Analysis of The Processing Technology for The Threaded Connection of Titanium Alloy Pipes

Titanium alloys have high specific strength, excellent heat resistance and corrosion resistance, and are widely used in aviation hydraulic systems. However, this material has poor thermal conductivity, strong chemical activity and a small deformation coefficient, which makes the processing of the threads for titanium alloy pipe joints extremely difficult. This paper, based on the material and processing characteristics of titanium alloys, analyzes the difficulties in thread processing, clarifies the key points of selecting the right thread turning tools, and optimizes the process from the dimensions of cutting parameters, tool structure, cooling and lubrication, and tooling and fixtures, providing a reference solution for high-precision titanium alloy thread processing.

 

There are mainly three types of titanium alloys: α-type (TA), β-type (TB), and α+β-type (TC). Among them, the TC type duplex alloy is the most widely used and is an important raw material in the aerospace industry. Its processing characteristics are mainly manifested as:

1. Low thermal conductivity: Poor heat dissipation, during processing, it is prone to generate high temperatures, resulting in large workpiece rebound and obvious deformation.

2. Small deformation coefficient: During cutting, the contact length between the chip and the tool is short, and the cutting force is concentrated at the cutting edge, which aggravates tool wear.

3. High chemical activity: At high temperatures, it is prone to react with oxygen, nitrogen, and the tool material, generating adhesion, causing "biting" phenomena.

4. High strength and good toughness: The cutting resistance is large, and the requirements for tool rigidity and wear resistance are high.

These characteristics make the processing of titanium alloy threads face prominent problems such as large tapping torque, easy breakage of the tap, and difficult control of thread accuracy.

For the difficulties in tapping titanium alloys, it is recommended to use serrated taps. This type of tap removes one tooth every other tooth, causing the teeth to be arranged alternately. During cutting, only one surface is in contact, effectively reducing the friction torque and preventing the tap from getting stuck or being damaged. The cutting thickness of the serrated tap is doubled, which can exceed the depth of the cold work hardened layer. Although the cutting force of a single tooth increases, the chip removal is smoother, the friction and adhesion are reduced, thereby improving the durability of the tap and the accuracy of the thread. When designing, it is important to ensure that the number of notches is an odd number to evenly distribute the force on the tooth edges.

In terms of material selection, it is recommended to use high-speed steel taps. They have high toughness, good resistance to deformation and excellent wear resistance. A pre-tapping with high-speed steel taps can be done first, followed by correction with carbide taps. In the future, as the development of tool materials progresses, it is expected that there will be more specialized taps specifically designed for titanium alloys.

1. Reasonable increase of bottom hole diameter

Appropriately increasing the diameter of the thread bottom hole can reduce cutting force and cutting heat. The increase amount should be comprehensively considered based on the requirements of thread contact rate and the number of threads. Slightly reducing the tooth height and increasing the inner diameter of the thread, especially for titanium alloy tapping, although it slightly reduces the contact rate, it can ensure reliability by increasing the length of the threaded connection.

2. Control of cutting speed and tool geometric parameters

For titanium alloy thread processing, low-speed cutting should be adopted. The recommended cutting speed is controlled at 200-300 mm/min. Too small a speed is prone to generating cold hard layers, while too large a speed will aggravate temperature rise and tool wear. In terms of tool geometric parameters: Select an appropriate rake angle to enhance the strength of the cutting edge; Use a larger relief angle to facilitate chip removal. For deep hole tapping, the number of chip retention grooves can be reduced to increase the chip retention space and improve the chip removal capacity.

3. Optimization of tap chuck and coolant

Machine tapping must be equipped with a dedicated tap chuck and operated with a wrench. The tail pin of the titanium alloy thread should be slightly longer than the standard length. It is recommended to set a retraction groove to prevent the tap from chipping when it reaches the bottom.

Coolant lubrication is crucial. Select a coolant with high activity and good lubrication performance, directly cooling the tap to prevent high temperatures from causing adhesion. It is recommended to use a coolant mixture of oil acid, sulfurized oil and kerosene in an appropriate proportion, or use F43 cutting oil. A coolant groove can also be opened on the back of the tap to ensure that the coolant can smoothly reach the cutting area.

The high-precision processing of the threaded joints of titanium alloy pipes requires the selection of appropriate cutting tools such as serrated taps and high-speed steel taps, as well as the optimization of process parameters such as bottom hole diameter, cutting speed, tool geometric parameters, cooling lubrication, and tap chuck. This can effectively avoid the weaknesses of titanium alloy, such as poor thermal conductivity and high chemical activity, and improve the processing accuracy and efficiency of the threads. With the development of materials science and processing technology, the titanium alloy thread processing technology will be further improved, providing support for the high-reliability manufacturing of aviation hydraulic systems.

 

Baoji Yibaite New Materials Technology Co., Ltd. specializes in the processing of titanium alloy pipe. Welcome to consult.