Anti-ablation wear-resistant tungsten alloy gradient coating and preparation method and application thereof

Abstract

The invention discloses an anti-ablation wear-resistant tungsten alloy gradient coating and a preparation method and application thereof.The anti-ablation wear-resistant tungsten alloy gradient coating is characterized in that high-specific-gravity tungsten alloy powder serves as a raw material, a wide-light-spot laser cladding technology is adopted, parameters are dynamically adjusted, the laser cladding power is gradually reduced from a bottom layer to an interlayer to a top layer, and the ablation resistance of the coating is improved; therefore, a three-layer transition structure sequentially comprising the bottom layer, the middle layer and the top layer in the direction from the base body to the coating is obtained, the size of tungsten crystal grains is gradually transited from thick to dispersed and fine from the bottom layer to the top layer, and meanwhile the solid solution phenomenon is gradually enhanced. According to the preparation method, the cladding efficiency is improved, uniform transition from hardness, strength and toughness to the thermal expansion coefficient is achieved for the performance of the prepared coating, the comprehensive performance of the coating is considered on the premise that the ablation resistance is improved, and a new way is provided for preparation of the alloy gradient coating.

Description

Technical Field

[0001] The present invention belongs to the field of metal surface modification, and particularly relates to an anti-ablative and wear-resistant tungsten alloy gradient coating and a preparation method thereof. Background Art

[0002] Tungsten alloys have advantages such as high density, high strength, and corrosion resistance, and are widely used in materials for kinetic energy armor piercing, balance weights, radiation shielding, etc. and extreme high-temperature service environments, which pose relatively high requirements for their hardness, wear resistance, and anti-ablative performance. However, tungsten has a high brittle-ductile transition temperature, low toughness, and low tensile ductility, which limits the application of tungsten alloys as structural materials. At present, liquid-phase sintering is a common technical means for manufacturing tungsten materials, and the obtained microstructure is relatively uniform, but it requires a long temperature cycle, resulting in grain growth, coarsening of intermetallic compounds, and still having defects such as high W-W connectivity, easy cracking of phase interfaces, and poor plasticity. Therefore, it is urgent to explore new preparation technologies to solve the above problems.

[0003] In terms of preparation processes, common technologies include liquid-phase sintering, selective laser melting, electron beam melting, laser cladding technology, etc. Among them, laser cladding has significant efficiency advantages in preparing complex and precision parts, and the technology related to the present invention is laser cladding. However, during the laser cladding process, due to the large differences in properties such as strength, toughness, thermal expansion coefficient, and thermal conductivity between the coating and the substrate, thermal stress is easily generated due to heat accumulation during the heat input process, resulting in quality defects such as pores and cracks in the prepared coating. In order to avoid stress concentration, gradient design of the coating structure can well solve this problem, but new problems arise when laser cladding the gradient coating structure. At extremely high temperatures, the difference in thermal expansion coefficients between the layers of the gradient coating may cause delamination. Coating delamination is an important manifestation of the failure of the coating-substrate interface, which will significantly affect the creep resistance of the material and the interface bonding quality, and the creep resistance and interface bonding quality of the coating are the core factors determining the anti-ablative performance, and the anti-ablative property is an important indicator for evaluating whether tungsten and its alloys can be used in high-end environments. In addition, the low efficiency of traditional laser cladding also restricts the wide use of tungsten and its alloy coatings.

[0004] Chinese Patent Application No. 202410541046.8 discloses a refractory high-entropy alloy coating with a gradient structure and a preparation method thereof. By using the laser cladding technology, a refractory high-entropy alloy powder with a deposition coating material composition expression of WaMobTacMd is used to achieve a single-phase solid solution structure on the outer surface of the coating in a synchronous powder feeding laser cladding + laser remelting manner. At the same time, the preparation of a gradient coating with a uniform transition from strength, hardness to thermal expansion coefficient is realized, and a coating material with excellent performance is obtained. However, this technology uses the laser cladding + laser remelting method to prepare the gradient coating, with complex process and low preparation efficiency. In addition, the highest tungsten content in the coating prepared by this technology is 50%, which will affect the ablation resistance of the coating. Chinese Patent Application No. 202410540407.7 discloses a method for microstructure control of laser selective melting W-Ni alloy. By reasonably controlling the mass percentage of W powder and Ni powder, the microstructure of the alloy is regulated, and a W-Ni alloy with high density and few defects is successfully prepared. Moreover, as the mass percentage of W increases, the W dendrite phase in the coating significantly increases; however, the increase in the W dendrite phase in the coating is not conducive to the synergistic improvement of the strength and toughness of the alloy. It is not advisable to prepare a high-density high-tungsten alloy coating at the cost of damaging the toughness of the coating. Therefore, how to design the coating structure and laser cladding process to balance the ablation resistance and comprehensive performance of tungsten and its alloy coatings is one of the problems faced in the current preparation of tungsten alloy coatings. Summary of the Invention

[0005] In order to improve the ablation resistance of tungsten and its alloy coatings and take into account their comprehensive performance, and at the same time improve the cladding efficiency, the present invention provides an ablation-resistant and wear-resistant tungsten alloy gradient coating, the performance of which realizes a uniform transition from hardness, strength, toughness to thermal expansion coefficient, and has a high cladding efficiency.

[0006] The present invention also provides a method for preparing the ablation-resistant and wear-resistant tungsten alloy gradient coating.

[0007] To achieve the above object, the technical solution adopted by the present invention is:

[0008] An ablation-resistant and wear-resistant tungsten alloy gradient coating, characterized in that it is prepared from high-specific-gravity tungsten alloy powder through a three-layer transition structure prepared by the wide-spot laser cladding technology and dynamic adjustment of process parameters; the three-layer transition structure is successively a bottom layer, an intermediate layer and a top layer along the direction from the substrate to the coating surface. From the bottom layer to the top layer, the size of tungsten grains gradually changes from coarse to fine and dispersed, and at the same time, the solid solution phenomenon gradually enhances; the dynamic adjustment of the process parameters refers to that from the bottom layer - intermediate layer - top layer, the power of laser cladding gradually decreases.

[0009] Furthermore, the tungsten grains at the bottom layer are mainly irregular particles with an average size of 50 - 100 μm. The tungsten grains in the middle layer are 5 - 10 μm in size and show a dendritic structure. The tungsten grains at the top layer are 200 - 500 nm in size and are evenly dispersed.

[0010] Furthermore, the chemical components in the high - specific - gravity tungsten alloy powder gold include W, Ni, Fe, and Co, and their mass percentages are respectively: W: 85% - 95%, Ni: 3% - 8%, Fe: 1% - 6%, Co: 0.2% - 1%, and the sum of the mass percentages of each component in the high - specific - gravity tungsten alloy is 100%.

[0011] Furthermore, the particle size of the W powder in the high - specific - gravity tungsten alloy powder is 53 - 105 μm, the particle size of the Ni powder is 53 - 105 μm, the particle size of the Fe powder is 15 - 45 μm, and the particle size of the Co powder is 15 - 45 μm.

[0012] The method for preparing the above - mentioned anti - ablation and wear - resistant tungsten alloy gradient coating provided by the present invention includes the following steps:

[0013] Step 1: Preparation of molten powder

[0014] Mix and grind the weighed W, Ni, Fe, and Co powders to the required fineness, and place them in a rotary synchronous powder feeder after vacuum drying for standby.

[0015] Step 2: First, use a master oscillator power amplifier laser (abbreviated as MOPA laser) to pre - treat the substrate to remove the surface rust layer and oxide film, remove the surface impurities with anhydrous ethanol to obtain the pre - treated substrate, and then pre - heat the pre - treated substrate with a constant - temperature electric hot plate to reduce the generation of cracks during the preparation of the gradient coating.

[0016] Step 3: Using the high - specific - gravity tungsten alloy powder as the deposited coating material, adopt the wide - spot laser cladding technology. Under an inert atmosphere, deposit the high - specific - gravity tungsten alloy powder on the surface of the pre - heated substrate in a synchronous powder - feeding manner to form multiple cladding layers on the surface of the pre - heated substrate. It is required that the laser cladding power gradually decreases from the bottom layer to the top layer.

[0017] Furthermore, in Step 3, the process parameters of the wide - spot laser cladding are as follows:

[0018] Bottom - layer laser power: 7500 - 8500 w, spot diameter is 10 - 30 mm, scanning speed is 3 - 5 mm / min, cladding overlap rate is 10% - 30%, powder - feeding speed is 0.5 - 1.5 r / min, and the gas flow rate of the protective gas is 5 - 10 L / min.

[0019] Intermediate layer laser power: 7000 - 7500w, spot diameter is 10 - 30mm, scanning speed is 3 - 5mm / min, cladding overlap rate is 10% - 30%, powder feeding speed is 0.5 - 1.5r / min, gas flow rate of the protective gas is 10 - 15L / min;

[0020] Top layer laser power: 6500 - 7000w, spot diameter is 10 - 30mm, scanning speed is 3 - 5mm / min, cladding overlap rate is 10% - 30%, powder feeding speed is 0.5 - 1.5r / min, gas flow rate of the protective gas is 10 - 15L / min.

[0021] Further, in step one, mixing is carried out using a ball mill equipped with zirconia balls. The weight ratio of zirconia balls to the prepared W, Ni, Fe, Co powders is 3:1, the rotation speed of the ball mill is 400r / min, the ball milling and mixing time is 4h, and vacuum drying is carried out for 2 hours.

[0022] Further, the substrate is any one of a carbon steel substrate, a stainless steel substrate, and a nickel-based alloy substrate.

[0023] Further, the temperature of the constant temperature electric hot plate is 150 - 300°C.

[0024] The anti-ablative and wear-resistant tungsten alloy gradient coating of the present invention is mainly used in component application scenarios that need to withstand composite harsh conditions such as high-temperature ablation, high-pressure impact, and mechanical wear. Such as aerospace materials, nuclear reactor divertors, etc.

[0025] The advantages of the present invention will be described below in combination with the preparation process

[0026] 1. When the present invention processes the substrate, first, a laser beam with a high energy density from a MOPA laser is used to irradiate the metal surface, causing the rust layer and oxide film on the surface to be instantaneously melted, ablated, evaporated, or peeled off by heat, thereby achieving rapid cleaning. This method does not use any chemical agents, and the residues generated during the cleaning process are very few and are solid harmless powders, which will not cause pollution to the environment. MOPA laser rust removal is a non-contact method, which does not generate mechanical force on the object to be cleaned, has no friction, and the heat conduction is very low, so it will not damage the surface of the metal object. Then, a constant-temperature electric hot plate is used to preheat the pretreated substrate, which can effectively reduce the residual stress in the laser cladding coating. Residual stress is generated during the laser cladding process due to rapid heating and cooling, which may cause the coating to crack. By preheating the substrate, the temperature gradient between the coating and the substrate can be reduced, thereby reducing the residual stress and further reducing the generation of cracks. Experimental studies have shown that as the preheating temperature increases, the average residual stress in the coating gradually decreases, indicating that substrate preheating can effectively reduce the cracking sensitivity of the laser cladding coating. Substrate preheating also leads to changes in heat transfer conditions, which in turn affect the microstructure and mechanical properties of the laser cladding coating. In addition, preheating can change the temperature distribution and cooling rate in the molten pool, thereby affecting the grain growth and phase transformation processes.

[0027] 2. In order to be able to achieve high-efficiency large-area preparation of tungsten alloy gradient coatings, the present invention uses a wide-spot laser cladding technology for preparation. It should be noted that the wide-spot laser cladding technology can accurately control the heat input, thereby achieving additive manufacturing of more complex geometries, with high forming efficiency and freedom, which is very suitable for the manufacturing requirements of complex components. And by changing the composition of the deposited material, the wide-spot laser cladding technology can efficiently produce functional gradient materials. The present invention uses a laser beam with a high energy density as the heat source, which can quickly melt the surface of the base material and the powder material to form a metallurgical-bonded cladding layer. And the laser spot length is longer, and the cladding area per unit time is greatly increased, thereby significantly improving the cladding efficiency, reducing material waste and subsequent processing volume. In addition, a metallurgical bond is formed between the cladding layer and the base material through the laser cladding technology. This bonding method makes the bonding strength between the cladding layer and the base material high, not easy to fall off, and effectively extends the service life of the workpiece. During the wide-spot laser cladding process, since the energy comes from a laser beam with a high energy density, the heat-affected zone of the cladding is small, the deformation of the substrate after cladding is small, and the thickness, width, and composition of the cladding layer can be accurately controlled, and the cladding layer can be customized according to the specific use requirements of the workpiece.

[0028] 3. To ensure that the tungsten alloy gradient coating prepared by wide-spot laser cladding can achieve the designed coating structure, the process parameters of multi-layer wide-spot laser cladding are dynamically adjusted. From the bottom layer to the intermediate layer and then to the top layer, the laser cladding power gradually decreases. This is because with the stacking of the cladding layers, the heat accumulation in the substrate and the deposited layers increases significantly. During each layer of cladding, the laser energy not only acts on the current layer but also generates a cyclic thermal effect on the underlying material through heat conduction. When the substrate temperature rises, the temperature of the molten pool will exceed the critical value under the same laser power, which may lead to an increase in the dilution rate, excessive melting of the substrate, intensified diffusion of alloying elements at the interface between the cladding layer and the substrate, and affect the coating performance; the microstructure coarsens, the cooling rate of the molten pool decreases, the grain size increases, and the hardness of the cladding layer decreases; thermal stress concentration occurs, the temperature gradient increases, and the crack sensitivity improves. To maintain a stable heat input to the molten pool, it is necessary to gradually reduce the laser power to offset the cyclic thermal effect. In terms of the protective gas flow rate, the heat accumulation is relatively large during the cladding of the intermediate layer and the top layer, and the tungsten grains in the intermediate layer and the top layer are fine and prone to oxidation at high temperatures. Therefore, the gas flow rate of the protective gas for the intermediate layer and the top layer is increased to prevent the oxidation of tungsten.

[0029] In summary, the present invention preheats the pretreated substrate using a constant-temperature electric hot plate. On the one hand, it ensures that the preheating temperature is uniform and controllable, and on the other hand, it reduces the generation of residual stress and cracks during the preparation of the gradient coating. The wide-spot laser cladding technology can achieve the preparation of high-efficiency large-area tungsten alloy gradient coatings. At the same time, it can make the refractory and high-density tungsten alloy powder melt as much as possible in the beam, resulting in good coating formation. The energy of the laser is mainly used to melt the powder, making the melting depth of relatively low-melting-point copper, aluminum, magnesium, titanium, and steel substrates controllable, with small deformation. And it can control the sinking and stratification of high-density tungsten alloy in the molten pool, while avoiding excessive mixing of substrate elements into the coating, ensuring the distribution of coating composition and obtaining the expected performance. The present invention finally prepares a tungsten alloy gradient coating on the surface of the substrate, including a three-layer transition structure of the top layer, the intermediate layer, and the bottom layer. Among them, the W grains in the bottom layer are mainly irregular particles, with an average size of 50 - 100 μm, and the NiFe binder phase is relatively independently distributed with a low solid solution degree; the solid solution reaction between W and NiFe begins to occur in the intermediate layer, forming a dendritic structure with a width of 5 - 10 μm, the grain boundaries become blurred, and the shape complexity increases, indicating the initial formation of the solid solution; by the top layer, the solid solution degree is further deepened, the elements of W and NiFe are highly mutually soluble, the degree of grain refinement is significantly improved, the size is 200 - 500 nm, showing a uniform and dispersed distribution, and the dendritic structure disappears, forming a dense W-NiFe solid solution structure. Thus, the coordinated improvement of the strength, toughness, ablation resistance, and wear resistance of the tungsten alloy gradient coating is achieved. Description of the Drawings

[0030] Figure 1 SEM and EDS images of the high-density tungsten alloy powders prepared in Examples 1 - 3 and Comparative Examples 1 - 3.

[0031] Figure 2 Laser confocal images of the surface morphologies of the tungsten alloy gradient coatings prepared in Examples 1 to 3 and Comparative Examples 1 to 3. From a to f are the laser confocal images of the surface morphologies of the tungsten alloy gradient coatings prepared in Example 1, Example 2, Example 3, Comparative Example 1, Comparative Example 2, and Comparative Example 3, respectively.

[0032] Figure 3 SEM and EDS images of the cross-section of the tungsten alloy gradient coating prepared in Example 1.

[0033] Figure 4 XRD patterns at different positions of the tungsten alloy gradient coating prepared in Example 1.

[0034] Figure 5 Friction and wear performance tests of the tungsten alloy gradient coating prepared in Example 1 under different loads in different regions.

[0035] Figure 6 SEM image of the cross-section of the tungsten alloy gradient coating prepared in Comparative Example 1.

[0036] Figure 7 SEM image of the cross-section of the tungsten alloy gradient coating prepared in Comparative Example 2.

[0037] Figure 8 SEM image of the cross-section of the tungsten alloy gradient coating prepared in Comparative Example 3. Detailed implementation manners

[0038] The technical solutions and positive effects of the present invention will be described in detail below with reference to the accompanying drawings and specific examples.

[0039] Example 1

[0040] This example provides an anti-ablation and wear-resistant tungsten alloy gradient coating, and the preparation method is as follows:

[0041] Step 1: Prepare high-density tungsten alloy powder:

[0042] (1) The chemical components in the high-density tungsten alloy powder include W, Ni, Fe, and Co. According to the mass ratio of W: 93%, Ni: 4.6%, Fe: 2.1%, Co: 0.3%, weigh high-purity metal elemental powders with a purity of ≥99.9%.

[0043] (2) Place the weighed W, Ni, Fe, and Co powders of the above mixture into a ball mill for mixing. In the ball mill, zirconia balls are used. When ball milling and mixing, the weight ratio of zirconia balls to the prepared W, Ni, Fe, and Co powders is 4:1. The rotation speed of the ball mill is 400 r / min. At room temperature, the ball milling and mixing time is 4 h to fully mix the powders.

[0044] (3) Put the above-mentioned well-mixed high-density tungsten alloy powder into a vacuum drying oven and dry it for 2 hours to remove the moisture on the surfaces of W, Ni, Fe, and Co powders.

[0045] (4) Place the dried high-density tungsten alloy powder in a rotary synchronous powder feeder for standby.

[0046] Step 2: Pretreatment of the substrate (select Q355 steel):

[0047] (1) Use a MOPA laser to treat the surface of the substrate to make it show a metallic luster. The process parameters of the MOPA laser are: power 1 mJ, pulse width 100 ns, frequency 50 Hz, to remove the rust layer and oxide film on the surface of the substrate.

[0048] (2) Rinse the substrate treated by the MOPA laser with absolute ethanol, and then ultrasonically clean it for 5 min to remove surface impurities. After drying, a pure and dry substrate is obtained.

[0049] (3) Place the above-mentioned substrate on a constant-temperature electric hot plate for preheating, and keep it warm at 300 °C for use.

[0050] Step 3, Wide-spot laser cladding gradient coating

[0051] (1) Use the wide-spot laser cladding technology to prepare the bottom tungsten alloy coating. The specific process parameters are: laser power: 8500 w, spot diameter 30 mm, scanning speed 5 mm / min, cladding overlap rate 10%, powder feeding speed 1.5 r / min, and gas flow rate of the shielding gas 10 L / min.

[0052] (2) Use the wide-spot laser cladding technology to prepare the intermediate tungsten alloy coating. The specific process parameters are: laser power: 7500 w, spot diameter 30 mm, scanning speed 5 mm / min, cladding overlap rate 10%, powder feeding speed 1.5 r / min, and gas flow rate of the shielding gas 15 L / min.

[0053] (3) Use the wide-spot laser cladding technology to prepare the top tungsten alloy coating. The specific process parameters are: laser power: 7000 w, spot diameter 30 mm, scanning speed 5 mm / min, cladding overlap rate 10%, powder feeding speed 1.5 r / min, and gas flow rate of the shielding gas 15 L / min. Finally, an anti-ablation and wear-resistant tungsten alloy gradient coating with a thickness of 1 - 1.5 mm is formed on the surface of the substrate.

[0054] Example 2

[0055] The technical solution adopted in Example 2 is different from that in Example 1 in terms of the laser power during the wide-spot laser cladding gradient coating in Step 3. Specifically, it is reflected in:

[0056] The laser power of the bottom tungsten alloy coating is 8000 W, the laser power of the middle tungsten alloy coating is 7250 W, and the laser power of the top tungsten alloy coating is 6750 W. The remaining cladding parameters are the same as those in Example 1.

[0057] Example 3

[0058] The technical solution adopted in Example 3 is different from that in Example 1 in that the process parameters of the wide-spot laser cladding gradient coating in Step 3 are specifically as follows:

[0059] The laser power of the bottom tungsten alloy coating is 7500 W, the laser power of the middle tungsten alloy coating is 7000 W, and the laser power of the top tungsten alloy coating is 6500 W. The remaining cladding parameters are the same as those in Example 1.

[0060] To prove the importance of dynamically adjusting the process parameters of the wide-spot laser cladding in the preparation of gradient coatings and its influence on the coating properties and structure, three comparative examples were made. The preparation of the high-specific-gravity tungsten alloy powder and the matrix pretreatment process in the comparative examples are the same as those in the three examples of the present invention, and the difference lies only in the process parameters of the wide-spot laser cladding.

[0061] Comparative Example 1

[0062] The difference in the process parameters of the wide-spot laser cladding in Comparative Example 1 from those in Example 1 is that the laser cladding rates of the bottom, middle, and top layers are all changed to 8000 W.

[0063] Comparative Example 2

[0064] The difference in the process parameters of the wide-spot laser cladding in Comparative Example 2 from those in Example 1 is that the laser scanning speeds (i.e., cladding rates) of the bottom, middle, and top layers are reduced to 2, and the powder feeding speeds are all changed to 1 r / min.

[0065] Comparative Example 3

[0066] The difference in the process parameters of the wide-spot laser cladding in Comparative Example 3 from those in Example 1 is that the laser power for preparing the bottom tungsten alloy coating is 7500 W, the laser power of the middle tungsten alloy coating is 7000 W, the laser power of the top tungsten alloy coating is 6000 W, and the scanning speeds of the bottom, middle, and top layers are all changed to 3 mm / min, and the powder feeding speeds are all changed to 1 r / min.

[0067] The cladding process parameters of the three examples and the comparative examples are summarized in Table 1.

[0068] Table 1

[0069]

[0070]

[0071] The morphology, microstructure and properties of the coating are characterized and tested through experiments below.

[0072] (I) Morphology characterization

[0073] Figure 1 The SEM and EDS images of the high-density tungsten alloy powders prepared for three embodiments and three comparative examples of the present invention are shown. It can be seen that the particle sizes of W powder and Ni powder are both 53 - 105 μm, and the particle sizes of Fe powder and Co powder are 15 - 45 μm. Moreover, the fluidity of the powder is good, which can ensure continuous and uniform feeding from the synchronous powder feeder during the experiment.

[0074] Figure 2 The laser confocal images of the surface morphologies of the tungsten alloy gradient coatings prepared for Examples 1 - 3 and Comparative Examples 1 - 3 of the present invention are shown. It can be seen from the figures the effects of different process parameters on the formability, roughness, pores and cracks of the coatings. In terms of formability, a low energy density will lead to incomplete melting of the powder, thereby reducing the metallurgical bonding strength between the coating and the substrate; while an excessively high energy density may cause excessive melting of the substrate, a significant increase in the dilution rate, and damage to the coating composition design. In terms of surface roughness, the surface tension of the molten pool dominates at a low energy density, and spherical protrusions are easily formed. When the energy density increases, the fluidity enhances and the surface tends to be flat. In terms of pores and cracks, when the energy density is relatively low, the fluidity of the molten pool is poor, and the gas escape is blocked, resulting in an increase in the porosity. A higher energy density will intensify the thermal stress concentration and solidification shrinkage, leading to crack propagation.

[0075] Figure 3 The image of the cross-section of the tungsten alloy coating prepared for Example 1 is shown. It can be seen from the figure that when the tungsten alloy coating is cladded by the wide-spot laser cladding technology, the coating shows an obvious gradient layered structure. Among them, the W grains at the bottom layer are mainly irregular particles, with an average size of 50 - 100 μm, and the NiFe binder phase is relatively independent and has a low solid solution degree; the solid solution reaction between W and NiFe starts to occur in the middle layer, forming a dendritic structure with a width of 5 - 10 μm, the grain boundaries become blurred, and the shape complexity increases, indicating the initial formation of the solid solution; the solid solution degree in the top layer of the gradient coating is further deepened, the W and NiFe elements are highly mutually soluble, the grain refinement degree is significantly improved, the size is 200 - 500 nm, showing a uniform dispersion distribution, and the dendritic structure disappears, forming a dense W-NiFe solid solution structure.

[0076] From Figures 6 - 8From the coating structure diagrams obtained from the three comparative examples, it can be seen that in Comparative Example 1, the cladding power did not undergo a gradient decrease. A large amount of fine-grained structure was generated at the top of the coating. However, due to the increase in the cyclic heat accumulation effect, the grains at the bottom of the coating were coarsened and a large amount of brittle dendritic structure was produced, which was not conducive to the improvement of the coating toughness. In Comparative Example 2, the cladding power underwent a gradient decrease, but the cladding rate (i.e., the scanning speed) was reduced outside the range required by the present invention, resulting in an increase in heat input. The thermal stress concentration caused microcracks in the coating. In addition, the increase in heat input made the grain structure of the coating coarser, which was not conducive to the performance improvement. In Comparative Example 3, the laser cladding power was relatively low, the melting degree of W grains was relatively low, a gradient structure could not be formed, and porosity defects were easily generated. The test results of the three comparative examples also confirmed the importance of the dynamic control of process parameters in the present invention from the side.

[0077] (2) Microstructure testing

[0078] Figure 4 Fig. is the XRD pattern at different positions of the tungsten alloy coating prepared in Example 1. The phase analysis of the W alloy sample formed by wide-spot laser cladding was carried out, and the crystal structures of the precipitated phases were obtained. It can be seen from the figure that the diffraction peaks of W, NiFe, Fe2W, and Fe7W6 precipitated phases all appeared in the gradient coating of the W alloy formed by wide-spot laser cladding. Among them, the structures of Fe2W phase and Fe7W6 phase are extremely similar, and there is partial angular overlap of the diffraction peaks; the difference in the positions of their XRD diffraction peaks is mainly reflected at 20-35°, and the diffraction peaks in this angular range are unique to the Fe2W phase only. W has a BCC crystal structure, the NiFe bonding phase has an FCC structure, and the Fe7W6 phase and Fe2W phase have TCP (topologically close-packed hexagonal) structure and HCP (hexagonal close-packed) structure respectively. Due to the small degree of solid solution at the bottom layer, the proportion of W phase is relatively high. The degree of solid solution at the top layer is relatively large, and part of the W phase is transformed into Fe2W and Fe7W6 phases, resulting in a decrease in the peak value of the W phase and an increase in the peak values of Fe2W and Fe7W6 phases.

[0079] (3) Performance testing

[0080] In order to analyze whether the cladding process adopted in the present invention can achieve uniform transition of coating performance, the following takes the coating hardness and coefficient of friction (COF) as examples to illustrate, as Figure 5 As shown in a, the microhardness curve of the coating is divided into three parts: the bottom layer, the middle layer, and the top layer. The microhardness shows an increasing trend layer by layer: the hardness of the bottom layer is 600±20HV, the middle layer rises to 750±20HV, and the top layer further increases to 850±30HV.

[0081] In order to explore the wear resistance of the wide-spot laser cladding gradient coating, the friction and wear performance tests under different loads were carried out on different regions of the coating, asFigure 5 As shown in Figure b. As can be seen from the figure, the top layer of the tungsten alloy gradient coating has the smallest wear volume, wear rate and lowest friction coefficient, showing excellent wear resistance. This is because the W content in the top layer is high and the W particles with greater hardness significantly enhance the wear resistance of the coating.

[0082] The following test is conducted to compare the ablation resistance of the tungsten alloy gradient coating prepared in Example 1 and Comparative Example 1. The specific test method is: using a stable oxygen-acetylene flame flow as a heat source, the flame flow (the flame flow temperature is about 3000°C) is directed at a 90° angle to burn onto a circular sample to ablate the material. At the same time, the back wall temperature of the sample is measured, and the thickness and mass changes of the sample after the test are measured to calculate the linear ablation rate, mass ablation rate and insulation index of the sample. Processed according to national or industry standard dimensions: diameter slightly less than 30mm, optimal size: diameter 29.5mm, reference standard GJB-323A96. Flame heat flux density: 4186.8±418.68Kw / m 2 , oxygen flow rate: 1512L / h, acetylene flow rate: 1116L / h, oxygen pressure: 0.4MPa, acetylene pressure: 0.095MPa, distance from the initial surface of the test to the flame nozzle: 10±0.2mm, flame ablation angle: 90°, flame nozzle diameter: 2mm. The test results are shown in Table 2, from which it can be seen that the ablation resistance of the three embodiments of the present invention is greater than that of the three comparative examples.

[0083] Table 2 (ablation rate of tungsten alloy gradient coating prepared in Examples 1-3 and Comparative Examples 1-3)

[0084] Comparative Example Mass Ablation Rate (%) Linear Ablation Rate (%) Example 1 0.009 0.21 Example 2 0.20 0.25 Example 3 0.31 0.28 Comparative Example 1 0.36 0.31 Comparative Example 2 0.38 0.30 Comparative Example 3 0.42 0.35

[0085] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that are not conceived through creative work should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope defined in the claims.

Claims

1. An ablative-resistant and wear-resistant tungsten alloy gradient coating, characterized in that, It is a three-layer transition structure prepared from high-density tungsten alloy powder through dynamic adjustment of wide-spot laser cladding technology and process parameters; the three-layer transition structure includes a bottom layer, an intermediate layer, and a top layer in sequence from the substrate to the coating surface. From the bottom layer to the top layer, the size of tungsten grains gradually changes from coarse to fine and dispersed, and the solid solution phenomenon gradually strengthens; the dynamic adjustment of the process parameters means that from the bottom layer to the intermediate layer to the top layer, the laser cladding power gradually decreases.

2. The anti-ablative and wear-resistant tungsten alloy gradient coating according to claim 1, wherein The tungsten grains in the bottom layer are mainly irregular particles with an average size of 50-100μm; the intermediate layer has a dendritic structure, and the tungsten grain size is 5-10μm; the tungsten grain size in the top layer is 200-500nm, showing a uniform and dispersed distribution.

3. The anti-ablative and wear-resistant tungsten alloy gradient coating according to claim 1, wherein The chemical components in the high-density tungsten alloy powder include W, Ni, Fe, and Co, and their mass percentages are: W: 85% - 95%, Ni: 3% - 8%, Fe: 1% - 6%, Co: 0.2% - 1%, and the sum of the mass percentages of each component in the high-density tungsten alloy is 100%.

4. The anti-ablative and wear-resistant tungsten alloy gradient coating according to claim 3, wherein The particle size of W powder in the high-density tungsten alloy powder is 53-105μm, the particle size of Ni powder is 53-105μm, the particle size of Fe powder is 15-45μm, and the particle size of Co powder is 15-45μm.

5. A method for preparing an ablative-resistant and wear-resistant tungsten alloy gradient coating according to any one of claims 1-4, characterized in that, It includes the following steps: Step 1: Preparation of molten powder Mix the weighed W, Ni, Fe, and Co powders and grind them to the required fineness. After vacuum drying, place them in a rotary synchronous powder feeder for standby. Step 2: First, use a master oscillator power amplifier laser to pre-treat the substrate, and then use a constant-temperature electric hot plate to preheat the pre-treated substrate. Step 3: Using high-density tungsten alloy powder as the deposited coating material, adopt wide-spot laser cladding technology. Under an inert atmosphere, deposit the high-density tungsten alloy powder on the surface of the pre-treated substrate in a synchronous powder feeding manner to form multiple cladding layers on the surface of the pre-treated substrate. It is required that the laser cladding power gradually decreases from the bottom layer to the top layer.

6. The preparation method of the anti-ablative and wear-resistant tungsten alloy gradient coating according to claim 5, characterized in that, In Step 3, the process parameters of wide-spot laser cladding are as follows: Bottom layer laser power: 7500-8500w, spot diameter is 10-30mm, scanning speed is 3-5mm / min, cladding overlap rate is 10% - 30%, powder feeding speed is 0.5-1.5r / min, and the gas flow rate of the protective gas is 5-10L / min; Intermediate layer laser power: 7000-7500w, spot diameter is 10-30mm, scanning speed is 3-5mm / min, cladding overlap rate is 10% - 30%, powder feeding speed is 0.5-1.5r / min, and the gas flow rate of the protective gas is 10-15L / min; Top layer laser power: 6500-7000w, spot diameter is 10-30mm, scanning speed is 3-5mm / min, cladding overlap rate is 10% - 30%, powder feeding speed is 0.5-1.5r / min, and the gas flow rate of the protective gas is 10-15L / min.

7. The preparation method of the anti-ablative and wear-resistant tungsten alloy gradient coating according to claim 5, characterized in that, In Step 1, the mixing is carried out using a ball mill equipped with zirconia balls. The weight ratio of the zirconia balls to the prepared W, Ni, Fe, and Co powders is 3:

1. The rotational speed of the ball mill is 400 r / min, the ball milling mixing time is 4 h, and vacuum drying is carried out for 2 h.

8. The preparation method of the anti-ablative and wear-resistant tungsten alloy gradient coating according to claim 5, characterized in that, The substrate is any one of a carbon steel substrate, a stainless steel substrate, and a nickel-based alloy substrate.

9. The preparation method of the anti-ablative and wear-resistant tungsten alloy gradient coating according to claim 5, characterized in that, The temperature of the constant temperature electric hot plate is 150 - 300 °C.

10. Application of an anti-ablative and wear-resistant tungsten alloy gradient coating as described in any one of claims 1 - 4 to components subjected to the combined severe conditions of high-temperature ablation, high-pressure impact, and mechanical wear.

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