Machining characteristics of titanium alloy
Turning and milling are particularly difficult when the hardness of titanium alloy is greater than HB350. When the hardness of titanium is less than HB300, the sticking phenomenon is likely to occur, and it is also difficult to cnc cutting. But the hardness of titanium alloy is only one aspect that is difficult to cut. 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: This is a significant feature of titanium alloy cutting processing, and the deformation coefficient is less than or close to 1. The sliding friction of chips on the rake face is greatly increased, which accelerates tool wear.
(2) The cnc cutting temperature of titanium alloy is high: Because the thermal conductivity of titanium alloy is very small (only equivalent to 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 transmitted, and it is concentrated in a small area near the cutting area and the cutting edge, and the cutting temperature is very high. Under the same cutting conditions, the cutting temperature can be more than twice as high as when cutting No. 45 steel.
(3) The milling force per unit area of titanium is large: The main cutting force of titanium is about 20% less than when cutting steel. Since 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 likely to cause chipping. At the same time, due to the small modulus of elasticity of titanium alloy, it is prone to bending deformation under the action of radial force during processing, causing vibration, increasing tool wear and affecting the accuracy of parts. Therefore, the process system is required to have good rigidity.
(4) The processing and hardening of titanium alloy is serious:Due to the high 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 temperatures; At the same time, plastic deformation during cutting will also cause surface hardening. Chilling phenomenon not only reduces the fatigue strength of parts, but also aggravates tool wear, which is a very important feature when cutting titanium alloys.
(5) Tools for processing titanium alloys are easy to wear and damage: After the blank is processed by stamping, forging, hot rolling and other methods, a hard and brittle uneven skin is formed, which can easily cause chipping of the turning tool, making the removal of the hard skin the most difficult process in titanium alloy processing. In addition, due to the strong chemical affinity of titanium alloy to the tool material, the tool is prone to bond wear under the conditions of high cutting temperature and high cutting force per unit area. When turning titanium alloy, sometimes the abrasion of the rake face is even more serious than that of the flank; When the feed rate f<0.1 mm/r, the wear mainly occurs on the flank surface; When f>0.2 mm/r, the rake face will be worn; When using cemented carbide tools for fine turning and semi-finishing turning, the wear of the flank face should be VBmax<0.4 mm.