Benefits of Co2 Cooling as a Cooling Technology for Drilling of Stackup Composite Structures

The use of composite materials and composite stackups in aerospace and automotive applications has been and will continue to grow at a very high rate due to the high strength and low weight of the materials. One key problem manufacturers have using this material is the ability to efficiently drill holes through the layers to install fasteners and other components. This is especially true in stackups of Carbon Fiber Reinforced Plastic (CFRP) and titanium (Ti) due to the desire of drilling dry for the CFRP layer and the need for cooling when drilling the high strength Ti layer. By using carbon dioxide (CO2) through tool cooling agent, it is possible to protect both layers. Previous studies have shown significant benefits of CO2 Cooling in CFRPTi stackups including: 30% productivity increases, 10% tool life improvements, and substantial energy savings [1, 2, 3]. This study extends the earlier work by comparing the performance of polycrystalline diamond (PCD) tooling in drilling of CFRP-Ti stackups with three types of coolant conditions: CO2 coolant only, Minimum Quantity Lubrication (MQL) only, and CO2 with MQL. Furthermore, these tests were conducted using portable Automatic Drilling Unit (ADU) which are common in many aerospace assembly processes. Parameters evaluated include roundness of composite and Ti, composite erosion, Ti surface finish, delamination of composite, PCD tooling chipping, and tool wear. Based on the results of a comparison of MQL drilling vs. MQL+CO2 and CO2 alone, the addition of CO2 coolant does not alter the average hole diameter in the Ti layer. However the addition of CO2 cooling generally results in a slightly small hole diameter in the CFRP material. Furthermore, CO2+MQL is expected to increase productivity by 3X without creating observed delamination issues within the CFRP layer.