With the continuous development of current science and technology, the application of titanium alloy can not only improve the firmness of its overall construction materials in space technology and marine technology, but also improve the quality of some civil titanium CNC parts, Therefore, it is very important to analyze the characteristics of titanium alloy which is difficult to process and find out how to process in CNC machining.
Problems and Challenges in CNC Titanium Machining
When the hardness of titanium alloy is higher than HB350, it is very difficult to cut. When the hardness is lower than HB300, it is easy to stick 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 comprehensive influence of the 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 remarkable feature of titanium alloy cutting, the deformation coefficient is less than or close to 1. The distance of the chip sliding on the rake face is greatly increased, which accelerates the tool wear.
(2) High cutting temperature: because the thermal conductivity of titanium alloy is very small, the contact length between chip and rake face is very short, the heat generated during cutting is not easy to transfer out, and it is concentrated in a small range near the cutting area and cutting edge, so the cutting temperature is very high. Under the same cutting conditions, the cutting temperature can be more than twice as high as that of 45 steel.
(3) The cutting force per unit area is large: the main cutting force is about 20% smaller than that of steel cutting. Because the contact length between chip and rake face is very short, the cutting force per unit contact area is greatly increased, which is easy to cause edge collapse. At the same time, because the elastic modulus of titanium alloy is small, it is easy to produce bending deformation under the action of radial force, which causes vibration, increases tool wear and affects the accuracy of parts. Therefore, the process system should have better rigidity.
(4) Severe cold hardening phenomenon: 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 temperature; at the same time, the plastic deformation in the cutting process will also cause surface hardening. The phenomenon of cold hardening can not only reduce the fatigue strength of parts, but also increase the tool wear, which is a very important feature in cutting titanium alloy.
(5) Tool is easy to wear: after the blank is processed by stamping, forging, hot rolling and other methods, it forms a hard and brittle uneven skin, which is very easy to cause the phenomenon of edge collapse, making the removal of 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 easy to produce adhesive wear under the 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; when the feed rate f < 0.1 mm / R, the wear mainly occurs on the rake face; when f > 0.2 mm / R, the rake face will be worn; when finishing and semi finishing with cemented carbide tools, VBmax < 0.4 mm is suitable for the wear of the rake face.
Machining hard alloy materials such as titanium alloy requires large cutting force, or high torque spindle. However, the spindle torque of high-speed CNC machine tools typically used in HEM-HSM machining of light alloy materials such as aluminum alloy is mostly less than 100nm, generally less than 200nm, which does not have the ability of machining hard alloy materials such as titanium alloy with high efficiency.
Generally, only low cutting speed is allowed for machining hard alloy materials such as titanium alloy, that is, only low spindle speed can be used. However, the spindle speed range of high-efficiency high-speed CNC machine tool typically used for HEM-HSM machining of light alloy materials such as aluminum alloy does not meet the requirements of current titanium alloy processing technology.