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Table 1 Cutting tool materials used for cutting titanium alloy Grade ISO HRA sbb YG3X K01 91.5 110 YG6A K10 92 140 YG6X K10 91 140 YG8 K30 89 150 YG8W K25 92 200 YG813 K20 91 160 YG643 K05 93 150 YG10HT K30 92 220 YL10.2 K35 HV1600 220 YS2T K30 91.5 180 YD15 K01 91.5 170 Y310 K05 92 125 Y320 K20 91.5 180 Y330 K30 90.3 200 YM051 K10 92.5 162 1 Selection of tool material and geometric parameters The thermal conductivity of titanium alloy is poor, which is 1/6 to 1 of No. 45 steel. /7. When selecting the tool material, the thermal conductivity of the tool material must be taken into account. The YG class is better than the YT class. Cutting titanium alloy generating unit chip, deformation coefficient is approximately 1, the contact length of the cutting chip is very short, the cutting force is concentrated near the cutting edge, the unit cutting force is large, easy to chipping. Requires better tool material toughness, sbb> 150kg/mm2, titanium alloy chemical properties live wave, easy to produce hard and brittle compounds with oxygen and nitrogen in the air, accelerate tool wear; titanium and tool elements are easy to affinity, processing titanium alloy use The tool material is less titanium or no titanium. YG cemented carbide is preferable, and YT cemented carbide is not used. The tool material used for cutting titanium alloys is shown in Table 1. Titanium alloy has a small elastic modulus, which is 1/2 of 45 steel, and it has a large deformation during processing. The back angle of the tool should be larger. Take a0=15°～18° to reduce the friction between the machining surface and the flank. The rake angle must also be larger than g0=10°. The tool is sharp, reducing the machining distortion, reducing the work hardening, and improving the quality of the machined surface. The main angle kr=90° reduces the radial force and prevents vibration. The radius of the tool tip arc determines the strength, heat dissipation, and wear of the tool tip. It is not easy to oversize and re = 1 to 2 mm. For cutting titanium alloys, the tool must be sharp and the blunt knife cannot be cut. The blunt radius of the tool must be small, that is, fine grained material is selected to reduce the radius of the obtuse circle. At the same time, the hardness of the tool material is increased, and the toughness does not decrease. Improve the sharpening quality of the tool, use diamond, cubic boron nitride grinding wheel. Before and after the blade surface roughness Ra ≤ 0.1μm, reduce the chip and rake face. The friction between the workpiece and the flank. The edge should be straight, without jaggies, to reduce the wear of the chipping. 2 Cutting Roughness The purpose of the titanium alloy is to remove large excesses and ensure greater tool life. VB = 0.4mm, cutting speed is not high, generally V ≤ 35m/min, tool durability T = 60min, cutting edge ap ≤ 4mm, cutting amount f ≤ 0.24mm / r, and increase flow solution cooling. The purpose of the precision car titanium alloy is to ensure dimensional accuracy and surface quality, generally ap ≤ 1mm, wear blunt standard VB = 0.2mm, try to use high cutting speed and the necessary tool life, cutting speed 35m/min ≤ V ≤ 140m/min Tool durability T=30min. With engine oil cooling and lubrication, reduce the friction coefficient between the cutters and improve the quality of the machined surface. Turning usage is shown in Table 2.
Table 2 Cutting Parameters NO 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 V(m/min) 13.2 14 21.1 23.3 30 31.7 35 37.7 40 45.2 45.8 50.2 69.9 70.7 76.4 89.4 111.8 127.2 130.2 141.3 f (mm/r) 0.24 0.24 0.24 0.24 0.2 0.2 0.2 0.16 0.12 0.16 0.1 0.2 0.13 0.13 0.1 0.1 0.1 0.1 0.1 ap (mm) 3 3 3 3 2.5 2.5 2.5 2.5 1.5 1 1 1 0.5 0.5 0.5 0.5 0.3 0.3 0.3 0.3 3 Causes of tool wear and tool wear. Titanium alloy machining. Tool wear occurs at the flank and tip. Titanium has a small elastic modulus, a large rebound during cutting, and a large contact area between the flank and the machining surface, which accelerates friction. The furrow can be seen under the microscope. The tip has poor heat dissipation conditions, high temperatures, and severe wear. Especially when finishing the car, when the wear amount of the flank does not reach the dullness standard, the tool tip wears too fast and the cutting performance of the tool is lost. No noticeable signs of wear can be seen on the rake face because of the short contact length of the chips. The main forms of wear are chipping and micro-disintegration. The sawtooth blade can be seen under the microscope. Processing titanium alloy knife is very serious. Both the high-speed cutting and the low-speed cutting have sticking knife problems. Dry cutting is more obvious. Adhesive wear is the main cause of wear of titanium alloys. When the cutting speed is high, the diffusion wear accounts for a large proportion when the cutting temperature reaches 600°C or more. The reaction of oxygen and nitrogen in the air with titanium alloys to produce hard and brittle compounds exacerbates abrasive wear on the tool. As a result, the blunt standard for machining titanium alloy tools is set to be lower, the tool life is lower, and the cutting speed is still relatively low. See Table 3 for specific values.
Table 3 Tool life and cutting speed VB(mm) V(m/min) T(m/min) 0.2 50~140 30 0.3 35~50 60 0.5 20~35 90 0.6 >20 120 YG class g0=10°, A0=18°, dry cutting, wet cutting, T double 4 parts processing technology and surface quality used lathe power is greater, the spindle rotation accuracy is higher, the feed mechanism has no crawling. The knife bar has enough rigidity. The parts should be firmly clamped. For example: When the vehicle Ø74 is cylindrical, the center is supported by a center frame, which improves the clamping rigidity and reduces the aspect ratio. The processing sequence is as follows: the Ø65 hole is machined first, then the Ø74 outer circle and the empty knife are machined. The hole can be cut with a single-edged trowel, allowing a large amount of knife. It is better to use a floating boring tool. After the hole has reached the dimensional accuracy, the chuck is used to clamp the outer circle of the car and the shock absorbing material is placed in the hole. The tool must be sharp. Once it is vibrating, change the tool changer to vibration source immediately. It is better to use the plastic expansion tire fixture to process the outer circle. The boring speed is not too high, V=20m/min is better. Control the amount of rebound. The minimum depth of cut is 0.05mm. When ap too small can not form a continuous chip, it is easy to produce vibration. When the clamping stiffness is good, the outer circle ap = 0.01 is feasible as a continuous chip. As long as the cutting titanium alloy does not vibrate, a surface roughness Ra of 1.6 μm can be achieved. The tool is sharp and the surface quality is also good. Use a sharp carbide turning tool to cut the titanium alloy and increase the flow lubrication liquid to cool and lubricate. When the cutting speed is V ≥ 100m/min, the surface roughness of the outer circle of the car can reach Ra0.8μm, the surface hardened is lighter and the residual stress is smaller. The relatively high size and lightness are easy to obtain. When ap=0.005mm, continuous soft chips can be formed to achieve the dimensional accuracy and roughness of the grinding. Boring has a certain degree of difficulty. The main reasons are: the rigidity of the process system composed of machine tools, fixtures, workpieces and tools is insufficient. Once vibrated, the wall quality immediately drops. Roughness Ra 1.6μm is still easy to reach. 5 Conclusions Thin-walled titanium alloy parts are easier to turn than milling. Turning for continuous cutting, no impact. Chips are unit chips and there is no problem with chip breaking. Titanium alloy has a small elastic modulus and a large yield-to-strength ratio; it has a large elasticity during cutting and requires large deformation energy. Select tool material hardness is higher, toughness is better, sbb ≥ 200kg/mm2, and does not contain TiC. The edge should be sharp, the back corner should be large, and the radius of the obtuse circle should be small. Avoid blunt knife cutting. Titanium alloy is hard and tough, thermal conductivity is poor; when roughing, to ensure the necessary tool durability, to determine the cutting speed; fine car, in order to complete the car generation grinding, as far as possible the use of high-speed cutting; can reduce the surface roughness, reduce the Machining surface work hardening and residual stress. To improve the stiffness of the process system consisting of machine tools, fixtures, workpieces and tools can solve the problem of turning deformation of titanium alloy thin-walled parts.
An aircraft engine wearing parts diagram (abbreviated), the minimum wall thickness of 2mm, a total length of 400mm, is a typical long thin-walled parts. Technical requirements are higher, the aperture tolerance is 0.046, the outer circle tolerance is 0.03, and the surface roughness Ra is 1.6. In order to ensure the difference in wall thickness, the hole and the outer circle must be coaxial and the processing is difficult. The material is imported from Russia under the designation BT20, and the yield strength and tensile strength sb=90-110kg/mm2. It is more difficult to process than TC4. Grinding titanium alloys tends to clog the grinding wheel and burn the machined surface. Therefore, it is only possible to use turning to replace the grinding wheel.