Experimental investigation on drilling machinability and hole quality of CFRP/Ti6Al4V stacks under different cooling conditions

Ti6Al4V alloys are often employed in conjunction with carbon fiber-reinforced polymers (CFRPs) to create hybrid structures for aerospace applications. The introduction of the fibrous composites brings significant challenges to the titanium alloys with extremely poor machinability. Since the minimum quantity lubrication (MQL) is reported as a technologically feasible method to improve the machinability of individual titanium alloys, the current work aims to investigate the impacts of the MQL on the machining behavior of titanium-related stacks through the comparison with the dry cutting condition. Drilling tests were carried out to evaluate the machinability and hole quality of CFRP/Ti6Al4V stacks using the TiAlN-coated and diamond-coated drills. The obtained results indicate that the MQL yields a reduction of the drilling torque and the minimization of specific cutting energy consumption during the drilling process of CFRP/Ti6Al4V stacks. The machining qualities are also improved by the help of the MQL, and better surface morphologies of CFRP holes and less titanium burr formation are achieved under the MQL condition. Moreover, the MQL drilling produces better geometrical accuracy of cut composite holes, and the diamond-coated drills outperform the TiAlN-coated ones in terms of higher geometrical accuracy of cut stack holes.