Study On Properties Of CuNi Alloy Coatings By Extreme High Speed Laser Cladding

Copper-nickel alloy has excellent corrosion resistance,processing performance and anti-biological antibacterial properties.It has a wide range of applications in the marine environment which is commonly used in ship pipe joints,flange valves,propellers and propeller shafts.The preparation of copper-nickel alloy coating on the surface of a low corrosion resistance workpiece can effectively improve the service time of the workpiece in the marine environment and broaden the application conditions of the workpiece.As a new type of advanced surface modification technology,extreme-high speed laser cladding has the characteristics of fast cladding speed,extremely low dilution rate,high coating surface smoothness,fine microstructure and high cladding efficiency.It has a wide application prospect in the field of coating surface modification.However,there are few studies on the preparation of copper-nickel alloy coatings by extreme-high speed laser cladding（EHLA）.Therefore,it is of great significance to explore the relationship between the preparation process of copper-nickel alloy coatings by extreme-high speed laser cladding and the microstructure of the coatings,as well as the corrosion resistance and corrosion mechanism of copper-nickel alloy coatings by extreme-high speed laser cladding.In this study,the extreme-high speed laser cladding combined with laser remelting technology was used.Firstly,the mechanically mixed Cu and Ni powders were used as cladding powders to clad on the Q355 steel.The preparation process of copper-nickel alloy coating and the influence of remelting speed on the solidification and crystallization behavior of the molten pool was studied by the finite element model.At the same time,the surface morphology,microstructure and corrosion resistance of the coating were tested and analyzed.Then,the copper-nickel alloy coating was prepared on the Q355 steel with copper-nickel alloy powder（adding different contents of h-BN）.The effects of h-BN on the microstructure and corrosion resistance of the coating were systematically studied.The properties of the passivation film were tested and analyzed by SEM and electrochemical methods,and the failure mechanism of coating passivation film was clarified.Finally,the corrosion test of copper-nickel alloy coating under different hydrostatic pressures was carried out by using a simulated deep-sea device,and the influence of hydrostatic pressure on the corrosion behavior of copper-nickel alloy coating was studied.The research results of this study were as follows:（1）In the process of remelting,with the decrease of remelting speed,the heat input in the molten pool increases,the uniformity of element distribution in the coating is improved,and the microcracks and pores in the EHLA coating are eliminated.The microstructure of different areas of the coating is equiaxed crystals,columnar crystals and planar crystals from top to bottom,and the transformation from columnar crystals to equiaxed crystals is observed between the columnar crystals area and equiaxed crystals area.A uniform and dense single-phase Cu Ni alloy coating can be obtained at a low remelting rate of 5 m/min,at which the coating has the highest microhardness(150 HV_0.2)and the best corrosion resistance.The pre-passivation treatment can further improve the corrosion resistance of the coating.（2）When the content of h-BN was 1 wt%,the Cu Ni alloy coating obtained the highest corrosion potential（-177.0 m V）and the lowest corrosion current density(63.00 n Acm^-2).The Rct value of the coating was increased from 329.2 kΩ/cm² to 2038.0 kΩ/cm²,and the corrosion resistance of the coating was improved.The passive film on the surface of Cu Ni alloy coating shows typical n-type semiconductor characteristics.After adding h-BN,the point defect concentration and vacancy diffusion coefficient of the passive film on the coating surface are relatively small,respectively 3.82×10¹⁹ cm^-3 and 1.8590×10^-16 cm²s^-1,resulting in the overall reduction of oxygen ion vacancy diffusion rate at the interface between the passive film and the coating.After the addition of h-BN,the microstructure of the coating changes,increasing active sites in the nucleation and growth process of the passive film,which rapidly forms a dense and continuous passive film.At the same time,the ability to adsorption of anions on the surface of the passive film is reduced,as the corrosion resistance of the coating is improved.（3）The corrosion potential and current density of Cu Ni alloy coating in a simulated deep-sea environment are-0.236 V and 4.83μAcm^-2,better than-0.180 V and 1.69μAcm^-2.The charge transfer resistance of the coating is 196600Ω,which is lower than 24970Ωat atmospheric pressure.In the simulated deep-sea environment,the size of pitting pits on the coating surface after long-term immersion is larger than that under normal pressure.The surface of Cu Ni alloy is mainly composed of.The results of the cyclic immersion test show that the corrosion resistance of Cu Ni alloy coating at the initial protection stage is lower than that at atmospheric pressure in the simulated deep-sea environment.