Impact variation of workpiece stiffness on drilling of CFRP/Ti thin stacks

High-quality drilling Carbon Fiber Reinforced Polymer (CFRP)/Titanium alloy (Ti) stacks is a significant challenge in the aerospace industry, especially for thin stacks. This study investigates the drilling behavior of CFRP/ Ti thin stacks under various stiffness conditions and reveals the impact of stiffness variation. Various quantified stiffness conditions were set for drilling tests and analysis. The results indicate that when the drill bit fully penetrates CFRP and still cuts Ti, an interlayer gap will be formed between CFRP and Ti layers, and lower stiffness will enlarge the interlayer gap. The maximum thrust force was found to decrease linearly by 3-4 % with every 70 % increase in clamping length, even though the thrust force increased over a longer period in lowstiffness cases. Ti cutting chips were consistently extruded into the interlayer in low stiffness cases, which consequently caused scratching and damaged the interlayer surface. In addition, the extruded chips lead to a reverse deformation of CFRP, which enlarges the gap and further aggregates chip extrusion and interlayer damage. Meanwhile, three additional CFRP removal phenomena caused by deformation and Ti chip scratching were observed, chip scratching was the most serious, and the mechanism of hole-shape formation was revealed. Compared to the CFRP hole, the shape of the Ti hole was barely affected by its deformation, but its position error was enlarged due to in-suit changing deformation and the position-based cylindricity errors of the stacks were also affected. More importantly, the beginning of Ti chip extrusion was proposed as a critical condition for lowstiffness stack drilling when the gap exceeds the Ti chip curl height, which has been proved by integrated simulation and experiment analyses. This critical condition and its correlated clamping distribution could be adopted as free interlayer damage drilling instruction for low stiffness CFRP/Ti stack.