| ?-TiAl alloys are one of the preferred new high-temperature structural materials in aerospace and other fields and are currently the research focus of TiAl intermetallic compounds.However,the ?-TiAl alloys are a difficult-to-process material with room temperature brittleness and low ductility,and the deformation of the material during processing is small.In addition,the ?-TiAl alloys have poor fracture toughness,which is easy to introduce microcracks and other defects in the machining process,so easy to fracture and fail in the service process.As a crucial means of ultra-precision processing technology,nano-cutting plays a vital role in the field of nano-manufacturing and is the basis for the development of atomic and close-to-atomic scale manufacturing technology.For this reason,based on the research results of modern physics such as molecular dynamics,this paper studies the nano-cutting process of single crystal ?-TiAl alloys analyzes the essential difference between nano-scale and macro-scale cutting of ?-TiAl alloys and explains the physical nature of residual stress.Explain the influence of low-temperature cutting on the machinability and surface integrity of materials,and provide theoretical support for realizing the control of residual stress by processing technology,and promote the development of low temperature cutting technology.The main research contents of this paper are as follows:(1)Based on the molecular dynamics method,the nano-cutting process of the single crystal ?-TiAl alloys are simulated,and the nano-cutting models of shear and extrusion were constructed respectively.Kinematics description is used to analyze the adhesion phenomenon between the tool and the contact atoms and found that the material removal rate of shear cutting is higher,while the extrusion cutting has a larger elastic deformation area.The morphology of the stagnant zone is described by layered colored markings,and the distribution and movement of cutting layer atoms under shear and extrusion cutting are analyzed in combination with the microscopic characterization of the surface morphology.(2)The essential difference between extrusion and shear cutting is comprehensively analyzed,and the influence of temperature,stress,and phase transformation on the formation of residual stress is studied.The cutting temperature distribution is analyzed and the correlation between cutting temperature and phase change is discussed.Comparing the distribution of hydrostatic stress and Von Mises stress of the workpiece and found that the high hydrostatic pressure distribution area corresponds to large inelastic deformation.Combining the common neighbor analyses method,the phase transformation that occurs during the cutting process is divided into two types: high stress-induced amorphization(HSIA)and elastic stress-induced dislocation(ESID),and the chips under shear and extrusion loading are respectively interpreted the removal mechanism.Besides,the relationship between the uneven temperature distribution,the tensile stress of the flank surface,the subsurface damage layer and the residual stress of the processed surface is clarified.(3)The effects of low temperature on the characteristics of single crystal ?-TiAl alloys materials are studied,and the effects of different low-temperature cutting conditions on the machinability and surface integrity of the materials are analyzed.Low-temperature will cause the increase of cohesive energy of single crystal ?-TiAl alloys,which is an unnecessary condition for the formation of residual compressive stress on the processed surface.In the stable cutting stage,the heat dissipation rate increases linearly with the increase of the cutting distance,which is consistent with the stability of the Newtonian layer temperature.Also,the evolution process of the dislocation band in the subsurface damage layer is explained from the atomic point of view.The surface quality under different low-temperature cutting is compared,and the optimal low-temperature cutting temperature is 173 K. |
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