| The superalloy 4169 is a niobium-added precipitation hardened Nickel-chromium-iron based alloy. It has high yield and tensile strength, high fatigue and corrosion resistance, high oxidation and radiation resistance, good processability and weldability. It can be made into plates, rings, blades, shafts, fasteners, elastic components, sheet metal structural parts, casing and other components in aerospace vehicles on long term use.Superalloy 4169 coatings prepared by laser cladding technique present the niobium segregation which results in Laves phases. The Laves phases are harmful to the microstructure and properties of the alloy coatings, therefore, limit the application of laser cladding in the remanufacturing. This paper studies the effect of laser power and substrate temperature on the niobium segregation in laser cladding, and the effect of niobium segregation on the microstructure and properties of the coatings.The microstructure observation indicates that superalloy 4169 showing characteristics of epitaxial growth in the solidification of laser cladding. The alloy coating is mainly composed of slender columnar dendrite which varying the growth direction. The microstructure is very fine due to the rapid melting of laser with high gradient and high speed solidification characteristics.DTA and Phase analysis suggests that the solidification process of the superalloy 4169 can be divided into three steps, which are primary solidification Lâ†'γ, eutectic-type Lâ†'(γ+NbC) reaction and termination of solidification by second eutectic-type Lâ†'(γ+Laves) reaction.The analysis of the effect of the laser power on the microstructure show that cracks are present in the 5KW laser cladding coatings but absent in the 1.5KW laser cladding coatings. The cooling rate is higher in 1.5KW laser cladding coating than 5KW laser cladding coating. The segregation of niobium is lower and the Laves phase is less in the 1.5KW laser cladding coating than in the 5KW laser cladding coating.The study of the effect of liquid nitrogen cooling treatment on the microstructure shows that the cooling treatment can increase the cooling rate in the laser cladding. The cooling rate in the solidification is 3124±517℃/s with liquid nitrogen cooling treatment and 941±122℃/s without liquid nitrogen cooling treatment. Liquid nitrogen cooling treatment can inhibit the segregation of the Nb in the laser cladding, subsequently make the size and volume of the Laves phase smaller in the superalloy 4169 coating.The analysis of the heat treatment indicates that heat treatment can increase the hardness of the superalloy 4169 coatings significantly. The hardness of the coatings fabricated in lower laser power is higher after heat treatment than in higher laser power (5KW: 400HV, 1.5KW: 460HV). The hardness of the coatings fabricated with liquid nitrogen cooling treatment is higher after heat treatment than without cooling treatment (without cooling: 460HV, without cooling: 482HV). |
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