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When the hardness of titanium alloy is greater than hb350, it is particularly difficult to process. When the hardness is less than hb300, it is easy to stick to the tool and difficult to cut. However, the hardness of titanium alloy is only one aspect that is difficult to be machined. The key lies in the influence of the combination of chemical, physical and mechanical properties of titanium alloy on its machinability. Titanium alloy has the following cutting characteristics:
(1) Small deformation coefficient: the remarkable feature of titanium alloy machining, the deformation coefficient is less than or close to 1. The sliding friction distance on the rake face of cutting is greatly increased, accelerating tool wear.
(2) High cutting temperature: the thermal conductivity of titanium alloy is very small (1/5~1/7 of 45 # steel), the contact length between chip and rake face is very short, and the heat generated during cutting is not easy to be transmitted, which is concentrated in a small range near the cutting area and cutting edge, and the cutting temperature is very high. Under the same cutting conditions, the cutting temperature can be more than twice that of 45 steel.
(3) The cutting force per unit area is large: the main cutting force is about 20% less than that when cutting steel. Because the contact length between the chip and the rake face is extremely short, the cutting force per unit contact area is greatly increased, which is easy to cause edge collapse. At the same time, the elastic modulus of titanium alloy is small, and it is easy to produce bending deformation under the radial force during machining, causing vibration, increasing tool wear and affecting the accuracy of parts. Therefore, the process system is required to have good rigidity.
(4) The phenomenon of cold hardening is serious: due to the great chemical activity of titanium, it is easy to absorb oxygen and nitrogen in the air to form a hard and brittle skin at high cutting temperature; At the same time, plastic deformation in the cutting process will also cause surface hardening. Cold hardening not only reduces the fatigue strength of parts but also aggravates tool wear, which is a very important feature when cutting titanium alloys.
(5) The tool is easy to wear: after the blank is processed by stamping, forging, hot rolling and other methods, the hard and brittle uneven skin is formed, which is very easy to cause the edge collapse phenomenon, making cutting the hard skin the most difficult process in titanium alloy processing. In addition, due to the strong chemical affinity of titanium alloy to tool materials, the tool is prone to bond wear under conditions of high cutting temperature and large cutting force per unit area. When turning titanium alloy, sometimes the wear of the rake face is even more serious than that of the rake face.
(6) Ignition risk: Generally speaking, there is no ignition risk when cutting titanium alloy. Only in micro cutting, the small chips cut can ignite and burn. Therefore, it is strictly prohibited to use oil-based cutting fluid. In order to avoid fire, it is necessary to pour a large amount of cutting fluid and prevent chips from accumulating on the machine tool. Replace the tool immediately after it is blunt, or reduce the cutting speed and increase the feed rate to increase the cutting thickness. In case of fire, fire extinguishers such as talc powder, limestone powder and dry sand should be used to put it out. Carbon tetrachloride and carbon dioxide fire extinguishers are strictly prohibited, and watering is not allowed. Water will accelerate combustion and even lead to a hydrogen explosion.
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