Titanium alloys are hard and high-strength. What is the trick to complete the processing of waste knives during processing?
Titanium and titanium alloys are a new material developed in recent decades, which belong to non-ferrous metals. Due to his performance and cutting characteristics, its machinability is relatively poor. But as long as he fully understands his performance and chooses the cutting conditions reasonably, he can successfully complete his cutting.
Titanium alloy workpiece is too hard, what should I do when processing waste knives? The factory master said: the processing method is useless
First understand what is the composition of titanium alloy?
Titanium alloy has a series of excellent mechanical and physical properties such as low density (about 4.5 g / cm3), high strength and corrosion resistance (acid, alkali, seawater and atmospheric medium). Therefore, it is more and more widely used in industries such as aviation, aerospace, nuclear energy, chemical industry, petroleum, metallurgy and medical equipment.
(1) Classification of titanium alloys. Titanium is an allotrope with a melting point of 1720 ° C and an isotope transition temperature of 882 ° C.
Below 882 ° C, titanium exhibits a close-packed hexagonal lattice structure, called alpha titanium. Above 882 ℃, it shows a body-centered cubic lattice structure, called β-titanium. Using the different characteristics of the above two structures, adding different alloy elements to gradually change the phase transition temperature and phase content, so as to obtain titanium alloys with different structures.
At room temperature, titanium alloys have three basic structures, which can be divided into the following three categories. ①Alpha titanium alloy. Single-phase alloy composed of α solid solution. The heat resistance is higher than that of pure titanium, the structure is stable, the oxidation resistance is strong, and the strength is maintained at 500 to 600 ℃. The creep resistance is strong, but it cannot be strengthened by heat treatment. Hardness is 240 ～ 300HB, tensile strength σb is 340 ～ 981MPa, elongation δ is 10% ～ 25%, impact value ak is (29.43 ～ 58.86) × 104J / m2, thermal conductivity к is 7.54 ～ 10.47W / m · K. ② Beta titanium alloy. Single-phase alloy composed of β-phase solid solution.
Has higher strength without heat treatment. After quenching and aging, the alloy is further strengthened, but the thermal stability is poor, so it is not suitable for use at high temperatures. The grades are TB1 and TB2. The hardness is 240 ～ 300HB, the tensile strength σb is 1 079 ～ 1 373MPa, the elongation δ is 18%, the impact value ak is 29.43 × 104J / m2, and the thermal conductivity к is 6.28W / m · K. ③α + β titanium alloy. Composed of α and β bidirectionally, there are both α stable elements and β stable elements, such as Cr eutectoid elements such as Cr, Co, Fe, Ni and Mn, which stabilize the β phase.
Both Sn and Zr elements have considerable solid solubility in the α and β phases and are effective strengthening agents. This type of alloy is stable in structure, has good high temperature deformability, toughness and plasticity, and can be quenched and aged to strengthen the alloy. The strength after heat treatment is increased by 50% to 100% compared to the annealed state. High temperature strength, can work for a long time at 400 ~ 500 ℃, thermal stability is second only to α titanium alloy. The grades are TC1 ～ TC10.
Hardness is 210 ～ 365HB, tensile strength σb is 589 ～ 1 059MPa, elongation δ is 10% ～ 15%, impact value ak is (29.43 ～ 44.15) × 104J / m2, thermal conductivity к is 5.44 ～ 9.63W / m · K.
Among the three titanium alloys, α titanium alloy has relatively good machinability, followed by α + β titanium alloy, and β titanium alloy is the worst. Among them, the most commonly used are α titanium alloy and α + β titanium alloy.
(2) Performance characteristics of titanium alloy.
- ①High specific strength. Titanium alloy has low density and high strength, so its specific strength (σb / ρ) is greater than that of ultra-high strength steel.
- ②High heat strength. Titanium alloy has good thermal stability, high temperature strength, and can work for a long time at 500 ℃, while aluminum alloy can only work below 200 ℃. The strength of the titanium alloy at a temperature of 300 to 500 ° C is about 10 times higher than that of the aluminum alloy.
- ③ Good corrosion resistance. Titanium alloy can work for a long time in humid atmosphere and seawater medium, its corrosion resistance is far superior to stainless steel. Strong resistance to pitting, acid etching and stress corrosion. It has excellent corrosion resistance to alkali, chloride, chlorine organic compounds, nitric acid and sulfuric acid. But titanium has poor corrosion resistance to media with reducing oxygen and chromium salts.
- ④ Great chemical activity. Titanium produces strong chemical reactions with O, N, H, CO, CO2, water vapor and nitrogen in the atmosphere. When the carbon content is greater than 0.2%, hard TiC will be formed in the titanium alloy. When the temperature is high, it will form TiN hard surface layer with N. Above 600 ° C, titanium can absorb oxygen to form a hardened layer with higher hardness.
Increased hydrogen content will also form a hardened layer. The depth of the hard and brittle surface layer produced by titanium absorbing gas can reach 0.1-0.15mm, and the degree of hardening is 20% -30% higher than the hardness of the substrate. Titanium also has a large chemical affinity, and is prone to stick to the friction surface.
- ⑤ Low thermal conductivity and small elastic modulus. Titanium has a very low thermal conductivity of 15.24 W / m · K, which is 1/4 of nickel, 1/5 of iron, and 1/4 of aluminum. The thermal conductivity of various titanium alloys is lower, generally 50% of titanium. For example, the к of TC4 is 7.95W / m · K, and the к of TC9 is 7.12W / m · K. The elastic modulus E of titanium and titanium alloy is 110 000 MPa, which is about 1/2 of that of general steel, so the rigidity is poor and easy to deform.
- ⑥ Low temperature performance is good. Titanium alloy maintains its mechanical properties under low temperature and ultra-low temperature (－100 ～ －253 ℃) conditions, it is an important low temperature structural material.
2. Cutting characteristics of titanium alloy
Titanium has a high melting point (1 720 ° C) and a large activation energy. It is not easy for lattice atoms to leave their equilibrium positions. Therefore, during cutting, the energy consumed for cutting off into chips is large. The more alloying elements added, the higher the strength and hardness of the alloy and the more difficult it is to cut. Tests have shown that when the hardness of the titanium alloy is greater than 350 HBW, the more difficult the cutting process.
When the hardness is less than 300HBW, it is easy to stick the knife, and it is also difficult to cut. The best cutting hardness of titanium alloy is 300 ~ 350HBW. The reason why the titanium alloy is difficult to cut is not only the hardness, but also the influence of other mechanical properties, physical properties and chemical properties. The cutting characteristics are as follows.
- (1) The deformation coefficient is small. The chip deformation coefficient of cutting titanium alloy is slightly less than 1 or close to 1, which is a significant feature when cutting titanium alloy. The small plastic deformation and the small chip shrinkage rate are due to the transition of titanium from α to β when cutting at high temperatures, and the increase in the volume of β titanium causes the growth of chips. , H and N, the titanium chips lose their plasticity, and the chips no longer shrink.
- (2) The cutting temperature is high. Due to the low thermal conductivity of the titanium alloy, it is difficult to transfer the cutting heat, resulting in an increase in the temperature of the cutting zone. So under the same cutting conditions, his cutting temperature is nearly double that of cutting 45 steel.
Is also due to its small plastic deformation, short contact between the chip and the rake face, and the cutting heat is concentrated in the narrow area of the cutting edge, which is not easy to spread out.
- (3) The cutting force per unit area is large. The main cutting force for cutting titanium alloy is about 20% smaller than that for cutting general steel. However, due to the short length of the contact between the tool and the chip, the cutting per unit area is greatly increased, and the tool is likely to cause chipping.
- (4) The tool is easy to wear. The titanium alloy blank is processed by forging, hot rolling and other methods to form a hard and brittle uneven skin, which is easy to cause tool wear. Furthermore, due to his high cutting temperature, short contact between the chip and the rake face of the tool, poor heat dissipation conditions, coupled with its small modulus of elasticity and large elastic recovery (about 2 to 3 times of cutting stainless steel), friction with the rake face of the tool The large area and large affinity make it easy to cause abrasive, adhesion, diffusion and oxidative wear of the tool.
- (5) It is easy to produce surface metamorphic pollution layer. Due to the large chemical activity of titanium alloy, it is easy to produce strong chemical reaction with various gas impurities. If O, N, H, C, etc. invade the surface layer of the titanium alloy, it is easy to form a TiC, TiN hard surface layer, resulting in an increase in the hardness and brittleness of the surface layer.
3. Tips for selecting processing conditions for cutting titanium alloy
(1) Cutting tool material for cutting titanium alloy. When cutting titanium alloys, tool materials with high hardness, good wear resistance, high bending strength, high thermal conductivity and low affinity with titanium alloys should be used.
- ① High speed steel. Aluminum-containing, cobalt-containing super-hard high-speed steel and high-vanadium high-speed steel should be preferred, because its hardness at room temperature and high temperature is 6-8HRC higher than that of ordinary high-speed steel, and the tool durability can be increased by more than 4 times. Brands include W6Mo5Cr4Ν2Al, W2Mo9Cr4ΝCo8 and W12Mo3Cr4Ν3Co5Si.
- ②Cemented carbide. YG-based fine-grained or ultra-fine-grained cemented carbide with TaC or NbC should be selected. This type of alloy not only has high hardness and wear resistance, but also has several times higher tool durability. The grades are YS2, YG813, YD15 and YG6x. Coated cemented carbide should not be TiN and TiC coatings, but TiAlN and TiAlSi coatings and diamond coatings should be selected.
- ③ Super hard tool material. Such as artificial polycrystalline diamond (PCD), artificial thick film brazed diamond (CND) and cubic boron nitride (PCBN). They can cut titanium alloy at high speed (Nc≥200m / min), the tool durability is dozens to hundreds of times of hard alloy, and the processing quality is good.
(2) The geometric parameters of the tool for cutting titanium alloy.
- ① Front corner. When the titanium alloy is cutting, the contact between the chip and the rake face of the tool is very short. If the rake angle is small, the contact area between the chip and the rake face can be increased, so that the cutting force and cutting heat are not excessively concentrated on the cutting edge of the tool, and the heat dissipation condition is improved. At the same time protect the blade, reducing the possibility of chipping. Generally γ0 = 5 ° ～ 15 °. High-speed steel cutter, γ0 = 10 ° ～ 15 °. For tools made of super-hard tools, γ0 = 0 ° ～ 5 °.
- ② Rear corner. Because the elastic modulus of the titanium alloy is small, which is 1/2 of that of general steel, the elasticity of the cutting surface and the processed surface is greatly restored during cutting. In order to reduce the friction and wear of the tool flank and the workpiece, generally α0≥ 15 °.
- ③ The main and auxiliary declination. When cutting titanium alloys, the cutting temperature is high and the blade contact is short. In order to increase the strength of the tip, improve the heat dissipation conditions and reduce the load per unit length of the blade, during roughing, κr≤45 °, κr ‘= 10 ° ～ 15 ° ; During finishing, κr> 75 °.
- ④ Blade inclination. Due to the uneven and hard skin of the rough skin, in order to increase the strength of the blade tip, λs = -5 ° ～ -3 ° during rough machining; λs = 0 ° ～ 3 ° during finishing. ⑤Arc radius of tool tip. Generally, rε = 0.8 ～ 1.2mm.
(3) Cutting amount for cutting titanium alloy. Due to the performance and cutting characteristics of titanium alloys, the main factor affecting tool durability is cutting temperature. Therefore, strive to choose the corresponding cutting speed at the optimal cutting temperature. The optimal cutting temperature of high-speed steel tools is 480 ～ 540 ℃, and the corresponding cutting speed is νc = 8 ～ 12m / min;
The best cutting temperature of cemented carbide tools is 650 ～ 750 ℃, and the corresponding cutting speed is Νc = 25 ～ 54m / min. However, the amount of feed also has a greater impact on the cutting temperature of titanium alloys. The large cutting temperature is high, so generally it should not be greater than 0.3mm / r. PCD and CND cutters have a Nc = 100-200m / min, but generally dry cutting takes a small value, while wet cutting takes a large value. When cutting titanium alloys, high-speed micro dry cutting should be avoided to avoid spontaneous combustion of tiny titanium chips in the air.
(4) Cutting fluid for cutting titanium alloy. In order to reduce the cutting temperature, a large amount of cooling cutting fluid should be poured in the cutting area. The thermal conductivity, specific heat, heat capacity, vaporization heat, vaporization speed, flow rate and flow rate of the cutting fluid are high.
Generally, the thermal conductivity of water is 3 to 5 times higher than that of oil, the specific heat is 1 times larger, and the heat of vaporization is about 10 times. In turning, milling and drilling, emulsions or aqueous solutions with extreme pressure additives (containing S, P and Cl) are often used.
The formula of extreme pressure aqueous solution is: chlorinated fatty acid, polyvinyl chloride 0.5% ～ 0.8%, trisodium phosphate 0.5%, triethanolamine 1% ～ 2%, sodium nitrite 1.2%, and the rest is water.
When reaming, reaming, broaching and tapping, extreme pressure cutting oil, castor oil, oleic acid, vulcanized oil and chlorinated oil with good lubricating properties should be used.
(5) Attention should be paid to cutting titanium alloy.
- ① When cutting titanium alloy, avoid micro cutting to avoid spontaneous combustion. In case of fire, talcum powder, lime powder and dry sand should be used to extinguish the fire. It is strictly forbidden to use carbon tetrachloride and carbon dioxide fire extinguishers, and watering is not allowed to avoid accelerated combustion and lead to hydrogen explosion.
- ② When milling, try to use down milling and asymmetric milling, so that when cutting off, the chip is very thin, it is not easy to stick to the blade, to avoid damage to the blade during cutting. The eccentricity of asymmetric milling e = (0.04 ~ 0.1) d0, d0 is the cutter diameter.
- ③Since the elastic modulus of the titanium alloy is 1/2 of the steel material, it is easy to deform during clamping, which directly affects the processing accuracy. Therefore, when installing the workpiece, the clamping force should be appropriate and not too large. If necessary, add auxiliary support.
- ④When drilling with high-speed steel drill bits, the drilling pattern is to use three-pointed and seven-edged group drills as far as possible. Small axial force and improved heat dissipation conditions. Νc≤12m / min, higher NC will accelerate the bit wear.
- ⑤Tapping is the most difficult process for cutting titanium alloys, especially small diameter threads. Because of its small elastic modulus, large springback, and the large contact area between the tap and the workpiece, the friction torque is increased, which is generally about 1 times that of 45 steel, which causes the tap to wear too fast and even be broken in the screw hole. In order to change this undesirable phenomenon, the back angle of the tap should be increased to about 15 ° with a diamond assorted file before tapping, and the bottom hole of the thread should be enlarged to make the tooth height rate about 70%.
Although titanium alloy is a difficult-to-cut material, as long as he is familiar with his performance and cutting characteristics, and chooses the tool material, tool geometric parameters, cutting amount, cutting fluid, and attention to the problems in cutting, he can successfully complete all kinds of Cutting.