Research On Responding Mechanism Of Microstructures And Fatigue Properties Of Friction Stir Processed Nickel Aluminum Bronze Alloys

The Nickel-aluminum bronze（NAB）alloy is widely used for ship propeller,pump and valves etc.due to the excellent combination of high strength and corrosion resistance.The cast NAB alloy presents complex microstructures and the defects such as coarser grain,porosities and segregation are prone to appear due to their poor casting properties.As cast NAB alloy is easily subjected to selective phase corrosion,cavitation erosion and erosion-corrosion etc.in complex ocean condition.Thus,further improvement of corrosion resistance and mechanical properties of as cast NAB alloy lies in the core of producing marine equipment with high service performance.In order to obtain the microstructures with better corrosion resistance and fatigue properties,it is necessary to know the responding relation between microstructures and properties of NAB alloy.Therefore,in this study,we modified the microstructures of NAB alloy using friction stir processing（FSP）and tried to obtain responding relation between microstructures and properties of NAB alloy.During the process,we optimized their process parameters and investigated deeply microstructural evolution and strengthening mechanism of FSPed NAB alloy.Optimizing microstructures were also performed via post heat treatment and improving FSP tool.Finally,we investigated corrosion resistance,fatigue and corrosion fatigue properties of FSPed NAB alloy and revealed their formation mechanism.The main experimental results and conclusions were as follow:（1）During FSP,the microstructures are refined significantly and uniformly distributed,which obviously improve the mechanical properties of as cast NAB alloy.More martensite nano-twins are also formed due to high strain rate and peculiar martensite structure.The strengthening mechanisms of FSPed NAB alloy involve grain refinement,secondary phases,solid solutions,dislocations and nanotwin strengthening.The microstructures of FSPed NAB alloy depends on process parameters.Cavity defects are prone to form in retreating side due to inadequate fill of flowing material when the heat input during FSP is low.（2）Theβˊphase in FSPed NAB alloy can be eliminated via post heat treatment and more coherent twin boundaries are formed,which is beneficial to corrosion resistance of FSPed NAB alloy.However,the microhardness of FSPed NAB alloy also decreases significantly during post heat treatment.The grain size of post heat treated NAB alloy gradually increases with increase of rotation rate.Highest microhardness value is obtained at the rotation rate of 800 rpm.Uniformly distributed secondary phases formed during FSP make a main contribution to its higher microhardness.（3）Decreasing shoulder diameter and increasing pin length can decrease the heat input during FSP,thus decrease significantly the content ofβˊphase in FSPed NAB alloy.The microstructures in below area of stir zone obtained using improved FSP tool mainly contain fine equiaxedα,few discontinuousβˊand fineκphases.The microstructures in the area has both fewβˊphase and high microhardness,which is optimal microstructures of NAB alloy.The peak temperature,stress rate and calculated Zener-Holloman parameters of the microstructures are⁸00?C,²7.9s^-1 and²5.3,respectively.（4）FSPed NAB alloy obtained by original FSP tool has inhomogeneous microstructures from top surface to the bottom that can be divided into three subregions:Widmanstattenα,fineκ_iv andβˊphases subregion,bandedαcolonies,fineκ_iv,andβˊphases subregion,and stream-likeαcolonies,fineκ_iv andβˊphases subregion.The equiaxedαand fineκphases could improve FCG resistance of FSPed NAB alloy.Whileβˊphase is detrimental to its FCG resistance.Compared to forged matrix alloy,FSPed NAB alloy exhibits a better FCG resistance only at highΔK levels,while matrix alloy has lower FCGR at lowΔK-levels.At lowΔK-levels,the fatigue crack deflection effect caused by coarserκ_ii particles in the matrix alloy makes a main contribution to its better fatigue crack growth resistance.With increasingΔK,aforementioned crack deflection effect gradually diminishes.In this situation,the intrinsic fatigue crack propagation resistance（ductility）play more role in the FCG resistance.The alloy obtained by improved tool has a better fatigue properties at both low and highΔK.This is because equiaxedαand fineκphases formed by improved tool could also deflect main crack,which improve FCG resistance of FSPed NAB alloy.In this situation,FCG resistance of FSPed NAB depend on its ductility.（5）As cast NAB alloy is subjected to severe local corrosion and selective corrosion.The most serious damage locates in theαphase of eutectoidα+κ_Ⅲstructure,then inβˊphase region,whereasκ_iii phase is remained.The FSPed NAB alloy is corrosed uniformly.Well protective film is formed rapidly on FSPed NAB alloy surface,which improve significantly its corrosion resistance property.Increasing rotation rate or decreasing advancing velocity can also improve the corrosion resistance property of FSPed NAB alloy.（6）During corrosion fatigue,βˊphase in FSPed NAB alloy is damaged severely due to the occurrence of dealuminium reaction.At crack tip,due to the coupling of acidification of the crevice and stress field,the protective film formed onαandβˊphases could be unstable,dissoluted or damaged.In this situation,βˊphase was corrosion damaged as anode during electrochemical corrosion.The microstructures obtained via improved FSP tool contains few discontinuousβˊand moreκphase.In this situation,κphase was corrosion damaged as anode during electrochemical corrosion.After improving FSP tool,the corrosion fatigue mechanism change from large areaβˊdamage to fineκphase damage,which improve the corrosion fatigue property of FSPed NAB alloy.