The cutting properties of titanium alloy are analyzed from three aspects: atmosphere, material properties and carbon content. At the same time, the corresponding improvement measures are put forward, it lays a theoretical and practical foundation for the machining of titanium alloy structure in the process of aircraft manufacture and repair. Titanium alloy is widely used in aerospace, petrochemical, marine, energy and Transportation Industries because of its high specific strength, high temperature resistance and corrosion resistance. With the development of modern aircraft, titanium alloys have been widely used in aircraft manufacturing, especially in the fields of stealth, fatigue resistance and high strength and lightweight, the proportion of machined parts on aircraft increases correspondingly, but based on the physical properties of titanium alloys, their machinability is poor, thermal conductivity is poor during machining, and the viscosity of materials is high during machining, as a result, it is very difficult to cut a large number of titanium alloy key parts and important parts, and the machining accuracy can not be guaranteed easily. Therefore, the problems in the precision cutting process of titanium alloy mentioned above are solved in the field of aircraft manufacture and repair, it is very important to break through the technical bottleneck of Titanium Alloy machining.

1. Titanium alloy is different from aluminum alloy, alloy steel and magnesium alloy in the cutting process of titanium alloy parts. It is impossible to guarantee the machining precision and quality under the high speed cutting condition. There are two main factors that cause the machining difficulty: one is the inherent property of the material, the other is the special performance requirement of the aeronautical structure. The inherent properties of titanium alloy are as follows: 1 because of its poor thermal conductivity and large friction coefficient, the cutting heat is not easy to be transmitted, and is concentrated in a small area near the cutting zone and cutting edge. The hardness of cutting edge decreases under high temperature, which shortens the tool life and directly affects the precision of machined parts. 2 The titanium element chemical property is active, above 300 °C, the titanium atom is easy to take strong chemical reaction with the hydrogen, oxygen and nitrogen in the atmosphere, forming the surface hardening layer, under the action of cutting heat, the temperature of this part rises, it reacts more easily with CO2 in the air and carbon in the grease, and when the carbon content is more than 0.12% , it forms TiC hard carbide, which accelerates tool wear in the cutting process. 3 Titanium Alloy has very strong chemical affinity to tool materials containing titanium element. 4 The value of elastic modulus e is small, and the ratio of tensile strength to yield strength is small, which leads to the tool springback along with the rotation of the workpiece in the machining process, thus increasing the cutting friction and accelerating the tool wear, affect the machining accuracy.

The special requirements of titanium alloy aviation structural parts are mainly embodied in the following three aspects. 1) the structure is complex, the size is big, the cutting precision and the quality have the strict request. 2) in order to meet the requirement of static load and dynamic load, the stiffener-stiffener structure and the thin-walled groove structure are common in aircraft structure, which can meet the requirements of tensile strength, yield strength, fatigue strength and elastic-plastic limit. If the stiffener-plate structure and the thin-walled groove structure are formed by mechanical processing, the amount of removal from the sheet to the finished product is large, and the processing process is easy to produce large deformation, and the product precision is difficult to guarantee. As shown in figure 1 for a type of aircraft TA15 titanium alloy protective grid local schematic, figure 2 for the thin-walled groove structure schematic. 3) the processing stress is big, easy to deform, the processing size is difficult to guarantee. As shown in Fig. 3, the long axis part of TC6 titanium alloy has the characteristics of high machining cost, precise dimension, complex shape, difficult machining, high chip removal rate, high machining stress and easy deformation. The influence of stress and deformation factors should be considered in the processing of products, otherwise it is easy to be discarded due to deformation. The key point of the research is the long axis turning and the guarantee of the coaxiality and circular runout in grinding.

2. Improvement Measures for titanium alloy cutting in order to ensure the cutting quality of titanium alloy parts, based on the factors affecting the cutting performance of such materials, the cutting performance can be improved from three aspects: Tool selection, cutting parameter adjustment and fixture design. (1) the selection of cutting tool materials is based on the properties of titanium alloy materials, and the selection of cutting tools should be considered from the aspects of reducing cutting temperature, reducing stick wear, etc. . According to the composition characteristics of cemented carbide tool materials, YT-type tools contain Ti elements, which have affinity to the titanium alloy parts to be processed, and are easy to “Stick the knife”and damage the tip, yt Type Carbide tools should not be selected. YG type cutting tools are selected when low speed cutting below l m/s is used. In order to ensure the high-speed cutting quality of titanium alloy and speed up the heat dissipation in the process of machining, YG8 tool can be used for rough machining and intermittent machining, and YG3 tool can be used for fine machining and continuous machining. Based on the above discussion, titanium alloy tool materials need to meet the following characteristics: 1 High Hardness and have sufficient strength and toughness. 2 High Wear Resistance. 3 The tool material has poor affinity with titanium alloy and does not deposit and diffuse with titanium alloy. (2) the selection of reasonable cutting parameters and the selection of cutting parameters in working procedure arrangement have an important influence on the reliability and quality of titanium alloy processing, especially the cutting rake angle, the cutting back angle and the cutting edge ARC radius. The cutting tool and the cutting rake angle γo and the cutting back angle αo are shown in Fig. 4.

During the cutting process, the cutting friction generates heat, part of the mechanical energy is converted into heat energy, and the thermal conductivity of the titanium alloy is poor and the heat is concentrated, which leads to the increase of the cutting workpiece temperature, when the cutting temperature rises, the fresh metal surface reacts with Oxygen, hydrogen, nitrogen and other elements in the air to form hard and brittle compounds. According to the mechanical calculation, the smaller Rake Angle γo can reduce the friction acting on the rake face, and then reduce the tool wear effectively. In addition, reducing the value of Rake Angle γo can increase the contact area between the chip and the rake face, reduce the stress concentration at the tip of the tool, accelerate the heat dissipation, and reduce the possibility of tool breakage in the cutting process. The results show that when the current angle γo is 28 ° ~ 30 ° , the tool durability is the highest. The larger the value of αo, the easier it is for the cutting edge to cut into the metal layer, but the higher the value of Αo, the easier it is for the cutting edge to break. According to machining experience, 15 ° Back Angle αo can not only ensure the sharpness of cutting edge, but also reduce the temperature of the tip and increase the durability of the tool. According to the practical production experience, the tool caving can be reduced by increasing the radius r of the cutting edge. This is because the curvature radius of the cutting arc increases, the cutting stress concentration factor decreases, and then reduces the processing stress, and improves the tool life.

(3) fixture design take the titanium alloy TC6 long shaft parts shown in figure 3 as an example, the parts need hard chrome plating after cutting to ensure φ34h8 requirements, through the tip of the two external grinding two φ34h8 measurement found, the circular runout of φ34h8 at both ends is out of tolerance, and the tip of machine tool is worn during titanium alloy cutting, but the tip of both ends of product is worn after hard tip is replaced. After repeated tests and tests, the wear of titanium alloy and the top of the machine tool makes the titanium alloy have the ability to absorb oxygen and hydrogen from the surrounding atmosphere, produce the α-transformation of the structure, and strengthen the machined surface, when the hardness of the replacement tip is greater than the surface hardening hardness, the product itself will wear, and in cylindrical grinder processing of all types of titanium alloy will have this problem, processing site as shown in figure 5.

In order to avoid mutual grinding between the top of the machine tool and the product, the external cylindrical grinder is reformed, and the two top points are refitted as the movable top points, as shown in figure 6. The workpiece and the top are rotated together, so that the friction between the machine tool and the part during the rotation is avoided, the coaxiality of the product is guaranteed, and the problem of the coaxiality of all titanium alloy cylindrical cutting is solved. 3. Conclusion the factors affecting the cutting quality of titanium alloys are: The elements such as carbon, hydrogen, oxygen and nitrogen in the atmosphere can cause the “Cold hardening phenomenon”; The introduction of carbon causes the decrease of machinability of titanium alloy and the acceleration of tool wear, while the chemical affinity of titanium alloy to the tool containing titanium element can accelerate tool wear. In view of the above factors, the cutting tool materials, cutting parameters, working procedures and fixtures are reasonably selected to improve the cutting performance. 

Source: metalworking

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