Investigation On Microstructural Evolution And Mechanical Properties Evaluation In High-Speed Milling Of Ti-6Al-4V Alloy

The recent advancement in the development of high-performance and improved machine tools made the milling process more flexible and eco-friendlier,which can replace the grinding process in the near future.Therefore,it becomes essential to know material behavior and its structural configurations in combination with technological adeptness to take advantage of its potential.The combined action of translation and rotational motion,as well as short and variable undeformed chip thickness and the friction between chip/tool interface,leads the microstructural evolution within machined subsurface and eventually result in mechanical properties changes of the manufactured part.As a result,the geometric stability,fatigue life,and the service performances of the manufactured component are affected.Accordingly,inquiry about microstructural progress mechanism and the mechanical properties changing in highspeed milling process of Ti-6Al-4V alloy is beneficial to understanding the precise estimation of chip and machined subsurface microstructures.In the current study,high-speed milling process of Ti-6Al-4V alloy was considered as a research objective fascinated on the effect of various machining parameters on cutting forces,cutting temperature,chip geometrical characteristics,the microstructural evolution of the chip,and machined subsurface and in-depth mechanical properties evaluation.The main research works can be explained as follow:First,with 5 factors at 5 levels,Taguchi’s design of experiments with standard L25(55)orthogonal array was implemented for high-speed milling of Ti-6Al-4V alloy.The machining forces and workpiece temperature were measured against each milling parameter,and range analysis was performed.It was found that the most dominant parameter which significantly influences the cutting forces and workpiece temperature is the cutting speed,followed by feed rate.In addition,a temperature-displacement coupled finite element(FE)model,which can replicate the actual milling process,was established for high-speed milling of Ti-6Al-4V alloy.In the anticipated model,the potentiality of Abaqus/Explicit has been utilized to link the material-damage and its fracture energy.Chips were divided into tiny foundations along a distinct cutter-rotation angles and discrete period intervals.The developed model is then validated through investigational results in terms of cutting forces,chip temperature,and chip geometrical characteristics.A good correlation was found between the numerically adopted model and experimentally obtained results.Furthermore,the physical phenomenon of the serrated chip formation and the effect of friction coefficient on shear stress is also highlighted and discussed.The developed FE model,acquired experimental data and optimized cutting parameters in this research can provide solid support for high-speed milling of Ti-6Al-4V alloy.Secondly,based on the cutting forces and cutting temperature,the chip and machined subsurface microstructural evolution,including white layer formation and plastic deformation,as well as phase transformation analysis is detected using optical and scanning electron microscopy and X-ray diffraction analysis,respectively.The chip back surface is coarsely separated into two distinct regions,with undeformed and plastic deformation regions.Correspondingly,the microstructure inside the chip adiabatic shear zone was also analyzed and compared with the outer microstructure,and the behavior of the shear band against various machining parameters was observed.Flow motion along adiabatic shear band(ABS)was found quite inhomogeneous since the microstructure inside ABS is stretches completely.In contrast,the microstructure outside the ABS remains undeformed.Similarly,the milled subsurface microstructure is divided into three precincts:bulk region,plastic deformation,and transform layer.Intensity of transformed layer is detected to upsurge with both cutting speed and feed rate.In contrast,the plastic deformation shows a slight variation among cutting speeds but increases gradually with the feed rates.In addition,the phase renovation analysis for both chip’s back and milled surface reveals that conversion of β-Ti into α-Ti is not time-dependent and can coincide.Current approach is beneficial for maintaining desired milled exterior veracity.Finally,the simulated and experimental approach was carried out to evaluate the in-depth residual stress in the milled part.The proposed simplified model was implemented,which is integrated by a linear elastic constituent to categorize the material behavior as a non-Newtonian fluid,and thermo-mechanical coupling effect on deformation.The developed model was then allowed to cool down to room temperature for stress-strain relaxation.The cutting insert is decelerated to a complete stop.Cutter is then removed from workpiece to ensure no interaction amongst workpiece and cutter.Finally,applied thermo-mechanical loads were removed and permissible to casual at ambient temperature.Experimental and simulated results showed that residual stress(RS)silhouette along both directions remains generally hook-shaped and reaches to an extreme compressive stress value after a dip of nearly 10 μm and then transiently approaches to close zero valves into a deep part of the workpiece.In addition,consequence of transformed layer on RS is also investigated,and it is acknowledge that RS remains tensile when no transformed layer is perceived on milled surface,and the shape of RS becomes compressive with foundation of a transformed layer.Furthermore,a nano-indentation procedure is hired to quantify the hardness of milled surface down to bulk region.The machined surface and sub-surface hardness were highly improved,which could be ascribed to grain refinement,deformation twinning,and material phase proportions.In addition,the relationship between hardness,white layer,and residual stress indicated that the residual stress tends to be more compressive with an increase in machined surface hardness.Here the improved machined surface hardness can be associated with the hardened structures existing in the white layer.The present exploration can provide a better understanding of milled surface integrity in HSM of Ti-6Al-4V alloy as well as optimize process parameters on component’s services performance.