Taking the laser remanufacturing and repair technology of 17-4PH precipitation hardened martensitic stainless steel blade as the background, nickel-chromium alloy and WC-Co composite powder were selected as cladding materials, and the composite coating was prepared on the surface of 17-4PH stainless steel by laser cladding technology. The experiment was carried out by adjusting the two process parameters of laser power and powder feeding speed. By observing the microstructure of the sample cladding layer and testing the mechanical properties of the cladding layer, the results show that when the laser power is 1700W, the structure of the coating is uniform and dense. The hardness and friction and wear properties of the coating are higher than those of the substrate. The optimal process parameters are laser power of 1700W and powder feeding speed of 30mg/s. The composite coating prepared under this process parameter can effectively improve the hardness and friction and wear resistance of 17-4PH stainless steel.
In the development of the coal mining industry, coal mining machinery and equipment are the prerequisite for safe and efficient production of coal mines. Coal mining machinery and equipment are in harsh working environment for a long time, and the machinery and equipment are easily damaged. The most economical and effective way to solve the above problems is to use surface engineering methods to prepare a wear-resistant coating on the surface of easily worn parts of coal mining machinery [1-4].
As the last stage blade of the turbine, 17-4PH stainless steel has high strength, excellent corrosion resistance and excellent processing performance, and is widely used in aerospace, energy and chemical industries. However, since its surface is susceptible to wear and corrosion, laser cladding technology, as an effective surface modification method, can form a layer of coating with excellent performance on the surface of the material, thereby improving the service life and performance of the material [5-61].
Shen Yanjin et al. prepared WC particle reinforced coating on 45 steel substrate by laser cladding, which enhanced the hardness and wear resistance, and both were higher than the substrate material. In order to solve the problem of poor wear resistance of TA2/TC4, Xu Miaomiao [8] prepared WC-12Co and WC-25Co powder wear-resistant coatings by changing the laser process parameters. Guo Hongli et al. [9] used laser cladding technology to prepare Ni60-WC-Co composite coatings and found that the optimal laser power was 1.5kW, with higher hardness and wear resistance. Duan Mengfan [10] explored the preparation of Ni60 laser cladding layers on Q235 substrates and their effects on friction and wear properties. Hu et al. used high-speed laser cladding technology to successfully prepare Ni-WC composite coatings with different components and uniform distribution on the surface of 304 steel, and showed good mechanical properties. Yang Jiaoxi [11] and Yao Jianhua [12] used laser broadband cladding technology to repair turbine blades with surface cavitation.
Research on the repair and strengthening of turbine blades was mainly based on welding repair in the early stage, but was later replaced by laser cladding repair technology due to relatively poor repair quality. Laser cladding technology has significant advantages in preparing high-performance cladding layers on low-cost metal substrates. Therefore, this paper selects nickel alloy powder and WC-Co composite powder as cladding materials, and uses fiber-coupled semiconductor lasers to perform laser cladding on the surface of 17-4PH stainless steel alloy to obtain a composite coating, and mainly studies the influence of laser technology on the structure and performance of the coating.
1 Experimental materials and methods
The experiment uses 17-4PH stainless steel (0Cr17Ni4Cu4Nb) as the substrate, with a size of 20mm×20mm×8mm. Table 1 shows the chemical composition of 17-4PH stainless steel. Before cladding, the cladding substrate was polished with 400, 600 and 1000 sandpapers, and then cleaned with alcohol to remove the surface oxide layer and stains to improve the laser energy absorption efficiency. The cladding material is 80% nickel-chromium alloy composite powder + 20% WC-Co composite powder (80% and 20% are the mass fractions of the corresponding substances, respectively).
The experiment uses a fiber-coupled semiconductor laser (model LDF4000-100), the laser wavelength range is between 980 and 1030 nm, the spot diameter is 4 mm, and the powder feeding method is lateral. Argon gas is used for protection during the experiment. The samples were obtained using laser powers of 1550 W and 1700 W and powder feeding speeds of 30 mg/s, 32 mg/s, and 34 mg/s, respectively, and the samples were numbered as shown in Table 2. After grinding and polishing, the samples were observed and tested for macroscopic morphology, organization and performance. The JSM-IT2000 scanning electron microscope was used to observe the morphology of the sample coating cross section, and the Aeris X-ray diffractometer was used to analyze the coating phase. The HXD-2000TMC/LCD digital microhardness tester was used to test the hardness. The test load was 0.981N and the holding time was 10s. The hardness of 7 points on the cross section of the cladding layer was tested to calculate the average value. The hardness of the substrate was also tested to calculate the average value. The HSR-2M high-speed reciprocating friction and wear tester was used to test the wear resistance of the surface coating and the substrate material of the polished cladding sample.
2 Results and analysis
2.1 Macromorphology analysis
The macromorphology of the cross-section of the cladding samples under different laser powers and powder feeding rates was analyzed. Figure 1-1 shows the cross-sectional morphology of the cladding samples under the laser power of 1550W and the powder feeding rates of 30mg/s, 32mg/s and 34mg/s, respectively. Figure 1-2 shows the cross-sectional morphology of the cladding samples under the laser power of 1700W and the powder feeding rates of 30mg/s, 32mg/s and 34mg/s, respectively. As can be seen from Figure 1, each cladding sample consists of dark and light areas with a clear dividing line in the middle. The light area is the cladding layer and the dark area is the substrate layer. When the laser power is 1.550W, there are some unmelted small particles in the cross section, and the cladding layer forms a good metallurgical bond with the substrate. In comparison, when the laser power is 1700W, the cladding layer penetrates the substrate more. This is because as the laser power increases, the input energy increases, and the cladding powder and the substrate are partially melted to form a molten pool. With the action of gravity and surface tension, a deeper and wider molten pool is formed. The powder feeding rate has no obvious regular effect on the macroscopic morphology.
2.2 Microstructure Analysis
The cross-section of the cladding samples under different laser powers and powder feeding rates was subjected to microstructure and point scanning analysis. When the laser power is 1550W and the powder feeding rate is 30mg/s, there is a clear boundary between the cladding layer and the base layer, and the cladding layer structure is lath-shaped and a small amount of granular. When the powder feeding rate is 32mg/s, the size of the lath-shaped structure becomes smaller. When the powder feeding rate is 34mg/s, the lath-shaped structure decreases, the granular structure increases, and a mesh-shaped structure is formed. As can be seen from Figure 2-1, with the increase of the powder feeding rate, the structure changes from lath-shaped to granular. When the laser power is 1700W and the powder feeding rate is 30mg/s, there is a light-colored structure in the transition zone between the cladding layer and the substrate. The cladding layer structure is fine granular and aggregates to form needles. Combined with XRD analysis, it can be seen that some fine particles are precipitated Cr7C3 phases. When the powder feeding rate is 32mg/s and 34mg/s, the structure is a granular network, as shown in Figure 2-2.
The dark and white areas of the cladding layer are scanned and analyzed to obtain the element distribution, as shown in Table 3. From the data in the table, it can be seen that the content of Fe, Cr, Co, Ni and W elements is higher in the white area, the content of Fe, Cr, Co and Ni elements is higher in the dark area, and the content of W element is lower than that in the white area. When the laser power is 1550W, the content of Co and Ni elements is generally higher than that at 1700W. It may be because the laser power is increased, and the elements can be melted more and better in the substrate.
2.3 Phase analysis
Figure 3 shows the XRD spectra of samples with different powder feeding rates at 1550W and 1700W laser powers. It can be seen that the diffraction peaks of the sample coating are mainly Fe-Cr, Al-Ni, Al.C, and WCx composite phases. The existence of W element in the cladding layer is mainly WCx. At 1700W, CrC phase is formed.
2.4 Hardness analysis
After testing and calculation, the hardness HV0.1 value of the substrate is 371.1. From Figure 4, it can be seen that the microhardness value of the cladding layer of the sample is higher than that of the substrate. When the laser power is 1550W, the hardness of the coating increases with the increase of powder feeding speed. When the laser power is 1700W, the law is the opposite. The powder feeding speed gradually increases, and the hardness of the coating becomes lower and lower. In addition, when the powder feeding speed is 30mg/s, the hardness of the coating increases with the increase of laser power. However, when the powder feeding speed is 32mg/s and 34mg/s, the hardness of the coating gradually decreases with the increase of laser power. At 32mg/s, this effect is not obvious, but at 34mg/s, the effect is more obvious. Through the analysis of the microstructure and phase composition of the coating, due to the high melting point and density of the W element, W or WCx forms a denser network structure in the coating, which improves the hardness of the coating. And when the laser power is 1700W and the powder feeding speed is 30mg/s, some fine particles appear in the organization as precipitated Cr and C phases, which enhances the hardness of the sample.
2.5 Wear resistance analysis
As shown in Figure 5, the friction coefficient of the substrate is maintained at about 1.1. Figure 6 shows the friction coefficient of the cladding layer under different process parameters, which varies greatly. When the laser power is 1550W, the powder feeding speed and the friction coefficient of the cladding layer are inversely proportional. When the powder feeding speed is 34mg/s, the friction coefficient finally remains at about 1.0. When the laser power is 1700W, the powder feeding speed and the friction coefficient of the cladding layer are in direct proportion. When the powder feeding speed is 30mg/s, the friction coefficient is finally maintained at about 0.9. When the powder feeding speed is 30mg/s, the laser power and the friction coefficient of the cladding layer are inversely proportional. However, when the powder feeding speed is 32mg/s and 34mg/s, the laser power and the friction coefficient of the cladding layer are in direct proportion. In summary, the wear resistance of samples 3 and 4 is improved. Therefore, the better process parameters are: laser power of 1700W, powder feeding speed of 30mg/s and laser power of 1550W, powder feeding speed of 34mg/s. Combined with the microstructure and phase composition of the cladding layer, the cladding layer forms small particles and a relatively dense mesh structure, which improves the wear resistance of the cladding layer.
3 Conclusion
1) By observing the macroscopic morphology of the cladding layer, the cladding layer and the substrate form a good metallurgical bond. As the laser power increases, the cladding layer penetrates more of the substrate, forming a deeper and wider molten pool. The powder feeding rate has no obvious regular effect on the macroscopic morphology.
2) The microstructure of the cladding layer of the samples under different process parameters is different, mainly lath-like structure, reticular structure and granular structure. The coating under different process parameters mainly consists of three phases: FeCr, AINi and WCx. When the laser power is 1700W and the powder feeding rate is 30mg/s, the cladding layer is better combined with the substrate, and there are fine particles in the microstructure, which are more evenly distributed and denser.
3) The microhardness and wear resistance of the cladding layer under different process parameters are slightly higher than those of the substrate. When the laser power is 1700W and the powder feeding rate is 30mg/s, the average microhardness HV0.1 of the sample is the highest, which is 675.33. At the same time, the wear resistance of the sample is the best.
4) When the laser power is 1700W and the powder feeding speed is 30mg/s, under this process parameter, the microstructure and performance tests of the coating are optimal.
