Research On The Microstructure And Corrosion Resistance Of Ni-Based Alloy Coatings By Laser Cladding On The Soft Steel

Ni-based self-fluxing alloy coatings on the surface of the soft steel (Q235) were prepared by three different process methods, which include thermal spray pre-coated coating +laser remelting, adhesive pre-coated coating + laser remelting and synchronous feeding powder laser cladding.The microstructure, element distribution, phase composition, microhardness and corrosion resistance of the cladding coatings have been measured and analyzed with optical microscope(OM), scanning electronic microscope(SEM), energy disperse spectroscopy(EDS), X-ray diffraction(XRD), microhardness-tester and electrochemical workstation.The comparative study on the laser cladding coatings and the oxygen-acetylene flame remelted coating in terms of microstructures, element distribution on section, phases, microhardness and corrosion resistance also has been carried out.The results show that there are different microstructures for different zones in the laser cladding layer due to different solidification mechanisms. At the bonding zone of the cladding layer and the substrate are plane crystals; at the bottom and the middle of the cladding layer are the cellular crystals and the dendrites perpendicular to the bonding surface; at the top of the cladding layer are the dendrites growing at various directions. The crystal grains gradually become finer and finer from the bottom to the top of the layer. The phases of laser cladding coatings are mainlyγ-(Ni,Fe), Ni₃B, Cr₇C₃, CrB and so on.The hardness distribution curve measured from the cladding layer to the substrate exhibits the form of ladder. The corrosion tests show that Ni-based alloy layers prepared have reached standard II. That is to say, the coatings provide good corrosion resistance.The influences of laser power and laser scanning speed on the microstructure, hardness and corrosion resistance are researched. As laser power increases, the grains would gradually become coarser, the hardness increases firstly and then reduces with the increase of laser power, and the corrosion rate decreases firstly and then increases. In this test the optimum corrosion resistance of the cladding layer can be obtained by use of laser power of 2.5kW. The grains would become finer and microstructure would become compact with increasing laser scanning speed. Both the hardness and corrosion resistance are improved. When laser scanning speed is 6mm/s, the optimum microstructure and properties are obtained.While in the oxygen-acetylene flame spray coating the microstructure shows uniformity in general, and the coarse grains display non-directional growth. The plane crystal at the interface is discontinuous. There is much larger the heat affected zone. In addition, phase analysis indicated that oxygen in the oxides of Cr₂O₃, NiCr₂O₄, Fe₂O₃ and so on, is very rich in the oxygen-acetylene flame spray coating.