Research On Drilling Process And Tool Wear Of Ceramic Matrix Composites

As a typical continuous fiber reinforced ceramic matrix composite,C_f/SiC composite has excellent characteristics of high temperature resistance,wear resistance,high specific strength and low coefficient of thermal expansion.They are widely used in high thrust weight ratio aeroengines,thermal protection components of hypersonic aircraft,reusable rocket propulsion system nozzles and other components.However,due to its excellent physical properties,it often causes serious tool wear.And due to its anisotropy,burr,delamination,tear and other machining quality problems will occur during drilling.It is a typical hard-to-machining material.In this paper,the removal mechanism of composite materials is studied based on single particle scratch experiment.Different scratch speed,feed rate and scratch depth are designed,the thrust force in the scratch process is analyzed,and the effects of different scratch parameters on surface machining quality are studied.The time domain and frequency domain characteristics of AE signal in the process of scratching are analyzed by using the combination of acoustic emission(AE)technology and ultra-deep field microscope observation,and the corresponding relationship between AE signal frequency band and machining damage is studied.It lays a theoretical foundation for the drilling process planning and material removal mechanism of C_f/SiC composites.Then,the experiments of high-efficiency drilling process of C_f/SiC composites are carried out.The drilling mechanical model of C_f/SiC composites is established through mathematical derivation,and the reliability of the model is verified by experiments.The effects of different machining parameters on machining quality and drilling damage are analyzed.The force and thermal signals in the drilling process are collected.The effects of thrust force on burr and tear defects in the machining process of C_f/SiC composites are analyzed.The surface morphology of the machined hole wall is analyzed by three-dimensional non-contact surface topography instrument and ultra-deep field microscope.The results show that the processing parameters of high speed and low feed can effectively improve the surface quality of the hole wall.Finally,this paper studies the tool wear in the machining process of C_f/SiC composites,designs PCD tools with different point and clearance angles,carries out the drilling experiment of C_f/SiC composites with different machining parameters,and analyzes the force signal and AE signal in the experimental process.The PCD tool wear and material processing quality were observed by SEM.The results show that the PCD tool with a point angle of 140°and a clearance angle of 20°has low tool wear in the machining process.At the same time,the machining parameters of low speed and low feed can effectively reduce tool wear and prolong tool life.By observing the hole wall fiber fracture mode and chip morphology at different wear stages of the tool,and combined with the frequency domain distribution analysis of AE signal,the reinforcement phase and matrix are discussed respectively,and the influence of tool wear on the material removal mechanism is summarized.Prior to tool wear,fiber fracture is flatter,matrix is mainly removed via crushing.After tool wear,fiber fracture is more uneven and it mainly occurs via bending.Matrix removal mechanism changes from crushing to friction removal,while the surface roughness is lower.This study makes a study on the efficient drilling process and tool wear of C_f/SiC composites,which has important theoretical and practical significance for improving the machining quality and tool life.