Why Is It Difficult To Machine Threads On Titanium Alloy Pipes?

Titanium alloy pipes are used for sealing and pressure-bearing in pipelines, and the quality of thread machining determines the service performance of the pipelines. Titanium alloys  are difficult to machine. Conventional tapping and turning are prone to heat accumulation, poor chip evacuation, workpiece springback and tool wear, which lead to thread defects, high consumable loss, low output and high scrap cost.

1. Unstable accuracy

Thermal deformation and springback cause out-of-tolerance of pitch, tooth profile and pitch diameter, which cannot meet the requirements of precision assembly;

2. Severe tool wear

Conventional tools are prone to sticking, wear and chipping, and tool breakage occurs frequently during fine thread machining;

3. Poor surface quality

Scratches and built-up edges damage the sealing performance of threads, which are prone to leakage and loosening during long-term use;

4. Low production capacity and yield

Only low-speed machining is available, resulting in low production efficiency, poor batch dimensional consistency and high scrap cost.

1. Traditional Machining Process

The mainstream process is: blank cutting → rough turning of outer circle/end face → drilling and reaming → finish turning of outer circle → rough and finish thread machining → deburring → inspection and warehousing. Ordinary CNC lathes and conventional tapping machines are used, matched with general high-speed steel taps and cemented carbide turning tools, and ordinary emulsions are used for cooling.

2. Five Core Defects of Traditional Processes

• Improper tools

General taps have large contact area and high cutting resistance, which are easy to stick to tools and break, and their poor chip evacuation structure leads to chip accumulation.

• Mismatched parameters,

Fixed general parameters, excessive speed aggravates high-temperature wear and tool sticking, while too low speed causes work hardening of materials.

• Insufficient cooling

Ordinary emulsions have poor high-temperature resistance and anti-adhesion performance, which cannot dissipate heat quickly and are difficult to inhibit the formation of built-up edges.

• Process defects

Most workpieces have no special thread relief grooves, resulting in tooth chipping and incomplete tooth profile at the finishing stage; there is a lack of segmented machining and tool retraction for chip evacuation processes.

• Single machining method

Pure rigid continuous cutting is likely to cause stress concentration, making it difficult to control workpiece springback deformation and resulting in poor dimensional consistency in batch machining.

1. Tool Optimization

Abandon general tools and adopt titanium alloy special tools made of TiAlN-coated ultra-fine grain cemented carbide/cobalt-containing high-speed steel. For internal threads, use spiral groove taps with optimized cutting cone rake angles and polished calibration sections; for external threads, use special turning tools with refined cutting edges. Standard dense-tooth taps are prohibited to solve the problems of tool sticking and chipping.

2. Customized Parameters

Adopt the low-speed, small-feed and layered stable cutting process. The linear speed for conventional threads is 3-6m/min, and the single cutting allowance is 0.05~0.15mm. Cooperate with pecking cutting cycle to evacuate chips, dissipate heat, release stress and inhibit springback.

3. Process Upgrade

Add double stress relief and thread springback compensation processes. The optimized process is: blank cutting → aging stress relief → rough turning → secondary stress relief → finish turning of datum → machining of relief grooves → three-stage thread machining → low-temperature deburring → inspection and warehousing, which eliminates stress and clamping deformation.

4. Cooling Upgrade

Replace ordinary emulsions with special extreme-pressure cutting oils for titanium alloys, adopt high-pressure precise spraying, and match internal cooling tools for deep hole internal threads to build an internal and external dual cooling system.

5. Structural Adaptation

Add relief grooves at the thread ends; optimize the fillets of thread crests and roots; optimize the datum structure of thin-walled fittings to reduce machining difficulty, stress concentration and cutting vibration.

1. Regularly inspect the cutting edge status of tools. Repair or replace them immediately in case of slight wear or passivation to prevent affecting thread accuracy and surface quality.

2. Adjust cutting parameters according to thread specifications and workpiece wall thickness. For fine threads and thin-walled threads, reduce cutting speed and single allowance to avoid vibration deformation.

3. Regularly filter and replace the cooling and lubricating medium to ensure its cleanliness, heat dissipation and lubricity, and prevent impurities from scratching the tooth surfaces.

4. Clean up chips and oil stains in the thread grooves in time after machining. Conduct batch random inspection of dimensions and tooth profile quality with special measuring tools, and establish a quality traceability system.

5. Strictly control the heat treatment stress relief process to ensure full release of internal stress and prevent thread failure caused by later deformation.

Ruihang offers optimal quality raw materials for your precision components production. Please feel free to contact us via email: Sam.Rui@bjrh-titanium.com