Study On Shot Peening Of Nickel-aluminum Bronze And Its Corrosion Resistance To Seawater

Nickel-aluminum bronze?NAB?is a multi-phase alloy mainly composed of Cu and added with Ni,Fe,Mn,Al and other elements.It has been extensively used in marine installations duo to it attractive combination of excellent mechanical properties and corrosion performance,becomes the mainstream material for marine propellers.However,the as-cast nickel-aluminum bronze alloys are susceptible to corrosion failure and fatigue failure due to coarse grains,uneven distribution of phases,and non-uniform composition,and are difficult to adapt to the service conditions of modern high-powered propellers.Corrosion failures and fatigue failures often originate from the alloy surface.Therefore,surface modification and strengthening is a necessary way to prepare high-power propeller materials.This article aims to enhance the mechanical properties and corrosion resistance of nickel-aluminum bronze alloys by mechanical shot peening?SP?and laser peening?LSP?in order to improve the service life of the nickel-aluminum bronze propellers.The surface modification of nickel-aluminum bronze alloy was performed by mechanical shot peening and laser peening.The influence of shot peening on the surface morphology and surface structure of the alloy was studied,and the mechanism of its transformation was analyzed.The results show that SP and LSP can strengthen the superficial microstructure of the alloy,including grain refinement,dislocation increment,and surface distortion.In addition,two kinds of shot peening methods can generate residual compressive stress field on the surface of nickel-aluminum bronze alloy.The laser peening treatment has the characteristics of small energy input and concentration,so that the residual compressive stress field depth of the LSP samples surface are higher than that of SP samples.During shot peening energy,when the input energy within a certain range,the degree of tissue strengthening and the depth of the residual compressive stress layer increase with the increase of energy input during shot peening,and when the input energy is too large,mechanical peening and laser peening will make the alloy micro cracks develop on the surface.Shot peening produces a structural strengthening and residual compressive stress field on the surface of the alloy,which changes the mechanical properties of the alloy.In this paper,the influence of shot peening on the mechanical properties of nickel-aluminum bronze alloy was evaluated by measuring the microhardness,tensile strength,plasticity and fatigue properties of the samples before and after shot peening.The results show that shot peening can increase the hardness of the nickel-aluminum bronze alloy surface,which is due to the combination of grain refinement,dislocation growth and residual stress.The ability of laser peening to obtain a deeper hardened layer relative to mechanical shot peening is mainly due to its deeper residual compressive stress layer.Under appropriate energy input conditions,mechanical shot peening can improve the tensile strength of the material,but the plasticity slightly decreases;laser peening can make the tensile strength and plasticity of the material improved;when the energy is too large,the mechanical properties of the material are significant.decline.Shot peening can suppress the initiation of fatigue cracks and slow the rate of crack propagation,thereby improving the fatigue properties of the material.The improvement of fatigue performance is mainly attributed to the fact that the residual compressive stress reduces the actual force of the material,and the presence of residual compressive stress reduces the effective stress ratio so that the crack propagation slows down.Under the appropriate shot peening parameters,shot peening can not only improve the comprehensive mechanical properties of the alloy,but also improve the corrosion performance of the alloy.In this paper,the corrosion susceptibility before and after shot peening was analyzed by atomic force microscope.The corrosion resistance of the alloy before and after shot peening was evaluated by electrochemical test,immersion corrosion test and stress corrosion test.Before shot peening,the surface potential of the sample was unevenly distributed,the surface potentials of?_I and?_II were the highest,the surface potentials of?_III and?_IVV were the second,and the surface potential of?phase was the lowest.This was due to the different chemical composition of each phase.After shot peening,the potential of each phase is still different,but the overall potential difference is small,only-0.06V⁺0.07V?less than-0.26V⁺0.30V?,reducing the corrosion sensitivity of the sample,making the material Selective corrosion tends to decrease,improving the electrochemical corrosion performance and soaking corrosion performance of the alloy.The shot peening treatment can enhance the stress corrosion resistance of the alloy.The pre-peening crack originates from the tip region of the sample surface and tends to expand along the?+?_IIIII phase boundary.The crack propagation rate is larger;the crack originates from the strengthening after shot peening.In the inner region of the layer,cracks originate from multiple locations,and the crack propagation path is tortuous and generates more secondary cracks.The improvement of the stress corrosion resistance is mainly due to the fact that the shot peening can reduce the tendency of the material to select the phase corrosion,the closing effect of the residual compressive stress on the crack tip and the high density of dislocations make the crack propagation rate low.