Co-authors​ :

 Nan GUO (CHINA), Yan CHEN , Yucan FU , Yinggang LI (CHINA) 

Carbon fiber reinforced plastic (CFRP)/Ti6Al4V stacks are widely used in the aircraft industry, contributing to enhance structural strength, to reduce overall weight, and to lower life-cycle costs. However, the drilling process involved in the assembly of CFRP/Ti6Al4V stacks is susceptible to thermal damage, leading to challenges such as CFRP delamination, large Ti6Al4V burr height, and poor machined surfaces quality due to high drilling temperature. To solve the drilling problems caused by high temperature, this research investigate the drilling performance of CFRP/Ti6Al4V stack under cryogenic conditions, which is characterized as cryogenic supercritical carbon dioxide (scCO2). The orthogonal cutting experiments of CFRP and Ti6Al4V under scCO2 cooling strategy were conducted first, compared with the condition without cooling. The force signals and machined surface were measured to analyze the cutting/thrust force, tool-workpiece friction and surface integrity. Then the drilling experiments were carried out under ScCO2 cooling on Seti-tec electric drilling unit (EDU) equipment commonly used in aircraft industry. Internal air cooling and minimum quantity lubrication (MQL) strategies were also used for comparison. Thrust force, torque, temperature and hole quality were measured in the experiments to evaluate the effect of cooling/lubrication strategies on the drilling performance during different tool wear stages. Experiments results showed that under ScCO2 cooling strategy cutting force and thrust force all increased, accompanied by a rise in the friction coefficient when compared to conditions without cooling. The machined surface indicated that ScCO2 cooling strategy benefited to CFRP and Ti6Al4V cutting, the depth of pits in reverse cutting direction was decreased 49.7% when ScCO2 cooling used compared to the depth without cooling. Drilling temperature of CFRP, interface and Ti6Al4V under internal air cooling was 51.2°C, 65.9°C and 114°C respectively, while the reductions were 15%. 72% and 33% under ScCO2 strategy and 27% when drilling Ti6Al4V under MQL. Thrust force and torque were 422.7N and 1.476Nm when drilling CFRP with internal air cooling and those were 1249N and 2.932Nm when drilling Ti6Al4V. With ScCO2 cooling, the thrust force and torque were increased by 5% and 33% when drilling CFRP, while the increases were 2.1% and 30.7 when drilling Ti6Al4V. CFRP exit delamination and Ti6Al4V burr height sharply decreased compared to the condition under MQL and internal air cooling strategies, and hole surface quality was also improved with ScCO2 cooling. These findings provide useful insights into improving the machinability of CFRP/Ti6Al4V stacks in the aerospace industry, particularly for lowering drilling temperature and improving tool life.