Since the discovery of the element titanium in 1790, mankind has gone through a hundred years of arduous exploration in order to obtain its extraordinary performance. It was not until 1910 that mankind first produced the metal titanium, but the road to the application of titanium alloys was full of ups and downs, and it was not until 1951, 40 years later, that industrialized production was finally realized. Titanium alloy has the characteristics of high specific strength, corrosion resistance, high temperature resistance and fatigue resistance, etc. Its weight is only 60% of steel, but its strength is higher than alloy steel. Therefore, titanium alloys are more and more widely used in aviation, aerospace, power generation equipment, nuclear energy, shipping, chemical industry and medical equipment.
However, the processing of titanium alloys is a very challenging task. The four characteristics of titanium alloys, such as low thermal conductivity, severe machining hardening, high affinity with tools, and small plastic deformation, are the essential reasons why they are difficult to machine. The thermal conductivity of titanium alloy is only about 16% of 45# steel, and it is difficult to conduct the heat generated in the processing in time, which leads to high temperature of the cutting edge locally, and easily triggers the diffusion wear of the tool. At the same time, the machining hardening phenomenon of titanium alloy is also very serious, and the affinity of the tool is high, it is easy to bond with the titanium-containing cemented carbide, exacerbating the wear of the tool. In addition, the plastic deformation of titanium alloy is small, about 1/2 of the modulus of elasticity of 45 steel, so the elastic recovery is large, the friction is serious, and the workpiece is also prone to clamping deformation.
For the machining difficulties of titanium alloy, we can take the following process know-how:
First, inserts with positive angle geometry are used to reduce cutting forces, cutting heat and workpiece deformation. This insert design can better adapt to the machining characteristics of titanium alloys and improve machining efficiency and quality.
Secondly, a constant feed is maintained to avoid hardening of the workpiece. In the cutting process, the tool should always be in the feeding state, and the radial draft should be 30% of the radius when milling. This ensures the stability of the cutting process and reduces the hardening of the workpiece.
Third, the use of high-pressure high-flow cutting fluid to ensure the thermal stability of the machining process. The cutting fluid can take away the heat generated in the cutting process in time, preventing the surface of the workpiece from being denatured and the tool from being damaged due to high temperature.
Fourth, keep the blade edge sharp. Dull cutting tools are the cause of heat collection and wear, easily leading to tool failure. Therefore, it is necessary to regularly check and replace the blade to ensure the sharpness of the edge.
Fifth, machine titanium alloys in their softest state possible. Hardened titanium materials become more difficult to machine, so machine the material in its softest state to improve machining efficiency and tool life.
Sixth, use a large tip radius or chamfer cut to put as much of the cutting edge into the cut as possible. This reduces cutting forces and heat at each point and prevents localized breakage. When milling titanium alloys, cutting speed has the greatest impact on tool life, with radial draft (milling depth) coming in second.
In addition to the above process know-how, we can also start from the blade to solve titanium machining problems. The insert groove wear that occurs during titanium machining is caused by the hardened layer left by the pre-machining. Chemical reaction and diffusion between the tool and the workpiece material occurs when the machining temperature exceeds 800°C, which is also one of the reasons for the formation of groove wear. Titanium machining therefore requires special insert materials and geometries to cope.
Tool structures suitable for titanium machining should focus on heat transfer. Large quantities of high-pressure cutting fluid have to be sprayed onto the cutting edge in a timely and precise manner in order to remove the heat quickly. There are unique structures of milling cutters on the market that are specifically designed for titanium machining that meet this need.
In terms of specific machining methods, turning, milling, tapping and reaming all have different machining characteristics and considerations. For example, in turning titanium alloy, it is necessary to choose suitable tool materials and geometric parameters, and adopt lower cutting speed and moderate feed; in milling titanium alloy, generally use the smooth milling method, and choose suitable tool materials and geometry; in tapping, it is necessary to give priority to the use of one in place of the jumping tap, and pay attention to the selection of the taper and the mode of operation; in reaming, it is necessary to choose the suitable reamer material and take the In reaming, choose the right reamer material and take appropriate process measures to improve the processing quality.
To sum up, although the processing of titanium alloy is full of challenges, as long as we have mastered the correct process know-how and choose the appropriate tools and processing methods, we will be able to achieve high efficiency and high quality processing.
