Modern aircraft engines involve knowledge and theories of many professional disciplines such as aerodynamics, thermal engineering, structure and strength, control, testing, computers, manufacturing technology and materials, and are known as the jewel in the crown of modern industry. Compressor blades are extremely important key components of aircraft engines. They are subject to centrifugal force and its bending moment, aerodynamic force and its bending moment, thermal load and vibration load, and are also threatened by damage from foreign objects, such as sand and flying birds. In addition, due to the large number of rotor blades in aircraft engines, compressor blades are extremely prone to wear, corrosion pits, falling blocks, deformation, cracks and fractures, which lead to blade failure and seriously threaten the reliability and safety of aircraft. Due to the high technical content, high cost, high processing difficulty and long maintenance cycle of compressor blades, the cost of repairing damaged blades is only 20% of the cost of directly replacing blades. Therefore, repairing damaged blades is a more economical, environmentally friendly and efficient choice.
Laser cladding is a popular surface modification technology in recent years. Compared with traditional surface modification technology, laser cladding has the advantages of high degree of automation, fine and uniform cladding layer structure, fine grains, high bonding strength between cladding layer and substrate, and small thermal deformation of substrate. Luo Kuilin et al. successfully applied laser cladding technology to the repair of a certain type of aircraft engine fan stator through experimental research and optimized cladding process, using TC4 spherical powder as cladding material, and passed the test run assessment; Shen Jingyi et al. used laser cladding technology to prepare FeCrNiB alloy cladding layer on TC4 titanium alloy blades. The cladding layer structure is mainly composed of fine and dense equiaxed crystals, dendrites and cellular crystals, and the microhardness is doubled compared with the substrate. Xu Xiangyu et al. studied the mechanical properties of TC4 titanium alloy cladding coating. The metallurgical bonding between the substrate and the cladding layer is good, and the mechanical properties of the cladding coating and the transition zone fully meet the use standards of aviation titanium alloys. A large number of studies have shown that the application of laser cladding technology in the repair of aircraft engine blades has a very broad prospect.
This paper takes Ti811 titanium alloy high-pressure compressor blades as the research object, uses coaxial powder feeding laser cladding technology, and uses TC4+Ni45+Y2O3 mixed alloy powder as cladding material to prepare cladding coatings on Ti811 alloy high-pressure compressor blades. The phase composition, microstructure and microhardness of the cladding layer are analyzed to provide a basis for the repair of titanium alloy compressor blades.
Experimental materials and test methods
The substrate used in the experiment is Ti811 titanium alloy high-pressure compressor blades. Table 1 Main chemical composition of Ti811 titanium alloy. The surface of the compressor blade is polished with emery paper to remove oxides, and the dirt is washed with anhydrous ethanol and dried. The laser cladding powder is a mixed alloy powder with a mass fraction of 65wt%TC4, 33wt%Ni45A and 2wt%Y2O3, and the powder diameter is between 50 and 120μm. Table 2 and Table 3 are the main chemical composition tables of TC4 and Ni45 respectively.
YLS-1000, according to the previous research foundation, the laser cladding process is set up: laser power is 350W, scanning speed is 7mm/s, powder feeding speed is 0.9g/s, laser spot diameter is 1mm, shielding gas flow rate is 17nl/min, powder feeding gas flow rate is, and both powder feeding gas and shielding gas are argon.
The macroscopic morphology of the cladding layer is observed by optical microscope, and the microstructure of the sample cladding layer is analyzed by GeminiSEM 460 scanning electron microscope (SEM). The microhardness of the cladding layer was tested using a Qness Q10 A+ electronic microhardness tester
Test results and analysis
Macromorphology of cladding layer
Porosity and cracks are the most common defects in laser cladding layers. The main reason for the generation of pores is that the powder feeding gas is not removed in time during the solidification of the molten pool and part of the cladding material is vaporized due to the excessively high laser temperature during the cladding process. In addition, the solidification shrinkage of the molten pool will also produce pores. The main reasons for the generation of cracks are excessive thermal stress, organizational stress and constraint stress; the formation, solidification and cooling of the molten pool during laser cladding are completed in a very short time, and the rapid cooling and heating process leads to a very large temperature gradient, which greatly increases the thermal stress; organizational stress is caused by the different specific heat capacities of the cladding material and the base material, and the uneven transformation during the phase change process; constraint stress is the tensile stress and compressive stress generated by the material due to thermal expansion and contraction, and is also an important part of the internal stress.
Figure 1 is a cross-sectional view of the cladding layer and cladding layer on the surface of the Ti811 titanium alloy blade. It can be seen that the surface of the prepared multi-pass cladding layer is continuous and uniform, the cladding layer has no visible defects such as pores and cracks, the internal structure of the cladding layer is dense and uniform, and the cladding layer forms a good metallurgical bond with the blade substrate. It can be seen that the implementation of the laser cladding process is good.
Microstructure of the cladding layer
Figure 2 is the XRD analysis diagram of the cladding layer. It can be seen from the figure that the cladding layer is mainly composed of intermetallic compounds Ti2Ni, reinforcement phases TiC, TiB2 and α-Ti.
Figure 3 is the SEM image of the cladding layer at different magnifications. It can be seen from the figure that the precipitated phase of the cladding layer is evenly distributed in the cladding coating, and the equiaxed spherical phase A1, irregular block phase A2, dendritic phase A3 and composite phase A4 are distributed in the cladding coating. Figure 4 is the EDS elemental composition analysis diagram of each different phase. From the analysis results in the figure, the equiaxed spherical phase A1 is mainly composed of Ti and C elements, and their atomic ratio is close to 1:1; the irregular block phase A2 is mainly composed of Ti and Ni elements, and their atomic ratio is close to 2:1; the dendritic phase A3 is mainly composed of Ti and B elements, and their atomic ratio is close to 1:2; the mass proportion of Ti element in the dark base A is greater than 80%. Combined with the analysis results of XRD, it can be concluded that A1 is TiC, A2 is Ti2Ni, A3 is TiB2, A4 is TiC and TiB2, and A5 is α-Ti.
TiC is a face-centered cubic NaCl-type structure formed by inserting smaller C atoms into the octahedral position of the Ti close-packed lattice. It generally generates primary crystals in the form of equiaxed spherical or nearly spherical particles, and exists in the matrix in the form of short rods and dendrites. Moreover, Y2O3 in the cladding material is moderately effective as a heterogeneous nucleation core of TiC, which can play a role in refining TiC grains. Therefore, the TiC precipitated in the cladding layer is mostly equiaxed spherical, which is consistent with the equiaxed spherical phase TiC observed on the way.
According to relevant research, the lattice mismatch between TiB2 and TiC is only 1.6%, which is much smaller than the critical mismatch of 16% for incoherent formation, which is conducive to the heterogeneity of TiC on the white surface of the precipitated TiB2 phase. At the same time, since the Gibbs energy of TiB2 is lower than that of TiC, TiB2 precipitates before TiC during the solidification of the molten pool, and TiB2 is a close-packed hexagonal crystal structure (C32). TiB2 usually precipitates in the melt in the form of flakes and dendrites. After TiB2 precipitates and fully grows, there is less B element and more C element in the molten pool, and Ti atoms and C atoms are enriched on the surface of TiB2. After the TiC nucleation conditions are met, TiC nucleates and grows on the surface of the TiB2 crystal, thus forming a composite structure phase of TiC+TiB2.
Microhardness
Figure 5 is a microhardness gradient diagram of the laser cladding layer on the surface of the Ti811 alloy blade. The horizontal axis is the distance from the top of the cladding layer, and the vertical axis is the microhardness of the blade cladding coating (HV0.3). It can be seen from the figure that the microhardness increases linearly from the blade substrate to the cladding layer. This is because the hard phases TiC and TiB2 and the intermetallic compound TiNi generated in large quantities during the cladding significantly increase the hardness of the cladding layer. The average hardness of the blade substrate is 442HV0.3, the maximum microhardness of the cladding layer can reach 982HV0.3, and the average hardness of the cladding layer is 906HV0.3. The hardness of the cladding coating is significantly greater than that of the blade substrate, which is about 2.04 times that of the blade substrate.
Wear resistance
Table 5 shows the wear performance of Ti811 substrate and cladding coating, and Figures 6 and 7 show the three-dimensional morphology of the wear of Ti811 substrate and cladding coating and the contour curve of the wear section. It can be seen that the wear rate of the cladding coating is 1.07×10-3mm2/(N·m), the wear rate of the substrate is 2.21×10-3mm2
/(N·m), and the wear rate of the cladding coating is reduced by 51.5% compared with the substrate. This is because the ceramic phases TiC and TiB2 and the intermetallic compound Ti2Ni distributed in the cladding
coating play a skeletal support role in the wear process, effectively delaying the invasion of abrasive particles, reducing the travel of the tearing shear point during the wear process, and significantly improving the wear resistance of the cladding coating. The wear scar depth of the cladding coating is about 30μm, and the wear scar depth of the substrate is about 55μm, which is reduced by 45.5%. From Figures 6 and 7, it can be seen that the worn surfaces of the Ti811 substrate and the cladding coating are accompanied by a small number of grooves, but the number of grooves on the cladding coating is less, and the worn surface is smoother. These grooves are caused by the harder WC surface particles invading the softer substrate and coating. According to the Archard model, when the sliding distance and normal load remain unchanged, the greater the microhardness, the better the wear resistance. The hardness of the cladding coating is much greater than the hardness of the substrate, which effectively improves the wear resistance of the coating. Therefore, there are fewer grooves on the worn surface of the cladding coating.
Summary
(1) This paper uses mixed alloy powder as cladding material on Ti811 titanium alloy high-pressure compressor blades, and uses laser cladding technology to prepare multiple cladding layers. The cladding layers are evenly distributed and have no macro defects such as pores and cracks. The phases precipitated in the cladding layer are mainly TiC, TIB2, Ti2Ni and α-Ti substrate.
(2) In the cladding layer, TiC is equiaxed spherical, Ti2Ni is irregular blocky, TiB2 is dendritic, TiC is heterogeneously nucleated on the surface of TiB2 to form a composite structure phase, and the precipitated phase significantly improves the microhardness and wear resistance of the cladding coating.
(3) The microhardness of the laser cladding coating can reach up to 982HV0.3, and the average microhardness is 906HV0.3, which is about 2.04 times that of the substrate. The wear rate of the cladding coating is 1.07×10-3mm2/(N·m), which is about 51.5% less than that of the substrate.
