Surface Morphology And Mechanical Properties Of AZ31B Magnesium Alloy Via Ultrasonic Rolling

Magnesium alloy is an ideal lightweight metal structural material,due to its low density,large specific strength and specific elastic modulus,good vibration attenuation and thermal conductivity.However,the hardness,tensile strength and yield strength of magnesium alloy are low.At present,it is mainly used in non-critical parts in the fields of automobile,aerospace and communication.It is necessary to improve its mechanical properties to expand the application range of magnesium alloy.Ultrasonic surface rolling technology can strengthen the surface and improve the overall mechanical properties.Therefore,the ultrasonic rolling process and surface strengthening effect of AZ31 B magnesium alloy were systematically studied in this paper.Response surface methodology(RSM)was used to optimize ultrasonic rolling processing parameters.The samples with representative macro and micro surface properties were obtained and tested for tensile and compressive properties.The effects of rolling parameters and surface macro and micro properties on mechanical properties were analyzed to reveal the surface strengthening mechanism.The surface of extruded AZ31 B magnesium alloy bars was treated with turning and ultrasonic rolling.The effects of ultrasonic amplitude,rolling passes and static pressure on the surface roughness and mechanical properties(hardness,tensile/compressive properties)of the samples were studied.The results showthat with the individual increase of ultrasonic amplitude,rolling pass or static pressure,the surface roughness,tensile strength and compressive strength firstlyincrease and then decrease.The Vickers hardness increases gradually,the cold work hardening effect is enhanced,and the compressive yield strength increases.The surface roughness of the sample is the smallest,when the individual factor takes the highest level.The tensile and compressive strength of the sample is highest when the individual factor takes the intermediate level.The effect of interaction of ultrasonic rolling multiple factors(ultrasonic amplitude,spindle speed,rolling pass and feed speed)on surface roughness and hardness was studied by response surface methodology.It was found that the square of rolling passes,the interaction of rolling passes and feed speed have obvious effects on both of them.The regression model of surface roughness and Vickers hardness was obtained by variance analysis,which was used to predict the response target value and provide the basis for ultrasonic rolling of AZ31 B magnesium alloy.Through multi-objective optimization,a set of optimal processing parameters and corresponding processing results to surface roughness and hardness are obtained.Compared with the turning sample,the surface roughness of the sample processed by the optimized parameters is reduced by 81.43%,and the Vickers hardness is increased by 48.78%,indicating that the ultrasonic rolling reduces the surface roughness of AZ31 B magnesium alloy and improves the microhardness.Six groups of microstructure test samples,six groups of tensile samples and six groups of compression samples were processed by using the turning parameters,the rolling parameters for maximum and minimum surface roughness samples,the rolling parameters for maximum and minimum hardness samples in the former multi-factor experiment,and the parameters for the dual-objective optimization result.The metallographic structure,tensile and compressive mechanical properties were tested.The results of microstructure test show that the deformation layer of ultrasonic rolling strengthening is the matrix,twin layer and grain refinement layer from inside to outside.The average grain size of the matrix is about 55?m.The depth of deformation layer of the sample with minimum surface roughness is 551?m.The thickness of twin layer is about 200?m and the average grain size is about 15?m.The thickness of grain refinement layer is about 351?m,and the average grain size is about 2?m.Compared with turning samples,the average grain size of twin layer and grain refinement layer decreased by 72.72% and 96.36%,respectively,and the average grain size of some region reached nanometer level.The depth of deformation layer of the sample with the largest Vickers hardness is 633?m.The thickness of twin layer is about 200?m and that of grain refinement layer is about 433?m.Compared with the sample with the smallest surface roughness,the thickness of twin layer and grain refinement layer increases,but the average grain size changes little.The tensile strength of the rolling sample with the maximum surface roughness,the minimum surface roughness,the minimum hardness,the maximum hardness and the optimized processing result is increased by 11.02 %,20.00 %,22.96 %,15.35 %and 17.96 %,respectively,compared with that of the turning sample.The compressive strength of the rolling sample with the maximum surface roughness,the minimum surface roughness,the minimum hardness,the maximum hardness and the optimized processing result is increased by 4.51%,7.96%,7.16%,9.28% and 6.37%,respectively,compared with that of the turning sample.It indicates that the tensile and compressive strength of the dual-objective optimization sample is between the maximum and minimum surface roughness samples.The lower surface roughness is more favorable for the improvement of tensile and compressive strength.The higher hardness is more conducive to the improvement of compressive strength,but not conducive to the improvement of tensile strength.In summary,the surface morphology,grain refinement and mechanical properties are improved by the plastic deformation induced by ultrasonic rolling.The results provide not only reference processing parameters for surface strengthening and mechanical properties improvement of AZ31 B magnesium alloy,but also material parameters for the design of strengthened AZ31 B components.