In recent years, the use of hybrid composite stacks, particularly CFRP/Al assemblies, and fiber metal laminates (FMLs) has progressively become a convincing alternative to fiber-reinforced polymers (FRPs) and conventional metal alloys to meet the requirements of structural weight reduction in the modern aerospace industry. These new structural materials, which combine greater mechanical properties with low specific mass, are commonly assembled by riveted and bolted joints. The drilling operation, which represents the essential hole-making process used in the aerospace industry, proves particularly challenging when it comes to achieving damage-free holes with tight tolerances for CFRP/Al stacks in one-shot operations under dry conditions due to the dissimilar mechanical and thermal behavior of each constituent. Rapid and severe tool wear, heat damage, oversized drilled holes and the formation of metal burrs are among the major issues induced by the drilling of multi-material stacks. This paper provides an in-depth review of recent advancements concerning the selection of optimized strategies for high-performance drilling of multi-material stacks by focusing on the significant conclusions of experimental investigations of the effects of drilling parameters and cutting tool characteristics on the drilling performance of aerospace assemblies with CFRP/Al stacks and FML materials. The feasibility of alternative drilling processes for improving the hole quality of hybrid composite stacks is also discussed.

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 (Y₁), Al7075-T6 thrust force (Y₂), CFRP hole surface roughness (Y₃), Al7075-T6 hole surface roughness (Y₄) as a function of the tool geometry were established. The results indicated that the proposed correlation models could be used to predict the performance indicators within the limit of factors investigated. The optimum twist drill geometry was established at 45° of chisel edge angle, 7° of primary clearance angle, and 130° of point angle for the drilling of CFRP/Al7075-T6 stack material in a single-shot process. The error between the predicted and actual experiment values was between 6.64% and 8.17% for the optimum drill geometry. The results from this work contribute new knowledge to drilling thrust force signature and hole quality in the single-shot drilling of composite-metal stacks and, specifically, could be used as a practical guideline for the single-shot drilling of CFRP/Al7075-T6 stack for aircraft manufacturing.

Drilling two different materials in a layer, or stack-up, is being practiced widely in the aerospace industry to minimize critical dimension mismatch and error in the subsequent assembly process, but the compatibility of the drill to compensate the widely differing properties of composite is still a major challenge to the industry. In this paper, the effect of customized twist drill geometry and drilling parameters are being investigated based on the thrust force signature generated during the drilling of CFRP/Al7075-T6. Based on ANOVA, it is found that the maximum thrust force for both CFRP and Al7075-T6 are highly dependent on the feed rate. Through the analysis of maximum thrust force, supported by hole diameter error, hole surface roughness, and chip formation, it is found that the optimum tool parameters selection includes a helix angle of 30°, primary clearance angle of 6°, point angle of 130°, chisel edge angle of 30°, speed of 2600 rev/min and feed rate of 0.05 mm/rev. The optimum parameters obtained in this study are benchmarked against existing industry practice of the capability to produce higher hole quality and efficiency, which is set at 2600 rev/min for speed and 0.1 mm/rev for feed rate.