Cutting characteristics of titanium alloy
When the hardness of titanium alloy is greater than HB350, cutting is particularly difficult. When it is lower than HB300, it is easy to stick to the knife and difficult to cut. However, the hardness of titanium alloys is only one aspect of the difficulty of machining. The key lies in the comprehensive influence of the chemical, physical and mechanical properties of the titanium alloy itself on its cutting performance. Titanium alloys have the following cutting properties:
(1) Small deformation coefficient: This is a remarkable feature of titanium alloy cutting. The deformation coefficient is less than or close to 1. The sliding friction distance of chips on the rake face is greatly increased, which accelerates tool wear.
(2) High cutting temperature: Since the thermal conductivity of titanium alloy is very small (equivalent to only 1/5 to 1/7 of No. 45 steel), the contact length between the chip and the rake face is extremely short, and the heat generated during cutting is not easily transferred. . Come out, concentrated in the cutting area and a small range near the cutting edge, the cutting temperature is very high. Under the same cutting conditions, the cutting temperature can be more than double that of cutting 45 steel.
(3) Large cutting force per unit area: the main cutting force is about 20% smaller than steel cutting. Due to the extremely short contact length between the chip and the rake face, the cutting force per unit contact area is greatly increased, which is likely to cause chipping. At the same time, due to the small elastic modulus of titanium alloy, it is prone to bending deformation under the action of radial force during processing, causing vibration, increasing tool wear, and affecting part accuracy. Therefore, the process system is required to have good rigidity.
(4) Severe cold hardening phenomenon: Due to the high chemical activity of titanium, at higher cutting temperatures, it can easily absorb oxygen and nitrogen in the air to form a hard and brittle outer skin; at the same time, plastic deformation during cutting can also cause Surface hardening. The phenomenon of work hardening not only reduces the fatigue strength of the part, but also increases tool wear, which is a very important characteristic when cutting titanium alloys.
(5) The tool is easy to wear: After the blank is processed by stamping, forging, hot rolling, etc., a hard and brittle uneven skin is formed, which can easily cause chipping, making the removal of the hard skin the most difficult process in titanium alloy processing. In addition, due to the strong chemical affinity between titanium alloy and tool material, the tool is prone to adhesive wear under the conditions of high cutting temperature and large cutting force per unit area. When turning titanium alloys, sometimes the wear of the rake face is even more serious than that of the flank face; when the feed rate f<0.1mm/r, the wear mainly occurs on the flank face; when f>0.2 mm/r, The surface of the tool on the front face will be worn; when using cemented carbide tools for finishing and semi-finishing, it is more appropriate for the flank wear VBmax to be less than 0.4 mm.
During milling, due to the low thermal conductivity of titanium alloy materials, the contact length between chips and the rake face is extremely short, and the heat generated during cutting is not easy to dissipate, and is concentrated in the cutting deformation zone and small area. range close to the cutting edge. During machining, extremely high cutting temperatures will be generated at the cutting edge, which will greatly shorten the life of the tool. For titanium alloy Ti6Al4V, when the tool strength and machine power allow, the key factor affecting the tool life is the cutting temperature, not the cutting force.
In the process of cutting titanium alloy, the matters that should be paid attention to are:
(1) Due to the small elastic modulus of titanium alloy, the clamping deformation and force deformation of the workpiece during processing are large, which will reduce the processing accuracy of the workpiece; the clamping force should not be too large when the workpiece is installed, and auxiliary supports can be added if necessary.
(2) If hydrogen-containing cutting fluid is used, it will decompose and release hydrogen at high temperatures during the cutting process, which will be absorbed by titanium and cause hydrogen embrittlement; it may also cause high-temperature stress corrosion cracking of titanium alloys.
(3) The chloride in the cutting fluid may decompose or volatilize toxic gases when used. Safety precautions should be taken when using it, otherwise it should not be used; parts should be thoroughly cleaned with chlorine-free cleaning agent immediately after cutting to remove chlorine-containing residues things.
(4) It is forbidden to use tools and fixtures made of lead or zinc-based alloys in contact with titanium alloys, and the use of copper, tin, cadmium and their alloys is also prohibited.
(5) All tools, fixtures or other devices in contact with titanium alloy must be clean; cleaned titanium alloy parts must be prevented from being contaminated by grease or fingerprints, otherwise it may cause stress corrosion from salt (sodium chloride) in the future.
(6) Under normal circumstances, when cutting titanium alloys, there is no risk of ignition. Only in micro-cutting, the small chips cut off have ignition and combustion. In order to avoid fires, in addition to pouring a large amount of cutting fluid, chips should also be prevented from accumulating on the machine tool. The tool should be replaced immediately after it is blunt, or the cutting speed should be reduced and the feed rate should be increased to increase the chip thickness. If a fire breaks out, fire extinguishing equipment such as talcum powder, limestone 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, because water can accelerate combustion and even cause a hydrogen explosion.
