Titanium alloy is different from other metals. Its own characteristics make the thread machining of titanium alloy pipe joint the most difficult part in the titanium alloy machining process. Based on the analysis of the metal characteristics and machining characteristics of titanium alloy, this paper proposes to realize the high-precision machining of the thread of titanium alloy pipe joint through the selection of titanium alloy machining tools and the improvement of machining technology.

　　Keywords titanium alloy; Thread machining; Tapping; Cutting tools; technology

　　0 Introduction

　　Titanium alloy pipe joint is a part that connects the pipeline in the hydraulic system or installs the pipeline on the hydraulic components. It is a general term for the connectors that can be assembled and disassembled in the fluid path. Pipe joint is a connecting tool between pipes and a detachable connecting point between components and pipes. It plays an indispensable and important role in pipe fittings. It is one of the two main components of hydraulic pipeline. Titanium alloy is an alloy composed of titanium and other metal elements. As a special material, titanium alloy is widely used in the field of aviation industry because of its light weight, high strength, high heat resistance and high corrosion resistance. Especially in the manufacture of aircraft and rocket spacecraft, titanium alloy is used as an important material to give full play to its characteristics. However, for the machining of titanium alloy, its poor machining performance directly affects the machining quality and efficiency of titanium alloy parts, especially in the machining process of thread. In this paper, the material processing characteristics of titanium alloy are deeply studied, the process suitable for titanium alloy thread processing is discussed, and the problems in titanium alloy tapping process are solved.

　　1 processing characteristics and characteristics of titanium alloy

　　The low thermal conductivity of titanium alloy directly leads to its poor heat dissipation. When processing threads, the temperature divergence and cooling performance are very poor, resulting in deformation due to large rebound after processing. Moreover, the cutting edge of the machining tool is severely worn, which reduces the service life of the tool. In addition, the small deformation coefficient of titanium alloy directly leads to the increase of tool loss. It has the characteristics of high chemical activity, which is easy to react with other metal materials under the condition of high temperature during processing, resulting in the bonding of cutting tools and taps, resulting in the phenomenon of "knife biting". In order to increase the strength of titanium metal elements, alloy elements are added to pure titanium to form titanium alloy. There are three types of titanium alloys: one is titanium alloy, represented by TA; One is titanium alloy, represented by TB; The other is + titanium alloy, represented by TC+ Titanium alloy is a two-phase alloy, which is widely used. It is an important titanium alloy raw material in the field of aviation industry. Titanium alloy has good metal properties, which are as follows: its strength is high and its density is small, but its strength is much higher than that of many alloy steels; Its heat resistance is good, its heat resistance strength is hundreds of times higher than that of aluminum alloy, and it has good thermal stability; Its low temperature performance is good, and it still has good performance under ultra-low temperature conditions; It has good corrosion resistance and strong resistance to acid, alkali, moisture and chloride; It has great chemical activity and can react with many chemical elements such as oxygen, nitrogen and carbon in the air; Its thermal conductivity is low, and its thermal conductivity is much lower than that of iron, aluminum and other metals.

　　2 selection of cutting tools for thread machining of titanium alloy

　　In titanium alloy thread processing, staggered taps are often used for tapping operation, that is, the cutting teeth of taps are removed at every interval and arranged in a staggered manner, so that the machined parts and taps can only contact on one side, so as to reduce mutual friction and reduce the torque caused by friction. This can effectively prevent the tap from being stuck or damaged, so as to improve the quality of thread processing. The use of the staggered tap can double the cutting thickness and the depth is greater than the cold work hardening layer. The increase of the cutting thickness directly leads to the increase of the cutting force of the tap teeth, but it is easier for the chip removal, reduce the friction and reduce the adhesion between the tap and the chip, so as to improve the durability of the tap and the thread accuracy. In the design of staggered tap, it should be noted that the final number of tooth slots is odd, so as to reduce the stress on the tooth edge. In the thread processing of titanium alloy materials, the use of staggered tap can effectively maintain the tapping stability and improve the thread accuracy. For thread machining of titanium alloy materials, it is recommended to use high-speed steel wire cone. The tap made of this material has high toughness and deformation resistance, and also has good wear resistance. For the tapping of titanium alloy materials, high-speed steel wire tap can be used for preliminary tapping, and then cemented carbide tap can be used for screw hole correction. With the in-depth study of tool materials, there will be more suitable materials to make taps for better processing of titanium alloy threads.

　　3 processing technology of titanium alloy pipe joint thread

　　The increase of threaded bottom hole can effectively reduce the cutting force and heat during machining. Titanium alloy pipes have high strength. The precondition for increasing the specific amount of the diameter of the threaded bottom hole lies in the requirements for the thread contact rate and the specific number of thread heads. From the aspect of processing technology, the requirements of thread inner diameter can be appropriately increased, which can reduce the thread height. Appropriately increasing the diameter of the thread is especially suitable for tapping special materials such as titanium alloy. Although the thread contact rate is reduced, the thread connection is still stable and reliable because of the increase of its length. In order to prevent the tap from breaking due to excessive pressure in machining, the machining process of machine tool tapping can be selected.

　　3.1 cutting speed and tool control

　　Because of the metal characteristics of titanium alloy materials, it is more conducive to thread machining to control the cutting speed and keep it at a small speed. However, it should be noted that the speed should not be too small. Generally, the speed should be kept at 200mm ~ 300mm per minute. When machining titanium alloy thread, the geometric dimension of the tool should be considered. The choice of appropriate rake angle can increase the strength of cutting edge and improve the durability of cutting tool; The selection of appropriate large back angle is conducive to chip removal during machining. In the deep hole tapping of titanium alloy pipe, the method of reducing the number of chip holding slots can be used to increase the chip holding space and enhance the chip removal ability of tap.

　　3.2 tap collet and coolant control

　　When tapping with a machine tool, it is necessary to use a special tap collet and then tap with a wrench. For the thread processing of titanium alloy, the thread tail buckle is usually longer than the standard length. It is best to design the tool withdrawal groove, so that even when the tap is tapped to the bottom, there will be no edge collapse. The active coolant with good lubrication function can be selected to directly cool the tap, so that the tap and chip will adhere due to the high temperature during the tap processing, which will affect the processing speed and precision of the tap. It is recommended to use the mixture of oleic acid, sulfide oil and kerosene with appropriate proportion to cool the tap. F43 cutting oil can also be selected to achieve ideal cooling effect. When machining the thread of titanium alloy material, a cooling groove can be opened on the edge back of the tap to ensure that the cooling can also reach the cutting edge smoothly.