Research On Surface Rolling Strengthening Of Magnesium Alloy AZ31B

In recent years,magnesium alloy has become a metal material with great prospects and application value in the automotive and aerospace industries,because of its low density,high specific strength,high specific rigidity,and good machining performance.However,its low strength,poor wear resistance,and poor corrosion resistance have been limited in industrial production.Surface rolling strengthening is a processing technology that realizes the surface strengthening of parts through mechanical processing,which can effectively improve the comprehensive performance of the surface of the parts and extend their service life.While rolling surface strengthening industrial production has been obtained in the preliminary application,but few studies can affect the mechanical properties of the surface of magnesium alloy,abrasion resistance,and corrosion resistance.Therefore,the effects of rolling process parameters on surface roughness,surface microhardness and surface residual stress were studied in this paper.The mechanism of rolling strengthening was revealed,and the reasons for improvement of friction and wear properties and electrochemical corrosion properties were analyzed.The experimental basis is provided for further research on the application of surface rolling strengthening technology to AZ31B magnesium alloy in industrial production.The effective rules of rolling process parameters on the surface integrity of AZ31B magnesium alloy specimens were studied.The surface rolling of AZ31B magnesium alloy can effectively reduce the roughness value,increase the surface hardness,and form the surface compressive stress.The test results show that the process parameter that has the greatest influence on the rolling strengthening effect is the number of repeated rolling.Comprehensive analysis of the test data summed up the best process parameter value range.Compared with the unrolled specimens,the surface roughness of the rolled specimens with the best process parameters is reduced by 74.3%,the maximum microhardness value is increased by 32.0%,and the maximum absolute value of the residual compressive stress can reach 95.36MPa.As the surface depth increases,the absolute value of the residual compressive stress exhibits a trend of first increasing,then decreasing,and then tending to be stable.The absolute value of the residual compressive stress reaches the maximum near 50?m from the surface,and the depth of the strengthening layer is about 280?m.The prediction model of surface roughness and residual stress is established.The correlation coefficient R-Sq between the actual measured value and the predicted value is more than 95%,which indicates that the correlation between the predicted value and the measured value is strong and the fitting degree is also high.The metallographic microstructure of the rolling specimens with the best process parameters was observed.The results show that rolling hardening can cause serious plastic deformation on the surface of the specimen,and the surface grains are significantly refined.The surface deformation zone is divided into grain refinement zone,transition twin zone,and coarse crystal fracture zone.Then the transmission electron microstructure observation was carried out,and then the rolling strengthening mechanism was explained.The details are as follows:in the area close to the matrix,dislocation slip and dislocation entanglement occur.Under the combined action of shear stress in different directions,deformation twins and twins intersect phenomenon gradually occur.With the increase of strain rate and the influence of temperature,dynamic recrystallization of twins occurs when twins nucleate at the intersection of twins.In the area close to the rolling surface,a large number of micro twins are generated in the dislocation cells,which gradually differentiate into sub-grains.The sub-grain boundaries continue to absorb dislocations,prompting the grain boundaries to gradually change from small-angle to large-angle grain boundaries.Nanocrystals with uniform size,equiaxed shape and an average size of approximately 4 nm are formed on the rolled sample surface.The friction and wear performance and electrochemical corrosion performance of AZ31B magnesium alloy samples before and after rolling treatment were tested.The results show that,compared with the unrolled specimens,the breaking-in period of the best process parameters of the rolled specimens decreased from 0?1100s to 0?400s.Correspondingly,the average friction coefficient and the amount of wear decreased by 12.66%and 72.97%,respectively.The depth of the wear scar is also significantly reduced.The wear form of the unrolled sample and the rolled sample are both composite wear forms of abrasive wear and adhesive wear.The difference is that the adhesive wear form of the sample after rolling changes from smearing to scratching.After rolling strengthening,the number of corrosion pits on the surface of the AZ31B magnesium alloy sample was significantly reduced,and the corrosion potential was increased from-1.3387V to-1.1984V,and the corrosion current density was reduced from 7.202×10^-5A/cm²to 4.285×10^-5A/cm².It indicates that after surface rolling treatment,the friction and wear performance and electrochemical corrosion performance of AZ31B magnesium alloy can be significantly improved.