Preparation And Properties Research Of Nickel-based Alloy Coatings By Laser Cladding On Nickel-aluminum Bronze

Nickel aluminum bronze（NAB）alloys become the mainstream material for marine propellers due to their excellent corrosion resistance and good fatigue resistance.Hover,with the increase of navigation demand and the deterioration of service environment,the performance of the propeller should be further improved.The purpose of this study is to improve the corrosion,cavitation-corrosion and fatigue resistance of nickel-aluminum bronze alloys,thereby extending the service life of marine propellers.In this research,the NAB alloys were heat-treated firstly,in order to improve the comprehensive mechanical properties of alloys.Microstructure of the alloys were controlled by different heat treatment methods,and their electrochemical corrosion resistance and anti-fatigue crack growth rate were analyzed to optimize the heat treatment process.The results showed that annealing caused the transformation ofβ’intoɑandκphases,which remarkably improved plasticity and corrosion resistance of the alloy,while its strength degraded slightly.The aged sample demonstrated the highest tensile strength,but its elongation and corrosion resistance diminished greatly.Alloys normalized properly could posse good combinations of strength and toughness,and their noncorrodibility kept good.Fatigue cracks in as-cast sample preferred to propagate throughαandκinterface.However,fatigue cracks in normalized alloy mainly propagated through the ductileαgrain,and had the lowest crack growth rate.For the aged sample,crack extended throughαandκinterface,as well as in theαgrain.This paper focuses on the application of several nickel-based alloy cladding layers on nickel-aluminum bronze,in order to further improve the corrosion resistance and fatigue performances of propeller blades.NiY coating was successfully prepared by laser cladding technique on the surface of nickel aluminum bronze alloy.The microstructure evolution was analyzed by x-ray diffraction,scanning electron microscope and transmission electron microscope,and also,the effect of rare earth element Y was investigated.Corrosion behavior of the cladding layer in 3.5%NaCl solution was evaluated by polarization curve and electrochemical impedance spectroscopy.The results showed that the top of the cladding layer was composed of uniform equiaxed grains,where there existed many subgrains.While the bottom of cladding layer was mainly composed of Cellular grains and rod-like precipitates,and Cellular grains contained a lot of dispersed nanoparticles.Compared with substrate,the corrosion potential of cladding layer was improved,and the corrosion current density was decreased by an order of magnitude.According to electrochemical impedance spectroscopy（EIS）data,the cladding layer formed a complete protective film on the surface,which enhanced its corrosion resistance.The microhardness through all the cladding was uniform,and the average value was two times more than that of substrate.Furthermore,rare earth element helped for deoxidizing and slagging,besides refining grains.In addition,NiCrFe cladding layer was prepared on the surface of nickel-aluminum bronze alloy.The results showed that the dilution rate reduced with the decrease of laser power,and the grain tended to refine.The cladding layer was mainly composed of Ni（Cr,Fe,Cu）,Co（Mn0.5Si0.5）,MoNi and Cr（Ni,Mo）,and the bonding zone consisted mainly of Cu-based solid solution and some precipitated phases.There was a large number of dispersed nanoparticles in the dendrites,thus showing a higher hardness,and the nanoparticles were speculated as NiCu phase by TEM analysis.The corrosion resistance of the cladding was significantly improved compared to the substrate,for a higher content of Ni and Cr.The average hardness of the cladding was more than three times that of the substrate,and the elastic recovery ratio was more than twice that of the substrate,which contribute to the improvement of cavitation corrosion resistance.The addition of Cu and Y could refine the dendrites of NiCrFe cladding,and reduced inhomogeneity of the microstructure.Moreover,it increased the wettability between the cladding and the substrate,thus improving the forming quality of the cladding.A higher content of Mo leaded to the existence of CrMo phase in the cladding,which further increased the hardness of the cladding to 750 HV_0.1.And also,Mo could effectively prevent the erosion of Cl^-,promoting Ni to form a dense protective film on the surface.