Abstract:

A new thread tapping method, called combined cut and form tapping, was successfully used for producing threads in high strength titanium alloy, which may not only reduce forming torque but also strengthen the thread root where initial fatigue cracks very often occur. The microstructure of the thread machined by form tapping showed a layer of fine fiber distribution along the thread profile. Residual stresses near the root measured by moire interferometry were about -120MPa, depending on different forming conditions. The result of fatigue tests showed that average fatigue lives of strengthened threads by forming tap were about three times longer than those of cut threads at the same stress level. That could be contributed to the high compressive residual stress, the high fiber density and the decrease in surface roughness.

Key words: form t apping, titanium alloy, work- har dening, r esidua l str ess