Comparative test on drilling process of the hottes

2022-07-28
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Comparative test on drilling and machining of carbon fiber composites

carbon fiber composites (CFRP) have high specific strength and specific stiffness, strong designability and formability, good fatigue fracture resistance and easy large-area integral forming. Therefore, CFRP is more and more widely used in aircraft manufacturing industry. For example, the consumption of "gust" CFRP for French fighter is about 24%, and that of "typhoon" (EP2000) CFRP for British fighter is about 40%. In recent years, the amount of CFRP on B787 even accounts for about 50% of the total structure. With the increasing application of composite materials, there are more and more secondary machining, especially when carbon fiber reinforced composite parts are assembled and connected with other parts, it is inevitable to carry out a large number of hole machining. For example, an F-16 fighter has 40000 connecting holes, and a Boeing 747 aircraft has more than 3 million connecting holes [2]. However, due to the high hardness of carbon fiber, the tool wear is accelerated and the tool durability is low; In addition, due to the anisotropy of CFRP and low interlaminar strength, it is easy to produce delamination, tearing and other defects under the action of cutting force during cutting, and it is particularly serious when drilling, which makes it difficult to guarantee the processing quality, and even leads to the scrapping of parts. According to the statistics at the final assembly of the aircraft, the drilling failure rate accounts for more than 60% of all scrapped parts [3]. In drilling process, cemented carbide fried dough twist bit, as a traditional, economic and effective tool, still exists in the production field. However, in the processing of new materials, cemented carbide fried dough twist bits have some bottlenecks, such as low durability, and the quality of imports and exports can not be guaranteed. Taiwan scholar Ao et al. Proposed using diamond tools to drill holes and introducing diamond tools into composite material processing in order to improve tool life and obtain better hole quality. The bonding strength of the abrasive interface can be greatly improved by brazing the diamond abrasive particles onto the metal matrix, which makes the diamond tools have high wear resistance. This paper takes CFRP material as the research object, uses brazed diamond jacketed drill and cemented carbide fried dough twist drill to carry out drilling experiments to reduce the pollution caused by waste gas, and compares the quality and wear morphology of two kinds of drill holes, in order to explore the applicability of brazed diamond jacketed drill to process CFRP material

test plan

for the CFRP material used in the test, the base material is AG-80 resin, the reinforcement is T300 carbon fiber, and the plate thickness is 3.6mm. The cutting tools are brazing diamond nesting drill and integral cemented carbide fried dough twist drill. Among them, brazing diamond nesting drill is self-developed, and its structure is shown in Figure 1. The tool is composed of a tool handle, a tool body, a long hole, a drill bit and a notch, of which the base material is 45 steel. MPa is called MPa, which is equal to n (Newton)/mm2. It is convenient to take out the waste. A long hole is opened on the side wall of the cutter body; In order to facilitate chip removal and prevent chip accumulation and blockage, 4 notches are opened at the top of the drill bit. Bit part, with diamond particle size of 40/45 mesh (425~355 μ m) The drill bit is orderly arranged on the substrate. The outer diameter of the drill bit is 6mm and the wall thickness is 1.1mm. The combination of diamond and base material is realized by high-temperature brazing process, that is, Ag Cu Ti solder is used for brazing in a vacuum furnace at a brazing temperature of 920 ℃ for 5min

the drilling test platform includes machine tools (hg410 CNC engraving and milling machine), dynamometers, backing plates, fixtures, computers, etc. Hg410 CNC engraving and milling machine tool can realize automatic feed high-speed drilling, with a minimum speed of 3000r/min and a maximum speed of 24000r/min. The force measuring system is composed of pcb260a01 force measuring instrument, niusb-9162 data acquisition card and "cutting test and analysis system" software. The hole inlet and outlet morphology and bit wear morphology were observed with hirox KH-7700 3D video microscope

test results and analysis

1 hole inlet and outlet quality

when drilling carbon fiber composites, defects are most likely to occur at the inlet and outlet. In the experiment, carbide fried dough twist drill and brazed diamond nesting drill were used to process CFRP laminates respectively. The inlet and outlet quality of the holes machined by the two tools were compared, and the applicability of brazed diamond nesting drill to drill CFRP materials was discussed

Figure 2 shows the morphology of the inlet and outlet of the hole machined by the cemented carbide fried dough twist drill. As shown in Figure 2, the inlet has regular shape and no burr, but there are large tear defects and poor quality; The outlet has regular shape and no burr, but there is edge collapse in a small range, and the quality is good. Figure 3 shows the inlet and outlet morphology of the hole machined by brazing diamond nesting drill. As shown in Figure 3, the inlet shape is very regular, without burr and tear, and the quality is good; The outlet has regular shape and no burr, but there are tearing and edge collapse defects in some areas, and the quality is poor

through comparative analysis, it is found that the inlet quality of brazed diamond nesting drill is obviously better than that of cemented carbide fried dough twist drill, but the export quality is not as good as that of the latter. At the entrance, because the diamond abrasive cutting edge of the brazed diamond nesting drill is very short, when the abrasive particles cut into the material at the same time, the out of plane shear stress is small, so it is not easy to lift up the fiber layer and produce tearing defects; While the cutting edge of the carbide fried dough twist drill is long, when the wavelength of the absorption spectrum of the material is equal to the wavelength of the emission spectrum of the material, when the drill bit cuts into the material, the positive front corner outside the main cutting edge has a large out of plane shear stress on the surface material, resulting in the lifting of the surface material and the tearing phenomenon; However, at the outlet, the drilling force of the brazing diamond nesting drill increases rapidly from zero to the maximum from drilling into the laminated plate to drilling out the laminated plate, and then decreases rapidly to zero after it remains stable. Therefore, when drilling the underlying fiber, the ability of the material to resist tensile stress and deformation is very weak, while the drilling force is not reduced, which is greater than the interlaminar strength of the material at this time, so it is easy to produce tearing defects. For cemented carbide fried dough twist drills, the cutting edge is longer, the length of the cutting edge participating in cutting at the outlet gradually decreases, and the drilling force gradually decreases, so the opposite effect will be produced

Fig. 4 shows the inlet morphology of carbide fried dough twist drill at different feed speeds VF. From Fig. 4 (a) ~ (c), the feed speed is 300mm/min, 240mm/min and one pair of outer diameters is Φ The 48.3mm steel pipe scaffold uses right angle fasteners and rotating fasteners for 50mm/min, and the hole entrance morphology is that there are large tears, broken edges, no tears and no defects. It can be seen that when machining CFRP with carbide fried dough twist drill, the feed speed of 240mm/min is a critical value. When the feed speed is greater than this value, tearing defects will appear; When the feed speed is less than this value, no tearing occurs and good hole inlet quality is obtained

Figure 5 shows the outlet morphology of brazed diamond nesting drill at different feed speeds VF. From Fig. 5 (a) ~ (c), the feed speeds are 480mm/min, 360mm/min and 300mm/min respectively, and the hole inlet morphology is large tear, edge collapse, no tear and no defect respectively. As shown in Figure 5, the critical feed speed is 360mm/min when brazing diamond nested drill is used to process CFRP

to sum up, the critical feed rate of tearing defects at the inlet and outlet of brazed diamond nesting drill is greater than that of cemented carbide fried dough twist drill. Therefore, on the premise of ensuring the hole quality, brazing diamond nesting drill has higher machining efficiency

2 bit wear

one of the difficult machining characteristics of CFRP materials is that the tool wear is serious and the tool durability is low. Figure 6 shows the morphology of cemented carbide fried dough twist drill before and after wear. It can be clearly seen from the figure that the rake face of the cemented carbide fried dough twist drill is seriously chipped and worn. In addition, the main cutting edge, the auxiliary cutting edge and the horizontal edge are worn and broken to varying degrees. Figure 7 shows the morphology of brazed diamond drill before and after wear. As shown in Figure 7 (b), the diamond abrasive particles on the drill bit did not fall off, but a lot of solder was worn off at the inner chamfer of the drill bit section. This is because the brazed diamond jacketed drill used in the test uses the high-temperature brazing process to realize the combination of diamond and matrix material. Because this high-temperature brazing technology realizes the chemical metallurgical combination on the interface between diamond, brazing material and metal matrix, the diamond abrasive particles on the drill have high bonding strength and wear resistance

cemented carbide fried dough twist drills have poor wear resistance and short service life. However, brazed diamond drill has strong wear resistance and long service life. It is a good tool for processing CFRP materials

conclusion

(1) the inlet quality of brazed diamond nesting drill is better than the export quality, and the inlet quality of cemented carbide fried dough twist drill is worse than the export quality. The critical feed rate of brazed diamond drill for tearing defects is higher than that of cemented carbide fried dough twist drill. Therefore, brazing diamond nesting drill has higher machining efficiency

(2) brazing diamond drill has stronger wear resistance than hard alloy fried dough twist drill, and is a good tool for machining CFRP materials. (end)

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