Titanium is an isomer.
At room temperature, titanium alloys have three kinds of matrix structures, which can be divided into the following three categories: α Titanium alloy, β Titanium alloy,（ α+β） Titanium alloy.
Titanium alloy has the advantages of low density, high strength and corrosion resistance, so it has been widely used in aviation, aerospace, power generation equipment, nuclear energy, shipbuilding, chemical industry, medical devices and other fields.
1. Classification of titanium alloys
The classification and characteristics of titanium alloys are shown in table 7-11.
Table 7-11 classification and characteristics of titanium alloys
| Type | Features |
| α Titanium alloy | The structure is α phase a structure with a single closely arranged hexagonal lattice. α titanium alloy is characterized by good high temperature performance (it can work at 500 ℃ for a long time), strong oxidation resistance, but it cannot be strengthened by heat treatment, and low strength at room temperature. Typical brands are TA7, TA8, etc.α titanium alloy is a kind of titanium alloy that is easier to process. |
| β Titanium alloy | The organization is a single body centered cubic lattice β organization. βtitanium alloy is characterized by good cold deformation plasticity, which can be strengthened by heat treatment. It has high strength at room temperature, but poor thermal stability, so it is not suitable to work under high temperature conditions.Typical brands include TB1, TB2, etc.β titanium alloy has poor machinability. |
| （ α+β） Titanium alloy | It has αand β duplex structure.It is characterized by high strength at room temperature and high temperature, good plasticity and toughness, and can be strengthened by heat treatment, so it is widely used. Typical brands include TC1, TC4, etc. The machinability of （α+ β）titanium alloy is between the first two categories. |
2. Reasons why titanium alloy is difficult to cut
The reasons why titanium alloys are difficult to cut are shown in table 7-12.
Table 7-12 reasons for difficult cutting of titanium alloy
| Reason | Instruction |
| Short knife chip contact length | Under the action of oxygen and nitrogen in the air, titanium alloy chips will form hard and brittle compounds, making the chips into short pieces, so the tool chip contact length is very short, the cutting force and cutting heat are concentrated near the cutting edge, and the tool is easy to collapse. |
| Poor thermal conductivity | The thermal conductivity of titanium alloy is small, only 1/6~1/7 of that of 45 steel, and the density is small. The cutting heat is concentrated near the cutting edge, the temperature in the edge area is high, and the tool wear is intense. |
| High chemical affinity | Bonding with cemented carbide containing Ti is serious |
| Small elastic modulus | It is about 1/2 of the elastic modulus of 45 steel, so the elastic recovery is large and the friction is serious.At the same time, the workpiece is also prone to clamping deformation. |
| Titanium chips are flammable | Under high temperature (600 ℃), titanium chips are easy to burn. |
| Severe cold hardening | Titanium has great chemical activity. At high cutting temperature, it is easy to absorb oxygen and nitrogen in the air, forming a hard and brittle skin. At the same time, plastic deformation in the cutting process will also cause surface hardening.Cold hardening phenomenon will not only reduce the fatigue strength of parts, but also aggravate tool wear. |
3. Specific measures for cutting titanium alloy
The specific measures for cutting titanium alloy are shown in table 7-13.
Table 7-13 specific measures for cutting titanium alloy
| Measure | Instruction |
| Selection of tool materials | Cemented carbide tools shall be used as far as possible, and YG (ISO class K) cemented carbide without Ti shall be selected. In case of intermittent cutting or impact, high-speed steel tools (such as W2Mo9Cr4VCo8) can also be used, YBG20 and YBG102 can be used respectively. Remarkable results can also be achieved by cutting titanium alloys with diamond and cubic boron nitride cutters. |
| Selection of tool geometric parameters | The rake angle and main deflection angle of the tool should be small, and an appropriate tool tip arc must be ground; The rear corner should be large. Generally, carbide turning tools can be γo=5 ° -8, αo=10 ° ~15 °, γo=-3–5 °, kt=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 ≤ 0.2m. |
| Selection of Cutting Parameter | Choose a lower cutting speed, a larger feed rate and back feed rate.When turning with cemented carbide tools, the back cutting amount ap=1~3mm, and the back cutting amount of rough turning must be greater than the oxide skin depth;Feed rate f=0.1~0.3mm/r; The choice of cutting speed should be based on the strength of the titanium alloy being processed and the amount of back cutting. The cutting speed of TC4 titanium alloy vc=26~60m/min. |
| cooling and lubrication | Generally, extreme pressure emulsion should be used for cooling, and the flow should be sufficient. However, if the fatigue strength of parts is required to be high, the cutting fluid should not contain sulfur and chlorine. At this time, ordinary emulsion should be selected.If chlorine containing cutting fluid is used, hydrogen will be released and absorbed by titanium at high temperature during cutting, causing hydrogen embrittlement; It may also cause high temperature stress corrosion cracking of titanium alloy. |
| Clamping of workpiece | The clamping force should not be too large to avoid workpiece deformation. If necessary, auxiliary support can be added to improve the clamping rigidity. |
| other | The machine tool should have good rigidity, and the clearance of all moving parts should be carefully adjusted, which is particularly important for rough machining, otherwise it is easy to cut. |