Longitudinal-Torsional Vibration Assisted Helical Milling with Cooling Temperature and Drilling Experimental investigation

CFRP/Ti6Al4V laminates are extensively utilized in the load-bearing structures of aerospace equipment, which inevitably requires hole machining. However, its cutting temperature and axial force can easily lead to defects such as thermal damage, resin matrix glass transition, delamination, and significant tool wear. To tackle these challenges, this study introduces a novel low-temperature (0 to -40°C) longitudinal-torsional ultrasonic vibration-assisted helical milling process. The method utilizes the synergistic effect of cryogenic cooling and ultrasonic vibration to specifically mitigate the aforementioned machining defects. A comparative investigation was conducted on the hole-making characteristics of three processes, conventional helical milling, longitudinal ultrasonic vibration-assisted milling, and the proposed longitudinal-torsional ultrasonic vibration-assisted milling, under low-temperature conditions, with emphasis on the evolution of interfacial temperature and axial force. The results demonstrate that the machining quality is directly governed by the temperature-dependent behavior of CFRP. Specifically, observations from hole geometry and micromorphology identify -35°C as the critical temperature for interlaminar resin bonding failure during longitudinal-torsional ultrasonic vibration-assisted processing, which readily induces the formation of interlaminar voids. Experimentally, the proposed process reduces the axial force in the CFRP layer and the titanium alloy layer by 16.28% and 20%, respectively, compared with conventional helical milling. Moreover, adopting a CFRP-to-Ti milling sequence at an ambient temperature of -15°C yields the minimum axial force and effectively minimizes interlaminar voids.