Machining in composite/metal stacks: conventional vs. ultrasonically-assisted drilling

Carbon-fibre-reinforced polymer/titanium stack components possess outstanding mechanical properties such as excellent strength-to-weight ratio, high corrosion resistance, stiffness, low density and excellent fatigue properties. These mechanical advantages translate to better fuel efficiency and energy saving. These hybrid materials are not only popular but also a substitute for metals in many structural applications.

Recently, a novel machining technique - ultrasonically assisted drilling (UAD) - was introduced to machine hard-to-cut materials, in which vibrations with a low amplitude, low energy and high frequency was superimposed on the drilling tool in the axial direction during the conventional drilling process. This hybrid machining technique brings along several advantages, including multi-fold reduction in forces, required to drill in difficult-to-cut materials such as glass, ceramics, CFRP or hard alloys, hence improving machinability, associated with high-quality surface finish, and meeting standards required for modern materials.

Finally, a novel control system is designed and manufactured for UAD machining technique, capable of switching between vibratory parameters for each material constituent in the CFRP/Ti stack was implemented. The development of the control system in UAD resulted in a significant reduction in the average drilling forces, both in the composite and metal phase - 33% – 88%. The developed process appeared to improve drilling in the CFRP/Ti stack, with enhanced surface quality observed, reducing (or, in some cases, eliminating) damages induced during the drilling process in comparison to CD, with a two-to-three-fold reduction in the surface roughness.