Analysis of defect detection technology of aero engine high pressure turbine blade
Turbine blade is a key component of aero engine, and its processing is complicated, which requires very high quality inspection. Laser direct writing (LDM) is used to manufacture domestic high pressure turbine blades. It has the characteristics of high precision, high density and high spectral resolution, and can be used for 3D measurement, non-destructive testing and 3D reconstruction of 3D products. With the application of laser direct writing technology in domestic high pressure turbine blade manufacturers, high pressure turbine blades have achieved mass production. This paper introduces the laser direct writing forming process and defect detection technology of domestic high pressure turbine blades, and analyzes the defect detection method and software.
With the rapid development of aero-engine technology, aero-engine puts forward higher requirements on blade quality. Turbine blade strength, fatigue life and surface mass complexity are important indicators to measure its performance. Due to the complex manufacturing process of high-pressure turbine blades, most of the blades are produced by laser direct writing. Laser direct writing technology uses a high-power semiconductor laser to continuously irradiate the laser in the area to be processed, so that it forms a uniform distribution of laser beams in the area to be processed. The traditional testing methods include machining and non-destructive testing, which have some problems such as high processing cost, low detection efficiency and easy to be disturbed by human. Therefore, in order to ensure that turbine blades have excellent mechanical properties, corrosion resistance and fatigue resistance, they must be accurately and quickly tested.
1.LDM process introduction
Laser direct writing (LDM) is a kind of laser beam with variable intensity to implement variable dose exposure on the resist material on the substrate surface, and form the required relief contour on the resist surface after development. Its main contents include: select the appropriate ceramic material, select the appropriate processing method, optimize the laser processing parameters. LDM technology is a method of using high-power laser to write different patterns on ceramic materials. It etched microstructures on the surface of ceramic materials to achieve complex morphology, hyperspectral resolution and digital product modeling, and integrated it with LDM process to generate rich surface details to meet the requirements of high precision and high stability of precision equipment such as aircraft engines. Laser direct writing technology is a set of laser processing, non-destructive testing, image processing, CAD/CAM in one of the new manufacturing technology, compared with the traditional process, the technology has the following advantages: ① high processing accuracy; ② Fast processing speed; ③ High material utilization rate; ④ Good surface quality; ⑤ can be personalized customization. LDM technology uses laser direct writing method to write the surface of ceramic materials, and photochemical reactions occur in the internal microstructure of the material (such as atoms, molecules, etc.) under the action of laser, thus changing the structure and properties of the material. There are many ways to achieve laser direct writing technology, and there are mainly three types for ceramic materials: the first type is the traditional method (such as chemical vapor deposition, melting rapid quenching, plasma enhanced CVD, etc.); Second, advanced technology (such as 3D printing, laser direct writing, etc.); The third is 3D printing + laser melting pole technology (such as: 3D printing + laser melting pole shift technology, etc.). There are three main methods of laser selective melting forming technology. One is to use laser to denude ceramic materials to make them have complex three-dimensional morphology. The second is etching, etching; The third is to use laser direct etching method on the surface of ceramic materials for graphic processing. The laser energy density used in LDM technology is high, and a high energy density is required to etch ceramic materials. At the same time, the laser ablation depth must be precisely controlled.
2. Defect detection technology
At present, the industrial detection of blade defects is mainly X-ray method, ultrasonic method and X-ray perspective method. X-ray method, ultrasonic method is a non-destructive testing method, it can detect the internal defects of the material, X-ray perspective method is a use of X-ray or gamma rays emitted by the source to irradiate the processed object, so as to detect the tiny defects inside the material, but the ray penetration ability is limited, can not detect the tiny defects. Therefore, in practical applications, X-ray method and ultrasonic method are the main detection means. However, with the development of technology, industrial CT detection with micro-focus has been widely used in the field of turbine blade manufacturing due to its non-destructive, high efficiency and high precision.
(a) Radiographic transillumination of the inlet edge
(b) Radiographic penetration of the exhaust edge
(c) The intake edge is transilluminated by digital radiography
2.1 X-ray detection X-ray detection is to use an X-ray tube to emit X-rays on the surface of the object under test, observe the defects on the surface of the object under test, and then use image recording to quantify and locate the object. According to the different penetration depth, X-ray can be divided into three methods: penetration depth, penetration width and penetration thickness. The transillumination method uses X-ray tube to irradiate the surface of the tested material to detect the internal defects of the material. Due to the limitation of equipment and technology, the method is difficult to achieve accurate quantification of the internal defects of complex structural parts. This method is suitable for the workpiece with smooth surface and uniform density, but it cannot accurately locate and quantify the complex internal components.
2.2 Ultrasonic detection The basic principle of ultrasonic detection is to use the ultrasonic detector and probe to emit ultrasonic waves, and the probe receives the echo for positioning. Ultrasonic detection technology is widely used in industrial fields because of its advantages of high sensitivity, high penetration, high precision and continuous detection. For metal materials, usually use straight head and oblique head two methods, straight head detection depth is generally 1mm, inclined head detection depth is generally 5mm, in practical applications, ultrasonic detection device according to the different objects to be measured, using different probes. The thermal conductivity of turbine blade material is high, so the probe with good thermal performance must be selected for ultrasonic detection. For low intensity ultrasonic signal, such as room temperature glass ceramic rod, because of its good thermal properties, can fully meet the detection requirements. For materials containing high-density defects or inclusions, a probe with strong penetration and high sensitivity should be selected, and for materials containing large-size defects, continuous emission method and pulse reflection method can be used to detect. In practical application, the coupling method of single longitudinal wave, double shear wave and longitudinal wave can be used, and it is feasible to use single longitudinal wave detection for materials containing cracks and other defects. At present, ultrasonic testing technology has been widely used, but because of its expensive testing equipment, it is not suitable for field testing.
2.3 Microfocus Industrial CT detection Microfocus industrial CT detection mainly uses X-ray or gamma ray transmission and reflection in the substance to form a ray beam, and then the detector receives the ray beam irradiation on the detected object to absorb energy, converted into X-rays or gamma rays, and then the detector converts the energy into electrical signals, and then the structure image of the object can be obtained after processing. During the detection, the object is first placed on the X-ray source, and then the signal formed by the X-ray beam passing through the object is received by scanning method. When the detection object is in a non-transparent state, the signal received by the detector will be spotty; Speckle is generated by the signal received by the detector when the detected object is transmitted. When the spot area is large, it indicates that there is a large defect in the detected object. When the spot area is small, it indicates that there is a small defect in the detected object. In order to eliminate the effect of speckle on the image quality, special methods can be used to eliminate the speckle effect and improve the image quality. For example, a color filter can be added in front of the detector to eliminate spots, in addition, speckle can be suppressed by changing the detector parameters, and linear scanning can be performed for small size defects; For large size defects, surface scanning is possible. For the detection of high-pressure turbine blades, appropriate test methods and test parameters should be selected according to the specific working conditions. Multi-beam light detection is usually adopted, and linear array detectors are used as the main detection unit in the image acquisition system. X-ray and gamma rays are mainly used for detection according to different blade materials.
3.Defect detection software introduction
This paper introduces a microfocal CT scanning software suitable for high pressure turbine blade defect detection. The software mainly performs the following functions: (1) scan data reading; ② Image measurement and analysis; ③ Automatic detection of defects; ④ Data management; ⑤ Quality control; ⑥ Three-dimensional reconstruction. Among them, the reading of scan data is a very important data, which determines the number, position, shape, size and other information of the image midpoint. Based on the detection results, the CT scan results can be adjusted according to different requirements. For scanning data processing, the software has defects classification, defects filtering, defects registration, defects correction, defects reconstruction and other functions. Table 1 CT scan parameters.
4.LDM blade detection test research
The actual operation data before and after mixing are shown in Table 6. It can be seen from Table 6 that under test conditions, when 100% natural gas is burned, the output power of the gas turbine is 179.8MW and the efficiency is 35.49%. The output power of the gas turbine is 169.0MW and the efficiency is 35.81%, which is basically consistent with the calculated value.
4.1 Secondary processing defects Secondary processing refers to the blade repair, grinding, polishing and other processing processes, in the secondary processing process may appear the following problems: (1) surface roughness is not up to standard: in the polishing process polishing equipment will produce a certain noise, so that the surface roughness after polishing can not meet the requirements. In order to eliminate this kind of noise, manufacturers generally use ultrasonic, electrolysis and other methods to remove it, ultrasonic, electrolysis can remove the surface roughness, but ultrasonic is more susceptible to the impact of dust or oil on the surface of the blade, therefore, whether ultrasonic or electrolysis, are not suitable for removing the surface roughness of the blade. In actual production, when the surface roughness of the blade does not meet the requirements, grinding can be used. Although the defects can be effectively eliminated, secondary processing is still needed after grinding. (2) Unqualified surface quality: In the production process of high-pressure turbine blades, if the surface quality of the blades fails to meet the standard, measures such as polishing and polishing can be taken to solve the problem. Although this method can eliminate defects, it reduces the performance of the blades. In order to improve its performance, manufacturers often polish and polish it many times in the production process, but when grinding and polishing, it is easy to produce secondary processing defects.
4.2 Material stratification In the manufacturing process of high-pressure turbine blades, due to the mismatch of process parameters, one or more raw materials or impurities enter the inside of the blades, resulting in material stratification. In the actual test, the high pressure turbine blade with delamination defects can be placed on the sample disk, and the sample disk can be compared with the ordinary sample disk to find the delamination defects of the material. If there is a problem during the positioning process, further inspection is needed to determine its specific location, so as to determine the specific type of defect.
4.3 Porosity and Slag inclusion defects such as porosity and slag inclusion are common quality problems in the production of high-pressure turbine blades. Porosity defect is the main cause of material strength decline, which has an important impact on the performance of high-pressure turbine blades. In actual production, defects are often characterized by small bubbles inside. Compared with other solid substances, the size of the bubble is very small compared with other solid substances, when the inner wall of the bubble is subjected to great stress, cracks will occur, in addition, the inner wall of the bubble is relatively weak, easy to rupture under the action of external stress. There are some heat transfer problems in the processing of high-pressure turbine blades, which will cause burning phenomenon to a certain extent. If the ablation part is not removed in time, inclusions may be formed. Slag inclusion is a common form of inclusions, and slag inclusion defects are more serious than porosity defects, which not only seriously affect the service performance and life of high-pressure turbine blades, but also may lead to blade strength decline or even failure. In actual production, if the slag inclusion area of high-pressure turbine blades is not large, the conventional industrial CT method can be used to detect it; if the slag area is large or there are obvious defects, the microcoke industrial CT should be used to detect and analyze. In the process of microfocus industrial CT detection, in order to prevent image blur, the image can be preprocessed and segmented to obtain clear and accurate defect information.
In summary, with the continuous development of aero-engine technology, quality detection of high-pressure turbine blades is becoming more and more important. This paper introduces several common high pressure turbine blade defect detection technologies. In practical applications, different defect detection technologies are different. When applying different defect detection technologies, it is necessary to select and combine them according to the specific conditions of the blades. The development of high pressure turbine blade defect detection technology still faces many challenges and difficulties. In the future, equipment accuracy, data processing capability and algorithm performance need to be further improved to better meet the requirements of aero-engine high pressure turbine blade defect detection.
