Tungsten carbide WC nickel-based alloy coating has excellent properties such as high temperature resistance, corrosion resistance, wear resistance, and high hardness. The use of laser cladding technology to prepare WC nickel-based alloy coating has obvious advantages and has become a hot topic of research at home and abroad. This paper reviews the research progress of laser cladding preparation of WC enhanced nickel-based alloy coating. First, the characteristics of WC enhanced nickel-based alloy coating and its laser cladding preparation method are introduced. Secondly, several commonly used cladding layer performance improvement measures and post-processing technologies are introduced. Finally, the problems and development trends of laser cladding preparation technology for WC enhanced nickel-based alloy coating are prospected. Nickel-based alloy has excellent toughness, wettability, lubricity, corrosion resistance and oxidation resistance. It is a commonly used coating material in industry. Adding an appropriate amount of ceramic particles can significantly improve the hardness, resistance and corrosion resistance of the coating. Tungsten carbide (WC) has the characteristics of high strength, high hardness, high melting point and low thermal expansion coefficient. It is an ideal coating performance enhancement material. Adding it to nickel-based alloy powder can improve the fluidity of the molten pool and enhance the density of the cladding layer, thereby forming a coating that is resistant to high temperature, corrosion, wear and high hardness.

Laser cladding technology uses a high-energy laser beam to melt the substrate and metal powder and quickly cool it to form a cladding layer with low dilution rate, narrow heat-affected zone and metallurgical bonding with the substrate, which can achieve the repair and surface strengthening of metal parts. The WC-enhanced nickel-based alloy coating prepared by laser cladding technology has a fine and uniform structure, and its hardness and resistance are effectively improved57. In the laser cladding preparation process, there are many factors that affect the coating performance, including process parameters, WC powder morphology, external field auxiliary type, etc.

In order to systematically understand the process of laser cladding preparation of WC nickel-based composite coatings and realize the preparation of high-performance composite coatings, this paper reviews and analyzes the influencing factors of coating preparation quality, optimization measures and the current research status of laser cladding layer post-processing technology at home and abroad, and looks forward to the future research focus, providing a reference for scientific researchers and engineering technicians engaged in the research and application of this technology.

1 Factors affecting the performance of WC nickel-based composite coatings

1.1 Laser cladding process parameters

The common methods for preparing WC nickel-based composite coatings by laser cladding include the pre-powder method and the coaxial powder feeding method. The principle is shown in Figure 1. The pre-powder method requires the use of a binder to lay the molten powder on the surface of the substrate in advance, and then the high-energy laser scans the melted powder according to the planned route and cools it to form a surface cladding layer. This method has a flexible powder laying method and does not require additional powder feeding equipment. However, the bonding efficiency is low, and it is difficult to control the thickness and uniformity of the coating. At the same time, the binder will produce gas under high temperature, which is easy to form defects such as pores and cracks in the molten layer, deteriorating the coating quality. The coaxial powder feeding method requires an additional powder feeding device. The powder feeding gas transports the cladding powder to the surface of the substrate through the powder feeding pipeline. After being melted by the high-energy laser, it forms a surface molten pool with the substrate, and then cools to form a surface cladding layer. Coaxial powder feeding can accurately control the powder delivery through the powder feeding equipment, and is more efficient in preparing surface coatings. It is currently a common method for preparing laser cladding surface coatings.

Laser process parameters have a great influence on coating quality. Before the laser cladding experiment, it is necessary to optimize the process parameter combination in advance, including laser power, spot size, scanning speed, powder feeding amount and overlap rate. “3-. Jing Zhenyu et al. ” adopted the single factor control method, took the three process parameters of laser power, powder feeding amount and scanning speed as the control variables, and took the dilution rate, height and width of the cladding layer as reference indicators. Single factor experiment and orthogonal range analysis were carried out, and the optimal process parameters for laser cladding of Ni35WC1 coating were obtained as laser power 1 500 W, powder feeding amount 2 g/s, and scanning speed 4 mm/s. The hardness, resistance and corrosion resistance of the cladding layer were tested and analyzed, and it was found that all three were improved. Yin Baojian! The influence of laser cladding process parameters on the microhardness and coating thickness of NiWC25 alloy powder coating was studied. The results showed that when any parameter of scanning speed, powder feeding voltage and laser power was fixed, the maximum microhardness could be obtained by adjusting the other two process parameters, and the coating thickness showed an increasing trend with the increase of laser power, powder feeding voltage and scanning speed.

The laser cladding surface molten pool has the phenomenon of rapid heating and rapid cooling, which leads to defects such as pores and cracks in the cladding layer. Reasonable combination of process parameters can inhibit the formation of defects. Zhou Jianbo et al.7 studied the influence of laser process parameters on the crack rate and microstructure of Ni60/WC coating. The results showed that the crack rate of the coating was proportional to the power and inversely proportional to the speed, while the overlap rate and powder feeding amount had no obvious effect on the crack rate, and each parameter had no obvious effect on the growth mode of the product. Yao et al.8 used circular spot and broadband spot to melt pre-alloyed WC-NiCrMo powder on the surface of SS316, analyzed the microstructure of the coating, and studied the wear behavior of the coating through dry sliding wear test. The results are shown in Figure 2. Partially dissolved WC particles are evenly distributed in the coatings formed by circular and broadband spots. The microstructures are composed of WC, MC and γ-(Ni,Fe) solid solution. Due to the dilution of Fe, the microstructures of the two coatings are different. Compared with the coating prepared by circular laser spots, the coating prepared by broadband laser spots has higher hardness and better wear resistance, and the wear surface is smoother, and the overall performance of the coating is better.

1.2 WC addition ratio and particle morphology

The change of WC addition ratio will also affect the quality of the cladding layer. Too much WC powder will cause incomplete melting, thus adhering to the coating surface and affecting the surface quality 9-0. Han Chengfu et al. used laser cladding technology to prepare Ni60A/WC composite coatings on the surface of H13 steel, and studied the influence of different WC contents on the microhardness and dust abrasion properties of the cladding layer. The results are shown in Figure 3. As the WC content increases, the microhardness and wear resistance of the cladding layer gradually increase, the medium-sized grooves in the wear morphology decrease, become shallower and narrower, and the micro-cutting effect weakens. When the WC content is 15%, the cladding layer has the best performance. At this time, the coating hardness reaches 1116.2HV0.2, which is about 4.6 times that of the substrate. Tian et al. P2 used fiber laser to prepare Inconel625+WC composite coating on 2Cr13 steel surface, and studied the influence of WC content on the structure, microhardness and corrosion resistance of the composite coating. The results are shown in Figure 4. The microhardness of the cladding layer increases with the increase of WC content. When the mass fraction of WC is 20%, the average hardness of the coating can reach 536.98HV1. It is about 2.64 times that of the substrate; when the mass fraction of WC is 10%, the coating has the best corrosion resistance, and its corrosion potential is 0.788 06 V higher than that of 2Cr13 steel, and the corrosion current density is only 0.86% of that of 2Cr13 steel.

0rtiz et al. 2 used CO2 laser to prepare NiCrBSi+WC cladding layers with 5 different WC contents on the surface of C45E steel, and analyzed the distribution of WC particles in the cladding layer. The results are shown in Figure 5. The average content of WC in the actual cladding layer accounts for about 80% of the powder feeding amount, and its concentration gradually increases from top to bottom along the depth direction of the cladding layer. The thickness of the cladding layer gradually decreases with the increase of wear time. At the same time, the proportion of WC particles reaching the contact surface gradually increases, and the wear resistance of the cladding layer is also enhanced. Yang Erjuan et al. prepared NiCrBSi-WC composite coatings on the surface of H13 hot working die steel plates by laser cladding, and studied the influence of different WC particle contents on the microstructure, microhardness, fracture toughness and wear resistance of the coating. The results show that with the increase of WC particle content, the hardness of the composite cladding layer gradually increases, and the fracture toughness and wear resistance of the cladding layer show a trend of increasing first and then decreasing. The increase in fracture toughness is due to the deflection mechanism of cracks caused by WC particles in the NiCrBSi matrix: and the wear resistance of the NiCrBSi/WC composite cladding layer with 60% WC content can reach 9 times that of the quenched H13 steel substrate. From the above research, it can be seen that the amount of strengthening particles added should be optimized according to the actual performance requirements of the coating to obtain the best ratio.

In addition to the WC addition ratio affecting the quality of the surface cladding layer, its size and shape also have a certain influence on the surface cladding layer. Zhang et al. studied the influence of spherical, irregular and flocculated WC particles on the structure of WC reinforced Ni60 composite coatings, and found that spherical WC particles accumulated at the bottom of the coating, irregular WC particles gathered in the middle and lower part of the coating, and flocculated WC particles were evenly dispersed in the coating; at the same time, the hardness, wear resistance and corrosion resistance of the three coatings were significantly improved compared with the stainless steel substrate, and the polarization curve showed that the cladding layer with the addition of flocculated WC had higher corrosion resistance and bonding than the cladding layer with the addition of spherical or irregular WC, as shown in Figure 6. Deschuyteneer et al. studied the effects of different tungsten carbide reinforcement particle size and morphology on the wear resistance of NiCrBSi-based composite materials and found that large spherical particles have better wear resistance, while smaller spherical particles help improve the sliding friction resistance of the cladding layer.

2 WC nickel-based composite coating performance optimization method

2.1 Adding rare earth elements

Rare earth elements are widely used in the industrial field and are called “industrial vitamins”. Adding a certain amount of rare earth elements to the cladding powder can refine the cladding layer structure and reduce defects such as pores and cracks. 2. Mohammed et al. P9 used a high-power diode laser to laser melt a coating combining 40%Ni-60%WC and La₂0,Ce0, on the surface of A36 steel. The study showed that the addition of rare earth elements further refined the coating structure, and the corrosion resistance and hardness were significantly improved; in contrast, the coating with La0; added had higher microhardness and corrosion resistance than the coating with Ce0, added. Zhang Leitao et al. [B0] modified Ni60/50%WC coating by adding Ce0, and studied the effect of Ce0 content on coating surface cracks and hardness. The results are shown in Figure 7. When Ce0 was not added, the coating surface had abundant cracks. When 1% Ce0 was added, the coating had no cracks, the structure was uniform and dense, and the WC particles were rounded. Compared with the coating without Ce0, the coating hardness increased by about 11.88%, and the wear rate also decreased significantly.

2.2 Preparation of gradient coating

Gradient coating refers to a gradient coating with at least two layers of different materials or different contents of the same material in the surface coating. The preparation of gradient coating can optimize performance and improve surface quality9. Wang et al. prepared a WC-enhanced Ni-based gradient composite coating on a 0345R steel substrate using laser cladding technology. The coating was designed as a four-layer structure, with a C276-10% WC coating on the bottom layer and a Ni60-WC coating with a higher hardness on the top layer (the WC mass fractions from bottom to top were 10%, 30% and 50%, respectively). The microhardness and wear properties of the coating were studied, and it was verified that the gradient combination design was beneficial to reducing the cracking tendency of the coating. The results are shown in Figure 8. From the substrate to the surface, the average microhardness of the coating increases continuously. Under the action of multiple remelting, the alloy elements are re-diffused in the formed sublayers, and the strengthening phase is evenly distributed in the entire gradient coating: the wear curves of the four-layer coating and the substrate in the gradient composite coating were tested, and the gradient composite coating can effectively improve the overall wear resistance.

Li Li et al. B studied the phase composition, wear resistance and wear morphology of Ni/Ni-WC gradient coating. The results show that the phase of the gradient coating Ni60A+35%WC wear-resistant layer is mainly composed of y-(Ni,Fe) solid solution, wSi, phase and various carbides and other hard phases; there are no defects such as cracks and pores between the gradient coatings and between the coatings and the substrate, and the interface shows good metallurgical bonding: the hardness of the Ni-WC gradient coating is significantly improved, mainly due to the solid solution strengthening and fine strengthening of the unmelted WC and the generated hard carbides: the coating and the substrate show different wear mechanisms, among which the coating is mainly fatigue wear and abrasive wear, and the substrate is mainly adhesive wear and particle wear. Through the above analysis, it can be seen that the gradient coating has a stratification effect, which can enhance the metallurgical bonding effect between the coating and the substrate while improving the hardness and wear and corrosion resistance of the coating.

2.3 External field assisted laser cladding

Adding external field assistance to the laser cladding experiment can refine the organization and suppress defects, thereby optimizing the organization and performance of the cladding layer. Commonly used external field assistance technologies include ultrasonic vibration assistance, induction heating assistance, electromagnetic field assistance, etc. Among them, ultrasonic vibration can produce “cavitation effect”, promote the flow of molten pool, refine grains, and inhibit the generation of defects such as cracks. Li et al.3 introduced ultrasonic vibration in the process of laser melting Ni-60%WC-0.8%a,0, coating. The results show that the introduction of ultrasonic vibration destroys the dendrites in the cladding layer, refines the grains, and the composite coating has the highest wear resistance at an ultrasonic vibration power of 800 W. Wang Xue et al. introduced 20 kHz ultrasonic waves into the surface of 316L steel to prepare molten WC-Ni coatings, and analyzed the effects of ultrasonic vibration on the microstructure, hardness, and element distribution of the coating. The results show that under the action of ultrasonic vibration, a large number of fine grains are formed in the coating, there are many equiaxed crystals and some dendrites, the overall microstructure is relatively dense, the hardness of the coating is significantly improved, and the friction coefficient is reduced by 0.17 compared with the coating without ultrasonic vibration.

Induction heating uses electromagnetic induction to heat the base material, reduce the temperature difference between the base and the cladding layer, thereby reducing residual stress, inhibiting crack defects, and improving coating quality3. Li et al.3 used induction cladding to prepare Ni60+25%WC reinforced steel-based surface composite materials and analyzed the effects of different induction heating current intensities on the cladding layer. The results show that within a certain range, the greater the induction heating current, the denser the cladding surface and the better the overall strength; when the current is within 860~890 A, the coating performance is optimal: when the induction heating current exceeds 890 A, overburning may occur, resulting in reduced bonding strength between the cladding layer and the substrate and damage to the substrate. Farahmand et al.P9 used induction heater-assisted laser cladding to prepare Ni-60%WC coatings. By combining numerical simulation with induction heating-assisted laser cladding on-site thermal monitoring, the influence of induction heaters on the coating was studied. The results are shown in Figure 9. The use of an induction heater can make the molten pool more uniform and smooth, reduce the stress concentration at the junction of the cladding layer and the substrate, reduce the crack sensitivity of the composite coating, and improve the wettability of the WC particles in the molten pool: during preheating, the higher the peak temperature in the cladding layer, the greater the solubility of WC in Ni, and the hardness of the substrate increases with the increase of the solubility of WC in the Ni melt.

3 Post-treatment of laser cladding surface

Due to the large surface roughness of laser cladding surface coating, the uneven surface of the cladding layer and the residual powder particles will lead to residual stress concentration, surface cracks, and reduce the wear resistance and corrosion resistance of the cladding layer. Post-treatment is needed to further optimize the surface quality of the cladding layer. At present, the commonly used coating post-treatments include organic processing technology, chemical electrochemical treatment, abrasive flow polishing and laser polishing. Mechanical processing is a method of mechanically removing the rough surface of the cladding layer using machining tools. It is a common laser cladding post-treatment method, including turning, milling, planing and grinding. An Yiwei et al. [0 used milling technology to process the surface of 316L stainless steel additive forming parts. The surface roughness after milling was less than Ra0.4 um, and the surface texture was clear, continuous and uniform. Zhang et al. used turning technology to process the surface of Cr-Ni-based stainless steel cladding layer. The surface roughness after processing can reach as low as Ra60 nm. The microhardness is also greatly improved. Beaucamp et al. additively manufactured titanium alloys through shape adaptive grinding, and the surface roughness can reach below Ra10 nm. It can be seen that mechanical processing has improved the surface quality of parts, but for parts with complex shapes, especially for processing in narrow areas, mechanical processing has certain limitations; in addition, there is a problem of tool wear in mechanical processing, which affects the consistency of processing quality. Chemical polishing is a method of using chemical reagents to specifically eliminate rough surfaces, and electrochemical corrosion is a method of using the workpiece as an anode and an insoluble material as a cathode to specifically eliminate materials in an electrolyte. These two methods can effectively reduce surface roughness, but chemical reagents are inevitably used in the process. Tyagi et al.: Chemically polished the surface of additively manufactured 316 stainless steel. After polishing, the surface changed from gray to bright, and the surface roughness reached Ra0.37 wm. Zhao et al. used electrochemical polishing technology to polish the inner hole manufactured by selective laser melting, reducing the surface roughness of the straight hole from Sa14.151 μm to Sa3.88 μm.

Abrasive flow machining (AFM) is a method of reducing the roughness of the material medium when it flows through the surface of the molten layer. Peng et al. used abrasive flow machining technology to polish the surface of AISi10Mg aluminum alloy parts manufactured by additive manufacturing. The surface roughness after treatment was reduced from Sa13 μm to Sa1.8 μm, and the surface defects caused by “spheroidization effect” and “powder adhesion” were removed, improving the surface integrity. Gao Hang et al. used abrasive flow machining to effectively remove the metal ball clustering phenomenon on the surface of the parts caused by the “spheroidization effect”, and effectively polished the outer surface and inner hole of the additively manufactured grille parts. The grille surface was bright and the texture was uniform.

Laser polishing (IP) is a method of using high-energy laser to melt materials, redistribute them and cool them to obtain a smooth surface. As a non-contact polishing method, it effectively avoids the impact of the polishing process on the parts. Xu et al. 47 studied the effect of laser polishing on the surface morphology and mechanical properties of additively manufactured TA1 components and found that the surface roughness of TiA1 can be reduced to about Ra1.76 μm by continuous laser polishing, and the microhardness, wear resistance and corrosion resistance are all improved. Chen et al. studied the effect of laser polishing on the surface modification and corrosion behavior of additively manufactured 316L and found that by optimizing the scanning speed and number of times, the surface roughness after polishing can be reduced by 92%, the microhardness can be increased from 1.82 GPa to 2.89 GPa, and the corrosion resistance can be significantly improved. i et al. studied the effect of laser polishing on the surface properties of selective laser melted titanium alloy and found that the surface roughness of the initial sample after laser polishing was reduced from Ra6.53 μm to Ra0.32 μm, and the microhardness and wear resistance were increased by 25% and 39%, respectively. Zhou et al. 50 used nanosecond laser to polish the surface of AISi10Mg powder bed fusion additively formed parts and found that good surface quality could be obtained by reducing the filling spacing, increasing the number of polishing times and selecting the appropriate laser direction, so that the microhardness of the parts increased from 112.3HV to 176.9HV, which was due to the grain refinement effect of laser polishing. Liu et al. 6 polished the surface of mconel718 high temperature alloy parts by single pulse laser polishing (SPLP) and hybrid laser polishing (HLP) technology combining pulsed laser and continuous wave laser. The results are shown in Figure 10. After SPLP and HLP treatment, the surface roughness of the alloy was reduced from Ra15.75 um to Ra6.14 wm and Ra0.23 respectively. um, the Young’s modulus of the surface layer was increased to (282+5.21)GPa and (304+5.57)GPa respectively, and the surface formed a mirror effect after polishing.

4 Conclusions

(1) When using laser cladding technology to prepare nickel-based coatings, the appropriate addition of WO reinforcing particles can effectively improve the quality of the cladding layer, significantly improve the hardness, wear resistance, corrosion resistance and high temperature resistance of the cladding layer, the shape, size, addition ratio and distribution state of WC particles will have a great influence on the organization and performance of the coating, and the microstructure, element composition and phase distribution of the coating under different laser process parameters are also different.

(2) Laser cladding is used to prepare WC nickel-based composite coatings. The coating has a strong bonding force with the substrate, but the workpiece is prone to deformation and cooling shrinkage. If the temperature gradient between the substrate and the coating is large, cracks will occur. By adding rare earth elements or preparing gradient coatings, residual stress can be reduced, cracks can be inhibited, and the porosity of the cladding layer can be reduced.

(3) External field-assisted laser cladding of WC Nickel-based composite coatings can refine grains, inhibit element segregation, promote the flow of the molten pool, help inhibit the generation of defects such as pores and cracks in the coating, and effectively improve the quality and performance of the coating.

(4) Laser cladding surface coatings can effectively improve the mechanical properties and service life of materials, but the surface of the cladding layer is prone to spheroidization effect, powder adhesion and high surface roughness, and post-processing is required to further improve the surface quality and mechanical properties.