructure and a macroscopic expression of the increase in microhardness.

3.5. Wear Resistance

The distribution of friction coefficients and average wear rate on the surfaces of different specimens are presented in Figure 13. It can be seen that the coefficient of friction of each specimen had similar trends, and all of them entered the stable wear stage with less fluctuation of the coefficient of friction after a more intense grinding wear stage. The average friction coefficients of the substrate, Inconel625-Ni/WC composite coating, and LRed Inconel625-Ni/WC composite coating were 0.738, 0.483, and 0.457, respectively. The average coefficient of friction of both Inconel625-Ni/WC composite coating and LRed Inconel625-Ni/WC composite coating was significantly lower than that of the substrate, decreasing by 34.55% and 38.08%, respectively. The average friction coefficient of LRed Inconel625-Ni/WC composite coating was the lowest, decreasing by 5.38% compared to that before LR.

In addition, it can be seen that the substrate specimen had the highest average wear rate of 4.86 × 10⁻⁶ mm³N⁻¹m⁻¹, while the average wear rates of Inconel625-Ni/WC composite coating and LRed Inconel625-Ni/WC composite coating were 4.17 × 10⁻⁶ mm³N⁻¹m⁻¹ and 2.08 × 10⁻⁶ mm³N⁻¹m⁻¹, which were 14.20% and 57.20% lower compared to the substrate, respectively. Although the wear resistance of both Inconel625-Ni/WC composite coating and LRed Inconel625-Ni/WC composite coating improved, the LRed Inconel625-Ni/WC composite coating had the lowest value of average wear rate, which decreased by 50.12% compared to that before LR, indicating that LRed Inconel625-Ni/WC composite coating had the best wear resistance, which was corresponding to the LRed Inconel625-Ni/WC composite coating having the smallest average friction coefficient. The analysis suggested that the Ni/WC layer in composite coating formed a new melt pool after LR, and the microstructure was remelted and solidified. The internal coarse dendrites were broken up to form finer equiaxed crystals distributed inside the melt pool, making the internal microstructure denser and finer. In addition, LR led to a more pronounced convection effect inside the coating and a more uniform distribution of elements inside the melt pool. Macroscopically, this was reflected in an increase in microhardness and wear resistance.

In order to further reveal the wear mechanism of different specimens, the SEM images of the wear surface of different specimens are demonstrated in Figure 14. It can be seen from Figure 14a,b that the substrate surface had wide scratches and deep grooves, and also appeared a large number of spalling pits, and obvious adhesion phenomenon, which showed adhesive wear. Figure 14c,d present the surface wear diagrams of Inconel625-Ni/WC composite coating, where shallow grooves can be seen on the surface, and some diffusely distributed white WC particles can also be seen on its wear surface, which was very beneficial for the improvement of the wear resistance of the material. The appearance of spalling pits, adhered debris, and adhesion phenomena indicated that the Inconel625-Ni/WC composite coating wear surface had multiple wear forms of both abrasive and adhesive wear, showing better wear resistance compared to the substrate. In Figure 14e,f, the LRed Inconel625-Ni/WC composite coating surface showed narrower scratches, both in terms of smoother wear surface and shallower surface grooves compared to the substrate or Inconel625-Ni/WC composite coating. Except for some white WC particles, which were mainly fine abrasive fragments, the overall performance was abrasive wear, showing excellent wear resistance.

The 3D morphology of the abrasion scars on the surface of different specimens is demonstrated in Figure 15, and the width and depth of the abrasion scars of different specimens were obtained by measurement, as shown in

Table 3. Depth and width of abrasion scars on the surface of different specimens.

Samples	Substrate	Inconel625-Ni/WC Composite Coating	LRed Inconel625-Ni/WC Composite Coating
Abrasion depth (µm)	808.87	732.71	653.17
Abrasion width (µm)	29.23	27.17	23.04

. From the graphs, it can be seen that the depth and width of the abrasion scars of the substrate were the largest, reaching 808.87 µm and 29.23 µm, respectively, while the width and depth of the abrasion scars of the Inconel625-Ni/WC composite coating were 732.71 µm and 27.17 µm respectively, which were reduced by 9.42% and 7.05% compared to the substrate. The depth and width of wear scars of LRed Inconel625-Ni/WC composite coating were further reduced to 653.17 µm and 23.04 µm, which were 10.86% and 15.2% lower than the Inconel625-Ni/WC composite coating, indicating that the surface wear resistance of the composite coating was further improved after LR. This was consistent with the SEM image of the wear surface in Figure 14.

In addition, EDS map scanning was performed on the wear surfaces of each specimen to investigate the differences in the elemental distribution between the substrate and the different coatings during the wear process. Figure 16 shows the results of the EDS map scanning spectra of the different specimens, and we can find that O elements were present on the wear surfaces of all specimens, indicating that oxidative wear was also present in the wear mechanism. The weight ratios of O elements on the wear surfaces of the substrate, Inconel625-Ni/WC, and LRed Inconel625-Ni/WC composite coatings were 2.5%, 2.0%, and 0.7%, respectively. It can be found that the Inconel625-Ni/WC composite coating clad on the substrate could reduce the content of oxide on the wear surface and play a role in inhibiting oxidative wear. For the LRed Inconel625-Ni/WC composite coating, the O content on the wear surface was reduced by 72% and 65% compared with the substrate and Inconel625-Ni/WC composite coating, respectively, and the degree of oxidative wear was further reduced. It can be concluded that the LR of the composite coating could also inhibit or reduce the occurrence of oxidative wear, which had a gaining effect on the improvement of wear resistance.

3.6. Impact Resistance

The impact toughness values of the substrate and Inconel625-Ni/WC composite coating before and after LR are shown in

Table 4. Impact energy and impact toughness values of different specimens.

Samples	Substrate	Inconel625-Ni/WC Composite Coating	LRed Inconel625-Ni/WC Composite Coating
Impact energy (J)	2.19	3.63	4.12
Impact toughness value (J/cm2)	2.74	4.54	5.15

. The impact toughness values of the substrate and Inconel625-Ni/WC composite coating were 2.74 J/cm² and 4.54 J/cm², respectively. After LR, the impact toughness value of the LRed Inconel625-Ni/WC composite coating reached 5.15 J/cm², an increase of 87.96% and 13.44% compared with the substrate and Inconel625-Ni/WC composite coating, respectively. The impact resistance performance of the composite coating was further improved.

The fracture morphology of the Inconel625-Ni/WC composite coating before and after LR was observed by SEM, and the specific images are shown in Figure 17. For the Inconel625-Ni/WC composite coating, a large number of small-sized dimples can be seen in the Incon625 layer at the bottom of the coating, and the fracture morphology of the Incon625 layer showed a ductile fracture. The fracture morphology of the top area of the Ni/WC layer showed some cleavage steps, river-like patterns, and carbide skeletons, and the overall fracture morphology showed cleavage fracture. The impact toughness value of Inconel625-Ni/WC composite coating was significantly higher compared with the substrate due to the presence of fine dimples in the fracture morphology of the Incon625 layer, which showed excellent toughness characteristics. For the LRed Inconel625-Ni/WC coating, the dimples at the fracture of the Incon625 layer at the bottom of the coating also proved that the fracture mechanism in this region was a ductile fracture. However, the fracture in the top area of the Ni/WC layer also appeared with some dimples in addition to cleavage steps and river-like patterns, which exhibited quasi-cleavage fracture. This was significantly different from the top area of the Ni/WC layer in Inconel625-Ni/WC composite coating, which indicated that the overall toughness of the Inconel625-Ni/WC composite coating was further improved after LR, which was consistent with the data change of impact toughness values in Table 4. The analysis suggested that it was mainly because the LR changed the microstructure of the Ni/WC layer, which led to the refinement of its internal microstructure and denser distribution, and the internal grain dislocations to be blocked, which contributed to the improvement of the impact resistance.

4. Conclusions

In this study, the Inconel625-Ni/WC composite coating was prepared by first-stage LC, followed by second-stage LR to further improve its microstructure, wear resistance, and impact resistance. The main conclusions are as follows:

(1)

The optimal laser energy density for the LR test was 25.0 J/mm², and the surface quality was significantly improved after LR. LR did not change the phases of the composite coating, and the surface phases of the coating remained γ-(Fe, Ni), FeNi₃, CrB, Cr₇C₃, Cr₂₃C₆, etc. The γ-(Fe, Ni) solid solution and FeNi₃ were mainly concentrated in the interdendritic structures, while the dendrites were mainly composed of eutectic compounds such as Cr₇C₃ and Cr₂₃C₆. The microstructure of the Ni/WC layer was more dense and finer, with cellular crystals, columnar crystals, and disordered growth of fine equiaxed dendrites in order from the bottom to the top.

(2)

The depth of influence of the LR was about 2.0 mm, and the average microhardness of the Ni/WC layer in the remelted composite coating increased slightly. The average friction coefficient and wear rate of LRed Inconel625-Ni/WC composite coating were 0.457 and 2.08 × 10⁻⁶ mm³N⁻¹m⁻¹, respectively, which were 5.38% and 50.12% lower than before LR. The wear surface became smoother, showing abrasive wear, and the wear resistance was significantly improved.

(3)

The impact toughness value of the Inconel625-Ni/WC composite coating reached 5.15 J/cm² after LR, which was 13.44% higher than Inconel625-Ni/WC composite coating. After LR, the fracture morphology of the Inconel625 layer at the bottom of the LRed Inconel625-Ni/WC composite coating still showed the characteristic of ductile fracture, but some dimples appeared in the top area of the Ni/WC layer of the composite coating. The impact resistance of the composite coating was further improved.