TC4 is widely used in the aerospace field due to its advantages such as high specific strength, good corrosion resistance and light weight. However, its disadvantages such as large friction coefficient and poor wear resistance greatly limit its application scope. Aiming at the shortcomings of TC4 such as low hardness and poor wear resistance, a 4 kW high-power semiconductor laser was used, and transition group refractory carbides HfC, TaC and ZrC were selected as the reinforcement phase. H13 steel-based powder was used as the base powder. Steel-based metal-ceramic hard coatings with different proportions were prepared on the surface of TC4 by laser cladding technology. Then, the macroscopic morphology, microstructure, phase composition and element distribution of coatings with different proportions were compared and analyzed by scanning electron microscope (SEM), EDS energy spectrometer, D/max-2500/PC X-ray diffractometer (XDR) and other experimental methods. The hardness of the coating was tested using a Qness Vickers microhardness tester and the change law of the microhardness of the cross section of the cladding sample was analyzed. The friction and wear properties of coatings with different material components were studied using an MMU-5G end face friction and wear testing machine. The research results show that the cladding layer of the specimen forms a good metallurgical bond with the substrate, and the organizational morphology of the cladding layer is mainly dendritic and blocky. The main phases of each cladding layer contain TiC. With the increase of the content of ternary ceramic powder, the content of MC also increases. When the addition amount of carbide mixed powder is 15% (mass fraction), the Hf0. 8Ta0. 2Fe2 ternary alloy phase is detected in the cladding layer. When the content of ternary ceramic powder is 10% (mass fraction), the cladding layer has the finest grains and the highest average hardness of the coating, which is about 763.43 HV, 2.29 times the hardness of the substrate. When the content of ternary ceramic powder is 5% (mass fraction), the coating has the best wear resistance, with a relative wear resistance of 25%.

TC4 is widely used in the aviation field due to its advantages of high specific strength, corrosion resistance and light weight. However, its disadvantages such as high friction coefficient and poor wear resistance greatly limit its application range. In order to effectively improve the wear resistance of the TC4 alloy surface, a wear-resistant coating can be prepared on the substrate surface by laser cladding and other technical means. The coating materials used for titanium alloy surface modification can be divided into several categories, including single elements (B, N, Si, etc.), intermetallic compounds (TiAl, Ti3Al, Ti5Si3, etc.), ceramic materials (WC, TiB2, TiC, Al2O3, etc.) and metal-ceramic composite materials (WC/Mo, NiCrBSi/TiN, etc.).

Feng Jinyu et al. prepared Ti-Al (-C, N) composite coating on the surface of TC4 substrate by laser cladding method. The study showed that the appropriate powder ratio can make the average hardness of the cladding layer 673.56 HV, which is 2.04 times that of the substrate. Qin Yang et al. prepared Ti3SiC2/Ni-based coatings with different Ni contents on the surface of TC4 by laser cladding technology. The coating with the highest microhardness was about 2.6 times that of the substrate, and the wear loss was less than that of the substrate. Qin Xin et al. used laser cladding technology to prepare NiCrCoAlY-Cr3C2 composite coating on the surface of TC4 titanium alloy. The maximum hardness of the composite coating was 1 344 HV, which was about 3.8 times that of the substrate; the friction coefficient was 0.2~0.3, which was significantly lower than that of the substrate. The composite coating was oxidized at a constant temperature of 850℃ for 100 h and the oxidation weight gain was 6.01 mg/cm’2, which was about 24% of the substrate. In summary, carbide ceramic powders are widely used in laser cladding, mainly WC, TiC, SiC and Cr3C2, but there is little research on transition metal carbides.

Transition metal refractory carbides generally have high hardness and elastic modulus, while H13 has good hardness and wear resistance at higher temperatures. In order to explore the strengthening effect of transition metal refractory carbide ceramic powder on laser cladding coating, this paper selects H13 powder as the base powder and HfC, TaC and ZrC transition group ternary ceramic powders as the reinforcing phase. Under the condition of unchanged laser process conditions, the addition ratio of ternary ceramic powder is changed to prepare modified coatings with different material components on TC4 plate. Through phase analysis, microstructure analysis, hardness test and friction and wear test of cladding layer, the influence of ternary ceramic powder content on the microstructure and properties of metal ceramic composite coating is studied, which lays the material and process foundation for the preparation of titanium alloy modified coating.

1 Experimental materials and methods

The substrate is TC4 alloy plate with a plate size of 50mm×50mm×8mm. The surface oxide film is removed by sandpaper before use. The coating material uses H13 steel-based powder as the base powder, and transition carbide HfC, TaC and ZrC powders as the reinforcing phase, with a purity of 99.5%. The micromorphology of the three carbide ceramic powders is shown in Figure 1. The thermal physical properties of TC4 and H13 metal powders and ceramic phase powders are shown in Table 1. A 4 kW high-power semiconductor laser was used to prepare a single-pass cladding layer on the surface of the TC4 plate. The cladding powder was a steel-based metal-ceramic composite powder containing an equal proportion of ceramic reinforcement phase. The powder ratio was shown in Table 2. The optimized laser cladding process parameters used in the experiment were: laser power 2 800W, scanning speed 300mm/min, spot size 2mm×11mm, and preset powder thickness 2mm.

The metallographic samples were prepared by grinding, polishing and corrosion, and their microstructures were observed by Hitachi-3400N scanning electron microscope (SEM). The microstructure composition was analyzed by EDS energy spectrum. The phase composition was analyzed by D/max-2500/PC X-ray diffractometer (XDR). The microhardness of the cross section was measured by Qness Vickers microhardness tester. Starting from the surface of the cladding layer, 3 points were tested vertically downward along the substrate at intervals of 0.2 mm, and the average value was taken as the average microhardness value of the depth. The test parameters were set as a loading load of 500 g and a loading time of 10 s. Multiple cladding coatings were prepared on the TC4 plate with an overlap rate of 60%, and friction and wear test specimens were prepared according to the specimen size requirements of the MMU-5G end face friction and wear tester. The material of the abrasive specimen was Cr12MoV, and its surface wear resistance was compared and analyzed. The test loading load was 50N and the rotation speed was 200. r/min, and the continuous wear time is 10min.

2 Experimental results and analysis

2.1 Macromorphology of cladding layer

Figure 2 shows the cross-sectional morphology of cladding samples of different materials. Each specimen substrate and cladding layer achieve good metallurgical bonding and have a clear dividing line. The surface roughness of the cladding layer increases with the increase of the content of ternary ceramic powder, mainly due to the high melting point of the ceramic phase. As the ceramic phase increases, the number of unmelted nodules on the surface increases. When ternary ceramic powder is added to the cladding powder, obvious cracks begin to appear in the cladding layer, and as the content of ternary ceramic powder increases, the cracks in the cladding layer also gradually increase. The main reason is that the melting point of the ternary ceramic powder is much higher than that of the H13 powder and the TC4 titanium alloy substrate, and the thermal conductivity, elastic modulus and thermal expansion coefficient of the H13 powder and the TC4 titanium alloy substrate are very different. During the laser cladding process, a large temperature gradient is caused inside the molten pool, which is easy to generate large stress locally, causing cracks in the cladding layer.

2.2 Phases of cladding layers

Figure 3 shows the X-ray diffraction patterns and phase analysis results of the S0-S3 cladding layers. As shown in Figure 3, under the irradiation of the high-energy laser beam, the cladding powder and the substrate surface melted simultaneously to form a molten pool. The Ti element flowed upward in the molten pool and preferentially reacted with the C element in the cladding powder to form TiC. The main phases of each cladding layer contained TiC. When the amount of ternary ceramic powder added increased, the content of MC formed increased. When the amount of carbide mixed powder added was 15% (mass fraction), the Hf0.8Ta0.2Fe2 ternary alloy phase was detected in the cladding layer. Due to problems such as instrument resolution or incomplete types of PDF cards (XRD standard database JCPDS), no compounds or solid solutions of Ta, Hf and Zr elements were detected in the S1 and S2 cladding layers, and EDS will be used for auxiliary analysis later.

2.3 Microstructure morphology and EDS analysis of cladding layer

Figure 4 shows the microstructure morphology of cladding layer of different material samples. The points marked in Figure 4 are EDS energy spectrum analysis detection points. As can be seen from Figure 4, the microstructural morphology of the bottom of the S0 cladding layer and the middle of the cladding layer is quite different. The bottom of the cladding layer is mostly fine dendrites and blocky structures. The direction of the dendrites is basically perpendicular to the substrate and grows toward the top of the cladding layer. The blocky structure in the middle of the cladding layer is significantly larger, and needle-shaped martensite can be observed at the junction of the substrate and the bottom of the molten pool. The microstructural morphology of the bottom of the S1 and S2 cladding layers is similar to that of the S0 cladding layer. The growth direction of the dendrites is the same. The bottom is a dendrite that grows perpendicular to the bonding zone toward the surface of the cladding layer. There are fine blocky structures near the middle of the cladding layer, but the size of the structure is relatively small. This is mainly because with the addition of ternary ceramic powder, the supercooling increases during the crystallization process, which in turn leads to an increase in the nucleation rate, and finally forms relatively small grains. There is no boundary between the substrate and the cladding layer of S3. The boundary line of the H13 powder is obvious, and obvious cracks can be observed in the cladding layer. The main reason is that the thermophysical properties of the H13 powder itself are quite different from those of the TC4 titanium alloy powder. With the increase of the proportion of carbide ceramic powder added, the difference in thermophysical parameters between the entire cladding powder and the substrate becomes larger, and the residual stress during the cladding process becomes larger.

The test results of the marked positions in Figure 4 are shown in Table 3.
According to Table 3, it can be seen that the Ti content at points 1 and 3 of the S0 cladding layer is relatively high. During the laser cladding process, the surface of the substrate TC4 titanium alloy is heated and melted and decomposes the Ti element. The Ti element moves to the middle and upper parts of the molten pool due to the force generated by the flow of the molten pool, and reacts with the elements decomposed by the molten H13 powder to generate some Ti-containing compounds or solid solutions. The white reticular structure at point 2 is mostly Fe-containing compounds. Similarly, the Al content at point 2 is higher than that at points 1 and 3, and there may be Fe-Al intermetallic compounds.

The white precipitate phase of the S1 cladding layer is metal carbide and a small amount of compounds or solid solutions containing Ta, Hf, and Zr, and there are a large number of Ti-Fe intermetallic compounds in the black area. Compared with the white precipitate phase, the Ta, Hf, and Zr elements in the black area are relatively small; compared with the white area, the C content in the black area is significantly reduced, indicating that during the laser cladding process, the C element decomposed by the carbide ceramic under heat combines with the Ti element decomposed by the TC4 titanium alloy to form TiC. TiC is a face-centered cubic structure with high density and high strength. It can effectively improve the performance of the cladding layer and exists in the cladding layer in the form of a white precipitate phase. This is a typical organizational feature of the distribution of hard precipitates.

Compared with the S1 cladding layer, the Hf, Ta, Zr, and C element contents detected in each area of ​​the S2 cladding layer increased significantly with the increase of the ternary ceramic powder content. Combined with the XRD phase analysis of S2, it can be seen that the white dendrites are mostly TiC, and the compounds of Zr, Hf, Ta and other elements are enriched in the dendrite structure. The black area has less Zr, Hf, Ta and other elements than the white dendrites, and is mainly Fe3C generated by the combination of C and Fe. Compared with point 1, the Fe content of the other points has increased significantly, and the content of Ti and C has decreased. Because Ti has a strong affinity with C, C decomposed from carbides combines with Ti to form TiC during the laser cladding process. At the same time, a small amount of C combines with Fe. Combined with the phase analysis of S2, it can be seen that the compound generated by the combination of C and Fe is Fe3C.

In the S3 cladding layer, points 1 and 4 contain a small amount of Zr, Hf and Ta elements. The contents of Zr, Hf and Ta at points 2 and 3 are greatly improved compared to points 1 and 4. The reason may be that the content of ternary ceramic powder in the cladding powder increases. Carbide ceramic powder has an ultra-high melting point. During the laser cladding process, the number of unmelted ceramic phase particles increases. The unmelted carbide particles are in contact with the molten pool. The carbides generated together form a massive phase.

2.4 Microhardness of cladding layer

Figure 5 is the microhardness curve of the S0-S3 cladding layer. It can be seen from the figure that with the addition of ternary ceramic powder, the hardness of the cladding layer increases significantly, but the difference between different addition amounts is not significant. The microhardness gradually decreases from the coating surface to the heat-affected zone, which helps to reduce the residual stress between the coating and the substrate. It also shows that the coating and the substrate form a good metallurgical bond. Among them, the average hardness of the S2 cladding layer is about 763.43 HV, the average hardness of the heat-affected zone is about 375 HV, the average hardness of the base material is about 333.2 HV, and the hardness of the cladding layer is 430.23HV higher than the base material. , which is 2.29 times the hardness of the base material.

2.5 Wear resistance of cladding layer

Table 4 shows the wear amount data of different samples under the same experimental conditions. Since the coating of sample S3 has large cracks, only the friction and wear properties of the base material, S0, S1, and S2 samples are comparatively analyzed. According to Table 4, it can be seen that under the same experimental conditions, the wear amount of the base material is the largest. With the addition of ternary ceramic powder, the wear amount gradually decreases; compared with the base material, the wear amount of S0, S1 and S2 specimens are all the same. The coating S1 with 5% mass fraction of reinforcing phase has the best relative wear resistance.

6 is a picture of the wear scar morphology of each coating. From Figure 6, it can be seen that there is partial peeling off on the surface of the base material and S0 cladding layer. The main reason is that the surface hardness of the base material is low and the surface of the base material was subjected to stress during the experiment.
The combined action of vertical force and tangential force, and the hardness of the upper friction pair is relatively high. Under long-term stress, slight adhesion occurs between the surface of the base material and the upper friction pair, and then plastic deformation occurs, causing local shedding and wear. The form is mainly adhesive wear; there is no local shedding on the surface of the S1 and S2 cladding layers, but there are slight furrows and falling wear debris. The main reason is that the carbide reinforcement phase is added to the cladding layer. The surface hardness is relatively high. The surface wear of S1 and S2 is mainly abrasive wear. There are traces of furrows and grinding in the cladding layer. It can be observed that the furrows of S1 are narrow and deep. The shed particles of S2 are large and the furrows are wide and shallow.

3 Conclusion

(1) The structure of the cladding layer is mainly dendrite structure and massive structure, and cracks gradually appear as the ceramic phase is added. When 10% (mass fraction) of ternary ceramic powder is added, the grains of the cladding layer are the smallest. , Hf, Zr and Ta elements evenly exist in each structure of the cladding layer.

(2) Through the performance analysis of the cladding layer, when the ternary ceramic phase is added at 10% (mass fraction), the microhardness of the coating is the highest, with an average hardness of approximately 763.43HV, which is 2.29 times the hardness of the base material; When the ternary ceramic powder is added at 5% (mass fraction), the coating has the best wear resistance, and the relative wear resistance is 25%.