Ultrasonic Rolling Study Based On Mixed Hardening Model Of 18CrNiMo7-6 Alloy Steel

Ultrasonic rolling is a type of surface plastic deformation technique that can remarkably reduce the surface roughness of metal materials,enhance the surface microhardness of the material and introduce compressive residual stress in the surface layer,thereby improving the anti-fatigue performance of the material.In this study,the mechanical response of 18 CrNiMo7-6 alloy steel during ultrasonic rolling is analyzed.Considering the characteristics such as work hardening,strain rate strengthening and cyclic loading,the mixed hardening model of18 CrNiMo7-6 alloy steel under ultrasonic rolling is established.The material parameters of the mixed hardening model were determined on the basis of relevant tests.The finite element model of ultrasonic rolling was established,and the residual stress field was simulated and analyzed.Compared with the ultrasonic rolling test results,the validity of the finite element model was verified.Then the process parameters affecting the surface residual stresses were optimized and analyzed by Taguchi method.Finally,the other evaluation indicators of material surface properties,such as roughness,surface topography and microhardness,were studied and the influence of each processing parameter on the surface properties was analyzed.The main work consists of the following:1.The mixed hardening mechanical model of 18 CrNiMo7-6 alloy steel under ultrasonic rolling is established.By conducting uniaxial tensile test,high strain rate compression test and cyclic tensile-compression test on 18 CrNiMo7-6 alloy steel,the unknown material parameters in the mixed hardening model are determined and verified,which provide material parameters for the finite element analysis of ultrasonic rolling.2.By analyzing the relative motion of the sample and the tool head during the ultrasonic rolling.The finite element model of ultrasonic rolling is established to simulate the ultrasonic rolling process and then analyze the residual stress field distribution in the surface layer of the material.The simulated results were compared with the experimental test results,it is discovered that the two trends are consistent,which shows that the established finite element model is effective and can be used for the optimization analysis of process parameters.3.Taking the workpiece surface residual stress after ultrasonic rolling as the optimization objective,the processing parameters affecting the workpiece surface residual stress are optimized and analyzed by Taguchi method combined with finite element analysis.The optimal combination of process parameters between static pressure,spindle speed,feed rate and tool head diameter is obtained,and the significant influence of each processing parameter on the surface residual stress is evaluated.The results indicate that the combination of processing parameters are: static pressure400 N,feed rate 0.10 mm/r,spindle speed 150 r/min and tool head diameter 6 mm,the surface residual stress reaches the maximum;The order of significance obtained by Taguchi analysis is: static pressure > spindle speed > feed rate > tool head diameter.4.Other evaluation indexes of surface properties,roughness,surface morphology and microhardness of 18 CrNiMo7-6 alloy steel after ultrasonic rolling with process parameters were analyzed and studied by experimental method.The results indicate that the variation of roughness is positively proportional to the feed and spindle speed and inversely proportional to the rolling times.The law of variation of roughness with static pressure and ultrasonic amplitude shows non-monotonicity.Microhardness is positively related to static pressure,spindle speed,ultrasonic amplitude,and rolling number,which is inversely related to feed rate.