Research On Thread Machining Process Of Titanium Alloys Pipes Joints

Titanium alloy pipe joints are core connectors in pipeline systems, and the reliability of threaded connections directly determines the safety and stability of the system. However, titanium alloy has poor machinability, with problems such as low elastic modulus, high cutting resistance, low thermal conductivity, easy work hardening, and tool adhesion, bringing numerous challenges to thread machining.

 

 

1.High cutting resistance and easy work hardening

Titanium alloy has high strength and high-temperature strength, resulting in large cutting forces during machining. The hardness of the surface hardened layer reaches 1.5~2 times that of the matrix, which aggravates tool wear and even tool breakage.

 

2.Extremely poor thermal conductivity

The accumulation of cutting heat causes high-temperature wear and deformation of the tool, and also triggers titanium alloy phase transformation, affecting the mechanical properties of the thread.

 

3.Severe tool adhesion

Titanium alloy has strong chemical activity at high cutting temperatures, and is easy to bonding with the tool to form built-up edges, resulting in thread burrs and scratches, reducing precision and accelerating tool loss.

 

4.Difficult control of thread precision

 Due to its low elastic modulus, the material is easy to elastic deformation and unloading springback during machining, leading to dimensional deviations. Most of them are precision threads like MJ, which have strict requirements on pitch, thread profile, and pitch diameter, increasing the difficulty.

 

 

 

1.Tool Selection

Material: The best choice is the materials with high hardness, wear resistance, high temperature resistance, and chemical stability. Commonly used materials include cemented carbide (for medium and low speeds, low cost), CBN (for high speeds, high brittleness), and PCD (for finishing, high price).

 

Parameters: Large rake angle and relief angle, reasonable edge radius, qualified thread profile accuracy + precision grinding of the cutting edge.

 

Chip flute: Design with large helix angle + wide flute to achieve rapid chip evacuation, preventing heat accumulation and scratches.

 

2.Optimization of Machining Parameters

Cutting speed: Excessively high speed is easy to temperature rise, tool damage, and material phase transformation; excessively low speed leads to low efficiency and increased work hardening. For cemented carbide tools, the speed is 5-20m/min; for CBN tools, it is 20-40m/min.

 

Feed rate: Balancing efficiency and quality, set according to the pitch. For rough machining: 0.15-0.3mm/r (for efficient material removal); for finishing: 0.05-0.15mm/r (to ensure precision and surface finish). Excessively large feed rate is prone to increasing cutting force and heat, resulting in rough surfaces and tool chipping; excessively small feed rate leads to low efficiency and excessive friction.

 

Cutting depth: Adopt layered cutting to prevent excessive cutting force. For rough machining: 0.2-0.5mm per pass; for finishing: 0.05-0.1mm per pass, reducing the impact of work hardening.

 

3.Selection of Machining Methods

Turning: Suitable for internal and external threads of shaft/sleeve parts. CNC turning ensures precision with step-by-step machining; high-rigidity tools are required for internal threads.

 

Milling: Suitable for complex parts, with significant advantages in processing large-pitch/multi-start/special-shaped threads and deep-hole internal threads. Control the entry and exit of the tool to prevent defects.

 

Tapping/die cutting: Suitable for small-size threads, requiring special tools + cooling and lubrication. Vibration tapping is preferred to reduce tool adhesion.

 

4.Cooling and Lubrication Measures

Cutting fluid: Requires good cooling and lubrication performance and chemical stability, divided into oil-based (strong lubrication) and water-soluble (strong cooling) types, selected as needed.

 

Lubrication method: High-pressure precise oil injection; internal cooling tools are used for deep-hole internal threads.

 

 

1. Process control

Strictly control clamping to prevent deformation, regularly replace worn tools, control ambient temperature; use SPC for quality monitoring in mass production.

2. Dimensional inspection

Must inspect pitch, thread angle, pitch diameter, etc. For general precision, use plug gauges/ring gauges; for high precision, use projectors and coordinate measuring machines (CMM); strictly control cumulative and pitch errors.

 

3.Surface inspection

The surface roughness Ra ≤ 1.6μm, check for burrs, scratches, and cracks; inspect the hardened layer to prevent impact on fatigue strength.

 

The raw materials for titanium pipes joints production

 

 

Ruihang,as a professional manufacturer, is specialized in R&D,production and sales for titanium and titanium alloy products. The competitive factory price can supply for the customers. For more details, please contact us via the Email:Sam.Rui@bjrh-titanium.com