Abstract
The nickel-based superalloy GH3128 with high plasticity, high long-lasting creep strength, good resistance to oxidation and stamping, and good welding performance is widely used in aircraft engine heat shields. The many holes that need to be machined on the heat shield are not only small in diameter but also dense, and GH3128 as a typical hard-to-process material has the problems of large cutting force, high cutting temperature, and serious hardening. Therefore, poor dimensional accuracy and residual burrs have become the main factors that limit the processing efficiency and processing quality. So, a novel combination of manufacturing processes was proposed. Firstly, laser cutting technology was used to process the base hole in a GH3128 plate, followed by reaming, and finally, using a magnetic abrasive finishing effector to remove burrs formed during the first two steps. The whole drilling process of the heat shields fully meets the requirements of the technical parameters. This study provides new reference for manufacturing the holes of a heat shield and other similar porous parts.
1. Introduction
GH3128 is a nickel-based superalloy with a solid solution of tungsten and molybdenum and is reinforced with boron, niobium, and zirconium. Ezugwu et al. in 2003 [1] pointed out that the nickel-based superalloy was the most widely used alloy, accounting for about 50 wt.% of materials used in an aerospace engine. Li in 2016 [2] also mentioned that GH3128 was used in an aircraft engine combustor flame tube, an afterburner housing, an adjusting plate, and in other high temperature parts. The afterburner of an aeroengine is equipped with a corrugated heat shield made of GH3128 material, which has many film holes on it to prevent the combustion chamber from overheating and oscillating. There are many holes in the heat shield that are densely arranged. Due to the small diameter and the higher position accuracy of these holes, it is difficult to create them using the traditional punching or drilling process in this thin-walled surface. Generally, precision punching, electrical discharge machining (EDM), or laser cutting technology was used for manufacturing thin-walled holes. Precision punching equipment is very expensive, and it has many limits for the shape and size of the workpiece. In addition, the process needs to manufacture special molds corresponding to different workpieces and the cost of mold is also expensive. Xu et al. in 2009 [3] mentioned that the materials with a high specific strength are generally machined using EDM, and five material-removal mechanisms of cemented carbides machined by ultrasonic vibration-assisted EDM are proposed, which are melting and evaporation, oxidation and decomposition, spalling, the force of high-pressure gas, and the affection of ultrasonic vibration. Moreover, there were microcracks on the cut surface; microcracks depend not only on electrical discharge parameters but also on the properties of a workpiece. D’Urso et al. in 2014 [4] mentioned that EDM is suitable for the machining of hard and high-strength materials, and they studied the process performance of micro-EDM drilling of stainless steel. High electrical conductivity results in a faster drilling process but in a lower dimensional and geometrical precision of the holes; the processing efficiency of EDM is relatively low. Sun in 2013 [5] confirmed that picosecond laser trepanning drilling of the directional solidification of a Ni3Al-based superalloy has been studied, and the experiment demonstrates that there are periodic stripes in the hole wall. Ko et al. in 2003 [6] claimed that the step drill performs front edge cutting before step edge cutting, and the burr formed during the first cutting can be removed during the second cutting by step edge. In conclusion, these conventional processing schemes generally suffer from problems such as expensive manufacturing cost, poor versatility, poor hole-positioning accuracy, microcracks on the wall of the hole, or low processing efficiency. And Huang et al. in 2010 [7] confirmed that if these processes are used alone, the hole-making technical requirements of the heat shield cannot be well satisfied, too.
This paper is aimed at manufacturing qualified holes in a heat shield with more than 1 m in diameter and with a corrugate profile. Made of GH3128 with a wall thickness of 0.8 mm, a total of 4200 holes with a diameter of Ø5 mm are required. The technical requirement is that the diameter deviation of the holes is ±0.1 mm, the positional accuracy of the hole is ±0.1 mm, there are no burrs at the hole exit, and the edge fillet of the hole is ≤0.2 mm. Because of large size and complex shape of the heat shield, it is difficult to manufacture the hole and meet all of technical requirements using a conventional drilling process. In summary, a novel combination process composed of three steps is proposed: (1) the base hole (diameter is 4.2 mm) is created using the laser cutting method; (2) reaming (diameter is 5 mm) is performed; and (3) burr removal is accomplished by the magnetic abrasive finishing method. The novel process was proposed for achieving the goal of making qualified holes in a heat shield.
2. Experimental Study on Cutting the Base Hole
The principle of making a hole by laser is based on the synthesis process of high temperature melting and shock wave induced by the photothermal effect. Through a series of optical focusing systems, fine beams of high parallelism are generated, resulting in the lattice vibration of materials bombarded by extremely high-energy and high-density photon beams. Instantly, the light energy can be converted into heat energy, and the irradiated area will be heated up rapidly. Bai in 2015 [8] has stated that the material will undergo metallographic changes, melting, gasification, and considerable thermal stress in order to achieve the purpose of removing the material. The material at its focal point is exposed to a high-power high-density laser spot, which produces a local high temperature of 10000°C or more. The material vaporizes instantaneously, and the vaporized metal is blown away with the auxiliary cutting gas, cutting the material into a small size. As the focus moves, numerous microholes are connected to complete the cutting process. Meng in 2011 [9] built a vaporization-melt ratio mathematical model for improving of the quality in laser cutting of 6063 aluminum sheets with a thickness of 0.5 mm. The analysis verifies that the model is feasible, and it makes a contribution to laser precision cutting. Lei et al. in 2009 [10] mentioned that the laser cutting of a 0.5 mm silicon steel sheet with a CO2 laser has been studied. Although neither reference mentions the GH3128 plate, it is all about laser-cut sheet metal. It can be seen that the laser cutting of GH3128 should be equally feasible. A 400 W Nd:YAG () laser was used to manufacture Ø4.2 mm holes in a GH3128 sheet with a thickness of 0.8 mm. Its matrix is austenite, and its main chemical compositions are shown in Table 1.
3. Experimental Equipment and Procedures of Base Hole
The laser cutting equipment JHM-1GY-400B is shown in Figure 1. The specific parameters were as follows: the laser wave length is 1.06 μm, the rated average power of the laser is 400 W, and the laser pulse frequency is 1-200 Hz (adjustable). The laser pulse width is 0.1-20 ms (adjustable). The focal length of the lens is 75 mm, and the diameter of the focal spot is 0.3-0.6 mm. O2 was used as the auxiliary gas.
Chen et al. in 2017 [11] aimed to analyze the effect of laser power, scanning speed, and processing times on the surface roughness of polymethyl-methacrylate microchannels with a CO2 laser LCJG-1290 cutting process. Arun and Avanish in 2013 [12] investigated the laser cutting performance of a 1 mm duralumin sheet with the aim to improve the quality of the cut by simultaneously optimizing multiple performances such as cut edge surface roughness, kerf taper, and kerf width. Aoud et al. in 2017 [13] focused on evaluating the effect of laser power, cutting speed, and gas pressure on the heat-affected zone and kerf width quality. Gvozdev et al. in 2015 [14] built mathematical models that provide an adequate description of the effect of laser cutting parameters on the roughness of the cut surface with no burr and the extent of the heat-affected area, and the lack of perpendicularity of the cut surface was ascertained. By summarizing the references on these laser cuttings, the main technical parameters were selected and the experimental steps in this paper are proposed as follow: First of all, according to the references of relevant laser cutting, five technological factors, such as defocusing distance— (mm), processing speed or cutting speed— (mm/min), current— (A), pulse width— (mm), and frequency— (Hz), were selected as the test parameters. The orthogonal test method of four levels and five factors was developed, as shown in Table 2. The process parameters corresponding to the 16 sets of experimental data were obtained. Then, the base hole was made by the laser cutting process according to the technological parameters. Lastly, the results of the orthogonal experiment and the analysis of the variance were done.
4. Intuitive Analysis
The comprehensive evaluation index was formulated as shown in where is the slag thickness (mm), is the cut length (mm), and is the total length of the cut (mm). is the cut ratio. The slag thickness at the hole entrance by the laser cutting method was smaller relative to that at the exit. Therefore, the slag thickness of the hole exit was only detected and analyzed. The thickness values of six points were measured, and the mean value of the slag thickness was calculated. The experimental data are shown in Table 3.
The best results were No. 8 and No. 16, and the slag thickness was 280 μm; the worst results were No. 4, No. 5, No. 6, and No. 9, and all holes did not penetrate the alloy plates as shown in Figure 2. It could be seen from the test photo of No. 4 that the reason for the uncut was that the starting point was not cut, which belonged to the early to middle part of the cutting. The first condition of laser cutting was that the starting point penetrates to form a keyhole effect, so that the material had a 100% absorption rate of the laser. The probability of cutting completeness was significantly increased. Therefore, it was necessary to stay at the cutting starting point to meet the above requirements. Then, the starting point did not cut the laser power density of the process parameters, which was insufficient in the stay time. When the laser power accumulated enough, it could indeed penetrate the plate. However, due to the movement of the numerical control platform, the instability of the laser and air pressure caused to the slag to be poor and thus causing the cutting failure.
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The data in Table 3 was analyzed, and the specific steps were as follows: (1) Summarize the values of level 1, level 2, level 3, and level 4 and use , , , , and ( is the representative level) to form them. (2) The average value of each factor was calculated, i.e., , , , , and . (3) The difference () between the maximum or the minimum was calculated separately. It was called the extreme difference. The analysis results are shown in Table 4.
According to the value, the influence of each factor on the experiment result could be judged. In Table 4, the extreme difference was the maximum difference between the mean value, and the range of the difference represented the degree of influence of this factor on the laser cutting process. The larger the difference, the more important it was. Thus, the order of the main and secondary factors could be determined. Therefore, the effect of the five factors on laser perforation was obtained: . In order to more vividly and more intuitively draw the analysis results, the method of drawing effect plots (trends) was adopted to get the correct comprehensive analysis conclusion. According to the mean value of four levels in Table 4, the effect curve was obtained, as shown in Figure 3. The optimum was -0.25 mm. The reason was that the closer one gets to the focal point, the greater was the laser power density and the better was the cutting ability and the cutting quality. Or, in the case of ensuring the power density, the increase of the distance from the coke would inevitably increase the spot size, so that the width of the slit widens to eliminate the slag. This was also the reason why the cutting quality when out of focus at -1 mm was better than that at -0.75 mm. The best was 200 mm/min; too low processing speed and too high energy density led to a poor slag, while too high processing speed and too low energy density led to poor cutting ability. The optimum was 210 A; when the current level was within the range of 190-220 A, it had no significant effect on cutting quality. The optimum was 0.8 ms and 0.9 ms. As the pulse width continued to increase, it would cause each laser pulse to accumulate excessive heat and accumulate too much slag. The optimal was 70 Hz. Excessively high frequencies will result in too many laser cuts, excessive heat accumulation, and severe slag levels. The optimum process parameters could be obtained by taking the best horizontal value of each factor: ; ; ; or 0.9 ms; and .
5. Analysis of Variance
In order to further verify the experimental results of the intuitionistic analysis, the analysis of variance (ANOVA) for the experimental data is carried out: where is the number of corrections; is the total number of trials, which is 16; and is the sum of 16 test scores. The total sum of squares of the variance of the test should be the sum of the variance squared of each factor and error, because the test factors fill the entire orthogonal table; there is no empty column and there is no error term in the variance analysis. In fact, the error term is an unmeasured null column, so the choice of the least factor of the influence factor as the error term can also be used to compare the significant effect. where is the sum of the squares of variance; is the sum of the variance of , is the sum of the variance of , and so on for (the specific representative factors of , , , , and are shown in Table 2); and is the sum of the squared error variance. where is a comprehensive test score and is the test number (as shown in Table 3). where is the sum of all the horizontal test scores of the factor; is the horizontal number of , is the horizontal number of , and so on for , , and ; is the horizontal repetition number of , is the horizontal repetition number of , and so on for , , and ; ; and . Total freedom of the experiment is . The degree of freedom (DOF) of the factor is . , , , and and , , , and are consistent with the meaning of and . The variance analysis results were calculated according to equation (3). The results are shown in Table 5.
6. Validation Experiment of Cutting Base Hole
It was shown that the optimum process parameters obtained by intuitionistic analysis could improve the number of laser-perforated slag. Because or 0.9 ms, it was necessary to do two series of experiments to compare and obtain the best process parameters. The experiment results were measured and shown in Figure 4.
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It was difficult to directly distinguish the slag level from macrophotos. Therefore, the KEYENCE VHX-500FE super-depth electron microscope was used to compare the slag thickness, as shown in Table 6.
From Table 6, it could be seen that the slag level in the right area of the entire hole photo was worse than elsewhere. The reason was the starting point of the laser cutting process at the right bottom side of the hole. Because at the beginning of the actual operation, it was necessary to stay at the starting point for a certain amount of time to stabilize the laser power and accumulate enough energy to form the small hole for the blackbody effect. Therefore, the slag was worst at the right bottom side of the hole at the beginning than at the other zones of the cutting. Similarly, the right side at the back was also a starting point, and the numerical control platform had an acceleration process from the starting point to the preset constant stable speed and there was a deceleration process at the end of the uniform state. The change in the moving speed of these two processes leads to the instability of the laser and air pressure, which makes the slag thickness poor. Therefore, the slag thickness near the starting point is poor. The process was as follows: the slag in the initial and accelerated stages of the cutting was poor, the number and height of the slag in the middle phase of cutting was gradual and steady, and the slag was poor during the deceleration stages of the cutting process. The top 6 slag areas around the whole hole were selected, and the height value was obtained through the software of the electron microscope. The difference between this value and the height of the workpiece was the thickness of the slag. As or 0.9 ms, the mean values of the slag thickness were 21.5 μm and 17.7 μm, respectively. By taking this parameter into equation (1), we arrive at the calculation results as shown in Table 7.
From Table 7, it could be seen that the intuitive analysis could significantly improve the cutting quality. Test scores on both No. 17 and No. 18 were higher than those on No. 8 and No. 16 in Table 3. The test results of No. 18 were slightly better than those of No. 17. Therefore, the optimal process parameters were as follows: ; ; ; ; and . It took 4.71 s to manufacture the Ø5 mm holes by the laser cutting process on a 0.8 mm thick GH3128 plate. After completing the manufacturing of one hole, the actuator was displaced and began to make the next hole. The moving speed of the actuator was 1000 mm/min, and the distance between the two holes was 12 mm, so it needed to take about 0.72 s to change to the next drilling position.
7. Reaming
Using the integral alloy drilling bit with a 118° rake angle of HSS material, the base holes manufactured by optimized laser cutting process were reamed by a vertical milling machine. The diameter of the base hole was 4.2 mm, and the reamed diameter of the hole was 5 mm. The reaming process parameters are shown in Table 8.
After the hole was reamed, the burrs at the entrance edge of the hole were relatively small, and the naked eye could hardly detect it. But there were still burrs at the exit edge, and the detection methods and results are shown in Figure 5. Ten typically reamed holes were measured. The height of the burr was about 40-80 μm, and the width was between 132 and 300 μm. The height of the burr was significantly lower than that of the direct drilling process. The height and width were about 58.4% and 74.1%, respectively, compared to the direct drilling process. The main reason is that there is a larger axial pressure between the drill bit and the workpiece in the conventional process.
8. Deburred by MAF
The principle of magnetic abrasive finishing is to use a magnetic field to restrain magnetic abrasive particles to form magnetic brushes. Yun et al. in 2016 [15] aimed to improve the efficiency and uneven texture by using ultra-assisted MAF, and the study reveals that the efficiency of the surface roughness and material removal of alumina ceramic tubes were achieved. Lin et al. in 2007 [16] employed MAF to conduct free-form surface abrasion of stainless SUS304 material operations, and Ramax yielded an even lower value similar to that of the mirror surface. Verma et al. in 2017 [17] presented experimental investigations into the internal MAF of SS304 pipes that resulted in a precise surface. Du et al. in 2015 [18] carried out large numbers of experiments on the electrolytic-MAF of the nickel-based superalloy GH4169, and the processing efficiency was improved. Moreover, Wang and Hu in 2005 [19] explained that the magnetic finishing process can also remove microcracks and have surface residual comprehensive stress, which can greatly improve the workpiece reliability and fatigue strength. The author of this paper, Jiao et al. in 2017 [20] pointed out that the relative motion between the magnetic brush and the burr is the main material-removal mechanism. After a comprehensive analysis of the mechanism of the magnetic finishing based on these references, the MAF process is feasible for the removal of burrs. The magnetic brush is flexible, and the reasonable design of the finishing pressure can completely avoid the deformation and will not cause new damage to the workpiece. The MAF method has many advantages, and many papers introduce the MAF principle for polishing pipes, plates, free-form surfaces, etc.; however, there is no related research on the edge deburring process for the hole exit. So in this paper, two kinds of plans based on MAF were proposed for burr removal after the reaming process. The principles are shown in Figure 6. Plan 1 is a magnetic pole self-rotation scheme. Here, the center axis of the magnetic pole is concentric to the center axis of the pass, the gap between the pole end and the workpiece is 2 mm, the magnetic particles fill in the gap, and the magnetic pole drives the magnetic particles to rotate clockwise. Compared with Plan 1, in Plan 2, the central axis of the magnetic pole deviates from the center axis of the hole. The magnetic pole rotates under its own rotation and revolves around the center of the hole. The specific process parameters are shown in Table 9. The height and width of the edge burr were detected every 30 s, and the average height value of the four higher typical burrs was calculated.
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Using Plan 2 after 60 s, burrs were almost removed clearly. The micrographs after 60 s of processing with Plan 2 are shown in Table 10. There was no significant difference in the final effect between the two plans. But when comparing the edge burr removal efficiency, when Plan 2 was adopted, the removal efficiency of the edge burr was three times that of Plan 1, as shown in Figure 7. It could be seen from the form that the removal effect of the edge burr could meet the technical requirements. Liu et al. in 2014 [21] pointed out that the specialized tool can machine titanium holes without the burr based on a helical milling specialized tool. Wang et al. in 2018 [22] investigated cutting parameters in the helical milling process of a carbon fiber reinforced polymer. Li et al. in 2017 [23] analyzed the reason for reducing the hole exit defects by helical milling and grinding. Lu et al. in 2016 [24] built the dynamical model of the micromilling process of a nickel-based superalloy. Based on these references, the helical milling process has been the focus of deburring research. But the direct contact between the tool and the workpiece causes damage and deformation easily. A major advantage of the magnetic abrasive finishing process is that the magnetic brush keeps a flexible contact with the workpiece, which can well avoid these defects. The test result shows that the complex motion of the particles accelerates the removal of the burrs. In other words, the adoption of the revolution leads the force of the magnetic brush on the burr to be further improved. And the force was also more directional than that of Plan 1. These versatile finishing forces, which acted on the burr, caused the edge burr to be bent off, worn off, or gradually ground away.
The novel hole-manufacturing process with the combination of the three steps was adopted by the automatic end effector. The consistency of the hole was guaranteed, and the efficiency of manufactured hole was about 70-75 s per hole. The efficiency can be further improved by using more advanced laser equipment and machine tools.
9. Conclusion
Aiming at the heat shield of an aeroengine, a composite hole-manufacturing technology is proposed based on a laser cutting process, a reaming process, and the MAF process. Combining theoretical analysis and experimental research, the main conclusions are as follows: (1)The best process parameters for laser cutting the base hole of the GH3128 plate are ; ; ; ; and . The conclusion of ANOVA was consistent with the result obtained in the intuitive analysis. The factors influencing the effect of the laser cutting process of the GH3128 plate are .(2)The edge burr level decreased obviously with the reaming process after the laser cutting process of the base hole. The height and width of the burrs are about 68.4% and 71.1%, respectively, compared to the conventional direct drilling process.(3)The introduction of the rotation of the main shaft will enable the force of the magnetic brush to have more direction to the edge burrs. This multidirectional finishing force brings higher removal efficiency.
Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This paper was supported by the “High-End CNC Machine Tools and Basic Manufacturing Equipment” (2016ZX04002005) of the National Major Science and Technology Projects, China.