Research On The Theory Of Ultrasonic Surface Rolling Processing And Mechanical Properties Of Metal Surface

Ultrasonic surface rolling processing (USRP) is a newly developed surfacenanocrystallization technology, which can greatly improve the comprehensiveperformance of metal surface.This paper first chooses the nonlinear isotropic-kinematic hardening constitutivemodel to describe the stress-strain relation of material subjected to ultrasonic surfacerolling and then carries out symmetrical strain controlled low cycle fatigue tests underdifferent strain amplitude and strain rate for as-supplied, quenched and tempered40Crsteel for the sake of drawing material parameters. Meanwhile, the nonlinearisotropic-kinematic hardening constitutive model is revised considering strain, strainrate and cyclic loading characteristics.Secondly, the dynamic response of40Cr suffered by ultrasonic surface rolling isinvestigated. In this connection, dynamic monitoring system based on the principle ofpiezoelectric effect is established and the frequency and amplitude of dynamic impactforce which varies according to sine function are obtained. It is found that the impactforce increases as static pressure and vibration amplitude increase. Compared withstatic pressure, the influence of vibration amplitude is more significant. The motionstate of processing tip is obtained by finite element simulation, and further used forsolving the elastic-plastic spherical stress wave problem generated in dynamic impactprocess. The stress-strain state and velocity of any mass point at any time are providedthrough characteristic method.Energy distribution during ultrasonic surface rolling process is studied using finiteelement analysis, as well as definition and calculation of USRP efficiency and power.Also, effects of USRP parameters on surface characteristics are discussed. Simulationresults show that residual stress distribution in surface layer is evener after USRPtreatment and the surface residual stress is compressive. Compressive stress firstincreases and then decreases with the increase of depth. The magnitude and depth ofmaximum residual stress and residual stress range increase as spindle speed and feedrate decrease and static pressure, vibration amplitude, processing times increase.Surface roughness rises as spindle speed and feed rate increase and first decrease andthen increase with the increase of static pressure and vibration amplitude. Workhardening is characterized through the equivalent plastic strain. Finally, nanoindentation experiments are performed on the surface layer of40Crtreated by USRP to obtain the mechanical properties, including elastic modulus, nanohardness, yield strength, strain hardening exponent and residual stress. Elasticmodulus, nano hardness and yield strength gradually reduce from surface to interior,and are significantly improved with the increase of static pressure, amplitude andprocessing times, whereas strain hardening exponent presents a contrary trend. Within500μm from the surface, residual stresses are compressive and reduce with theincrease of distance from the surface.