Turning of titanium alloy and superalloy

In the past few years, the application of titanium alloys and superalloys in the aerospace industry has become popular, especially in turbine blades, compressors, and frame parts. After the advantages of materials have been widely recognized, they have now been extended to automobiles, medicines, chemicals, small parts and other industrial fields. With titanium alloys, high-temperature alloys are more and more widely used in the industrial field, and the processing of such difficult-to-process materials has become a problem that people are very concerned about.
High strength/weight ratio makes titanium have high durability. Compared with alloy steel and cast iron, titanium has better strength, toughness and ductility. In addition, titanium also has two important properties for mechanical parts, oxidation resistance and corrosion resistance. Despite the above-mentioned advantages, its difficulty in processing is prohibitive.
The same problem also appears in the processing of superalloys. The difficulty lies in the local high temperature associated with high cutting forces during the  titanium cutting machining. Due to local high temperature, the cutting edge will be damaged, which is mainly manifested as peeling and deformation.
Although the cutting force of machining titanium is not much higher than that of machining steel when machining workpieces of the same hardness, the machining process of titanium alloys is more complicated and difficult, as listed below:
The low thermal conductivity of titanium makes most of the cutting heat generated remain on the tip of the tool. In the turning process, work hardening will occur on the surface of the part, which will affect the uniformity of the titanium content on the surface of the part, reduce the fatigue strength, and reduce the accuracy of the part’s geometric dimensions. In very rare cases, cutting heat can even cause sparks. In heavy processing, the low elastic modulus of titanium can cause deformation and vibration.
In addition, according to the chemical properties, titanium has the characteristics of a chemical reaction with the tool material, which will accelerate the formation of crescent craters.
Under normal circumstances, tool wear will appear in the form of boundary wear and built-up edge. Boundary wear occurs along the cutting edge, simultaneously on the flank face and the rake face, and varies with the depth of cut.
Castings, forgings, after heat treatment, the hardened surface layer created by the previous process is also one of the causes of blade wear. The local temperature of the cutting edge often exceeds 800°C, which will cause a chemical reaction and physical diffusion between the blade material and the workpiece material, and eventually cause boundary wear.
Another situation is that during the machining process, when the contact surface of the tool and the workpiece reaches a certain temperature and the pressure is high, under the action of cold welding, the workpiece material adheres to the rake face to form a built-up edge. The built-up edge continues to form and peels off from the blade. Each peeling will peel off part of the alloy material on the blade at the same time, accelerating the wear of the tool.
In actual processing, ISCAR’s solution:
1. Blade coating: ISCAR strongly recommends the use of ultra-fine substrate TiAIN PVD coating to process titanium alloys and high-temperature alloys. The thin and smooth coating surface brought by PVD and the strong adhesion between the substrate and the substrate improve the ability of the tool to resist peeling and crescent craters. Therefore, PVD coating makes the blade have better wear resistance, chemical stability, and anti-swarfing performance.
2. Coating alone is not enough to solve the processing problems, and a correct processing plan is needed:
1. The large rake angle tool is used to effectively reduce the cutting force and cutting heat, while the size deviation of the workpiece is minimized;
2. Constant feed is an effective method to effectively prevent work hardening of the workpiece surface.