Surface Strengthening Mechanism Of Ti-6Al-4V Machined By Rotary Ultrasonic Burnishing

The titanium alloy Ti-6Al-4V has high specific strength and excellent corrosion resistance.It has been always used as the main material for turbine blades and other parts in the aircraft engine.As the demand for engine service performance becomes more and more demanding,the technical requirements for the surface of parts are getting higher and higher.The traditional mechanical processing has been difficult to meet the service requirements of Ti-6Al-4V parts.As an auxiliary processing method,ultrasonic vibration processing has been widely used in the field of mechanical processing.The ultrasonic softening and residual hardening effects can effectively reduce the burnishing force and improve the physical and mechanical properties of Ti-6Al-4V surface.The Ti-6Al-4V surface processed by rotary ultrasonic burnishing is taken as the research object of this paper.This work reveals the effect of ultrasonic vibration on the two-phase(?+?)distribution and microstructure in the surface layer of Ti-6Al-4V processed by rotary ultrasonic burnishing.It has a guiding significance on the preparation of Ti-6A1-4V with different phase fractions by ultrasonic assisted machining.The paper explains synergistic mechanism of the two-phase volume fractions,the mean grain diameter of ?-Ti,the mean width of ?-Ti on the Ti-6Al-4V surface microhardness processed by rotary ultrasonic burnishing.It provides a theoretical basis for the development of Ti-6Al-4V surface strengthening technology.Firstly,the contact behavior between roller and Ti-6Al-4V material is analyzed.Combined with the ultrasonic softening effect,a rotary ultrasonic burnishing force prediction model for Ti-6Al-4V is developed.The rotary ultrasonic burnishing force prediction model reveals the deformation process of Ti-6Al-4V material during rotary ultrasonic burnishing.The deformation mechanism and ultrasonic burnishing force of Ti-6Al-4V material in the rotary ultrasonic burnishing finishing area are studied respectively.The rotary ultrasonic burnishing finishing area can be divided into the initial ultrasonic burnishing area and the repeated ultrasonic burnishing area.Secondly,the paper researches the deformation process of Ti-6Al-4V material in the surface layer processed by rotary ultrasonic burnishing.The influence of ultrasonic vibration on the deformation of Ti-6Al-4V surface is analyzed from the aspects of displacement,deformation area and Mises stress in the two-dimensional simulation model.The plastic flow behavior of Ti-6Al-4V surface material along the radial direction of roller,the axial direction of roller and the normal direction of workpiece surface is analyzed in the three-dimensional simulation model.The formation mechanism of Ti-6Al-4V surface topography processed by rotary ultrasonic burnishing is also analyzed.Thirdly,the mapping relationship among the microstructure,two-phase distribution and microhardness of the Ti-6Al-4V surface under various ultrasonic amplitudes is analyzed from the aspects of two-phase volume fractions,average diameter of a-Ti grains and average width of ?-Ti grain.The Ti-6Al-4V surface hardening model of rotary ultrasonic burnishing is developed.The volume fractions and grain orientation evolution of ?-Ti and ?-Ti,grain size of a-Ti and average width of ?-Ti are analyzed to explain the residual hardening mechanism of Ti-6Al-4V surface in rotary ultrasonic burnishing with different amplitudes.Finally,the effect of ultrasonic amplitude on the microstructure of the Ti-6Al-4V surface layer of rotary ultrasonic burnishing is studied including the grain size,surface texture,local misorientation and grain types.Combined with the distribution of microhardness and residual stress in the Ti-6Al-4V surface layer of rotary ultrasonic burnishing with various ultrasonic amplitudes,the relationship among the microstructure,two-phase distribution and mechanical properties of the Ti-6Al-4V surface layer are established and analyzed.The influence of ultrasonic vibration on the crystal rotation,the intragranular dislocation movement,the formation of sub-grain boundary and the grain refinement in the Ti-6Al-4V surface layer are also explored.