Titanium alloys are widely used in aerospace, medical equipment, and other fields due to their high strength, corrosion resistance, and heat resistance. However, their low thermal conductivity, high chemical activity, and significant rebound effect lead to problems such as work hardening and tool adhesion during tapping, especially in small-diameter thread processing. Taking TC4 titanium alloy as an example, this paper discusses the key process parameters for improving the efficiency of small-diameter titanium alloy thread tapping from four dimensions: tap material, structure optimization, bottom hole design, and cutting fluid selection.
Analysis of the core factors affecting tapping efficiency
1. Selection of tap materials
The high strength of small-diameter titanium alloy thread tapping (TC4 tensile strength is above 900 MPa) and chemical activity require that tap materials meet the following characteristics:
High hardness and wear resistance: It is recommended to use cobalt high-speed steel (such as M42 alloy) with a hardness of ≥65 HRC, which has a 20% higher red hardness than ordinary high-speed steel;
Low affinity: Selecting taps with titanium aluminum nitrogen (TiAlN) coating on the surface can reduce the material adhesion rate by 30%-40%.
Fatigue resistance: The grain size must reach ASTM grade 10 or above through the vacuum heat treatment.
2. Optimized design of tap structure
Differentiated structural solutions are recommended for different processing scenarios:
Spiral tip taps are suitable for through holes or blind holes with an aspect ratio of less than 2. The 15° front angle design can optimize chip removal efficiency.
Spiral groove taps perform well in deep blind holes with an aspect ratio ≥ 2, and the 35° helix angle can achieve upward chip removal.
Skip-tooth taps are suitable for fine thread processing below M3, and the spacing tooth design reduces the cutting contact area by 30%.
Extrusion taps are suitable for M2-M6 high-precision threads, and the chip-free processing characteristics can reduce the risk of breakage.
3. Thread bottom hole parameter control
The rebound of small-diameter titanium alloy thread tapping can reach 2-3 times that of ordinary steel. The bottom hole diameter is recommended to be calculated as follows:
D bottom hole = D nominal - P * (0.65 - 0.75)
Where P is the pitch. When processing M2×0.4 thread, the bottom hole needs to be enlarged to Φ1.72±0.02 mm, which can reduce the tapping torque by 40%.
4. Cutting fluid matching solution
It is recommended to use a composite cutting fluid containing sulfur-chlorine extreme pressure additives, which must meet the following requirements:
High temperature lubricity: extreme pressure additive concentration ≥8%;
Quick heat dissipation: 40℃ kinematic viscosity is controlled at 12-15 cSt;
Corrosion resistance: pH value is maintained at 8.5-9.5, and benzotriazole corrosion inhibitor is added.
Process verification and optimization effect
Verified by orthogonal test (cutting speed 3-5 m/min, feed rate 0.05 mm/r), the optimization scheme has a significant effect:
1. The life of cobalt high-speed steel taps is increased to 150-200 holes (ordinary taps are only 50-80 holes)
2. The taper breakage rate of spiral groove taps is reduced from 12% to 2.5% when processing M2×0.4 deep blind holes (L/D=3)
3. Special cutting fluid reduces the processing temperature by 60-80℃, and the surface roughness Ra≤1.6 μm
Conclusion
Through the system optimization of material-structure-process, the processing bottleneck of small-diameter titanium alloy threads can be effectively broken. It is recommended that fine threads should preferably use skip-thread taps with extrusion molding process, and deep blind hole processing recommends 35° spiral groove taps with high penetration cutting fluid. Subsequent research can further explore the synergistic effect of PVD multi-layer coating and ultrasonic vibration-assisted tapping.
