Investigation of Drilling of CFRP-Aluminum Stacks Under Different Cooling Modes

Drilling of stacks poses great challenges due the heterogeneity and abrasiveness of the composites, the chip evacuation through the stack, in addition to the difference in properties between the metallic and the composite materials. The objective of this paper is to investigate the effect of drilling conditions such as tool material and geometry and lubrication mode on the hole quality as well as the tool wear in drilling of composite stacks (Carbon Fiber Reinforced Plastics CFRP-Aluminum). The thickness of each material was 19 mm. A 2-flute uncoated drill was used. Four different cooling modes were applied namely dry, minimum quantity lubrication (MQL) with low pressure (<1.5 bar) and high flow rate (400 ml/hr), MQL with high pressure (4.25 bars) and low flow rate (10 ml/hr), and finally flood cooling. The process control parameters, namely the forces and temperatures were measured using a special fixture design using a Kistler dynamometer and a reflective system with an infrared camera. The quality of the holes was compared in terms of delamination, surface roughness, circularity, concentricity, and diameter errors. The resultant cutting forces were found to be much lower than the thrust forces. The mean forces in the Aluminum were more than double those in the CFRP. Negligible tool wear was observed (less than 60 μm). No indication of thermal damage was found on the circumference of the holes in all the tested conditions. Due to the fact that the CFRP was supported by the Aluminum stack, the exit of the holes was mostly free from delamination. The dry and flood conditions produced holes free from entry delamination, while the holes drilled with MQL had delamination within 24% of the hole diameter. Both MQL cooling modes resulted in comparable temperatures, forces and hole quality.