[Abstract] 17–4PH stainless steel is widely used in key parts such as turbine blades in the aerospace field. In order to achieve the repair and remanufacturing of damaged 17–4PH parts for aviation, a 15–5PH coating was prepared on the surface of 17–4PH by laser cladding technology, and the phase, microstructure, microhardness, wear resistance and corrosion resistance of the coating were analyzed. The results show that the coating is mainly composed of Fe–Cr, martensite and α–Fe; the bonding area between the coating and the substrate is planar crystal, the bottom and middle parts are mainly columnar crystals, and the top is composed of columnar crystals and a small amount of equiaxed crystals. The average microhardness of the coating and the substrate are 408.7HV0.5 and 347.5HV0.5, respectively. The hardness of the coating is 17.6% higher than that of the substrate. The average friction coefficients of the coating and the substrate are 0.3051 and 0.3754, respectively. The wear cross-sectional areas are 813.74 μm2 and 2058.12 μm2, respectively. The wear resistance of the coating is significantly better than that of the substrate. The self-corrosion potential (Ecorr) of the coating and the substrate are –1.0780 V and –1.0975 V, respectively. The self-corrosion current density (Icorr) is 1.229×10–3mA/cm2 and 0.907×10–3mA/cm2, respectively. The corrosion resistance of the coating is comparable to that of the substrate. The results of the microstructure and surface properties of the coating show that the laser cladding 15–5PH coating can be used for the repair and remanufacturing of 17–4PH parts for aviation.
Keywords: laser cladding; remanufacturing; 17–4PH; 15–5PH; corrosion resistance
17–4PH stainless steel is often used to manufacture key aviation parts, including aircraft landing gear, aircraft engine parts and turbine blades, due to its excellent mechanical properties, corrosion resistance and wear resistance. In actual applications, such parts need to withstand extreme working environments, such as continuous operation under high temperature and high pressure conditions, high-speed friction and exposure to corrosive media. These harsh conditions easily lead to wear, corrosion and fatigue failure of parts, which do not meet the requirements of long-term service. Therefore, the repair and remanufacturing methods for damaged 17–4PH parts for aviation provide a practical way to extend their service life and improve resource utilization. As an advanced surface repair and remanufacturing technology, laser cladding technology uses a high-energy laser beam to quickly melt powder materials and damaged surfaces, and after cooling and condensation, a coating with a dense structure and excellent performance is formed. Compared with traditional repair and remanufacturing technologies such as thermal spraying, arc cladding technology and plasma spraying, laser cladding technology has the advantages of high energy density, low energy consumption and small heat-affected zone. It can repair and remanufacture damaged parts in any area and complex shapes.
At present, a large number of scholars at home and abroad have used laser cladding technology to carry out research on repair and remanufacturing technology in many fields. Li Sheng et al. used laser cladding technology to prepare an improved nickel-based high-temperature wear-resistant alloy coating of Inconel 718 on the surface of Inconel 718 alloy to repair and remanufacture the sealing surface of the nickel-based alloy valve of the supercritical unit. Chen et al. used laser cladding technology to prepare titanium carbide and titanium diboride reinforced composite coatings on the damaged carbon steel surface to repair and remanufacture carbon steel parts. Zhang et al. used cobalt-based and nickel-based alloys as raw materials to perform laser cladding on the surface of 1Cr12 martensitic stainless steel to repair and remanufacture pump shafts and valve stems in nuclear reactors. Li Yundong et al. used laser cladding technology to clad self-made iron-based alloy powder on a 28CrMoNiV steel substrate to repair and remanufacture turbine rotors. Among them, some scholars used laser cladding technology to repair and remanufacture parts in the aerospace field. Ren Weibin et al. used damaged rotor blades as the repair and remanufacturing objects, and laser clad self-made alloy powder on the surface of TC4 alloy. The results showed that the wear resistance of the blades was improved after repair. In order to realize the repair and remanufacturing of aircraft engine parts, Gao Xuesong et al. used laser cladding technology to prepare Al2O3+13% TiO2 (mass fraction) coating on the surface of titanium alloy, which greatly improved the corrosion resistance of the repaired parts. Wang Xiaoyang et al. conducted a study on the repair and remanufacturing of 2A50-T6 aviation aluminum alloy, and used laser cladding technology to prepare AlSi10Mg coating on the surface of 2A50-T6 aluminum alloy; the results showed that the cladding layer had only a small amount of pores, the density reached 99.96%, and the tensile strength of the aluminum alloy repaired by laser cladding reached 93.18% of the substrate. Zhang et al. conducted a study on the repair and remanufacturing of Inconel 718 high-temperature parts for aerospace, and laser clad Inconel 718 alloy powder on Inconel 718 alloy with prefabricated trapezoidal grooves to prepare a coating. The tensile strength and elongation of the repaired specimens were 736.6 MPa and 12.5%, respectively, and the tensile strength was significantly improved. Laser cladding technology has been widely used to repair and remanufacture parts in the aerospace field, but most of the research is on materials such as titanium alloys, aluminum alloys and high-temperature alloys (such as Inconel 718, Inconel 625). There are few studies on 17-4PH stainless steel, which is also widely used.
Cladding powder materials have an important influence on the structure and properties of remanufactured coatings. Commonly used cladding materials for laser cladding are cobalt-based, nickel-based and iron-based. Compared with cobalt-based and nickel-based materials, iron-based materials have better wettability and relatively lower engineering costs. 15-5PH is a typical iron-based powder, which is improved on the basis of 17-4PH by reducing the chromium and copper content and increasing the nickel content;
Not only does it have high strength and hardness, but studies have shown that the low chromium/nickel ratio makes 15-5PH have high toughness and good deformation properties. These characteristics make 15-5PH more advantageous in friction, tension and compression conditions, and can effectively alleviate the risk of cracking of 17-4PH aviation parts during service.
This study intends to clad 15–5PH powder on 17–4PH substrate, and compare and analyze the physical phase, microstructure and corrosion resistance of the cladding coating to verify the feasibility of using laser cladding 15–5PH powder to repair 17–4PH parts, and provide an experimental basis for the repair and remanufacturing of 17–4PH parts for aviation.
1 Test and Methods
1.1 Test Materials
The substrate was made of 17–4PH stainless steel (TISCO Group); the cladding powder was made of 15–5PH stainless steel (AVIC Mate) with a particle size of 53~150 μm. Before the test, the substrate surface was sanded to remove the oxide layer, and the cladding powder was placed in an oven at 200 °C for 120 min to ensure the fluidity of the powder. The main components of the substrate and cladding powder are shown in Table 1.
1.2 Experimental equipment and scheme
The laser cladding system is shown in Figure 1, which consists of OFM-TS-0601 laser cladding head (Gangchun Technology Company), RFL-C3000 fiber output semiconductor laser (Raycus Intelligent Technology Company), FANUC M-20iD/25 robot arm, TFLW-3000 water cooling device (Sanhe Tongfei Refrigeration Company), ECPF2-2LC multifunctional powder feeder (Shanghai Pairmid Machinery Technology Company), working platform and protective gas device. The maximum output power of the laser is 3000 W, the laser wavelength is (915±10) nm, the frequency is 50 Hz, and the coaxial powder feeding method is used for cladding. In order to prevent the coating from oxidation, high-purity argon gas with a purity of 99.999% is used to protect the molten pool throughout the process. The laser cladding process is shown in Figure 2. According to the results of the previous tests, the process parameters selected in this study are laser output power of 1700 W, laser scanning speed of 13 mm/s, powder conveying speed of 1.1 r/min, and overlap rate of 40%.
After the sample was clad, the middle section of the selected sample was cut into 10 mm×10 mm×12 mm block samples using DK-7735 high-speed wire cutting equipment (Taizhou Aier CNC Machine Tool Company) and inlaid, and then the block samples were polished and polished using 400#~2000# sandpaper in turn.
The surface of the polished sample was corroded with a mixed solution of FeCl3∶HCl∶H2O=5∶50∶100 by volume, and then cleaned and dried. The corroded sample was placed on the surface of the Leica DMILM metallographic microscope workbench (Leica, Germany) to observe the coating microstructure and study the evolution mechanism of the coating structure. The coating phase composition was detected by DX-2700B X-ray diffractometer (Dandong Haoyuan), and the scanning angle range (2θ) was set to 30°~100°, and the step angle was 0.02°/s; then the coating phase was analyzed by MDI Jade 6 software. The microhardness change from the coating to the substrate was measured by HV-1000B Vickers hardness tester (Laizhou Huayin Testing Instrument Co., Ltd.), and the test load was set to 500 g and the holding time was 10 s; the test point position and interval are shown in Figure 3. The corrosion resistance of the sample surface was tested by LK98BII electrochemical workstation (Lanlike Chemical Electronics Co., Ltd.), and the initial potential was set to -2 V, the end potential was 2 V, the scanning speed was 0.02 V/s, and the waiting time was 10 s. The wear resistance test was carried out using the MS-T3001 friction and wear tester (Lanzhou Huahui Instruments). The applied load was a 500 g weight, the speed was set to 200 r/min, the friction time was 10 min, and the friction radius was 3 mm. The friction pair used a GCr15 steel ball (HRC63). The friction force signal was obtained through the ball-disc friction principle and microcomputer automatic control technology. After the wear was completed, the Zate white light interferometer (KLA) was used to measure the wear morphology of the sample, and the scanning electron microscope (TESCAN VEGA) was used to analyze the wear form of the sample.
2 Results and discussion
2.1 Phase analysis
The X-ray diffraction (XRD) spectrum of the 15-5PH coating is shown in Figure 4. The analysis shows that the phase structure of the 15-5PH coating is mainly composed of Fe-Cr, martensite and α-Fe. The presence of α-Fe phase indicates that the coating contains a certain amount of ferrite, which can improve the toughness of the coating, but also affects the hardness of the coating. Other studies have shown that nickel has a positive effect on the transformation of ferrite to austenite. 15-5PH increases the nickel content on the basis of 17-4PH, so that ferrite is transformed to austenite to a greater extent, which is beneficial to improve the hardness of the coating. As can be seen from Figure 4, the XRD spectrum of the 15-5PH coating is dominated by martensite diffraction peaks, and no obvious austenite diffraction peaks are found. Due to the rapid cooling and heating characteristics of the laser cladding process, most of the austenite is transformed into martensite during the cladding process. As a hard phase, martensite usually has higher strength and hardness. In addition, as shown in Table 1, the iron and chromium contents in the 15-5PH alloy are high, so the Fe-Cr phase is formed, which can improve the strength and corrosion resistance of the material.
2.2 Microstructure
The microstructures of different areas of the coating are shown in Figure 5. During the laser cladding process, the temperature of the molten pool is high and the cooling rate is fast. The microstructure of the cladding layer is also affected by the direction of heat flow loss and the crystallization parameter G/R (G is the temperature gradient; R is the solidification rate). The microstructure of the bonding zone is shown in Figure 5 (d). The molten powder material is in direct contact with the surface of the substrate with a lower temperature. All the heat is transferred to the interface of the substrate. The temperature gradient is the largest and the solidification rate is small. The nucleation rate at the interface is lower than the grain growth rate, thus forming a thinner plane crystal structure. There is a large positive temperature gradient in the process of heat diffusion along the substrate at the bottom of the coating. Under the positive temperature gradient, the columnar crystals formed by the rapid cooling of the molten pool are perpendicular to the melting line and grow in the opposite direction of the heat flow diffusion, forming a columnar crystal belt perpendicular to the melting line at the bottom of the coating (Figure 5 (c)). As the columnar grains at the bottom grow to a certain extent, the heat flow through the substrate is no longer dominant, the temperature difference inside the remaining liquid metal in the middle of the coating decreases, the heat dissipation direction is not obvious, and it tends to be in a uniform cooling state, resulting in a more chaotic growth direction of the grains in the middle (Figure 5 (b)); since the temperature gradient in the middle of the coating is the smallest, the solidification rate is second only to the bottom, which can also provide good conditions for grain growth, thus forming columnar grains. The microstructure of the top of the coating is shown in Figure 5 (a), which is mainly columnar crystals, doped with a small amount of equiaxed crystals, and the grain size is relatively small; this is because the top of the coating is far away from the molten pool, and is impacted and strongly cooled by the environment and supplementary powder, resulting in a large temperature gradient and a small solidification rate, which restricts the growth of columnar crystals and forms a small amount of equiaxed crystals.
2.3 Microhardness
From the microhardness change curve of the cladding sample in Figure 6, it can be seen that the microhardness of the coating and the substrate are relatively stable, and the microhardness of the coating is significantly better than that of the substrate. The average microhardness of the coating is 408.7HV0.5; as the test point gradually moves away from the coating, the microhardness of the substrate tends to be stable, with an average value of 347.5HV0.5, and the microhardness of the coating is 17.6% higher than that of the substrate.
Combining the coating phase and microstructure, it can be seen that there is martensite in the coating phase, which has a high hardness. The grains inside the coating are small, and the number of crystal interfaces increases, which hinders the dislocation and slip movement, thereby inhibiting the plastic deformation of the grains; and because laser cladding has the characteristics of rapid melting and solidification, the solid solubility limit of the solid solution in the coating is improved, thereby enhancing the solid solution strengthening effect of the coating and increasing the hardness of the coating. The microhardness of the heat-affected zone shows a linear decrease trend, and the side close to the coating has the largest microhardness, reaching 477.7HV0.5. This is because under the influence of the fast-moving laser heat source, the heat-affected zone is subjected to a heat effect similar to quenching, which changes the microstructure and grain size of the heat-affected zone; as the distance from the coating increases, the influence of the laser heat source decreases, and the microhardness decreases rapidly.
2.4 Wear performance
Figure 7 shows the friction coefficient curve of the 15-5PH coating and the 17-4PH substrate. At the beginning of the friction and wear test, the friction pair and the test surface are in the running-in stage, and the friction coefficients of the coating and the substrate show a trend of substantial increase; when the test is about 3 minutes, the wear enters the stable stage, and the friction coefficients of the coating and the substrate gradually stabilize within a certain range. This is because at the beginning of friction, the friction pair and the contact surface are in point contact, and the extremely hard GCr15 steel ball is easily pressed into the test piece under the action of compressive stress. The material that falls off remains in the friction track, and the stress is large. Therefore, the friction coefficient fluctuates greatly. As the contact area between the friction pair and the material surface increases, the stress gradually decreases, and the friction enters a stable state. The average friction coefficient of the coating is 0.3051, which is lower than the average friction coefficient of the substrate (0.3754); this is mainly due to the process characteristics of laser cladding, which improves the density of the coating and optimizes the microstructure, thus significantly improving the wear resistance of the coating and reducing the friction coefficient.
In order to further explore the wear form of the coating and the substrate, this study used TESCAN VEGA scanning electron microscope to observe the wear morphology of the coating and the substrate, and the results are shown in Figure 8. It can be seen that both the coating and the substrate have obvious wear morphologies of gullies and stratification. The gullies come from the “plow-shaped” scratches caused by the relative movement of the detached materials remaining in the friction track and the friction pair at the beginning of the friction; at the same time, the large cyclic stress in the contact area exceeds the fatigue strength of the material, and new cracks are constantly generated in the friction track. The cracks gradually expand, and finally crack and peel off, forming a distinct layering. As shown in Figure 8 (b), a certain number of “pits” were also observed in the wear morphology of the 17-4PH substrate, which is because the tiny particles adhered to the surface of the material fall off due to stress. Combined with the above analysis, it can be seen that the wear forms of the coating and the substrate are mainly adhesive wear and fatigue wear.
The Zate white light interferometer was used to measure the wear groove of the sample after wear, and the measurement results were analyzed using the Vision64 software. Figure 9 shows the two-dimensional profile of the wear scar of the coating and the substrate. Among them, the average depth of the surface wear scar of the coating and the substrate is 1.443 μm and 3.054 μm, respectively. The wear scar profile is integrated, and the wear cross-sectional area is 813.74 μm2 and 2058.12 μm2, respectively. Based on the above data, it can be seen that the wear resistance of the coating is significantly better than that of the substrate.
2.5 Corrosion resistance
The dynamic polarization curves of the coating and the substrate are shown in Figure 10. As the potential changes, the self-corrosion current density change curves of the coating and the substrate are basically consistent (Figure 10 (a), where Ecorr1 and Ecorr2 are the self-corrosion potentials of the substrate and the coating, respectively; Epit1 and Epit2 are the pitting potentials of the substrate and the coating, respectively. The local enlarged view is shown in Figure 10 (b). At this potential, the corrosion and anti-corrosion processes on the metal surface reach a state of equilibrium. The more negative the self-corrosion potential of the material, the weaker its corrosion resistance. The increase in the self-corrosion current density (Icorr) indicates that the corrosion rate of the coating increases. Therefore, the greater the self-corrosion current density, the faster the corrosion rate of the coating, and its corrosion resistance is correspondingly lower. At this potential, pitting corrosion begins to occur on the metal surface.
As shown in Figure 10 (a), obvious passivation areas are formed in both the coating and the substrate, and a stable passivation film is formed on the metal surface, which can prevent further corrosion. By Tafel The self-corrosion potential and self-corrosion current density of the coating and the substrate were obtained by linear extrapolation, as shown in Table 2. Comparative analysis shows that the difference between the self-corrosion current density and self-corrosion voltage of the coating and the substrate is very small, which reflects that the corrosion resistance of the coating and the substrate is similar, and can meet the corrosion resistance requirements of the substrate.
3 Conclusion
In this study, 15-5PH coating was prepared on the surface of 17-4PH by laser cladding technology. The performance difference of the material before and after laser cladding was analyzed by observing the phase composition and microstructure of the coating, and testing the microhardness, wear resistance and corrosion resistance of the coating and the substrate.
(1) The 15-5PH coating is composed of Fe-Cr, martensite and α-Fe phases. The bonding area between the coating and the substrate is a thin layer of planar crystal structure. The bottom and middle of the coating are mainly columnar crystals, and the top is composed of columnar crystals and a small amount of equiaxed crystals. The grain size is relatively small. The average microhardness of the 15-5PH coating and the 17-4PH substrate is 408.7HV0.5, respectively. and 347.5HV0.5, the hardness of the coating is significantly higher than that of the substrate.
(2) The average friction coefficients of the coating and the substrate are 0.3051 and 0.3754, respectively; the average wear depths of the coating and the substrate are 1.443 μm and 3.054 μm, respectively, and the wear cross-sectional areas are 813.7 μm2 and 2058.12 μm2, respectively. The wear resistance of the coating is significantly better than that of the substrate, and both are mainly adhesive wear and fatigue wear.
(3) The self-corrosion potential of the 15–5PH coating is –1.0780 V, and the self-corrosion current density is 1.229×10’–3mA/cm2; the self-corrosion potential of the 17–4PH substrate is –1.0975 V, and the self-corrosion current density is 0.907×10’–3mA/cm2; the electrochemical parameters of the coating and the substrate are very small, and the corrosion resistance is comparable.
This study The organization and performance of the laser cladding 15–5PH coating on 17–4PH parts were analyzed, focusing on the hardness, wear resistance and corrosion resistance of the coating. However, in actual applications, such parts may also be subject to severe impact and other working conditions such as tension and compression. In subsequent research, impact and tensile tests of laser cladding coatings will continue to be carried out to further explore the impact resistance and tensile strength of remanufactured parts. In addition, in order to improve the stability and reliability of the repair layer, subsequent research will focus on the interface bonding state and physical and chemical compatibility to meet the long-term service requirements of the repaired parts.
