The experiment selected WC hard particles with high hardness and high melting point and Ni powder with good toughness as cladding materials, and prepared a dense WC reinforced Ni-based composite coating on the surface of Ti-6Al-4V titanium alloy by laser cladding technology. The microstructure, phase, composition and hardness of the cladding coating were studied by optical microscope, X-ray diffractometer, scanning electron microscope, energy spectrometer and microhardness tester. The results show that the WC metal-based composite coating was prepared on the Ti-6A1-4V substrate by using a powder ratio of WC: Ni with a mass ratio of 7:3. The coating is mainly composed of Ti2Ni, NiTi, WC, W2C, TiC, α-Ti, etc. Rod-shaped and dendrite-shaped black and bright TiC and block-shaped WC are distributed in the entire coating and some needle-shaped objects are distributed on the side close to the interface substrate. The average microhardness of the coating is about 760 HV, and the hardness of the substrate is about 380 HV. The hardness of the coating is about 2 times higher than that of the substrate, and there is a smooth transition hardness gradient distribution from the coating surface to the substrate.
Titanium alloys are widely used structural materials in aerospace, aviation, chemical industry, biomedicine and other industrial sectors due to their high specific strength, excellent corrosion resistance, good oxidation resistance and high temperature performance. Laser cladding is an excellent surface modification technology. Compared with traditional technologies, it has better controllability and can obtain better surface quality and bonding strength57. WC, as a second phase particle reinforcement phase, is usually used together with Ni to form a cladding layer to increase strength, friction resistance and other properties. At present, Zhang Leitao et al. used WCNi coating to prepare a high-performance coating on the surface of 45 steel by laser remelting technology. The results showed that the hardness and wear resistance of the coating were improved. Liu Ziping et al. used nickel-based WC coating to successfully prepare an enhanced coating on vermicular graphite cast iron by laser cladding. The results showed that the friction coefficient was significantly improved. Zhang Zhiqiang used different proportions of Ni-based WC coating on Q345R steel for laser cladding. The results showed that the use of coatings can increase the wear resistance of the steel surface. From the current research status, it can be seen that most of the research is on the surface of steel, while there is less research on titanium alloy as a matrix. Therefore, in order to increase the research samples and provide a basis for subsequent researchers, this paper uses WC and Ni to make composite cladding layers, synthesizes metal-based composite coatings through laser cladding technology, studies the structure and hardness of the coating, and compares the hardness changes of the coating and the substrate.
1. Experimental materials and methods
1.1 Experimental materials
The experimental substrate material is Ti-6AI-4V alloy, and the composition is shown in Table 1. The size of the laser cladding sample is Φ50 mmx10 mm. The cladding material is WC/Ni powder with a purity greater than 99%. The cladding layer powder is evenly adhered to the surface of the sample through a polyvinyl alcohol binder solution. The pasted sample is pressed by a press and finally dried.
1.2 Experimental method
The dried sample is laser clad on the surface of the above sample coating using an HL-5000 CO2 laser machine with a power of 2.5 kW and a spot diameter of Φ4mm. The experiment is repeated 3 times for each sample. The results are shown in Figure 1. Metallographic samples were prepared by wire cutting, and the corrosive agent was (4 mL concentrated HNO3, 6 ml H2O, 1 ml HF), and the corrosion time was 10s. The samples were subjected to OM and SEM image shooting, XRD phase analysis and microhardness test.
2 Results and analysis
2.1 Morphology of cladding layer
Figure 2 shows the overall image of the cladding layer under an optical microscope at 100 times. It can be seen that the cladding layer has an uneven structure, and the thickness of the cladding layer is about 1mm. During the corrosion process, obvious corrosion unevenness occurs; there are two obvious cracks in the cladding layer, one perpendicular to the inner interface, and the other on the surface of the cladding layer and parallel to the surface. This is mainly because during the laser cladding process, due to its rapid heating and rapid cooling characteristics, the cladding layer has a large residual internal stress, and the cracking sensitivity is significantly increased, causing the cladding layer to crack. There is a white bright band between the cladding layer and the substrate, which is a typical metallurgical bonding feature, indicating that the cladding layer is well bonded to the substrate. In addition, there are pores at the interface between the cladding layer and the substrate. This is mainly because the rapid cooling during the laser cladding process prevents the gas generated by the reaction from being removed in time.
Figure 3(a) is a 500-fold magnification of the upper boundary of the titanium alloy cladding layer. It can be observed that the dendritic phase is dispersed in all directions, accounting for the majority, and there are obvious dendritic products in the cladding layer. Some dendritic products are relatively coarse, such as the cross-shaped dendrites in the figure, with a size of 5~10 mm. There are also equiaxed phases dispersed. Figure 3(b) is a high-power microscope laser cladding layer. The organization size is uniform and evenly distributed, with a size of 2~5mm, and local cavities are dispersed in black. The optical microscope photo with a magnification of 1000 times. Figure 3(c) is a high-power microscope organization diagram of the cladding area, in which there are a large number of dendritic structures, which are unevenly arranged horizontally and vertically, with a size of 7~60 mm, and there is no obvious regularity in distribution, showing a chaotic distribution, with a magnification of 1000 times. Figure 3(d) is an enlarged view of the boundary between the laser zone and the substrate. There is an obvious white bright band at the boundary. The upper part is the cladding layer, and the lower part is the titanium alloy substrate. The cladding layer and the substrate have good metallurgical bonding. At the interface of the bonding zone, on the side of the titanium alloy substrate, obvious needle-shaped and plate-shaped martensite appear, and on the side of the interface cladding layer, there are rod-shaped structures closely arranged perpendicular to the interface.
2.2 XRD analysis of cladding layer
Figure 4 is the XRD spectrum of the laser cladding layer. It can be seen that WC substances appear in the XRD diagram. This is because the molten pool phenomenon was found during the cladding process. By comparing the PDF card, it is found that the 0 value is offset. From the formula 2dsinθ=λ, it can be seen that the d value is also offset. The coating is mainly composed of carbides and Ni compounds: carbides mainly include WC, W2C, Ni2W4C, etc. Among them, TiC, W2C and Ni2W4C are generated by chemical reactions after partial WC dissolution. They have good melting points and hardness. The compounds of Ni are mainly newly generated TiNi and Ti2Ni intermetallic compounds, which have good hardness and excellent toughness.
2.3 Analysis of the second organization of the cladding layer
Figure 5 is the surface scanning energy spectrum analysis of the cladding layer. It can be seen that the dark cellular structure has a relatively small Ni content, and the distribution of the other elements A, C, W, Ti, and V is relatively uniform. It is preliminarily inferred that the network phase is Ti2Ni and the cellular phase is NiTi.
From the point scanning results shown in Figure 6 and Table 2, it can be seen that the white phase organization at point A is mainly C, Ti, and W elements. Among them, the content of C and W is the highest and almost the same. Combined with the materials used in this experiment and the experimental principle, we can judge that the white phase in the coating is determined to be WC or a mixed reinforcement phase of WC and W2C. The main components of the dark gray organization at point B are C, Al, Ti, V, Ni, and W. Among them, the atomic ratio of Ti:Ni is 1.72. Combined with the XRD analysis results, it can be judged that the dark gray organization in the coating is a TiNi intermetallic compound. The light gray structure at point C is mainly composed of four elements: C, Al, Ti, V, and Ni. Among them, the content of Ti and Ni is the highest, and the Ti:Ni:C atoms are close to 1:1:1. Combined with the XRD results, it is inferred that it is a Ni-Ti-C intermetallic compound. The specific composition phase cannot be determined according to the current experiment. The main components of the black structure at point D are C, Al, Ti, Ni, and W. Among them, the content of ℃ and Ti is the highest. Combined with the XRD spectrum analysis, the black spherical phase is determined to be TiC.
2.4 Microhardness of the cladding area
Figure 7 is the hardness gradient curve from the outside of the laser cladding area to the substrate. It can be seen that the hardness of each 0.1 mm before 1.2 mm is about 850~670HV, and there is an obvious decline in the hardness gradient curve from 1.2~1.5mm. After 1.5 mm, the hardness is approximately stable at a constant value of about 380 HV. This shows that the first 1.2 mm is the laser cladding layer, the 1.2-1.5 mm is the heat affected zone, and the 1.5 mm is the substrate. The coating studied is about 2 times harder than the substrate, with an average microhardness of about 760 HV and a substrate hardness of about 380 HV. The hardness gradient distribution shows a smooth transition from the coating surface to the substrate.
3. Conclusion
(1) The WC metal composite coating was prepared on the Ti-6Al-4V substrate using a WC:Ni powder ratio of 3:7.
(2) The coating studied is mainly composed of Ti2Ni, NiTi, WC, W2C, TiC, α-Ti, etc. The samples were observed under an optical electron microscope and a scanning electron microscope, and a large number of rod-shaped and dendrite-shaped black and bright TiC and block-shaped WC were distributed in the entire coating, and some needle-shaped objects were distributed on the side close to the interface substrate. This is because laser cladding is a process of rapid heating and rapid cooling, which leads to uneven energy distribution and even uneven component distribution.
(3) The coating studied is about twice as hard as the substrate. The average microhardness of the coating is about 760HV, and the hardness of the substrate is about 380HV. There is a smooth transition hardness gradient distribution from the coating surface to the substrate.
