Causes of poor machinability of titanium alloy
Material machinability is often measured by the cutting speed allowed for cutting a material with a certain tool durability. Usually T = 60 min, then vT is written as v60. If the v60 used for cutting carbon steel in cemented carbide turning tools is used as a reference, the cutting speed allowed for cutting other materials is v60'. Then the ratio of v60' to v60 is called the relative machinability of the material.
The main reason for the poor machinability of titanium alloys is due to the extremely high chemical affinity of titanium alloys and the extremely poor thermal conductivity:
1. Strong chemical affinity. Titanium alloys are easily “affinitive” with tool materials at temperatures above 300 °C. Some of the elements in the tool material during cutting are not dissolved in titanium and are chemically reactive with titanium.
2. Poor thermal conductivity. Titanium alloys have poor thermal conductivity, low thermal diffusivity, and high cutting temperatures. The thermal conductivity of titanium alloy is half of that of industrial pure titanium, and the thermal conductivity is 1/5 to 1/7 of 45 steel. Therefore, under the same cutting conditions, the cutting temperature of titanium alloy is more than 1 times higher than that of 45 steel.
3. Although the cutting force of titanium alloy is not large, it is about 75% of carbon steel. However, the length of contact between the chip and the rake face of the tool is much smaller than that of carbon steel, which is about half that of carbon steel. Thus, the tool tip is subjected to a stress of about 1.5 times that of the carbon steel when cutting the titanium alloy.
4, Titanium alloy work hardening phenomenon is not very serious, about the same as low carbon steel.
The main reason for the poor machinability of titanium alloys is due to the extremely high chemical affinity of titanium alloys and the extremely poor thermal conductivity:
1. Strong chemical affinity. Titanium alloys are easily “affinitive” with tool materials at temperatures above 300 °C. Some of the elements in the tool material during cutting are not dissolved in titanium and are chemically reactive with titanium.
2. Poor thermal conductivity. Titanium alloys have poor thermal conductivity, low thermal diffusivity, and high cutting temperatures. The thermal conductivity of titanium alloy is half of that of industrial pure titanium, and the thermal conductivity is 1/5 to 1/7 of 45 steel. Therefore, under the same cutting conditions, the cutting temperature of titanium alloy is more than 1 times higher than that of 45 steel.
3. Although the cutting force of titanium alloy is not large, it is about 75% of carbon steel. However, the length of contact between the chip and the rake face of the tool is much smaller than that of carbon steel, which is about half that of carbon steel. Thus, the tool tip is subjected to a stress of about 1.5 times that of the carbon steel when cutting the titanium alloy.
4, Titanium alloy work hardening phenomenon is not very serious, about the same as low carbon steel.
