Modern aero-engine involves many professional disciplines such as pneumatics, thermal engineering, structure and strength, control, testing, computer, manufacturing technology and materials, and is hailed as the pearl in the crown of modern industry. The compressor blade is one of the most important components of aeroengine. Centrifugal force and its bending moment, aerodynamic force and its bending moment, thermal load and vibration load, while facing the threat of damage by foreign objects, such as sand, flying birds, combined with a large number of aero-engine rotor blades, compressor blades are extremely prone to wear, corrosion pits, falling blocks, deformation, cracks and breaking and other damage leading to blade failure,which seriously threaten the reliability and safety of aircraft. Due to 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. So 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 structure of the cladding layer, fine grain, high bonding strength between the cladding layer and the matrix, and small thermal deformation of the matrix.
In this paper, the Ti811 titanium alloy high-pressure compressor blade is taken as the research object, and the cladding coating is prepared on the Ti811 alloy high-pressure compressor blade by using the coaxial powder delivery laser cladding technology, and TC4+Ni45+Y2O3 mixed alloy powder is used as the cladding material. The phase composition, microstructure and microhardness of the cladding layer are analyzed. It provides a basis for the repair of titanium alloy compressor blades.
Test materials and test methods
The substrate used in the experiment was Ti811 titanium alloy high-pressure compressor blade. Table 1 shows the main chemical composition of Ti811 titanium alloy. The surface of the compressor blade was polished with emery paper to remove oxides, and washed with anhydrous ethanol and dried. Laser cladding powder is 65wt%TC4, 33wt%Ni45A and 2wt%Y2O3 mixed alloy powder, powder diameter between 50~120μm. Table 2 and Table 3 show the main chemical components of TC4 and Ni45, respectively.
Table 1 Chemical component of Ti811 alloying (Wt, %)
|
Al |
V |
Mo |
C |
Fe |
N |
O |
Ti |
|
8.1 |
0.99 |
1.05 |
0.03 |
0.01 |
0.05 |
0.06 |
Bal |
Table 2 Chemical composition of TC4 (wt, %)
|
Al |
V |
Fe |
C |
N |
O |
Ti |
|
|
5.5~6.8 |
3.5~4.5 |
0.3 |
0.1 |
0.05 |
0.2 |
Bal |
Table 3 Chemical composition of Ni45 (wt, %)
|
C |
B |
Si |
Cr |
Fe |
Ni |
|
0.3~0.6 |
2.0~3.0 |
3.0~4.5 |
11.0~15.0 |
5 |
Bal |
The laser power rate is 350W, the scanning speed is 7mm/s, the powder feeding speed is 0.9g/s, the laser spot diameter is 1mm, the protection gas flow rate is 17nl/min, the powder gas flow rate is, the powder gas and the protection gas are argon.
The macro view of cladding layer was observed by optical microscopes. GeminiSEM 460 scanning electron microscope (SEM) was used to analyze the microstructure of the cladding layer. The microhardness of cladding layer was measured by Qness Q10A + electronic microhardness tester.
Test results and analysis
Pores and cracks are the most common defects in laser cladding layers. The main reasons for the formation of pores are that the powder gas is not removed in time during the solidification process of the molten pool, and partial cladding materials are vaporized due to the high laser temperature during the melting process. The main causes of cracks are excessive thermal stress, structure stress and confinement stress. In the process of laser cladding, the formation, solidification and cooling of the molten pool are completed in a very short time, and the fast cooling and fast heating process leads to a very large temperature gradient, which greatly increases the thermal stress. The microstructure stress is caused by the difference of specific heat capacity between the cladding material and the base material at the same time, and the uneven transformation during the phase transition. Confining stress is the tensile stress and compressive stress caused by thermal expansion and cold contraction of materials, and it is also an important part of internal stress.
FIG. 4 shows the cross section of cladding layer and cladding layer on Ti811 titanium alloy blade surface. It can be seen that the surface of the prepared multi-channel cladding layer is continuous and uniform, the cladding layer has no porosity, cracks and other visible defects, and the internal structure of the cladding layer is dense and uniform, and the cladding layer forms a good metallurgical combination with the blade matrix. It can be seen that the implementation effect of the laser cladding process is good.
(a) blade cladding layer (b) cladding layer cross section
Conclusion
1. In this paper, mixed alloy powder is used as cladding material on the blades of Ti811 titanium alloy high-pressure compressor, and multi-channel cladding layer is prepared by laser cladding technology. The cladding layer is evenly distributed without macroscopic defects such as pores and cracks. The precipitated phases in the cladding layer are mainly TiC, TIB2, Ti2Ni and α-Ti substrates.
2. In the cladding layer, TiC is equiaaxial spherical, Ti2Ni is irregular massive, TiB2 is dendrite phase, TiC nucleates heterogenous on the surface of TiB2, forming a composite structural phase, and the precipitated phase significantly improves the microhardness and wear resistance of the cladding coating.
3. The microhardness of the laser cladding coating is 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.