Titanium alloy has the advantages of low density, high strength, corrosion resistance, etc., so it has been more and more widely used in aviation, aerospace, power generation equipment, nuclear energy, ships, chemical industry, medical equipment and other fields.
At room temperature, titanium alloys have three types of matrix organisation, and titanium alloys are divided into the following three categories: α titanium alloys (representative grade TA3, etc.), β titanium alloys (representative grade TB6, etc.), (α + β) titanium alloys (representative grade TC4, etc.).
α titanium alloy
①The organisation is a single dense hexagonal lattice of α-phase organisation.
② α titanium alloy is characterised by good high-temperature performance (can work at a high temperature of 500 ℃ for a long time), oxidation resistance, but can not be heat-treated to strengthen, low strength at room temperature.
③ α titanium alloy is a class of titanium alloy that is easier to process, typical grades are TA7, TA8 and so on.
β titanium alloy
①The organisation is a single body-centred cubic lattice of β organisation.
② β titanium alloy is characterised by good cold deformation plasticity, can be strengthened by heat treatment, high strength at room temperature, but poor thermal stability, not suitable for work at high temperature conditions.
③ β titanium alloy cutting and processing of poor, typical grades are TB1, TB2, etc..
(α + β) titanium alloy
① It has α and β dual-phase organisation.
② Characterised by high strength at room temperature and high temperature, good plasticity and toughness, can be strengthened by heat treatment, and therefore more widely used.
③ (a + β) titanium alloy cutting and machining between the first two types, typical grades are TCI, TC4, etc..
The reason why titanium alloy is difficult to cut
Titanium alloy is widely used in various fields because of its excellent qualities, however, it is "difficult to cut" and its excellent characteristics are "inseparable".
Titanium chips are flammable: at high temperature (600C), titanium chips are easily burned.
④poor thermal conductivity: the coefficient of thermal conductivity of titanium alloy is small, only 1/6-1/7 of 45 steel, and density is small, the cutting heat is concentrated in the cutting edge near the cutting edge, edge temperature is high, tool wear is intense.
Chemical affinity: serious bonding with cemented carbide containing Ti, and many tool coatings contain Ti.
Small modulus of elasticity: about 1/2 of the modulus of elasticity of 45 steel, so the elastic recovery is large. Friction is serious. At the same time, the workpiece is also easy to clamping deformation.
⑤ cold hard phenomenon is serious: the chemical activity of titanium is large, in the high cutting temperature, it is easy to absorb the oxygen and nitrogen in the air, the formation of hard and brittle skin, and the plastic deformation in the cutting process will also cause surface hardening. Cold hard phenomenon will not only reduce the fatigue strength of the parts, but also exacerbate tool wear.
⑥short chip contact length: titanium alloy chips in the air under the action of oxygen and nitrogen, will form hard and brittle compounds, so that the chips into short fragments, and thus the knife - chip contact length is very short, the cutting force and cutting heat is concentrated in the vicinity of the cutting edge, the tool is prone to chipping.
Specific measures for cutting titanium alloy
Want to attack titanium alloy, just know it is difficult to cut the reason is far from enough, but also according to its characteristics of an article on the right medicine.
Selection of tool material: Use Ti-free YG (ISO K) carbide, such as diamond and cubic boron nitride tools. At present, each tool supplier has its own corresponding titanium alloy cutting tools, readers can choose according to each sample book. Cutting home app has more than 1,000 cutting tool manufacturers' samples, if necessary, you can go to WeChat app to search for "cutting home" to use.
The choice of tool geometric parameters: the tool front angle and the main offset angle should be small, and must be ground out the appropriate tip arc, the back angle should be larger. General carbide turning tool can take
γ. = 5 ° ~ 8 ° α. = 10 ° ~ 15 °, λ. = -3 ° ~ -3 °.
λ.= -3°~ -5° kr=45° ~75
re=0.5~1.0mm
In addition, the roughness value of the front and back of the tool should be small, generally Ra≤02μm.
Cutting dosage selection: choose lower cutting speed, larger feed and back draft. When turning with carbide tools, the back draft should be 1-3mm, roughing back draft must be greater than the depth of the oxide skin, feed 0.1 ~ 0.3mm / r. The choice of cutting speed should be based on the strength of the titanium alloy processed and the maximum small back draft to decide, TC4 titanium alloy cutting speed should be 26 ~ 60m / min.
Cooling lubrication: Generally, extreme pressure emulsion should be used for cooling, and the flow rate should be sufficient. However, if the fatigue strength of the parts requires high, the cutting fluid should not contain sulfur, chlorine, at this time should be used ordinary emulsion. If the use of chlorine-containing cutting fluid, the cutting process at high temperatures will release hydrogen, which is absorbed by the titanium, causing hydrogen embrittlement: it may also cause high-temperature stress corrosion cracking of titanium alloys.
Workpiece clamping: the clamping force should not be too large, so as to avoid deformation of the workpiece, if necessary, can increase the auxiliary support to improve the clamping rigidity.
Other: the machine tool should have good rigidity, the clearance of the moving parts should be carefully adjusted, which is particularly important for rough machining, otherwise it is easy to hit the knife.
