Characteristics and applications of titanium alloys
Titanium alloy has much higher toughness, ductility, and especially strength than other metal materials, making it possible to manufacture product components with high unit strength, good rigidity, and light weight. In recent years, titanium alloy has been widely used in aircraft to replace aluminum alloy. The reason for this is that titanium alloy has good thermal stability and high temperature strength. At 300-500 ℃, its strength is about 10 times higher than aluminum alloy, and the working temperature can reach 500 ℃. Titanium alloys have superior corrosion resistance to organic substances such as alkali, chloride, chlorine, nitric acid, sulfuric acid, etc. At the same time, their resistance to pitting corrosion, acid corrosion, and stress corrosion in humid environments and seawater media far exceeds that of stainless steel. Products made from titanium alloy processing also have characteristics such as high hardness, high melting point, non toxicity, and no magnetism.
Based on the above series of excellent properties, titanium alloy was first applied in aviation. In 1953, Douglas Corporation of the United States applied titanium containing materials to the DC2T engine nacelles and fire walls for the first time, achieving good results. In the aerospace field, titanium alloy is used as a key material in the fan, compressor, skin, fuselage, and landing gear of aircraft engines, resulting in an overall weight reduction of about 30% to 35%. Titanium alloy has also been successfully applied in pressure resistant shells of nuclear submarines, seawater pipeline systems, condensers and heat exchangers, blades of exhaust fans, propellers and shafts, springs, firefighting equipment on aircraft carriers, propellers, water jet propulsion devices, rudder and other marine components. In addition, titanium alloy has become a suitable biomedical metal material due to its good biocompatibility, corrosion resistance, mechanical properties, and processing properties. It has been successfully applied in artificial knee joints, femoral joints, dental implants, dental roots, and denture metal brackets. Among them, Ti6Al4V is commonly used as a medical implant material, and Ti3Al-2.5V alloy is also used as a replacement material for the femur and tibia in clinical practice due to its good cold forming, corrosion resistance, and mechanical properties.
Difficulties in titanium alloy processing
(1) Small deformation coefficient: This is a significant feature in the cutting process of titanium alloy materials. In the process of cutting, the area of contact between the chips and the front cutting surface is too large, and the chip's travel on the front cutting surface of the tool is much larger than that of ordinary materials. Such long-term walking will cause severe tool wear, and friction will also occur during the walking process, leading to an increase in tool temperature.
(2) High cutting temperature: On the one hand, the small deformation coefficient mentioned earlier will lead to a partial increase in temperature. The main reason for the high cutting temperature during the titanium alloy cutting process is due to the very small thermal conductivity of the titanium alloy, and the short contact length between the chip and the front cutting surface of the tool. Under the influence of these factors, the heat generated during the cutting process is difficult to transfer and mainly accumulates near the tool tip, causing local temperatures to be too high.
(3) The thermal conductivity of titanium alloy is very low: the heat generated by cutting is not easily dissipated. The turning process of titanium alloy is a process of high stress and strain, which generates a large amount of heat. The high heat generated during processing cannot be effectively diffused. At the same time, the contact length between the cutting edge of the tool and the chip is short, causing a large amount of heat to accumulate on the cutting edge. The temperature rises sharply, the cutting edge softens, and tool wear is accelerated.
(4) Titanium alloy has great chemical properties: at high temperatures, titanium alloy easily reacts with tool materials, accelerating the formation of crescent pits. However, the cutting process of titanium alloys is basically carried out at high temperatures. When the cutting temperature reaches a certain level, molecules such as nitrogen and oxygen in the air can easily react with titanium materials, resulting in the formation of a brittle and hard skin. Moreover, during the cutting process of titanium materials, the plastic deformation on the machined surface of the workpiece leads to the occurrence of cold hardening, resulting in hardening on the machined surface of the workpiece material. These phenomena can exacerbate tool wear and reduce the fatigue strength of titanium materials.
(5) Cutting tools are prone to wear: The wear of cutting tools is the result of many comprehensive factors. In the cutting process of titanium alloy materials, it is very easy to cause the phenomenon of tool breakage. Titanium materials generally exhibit strong chemical affinity between tool materials under high temperature conditions. Additionally, the tool is prone to bonding with titanium alloy materials at high temperatures, which leads to a short service life of the tool. So there are two aspects that must be paid attention to when cutting titanium alloy materials, which are maintaining a low cutting temperature and increasing the stiffness of the tool/material being cut. Coated tools are one way to improve tool stiffness.
Current status of coating for titanium alloy cutting tools
Due to the high chemical activity and low thermal conductivity of titanium alloys, the cutting temperature is high during the cutting process, and chemical reactions are severe, resulting in rapid tool failure, short tool life, and high processing costs. The reasons for tool wear include mechanical friction, physical and chemical reactions under the action of cutting force and cutting temperature. In response to the difficulty of titanium alloy cutting, the selected tool material must meet the requirements of high hardness, high strength, high thermal conductivity, chemical stability, and good red hardness [10]. Currently, diamond cutting tools are widely recognized in the industry for processing titanium alloys, but due to their high price, hard alloy coated cutting tools still occupy the main position in the titanium alloy cutting market, as shown in Figure 1.

Figure 1 Modification effect provided by hard alloy tool coating
The traditional cutting theory believes that coated cutting tools are not suitable for processing titanium alloys. This is because traditional coatings are mostly binary TiC or ternary TiCN coatings, and the Ti element in the coating is prone to affinity with the workpiece, leading to rapid tool failure. Ezugwn et al. used CrN and TiCN coatings on high-speed steel substrates and uncoated tungsten cobalt hard alloy tools to cut TC4 titanium alloy. The study showed that the service life of CrN coated tools was longer than that of TiCN coated tools and uncoated hard alloy tools. At the same time, cutting experiments were conducted on Ti6A14V titanium alloy using single-layer TiN coating and multi-layer TiN/TiCN/TiN coating under the same cutting conditions. The results showed that the temperature generated by the multi-layer coated tool during cutting was lower than that of the single-layer coating, and the tool service life was also higher than that of the single-layer coating.
In recent years, due to the continuous maturity and progress of coating processes and methods in foreign countries, the composition of tool coatings has gradually become diversified, and there is even a suitable coating material for each workpiece material. In addition, better performance coatings such as soft coatings, superhard coatings, multi-layer coatings, and nanocomposite coatings have emerged. Therefore, more and more coated tools have been applied to high-speed cutting of titanium alloys, demonstrating good advantages. Figure 2 shows commonly used hard alloy coated tools in recent years.

Figure 2 Typical Coating Structure
The future development direction of hard alloy coated cutting tools
(1) The coating components of cutting tools are diversified. The multi-component tool coating material prepared by adding new elements to a single coating (such as Zr and V to improve coating wear resistance, Si to increase coating hardness and prevent chemical diffusion, B to enhance coating toughness, Al and Cr to improve coating oxidation resistance, etc.) greatly improves the overall performance of the tool. Coating materials have also evolved from TiN, TiAlN, TiCN, to dozens of coating materials such as TiSiN, TiSiCN, TiAlSiN, AlCrN, AlCrSiN, TiBN, CrN, ZrN, etc. In order to meet the diverse needs in cutting processing, new coating materials will also have great development prospects, as shown in Figure 3.

Figure 3 Development history of hard alloy tool coatings
(2) Greenization of cutting and processing. At present, wet cutting method is mainly used in mechanical processing in China, which means that during metal cutting, cutting fluid is continuously used to cool and lubricate the parts in contact with the tool and workpiece. However, the excessive use of cutting fluid can cause environmental pollution problems. At the same time, the oil fumes generated by the cutting fluid during the cutting process can directly harm human health and may trigger various diseases. Moreover, in terms of processing costs, cutting fluid accounts for a significant proportion of the total cost
14%~16%, tool costs only account for 2%~4%. Research has shown that if 20% of cutting is done using coated tools for dry machining, the total manufacturing cost can be reduced by 1.6% [16]. In addition, using high-speed dry cutting can significantly improve machining efficiency, improve machining accuracy, reduce surface roughness, and is more suitable for machining thin-walled parts. Therefore, whether from an environmental perspective or from a processing performance and economic perspective, manufacturing coated cutting tools that can meet dry cutting conditions is an important development goal for green machining.
Summary
With the proposal of "Made in China 2025", the manufacturing industry is experiencing rapid development, and the application of titanium alloys will be increasing. The requirements for quality and accuracy will also continue to improve. How to choose suitable titanium alloy cutting tools, improve machining efficiency, and reduce production costs is of great significance for the development of important industrial sectors such as automobiles, aerospace, and energy, as well as the overall level of manufacturing industry.