Low Damage Drilling of CFRP/Titanium Compound Materials for Fastening

Fiber-metal compound materials are characterized by excellent mechanical properties and enable innovative and new designs, which are increasingly used e.g. in the aircraft industry. Boreholes for riveting and bolts or fasteners are required to join the initially separated material layers. Due to the considerably different machining properties of fiber reinforced plastics and metallic materials the drilling process of compound stacks is a very challenging task. High cutting temperatures and insufficient chip extraction lead to damages and disturbances of the surface integrity especially in the anisotropic and inhomogeneous CFRP layer. In the present study, low frequency assisted vibration drilling was applied for a CFRP/Ti6Al4 V [10/10 mm] material stack. The axial oscillations cause an interrupted cut, which leads to small chip segments in the metallic material (Ti6Al4 V). It was found that the distinctive mechanical damage of the CFRP borehole surface could be significantly reduced due to the improved extraction of the small chip segments. Thermographic images of the drilling process in the titanium alloy are showing that the cutting temperatures can be reduced by more than 40% when using low vibration assisted drilling compared to conventional drilling. Consequently thermal damage of the matrix material was not found when applying low frequency assisted vibration drilling.