Investigation of the quality of Al-CFRP stacks when drilled using innovative approaches

The usage of metal/composite stacks such as those made of aluminum alloy (Al) and carbon fiber-reinforced polymer (CRFP) has recently increased in the aerospace industry thanks to their high strength-to-weight ratio. As drilling high-quality holes in metal/composite stacks is still a challenge, innovative machining processes, such as ultrasonic vibration-assisted drilling and cryogenic drilling, have gained increasing attention for the part quality they promise. The feasibility of using the synergistic ultrasonic cryogenic drilling approach on Al/CFRP stacks is investigated for the first time, and the influence of the aforementioned innovative drilling approaches at varying feed in relation to the hole quality declined in terms of geometrical accuracy, surface roughness, delamination, and surface defects evaluated. The cutting forces are acquired as well. The obtained results show that, at increasing feed, the hole quality drastically worsens, nevertheless, it improves when applying innovative drilling technologies with respect to the standard one. Overall, ultrasonic cryogenic drilling carried out at the lowest investigated feeds represents the best drilling scenario thanks to the simultaneous application of ultrasonic vibrations, which enhances the surface finish as a consequence of the decreased contact between the workpiece and tool, and cryogenic cooling, which makes the surfaces harder.