In-depth tool wear analysis in drilling of CFRP-Ti stacks by DLC coated drills

Spurred by the growing trend towards sustainability and use of green material alternatives, Carbon Fiber Reinforced Polymer paired with Titanium alloys (CFRP-Ti stacks) are employed in aerospace industry for their high strength-to-weight ratio and good galvanic corrosion resistance. However, production of holes for bolt or rivet connections, imperative for components assembly depicts unmatched difficulties in their machinability due to superior mechanical characteristics and distinct nature of stack components, causing rapid tool wear and compromised hole quality. In this work, in-depth tool wear analysis of Diamond-like-Carbon (DLC) coated drills were analysed periodically to evaluate tool wear growth and associated wear mechanism while drilling of CFRP-Ti stacks. Qualitative analysis was performed after drilling set number of holes by two DLC coated drills (DLC-Ar and DLC-Bn), and results compared with uncoated solid carbide (WC) drills tested under similar dry machining conditions. The wear progress was observed by SEM and EDX; and for extensive wear mechanism investigation, Focused Ion Beam (FIB) cross-sections were analysed. Moreover, morphological examination of etched drills after drilling experiment was performed to observe active cutting regions for overall damage extent, gain insight about coating reminiscences and underlying wear mechanism. The results demonstrate that DLC-Bn drill showed best performance with reduced thrust forces and torque, reduced wear of cutting edges and no fracture/chipping of drill corner compared to DLC-Ar and WC drill. Although, DLC-Ar drill does alleviate corner chipping but suffer premature coating failure, undergoes intense tribo-chemical wear, excessive removal of carbide grains and generation of highest thrust forces compared to WC drill.