Effects Of Tool Wear State On The Machined Surface Integrity Of Titanium Alloy Ti-6Al-4V

Tool wear process affects the contact states between the tools and workpieces,reducing the effective tool clearance angle,increasing the interface contact area,and changing the stress states and heat distribution.The additional thermo-mechanical loads directly change in the surface topography,microstructure and mechanical properties of the machined surface,which affects the final performance and service life of the manufacturing parts.Therefore,revealing the effects of tool wear on the machined surface integrity has become a key issue that needs to be solved urgently in the fields of mechanical processing and equipment service.In order to solve the challenges in machining Ti-6Al-4V and obtain high-quality machined surface,this work focused on the effects of tool wear on the machined surface integrity during orthogonal turning Ti-6Al-4V.Based on the thermophysical properties changes of contact interface between the tool and Ti-6Al-4V,the tool wear process induced by physical processes,chemical reactions and thermo-mechanical phenomena was analyzed,and the tool failure modes and wear mechanisms were revealed.Based on the mapping relationship between the tool wear and corresponding machined surface,the evolution mechanisms of surface integrity of Ti-6Al-4V induced by tool wear were analyzed.The additional thermo-mechanical loads considering wear induced tool geometrical alterations were analyzed,and the multiphysics prediction models between tool wear and surface integrity characteristics were established.Firstly,the high-temperature interface tribological characteristics and tool wear process of WC-6Co/Ti-6Al-4V were studied through high-temperature tribological experiments and cutting experiments.The high-temperature tribological behaviors of Ti-6Al-4V/WC-6Co interaction couples at the temperature ranging 20℃,320℃,620℃,and 920℃ were analyzed including coefficient of friction,tribological topography,element diffusion,phase compositions,and nano-indentation hardness.With the increase of the tribological temperature,the tribological behaviors were gradually changed from mechanically induced wear under the low-temperature conditions into the coupled mechanical-chemical induced wear under the high-temperature conditions.The tool failure modes were classified into the normal wear and abnormal wear failure forms at the various wear regions(cutting edge,rake face and flank face).The thermo-mechanical coupling tool wear process was analyzed through the interface between WC-6Co and adhesion Ti-6Al-4V,which revealed the comprehensive wear mechanisms of adhesive wear,dissolution-diffusion wear and chemical wear.Secondly,the correlation between tool wear and machined surface topography was evaluated by the surface analysis technology,and the evolution of the machined surface topography characterization emphasizing tool flank wear effects was analyzed.There was a good correlation between the evolution of machined surface topography and tool wear.The regularity of surface topography and the randomness of surface defects were the mapping of tool geometry and cutting edge motion trajectory on the machined surface.Based on the visual analysis of the three-dimensional(3D)machined surface topography,the detailed comparison of 3D surface parameter characteristics in terms of amplitude,spatial,hybrid,functional,and volume parameters was analyzed under various tool wear conditions.The forms and causes of undesirable surface defects,such as plowing grooves,adhered material particles,surface tearing,surface burning,lateral flow,plastic smearing,plastic shearing and surface ablation were discussed on the basis of tool wear patterns and additional thermo-mechanical loads.Then,the evolution mechanisms of microstructure in the localized deformation layers of the machined surface under the tool wear states were explored.Tool wear played a decisive role in the microstructure characteristics of the machined surface.The plastic deformation,grain boundary distribution and grain size in the localized deformation layers were studied,indicating the plastic activities were enhanced by the additional thermo-mechanical loads because of tool wear.With the increase of tool wear,the depth and degree of plastic deformation increased,high-density low-angle grain boundaries accumulated in the localized deformation layers.The percentages of small grains increased and various degrees of refinement occurred with the increase of tool wear.The texture enhancement regions indicated that the preferential deformation texture was modified by the retained shearing with C-type fibre textures.The volume fraction of a phase on the machined surface showed an increasing trend,and the intensity of the phase diffraction peak decreased overall.The gradient microstructure distribution underwent a gradual transition process along the depth direction,which was determined by the thermo-mechanical conditions associated with the tool wear.By analyzing the nanocrystals,dislocation slip,twinning structure and martensite transformation in the localized deformation layers,the dynamic recrystallization mechanism induced by tool wear was analyzed.Finally,the additional heat sources and mechanical loads were analyzed through the changes of tool geometries and contact modes under tool wear conditions.And the multiphysics relationship models between tool wear and mechanical properties of the machined surface were established.Based on the crystallization kinetics equation JMAK model and the hardness empirical formula Hall-Petch fine crystal strengthening principle,the hardness prediction of the machined surface under tool wear conditions was visualized through Abaqus user subroutine.Based on the analysis of additional stresses distribution and relaxation process,the prediction model of residual stress distribution on the machined surface was established with the considerations of tool wear.The accuracy of the proposed model was verified by turning Ti-6Al-4V experiments,and the evolution mechanisms of hardness and residual stress under tool wear conditions were revealed.The thesis research is supported by the National Natural Science Foundation of China "High-temperature Materials of Aero-engine/Scientific Basi of Advanced Manufacturing and Fault Diagnosis"(91860207)and the National Natural Science Funding "Machining,grinding processes and equipment"(51425503).