Numerical Simulation And Experimental Research On The Surface Integrity Of Ultrasonic-assisted Burnishing Shaft Parts

Ultrasonic-assisted burnishing process is a new surface treatment technology.Vibration impacts are superimposed onto the traditional burnishing process and the purpose of burnishing and strengthening the surface is achieved.Compared with other surface treatment techniques,ultrasonic-assisted burnishing process technology has the advantages of easy operation,uniform coverage,less consumption,less pollution and high processing efficiency.The friction and temperature change are ignored in analytical model and the experiment period is long and the cost is high.Aiming at these limitations and shortcomings of analytical analysis method and experimental research,a finite element simulation model of ultrasonic-assisted burnishing shaft parts is presented in this thesis,and the influence of processing parameters,dimension parameters and ultrasonic vibration parameters on the evaluation system of surface integrity such as residual stress,surface roughness and surface work hardening is comprehensively studied by using this model,and the optimal process parameters are determined by Taguchi method.Firstly,the motion state of the burnishing tip was determined by the testing of the dynamic impact force.The minimum mesh size in the contact area of the finite element model was determined by mesh convergence analysis.Based on Hertz contact theory and single-point impact finite element simulation,the penetration depth under different burnishing ball diameters and static forces were obtained and compared.The comparison of finite element simulation results of single point impact with the analytical calculation results shows the feasibility and effectiveness of the finite element model.Secondly,the kinematic analysis of continuous ultrasonic-assisted burnishing was carried out,the simulated processing parameters were determined,and the finite element model of continuous ultrasonic-assisted burnishing shaft was established.The established finite element model data was compared with the experimental results under the same processing parameters to verify the effectiveness of the finite element model.Then different processing parameters were combined to simulate the residual stress and equivalent plastic strain in the contact area under different burnishing tip diameters,static forces,heating temperatures,spindle speeds,feed rates,vibration amplitudes,friction coefficients and ultrasonic frequency parameters.The results show that after ultrasonic-assisted burnishing process,the surface of shaft parts has formed two directions of residual stress,which are tangential residual stress and axial residual stress,respectively.Under the same processing conditions,the tangential residual stress is usually smaller than the axial residual stress.With the increase of static forces,ultrasonic frequencies and vibration amplitudes,the maximum residual compressive stress in both directions also increases;with the decrease of burnishing tip diameters,spindle speeds,feed rates and friction coefficients,the maximum residual compressive stress in both directions increases.With the change of heating temperature in the contact area,the maximum residual compressive stress in both directions does not change much.The maximum value of equivalent plastic strain increases with the increase of static forces,spindle speeds,ultrasonic frequencies,vibration amplitudes and friction coefficients and the decrease of burnishing tip diameter,feed rates,and heating temperatures.Based on the Taguchi method,the parameters of ultrasonic-assisted burnishing process in the finite element model were optimized by orthogonal experimental design with surface roughness as the evaluation system.Significant factors and contribution rates of different processing parameters were determined by the analysis of variance(ANOVA).The results show that the optimum surface roughness parameters are as follows: static force of 150 N,spindle speed of 200 r/min,feed rate of 20 mm/min.The significant order is feed rate > static force > spindle speed,and the results of Taguchi analysis are consistent with ANOVA analysis.Under the analysis of variance with 95% confidence,it is concluded that the factor has the greatest influence on the surface roughness parameters is feed rate,and the contribution rate is 43.6%;followed by static force with the contribution rate of 30.7%;and then,the spindle speed with the contribution rate of 19.1%.Finally,the ultrasonic-assisted burnishing process experiment was carried out to study the influence of feed rate,static force and vibration amplitude on the maximum value of residual compressive stress,surface roughness and surface microhardness.The data indicate that the maximum value of residual compressive stress increases gradually with the decrease of feed rate,and the maximum value of residual compressive stress increases gradually with the increase of static force and vibration amplitude.With the increase of feed rate,the surface roughness gradually increases;with the increase of static force and vibration amplitude,the surface roughness decreases first and then increases gradually;and with the decrease of feed rate,the static force increases and the vibration amplitude increases,the surface microhardness increases gradually.As a result,the validation of the numerical simulation results is verified.