Titanium alloy is different from other metals, and its own characteristics make the threading of titanium alloy pipe joints the most difficult part in the processing of titanium alloy. This paper analyzes the metal characteristics and machining characteristics of titanium alloys, and proposes to achieve high-precision machining of titanium alloy pipe joint threads through the selection of titanium alloy machining tools and the improvement of machining technology.

Titanium alloy pipe joints are the parts that connect the pipelines or install the pipelines on the hydraulic components in the hydraulic system. The pipe joint is a connection tool between the pipeline and the pipeline, and is a connection point that can be disassembled and assembled between the component and the pipeline. It plays an indispensable and important role in pipe fittings. It is one of the two main components of hydraulic pipes. Titanium alloy is an alloy composed of titanium metal elements and other metal elements. As a special material, titanium alloy is widely used in the aviation industry due to 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 fully exploit its characteristics. However, for the mechanical processing of titanium alloy, its poor processing performance directly affects the processing quality and processing efficiency of titanium alloy parts and products, especially in the thread processing technology, there are considerable difficulties. This article conducts an in-depth study on the processing characteristics of titanium alloy materials, discusses the process suitable for the processing of titanium alloy threads, and solves the difficulties in the titanium alloy tapping process.

1 Processing characteristics and characteristics of titanium alloy
The low thermal conductivity of titanium alloy directly leads to its poor heat dissipation. When the thread is processed, the temperature dispersion and cooling performance is very poor, resulting in deformation due to the large amount of springback after processing. Moreover, the cutting edge of the machining tool is worn out greatly, which reduces the service life of the tool. In addition, the small deformation coefficient of titanium alloy directly leads to increased tool loss. Its large chemical activity is prone to chemical reaction with other metal materials in the case of high temperature during processing, resulting in the bonding of the tool and the tap, and the phenomenon of “biting the knife”. In order to increase the strength of titanium metal elements, alloy elements are added to pure titanium to form titanium alloys. There are three types of titanium alloys: one is a titanium alloy, represented by TA; one is a titanium alloy, represented by TB; and the other is a + titanium alloy, represented by TC. + Titanium alloy is a dual-phase alloy, which is the most widely used, and is an important raw material for titanium alloy in the aerospace industry. Titanium alloys have good metal performance characteristics, which are specifically manifested in: its strength is high and its density is small, but its strength is much greater than many alloy steels; its heat resistance is good, and its heat resistance strength is hundreds of times higher than that of aluminum alloys. Good thermal stability; its low temperature performance is good, and it still has good performance under ultra-low temperature conditions; its corrosion resistance is good, its resistance to acids, alkalis, humidity, chlorides, etc. is strong; its chemical activity is large, capable of It reacts with oxygen, nitrogen, carbon and other chemical elements in the air; its thermal conductivity is low, and its thermal conductivity is much lower than metals such as iron and aluminum.

2 Selection of thread cutting tools for titanium alloy
For the processing of titanium alloy threads, the wrong tooth taps are used for tapping operations. That is, the cutter teeth of the taps are removed one at a time, in a staggered arrangement, so that the processed parts and the taps only contact on one side to reduce friction between each other and reduce the friction The torque produced. This can effectively prevent the tap from being stuck or damaged, thereby improving the quality of thread processing. The use of such staggered taps can double the cutting thickness and the depth is greater than the cold hardened layer. The increase in cutting thickness directly leads to an increase in the cutting force of the tap teeth, but it is easier for the chip removal of the cutting, making the friction Reduced, the adhesion of the tap and the chip is reduced, thereby improving the durability and thread accuracy of the tap. In the design of the wrong tooth tap, it should be noted that the final number of tooth slots is an odd number to reduce the stress on the tooth edge. In the thread processing of titanium alloy materials, the use of staggered taps can effectively maintain the stability of tapping and improve thread accuracy. For thread machining of titanium alloy materials, high-speed steel tapers are recommended. 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 taps can be used for preliminary tapping, and then the tapping holes can be corrected with carbide taps. With the in-depth study of tool materials, there will be more suitable materials made of taps for better processing of titanium alloy threads.

3 Processing technology of titanium alloy pipe joint thread
The increase of the thread bottom hole can effectively reduce the cutting force and heat generated during machining. The strength of the titanium alloy pipe is large, and the prerequisite for increasing the specific quota of the diameter of the thread bottom hole is the requirement for the thread contact rate and the specific number of thread heads. From the perspective of processing technology, the inner diameter of the thread can be appropriately increased to reduce the thread height. Properly increasing the diameter of the thread is especially suitable for tapping special materials such as titanium alloys. Although the thread contact rate is reduced, the thread connection is still stable and reliable because of the increase in its length. In order to prevent the tap from breaking due to excessive pressure during processing, the processing technology of machine tool tapping can be selected.

  •  Cutting speed and tool control
    Because of the metal characteristics of titanium alloy materials, the cutting speed during processing is controlled to keep the cutting at a lower speed, which is more conducive to the threading work. But pay attention to the speed should not be too small, generally keep the speed between 200mm ~ 300mm per minute is appropriate. When threading titanium alloys, the geometry of the tool must be considered. The choice of a suitable rake angle can be to increase the strength of the cutting edge and improve the durability of the tool; the choice of a suitable large clearance angle is conducive to chip evacuation during processing. In the deep hole tapping of titanium alloy pipes, the method of reducing the number of chip flutes can be used to increase the chip volume and enhance the chip removal ability of the tap.
  •  Tap chuck and coolant control
    When using the machine tool for tapping, you need to use a special tap chuck, and then use a wrench to tap. For thread processing of titanium alloy materials, the thread tail buckle is usually longer than the standard length. It is best to design the retreat slot, so that even if the tap taps to the bottom, it will not cause chipping. Choose a highly active coolant with good lubrication function to directly cool the tap, and the excessive temperature generated during the tap processing will cause the tap to stick to the chips, which affects the processing speed and precision of the tap. It is recommended to use a mixture of oleic acid, vulcanized oil and kerosene that is proportioned in an appropriate ratio to cool the tap. You can also choose to use F43 cutting oil, which can also achieve the desired cooling effect. When processing the thread of the titanium alloy material, a cooling groove can be opened on the back of the edge of the tap to ensure that the cooling can also reach the cutting edge smoothly.

4 Conclusion
In summary, for the thread processing of titanium alloy pipe joints, we must first fully understand the metal characteristics and processing characteristics of titanium alloy materials, in order to adopt appropriate tap design and tap material selection for their characteristics. Secondly, appropriate and effective processing techniques must be adopted to effectively avoid the weaknesses of titanium alloy materials during processing. Through the cooperation of the cutting tool and the processing technology, the thread machining accuracy and processing speed of the titanium alloy are improved. With the in-depth study of metal materials and the development of processing technology, there will be better titanium alloy processing technology.