Application of different types of laser drilling in the aviation field

This paper studies the practical application of continuous drilling using high peak power pulsed (about 20kW) lasers with direct and fiber transmission. A large number of holes were machined in superheat-resistant nickel-based alloys using various lasers and operating parameters. The relevant data on processing time, recast layer, taper, oxide layer and cracking were statistically analyzed.

Blades, nozzle blades, combustion chambers and other parts on aircraft gas turbines need to be cooled when in operation, so people punch thousands of holes on the surface of these parts to ensure that the surface of the parts is covered with a thin layer of cooling air. This layer of cooling air can not only extend the service life of the parts, but also improve the performance of the engine. A typical advanced engine surface will have 100,000 such holes. With the development of drilling technology, the industry currently generally uses high peak power pulsed Nd-YAG lasers for processing, and trepanning and pulse drilling (percussion) technology has been successfully applied.

Figure 1 Device after laser perforation

EDM hole processing and laser drilling

At present, the temperature of gas used in jet engines in the aviation field can reach 2000℃, which has exceeded the melting point of turbine blades and combustion chamber materials, namely nickel alloy. Therefore, people generally use boundary layer cooling to solve this problem, that is, processing holes on the surface of gas turbines, nozzle blades and combustion chambers (see Figure 1), where the number of holes on each part ranges from 25 to 40,000 (the parameters of specific parts are shown in Table 1). The cooling gas can cover the entire surface of the part through the small holes on the part to isolate the external temperature, thereby playing a protective role.

Table 1 Typical applications of cooling holes

Cooling holes can be machined by electrospark machining (EDM) or laser machining. Although the EDM method can produce small holes of qualified quality, the machining efficiency is significantly lower than that of laser machining.

EDM also has the following three disadvantages:

1. Suitable for situations with low incident angle and incident angle variation.

2. Various consumables, such as electrolyte, are required, which increases processing costs.

3. In order to improve heat resistance, the blade surface needs to be coated with insulating ceramic, but EDM is not suitable for drilling holes in ceramic coating materials.

At present, the use of pulsed Nd:YAG lasers has become the first choice for drilling equipment in the aerospace field, mainly due to its following advantages:

1. Using 1.06μm 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 high aspect ratio cooling holes on the surfaces of various materials (including materials with heat-resistant coatings) (see Figure 2).

Figure 2 A stator blade of a turbine (Siemens generator), with a thermal isolation coating material YSZ (zirconia) sputtered by plasma on the surface

Laser drilling and quality control

There are two basic laser drilling methods in the aviation field: trepanning and laser pulse drilling. Trepanning is to use laser pulses to drill a hole in the center of the hole first, and then move the laser beam to the circumference of the hole or rotate the part to make a hole. Laser pulse drilling does not require moving the laser beam or the part. It can only use continuous laser pulses to make a hole, and the diameter of the hole can be adjusted by controlling the pulse energy during the process. Therefore, it can greatly shorten the processing cycle of the part, especially when processing symmetrical parts such as combustion rings and combustion chambers, the processing time can be further shortened. 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 are completely simultaneous and in a specific arrangement to make all the holes. 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 the hole is very critical. The quality of the hole processed by laser can be judged by different characteristics. From the perspective of geometric factors, it can be judged by the changes in the roundness, taper and entrance diameter of the hole. From the metallographic point of view, it can be judged by the changes in the structural organization such as the recast layer and the oxide layer. Among them, the formation of the recast layer is due to the fact that the molten metal is not ejected by the gas 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. Microcracks will appear on the surface of this metal coating, so that it will directly spread to the body. The standards used by airlines have been constantly striving to improve the quality of holes. For example, Rolls-Royce Airlines has established the maximum thickness standards of acceptable oxide layers and recast layers based on actual conditions, so that the geometric dimensions of the holes on the workpiece have an acceptable maximum deviation value range before the workpiece is used. Other airlines judge the quality of the processed holes by the gas flowability of the parts.

At present, most of the drilling of aviation parts adopts direct beam transmission system, but due to many technical reasons, the application of fiber optic light output system in laser drilling has been slow. There are two main reasons for this: one is that the damage threshold of optical fiber is relatively low; the other reason is the quality of the transmitted beam. 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 be produced using the correct pulse parameters. Therefore, the fiber optic application system has certain advantages over the direct beam transmission system, which are mainly reflected in:

1. Laser beam delivery systems provide options for laser transmission on CNC machine tools.

2. Energy homogenization brings about the characteristics of Top hat, improving the roundness and consistency of the holes.

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

Pulse piercing

The following mainly discusses the application of pulsed perforation in direct beam delivery and fiber-optic delivery systems using high peak power (up to 20kW) pulsed Nd:YAG lasers. We choose to drill holes on nickel-based alloys with different lasers and parameters to study the range of parameters such as recast layer, taper, oxide layer cracks, and processing time.

1. Drilling test

(1) Laser

Table 2 Laser parameters of JK704

The JK704 laser was selected for direct transmission beam drilling. This laser can provide high peak power (see Table 2) and good pulse stability, which is very suitable for processing small diameter holes (0.25-0.90 mm). The laser's Gaussian beam quality (see Figure 3) and enhanced control and pulse shaping characteristics provide greater flexibility in processing aerospace materials, including materials with thermal insulation coatings.

Figure 3. Beam quality of JK704

Table 3 JK 300D parameter table

Figure 4 Top hat beam characteristics of JK300D laser

This fiber transmission drilling test will be completed using GSI's latest high peak power pulse laser JK300D (parameters see Table 3). This laser has high peak power and Top hat characteristics (see Figure 4), which is suitable for pulse drilling of aviation alloy materials. The beam emitted by the laser is transmitted in a 10m×300μm diameter optical fiber and output through a 160mm right angle collimation system and optical focusing mirror. (2) Perforation test

We used two laser systems to perform drilling tests using various laser and operating parameters (see Table 4). These parameters were used to compare the performance of the two laser systems in drilling holes in aerospace nickel-based alloys.

4 Perforation test parameters

(3) Results and discussion

Since the designer of the part first considers sufficient airflow through the cooling hole to achieve appropriate cooling, and the airflow size is mainly determined by the size and shape of the hole on the surface of the part, the size, roundness and taper of the hole must be strictly controlled. There are other factors to consider. Because the holes are relatively close to each other, any deviation in the size of the hole may affect other holes in the area, resulting in local deviation of the part. Except for the recast layer and the thermomechanical affected zone, excessive taper and surface convex grooves are not allowed.

Figure 5 Drilling time at different pulse widths (20° to the surface, JK300D, O2 assisted)

Figure 6 Drilling time at different pulse widths (10° to the surface, 300μm spot, O2 assisted)

Figure 7 Drilling time at different pulse widths (20° to the surface, JK704LD1, O2 assisted)

2. Drilling time

Both lasers take less than 0.5 seconds to process a vertical hole in a 2 mm thick material. Figures 5 to 8 show the time to process 10 and 20 holes on the surface using a fiber-optic transmission system. It can be seen that the better focus depth of the 160 mm long focal length and 300 μm diameter spot is shorter than the 120 mm focal length beam. The same graph also shows the correlation between pulse width and processing time. Long pulse width and therefore higher pulse energy laser drilling is faster than short pulse width and therefore lower pulse energy. We use the JK704 LD1 laser to demonstrate this experiment because its laser beam quality M2 = 8 is better than the JK300D M2 = 16, which makes the processing time shorter. High-quality beams can achieve longer focal lengths (200-250 mm) while ensuring the energy density required for fast drilling. The main advantage of using a long focal length laser is that it can reduce damage caused by spatter during processing, thereby extending the life of the protective lens. In addition, high-quality beams can provide a good depth of focus, which provides a larger error range for various workpieces or motion systems.

Figure 8 Drilling time at different pulse widths (10° to the surface, JK704LD1, O2 assisted)

3. Taper

Figures 9 and 10 show typical tapers of holes of different angles machined in 2 mm thick material by two lasers. Although the tapers produced by the two systems are very similar, it can be seen that the roundness of the holes machined by the fiber delivery system is better than that of the beam delivery system because the fiber can make the laser distribution more uniform. Figure 11 shows the cross-section of the holes machined by the two lasers. It can be seen that the tapers of the vertical holes machined by the two lasers are not the same in the depth direction, especially in the center of the hole. The figure gives us feedback on the taper difference caused by laser parameters and the effect of laser peak power density on hole shape. Current research shows that the generation of surface convexity is mainly in the center of the hole, and more often occurs under high energy density. It is speculated that it may be because the formation of plasma significantly reduces the effect of evaporation and removal of materials during the hole forming process. No surface convexity is produced when a sharp-angle hole is machined on the surface, which may be because the 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)

4. Recast layer

除了氧化层，重铸层是激光钻孔在金相方面的主要特点，并且已经在光纤系统中经过全面的研究。结果表明在表面加工90的孔时，光纤传输激光系统重铸层的典型厚度大概为25～35μm。这个重铸层与光束直接传输激光系统非常类似。而氧化层大概在10～15μm，两种激光器得出的测试结果都在此范围内。如果在表面上加工一个锐角的孔，那么重铸层厚度随位置变化非常显著。在入口处会有更厚的重铸层，可能是由于在脉冲钻孔过程中大量地熔化了的材料从这个地方喷出而遗留下来的。同时我们也可以预测出，在低能量和低峰值功率的情况下重铸层的厚度会增加。

Conclusion

GSI Group has been producing laser drilling machines for the aerospace industry since the 1980s, and the JK704 laser has set the standard in industrial laser drilling. The new high-power fiber-delivered laser has many advantages over direct beam delivery systems (drilling results shown in Figure 11):

Figure 11 Drilling of two systems

1. Simple, low-cost and high-power fiber-optic transmission laser drilling machine.

2. Pulse drilling has a certain range of applications in the aviation field, usually with a hole diameter of 0.4-0.8mm and a thickness greater than 6mm.

3. The processed holes can achieve very ideal roundness.

4. High quality beam, can be transmitted through 300mm optical fiber. Typical focal length range is 120～160mm, and has the following advantages:

(1) High material removal rate;
(2) Ability to process shallow-angle holes;
(3) Excellent depth of focus;
(4) Effectively reduce optical device loss caused by sputtering during the processing.

5. It can process holes with a minimum angle of 10 with the surface.

6. Simple laser integrated motion system enables the robot to have the functions of transmission and multi-channel time-sharing processing.

7. On-the-fly drilling technology.