Drilling of Ti/CFRP/A1 multilayer stack materials

Multilayer metallic/composite stacks are increasingly being used in wing and tail plane sections of modern commercial aircraft, with component assembly primarily through mechanical joining, hence the requirement for fixation holes. Currently, the individual material sections (titanium, aluminium and CFRP) are machined independently prior to assembly however; there is growing demand within the industry to produce holes through the stack in a single operation. The research detailed in the thesis involves evaluating the effect of operating parameters, drill geometry, tool materials/coatings and cutting strategy when single shot drilling three-layer Ti/CFRP/Al stacks. Performance was assessed against various process measures including thrust forces/torque, hole accuracy/quality, tool wear/life, burr formation and hole surface integrity (microhardness and microstructure). Statistical design of experiments and associated analysis techniques (main effects plots, ANOVA etc.) were employed to identify the significance of variable factors and preferred operating levels with respect to different responses. Based on the experimental results, a bespoke drill design was formulated, which was validated against current commercially available drills recommended for the drilling of multilayer stacks. Finally, the influence of cutting fluid pressure on temperature and hole quality was investigated