Investigations On Mechanism And Numerical Analysis Of Ultrasonic Rolling On 18CrNiMo7-6

Ultrasonic rolling process,as a new surface strengthening technology,combines the traditional rolling technology with ultrasonic technology to introduce compressive residual stress(RS)into the metal surface,so as to improve the metal comprehensive performance.To date,much research has been focused on ultrasonic rolling.However,the current research on ultrasonic rolling is mostly theoretical analysis and experimental research,few scholars focus on the formation and coupling mechanism of residual stress field(RSF)after ultrasonic rolling.Therefore,the formation and coupling mechanism of RSF after ultrasonic rolling were carried out by combining theoretical analysis and finite element simulation in this paper.The influence of processing parameters on the strengthening effect was further studied.On the basis of ensuring the magnitude and depth of compressive RSF and considering the surface topography of the workpiece,whereby to optimize the process parameters.The main research contents are as follows:(1)Firstly,the mechanism of ultrasonic rolling strengthening was analyzed,and then the formation and coupling mechanism of RSF after ultrasonic rolling were investigated.Finally,the mathematical model of RSF after ultrasonic rolling was established.The results show that the effect of strengthening is under the combined action of static and ultrasonic vibration,and the RSF is the result of the elastic-plastic deformation of the workpiece.The ball per cycle moving distance L and the step-over distance S influence the RSF and the surface topography.When L and S are small,the coupling effect of RSF is better,and the surface is smoother after strengthening.(2)The single point finite element model of ultrasonic rolling process on18 Cr Ni Mo7-6 was established by using ABAQUS software.The transient evolution and steady distribution in single point process were analyzed,and the influence of ball radius R,static pressure F,ultrasonic vibration A and ultrasonic frequency f on RSF was investigated.The results show that,with the decrease of R,the maximum value of compressive RS increases,and the depth of compressive RS layer is almost constant.With the increase of F,the maximum value of compressive RS and the depth of compressive RS layer increase significantly.With the increase of A,the maximum value of compressive RS and the depth of compressive RS layer increase,and compared with R and F,the effect of A on the mechanical properties is small.In addition,the influence of A on compressive RS is small when F in a big value,but in a small value of F,the influence of A on compressive RS increases.Compared with the above three process parameters,the influence of A on compressive RS is the least in the single point ultrasonic rolling process.And the maximum value of the compressive RS and the depth of the compressive RS layer do not change with the change of the f.(3)Based on the single point process,the finite element models of a single pass and multiple passes ultrasonic rolling process were established.The influences of L and S on the RSF and the surface roughness were investigated.The results show that the stress and strain of the workpiece varies in the process,and the stress field of the previous period will be affected by the latter period.With the decrease of L,the coupling of the RSF generated by two adjacent periods becomes more sufficient in a single pass ultrasonic rolling process.After multiple periods of strengthening,the RSF becomes more continuous and uniform along the depth direction.With the decrease of S,the coupling of the RSF generated by the two adjacent passes ultrasonic rolling becomes more sufficient and the surface of workpiece is smoother.After multiple passes ultrasonic rolling,the range of the RSF becomes larger and the RSF becomes more continuous and uniform along the depth direction.(4)On the basis of ensuring the magnitude and depth of compressive RSF and considering the surface topography of the workpiece,the optimal process parameters of F=200N,A=6?m,R=2mm,f=30KHz,L=0.1mm and S=0.05 mm are eventually determined.Under this parameter,the maximum value,the maximum depth and the depth of compressive RS is-676.54 MPa,0.05 mm and 0.54 mm,respectively.After the multiple passes ultrasonic rolling process,the mathematical model,the simulation model and the experimental model of RSF are in relatively good agreement,which verifies the accuracy of the mathematical and simulation model.