Laser drilling in the hottest aviation field

2022-08-16
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Laser drilling in the field of aviation

this paper studies the practical application effect of continuous drilling with direct transmission and optical fiber transmission peak power pulse (about 20kW) lasers. At the same time, a large number of holes are processed on super heat-resistant nickel base alloy with various lasers and working parameters, and the relevant data of processing time, recast layer, taper, oxide layer and cracking are counted

the blades, nozzle blades, combustion chambers and other components of aviation gas turbines need to be cooled during working conditions. Therefore, thousands of holes are punched on the surface of these components to ensure that the surface of these components is covered by a thin layer of cooling air. This layer of cooling air can not only prolong the service life of parts, but also improve the working performance of the engine. A typical advanced engine surface will have 100000 such holes. With the development of drilling technology, the industry usually uses peak power pulsed Nd YAG laser to process, and trepanning and pulse drilling technology have been successfully applied

EDM hole processing and laser drilling

at present, the gas temperature used in jet engines in the aviation field can reach 2000 ℃, which has exceeded the melting point of turbine blades and combustion chamber materials, that is, nickel alloy. Therefore, people generally use the method of boundary layer cooling to solve this problem, that is, machining holes on the surface of pneumatic turbines, nozzle blades and combustion chambers (see Figure 1), Among them, there are 25 to 40000 holes on each part (the parameters of specific parts are shown in Table 1). The cooling gas can cover the surface of the whole part through the holes on the part to isolate the external temperature, so as to play a protective role. Table 1 typical applications of cooling holes

Figure 1 devices after laser perforation

cooling holes can be processed by EDM or laser. Although EDM can be used to process qualified holes, the processing efficiency is significantly lower than that of laser processing. In addition, EDM has the following three disadvantages:

1 It is suitable for occasions with low incident angle and variable incident angle

2. The use of various consumables, such as electrolyte, increases the processing cost

3. In order to improve the heat resistance, the blade surface needs to be coated with insulating ceramics, but EDM is not suitable for perforation on the ceramic coating material

at present, pulsed nd:yag laser has become the first choice of drilling equipment in aerospace field, mainly because it has the following advantages:

1 Use 1.06 μ M wavelength has a good effect on material processing

2. It has the characteristics of high pulse energy and peak power

3. It can quickly process cooling holes with high aspect ratio on the surface of various materials (including heat-resistant coating materials) (see Figure 2)

Fig. 2 a stator blade of a turbine (Siemens generator), surface plasma sputtered thermal isolation coating material YSZ (zirconia)

laser drilling and its quality control

there are two basic laser drilling methods in the aviation field: casing and laser pulse drilling. Casing is to drill a hole in the center of the hole with a laser pulse, and then move the laser beam to the circumference of the hole or process a hole by rotating the part. Laser pulse drilling requires neither moving the laser beam nor moving the parts. The hole can be processed only by continuous laser pulses, and the diameter of the hole can also be adjusted by controlling the pulse energy in the processing process. Therefore, the processing cycle of parts can be greatly shortened, especially when the experimental results are very stable, and the processing time can be further shortened when processing parts with symmetrical structures such as combustion rings and combustion chambers. Laser pulse drilling has become a very important application technology in the aviation industry. The pulse frequency of the laser is synchronized with the rotation frequency of the workpiece, and the laser pulses process all the holes in a specific arrangement at the same time. However, although this "drill on the fly" technology shortens the processing time, the quality of the processed holes is usually not ideal

The quality of holes is very critical. The quality of holes processed by laser can be judged by different characteristics. Considering the geometric elements, it can be judged by the changes of the roundness, taper and inlet diameter of the hole. From the perspective of metallography, it can be judged by the structural changes of recast layer and oxide layer. Among them, the recast layer is formed because the molten metal is not sprayed out by the air pressure generated by the laser pulse, but is left in the hole, so a thin layer of solid metal coating is left on the hole wall, and the surface of this layer of metal coating will produce microcracks, which will directly spread to the body. For a long time, the standards used by airlines have been constantly trying to improve the quality of holes. For example, Rolls Royce Airlines has established the maximum thickness standard of acceptable oxide layer and recast layer according to the actual situation, so that the geometric dimension of the hole on the workpiece has the maximum acceptable offset value range before the workpiece is used. Other airlines judge the quality of machined holes through the gas fluidity of parts

at present, direct beam transmission system is mostly used in drilling of aviation parts, but due to many technical reasons, the application of optical fiber output system in laser drilling has been developing slowly. There are two main reasons for this: first, the damage threshold of optical fiber is relatively low; Another reason is the transmitted beam quality. The diameter of the optical fiber will lead to the deterioration of the beam quality M. However, when M2 = 25 or better, qualified holes can also be produced by using the correct pulse parameters. Therefore, the optical fiber application system has certain advantages over the direct beam transmission system, mainly reflected in:

1 The laser beam transmission system provides options for laser transmission on CNC machine tools

2. The energy homogenization brings the characteristics of top hat and improves the roundness and consistency of holes

3. Transmission pulse drilling technology greatly shortens the processing time in high-quality perforation, which is conducive to improving production efficiency and reducing processing costs

pulse perforation

the following mainly discusses the application of pulsed nd:yag lasers with peak power (up to 20kW) in direct beam transmission and optical fiber transmission systems, respectively. We choose to drill holes on nickel base alloy with different lasers and parameters, so as to study the range of parameters such as recast layer, taper, oxide layer crack and processing time

1. Drilling test

(1) laser

jk704 laser is selected for direct beam drilling in the test. This kind of laser can provide high peak power (see Table 2) and good pulse stability, which is very suitable for machining small diameter holes (0.25 ~ 0.90mm). The Gaussian beam quality (see Figure 3) and enhanced control and pulse shaping characteristics of the laser provide greater flexibility in processing aerospace materials, including materials with thermal insulation coatings

Figure 3 beam quality of jk704

Table 2 laser parameters of jk704

this fiber transmission drilling test will be completed with the latest peak power pulse laser jk300d of GSI (see Table 3 for parameters). This kind of laser has high peak power and top hat characteristics (see Figure 4), which is suitable for pulse perforation of aviation alloy materials. The laser beam is 10m × 3 selection of control system except user elements 00 μ M diameter optical fiber, and output through 160mm right angle collimation system and optical focusing lens. Table 3 JK 300D parameter table

Figure 4 top hat beam characteristics of jk300d laser

(2) perforation test

we use two laser systems to carry out drilling test with various lasers and working parameters respectively (see Table 4). Through these parameters, the drilling performance of the two laser systems on aviation nickel base alloy is compared. Table 4 perforation test parameters

(3) results and discussion

because the designer of the part first considers that sufficient air flow can achieve appropriate cooling through the cooling hole, and the size of the air flow is mainly determined by the size and shape of the hole on the surface of the part, so the size, roundness and taper of the hole should be strictly controlled. There are other factors to be considered, because the positions between holes are relatively close, so any deviation of hole size may affect other holes in the area, resulting in local deviation of parts. Except for recast layer and heat engine affected area, excessive taper and surface convex groove are not allowed

2. Drilling time

the time for both lasers to process a vertical hole on 2mm thick material shall not exceed 0.5s. Figure 5 ~ 8 shows the time of machining 10 and 20 holes on the surface with optical fiber transmission system to meet the growing future demand. It can be seen that with 160mm long focal length and 300 diameter μ The better focusing depth of M spot is shorter than that of 120mm focal length beam. Similarly, the graph also shows the correlation between pulse width and processing time. Laser drilling with long pulse width and therefore higher pulse energy is faster than that with short pulse width and therefore low pulse energy. We use jk704 LD1 laser to demonstrate this experiment, because its laser beam quality m2=8 is better than that of jk300d m2=16, which makes the processing time shorter. The high-quality beam can reach a longer focal length (200 ~ 250mm), while ensuring the energy density requirements of fast drilling. The main advantage of using long focal length laser is that it can reduce the damage caused by splashing in the processing process, so as to prolong the service life of the protective lens. In addition, the high-quality light fixture must be installed in the high and low temperature box. The beam can provide a good depth of focus, so as to provide a larger error range of various workpieces or motion systems

Fig. 5 drilling time at different pulse widths (20 angles to the surface, jk300d, O2 assistance)

Fig. 6 drilling time at different pulse widths (10 angles to the surface, 300 μ M spot, O2 assisted)

Figure 7 drilling time at different pulse widths (20 angles to the surface, jk704ld1, O2 assisted)

figure 8 drilling time at different pulse widths (10 angles to the surface, jk704ld1, O2 assisted)

3 Taper

Figure 9 and figure 10 show the typical taper of two lasers processing holes with different angles on 2mm thick materials respectively. Although the taper produced by the two systems is very similar, it can be seen that the roundness of the hole processed by the optical fiber transmission system is better than that processed by the beam transmission system, because the optical fiber can make the laser distribution more uniform. Fig. 11 shows the cross-section of the hole processed with two kinds of lasers. It can be seen that the taper of the vertical hole processed with two kinds of lasers is different in the depth direction, especially in the center of the hole, which changes greatly. The diagram gives us the taper difference caused by laser parameters and the influence of laser peak power density on the hole shape. Today's research shows that the generation of surface convexity is mainly in the center of the hole, and more often in the case of high energy density. It is speculated that the formation of plasma may significantly reduce the role of evaporation in the process of hole forming. When machining a hole with an acute angle on the surface, there is no surface bulge, which may be due to the fact that the light spot is elongated at an angle, which reduces the energy density

Figure 9 taper% and peak power (jk300d)

Figure 10 taper% and peak power (jk704ld1)

Figure 11 drilling of two systems

4 Recast layer

in addition to the oxide layer, the recast layer is laser drilled in gold

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