Surface Quality Adjustment And Control In Machining Titanium Alloy

Titanium alloy has superior performance such as high strength,high corrosion resistance and high heat resistance.It is widely used in aerospace and other fields.However,titanium alloy is typical difficult-to-machine material,and its small thermal conductivity and elastic modulus make it difficult to cut.In this thesis,experiments and finite element simulation methods were used to adjust the titanium alloy processing by multi-step cutting and pre-stress cutting,in order to control the surface quality of titanium alloy processing.Simulation models of titanium alloy multi-step cutting and pre-stress cutting were established.The influences of multi-step cutting and pre-stress cutting process control on chip shape,cutting force and residual stress of the machined surface layer were studied.The results show that both multi-step cutting and pre-stress cutting can increase the compressive residual stress of the finishing surface.Multi-step cutting changes the position of the maximum compressive residual stress in the depth direction and increases the segmentation degree of finishing chips;the compressive residual stress of pre-stress multi-step cutting increases significantly.The texture simulation of the machined surface shows that the texture density of the multi-step finishing surface is smaller,and the pre-stress processing has little effect on the simulated texture density.A two-step milling experiment of titanium alloy was carried out,and the influence of roughing parameters on the cutting force during roughing and finishing as well as the residual stress of finishing surface was studied.The results show that as the cutting speed increases,the cutting force first increases and then decreases;as the radial depth of cut and feed increase,the cutting force increases monotonically.The increase of roughing speed makes the finishing cutting force first increase and then decrease;the increase of the roughing radial depth of cut increases the finishing cutting force;the roughing feed has little effect on the finishing cutting force.A lower roughing cutting speed is conducive to obtaining a larger compressive residual stress on the finishing surface;at a higher roughing cutting speed,the compressive residual stress on the finishing surface first increases and then decreases.The influence of roughing parameters on the phase composition,phase distribution and nano-scale microcrystal size of the roughing and finishing surface layer material was studied.The microscopic mechanism of surface quality adjustment and control of titanium alloy machining was revealed.The results show that the peak intensity and width of the diffraction pattern of the machined surface is significantly larger than that of the matrix,which shows that the crystallinity is enhanced and the dislocation slip is obvious.Cutting speed and radial depth of cut have no obvious influence on the phase distribution of the machined surface,and the increase of the feed rate will increase the β-phase distribution.With the increase of the roughing cutting speed,the β-phase distribution of the finishing surface increases first and then decreases.With the increase of the roughing feed,the β phase distribution on the finishing surface decreases.The Scherrer formula was used to calculate the nano-scale microcrystal size.The microcrystal size increases with the increase of cutting speed;the microcrystal size first increases and then decreases with the increase of radial depth of cut and feed.As the depth of cut and feed increase,the size of the microcrystal on the finishing surface increases first and then decreases.With the increase of the roughing cutting speed,the finishing surface microcrystal fluctuates first at a smaller size.When the roughing cutting speed becomes higher,the finishing surface microcrystal size is also larger.