Study On Shot Peening Induced Strengthening Of Nickel-aluminum Bronze

The cast nickel-aluminum bronze?NAB?was shot peened at three different shot peening intensities?SPI?.Influence of shot peening?SP?parameters on the surface properties including residual stress,microhardness,surface topography,surface roughness and microstructure are investigated.The domain size and microstrain of material in the surface layer were calculated from the full width at half maximum of Cu?420?peak.The polarization curves of NAB before and after SP are tested to reflect the influnence of SP on the electrochemical properties of NAB.In terms of the distributions of compressive residual stress?CRS?,the variations of CRS curves show a similar trend that can be generally described as follows:the CRS first increases with depth until it reaches a peak value,then gradually decreases.What's more,at the same depth,the larger the SPI,the higher the CRS.There are four elemental parameters to characterize the CRS field?CRSF?,namely the surficial CRS?SCRS?,the maximum CRS?MCRS?,the depth of MCRS?DMCRS?,and the depth of CRSF?DCRSF?at which CRS attains to zero.There is no obvious discrimination among the values of SCRS at three SPIs.the DMCRS of higher SPI is quite larger than that of lower SPI.When it comes to the MCRS,it increases with SPI to a small extent which is around 8%.Besides,at SPI of 0.20 mmA and 0.25 mmA,the value of CRS reaches the maximum at the same depth,50?m precisely.After SP,the values of FWHM and microstrain at three different SPIs are all improved.And the trends are similar:they reach the maximum on the top surface,then gradually decrease along the depth and stabilize around the value close to that of un-peened sample finally.On the contrary,the microstrain and the microhardness reach maximum on the top surface and decrease with depth at the same SPI.As a result of combination of CRS and strain-hardening effect,the depth of microhardness-improved zone is slightly higher than the values of FWHM-improved area.Three-dimensional appearance of the surface topography and the surface roughness is investigated using AFM.The microstructure is observed by OM and SEM.Experimental results show that the surface is rougher and the roughness is highet at higher SPI.Microcracks are observed at the highest SPI of 0.25mmA and much more than 0.20mmA.Polarization curves and Nyquist plots of NAB before and after SP are measured.The results show that the self corrosion potential is lower and the AC Impedance is smaller at higher SPI,which indicates NAB is more vlunerable at higher SPI.However,the corrosion current density is lower at high SPI which means the corrosion rate is slower.That is attributed to the protective layer consisting of Cu₂O and Al?OH?_3.Compared with the SPI of 0.15mmA,the hardening effect is better at 0.20mmA.On the contrary,the improvement of hardening effect of 0.25mmA is not obvious while the roughness and number of microcracks are much more than0.20mmA.As result of that,the SPI of 0.20mmA seems to be more effective.