Laser cladding preparation method of wear-resistant and corrosion-resistant refractory high-entropy alloy coating on surface of Ti6Al4V matrix

Abstract

The invention discloses a laser cladding preparation method of a wear-resistant and corrosion-resistant refractory high-entropy alloy coating on the surface of a Ti6Al4V matrix, which comprises the steps of powder pretreatment, compact preset layer preparation and laser cladding: selecting high-purity Zr, Hf, Nb, Ta and Mo powder, proportioning according to a chemical formula (ZrHfNbTa) Mox, screening, cleaning, ball-milling and mixing to obtain uniformly mixed powder; the mixed powder is preset on the surface of a Ti6Al4V matrix through a binder, and a compact preset layer is prepared; and an optimized laser cladding process is adopted for cladding the preset layer, and the Ti-Zr-Hf-Nb-Ta-Mo refractory high-entropy alloy coating containing Mo is synthesized in situ. According to the method, the content of the Mo element is precisely regulated and controlled, a BCC + HCP double-solid-solution lamellar structure and interface twin crystals are formed through induction, the synergistic effect of lattice distortion strengthening, solid solution strengthening and fine grain strengthening is achieved, the microhardness of the coating is larger than or equal to 650 HV and is improved by 85% or above compared with a base body, and the wear-resisting and corrosion-resisting performance is synergistic and excellent.

Description

Technical Field

[0001] This invention relates to the fields of metal surface engineering and laser additive manufacturing, specifically to a wear-resistant, corrosion-resistant, refractory, high-entropy alloy coating on a Ti6Al4V substrate and its laser cladding preparation method, applicable to fields with stringent requirements for material surface performance, such as aerospace, marine engineering, and high-end machinery manufacturing. Background Technology

[0002] Ti6Al4V (TC4) titanium alloy, as a typical α+β type titanium alloy, has been widely used in aerospace engine blades, marine engineering equipment components, and core components of high-end machinery manufacturing due to its outstanding advantages such as light weight, high specific strength, excellent corrosion resistance, and biocompatibility. However, the Ti6Al4V matrix itself has inherent performance shortcomings: its microhardness is only about 350 HV, resulting in insufficient wear resistance, making it prone to surface scratches, wear spalling, and other failures under friction and wear conditions; at the same time, when serving in highly corrosive environments such as the ocean, although it has a certain degree of corrosion resistance, it still faces the risk of localized corrosion failure when exposed to complex corrosive media for a long time. These problems seriously limit the service life and application range of Ti6Al4V titanium alloy under harsh working conditions.

[0003] To address the insufficient surface properties of the Ti6Al4V matrix, surface strengthening technology has become a key research direction. Laser cladding technology, as an advanced surface modification technique, has significant advantages such as forming a metallurgical bond between the coating and the substrate, high bonding strength, dense microstructure, and strong controllability of coating composition, and has become one of the mainstream technologies for strengthening the surface of titanium alloys.

[0004] Refractory high-entropy alloys, composed of various high-melting-point metallic elements, possess characteristics such as high hardness, high wear resistance, excellent high-temperature stability, and corrosion resistance, making them an ideal material choice for laser cladding coatings. TiZrHfNbTa-based refractory high-entropy alloys, as a typical refractory high-entropy alloy system, have been attempted for surface strengthening of Ti6Al4V substrates, but several problems remain in practical applications: First, the hardness improvement is limited, making it difficult to meet the demand for high-hardness surfaces under harsh working conditions; second, the wear resistance is insufficient, resulting in a high wear rate and hindering long-term stable service; third, the elemental ratio optimization is unclear, particularly the amount of Mo added lacks systematic and in-depth research.

[0005] Specifically, Mo, as an important alloying element, significantly affects the performance of TiZrHfNbTa-based refractory high-entropy alloy coatings. Insufficient Mo (e.g., no Mo) results in insufficient lattice distortion, weak solid solution strengthening, and minimal improvement in coating hardness and wear resistance. Excessive Mo, during high-temperature cladding and subsequent service, readily combines with oxygen to form weakly protective oxides like MoO2. These oxides have a porous structure and cannot form a dense oxide film, leading to oxide film peeling during friction. This not only reduces the coating's wear resistance but also further affects its corrosion resistance. Therefore, there is an urgent need to develop a laser cladding method for preparing wear-resistant and corrosion-resistant refractory high-entropy alloy coatings on Ti6Al4V substrates that can achieve a synergistic improvement in coating hardness, wear resistance, and corrosion resistance through precise control of Mo content. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing TiZrHfNbTa-based refractory high-entropy alloy coatings, such as insufficient hardness, poor wear resistance, and unreasonable element ratios, and to provide a laser cladding method for preparing a wear-resistant and corrosion-resistant refractory high-entropy alloy coating on a Ti6Al4V substrate. This is achieved through precise optimization of (ZrHfNbTa)Mo... x By determining the molar fraction range of Mo in the system, and combining specific powder pretreatment processes, dense pre-layer preparation processes, and laser cladding process parameters, a high-performance refractory high-entropy alloy coating with a BCC+HCP double solid solution lamellar structure and an interfacial twin structure is constructed on the surface of Ti6Al4V substrate using in-situ laser cladding reaction. This achieves synergistic optimization of coating hardness, wear resistance, and corrosion resistance, significantly improving the service performance and service life of Ti6Al4V substrate under harsh working conditions.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A laser cladding method for preparing a wear-resistant, corrosion-resistant, refractory high-entropy alloy coating on a Ti6Al4V substrate includes the following steps: (1) Powder pretreatment: Zr, Hf, Nb, Ta, and Mo powders with a purity ≥ 99.9 wt.% were selected as raw materials; according to the chemical formula (ZrHfNbTa)Mo x The powder is weighed according to a ratio of 1:1:1:1, where x is the molar fraction of Mo relative to any one of Zr, Hf, Nb, or Ta, and 0.2 ≤ x ≤ 1.0. The weighed powder is then sieved, washed, and ball-milled to obtain a mixed powder with a uniformity > 95%. (2) Preparation of dense pre-layer: The surface of Ti6Al4V substrate is polished and cleaned; the mixed powder prepared in step (1) is pre-placed on the substrate surface using an adhesive, and after drying, a dense pre-layer with a density >90% is prepared. (3) Laser cladding: The pre-placed layer is clad using laser cladding technology under the protection of inert argon gas. Ti elements in the matrix diffuse into the coating through melting in the molten pool, and a refractory high entropy alloy coating containing Mo, TiZrHfNbTaMo, is synthesized in situ. The refractory high entropy alloy coating is mainly composed of BCC solid solution phase and HCP solid solution phase. The microstructure presents a lamellar structure, and a twin structure is formed at the interface between the coating and the matrix. The microhardness of the coating is ≥650HV.

[0008] Furthermore, in step (1), the preferred range of the molar number x of the Mo element is 0.5≤x≤0.8, and the most preferred value is x=0.75.

[0009] Further, in step (1), the sieving operation specifically removes particles with a particle size >150μm and <1μm; the cleaning is ultrasonic cleaning with ethanol for 15-25 minutes; the ball milling is carried out under argon protection, and the ball milling parameters are: rotation speed 200-400rpm, ball milling time 10-14 hours.

[0010] Further, in step (2), the adhesive is a polyvinyl alcohol aqueous solution with a mass fraction of 3-5%; the drying conditions are: drying temperature 70-90℃, drying time 0.5-2 hours; the thickness of the pre-formed layer is controlled at 0.6-1.0 mm.

[0011] Furthermore, in step (2), the mixed powder is pre-placed on the surface of the substrate by pressing, with a pressing pressure of 25-35 MPa and a holding time of 2-4 minutes.

[0012] Furthermore, in step (3), the specific process parameters of the laser cladding are: laser power 2.5-3.5kW, scanning speed 4-6mm / s, defocusing amount +10~+20mm, and spot diameter 5-7mm.

[0013] Furthermore, the laser cladding employs a fiber laser system, and the laser beam is a continuous wave laser.

[0014] Furthermore, in step (3), the protective flow rate of argon gas is 10-20 L / min to ensure that the molten pool and coating are not oxidized during the cladding process.

[0015] Furthermore, a wear-resistant and corrosion-resistant refractory high-entropy alloy coating on the surface of a Ti6Al4V substrate obtained by the above preparation method is provided. The coating has a microhardness of 650-760 HV, a friction coefficient that is more than 25% lower than that of the Ti6Al4V substrate under dry friction environment, and a corrosion potential ≥-0.35V in 3.5wt.% NaCl solution.

[0016] Furthermore, when the molar fraction of Mo is x=0.75, the microhardness of the coating reaches 718-720HV, the coefficient of friction is reduced by 35% compared with the Ti6Al4V matrix under dry friction environment, and the corrosion potential in 3.5wt.%NaCl solution is about -0.289V.

[0017] Compared with the prior art, the present invention has the following significant advantages: By precisely controlling the amount of Mo and combining it with an optimized preparation process, a BCC+HCP double solid solution lamellar structure and an interfacial twin structure were successfully constructed on the surface of a Ti6Al4V substrate. The lamellar structure significantly refined the coating grains, increased grain boundary density, and hindered dislocation slip; the interfacial twin structure effectively alleviated the residual stress generated during laser cladding, preventing defects such as cracks in the coating, while also improving the structural stability of the coating. Compared with traditional TiZrHfNbTa coatings, the coating of this invention has a more uniform and dense microstructure, laying a solid structural foundation for performance improvement.

[0018] The coating of this invention achieves a significant increase in hardness through a synergistic strengthening mechanism involving lattice distortion strengthening, solid solution strengthening, and grain refinement strengthening. The appropriate addition of Mo induces significant lattice distortion, creating a distorted stress field in the crystal structure and hindering dislocation movement. Mo dissolves in BCC and HCP solid solutions, forming a solid solution strengthening effect. The lamellar and twinned structures refine the grains, resulting in grain refinement strengthening. Under the synergistic effect of these three factors, the coating's microhardness is ≥650 HV, an increase of over 85% compared to the Ti6Al4V matrix (approximately 350 HV). When x=0.75, the hardness reaches 718.24 HV, an increase of 103.7% compared to the matrix, far exceeding the hardness level of existing TiZrHfNbTa coatings.

[0019] The lamellar and twinned structures of the coating in this invention enhance its wear resistance and reduce material loss during friction. Simultaneously, the high-hardness coating surface effectively resists abrasive cutting and grooving, reducing the wear rate. Under dry friction conditions, the average coefficient of friction of the coating is reduced by more than 25% compared to the Ti6Al4V substrate, with a 35% reduction at x=0.75. The wear mechanism is characterized by mild adhesive and oxidative wear, with no significant peeling.

[0020] In terms of corrosion resistance, the BCC+HCP double solid solution structure formed by the coating exhibits excellent chemical stability. Furthermore, it can form a dense and stable composite oxide film during service, effectively preventing the intrusion of corrosive media. In a 3.5 wt.% NaCl solution, the corrosion potential of the coating is ≥-0.35V, reaching -0.289V at x=0.75. This demonstrates superior corrosion resistance compared to the Ti6Al4V matrix and coatings without Mo addition, effectively resisting erosion in corrosive environments such as marine environments.

[0021] The preparation process is stable, reliable, and highly applicable. The preparation method of this invention has a clear flow, and each process parameter has been systematically optimized, exhibiting good repeatability and stability. The powder pretreatment process ensures raw material quality, the dense pre-layer preparation process improves the bonding quality between the coating and the substrate, and the laser cladding process parameter window is reasonable, facilitating industrial application. This method is suitable for surface strengthening of Ti6Al4V substrate components in aerospace, marine engineering, and high-end machinery manufacturing fields. It can optimize coating performance by adjusting the Mo element content (within the range of 0.2 ≤ x ≤ 1.0) according to different working conditions, demonstrating broad applicability.

[0022] With a scientifically designed composition, this invention addresses the pain points of existing technologies. Through a systematic study of the influence of Mo addition on coating performance, it clarifies the optimal molar ratio range of Mo (0.2≤x≤1.0, preferably 0.5≤x≤0.8, most preferably x=0.75), solving the problem of unclear Mo addition amounts in existing TiZrHfNbTa coatings. This approach avoids the insufficient hardness and wear resistance caused by too little Mo, while overcoming the defects of oxide film peeling, decreased wear stability, and corrosion resistance caused by too much Mo. It achieves the optimal balance between coating hardness, wear resistance, and corrosion resistance, providing a scientific basis for the composition optimization of refractory high-entropy alloy coatings.

[0023] To more clearly illustrate the structural features and effects of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Attached Figure Description

[0024] Figure 1 The bar chart shows the microhardness comparison of refractory high-entropy alloy coatings with different Mo contents (x=0~1.0) and Ti6Al4V substrates prepared in the embodiments and comparative examples of the present invention. Figure 2 This is a bar chart showing the average friction coefficient of coatings with different Mo contents and Ti6Al4V substrates prepared in the embodiments and comparative examples of the present invention under dry friction environment; Figure 3The diagram shows a comparison of the potentiodynamic polarization curves of coatings with different Mo contents and Ti6Al4V substrates prepared in the embodiments and comparative examples of the present invention in 3.5 wt.% NaCl solution. Figure 4 The image shows a cross-sectional microstructure (SEM) of the coating (Mo0.75) of Example 1 of this invention, revealing its characteristic lamellar structure and interfacial twins. Detailed Implementation

[0025] The present invention will now be further described in conjunction with the accompanying drawings and relevant knowledge, and will be described clearly and completely. Obviously, the described applications are only some embodiments of the present invention, and not all embodiments.

[0026] This invention discloses a laser cladding method for preparing a wear-resistant, corrosion-resistant, refractory high-entropy alloy coating on a Ti6Al4V substrate, comprising powder pretreatment, dense pre-layer preparation, and laser cladding steps: selecting high-purity Zr, Hf, Nb, Ta, and Mo powders, and cladding them according to the chemical formula (ZrHfNbTa)Mo x A homogeneous powder was obtained by sieving, washing, and ball milling in a ratio of (0.2≤x≤1.0). The powder was then pre-placed onto the surface of a Ti6Al4V substrate using a binder to prepare a dense pre-placed layer. Under argon protection, an optimized laser cladding process was used to clad the pre-placed layer, resulting in the in-situ synthesis of a Mo-containing TiZrHfNbTaMo refractory high-entropy alloy coating. This invention, through precise control of the Mo element content, induces the formation of a BCC+HCP double solid solution lamellar structure and interfacial twins, achieving a synergistic effect of lattice distortion strengthening, solid solution strengthening, and grain refinement strengthening. The coating exhibits a microhardness ≥650HV, an improvement of over 85% compared to the substrate, and excellent synergistic wear and corrosion resistance. The preparation process of this invention is stable and reliable, suitable for aerospace, marine engineering, and other fields, and has broad application prospects.

[0027] Specifically, this includes: reference Figures 1-4 As shown, a laser cladding method for preparing a wear-resistant, corrosion-resistant, refractory high-entropy alloy coating on a Ti6Al4V substrate includes the following steps: (1) Powder pretreatment: Zr, Hf, Nb, Ta, and Mo powders with a purity ≥ 99.9 wt.% are selected as raw materials. High-purity raw materials can avoid coating defects (such as pores and cracks) caused by the introduction of impurity elements, and ensure the uniformity of coating composition and performance stability; according to the chemical formula (ZrHfNbTa)Mo xThe powder was weighed and proportioned, with the molar ratio of Zr, Hf, Nb, and Ta being 1:1:1:1. These four high-melting-point elements form the matrix framework of the refractory high-entropy alloy, giving the coating good high-temperature stability and structural foundation. x represents the molar fraction of Mo relative to any one of Zr, Hf, Nb, or Ta, and 0.2 ≤ x ≤ 1.0. This range ensures that Mo plays an effective strengthening role while avoiding performance shortcomings caused by excessive or insufficient amounts. The weighed powder was then sequentially sieved, washed, and ball-milled to obtain a mixed powder with a uniformity >95%. The high uniformity of the mixed powder ensures that the elements react fully during the subsequent cladding process, avoids component segregation, and ensures the consistency of coating performance.

[0028] Preparation of dense pre-layer: The surface of Ti6Al4V substrate is polished and cleaned to remove surface oxide scale, oil and impurities, improve the surface roughness and cleanliness of the substrate, and enhance the bonding force between the pre-layer and the substrate; the mixed powder prepared in step (1) is pre-placed on the substrate surface using an adhesive, and after drying, a dense pre-layer with a density >90% is prepared. The high-density pre-layer can avoid powder splashing and coating cracking during the cladding process, ensure that the powder is fully melted during cladding, and reduce the dilution rate of the molten pool, thus ensuring the design composition and performance of the coating.

[0029] (3) Laser cladding: The pre-placed layer is clad using laser cladding technology under the protection of inert argon gas. Argon protection can effectively isolate air and prevent oxidation of the coating in the molten pool and during the solidification process, ensuring the stability of the coating's chemical composition and microstructure. Through molten pool melting, Ti elements in the matrix diffuse into the coating, and a Mo-containing TiZrHfNbTaMo refractory high-entropy alloy coating is synthesized in situ. The metallurgical bonding method greatly improves the bonding strength between the coating and the matrix, avoiding coating peeling during service. The refractory high-entropy alloy coating is mainly composed of BCC solid solution phase and HCP solid solution phase. The microstructure presents a lamellar structure, and a twin structure is formed at the interface between the coating and the matrix. The lamellar structure can refine the grains, increase the grain boundary area, and hinder dislocation movement. The twin structure can relieve residual stress and improve the coating toughness. The two work together to improve the mechanical properties of the coating. The microhardness of the coating is ≥650HV, which is more than 85% higher than that of the matrix (about 350HV), significantly enhancing the coating's wear resistance.

[0030] In a preferred embodiment of the present invention, in step (1), the preferred range of the molar fraction x of the Mo element is 0.5≤x≤0.8, and the most preferred range is x=0.75. This preferred range can cause the Mo element to induce significant lattice distortion, enhance the solid solution effect, and induce the formation of BCC+HCP lamellar structure and interface twins, effectively relieving residual stress, so that the microhardness of the coating can reach more than 2 times that of the substrate, and the wear resistance and corrosion resistance achieve the best synergy. Among them, the comprehensive performance is optimal when x=0.75.

[0031] In a preferred embodiment of the present invention, in step (1), the sieving operation specifically removes particles with a particle size >150μm and <1μm. Particles with a particle size >150μm are prone to uneven mixing, poor density of the pre-formed layer, and are difficult to completely melt to form unfused defects. Particles with a particle size <1μm are prone to agglomeration, affecting the uniformity of mixing and the quality of the pre-formed layer. After sieving, powder with good flowability and suitable for cladding process can be obtained. The cleaning is ultrasonic cleaning with ethanol, and the cleaning time is 15-25 minutes. This cleaning method can efficiently remove oil stains, dust and other impurities adsorbed on the powder surface, further improve the purity of the powder, and avoid the adverse effects of impurities on the cladding reaction and coating performance. The ball milling is carried out under argon protection. The ball milling parameters are: rotation speed 200-400rpm, ball milling time 10-14 hours. Argon protection can prevent powder oxidation during ball milling. Specific ball milling parameters can ensure that the powder mixing uniformity is >95%, while avoiding excessive powder breakage or cold welding agglomeration.

[0032] In a preferred embodiment of the present invention, in step (2), the adhesive is a 3-5% (w / w) polyvinyl alcohol aqueous solution. This adhesive has excellent bonding performance and can be completely decomposed during drying and cladding, leaving no residual impurities to contaminate the coating. The drying conditions are: drying temperature 70-90℃ and drying time 0.5-2 hours. These parameters can fully remove moisture from the pre-formed layer, avoid moisture vaporization during cladding leading to porosity defects in the coating, and ensure that the adhesive plays a stable bonding role. The thickness of the pre-formed layer is controlled at 0.6-1.0 mm. This thickness range can ensure that the powder is completely melted during laser cladding to form a coating thickness that meets the requirements for use, while avoiding the reduction in cladding efficiency due to excessive thickness or the insufficient performance due to excessive thinness.

[0033] In a preferred embodiment of the present invention, in step (2), the mixed powder is pre-placed on the surface of the substrate by pressing. The pressing pressure is 25-35 MPa and the holding time is 2-4 minutes. The pressing process can further improve the initial density and compactness of the pre-placed layer, enhance the adhesion between the pre-placed layer and the substrate, avoid the pre-placed layer from falling off and cracking during the drying and cladding process, and provide a stable reaction substrate for subsequent laser cladding.

[0034] In a preferred embodiment of the present invention, the process parameters of the laser cladding in step (3) are as follows: laser power 2.5-3.5kW, scanning speed 4-6mm / s, defocusing amount +10~+20mm, and spot diameter 5-7mm. This parameter window is optimized to ensure that the molten pool is fully melted without overheating, avoids excessive dilution caused by excessive melting of the substrate, and provides suitable thermodynamic and kinetic conditions for the formation of BCC+HCP layered structure and twins, ensuring the optimization of the coating microstructure and the stability of its performance.

[0035] In a preferred embodiment of the present invention, the laser cladding adopts a fiber laser system, and the laser beam is a continuous wave laser. Fiber lasers have the characteristics of high energy density, good stability and excellent beam quality. Continuous wave lasers can provide a continuous and stable heat source, ensuring a uniform temperature field in the molten pool, avoiding problems such as uneven cladding layer and poor bonding that may be caused by pulsed lasers, and improving the repeatability and reliability of coating preparation.

[0036] In a preferred embodiment of the present invention, in step (3), the protective flow rate of argon gas is 10-20 L / min. This flow rate range can form a stable protective atmosphere, completely covering the cladding area, ensuring that the molten pool and coating are not oxidized during the cladding process, avoiding oxidation products (such as MoO2) from affecting the microstructure and performance of the coating, and ensuring the wear and corrosion resistance of the coating.

[0037] In a preferred embodiment of the present invention, a wear-resistant and corrosion-resistant refractory high-entropy alloy coating on the surface of a Ti6Al4V substrate obtained by the above preparation method is provided. The microhardness of the coating is 650-760 HV, which is significantly improved compared to the substrate. It can effectively resist friction and wear. The coefficient of friction in a dry friction environment is reduced by more than 25% compared to the Ti6Al4V substrate, reducing energy loss and material wear during service. The corrosion potential in 3.5wt.% NaCl solution is ≥-0.35V, exhibiting excellent corrosion resistance. It can effectively resist the erosion of corrosive environments such as the ocean and extend the service life of the substrate.

[0038] In a preferred embodiment of the present invention, when the molar fraction of Mo is x=0.75, the microhardness of the coating reaches 718-720 HV, which is more than 100% higher than that of the substrate, and the hardness enhancement effect is optimal. Under dry friction environment, the coefficient of friction is reduced by 35% compared with Ti6Al4V substrate, the wear rate is significantly reduced, and the wear mechanism is more moderate (mainly adhesive wear and oxidative wear). The corrosion potential in 3.5wt.% NaCl solution is about -0.289V, with the lowest corrosion tendency. The formed composite oxide film is dense and stable, and the wear resistance and corrosion resistance achieve the best synergistic effect, meeting the use requirements under harsh working conditions.

[0039] The more specific preparation process of this invention is as follows: A laser cladding method for preparing a wear-resistant, corrosion-resistant, refractory high-entropy alloy coating on a Ti6Al4V substrate includes the following steps: For powder pretreatment, Zr, Hf, Nb, Ta, and Mo powders with a purity ≥ 99.9 wt.% are selected as raw materials. High-purity raw materials can avoid the adverse effects of impurity elements on coating performance and reduce defects caused by impurities (such as porosity and cracks). According to the chemical formula (ZrHfNbTa)Mo xThe proportions and weighings are performed, with the four high-melting-point elements Zr, Hf, Nb, and Ta maintained in an equimolar ratio (1:1:1:1). These four elements constitute the matrix framework of the refractory high-entropy alloy, ensuring that the coating has good high-temperature stability and structural integrity. x is the molar fraction of Mo relative to any one of Zr, Hf, Nb, or Ta, and its value ranges from 0.2 to 1.0.

[0040] The weighed powder is sieved to remove particles with a diameter >150μm and <1μm. Particles with an excessively large diameter are prone to uneven mixing, poor density of the pre-formed layer, and are difficult to completely melt during laser cladding, resulting in incomplete fusion defects. Particles with an excessively small diameter are prone to agglomeration, which also affects the uniformity of mixing and the quality of the pre-formed layer. After sieving, powder with a diameter of 1-150μm is selected, preferably powder with a diameter of 48-150μm. Powder in this diameter range has good flowability, which is convenient for subsequent mixing and pre-formed layer preparation.

[0041] The sieved powder is placed in ethanol for ultrasonic cleaning for 15-25 minutes, preferably 20 minutes. Ultrasonic cleaning effectively removes oil, dust, and other impurities adsorbed on the powder surface, further improving powder purity and preventing impurities from forming defects during the cladding process. After cleaning, the powder is dried to remove any residual ethanol from the surface.

[0042] The dried powder is placed in a ball mill and mixed under argon protection. The ball milling parameters are: rotation speed 200-400 rpm, ball milling time 10-14 hours. Argon protection can prevent the powder from being oxidized during ball milling, ensuring the chemical activity of the powder; the specific ball milling speed and time can ensure that the powder is mixed evenly, with a mixing uniformity >95%, so that the elements can fully react in the subsequent cladding process to form a coating with uniform composition.

[0043] The preparation of the dense pre-coated layer involves polishing the surface of the Ti6Al4V substrate using sandpaper, progressing from coarse to fine, to remove oxide scale, rust, and oil, thereby increasing the surface roughness and enhancing the adhesion between the subsequent pre-coated layer and the substrate. After polishing, the substrate is cleaned in a cleaning agent to remove residual polishing debris and oil, and then dried for later use.

[0044] A 3-5% (w / w) polyvinyl alcohol aqueous solution was selected as the binder. Polyvinyl alcohol aqueous solution has good bonding properties and can be completely decomposed during subsequent drying and laser cladding processes, leaving no residual impurities that affect the coating performance. The mixed powder prepared in step (1) was thoroughly stirred with the binder and then pre-placed on the surface of the Ti6Al4V substrate by pressing. The pressing pressure was 25-35 MPa, and the holding time was 2-4 minutes. The pressing process can improve the initial density of the pre-placed layer and avoid cracking and peeling during the drying and cladding processes.

[0045] The substrate pre-mixed with powder is placed in a drying oven for drying at 70-90℃ for 0.5-2 hours. This drying process removes moisture from the pre-mixed layer, further improving its density, ultimately producing a dense pre-mixed layer with a density >90% and a thickness of 0.6-1.0 mm. Controlling the thickness of the pre-mixed layer within this range ensures complete powder melting during laser cladding, while avoiding reduced cladding efficiency or poor adhesion between the coating and the substrate due to an excessively thick layer. Conversely, an excessively thin layer will not achieve the required coating thickness.

[0046] Laser cladding employs a fiber laser system (preferably the YSL-5000 fiber laser system), with a continuous-wave laser beam. The pre-formed layer is clad under the protection of an inert argon gas atmosphere. The argon gas flow rate is 10-20 L / min, effectively isolating the molten pool and the solidifying coating from air oxidation, thus ensuring the chemical composition and performance stability of the coating.

[0047] The specific process parameters for laser cladding are: laser power 2.5-3.5kW, scanning speed 4-6mm / s, defocusing distance +10~+20mm, and spot diameter 5-7mm. This parameter window has been optimized through extensive experimentation to ensure sufficient melting of the molten pool without overheating. Too low a laser power results in incomplete powder melting, leading to incomplete fusion defects; too high a power causes excessive melting of the substrate, resulting in excessive dilution and affecting coating performance, and may also cause defects such as cracks. Too slow a scanning speed leads to excessive heat input and coarse coating grains; too fast a scanning speed results in excessively rapid molten pool solidification and insufficient element diffusion, also affecting coating performance. Reasonable control of the defocusing distance and spot diameter ensures uniform laser energy distribution and a stable action area, which is beneficial for the formation of the lamellar structure of the BCC+HCP double solid solution and interfacial twins.

[0048] During the laser cladding process, the pre-placed powder melts rapidly under the high temperature of the laser to form a molten pool. At the same time, micro-melting also occurs on the surface of the substrate. The Ti element in the Ti6Al4V substrate diffuses into the molten pool and reacts in situ with the Zr, Hf, Nb, Ta, and Mo elements in the molten pool, eventually solidifying to form a Mo-containing TiZrHfNTa-Mo refractory high-entropy alloy coating.

[0049] In this invention, by precisely controlling (ZrHfNbTa)Mo xThe molar percentage (x) of Mo in the system allows for the control of the coating's microstructure and properties. Equal molar proportions of Zr, Hf, Nb, and Ta constitute the refractory high-entropy alloy matrix framework, ensuring the coating's high-temperature stability and structural foundation. Mo, as a key alloying element, enhances the coating's hardness and wear resistance through solid solution strengthening and lattice distortion, while simultaneously controlling the coating's phase composition and microstructure.

[0050] When x = 0.2-1.0, a BCC+HCP dual solid solution structure can be formed, and the coating microhardness is ≥650HV, which is more than 85% higher than that of the Ti6Al4V matrix (approximately 350HV). When x = 0.5-0.8, the coating performance reaches its optimal level, with a microhardness ≥655HV and synergistic improvement in wear resistance and corrosion resistance. When x = 0.75, the coating microhardness reaches more than 718HV, which is more than 103% higher than that of the matrix, the dry friction coefficient is reduced by 35%, and the corrosion potential in 3.5wt.% NaCl solution reaches -0.289V, exhibiting the best comprehensive performance.

[0051] When x≥0.2, the addition of Mo can produce a significant lattice distortion effect, which can improve the hardness and strength of BCC and HCP solid solutions through solid solution strengthening. When x≤1.0, excessive Mo can avoid the formation of weakly protective oxides such as MoO2, ensuring the density and stability of the oxide film and preventing the oxide film from peeling off during friction.

[0052] In this invention, the purity, uniformity and flowability of the powder are improved through steps such as sieving, washing and ball milling, providing high-quality raw materials for subsequent preparation of dense pre-layer and laser cladding.

[0053] Screening removes particles of unqualified size to avoid defects such as pores and incomplete fusion in the pre-formed layer; ultrasonic cleaning with ethanol removes impurities from the powder surface and improves the chemical purity of the powder; ball milling under argon protection ensures that the powder uniformity is >95%, guaranteeing that all elements react fully during the cladding process to form a coating with uniform composition and stable performance.

[0054] In this invention, the combination of high-purity, highly uniform powder and optimized composition design ensures that the role of Mo is fully utilized, avoiding uneven distribution of Mo due to powder impurities or uneven mixing, which would affect the lattice distortion effect and phase composition. At the same time, high-quality powder helps to improve the compactness of the pre-formed layer, providing favorable conditions for the stable formation of the molten pool and element diffusion during laser cladding.

[0055] In this invention, a high-density, uniformly thick pre-layer is prepared to enhance the bonding force between the pre-layer and the substrate, providing a stable reaction area for laser cladding and reducing defects in the cladding process.

[0056] Polyvinyl alcohol aqueous solution is used as a binder to ensure the bonding strength of the pre-formed layer and to ensure that there are no residual impurities. The pressing process and drying treatment ensure that the density of the pre-formed layer is >90% and the thickness is controlled at 0.6-1.0mm to avoid problems such as powder splashing and coating cracking during the cladding process, while ensuring the metallurgical bonding quality between the coating and the substrate.

[0057] The dense pre-layer enables the laser energy to be applied more concentratedly to the powder melting. Combined with optimized laser cladding process parameters, it ensures that the molten pool is fully melted without overheating, which is conducive to the formation of BCC+HCP layered lamellar structure and interface twins. At the same time, the good bonding between the pre-layer and the substrate can promote the diffusion of Ti elements from the substrate to the molten pool and fully react with Zr, Hf, Nb, Ta and Mo elements to achieve in-situ synthesis of high-performance coatings.

[0058] In this invention, by controlling parameters such as laser power, scanning speed, and defocusing amount, the temperature field, flow field, and solidification rate of the molten pool are adjusted, thereby achieving microstructure optimization and performance improvement of the coating.

[0059] The combination of parameters—2.5-3.5kW laser power, 4-6mm / s scanning speed, and +10~+20mm defocusing amount—ensures that the molten pool is fully melted and the dilution rate is moderate, avoiding over-melting of the substrate or incomplete fusion of the powder. This parameter window is conducive to twin growth and the formation of lamellar structures, refining the grains and further improving the coating's hardness and wear resistance.

[0060] Optimized laser cladding process parameters, combined with powder composition design, can maximize the lattice distortion effect and solid solution strengthening effect of Mo, inducing the formation of BCC+HCP double solid solution structure. At the same time, combined with a dense pre-layer, it can reduce oxidation and defects during the cladding process, ensure the density and stability of the coating, and achieve a synergistic improvement in hardness, wear resistance and corrosion resistance.

[0061] In this invention, the lamellar and twinned structures refine the grain size, improve the strength and hardness of the coating, and alleviate residual stress generated during the cladding process, thereby enhancing the coating's toughness and wear resistance. The lamellar structure significantly refines the grain size, increases the grain boundary area, hinders dislocation movement, and improves the coating's hardness; the twinned structure disperses stress concentration, preventing cracking and peeling of the coating under frictional and corrosive conditions, while also improving the coating's wear resistance and corrosion resistance.

[0062] This microstructure is the result of precise control of Mo elements, high-quality powder pretreatment, preparation of a dense pre-layer, and optimized laser cladding process parameters. The appropriate addition of Mo is the core factor inducing this structure, while the optimized powder pretreatment and pre-layer preparation processes provide a sound material basis for its formation. Reasonable laser cladding process parameters provide suitable thermodynamic and kinetic conditions for the structure's formation. These technical features work together to ultimately achieve synergistic optimization of the coating's microstructure and properties.

[0063] The present invention will be further described in detail below with reference to specific embodiments and accompanying drawings, in order to more intuitively demonstrate the rationality, performance advantages and microstructural features of the technical solution.

[0064] Example 1 (x=0.75, optimal solution) 1. Preparation process Raw material preparation: Select Zr, Hf, Nb, Ta and Mo powders with a purity ≥99.9wt.%, and control the powder particle size to 48~150μm (this particle size range is obtained after sieving to remove particles >150μm and <1μm, which ensures flowability and avoids agglomeration or non-fusion defects).

[0065] The powder was precisely weighed according to the stoichiometric ratio Zr:Hf:Nb:Ta:Mo=1:1:1:1:0.75 to ensure that the molar fraction of Mo is x=0.75, which is in line with the optimal performance range of the present invention.

[0066] Powder pretreatment: The weighed mixed powder is placed in an ethanol solution and ultrasonically cleaned for 20 minutes to thoroughly remove oil, dust and other impurities adsorbed on the powder surface; after cleaning and drying, it is placed in a ball mill and ball-milled at 300 rpm for 12 hours under an argon protective atmosphere to finally obtain a mixed powder with a mixing uniformity of >95%, ensuring uniform diffusion and reaction of each element in the subsequent cladding process.

[0067] Preparation of dense pre-formed layer: First, the surface of Ti6Al4V substrate is polished step by step (to remove oxide scale and rust), and then cleaned and dried to improve the cleanliness and roughness of the substrate surface; then, 4wt.% polyvinyl alcohol aqueous solution is applied as a binder, and the above mixed powder is evenly spread on the substrate surface, pressed with 30MPa pressure and held for 3 minutes, and finally dried in an 80℃ drying oven for 1 hour to form a dense pre-formed layer with a thickness of 0.8mm and a density of 92%, which provides a stable reaction substrate for laser cladding.

[0068] Laser cladding: A YSL-5000 fiber laser system is used for single-pass cladding under argon protection (to prevent oxidation of the molten pool). The laser cladding process parameters are precisely set as follows: laser power 3kW, scanning speed 5mm / s, defocusing amount +15mm, and spot diameter 6mm. This combination of parameters ensures that the molten pool is fully melted without overheating, providing a suitable thermodynamic environment for microstructure optimization.

[0069] Performance testing and analysis: (1) Microhardness analysis Figure 1 The bar chart shows the microhardness comparison between coatings with different Mo contents and the Ti6Al4V substrate (labeled TC4). The microhardness of the Ti6Al4V substrate is only 352.51 HV, while the microhardness of the coating in this embodiment (x=0.75) reaches 718.24 HV, an increase of 103.7% compared to the substrate, effectively doubling the hardness. Compared with other groups: the group without Mo addition (x=0) has a hardness of only 552.74 HV, with limited improvement; the group with excess Mo (x=1.0) has a hardness of 755.42 HV, but its subsequent wear resistance stability is insufficient; the hardness of this embodiment is in the optimal range, and it forms a synergistic advantage with wear resistance and corrosion resistance, verifying the rationality of x=0.75.

[0070] (2) Wear resistance analysis Figure 2 The bar chart shows the average friction coefficients of coatings and substrates with different Mo contents under dry friction conditions. A lower friction coefficient indicates better wear resistance. The average friction coefficient of the Ti6Al4V substrate is 1.22, while the average friction coefficient of the coating in this embodiment (x=0.75) drops to 0.78, a 35% reduction compared to the substrate, resulting in a significant decrease in wear rate. Figure 2 It can be clearly seen that the friction coefficient of group x=0 (without Mo) is 1.13, which is only slightly lower than that of the substrate; the friction coefficient of this embodiment is the lowest of all groups, and no oxide film peeling occurs during the wear process. The wear mechanism is mild adhesive wear and oxidative wear, which reflects the best wear resistance stability.

[0071] (3) Corrosion resistance analysis Figure 3 The diagram shows a comparison of the potentiodynamic polarization curves of coatings and substrates with different Mo contents in a 3.5 wt.% NaCl solution. A more positive corrosion potential (closer to 0V) indicates better corrosion resistance. In this example (x=0.75), the corrosion potential of the coating is -0.289V. The composite oxide film formed by the coating in this example is dense and stable, effectively blocking Cl. - Erosion resistance significantly enhances its serviceability in corrosive environments such as the ocean.

[0072] (4) Microstructure analysis Figure 4 The SEM image (including local magnification and crystallographic calibration) of the cross-sectional microstructure of the coating in this embodiment visually demonstrates the characteristic structure of the coating: The coating exhibits a typical lamellar structure with significantly refined grains and a greatly increased grain boundary area, which effectively hinders dislocation movement and provides structural support for increased hardness. A twin structure is clearly observed at the interface between the coating and the substrate. The twin structure can effectively alleviate the residual stress generated during laser cladding, prevent coating cracking, and further optimize mechanical properties. Crystallographic calibration shows that the coating is mainly composed of BCC solid solution phase and HCP solid solution phase. The two-phase structure works synergistically to balance hardness and toughness.

[0073] Example 2 (x=0.5, preferred solution) 1. Preparation process Raw material preparation: Powder was weighed according to the molar ratio of Zr:Hf:Nb:Ta:Mo=1:1:1:1:0.5. The other powder pretreatment, dense pre-layer preparation and laser cladding process parameters were completely consistent with those in Example 1, ensuring that the single variable was the Mo element content.

[0074] 2. Performance Testing and Analysis Depend on Figure 1 As can be seen, the microhardness of the coating in this embodiment (x=0.5) is 655.81 HV, which is 86% higher than that of the Ti6Al4V substrate (352.51 HV). Although it is lower than that of Example 1, it is significantly higher than that of the Mo-free group (x=0), meeting the core requirement of hardness ≥650 HV. The average coefficient of friction of the coating in this embodiment is 0.97, and its wear resistance is better than that of the Mo-free group, but lower than that of Example 1. The corrosion potential of the coating in this embodiment is lower than that of Example 1, but higher than that of the Mo-free group, still exhibiting excellent corrosion resistance.

[0075] Comparative Example 1 (x=1.0, Mo in excess) 1. Preparation process Raw material preparation: Weigh powder according to the molar ratio of Zr:Hf:Nb:Ta:Mo=1:1:1:1:1.0, and other process parameters are the same as in Example 1.

[0076] 2. Performance Testing and Analysis Microhardness: The microhardness of this comparative coating reached 755.42 HV, the highest among all groups, but the increase in hardness came at the cost of sacrificing wear resistance stability. Wear resistance: Excess Mo generates more MoO2 at high temperatures, which has weak protective properties and a loose structure. Corrosion resistance: Excess Mo leads to a decrease in the density of the oxide film and impairs corrosion resistance, verifying the necessity of the upper limit of Mo content (x≤1.0).

[0077] Comparative Example 2 (x=0, no Mo added) 1. Preparation process Only Zr, Hf, Nb, and Ta powders (molar ratio 1:1:1:1) were weighed, with no Mo added, and the remaining process parameters were the same as in Example 1.

[0078] 2. Performance Testing and Analysis Microhardness: The microhardness of the coating in this comparative example is only 552.74 HV, which is higher than that of the substrate, but does not reach the ≥650 HV required by this invention, and is significantly lower than that of the Mo-containing group. This directly verifies the core role of Mo addition in improving hardness—without Mo, the lattice distortion effect is weak, solid solution strengthening is insufficient, and the hardness improvement is limited. Wear resistance: The average friction coefficient of this comparative example is 1.13, with minimal improvement in wear resistance, indicating that the lattice distortion and microstructure regulation of Mo are key to improving wear resistance. Corrosion resistance: Approaching the corrosion potential of the substrate, the corrosion resistance was not effectively improved, further demonstrating the optimizing effect of Mo on the density of the composite oxide film.

[0079] In conjunction with the above embodiments, comparative examples, and appendices Figures 1-4 It can be concluded that the present invention achieves the best synergy between the coating microstructure (lamellar + BCC + HCP dual phase + twins) and performance (hardness, wear resistance, corrosion resistance) by precisely controlling the molar fraction of Mo x in the range of 0.2 to 1.0, especially when x = 0.75.

[0080] Those skilled in the art should consider the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art, all of which fall within the protection scope of this invention.

Claims

1. A laser cladding method for preparing a wear-resistant, corrosion-resistant, refractory high-entropy alloy coating on a Ti6Al4V substrate, characterized in that, Includes the following steps: (1) Powder pretreatment: Zr, Hf, Nb, Ta, and Mo powders with a purity ≥ 99.9 wt.% were selected as raw materials; according to the chemical formula (ZrHfNbTa)Mo x The powder is weighed according to a ratio of 1:1:1:1, where x is the molar fraction of Mo relative to any one of Zr, Hf, Nb, or Ta, and 0.2 ≤ x ≤ 1.

0. The weighed powder is then sieved, washed, and ball-milled to obtain a mixed powder with a uniformity > 95%. (2) Preparation of dense pre-layer: The surface of Ti6Al4V substrate is polished and cleaned; the mixed powder prepared in step (1) is pre-placed on the substrate surface using an adhesive, and after drying, a dense pre-layer with a density >90% is prepared. (3) Laser cladding: The pre-placed layer is clad under the protection of an inert gas using laser cladding technology. The Ti element in the matrix diffuses into the coating through the melting pool, and the refractory high entropy alloy coating containing Mo is synthesized in situ. The refractory high entropy alloy coating is composed of BCC solid solution phase and HCP solid solution phase. The microstructure presents a lamellar structure and a twin structure is formed at the interface between the coating and the matrix. The microhardness of the coating is ≥650HV.

2. The preparation method according to claim 1, characterized in that, In step (1), the molar number x of the Mo element is in the range of 0.5 ≤ x ≤ 0.

8.

3. The preparation method according to claim 1, characterized in that, In step (1), the sieving operation specifically removes particles with a diameter >150μm and <1μm; the cleaning is ultrasonic cleaning with ethanol for 15-25 minutes; the ball milling is carried out under argon protection, and the ball milling parameters are: rotation speed 200-400rpm, ball milling time 10-14 hours.

4. The preparation method according to claim 1, characterized in that, In step (2), the adhesive is a polyvinyl alcohol aqueous solution with a mass fraction of 3-5%; the drying conditions are: drying temperature 70-90℃, drying time 0.5-2 hours; the thickness of the pre-formed layer is controlled at 0.6-1.0 mm.

5. The preparation method according to claim 1, characterized in that, In step (2), the mixed powder is pre-placed on the substrate surface by pressing, with a pressing pressure of 25-35 MPa and a holding time of 2-4 minutes.

6. The preparation method according to claim 1, characterized in that, In step (3), the specific process parameters of the laser cladding are: laser power 2.5-3.5kW, scanning speed 4-6mm / s, defocusing amount +10~+20mm, and spot diameter 5-7mm.

7. The preparation method according to claim 1, characterized in that, The laser cladding uses a fiber laser system, and the laser beam is a continuous wave laser.

8. The preparation method according to claim 1, characterized in that, In step (3), the protective flow rate of argon gas is 10-20 L / min.

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