Study On Microstructure And Properties Of Laser Cladding Ni60-WC Composite Coating On 17-4PH Martensitic Stainless Steel Surface

The extreme conditions such as high temperature,high pressure,high load and corrosive medium put forward higher requirements for the wear and corrosion resistance of the coating,but the pure nickel-based self-fluxing alloys can no longer meet the needs of the industry.The preparation of composite coatings by adding high melting point hard compounds such as borides,oxides,and carbides into self-fluxing alloy powders has become a hot spot in the research and development of laser cladding.As a hard material with excellent wear resistance,WC has good chemical stability at room temperature.In thermal spraying and laser cladding,WC is often used as a hard reinforcing phase in the coating,which can significantly improve the oxidation resistance,wear resistance,and corrosion resistance of the workpiece surface.Aiming at the surface modification of ball valve material 17-4PH martensitic stainless steel,the microstructure and properties of Ni60+WC composite coating were studied by laser cladding technology.The effects of WC with different morphologies and different scales on the microstructure,microhardness,friction and wear,corrosion resistance,and other important performance indexes of the composite coating were revealed.The dissolution mechanism and action mechanism of WC particles in the composite coating were clarified,which provided the theoretical basis and experimental basis for the laser cladding preparation of Ni60+WC composite coating.The main contents include the following aspects.Firstly,the microstructure of spherical and irregular WC and Ni60 laser cladding composite coatings was studied.The metallographic results show that the growth law of the microstructure of laser cladding Ni60,Ni60+30wt.%irregular WC and Ni60+30wt.%spherical WC coatings from the fusion line to the top of the cladding coating is:plane crystal,columnar dendrite,and equiaxed crystal.XRD and TEM results show that the coating is mainly composed ofγ-Ni,eutectic structure,Cr B,M₂₃C₆,and unmelted WC.The eutectic structure is uniformly distributed aroundγ-Ni in lamellar form.The carbides M₂₃C₆are distributed in block and needle shapes in Ni60+30 wt.%irregular WC coating and Ni60+30 wt.%spherical WC coating,respectively.At the same time,it was found by scanning electron microscopy that the grains around the undecomposed WC in the coating tend to be equiaxed,indicating that WC can be used as a nucleation site forγ-Ni and M₂₃C₆to refine the structure.Secondly,the microhardness,friction,wear,and corrosion properties of WC/Ni60composite coatings with different morphologies were studied.It was found that the average microhardness of the nickel-based coating with 30 wt.%spherical WC reached 622.27HV_0.2,which was 126.63 HV_0.2 higher than that of the Ni60 coating and increased by25.50%.The average microhardness of the cladding layer with 30 wt.%irregular WC reached 545.34 HV_0.2,which was 49.70 HV_0.2 higher than that of the Ni60 coating and increased by 10.00%.Under the same wear conditions,the volume wear rate of Ni60coating is 8.31E-6 mm³/（N·m）,and the wear mechanism is mainly abrasive wear.The volume wear of Ni60+30 wt.%irregular WC coating is 6.20E-6 mm³/（N·m）due to the formation of carbides and the presence of undecomposed irregular WC,and the wear mechanism is mainly abrasive wear.The wear resistance of Ni60+30 wt.%spherical WC coating is further improved due to the formation of more carbides,and the volume wear is3.59E-6 mm³/（N·m）.The wear mechanism is mainly adhesive wear and abrasive wear.Through electrochemical impedance and polarization curves and fitting results,it is considered that the addition of WC can improve the corrosion resistance of the coating.To further study the performance of the coating,different contents of nano-WC were added to Ni60 powder particles for the cladding test.It was found that the addition of nano-WC not only promoted the fine-grain strengthening of the coating but also increased the content of carbides in the coating.The hardness of the coating increases with the increase of nano-WC content,which is related to the large number of carbides in the coating.At the same time,the increase of nano-WC content also improves the wear resistance of the coating.The wear mechanism of the coating changes from adhesive wear to abrasive wear.When the nano-WC content is 20 wt.%,the coating is fatigue wear.The corrosion resistance experiment of the coating shows that the corrosion resistance of the coating increases with the increase of nano WC content.