Machinability of titanium alloy and selection of band saw blade type
Sawing performance of titanium and titanium alloy
Titanium is an isomer with a melting point of 1720 ° C. When the temperature is lower than 882 ° C, it has a close packed hexagonal lattice structure, which is called α titanium alloy; when the temperature is higher than 882 ° C, it has a body centered cubic lattice structure, which is called β titanium. There are three kinds of matrix structure in titanium alloy at room temperature. Titanium alloy is also divided into the following three categories: α – titanium alloy, β – titanium alloy and α + β – titanium alloy. The national standard indicates TA, TB and TC respectively.
The machinability of α – titanium alloy is the best, followed by a + β – titanium alloy, and β – titanium alloy is the worst. Even for the same type of titanium alloy, due to its different tensile strength and composition, its cutting performance is also different. The following table is a part of the general tensile strength and hardness table of titanium and titanium alloy.
Although commercial pure titanium is similar to austenitic stainless steel, most titanium alloys are not easy to process. When the hardness of titanium alloy is more than hb350, it is very difficult to cut. When it 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 cut. The key lies in the influence of the synthesis 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, and the deformation coefficient is less than or close to 1. The distance of chip sliding and friction on the front face is greatly increased, which accelerates the wear of band saw blade.
- (2) High cutting temperature: due to the small thermal conductivity of titanium alloy (only equivalent to 1 / 5-1 / 7 of 45 steel), the contact length between chip and front cutter face is very short, and the heat generated during cutting is not easy to transfer out, 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 as high as when cutting 45 steel.
The table below shows the comparison of relative cutting properties of titanium alloy and other materials:
- (3) The cutting force per unit area is large: the contact length between the chip and the rake face is very short, and the cutting force per unit contact area is greatly increased, which is easy to cause edge collapse. At the same time, due to the small elastic modulus of titanium alloy, it is easy to produce bending deformation under the effect of radial force, which will cause vibration, increase the wear of band saw blade and affect the accuracy of parts. Therefore, the process system should be rigid.
- (4) Severe cold hardening: due to the high chemical activity of titanium, under high cutting temperature, it is easy to absorb oxygen and nitrogen in the air to form a hard and brittle skin; 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 increases the wear of band saw blade, which is a very important feature in cutting titanium alloy.
- (5) The tooth tip of band saw blade is easy to wear: after the rough 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 phenomenon of edge collapse, making the hard skin removal the most difficult process in titanium alloy processing. In addition, due to the strong chemical affinity of titanium alloy to the blade tip material, the blade is easy to produce bond wear under the condition of high cutting temperature and large cutting force per unit area. When cutting titanium alloy, sometimes the wear of the front face is even more serious than the back face. In the sawing process, due to the low thermal conductivity of titanium alloy material, and the short contact length between the chip and the front face, the heat generated in the cutting process is not easy to spread out, which is concentrated in the cutting deformation area and the small range near the cutting edge. During the cutting process, the cutting temperature at the cutting edge will be very high, which will greatly shorten the life of the band saw blade. For titanium alloy TC4, the cutting temperature is the key factor affecting the life of the saw blade, not the cutting force, when the strength of the tip and back of the saw blade and the power of the saw machine are allowed.
Source: China Titanium Alloy Flange Manufacturer: www.titaniuminfogroup.com