Semi-analytical torque modeling of Ti-6Al-4V-alloy internal trapezoidal thread extrusion forming with an emphasis on low-frequency torsional vibration

Conventional cold extrusion is generally considered to be less practical for trapezoidal threads, requiring a relatively large form tapping torque. The machining of it is always based on cutting, but the actual demands of high strength and long life have brought this process to its limits. These requirements drove the present work, which proposes a new process for the fabrication of a Ti-6Al-4V-alloy trapezoidal thread using low-frequency torsional vibration extrusion (LTVE) through a forming tap. A semi-analytical model of torsional vibration extrusion with friction was established by solving the problem of mechanical friction and slip-line field. The influence of low-frequency torsional vibration on the form tapping torque was studied, and the total amounts were determined quantitatively. The conclusion shows that the total tapping torque was composed of the positive pressure load and the friction load on the forming tap. The friction load was generated by the tool/workpiece friction, which is the largest component, accounting for more than 90 %. The introduction of vibration can reduce the friction in the thread form tapping so as to obtain high rheological properties, which is applicable for the low-frequency stage. Finally, a mechanical LTVE tapping excitation device was proposed based on this process method, and an experimental prototype that can meet the requirements of titanium alloy trapezoidal thread machining was developed. Experimental verification indicated that the LTVE forming process had reduced the total form tapping torque. It also proved the method’s overall reliability and effectiveness and the underlying insights needed to guide the thread manufacturing.