Investigation On Microstructure Characteristics, High Temperature Oxidation And Corrosion Properties Of Laser Cladding Cryomilled NiCrAlY Coatings

Compared to plasma spraying, electron beam-physical vapor deposition (EB-PVD), laser cladding can produce NiCrAlY coatings with low dilution, metallurgical bonding to substrate, fine microstructure, porosity-free and crack-free, and excellent high temperature oxidation resistance. Therefore, laser cladding technique has a promising application in surface strengthening and repairing the hot end of gas turbines. At present, most of NiCrAlY powder used by laser cladding is only suitable for thermal spraying techniques (plasma spraying and HVOF). However, laser cladding technique has some inherent characteristics such as rapid heating and rapid solidification, which can lead to crack formation in the cladding layer and the oxidation resistance of NiCrAlY coatings need to be improved further. In order to obtain NiCrAlY powder suitable for the characteristics of laser cladding and overcome the cracks of cladding layer, the process of high energy ball milling was adopted to produce NiCrAlY powder and the corresponding NiCrAlY coatings were produced by laser cladding. The particle size and morphology of cryomilled NiCrAlY powder were analyzed. Moreover, the microstructure, high temperature oxidation and corrosion properties of laser cladding cryomilled NiCrAlY coatings were investigated. The main results obtained were described as follows:With increasing the milling time, the particle size of NiCrAlY powder first decreased and then increased, the residual Al content in NiCrAlY powder decreased gradually and the corresponding endothermic peak area in DTA curve also decreased, indicating that the degree of mechanical alloying increased. Therefore, the semi-quantitative formula on evaluating the degree of mechanical alloying for cryomilling NiCrAlY powder was put forward as follows:D=S0Si/S0×100%. The Gibbs energy of the Ni-Al system was also calculated, the results showed that Ni3Al phase can be formed preferentially and then followed by NiAl phase, which agreed with the results of X-ray diffraction analysis of cryomilled NiCrAlY powder. Moreover, the process model of mechanical alloying NiCrAlY powder was also proposed.For laser cladding non-cryomilled NiCrAlY coating, the planar growth, cellular crystals, dendrites and equaixed crystals were observed in the interface of coating-substrate. When NiCrAlY power was cryomilled, NiCrAlY coatings prepared by laser cladding had no porosities and were free of cracks, whose microstructure is mainly composed of planar growth and columnar dendrites and a small amount ofequiaxed crystals. When the cryomilling time was25h, NiCrAlY powder had a100%degree of mechanical alloying, crystal dislocation、vacancy increased to a maximum andcrystals is multifaced, in the process of laser cladding the laser refracted. As a result, thelaser absorption of cryomilled NiCrAlY powder increased, the temperature of moltenpool also increased and the temperature gradient increased, which could coarsen thegrain size and retard the columnar to equiaxed transition (CET) and in turn the morecolumnar dendrites could obtain.Compared to the polycrystalline structure in laser cladding non-cryomilledNiCrAlY coating, the columnar dendrites in laser cladding cryomilled NiCrAlYcoatings can reduce the diffusion pathways of alloy elements such as Al, Cr and Niduring high temperature oxidation. Therefore, when both NiCr5AlY and NiCr10AlYpowders were cryomilled and obtained a100%degree of mechanical alloying, thecorresponding coatings had the highest oxidation resistance, showing that thecryomilling processs can improve the oxidation resistance of laser cladding NiCrAlYcoatings. In addition, the diffusion model of laser cladding NiCrAlY coatings duringhigh-temperature oxidation was established and the oxide products were formed orderlyaccording to the sequence θ-Al2O3�'α-Al2O3, Cr2O3and spinel structure NiCr2O4.The electrochemical experiments were performed to evaluate the electrochemicalresistance of laser cladding NiCr5AlY and NiCr10AlY coatings. The results showedthat the electrochemical resistance of laser cladding cryomilled NiCrAlY coating wasworse than that of laser cladding non-cryomilled NiCrAlY coating. Furthermore, theelectrochemical resistance of laser cladding cryomilled and non-cryomilled NiCrAlYcoatings was also worse than that of GH4169and304stainless steel. Therefore, thecryomilled process could not improve the electrochemical resistance of laser claddingNiCrAlY coatings. The tensile mechanical experiments were performed to evaluate thebonding strength of laser cladding NiCrAlY coatings. The results showed that lasercladding NiCrAlY coatings took on the brittle fracture during tensile, Al content couldnot change their tensile properties and that the Y addition can improve the tensilestrength.