Research On Strengthening Technology And Cavitation Behavior Of Nickel Aluminum Bronze

Nickel aluminum(Ni-Al)bronze has been widely used in marine equipment such as deep-sea locators and ship propellers due to its excellent mechanical properties and corrosion resistance.Its machining surface integrity and cavitation resistance are fundamental research issues.In the process of high-performance manufacturing of Ni-Al bronze propellers,the machining surface integrity of the complex curved surface directly affects the service life and propulsion efficiency of the propellers.Due to the variety of alloy elements and inhomogeneous microstructure,selective corrosion and cavitation corrosion easily occur under marine service conditions.In particular,cavitation failure of propellers often causes tremendous damage to ships and locators.Therefore,the relationship between Ni-Al bronze’s machining surface integrity and high-performance manufacturing and the strengthening mechanism of anti-cavitation are studied in the dissertation,which would provide theoretical support for the regulation of Ni-Al bronze’s machining surface integrity and anti-cavitation performance.The main results are as follows:(1)The relationship between the flow stress and the strain during the high strain rates and temperature deformation has been studied.Based on the stress-train curves under various strain rates and deformation temperatures,the Johson-Cook constitutive model of Ni-Al bronze with constant variables has been obtained.The Johson-Cook constitutive model with coefficient as a variable is established by modifying the strain rate strengthening factor and thermal softening coefficient,and the accuracy of the modified model is verified by error analysis.Combined with dislocation theory and the change of microstructure,the strengthening mechanism of high strain rate during plastic deformation of Ni-Al Bronze is revealed,and the thermal softening mechanism of high-temperature deformation is expounded.At 298～473K,the primary softening mechanism is the dynamic recovery,while at 673～1073K,the softening mechanism is the dynamic recrystallization.Combined with the modified Johson-Cook constitutive model,the coupling mechanism of strain rate and temperature on flow stress has been established.This study provides a theoretical basis for manufacturing the Ni-Al bronze under high temperatures and strain rates.(2)Study on cavitation corrosion resistance of Ni-Al Bronze strengthened by quenching and aging heat treatment.The quenching and aging heat treatment tests of nickel aluminum bronze were carried out at different quenching and holding temperatures.Based on the microstructure characteristics of the samples at different heat treatment temperatures,the microstructure evolution law of Ni-Al Bronze during quenching and aging treatment is expounded.The electrochemical and cavitation corrosion behaviors of heat-treated samples in 3.5% Na Cl solution were characterized by polarization curves and cavitation corrosion mass loss.Based on the evolution of microstructure,mechanical behavior,and cavitation corrosion behavior,the composite strengthening mechanism of quenching and aging treatment on cavitation corrosion resistance of Ni-Al Bronze was established.(3)The effect of shot peening on the surface integrity and cavitation behavior of Ni-Al bronze has been investigated.The surface strengthening layers with different depths are obtained on the Ni-Al bronze by adjusting the input currents and shot peening intensity.Based on the corrosion surface morphology and polarization curve parameters,peeningtreated samples’ surface roughness increases,and the electrochemical corrosion performance decreases.Combined with the microhardness distribution of the strengthened layers and the cavitation performance,the microstructure evolution model of shot peening has been established.The synergistic strengthening effect of refinement and dislocation strengthening counteracted the increasing corrosion effect of surface roughness,where the strengthening mechanism is dominant,leading to improved cavitation resistance of Ni-Al bronze.(4)The corresponding relationship between laser energy regulation and the microstructure and cavitation behavior of Ni-Al bronze has been investigated.The strengthening layers of the homogeneous martensitic phase are obtained by adjusting the laser power and scanning speed.Based on the microstructure analysis under micron,submicron and nanoscale,a three-level hierarchical martensite structure composed of the packet,block,and laths has been revealed.Meanwhile,nano precipitates with amorphous transition zone are dispersed between the laths.Based on the structure,corrosion,and cavitation behavior characteristics,the mechanism of laser energy strengthening under optimized parameters is explored.The strengthening mechanism of laser surface melting on cavitation resistance is clarified.The composite strengthening mechanism of grain refinement strengthening,dislocation strengthening,and dispersion strengthening has been established.(5)Laser cladding is carried out to prepare the Cobalt-based alloy(Stellite6)coating,Ni-Y alloy coating,and the composite coating on the surface of Ni-Al bronze.Based on the polarization curves,cavitation mass losses,and the corrosion morphology characteristics,combined with the microstructure and hardness distribution of the laser cladding coatings,the strengthening mechanism of the composite cladding coating Stellite6 + Ni Y is revealed.The composite strengthening mechanism of solid solution strengthening,grain refinement,and uniform corrosion improves the surface strength and corrosion resistance of Ni-Al bronze and further enhances the cavitation resistance.This paper clarifies the strengthening mechanism and microstructure evolution of NiAl bronze during plastic deformation under high temperatures and strain rates by studying the rheological,mechanical behavior.The cavitation resistance of Ni-Al bronze has been enhanced by four technologies,i.e.,integral heat treatment,shot peening,laser surface melting,and laser cladding.The effects and practical application possibilities of the four strengthening methods have been compared.The results show that laser surface melting and laser cladding have a more significant improvement in the cavitation resistance.In contrast,the application cost of laser melting is lower,and the application prospect is more promising.