Effect Of Ultrasonic Surface Rolling Process On Mechanical Properties And Corrosion Resistance Of AZ31B Mg Alloy

Mg-Al alloys have low density,high specific strength,high specific stiffness and good machinability.Recently,they become promising metal material used in automobile and aerospace industry However,the application of the Mg alloys are greatly limited by its drawbacks,such as the low strength,poor wear resistance,high chemical reactivity and poor plastic deformation ability because of their hexagonal close-packed(hcp)crystal structure.Ultrasonic surface rolling process(USRP)is a novel surface nanocrystallization technique based on high frequency impact of tool head.Compared with other surface processing techniques,USRP is able to improve the surface performance,mechanical properties and corrosion resistance of the material at the same time.Although USRP has been developed for industrial applications,there is little research about the effect of USRP on mechanical properties and corrosion resistance of magnesium and its alloy especially for AZ31B Mg alloy.Therefore,in this paper,a large number of experiments were carried out to certify whether USRP can also improve the surface performance,mechanical properties and corrosion resistance of AZ31B Mg alloys at the same time.It is hoped that these effective experimental data will further expand the application of AZ31B Mg alloy in industry.In this paper,the rolled AZ31B Mg alloy sheet was treated by USRP with different amount of rolling.Surface roughness of AZ31B Mg alloy was examined by using surface roughness measuring instrument.The phase constitution and crystalline structure of the Mg alloys was investigated by the XRD.The microstructure was observed by an optical microscope(OM)and SEM.The effect of amount of rolling on mechanical properties and corrosion resistance of AZ31B Mg alloy after USRP was analyzed.The microhardness,tensile properties,residual compressive stress,wear resistance of AZ31B.Mg alloy were investigated by nano-indentation tester,tensile testing machine,X-ray diffractometer,vertical friction and wear testing machine.After USRP treatment,SEM and EDS analysis were used to investigate the change of corrosion rate V and corrosion mechanism of AZ31B Mg alloy with different amount of rolling.The corrosion rate V is evaluated by the weight loss method.With the increase of the amount of rolling,the surface roughness of AZ31B Mg alloy samples decrease gradually after USRP.When the amount of rolling is 0.06mm,the surface roughness of USRPed sample reaches about Ra 0.1009μm which is the minimum surface roughness in all samples.The surface roughness shows a remarkable decline compared with the untreated surface which surface roughness is Ra 1.2326μm,and the surface roughness is reduced by 91.8%.the surface of untreated sample is covered by deep "peaks" and "valleys",and has obvious processing marks.With the increase of the amount of rolling,the "peaks" and"valleys" gradually become shallow,and finally disappear.However,when the amount of rolling is 0.07mm,a certain number of pits appear on the surface of the sample due to excessive pressing.After USRP,remarkable grain refinement and severe plastic deformation are seen in the surface layer of AZ31B Mg alloy,and the average grain size is far less than interior grains.The depth of the grain refinement after USRP reaches about 572.71±13.62μm.Three layers are formed after USRP:Ultra-fine layer,Fine grains layer and coarse grains layer.The average grain size of Ultra-fine layer is smaller than 1 μm,which indicates an excellent grain refinement,and the minimum grain size measured by SEM can reach 338.8nm.Compared with the coarse grains layer,the average grain size of the Ultra-fine layer is decreased by more than 50 times.With the increase of the amount of rolling,the mechanical properties of AZ31B Mg alloy samples increase greatly after USRP.The maximum surface microhardness value of the USRPed samples can reach 133.5±8.63 HV.Compared to the untreated sampl whose microhardness value is 96.3±6.09 HV,the microhardness of USRPed specimen is about 38.63%higher than that of the untreated specimens.The microhardness of the depth direction gradually decreases from the surface to the inner part.The surface microhardness of USRPed specimen is about 83.81%higher than the microhardness of inner part.The yield strength(YS)of AZ31B Mg alloy increases from 206.41±1.78MPa to 256.12±1.47MPa,which is increased by 24.08%compared with the untreated sample.In addition,the ultimate tensile strength(UTS)of USRPed samples is about 19.33%higher than that of the untreated sample.On the contrary,the elongation shows a continuous decline,which is only 47.45%of the untreated sample.The residual compressive stress test shows that the stress state of the material surface changes after USRP,and the original residual tensile stress becomes residual compressive stress.The maximum residual compressive stress can reach-85.2±10.8MPa.The residual compressive stress increases with the increase of the amount of rolling.The surface grain refinement and severe plastic deformation caused by USRP are the main reasons for improving the mechanical properties of AZ31B Mg alloy.The wear resistance of AZ31B Mg alloy increase greatly after USRP.The untreated Mg alloy sample has a severe wear circumstance under the 100 N external loads for 1200s.The wear mass loss of the untreated sample is 32.44mg,and the USRPed samples minimum mass loss is only 9.07mg.Grain refinement greatly improves the strength and hardness of material,and enhances the deformation resistance of material.It will greatly help to reduce the degree of the adhesion between the friction pair and samples.As the degree of adhesion decreases,the friction coefficient reduces.Under the same friction condition,the average friction coefficient of untreated sample is 0.337,which is 1.21 times higher than that of USRPed sample(0.279).According to the SEM images of the typical worn debris morphologies,it can be seen that the worn debris of the USRPed specimen presents flakes or granules,which is far less than the block worn debris of untreated specimen.Adhesive wear mechanism may be the main failure mode of untreated sample.Comparatively,there are some narrow and shallow parallel grooves in surface of USRPed sample,and the main failure mode is abrasive wear mechanism.The corrosion resistance of AZ31B Mg alloy increase greatly after USRP.The corrosion rate V is evaluated by the weight loss method.SEM and EDS analysis were used to investigate the effect of amount of rolling,surface roughness and residual compressive stress on corrosion rate V of AZ31B Mg alloy.When the amount of rolling is 0.06mm,the corrosion rate V is minimal,and the average mass loss by corrosion is 0.00407g.According to the SEM images,there are only some small and narrow corrosion cracks are found in the surface.Compared with the untreated sample,the corrosion rate is reduced by 64.15%.However,when the amount of rolling is 0.07mm,we can observe that the corrosion rate increases,which corresponds well to surface roughness and residual compressive stress of samples.The surface strengthening of AZ31B Mg alloy was used by USRP in this paper.It has an important theoretical and practical significance in mechanical properties and corrosion resistance of AZ31B Mg alloy and lays a foundation for the further research and application of magnesium alloys.It is hoped that these effective experimental data will further expand the application of AZ31B Mg alloy in industry.