Effect of Holding Time on the Extrusion Force and Microstructure Evolution during the Plastic Forming of Ti-6Al-4V Micro-Gears.

The application of titanium alloy micro-gears in microelectromechanical systems has been severely restricted, as the graphite mold is prone to abrasion or even to crack at high temperatures, mainly due to the forming load. We aimed to manufacture Ti-6Al-4V alloy micro-gears through hot extrusion under an electric field and to clarify the influence of holding time on the extrusion force. The results suggest that the formed gears had a complete filling and clear tooth profile. Moreover, the contact resistance and current density caused a gradient temperature distribution inside the billet, resulting in a carburized layer and inhomogeneous ß grains. The extrusion force increased with an increased holding time, which can be ascribed to the increase in the thickness of the carburized layer and the ß grain size. Among these two factors, ß grain size played a leading role in the extrusion force. Continuous dynamic recrystallization dominated the deformation in a single ß phase, and the misorientation of the transformed α laths from ß grains followed the Burgers orientation relationship. This study may pave the way for the extrusion forming of other titanium alloy micro-components.