Controllable fabrication of microstructures on the metallic surface using oblique rotary ultrasonic milling

Rotary ultrasonic milling emerges as a new and efficient fabrication method for surface microstructures. However, a phenomenon of secondary back-cutting interference exists in conventional rotary ultrasonic milling. It destroys the fabricated microstructure and leads to poor controllability of the surface geometries. In this study, oblique rotary ultrasonic milling (ORUM) is proposed to improve the process controllability by eliminating secondary back-cut interference. In the ORUM, the tool rotation axis is slightly inclined with an oblique angle of around 0.15°. In this way, surface texturing can be established by face milling without secondary back-cutting interference. A kinematic model was developed to predict the surface geometries of fabricated microstructures. Finite element analysis was performed to explore the material removal behavior for different vibration trajectories. Moreover, surface texturing tests were conducted on aluminum and titanium alloy to verify the efficacy of the proposed texturing process. The experimental results showed that the ORUM could texture microstructures on the metallic surface with much better geometric regularity than conventional ultrasonic milling. The model-predicted surface morphology of microstructure agrees well with the experimental results, verifying the high process controllability of ORUM. The effects of process parameters, including the depth-of-cut, spindle speed, and feedrate were presented to further demonstrate the process flexibility of ORUM in terms of the geometry adjustment of microstructures. Finally, fish-scale microstructures were fabricated on freeform surfaces like aircraft wings and implant hip joints, which implies the potential of ORUM for industrial applications.