Longitudinal amplitude effect on material removal mechanism of ultrasonic vibration-assisted milling 2.5D C/SiC composites

Abstract

The anisotropic, heterogeneous, brittle and hard nature of ceramic matrix composites (CMCs) challenge machining process. Ultrasonic vibration-assisted machining may be an excellent method for processing these CMCs. The paper aims to investigate the effect of longitudinal amplitude on material removal mechanism of ultrasonic vibration-assisted milling (UVAM) 2.5D C/SiC composites. Firstly, the UVAM experiments of 2.5D C/SiC composites are performed at different longitudinal amplitudes. Secondly, the machined surface defects, surface roughness and cutting force are measured and analyzed. The UVAM experimental results show that the longitudinal amplitude of tool changes the ratio of local fragmentation fracture and abrupt macro fracture in carbon fiber removal modes to affect surface quality. Finally, a novel micro-mechanical model of UVAM carbon fiber in 2.5D C/SiC composites is proposed. The proposed model demonstrates that the longitudinal amplitude induces alternate impact load in carbon fiber and is positively correlated with alternate impact load frequency. The strain rate increases with the alternate impact load frequency, which leads to the embrittlement and local fragmentation of the carbon fiber at the stress point. On the other hand, the cutting force increases at the excessive longitudinal amplitude, which makes the carbon fiber more prone to abrupt macro fracture during machining process. A comprehensive evolution mapping of the carbon fiber fracture process is established to explain the relationship between machining process, removal mechanism, and surface defects. This work suggest that the machined quality of CMCs component can be improved by selection of optimized longitudinal amplitude by control fracture-mode percentage in UVAM.