Analysis of machining induced damage when drilling hybrid carbon fiber-reinforced aluminum laminates

Carbon fiber-reinforced aluminum laminates (CARALL), a hybrid composite, hold significant promise for use in aircraft structures. However, the distinct behaviors of the materials in this multi-layered configuration pose considerable challenges during drilling. High drilling forces and elevated temperatures can cause various forms of damage, potentially compromising the performance of CARALL structures in service. This research involved performing drilling experiments on CARALL samples produced inhouse, examining various combinations of cutting speeds (30, 60, and 90 m/min) and axial feed rates (0.05, 0.15, and 0.25 mm/rev) to assess their impact on thrust force, drilling torque, machining temperature, and the quality of the hole surface. Increasing the cutting speed from 30 m/min to 90 m/min while reducing the axial feed from 0.25 mm/rev to 0.05 mm/rev resulted in a 40.4% reduction in thrust force and a 59.8% decrease in drilling torque, but also caused a 70.4% rise in machining temperature. At elevated cutting speeds, the rise in temperature leads to a decrease in cutting force, demonstrating the relationship between force and temperature. An increase in axial feed led to rougher bore surfaces. Although the surface finish initially improved with increasing cutting speed, a further increase caused degradation once the machining temperature exceeded the glass transition temperature of the epoxy resin. An acceptable surface finish ( R a ) of 1.93 µm was achieved when an intermediate cutting speed of 60 m/min and a lower feed rate of 0.05 mm/rev were employed. Typical defects observed in the drilled holes included grooves, fiber bending, fiber exposure, matrix degradation, material smearing, adhesion, and delamination. The observed delamination was mainly due to matrix degradation at temperatures beyond the glass transition temperatures.