| Carbon fiber reinforced silicon carbide(Cf/SiC)ceramic matrix composites have been widely used in cutting-edge science and technology fields as a new material with excellent properties such as high temperature resistance,corrosion resistance,high strength,high hardness,oxidation resistance and damage resistance.Among them,the needled fibers in 2.5D Cf/SiC composites can enhance the binding effect of the interfacial layer and effectively reduce the interface disbonding,which has greater development potential.However,the anisotropy and brittleness of 2.5D Cf/SiC composites lead to the phenomenon of high grinding force,serious tool wear and high surface damage during grinding,which makes it difficult to guarantee the quality of machining surface,and seriously hinders its application and development.In addition,it is difficult to effectively reveal the grinding mechanism of 2.5D Cf/SiC composites through traditional research methods,so it is urgent to combine the grinding theory and the new numerical modelling method to study it.In this thesis,based on the material properties,grinding heads and processing technology,a three phase numerical grinding model suitable for 2.5D Cf/SiC composites is established to explore the grinding characteristics and removal mechanism of 2.5D Cf/SiC composites.Combined with the design of new grinding heads and the ultrasonic vibration grinding process,the influence of process parameters on the grinding force and surface integrity of 2.5D Cf/SiC composites is investigated,so as to explore a new grinding process of ceramic matrix composites.Firstly,based on the brittleness of SiC matrix,the anisotropy of fibers and the mechanical properties of the interface,a single grit scratching numerical model for the mechanical properties and microstructure of 2.5D Cf/SiC composites was established.The accuracy of the model was verified by the scratching force and the scratching surface morphology.In addition,the transverse,normal and longitudinal cutting modes were defined according to the fiber orientation,the effects of fiber orientation on grinding characteristics,crack extension and removal mechanism were analyzed and the morphological differences of chips in different fibre cutting modes were also explained comprehensively combined with grit scratch experiments and simulations.The results show that the scratching force fluctuates periodically due to the arrangement of fibers,and the order of average scratching force is MA>MC>MB.The crack is easy to grow along the fiber axis,and the needled fiber can hinder the crack extension,which results in the largest damage layer for transverse fibers and the smallest for longitudinal fibers.The removal modes of transverse fibers are worn,fracture and peel-off,in which normal fibers are pullout and outcrop and the longitudinal fibers are worn and push-off.In addition,the different contact areas between grit and fibers lead to different fiber failure behaviors.Secondly,two kinds of segmented grinding head with different types of straight groove and spiral groove were designed.The influences of groove shape,groove width,groove number,spiral angle and process parameters on grinding force and surface roughness of 2.5D Cf/SiC composites were systematically studied.The results show that compared with the traditional grinding head,the straight groove grinding head can reduce grinding force but increase surface roughness,while the spiral groove grinding head has obvious effect on the reduction of grinding force and surface roughness,and at low rotational speed,the spiral groove grinding head can reduce grinding force and surface roughness faster,but at high rotational speed,it will lead to the increase of roughness.In addition,increasing the number and width of grooves can reduce the grinding force,but lead to the increase of surface roughness,while increasing the spiral angle can reduce the surface roughness.Finally,based on longitudinal ultrasonic vibration grinding experiments,the effect of ultrasonic amplitude on grinding force was studied,and the difference of fiber damage under different ultrasonic amplitudes was discussed at a microscopic level,so as to explain the mechanism of ultrasonic amplitude on surface roughness.The influence of the process parameters and the difference of macroscopic and microscopic defects on the grinding surface were compared between the two machining methods.The finite element model of ultrasonic vibration grinding was established and the removal mechanism of ultrasonic vibration grinding was analyzed by simulation.The results show that suitable ultrasonic amplitude can effectively reduce grinding force and surface roughness.Ultrasonic vibration can inhibit the crack extension,reduce burr,edge breakage and fiber delamination,and weaken the influence of process parameters.In addition,ultrasonic vibration can make the fiber shear fracture,and the fracture section is smooth,and the broken fiber dispersed to both sides of the feed direction,reduce fiber extrusion damage,promote chip discharge. |
PDF Full Text Request