Investigation on the Microstructure Transformation Mechanism of Machined Materials in Micro-cutting with Dislocation Dynamics Simulation

Author:Bai Jin Xuan

Supervisor:bai qing shun


Degree Year:2019





Ultra-precision machining technology is the frontier and hot point of machinery manufacturing,which has exhibited good application prospects and strategic value in aerospace,weapon equipment,energy and medical fields.Micro-cutting technology is one of typical ultra-precision machining methods with micro-tools,which holds the unique distinctions,such as high efficiency,diverse workpiece materials and complex machined surface shapes.However,with the reduction of component size,the changes of material physical properties and failure mechanisms have brought new challenges for micro-fabrication.The basement for the design and manufacture of micro-devices is to master the failure behavior of micro-devices.The properties of materials always exhibit certain temporal and spatial scale effect in small scale,such as atomic vibration,creep,dislocation pile-up,crack propagation,crystal cleavage,and so on.Hence,in order to grasp the roots of micro-component failure and reveal the intrinsic mechanism of workpiece size effect,it is necessarey to research the multi-scale characteristics of failure behavior.Due to the existence of scale effect,traditional macro-analytical methods cannot be applied to the numerical computation of micro-cutting process directly.In addition,the universal molecular dynamics simulation is difficult to be verified due to smaller model size.Hence,developing a new multi-scale framework for evaluating the quality of finished surface and subsurface and further revealing the formation and evolution mechanisms of the micro-defects of workpiece have a very important theoretical and practical significance.Depending on dislocation theory,crystal plasticity theory as well as micromechanics theory,present dissertation focuses on the microstucture evolution mechanisms of workpiece and the failure behaviors of diamond micro-tool in micro-cutting.The mainly contributions of this thesis include the following aspects:On the basis of dislocation theory,the control equations of dislocation evolution are obtained,and the discrete dislocation dynamics simulation model is established.The mechanisms of dislocation multiplication,movement and coupling are revealed.The evolution processes of dislocation slip,climb and annihilation are explained.The characterstics of disloction pile-up and interaction are explored.The constitutive rules of three-dimensional(3D)dislocation are intergrated into the two-dimensional(2D)plane simulation.A dislocation dynamics simulation software has been developed independently to lay a foundation for acquiring the machined material microstructural transformation mechanism during micro-cutting process.A physical detection platform for machining monocrystalline silicon experiment is estabished.The influence of micro-milling parameters on the machined surface morphologies is studied.The nucleation and movement processes of linear defects at nanosecond scale and micron scale have been simulated on the basis of bottom-up analysis.In order to further declare the failure mechanisms of silicon crystal in brittlemodel cutting,the evolution of dislocation patterns under transient and continuous impact waves is revealed.A multi-scale coupling method between discrete dislocation dynamics and continuum mechanics is developed.The microstructural changes of titanium alloy grain in micro-machining are discussed.Meanwhile,the effect of grain refinement on material mechancial properties has been studied.The formation and transformation processes of subsurface damage layer of workpiece are explored.The distribution mechanisms of dislocation and micro-internal stress at mesoscopic scale are described quantitatively.A dislocation density-based strain gradient model is proposed,and the influence of the parameters selection of constitutive equation on simulation results is researched.In addition,X-ray diffraction(XRD)is used to gain the micro-internal stress of workpiece.Qualitative comparison has been implemented between the simulated internal stress and experimental results.The damage and failure behaviors of polycrystalline diamond(PCD)micro-tools in micro-cutting titanium alloy are revealed,and the influence of tool structural parameters on machined surface quality was expounded.The integrities of subsurface damage layer are taken as an important evaluation indexs of the cutting performance of micro-milling cutters.The effect of cutting edge radius,rake angle and clearance angle on subsurface damages depth,damages area and dislocation distribution has been investigated.The collaborative simulation method between discrete dislocation dynamics and physical-based material plastic model is developed.The dislocation initiation and annihilation processes of workpiece under high strain rate compression is revealed by using three-dimensional discrete dislocation simulation technology.The dislocationrelated intrinsic constants in the physical-based constutive equation are calculated quantitatively.In addition,the grain size and dislocation density have been adoped as the characteristic variables of constitutive equation.The transformation mechanism of grain size in chip and machined surface has been revealed under complex working conditions.Predicted data is verified by existing experimental results.