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As a new type of metal-based particle reinforced composite material, boron aluminum alloy has a hardness (9.36 Mohs hardness) second only to diamond, so it is easy to be squeezed and crack in the milling process, thus affecting the processing quality of parts. Boron aluminum alloy is a kind of doped aluminum metal, high hardness B4C particles of new metal base particle reinforced composite material, through in-depth study of its turning, milling, grinding, electrical discharge and pliers processing performance, solve the processing problem of boron aluminum alloy, can make boron aluminum composite instead of titanium alloy, so as to be widely used in the need to reduce weight and require small deformation of parts.
1. Properties of boron aluminium alloys
Boron aluminum alloy is a composite material which can meet the requirements of lightweight, good thermal stability and low expansion coefficient, and has suitable mechanical properties. Boron-al alloys with different B4C contents are shown in Figure 1, and their mechanical properties are shown in Table 1. Table 2 shows the comparison of various properties between boral and common aluminum alloy 2A12, steel 2Cr13 and titanium alloy TC4.
B4C content /% Annealing T4 yield strength /MPa tensile strength /MPa elongation /% Yield strength /MPa tensile strength /MPa elongation /% elastic modulus /GPa151402307 ~ 93405005 ~ 8105±3201502506 ~ 83605204 ~ 6115±32517026 04 ~ 63705003 ~ 4125±3311602203 ~ 43604602 ~ 3131±3
Name density /g·cm-3 elastic modulus /GPa yield strength /MPa tensile strength /MPa linear expansion coefficient /10-6K-1 thermal conductivity /W·(m·K)-1 elongation /% boron aluminum (30% particle content)2.64131360460131123 ~ 4 aluminum 2A122.86827042023.6121 -- Steel 2Cr137.7522851070610.5 -- titanium alloy TC44.441098258959 --
Table 2 shows that compared with aluminum alloy, boron aluminum alloy can lose 7% of weight, increase the elastic modulus by 91%, yield strength by 33.3%, tensile strength by 9.2%, and linear expansion coefficient is only 55% of that of aluminum alloy. Compared with titanium alloy, the elastic modulus of boron aluminum alloy increases by 20%, the linear expansion coefficient is equivalent to titanium alloy, and the density is only 59% of titanium alloy, so the specific stiffness of boron aluminum alloy is higher than titanium alloy, more easy to resist deformation.
2 Processing of boron and aluminum alloy parts
In this paper, bracket parts as shown in Figure 2 are selected as test parts to study the processing technology of boron aluminum alloy. The raw material is block material, the material composition is B4C/Al composite material, the state is extrusion state, the heat treatment state is annealing state.
Parts processing technology is as follows: Milling (milling six square, square convenient clamping positioning, do the reference)→ wire cutting (contour of wire cutting parts)→ numerical control milling (semi-finishing, with precision requirements to leave 0.5 mm margin)→ pliers (using the grinding platform, pliers grinding datum)→ numerical control milling (finishing precision required parts to size, drilling)→ heat treatment (removing stress, Stable size)→ tongs (drilling, tapping)→ surface treatment.
3. Key processing technology of boron aluminum alloy
3.1 Milling
3.1.1 Selection of cutting tools
For the metal matrix particle reinforced composite material, when the carbide tool cutting, B4C particles are extruded and crack, easy to cause sharp edge collapse parts; When diamond cutting tool is used, B4C particles are often split and broken.
At high speed and cutting speed, the non-diamond tool will wear out quickly, and the machining quality will be manifested as the height difference at the overlap of the tool rails, or the tool grain will be quivering or fish scale. Compared with hard alloy tool, diamond knife has high durability, diamond thermal conductivity (146.5 W/(m·k)) is 1.31 times of boron aluminum alloy, 13.99 ~ 26.9 times of titanium alloy; Its high hardness, cutting edge can be sharpened very sharp; The cutting heat generated during cutting is less, and the cutting tool can transmit most of the cutting heat; Diamond tool allows higher cutting speed, but is less likely to produce bonding and diffusion between it and the cutting material. Therefore, it is more suitable for boron and aluminum alloy milling because of smaller bonding wear and diffusion wear generated by the cutting tool during cutting [2].
Compared with carbide milling cutter, the diamond cutter can ensure the machining quality of sharp edge and avoid the edge breakage effectively. In this test, the insert blade straight shank diamond milling cutter with diameter of 8 mm and length of 80 mm was selected as the cutting tool. The milling surface quality was shown in Figure 3.
The experimental results show that the most suitable milling parameters for boron-Al alloy materials are milling speed F=100 mm/min and spindle speed S=400 r/min.
3.1.3 Path of tool rail
When diamond cutter is used for milling, the appropriate cutting mode should be considered to reduce the occurrence of plane edge breakage. The tool way to choose along milling, the tool in the parts plane edge milling, the center of the tool should be in the edge, so as to ensure that there is more tool area involved in cutting, at the same time the direction of the material force for the material side, so that the material by extrusion pressure, rather than separation force, so that brittle materials in processing will not be broken edge edge. The appropriate tool path is shown in Figure 4.
In order to provide a good surface machining basis for semi-finishing, the datum surface of the test part was manually ground. The manual grinding processing site was shown in Figure 5. The experimental results show that the grinding parameters and machining effects of boron and aluminum alloy are similar to those of aluminum alloy 2A12 -- T4.
The wire cut finished product is shown in Figure 6. The experimental results show that the linear cutting parameters and machining effects of boron aluminum alloy are no different from those of ordinary aluminum alloy.
According to the characteristics of boron-Al alloy material, the strength of annealed boron-Al alloy is slightly lower than that of T4 state. Annealed blank material is used to facilitate parts processing. After rough milling and before finishing, the parts are treated with solution or stabilization. Solution treatment can improve the properties of the alloy, and obtain higher strength, better plasticity and corrosion resistance. The microstructure and dimensional stability of the alloy can be improved by the stabilization treatment.
After finishing the test, solid solution treatment was carried out. Before heat treatment, the hardness of the material is 80 HRB; After hardening, the hardness reached 88 HRB.
3.5 CNC milling and finishing
Annealed boron aluminum alloy is used in NC milling semi-finish machining, and 0.5mm allowance is left for key parts. Heat treatment is carried out after processing, and finally finish machining. Using polycrystalline diamond tool and diamond bit. The test results show that the most suitable milling parameters are milling speed F=100 mm/min, spindle speed S=400 r/min, milling depth H=2 mm. Point drilling parameters are S=400 ~ 600 r/min, F=20 ~ 50 mm/min, H=2 mm. Boring parameters are S=400 ~ 600 r/min, F=30 ~ 60 mm/min, H=0.02 ~ 0.2mm.
The finished parts are shown in Figure 7. After finishing the parts, the three coordinate inspection is basically qualified.
FIG. 7 Parts after finishing CNC milling
3.6 Drilling and tapping
In the test, it is found that the ordinary twist drill will be very difficult to drill boron and aluminum alloy, often a bottom hole is not drilled, the cutting edge of the bit has been worn, can not work, if continue to drill, it will cause the material collapse. Similarly, for holes with more than 10 pitches of thread, the ordinary tap is only adequate for the first half of the thread depth, and the second half of the thread depth is almost impossible to complete. For this purpose, specially purchased diamond drill bit and German EMUGE company tap, successfully completed drilling and tapping. Boron-aluminum alloy drilling and tapping are shown in Figure 8.
Figure 8 Boron-Al alloy drilling and tapping
3.7 Surface Treatment
In order to verify the surface treatment performance of boron aluminum alloy, the processed residues were respectively treated with conductive oxidation, black anodizing, natural anodizing and yellow anodizing (see Figure 9). The anodizing time was 1 h, the film thickness was 20 μm, and the surface treatment process of 2A12-T4 duralumin was used.
Figure 9. Four kinds of finished products of boron-Al alloy surface treatment
Salt spray test was conducted on boron-al alloy surface treated samples (see Figure 10). The conditions of salt spray test were as follows: at 35 ºC, NaCl solution with a concentration of 5% and a pH value of 6.5 ~ 7.2 was selected, and the sedimentation rate of salt spray was 1 ~ 2 mL/80 cm2·h. The test was conducted for 96 h continuously according to the standard of GJB 150.11-1986. The test results showed that no abnormality was found in the appearance of the test samples before, during and after the test.
FIG. 10 Salt spray test of four surface treated products of boron-Al alloy
According to GJB 150.9 -- 1986, under the condition of alternating temperature of 30 ~ 60 ºC and relative humidity of 95% for 240 h(including 6 h at 60 ºC and 8h at 30 ºC), 4 groups with 2 boron-aluminum alloy test pieces in each group were tested with humid heat (see Figure 11). The results showed that no abnormality was found in the appearance of the test samples before, during and after the test.
3.8 Grinding performance of turning and surface grinder
In order to verify the turning performance of boro-Al alloy and explore the appropriate turning parameters, diamond blades were used to turn the shaft parts of boro-Al alloy (see Figure 12). The turning results show that the surface roughness can reach about Ra3.2μm by using diamond turning blade. The turning parameters can refer to common aluminum alloy, that is, S=200 ~ 2 000 r/min; F=0.05 ~ 0.20 mm/r; ap=2 to 5 mm. Good turning performance.
FIG. 12 Turning of boron and aluminum alloy with diamond turning blade
When using the surface grinder to do mechanical grinding, boron aluminum alloy is wear-resistant than aluminum alloy, suitable for relatively small cutting depth, and the processing effect is close to that of aluminum alloy 2A12-T4. The grinding site of the surface grinder is shown in Figure 13.
FIG. 13 Grinding site of surface grinder
4 Conclusion
The processing technology of boro-Al alloy is summarized as follows: 1) Boro-Al alloy has higher elastic modulus and lower density than titanium alloy, so it can be used as a substitute for titanium alloy; 2) The CNC milling, drilling, tapping and boring of boron and aluminum alloys all require customized diamond tools, and the processing parameters are lower than those of ordinary aluminum alloys, generally: F=40 ~ 100 mm/min,S=400 r/min; 3) The grinding, grinding and linear cutting performance of boron aluminum alloy is similar to that of ordinary aluminum alloy, because the annealed billet strength is low, so the blank is suitable for annealed material, and after NC roughing, before finishing, the parts should be dealt with solution treatment. Boron aluminum alloy surface treatment refer to 2A12-T4 aluminum alloy process parameters.
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