Multi-Objective Optimization in Single-Shot Drilling of CFRP/Al Stacks Using Customized Twist Drill.

In recent years, the use of CFRP with titanium and/or aluminum to form materials for stacking has gained popularity for aircraft construction. In practice, single-shot drilling is used to create perfectly aligned holes for the composite-metal stack. Usually, standard twist drills, which are commonly available from tool suppliers, are used for practical reasons. However, existing twist drill bits exhibit rapid wear upon the drilling of composite-metal stack layers in single shot, due to the widely contrasting properties of the composite-metal stack, which causes poor surface quality. The stringent quality requirements for aircraft component manufacturing demands frequent drill bit replacement and thus incurs additional costs, a concern still unresolved for aircraft component manufacturers. Owing to highly contrasting properties of a composite-metal stack, it is obvious that standard twist drill cannot fulfil the rigorous drilling requirements, as it is pushed to the limit for the fabrication of high-quality, defect-free holes. In this work, customised twist drills of a tungsten carbide (WC) material with different geometric features were specially fabricated and tested. Twenty drill bits with customised geometries of varying chisel edge angle (30-45°), primary clearance angle (6-8°), and point angle (130-140°) were fabricated. The stacked-up materials used in this study was CFRP and aluminum alloy 7075-T6 (Al7075-T6) with a total thickness of 3.587 mm. This study aims to investigate the effect of twist drill geometry on hole quality using drilling thrust force signature as indicator. All drilling experiments were performed at spindle speed of 2600 rev/min and feed rate of 0.05 mm/rev. Design of experiments utilising response surface methodology (RSM) method was used to construct the experimental array. Analysis of variance (ANOVA) was used to study the effect of parameters and their significance to the thrust force and thus the hole quality. The study shows that the most significant parameter affecting the drilling thrust force and hole surface roughness is primary clearance angle, followed by chisel edge angle. Correlation models of CFRP thrust force (