Numerical Investigate On Mechanical Behavior And Characteristics Of Titanium Alloy Processed By Abrasive Flow Machining At Microscopic Scale

The material with characteristic of lightweight and high temperature resistance has always been the focus of researchers at home and abroad.In the past,nickel-based alloys have been widely used in aerospace and other fields,but their high density makes it difficult to meet lightweight standards.The emergence of titanium alloys,especially?-Ti Al alloys,has solved this problem.The?-Ti Al alloy,as a new lightweight high temperature structural material,has been considered as an excellent candidate material for high-temperature applications.However,the brittleness of?-Ti Al alloys in room temperature restricts their wide application.Abrasive flow machining,as a micro machining technology,is able to execute the plastic removal of brittle materials.Therefore,it is of great practical significance to study the plastic deformation behavior and material removal characteristics of?-Ti Al alloy in abrasive flow machining process.However,the micro-cutting process is a dynamic process at the micro scale.It is difficult to observe the micro details in real-time during the cutting process by means of traditional experimental methods.Consequently,it is imperative to find a new method to investigate the abrasive flow machining process at the micro scale.The abrasive flow machining process was investigated from the microscopically by means of molecular dynamics simulation.The mechanical behavior and micro cutting properties of abrasive flow machining of?-Ti Al alloy were revealed.Now most researches on?-Ti Al alloy have focused on the smooth material surface in vacuum environment,but the actual abrasive flow machining process is not the case.Therefore,the molecular dynamics model of single crystal?-Ti Al alloy with rough surface was constructed in the fluid medium environment,which is more suitable for practical processing.Considering the extremely complex component of aviation kerosene,dodecane was set as a fluid medium,and the CH₃ and CH₂ groups in the dodecane molecule were regarded as a single interactive unit in the joint atomic model to improve the calculation efficiency,and the texture surface was used as a special rough surface.Based on the established abrasive micro-cutting single crystal?-Ti Al alloy model with rough surface,the material removal,fluid medium distribution,cutting force changes,temperature and energy changes,and the evolution of subsurface defects during the micro-cutting process were analyzed and discussed in detail.It is found that under the extrusion and shear action of the abrasive particles,the atoms of the workpiece pile up to form chip,while the atoms of the workpiece move downward to form the machined surface.Due to dislocation movement,nucleation and annihilation,a host of defect structures are nucleated on the subsurface of the workpiece,such as atomic clusters,stacking dislocation,dislocation rings,prismatic dislocation loop and a large number of point defect structures.The four stacking faults with different orientations located on the adjacent close-packed surfaces slide along there Burgers vector until they reach the intersection of the slip plane,and interact to produce Lomer-Cottrell and Hirth dislocations,which prevent the stacking faults sliding further.The prismatic dislocation loop consists of the above defects and Shockley dislocations.The prismatic dislocation loop structure is easy to stably exist on the subsurface of the workpiece due to the existence of the Stair-rod dislocation that hinders slippage.Then,taking into account the cutting depth,particle size and particle type,the influence of different machining parameters on material removal and machining quality in the micro-cutting process was analyzed.The simulation results indicated that selecting a smaller cutting depth will reduce the number of atoms removed,but it will greatly reduce the deformation of the subsurface of the workpiece.Although the material removal efficiency will be reduced,the nucleation of dislocations will be reduced and a better machined surface will be obtained by choosing a smaller particle size.Selecting CBN abrasive particles can inhibit dislocation nucleation and reduce the possibility of defects,so it can effectively replace the diamond abrasive particles which is extremely expensive.In addition,the molecular dynamics model of nano-indentation was established.The thesis detailed analyzes the evolution process of prismatic dislocation loops that stably exists in machined materials,and compares the surface properties of the workpiece before and after processing.It was found that although nano indentation can make the original defects disappear,there will be more vacancy defects under the indentation area.The original defects in the workpiece increase the hardness of the material,which verifies the work hardening phenomenon of the machined surface caused by the defects in the material.