Research On High-precision And Low-damage Hole-making Process And Mechanism Of C_f/SiC Utilizing Abrasive Waterjet

Aiming at the existing problems in the abrasive waterjet machining of carbon fiber reinforced silicon carbide ceramic matrix composites(Cf/SiC),such as unclear control method and mechanism,unclear classification of surface damage and imperfect damage suppression strategy,this paper mainly studied the hole shape control and damage suppression of Cf/SiC machined by abrasive waterjet.Through the hole making experiment utilizing abrasive waterjet based on response surface method,the influence law and mechanism of cutting parameters on hole shape were revealed.A new Cf/SiC model based on three-dimensional four-way braid was created,and the damage formation process of Cf/SiC hole making by abrasive waterjet was analyzed by finite element model.The generation of interlayer cracks was observed,and the influence of material structure on impact stress propagation was revealed.A new damage quantitative method based on the principle of area invariance was proposed by analyzing the causes and distribution of various surface damage.Based on the single factor experiment of damage control,three kinds of surface damage suppression strategies were proposed for adjusting common cutting parameters,preparing prefabricated holes and staying away from vulnerable areas of materials.These research results will provide theoretical basis and technical support for the further research and application of high precision and low damage hole-making of Cf/SiC using abrasive waterjet.The roundness control and taper control of Cf/SiC hole making by abrasive waterjet were studied.The cutting trajectory was improved,the taper control prediction model was established,and the law and mechanism of abrasive waterjet process parameters affecting hole shape were revealed.It is found that the over erosion phenomenon at the beginning and end of the jet trajectory is the main reason that affects the roundness of the hole.The improvement scheme of the jet trajectory is put forward to cut out the over erosion area and optimize the roundness of the hole.The prediction model of the taper of Cf/SiC utilizing abrasive waterjet was established,and the influence trend and mechanism of the main process parameters(traverse speed,waterjet pressure,standoff distance and abrasive flow rate)on the taper of Cf/SiC using abrasive waterjet were studied.The results show that the standoff distance is the most important factor affecting the taper,followed by the traverse speed.The diameter difference of the inlet and outlet of the hole increases with the increase of the two factors.The influence of waterjet pressure and abrasive flow rate on the taper is not significant.The traverse speed,waterjet pressure and abrasive flow rate all affect the hole characteristics by changing the total jet energy.Besides the above mechanism,the standoff distance also affects the hole characteristics by changing the effective impact area.The common damage of abrasive waterjet perforating Cf/SiC is studied and a new quantitative damage method is developed.The distribution law and formation mechanism of common surface damage in abrasive waterjet hole making of Cf/SiC are studied.It is found that the tear damage is formed by the extension and intersection of cracks,so the crack damage can be regarded as incomplete tear damage,and the direction of crack extension is parallel to the direction of fiber bundle.Peripheral compound damage generally occurs around each hole,and includes two main mechanisms:high-pressure water and abrasive particles flooding into the material layer,and jet impact with insufficient machining capacity.Burr damage occurs less and has little influence in the process of Cf/SiC hole making by abrasive waterjet,which is not the main factor affecting the hole machining performance.A new damage quantitative analysis method based on the principle of constant damage area is proposed.This method has the advantages of low difficulty in observation and low cost in measurement,and can be applied to various fiber reinforced ceramic matrix composites by considering all damage effects comprehensively.A solid model of Cf/SiC was constructed and the damage formation process of Cf/SiC impacted by high speed abrasive particles was studied based on the model.Based on the microcomputed tomography(Micro-CT)image of Cf/SiC and the analysis of the material braiding method,the accurate internal structure of Cf/SiC braiding in three dimensional four-way braiding was explored,and the material solid model consistent with the real material was constructed.The finite element model of Cf/SiC impacted by high speed abrasive particles was established,and the formation process of Cf/SiC interlayer crack was observed.The stress distribution of a single high-speed abrasive particle impacting Cf/SiC with different velocities is studied.It is found that the presence of fiber bundles significantly hinders the stress propagation.Three kinds of damage suppression strategies are proposed and the internal mechanism of their effective damage suppression is analyzed.It is found that the surface damage increases with the increase of standoff distance and decreases with the increase of waterjet pressure and abrasive flow rate.The influence of standoff distance on the damage is the most significant,followed by waterjet pressure and abrasive flow rate,and the influence of traverse speed is not obvious.The damage suppression strategy of prefabricated holes is proposed,that is,the prefabricated holes are prepared before the formal fabrication.It is found through experiments that the amount of damage produced by using prefabricated holes is lower than that without any measures within a certain range,no matter how the allowance is.Three mechanisms of damage suppression by using the prefabricated hole strategy were revealed,namely,reducing the energy of the last machining step,smoothly removing high-pressure water and abrasive particles,and reducing the impact force.The influence mechanism of the vulnerable damaged area controlled by the material structure on the surface damage of abrasive waterjet perforating Cf/SiC was explored.It was found that the end of the fiber bundle should be avoided as far as possible and the pure matrix area should be far away from or completely removed when selecting the location of the hole-making area.