Analysis and Research on milling characteristics o

2022-10-24
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Analysis and Research on milling characteristics of titanium alloy in nitrogen medium

today, with the development of high technology, environmental protection has attracted more and more attention. Based on the balanced development of technology, economy and ecology, green manufacturing has become the topic of discussion today. In the field of mechanical manufacturing, cutting fluid is the largest source of pollution, especially in the aviation industry. In order to improve the processing quality of aviation parts, a large amount of cutting fluid is often used. In order to realize green manufacturing, people often focus on dry cutting, but the mechanical properties, machining efficiency and tool life of parts and components under dry cutting conditions will be greatly affected. In this paper, a green machining technology using nitrogen as cutting medium is proposed and explored. This paper mainly compares and analyzes the flank wear and milling force when milling titanium alloy under dry cutting and nitrogen medium

1 material properties of titanium alloys

because titanium alloys have a series of advantages, such as high specific strength, good thermal strength, high corrosion resistance, rich resources and so on, they are more and more widely used in aviation, aerospace and other industrial departments. They are used to make major components in aeroengines, and in aircraft structures, they are used to make important components such as wing beams, spacer frames and joints. Based on the wide application of titanium alloy, the main advantages are as follows:

· high specific strength: the density of titanium alloy is only 4.5g/cm3, much smaller than iron, and its strength is similar to that of ordinary carbon steel

· good thermal strength: the melting point of titanium alloy is 1660 ℃, which is higher than that of iron. It has higher thermal strength, can work below 550 ℃, and shows better toughness at low temperature

· good corrosion resistance: under 550 ℃, titanium alloy surface is easy to form a dense oxide film, so it is not easy to be further oxidized. It has high corrosion resistance to atmosphere, seawater, steam and some acid, alkali and soft media

on the other hand, the machinability of titanium alloy is relatively poor. The main reasons are as follows:

· poor thermal conductivity, resulting in high cutting temperature and reduced tool durability

when the temperature is above 600 ℃, a hard oxide layer is formed on the surface, and the knife has a strong wear effect

· low plasticity and high hardness increase the shear angle, the contact length between the chip and the rake face is very small, the stress on the rake face is very large, and the blade is easy to be damaged

· low elastic modulus, large elastic deformation, and large resilience of the workpiece surface close to the flank, so the contact area between the machining surface and the flank is large and the wear is serious

· these characteristics in the cutting process of titanium alloy make it very difficult to process, resulting in low machining efficiency and high tool consumption

2 selection of tool materials

the milling of titanium alloy is more difficult than turning. The main problem is that the cutting edge area is easy to bond with titanium, the cutter teeth are easy to collapse, and the tool durability is low. The amount of titanium alloy bonded in the cutting edge area of the milling cutter is directly proportional to the cutting thickness. When the cutter teeth cut into the workpiece surface again, the bonded titanium alloy peels off, resulting in the wear area of the cutter. With the increase of bonding amount, the wear area also increases; In serious cases, it will even cause the cutting edge to collapse and damage the milling cutter. End mills, cylindrical mills, three edge mills, etc. for milling titanium alloys are generally made of high-speed steel, especially forming mills. The commonly used tool material is HSS Co, whose main component is w2mo9cr4vco8; HSS Al, whose main components are W6Mo5Cr4V2Al and carbide, whose main components are YG8, yg10h, y300, etc

3 test scheme

taking titanium alloy material TC4 (Ti-6Al-4V) as the main research object, the test scheme is as follows:

· through orthogonal test, the changes of milling force under dry cutting conditions and cutting conditions with nitrogen as the medium are compared. The milling mode is selected as forward milling. The specific cutting parameters and parameter values are shown in Table 1. Table 1 orthogonal cutting test parameters of milling force

cutting parameter values milling speed V (m/min) axial depth of cut AP (mm) 1234 radial depth of cut AE (mm) 0.51.01.52.0 feed per tooth FZ (mm/z) 0.050.10.150.20 tool material W6Mo5Cr4V2Al tool diameter (mm) 12

· under dry cutting conditions and cutting conditions with nitrogen as the medium, analyze the wear of the back face of the milling cutter, and the wear amount of the back face is 0 2mm is the judgment standard. Parameter values are shown in Table 2. Table 2 wear test parameters

cutting parameter values milling speed V (m/min) axial cutting depth AP (mm) 6 radial cutting depth AE (mm) 1 feed per tooth FZ (mm/z) 0.08 tool material W6Mo5Cr4V2Al tool diameter (mm) 12

the above test machine tool is Mikron UCP 710 five coordinate machining center. The milling force is measured by Kistler three-way dynamometer, and the wear of the flank of the milling cutter is observed and measured under the microscope

4 milling force and its result analysis

milling schematic diagram is shown in Figure 1. According to the milling force data under nitrogen cutting medium and dry cutting conditions, a comparison diagram as shown in Figure 2 is established

Figure 1 milling schematic diagram Figure 2 milling force comparison diagram

Figure 2 milling force is the synthesis of the maximum forces in X, y and Z directions. The test results of milling force are analyzed as follows:

· it can be seen from Figure 2 that under the two cutting conditions of dry cutting and nitrogen as the medium, with the change of cutting conditions, the wear of the flank behind the milling cutter is observed under the microscope, and the two segments of the tensile broken specimen aligned at the fracture with the change trend of force measurement is basically the same

· in the first group of tests, the milling force under nitrogen medium is less than that under dry cutting conditions. In the next 15 groups of tests, the milling force under nitrogen medium is about 100 Newtons larger on average than that under dry cutting conditions

· preliminary analysis shows that the reason for the above results is the formation of hardened layers of nitrogen and titanium during the cutting process. At the beginning of cutting in nitrogen medium, that is, in the first group of tests, there is no hardened layer. At this time, due to the cooling and lubrication effect of nitrogen, the cutting force is relatively small; Since the second group of tests, the hardened layer has been formed. Although nitrogen still has the cooling and lubrication effect in the subsequent tests, the influence of the hardened layer on the milling force is more obvious, so the milling force under nitrogen medium is greater than that under dry cutting conditions

· under the cutting condition of nitrogen medium, the impact of nitrogen gas pressure on the dynamometer is also a factor for the increase of cutting force. After measurement, the gas pressure of nitrogen is only about 5 Newtons. It can be seen that the self-reported impact force of nitrogen pressure ABS is not the main factor causing the increase of milling force under nitrogen medium

5 milling cutter flank wear and its result analysis

based on the test data under nitrogen medium and dry cutting conditions, the wear curves shown in figures 3 to 8 are established

Fig. 3 wear curve of flank under dry milling condition Fig. 4 wear loss curve of flank under nitrogen medium

Fig. 5 Comparison of 30m/min wear curve Fig. 6 Comparison of 40m/min wear curve

Fig. 7 Comparison of 50/min wear curve Fig. 8 Comparison of 60m/min wear curve

analysis of test results:

· through the analysis of Fig. 3 and Fig. 4, it can be concluded that cutting speed is a crucial factor affecting the wear of flank of milling cutter, Under the conditions of the above two cutting media, when the cutting speed increases to 2 times, the tool life will be reduced to about 4% respectively, and the milling length will also be sharply reduced

· according to the analysis of figures 5 to 8, under the same cutting conditions, when the transportation distance of the vehicle using magnesium alloy material as a component reaches 128000 kilometers, the milling length with nitrogen as the cutting medium is much longer than that under dry cutting conditions

· preliminary analysis shows that the reason why the tool life can be prolonged under nitrogen medium is that its effective cooling and lubrication function reduces the temperature of the cutting area and reduces the friction between the tool and the workpiece. Its specific mechanism needs further study. At the same time, because there is no oxygen in the cutting area under nitrogen medium, the machined surface has higher surface integrity

6 conclusion

· compared with milling titanium alloy under dry cutting condition, the milling force has an increasing trend under nitrogen medium cutting condition

· when nitrogen is used as cutting medium, the tool life is greatly improved compared with dry cutting

· the hardened layer generated under nitrogen medium increases the cutting force and has a certain impact on tool wear (abrasive wear), but the cooling and lubrication effect of nitrogen greatly reduces the temperature of the cutting area and reduces the friction between the machined surface and the flank, so the tool life is still greatly improved

· high nitrogen pressure can blow chips away from the cutting area and reduce the impact of chips on the surface quality of the workpiece

the price of nitrogen is low and the processing cost is low. Using nitrogen as cutting medium can eliminate the environmental pollution caused by the large use of cutting fluid, and achieve green manufacturing in the true sense. The Green high-speed machining technology with nitrogen as the cutting medium has broad application prospects in the aviation field and high-speed machining field, especially in the machining of difficult to machine materials, which will promote the "reform" and "reform" to become the social resonance at that time and play a more important role. (end)

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