Study On Shot Peening Of Nickel Aluminum Bronze And Its Characterization

Nickel aluminum bronze(NAB)is one kind of aluminum bronze together with other alloying additions.It has been widely used as landing gear bearings and ship propellers due to its unique combination of high strength and excellent corrosion resistance.Improving the lifespan of NAB by further enhancing its superficial mechanical and fatigue properties is of great interest.The present study was aimed firstly at investigating the residual stress distribution between different phases of QAl10-4-4 using finite element modeling(FEM).Based on the simulation results,the premium shot peening(SP)parameters were picked out.Effects of SP techniques such as dual peening and pre-stress peening on the in-depth distribution of residual stress were evaluated.Microstructure and mechanical property were properly characterized.Further focus was put on the stability of residual stress,and the recovery and recrystallization behavior of peened QAl10-4-4.Finally,the shot peening strengthening mechanism was discussed in detail.ANSYS software was employed to simulate the single and multi-SP process.The results showed that residual stress was distributed inhomogeneously between the base metal and the second phase.Residual stress became discontinuous between different phases.Base metal showed the compressive residual stress(CRS)but the second phase possessed the opposite.Magnitude and depth of CRS were increased with the increasing shot velocity and/or coverage.Similar in-depth profiles of residual stress were obtained though the shot peening technique was different.Compressive residual stress was first increased and then reduced to near the stress-free state gradually with the increasing depth.Further increasing the peening intensity can improve the magnitude and depth of CRS.Dual peening improved the surface CRS and the maximum CRS as well as the homogeneity.It could also reduce the surface roughness in comparison with the single peening treatment.Comparing with the traditional SP,Pre-stress SP could improve the value other than the anisotropy of CRS.Specifically,the surface CRS was improved by 11%-54%,and the maximum CRS was improved by 17%-46% after peening under 0.15 mmA with pre-stresses of 100-250 MPa.No phase transition was observed after peening treatment.However,SP refined the domain size and weakened the original texture.There were numerous dislocation tangles and bands and sometimes deformation twins along {111}[112] directions in the surface layer.Results obtained by XRD line profile analysis revealed the same trend of microstructure along the depth.Domain size increased but microstrain and dislocation density decreased with increasing depth.This situation became aggressive as the peening intensity increased.Relaxation of CRS and the recovery and recrystallization as well as domain growth under heating environment were the function of annealing temperature and holding time.CRS faded significantly during the initial stages,and slowed its pace with the increasing time.The higher the temperature or the longer the aging time,the greater the relaxation of CRS.No phase transition or evident oxidation in the surface layer were detected by XRD.With the increasing temperature or time,dislocation tangles rich regions reduced and therefore the dislocation density.XRD reflection enhanced and peaks became sharper and narrower.It was interesting to found that the recovery and recrystallization exhibited inhomogeneity and asynchronism feature.Based on linear regression analysis,the activation energies of CRS,domain growth and microstrain relaxation were 166.9,199.1 and 175.4 kJ/mol,respectively.Short circuit diffusion was the main reason resulting in these activation energies less than that of pure Cu.Relaxation of CRS under external loading including static loading and alternating loading was also addressed.In the initial stages,CRS reduced rapidly.With the loading amplitude and number of cycle increasing,CRS decreased gradually before it becoming steady due to the reduced plastic deformation resulted from Bauschinger effect.With the applied loading of 100-250 MPa,the CRSs reduced by 15.3%-39.2%,respectively.Full width at half maximum and microhardness increased with peening intensity,but decreased with depth before they reached stable states.SP improved the surface yielding strength and microhardness by 21% and more than 30%,respectively.Fracture morphology showed that the dimple size diminished in the deformation layer.Along with the increasing facets implied the transformation of fracture feature from ductile to brittle.It meant that SP improved the strength,to some extent,at the cost of ductility.CRS and microstructure were two main strengthening mechanisms introduced by SP.CRS can effectively reduce the mean stress of NAB when it suffering the applied loading,and improve the allowable alternating loading.This can push the fatigue crack initiation towards the interior of bulk material,and thus improve its fatigue strength.Ultrafine domains(or grains)and soared dislocation resulted in the grain size strengthening and dislocation strengthening,which imparted surface layer the enhanced microhardness and yield strength.