As the performance of aircraft engines continues to improve, the operating temperature of the combustion chamber also becomes higher and higher. In order to improve the performance of aircraft engine turbine blades under high temperature conditions, air film holes are usually made on the blade surface. The hole processing quality of the air film is crucial to its bearing capacity and service life. Laser processing technology is currently one of the main air film hole preparation methods. The laser light sources used are mainly divided into long pulse laser, short pulse laser and ultra-short pulse laser. Long pulse laser and short pulse laser will produce microcracks and recast layers during the hole making process. The ultra-short pulse laser has almost no thermal damage to the material, but it has higher requirements for processing equipment and working environment. the treatment effectiveness is not high. Therefore, in order to meet the needs of high-quality air film hole processing, a water-guided laser processing method was proposed, a water-guided laser processing system was constructed by us themselves and a hole making test of the DD6 alloy was carried out. . The test successfully produced 400 µm diameter straight holes and 45° inclined holes. The processed air film holes had almost no recast layers, microcracks and no heat-affected zones, providing an effective technical reference for air film hole processing.
01
Preface
With the rapid development of aviation technology, high thrust-to-weight ratio and low fuel consumption have become the main development trends of aircraft engines. Therefore, the working environment of turbine blades has become more and more rigorous, which has imposed higher design requirements. , manufacturing and performance of turbine blades.[1]. At present, turbine blades are mainly made of nickel-based single-crystal superalloys, and air film holes are made on their surfaces to meet the requirements of high temperature resistance.[2]. Research shows that film cooling technology plays a 60-70% role in increasing the operating temperature of turbine blades. Therefore, the production of high-quality air film holes is crucial to improve the bearing capacity and service life of aircraft engines.
Laser processing technology is one of the main processing methods for preparing air film holes in aircraft engine turbine blades. Researchers at home and abroad have carried out a lot of research on laser processing of air film holes.[3]. At the beginning, the laser light source for laser processing of air film holes mainly used millisecond (ms) laser.[4]A millisecond laser is used to create air film holes on the surface of the DD6 alloy. The obtained air film holes have thermal defects such as recast layer and microcracks, and the surface roughness is poor. With the emergence of ultrashort pulse laser, its use for treating air film holes has been widely studied. Zhang Ruifeng et al.[5]The picosecond laser is used to process the gas film holes on the surface of the nickel-based single crystal alloy. The resulting hole wall has no recast layer or heat affected zone. However, due to the impact of the plasma, slight cracking is caused on the hole. wall and surface. The femtosecond laser has great advantages in material processing due to its extremely short pulse time and extremely high peak power.[6]A femtosecond laser hole creation test was carried out on a nickel-based alloy sample, and the relationship between the energy parameters (femtosecond laser pulse width, laser wavelength and repetition frequency) and the diameter and depth of the micro-holes was studied quantitatively. No recast layer, microcracks and heat affected zone. However, the femtosecond laser has higher requirements for equipment and environment, and the processing efficiency is low.
In response to the above problems, this article proposes water-guided laser processing technology. Water-guided laser processing technology uses the principle of total reflection of the laser in the water beam and uses water as an optical fiber to guide the laser to the surface. of the workpiece.[7]. Due to the scrubbing and cooling effect of the high-speed jet, the water-guided laser has a significant inhibitory effect on the thermal damage caused by the treatment. The processed holes have high processing quality, extremely low heat affected zones and minimal recast layers. .[8，9]. At the same time, water-guided lasers mainly use nanosecond lasers as the laser light source, which have higher processing efficiency. Therefore, this paper uses water-guided laser processing technology to process the air film holes of aircraft engine turbine blades. By comparing the quality achieved using conventional laser processing methods, the processing advantages of water-guided laser are established. The impact of different powers on the processing quality was studied, and straight holes and 45° inclined holes were successfully produced, providing a new solution for the field of air film hole processing.
02
Experimental analysis
2.1 Material analysis
The ultimate goal of this research is to produce high-quality air film holes on aircraft engine turbine blades. The structure is shown in Figure 1. Turbine blades are often cast from single-crystal nickel-based high-temperature alloys. This type of alloy has high strength and hardness as well as excellent resistance to high temperatures and corrosion. The test uses the nickel-based high-temperature single-crystal alloy brand DD6 as the research object, with a specification of 20mm × 20mm × 2.6mm. The chemical composition and physical properties of the material are shown in Table 1 and Table 2.
Figure 1 Structure of turbine blades
Table 1 Chemical composition of nickel-based single-crystal alloy DD6 (mass fraction) (%)
Table 2 Physical properties of nickel-based single-crystal alloy DD6
Before testing, the sample must be cleaned with absolute ethanol. Once the surface is free of stains, it is placed in an ultrasonic cleaner. The temperature is set at 50°C and the frequency is 40 kHz. After ultrasonic cleaning for 15 minutes. , it is dried. After the test, in order to facilitate the observation of the degree of laser ablation on the sample surface, the sample surface was ground and polished.
2.2 Hole making characteristics
The laser hole making test adopts the ring cutting hole making method, that is, controlling the XY working platform to control the movement trajectory of the workpiece in a series of circles concentric. The processing process is shown in Figure 2, and the air film hole quality requirements are shown in Table 3.
Figure 2 Schematic diagram of ring cutting and hole making processing process
Table 3 Quality requirements for air film holes (unit: μm)
2.3 Test equipment
The light source of the processing system used in this article is Pulse 532-50-LP laser produced by Suzhou Yinggu. Table 4 shows the relevant laser parameters. The nozzles involved in the test include three specifications: 60 μm, 80 μm and 100 μm (diameter). The diameter of the focused spot in the center of the nozzle is about 30 µm, which meets the coupling requirements.
Table 4 Laser Parameters
03
Water guide laser treatment method
3.1 Principle of water-guided laser action
When light is projected from water to air, if the angle of incidence is greater than a certain value, the light will be completely reflected at the water-air interface and transmitted with the jet. The working principle of water-guided laser processing technology is shown in Figure 3.[10]. The focused laser passes through the glass window and water layer and converges on the top surface of the nozzle, where it couples with the high-pressure jet emitted by the nozzle to form a coupled energy beam. The laser energy is not focused directly on the workpiece surface, but is transmitted to the workpiece surface through the water jet in capillary laminar flow. After the water jet in the stable range collides with the workpiece, the laser energy guided by the water jet is absorbed by the material surface, causing the material in the area to melt and evaporate. ablation. At the same time, the high-speed jet removes molten material, excess heat and residue, effectively scouring and cooling the treatment area.[11]。
Figure 3 Working principle of water-guided laser treatment technology
3.2 Water guide laser treatment system
The water guided laser processing system mainly includes 4 parts, namely coupling system, optical transmission system, water supply system and motion control system. The core part is the coupling system, which is used to generate fine water jets, couple the laser to the water beam, and observe and adjust the coupling state in real time. The optical transmission system includes lasers and adjustment lens groups such as collimator beam expanders, which are used to shape the beam generated by the laser into a coupleable beam. The water supply system provides high-pressure fine water jets, and the motion control system is used to control the motion status of the machine tool. The principle and actual construction of the water-guided laser processing system are shown in Figures 4 and 5.
Figure 4 Principle of water-guided laser treatment system
1—Laser 2—Shutter 3—Beam expander 4, 5—Reflector
6—Visible light source 7—CCD camera 8, 10—Thin film beam splitter 9—Attenuator
11—Focus lens 12—Coupling device 13—High pressure water inlet
14—Water-guided laser coupling energy beam 15—DD6 alloy
Figure 5 The actually constructed water-guided laser processing system
04
Comparison of the effects of water-guided laser and other treatment methods
To establish the processing advantages of water-guided laser, microsecond pulse laser, femtosecond laser, and water-guided laser were used to drill holes in DD6 alloy. The surface morphology of the obtained holes was compared. The results are presented in Figure 6.
a) Microsecond laser b) Femtosecond laser c) Water guide laser
Figure 6 Surface morphology of holes produced by different laser processing methods
Figure 6a shows that there is a large heat-affected zone on the surface of the hole processed by microsecond laser, the hole edge continuity is poor, and there is obvious molten material deposition. This is because the pulse width and power of the microsecond laser are large and the material cannot be completely cooled after the pulse is completed, and the heat continues to be transferred into the material, resulting in an affected area thermally on a large scale; the ablated material cannot be effectively discharged in time, deposited on the material surface to form droplet-like molten deposits. The hole surface processed by the femtosecond laser has almost no processing damage such as heat affected zone, material melting and redeposition. This is because the pulse width time of the femtosecond laser is shorter than the heat transmission time between the arrays. femtosecond laser processing This is a rough “cold working” process, resulting in better quality holes. However, due to the Gaussian distribution of laser energy, the taper of femtosecond laser holes is difficult to control, and the roundness of processed holes is poor and the processing efficiency is low, so it is difficult to process large quantities of holes. In contrast, the water-guided laser removes the molten material in time during the processing process, thereby overcoming the problem of molten metal splashing in traditional microsecond laser processing. At the same time, the jet effectively washes the cooling treatment wall, which can greatly. improve surface quality and reduce heat affected zone. During the water-guided laser processing process, the transverse energy distribution of the coupled energy beam is uniform, which makes the material surface uniformly heated during the processing process, enabling vertical processing over a longer period of time. distance, as well as the roundness and taper of the hole are also well controlled. In addition, water-guided lasers use nanosecond lasers, which can produce high energy power, significantly improving the efficiency of hole making compared with femtosecond lasers. In summary, compared with microsecond pulse laser and femtosecond laser, water-guided laser has significant processing advantages in the field of DD6 alloy hole making.
05
Processing straight holes
A nozzle with a diameter of 60 μm was used, the jet pressure was set at 15 MPa, the scanning speed was 1 mm/s, the average power was 50 W and 40 W, respectively, and a hole with a diameter of 400 µm was made on the DD6 high-temperature alloy. The surface morphology of the pores was characterized and the results are shown in Figure 7.
a) 50 W, upper surface b) 50 W, lower surface
c) 40 W, upper surface d) 40 W, lower surface
e) 40W, cross section
Figure 7 Morphological characteristics of holes treated under different average powers
The processing results show that there is almost no slag splashing on the hole surface obtained by processing DD6 under different average powers, the hole entrance edge is sharp, the roundness is high, and the affected area thermally is extremely small. However, comparing the morphology of the inlet and outlet obtained at different power levels, it is found that there is a small amount of molten metal deposition on the surface of the hole processed under a power of 50 W, and that there is a small amount of scales and material. overhaul near the exit; while the hole processed under 40W average power has a small amount of molten metal deposition, the surface roundness of the hole exit and entrance is higher, there is no layer of recast nor heat affected zone, and there is only a small amount. quantity of material deposited on the entry surface. Analyzing the reasons for this phenomenon, it is believed that under the same conditions of operating frequency and pulse width, higher average power corresponds to higher single pulse energy and peak power, and the rate erosion of the material is higher. The water jet formed by the water pressure of 15 MPa is not enough to completely cool and flush the processing area under these conditions, which prevents part of the molten material from being discharged in time, forming a small amount of deposits on the surface of the hole, and causing the formation of a recast layer. When the average power is reduced to 40 W, the cooling and scrubbing effects of the jet on the treatment area are more sufficient in the pulse space. Material deposition on the hole surface is significantly reduced. The recast layer is almost invisible on the surface. bottom of the hole, and the roundness of the hole inlet and outlet is significantly improved.
The hole treated with a power of 40 W was cut vertically from the middle, and the obtained half-hole morphology is shown in Figure 7e. It can be seen that the hole wall is relatively smooth, has good continuity, and has almost no recast layers or heat-affected zones. In addition, the diameter difference between the hole entrance and exit is not obvious, which proves that the hole obtained by water-guided laser processing has a smaller taper than other laser processing methods. This difference is mainly due to the fact that most traditional laser energy has a Gaussian distribution. Once the laser is focused, the energy quickly diverges and processing capacity is lost. The guided laser has a uniform distribution of energy density in the cross section of the jet within the working distance, and the processing depth of field is larger. The taper of the resulting hole is smaller.
06
Treatment of oblique holes
Most motor blades have a curved surface structure, and the tilt angles in different positions are very different. It is often necessary to process oblique holes on the blades. In order to ensure the integrity of the inclined hole, it is necessary that the exit and entrance beveled edges of the produced inclined hole are smooth and free of defects. Use a water-guided laser to make a 45° inclined hole on 2.6mm thick DD6 alloy. The diameter of the target hole is 400 µm. A concentric circular tool path is used. The selected parameters are: average laser power 40 W, scanning speed 1 mm/. s, water pressure 15MPa. Characterize the top and bottom surfaces and cross-sectional morphology of the treated hole, as shown in Figure 8.
a) Upper surface b) Lower surface
c) Section
Figure 8 Morphology of oblique holes obtained by water-guided laser treatment
For samples of the same thickness, the processing distance of oblique holes is longer than that of straight holes, and the processing is more difficult. However, observing the characterization results, it was found that the roundness of the inclined hole entrance was very good, and there was no molten material on the surface, which was consistent with the surface untreated. The edge of the hole is not missing or broken, but there is some damage at the end where the angle between the hole axis and the coating surface is acute. This is because the material at the end of the acute angle is thinner and is easily removed. by high speed water jets, causing minor defects. The end of the hole where the angle between the hole axis and the surface is an obtuse angle is sharp and no collapse occurs. The morphology of the exit surface is consistent with that of the entrance. However, as drainage at the bottom of the hole is more difficult and scrubbing and cooling performance is poorer, the surface quality obtained is reduced accordingly. The cross-section quality of the inclined hole is basically the same as that of the straight hole, the hole wall is smooth and continuous, and there is almost no recast layer and heat-affected zone, which meets the production requirements of air film. turbine blade hole. Tests have shown that the water-guided laser can produce high-quality straight holes with a diameter of 400 μm and 45° inclined holes in 2.6 mm thick DD6 nickel-based superalloy, which which has significant advantages in the field of air film hole production. in the turbine blades.
07
Conclusion
A water-guided laser was used to perform hole processing on a 2.6 mm thick DD6 nickel-based high-temperature alloy. The treatment quality and efficiency were compared with those of a microsecond pulse laser and a femtosecond laser, as well as the treatment advantages of the water-guided laser. were established. The 400μm straight holes and 45° inclined holes processed by water-guided laser processing are of excellent quality, with good hole surface roundness and almost no defects caused by material deposition, areas affected by heat and chipping. They meet hole making needs. industrial production and are ideal for turbines. Producing air film holes in the blades provides a better processing method.
Daguang focuses on providing solutions such as precision CNC machining services (3-axis, 4-axis, 5-axis machining), CNC milling, 3D printing and rapid prototyping services.
