Multi-scale And Multi-physics Evolution Mechanism In Two-step Machining Deformation Of Titanium Alloy

Titanium alloy has the advantages of high strength,corrosion resistance and fatigue resistance,which is widely used in the field of aero engine and aero structural parts.However,in the process of processing,the multi-scale deformation behavior of the material under the action of high temperature and high strain rate will have a significant impact on the macroscopic mechanical properties.In this paper,the time-space evolution process of stress,strain,temperature,grain size,dislocation and hardness in two-step machining of titanium alloy at micro,mesoscopic and macro scales was studied by combining experiment and simulation,so as to reveal the evolution mechanism of multi-scale and multi-physical fields.The rapid modeling system of cutting simulation was developed,and the simulation model of two-step orthogonal cutting of titanium alloy was established.The influence law of cutting speed and working step action on cutting morphology,plastic strain and cutting force was studied.The results show that the two-step two-dimensional cutting simulation model of titanium alloy is in good agreement with the experimental results.Under the influence of the previous step,the chip is serrated even at low speed.With the cutting speed increasing,the plastic strain and degree of chip serration increase.Conversely,the cutting force decreases.The spatial evolution mechanism of grain size in two-step cutting of titanium alloy was studied by developing the user subroutine of Zener Hollomon grain size prediction.The results show that plastic strain and temperature significantly affect the spatial distribution of grain size.,which is mainly concentrated in the main shear zone and the tool-chip contact area.what is more,the grain size of tool-chip contact area is slightly larger.The residual strain changes the initial strain distribution,resulting in the change of the grain size distribution on the free surface of the second step.Moreover,With the increase of cutting speed,the grain size variation zone of the free chip surface decreases,and the grain size of the main shear zone and the cutter-chip contact zone decreases.Based on the dynamic recrystallization simulation of microstructure with cellular automata model,the time-space evolution mechanism of the grain in the chip shear zone of two-step machining of titanium alloy was studied.The results show that due to the severe deformation,the grains in the shear band are broken up and appear a circular diffraction ring.With the increase of cutting speed,the grains become more broken and the diffraction ring becomes more continuous.Under the influence of grain nucleation and growth process,the flow stress induced by dynamic recrystallization increased first and then decreased,which is lower than the Mises stress.The temporal and spatial gradient evolution of the physical fields,such as strain and temperature,contributes to the dynamic recrystallization of shear band in the first step is as follows:tool tip region,middle region and free surface region.The spatial and temporal distribution of the initial strain in the second working step affected by the residual strain,resulting in the dynamic recrystallization sequence,which is as follow:tool tip region,free surface region and middle region.Based on the relationship between grain size and microhardness,the subroutine of microhardness distribution prediction in two-step cutting of titanium alloy was developed,and the influence mechanism of mesoscopic variation in shear zone on microhardness and crack formation was revealed.The results show that the variation area of microhardness is consistent with the grain size,but the variation trend is opposite.With the increase of cutting speed,the residual high energy dislocation increases,leading to the increase of microhardness.In the first cutting step,the spatial distribution of dislocations,grains and other physical fields is unbalanced,which leads to the crack initiation from the free surface and extension to the tool tip under the action of tension and compression.For the second step,the uniform distribution of dislocations,grains and microhardness under the influence of residual strain at low speed inhibited the crack initiation and propagation.However,with the increase of cutting speed,the grain refinement area on the free surface decreases,and the crack formation mechanism is the same as that in the first step.