Study On Strengthening Mechanism And Properties Of Gradient Nanostructures By Ultrasonic Rolling

Titanium alloy is widely used in aerospace,marine engine blades,high strength shell and other key components owing to the extraordinary characteristics of high specific strength,low density,high temperature resistance and corrosion resistance.However,its low hardness and poor wear resistance seriously restrict its service performance in harsh and complex environment.In this paper,gradient nanostructures are prepared on the surface of TC4 titanium alloy by ultrasonic rolling process,which can effectively improve the surface integrity and wear resistance.It is worth noting that the evolution law and strengthening mechanism of gradient nanostructure on the surface layer of materials under severe plastic deformation by ultrasonic rolling are revealed by hybrid means of mechanical analysis,process simulation,microanalysis and experiment.In addition,the influence of ultrasonic rolling on the surface integrity and wear resistance are particularly studied.Firstly,based on the mechanism analysis of ultrasonic rolling,the mechanical model of ultrasonic rolling is analyzed,and the plastic deformation mechanism of material surface is clarified.Thereafter,the finite element simulation model of ultrasonic rolling is established to study the variation of stress-strain and plastic deformation in the process of machining.The results indicate that the ultrasonic rolling causes obvious plastic deformation on the surface of the material,resulting in high residual compressive stress.The plastic deformation strain rate decreases gradually along the depth direction,and the maximum residual compressive stress decreases with the increase of rolling times.Secondly,the surface microstructure of titanium alloy with different rolling times and layer depth is analyzed by means of TEM and EBSD.Combined with the above finite element simulation analysis,the influence mechanism of ultrasonic rolling plastic deformation on the surface microstructure evolution is analyzed.The results show that the surface grain size of the material is about 20 nm after ultrasonic rolling,and the grain refinement degree along the depth direction is weakening,thus forming gradient nanostructure.The rolling times have a significant effect on the grain refinement degree.With the increase of the rolling times,the grain refinement layer deepens and the grain refinement degree increases.The mechanism of fine grain strengthening,strain strengthening and stress strengthening by ultrasonic rolling is revealed from the microstructure of dislocation density and grain size.Thirdly,the influence of ultrasonic rolling parameters and gradient nanostructures on the surface integrity of TC4 titanium alloy is studied based on the analysis of microstructure change and strengthening mechanism.The results indicate that the surface microhardness changes in accordance with the mechanism of ultrasonic rolling that the surface microhardness increases significantly and decreases along the depth direction.Ultrasonic rolling can effectively reduce the surface roughness of the material,and the minimum surface roughness value can be reduced to about Ra0.047.A higher residual compressive stress is introduced into the surface of the material,which first increases and then decreases along the depth direction.Besides,the maximum residual compressive stress gradually decreases with the increase of rolling times.Finally,the mapping relationship between rolling times,gradient nanostructure,friction and wear properties of titanium alloy layer under dry friction condition is established based on the analysis of ultrasonic rolling strengthening mechanism.The wear mechanism of the material is revealed by analyzing the evolution of friction coefficient,wear amount and wear morphology.It is found that ultrasonic rolling can effectively enhance the wear resistance of the material surface,and the wear resistance increases with the increase of rolling times.Similarly,the wear resistance is positively related to the degree of micro grain refinement,which provides a theoretical basis for the application of wear resistance of titanium alloy surface.