Preparation And Property Research Of Twin-wire Arc Spraying Ni-Al Coating

AbstractIn this dissertation, properties of twin-wire arc spraying(TWAS) used for preparing Ni-Al coating were systematically investigated, and the TWAS process was studied according to the service characteristics of large ship jet blast deflector. Using VOF dual-phase flow model and the standard k-ε model, theoretical model and numerical method were finally established to calculate the droplet deformation, breakup and solidification process in air flow. The deformation and breakup was analyzed under different diameters of the droplets and different atomizing gas pressure, and the effect of Weber number on the droplet breakup process. Meanwhile, different velocity of droplets impinging on the substrate deformation and temperature field in the solidification process were analyzed in detail, and revealing gas flight dynamics law of the TWAS particle velocity. A variety of structural coatings were prepared from the coating preparation method, technology, coating structure and coating materials etc. Relationship between the different spraying parameters and particle size was analyzed. The performance of bonding strength, friction coefficient and wear resistance was preliminarily analyzed. The correlation between coating structures and micro-hardness, bonding strength, impact resistance, high temperature resistance and corrosion resistance were also studied. Some properties including micro structure, surface roughness and wear resistance were further obtained after obtaining optimal Ni-Al composite coating technique. Studying for the effect of time and temperature of heat treatment on the phase structure and microstructure for Ni-Al coating through different heat treatment process, we can discuss the reaction mechanism of the coating/substrate interface of aluminum alloy, and analyzed the diffusion behavior of the interface element between the substrate and the coating. Finally we can obtain the following results:A series of coating structures including nine types were prepared by using TWAS spraying and flame spraying on the substrate of 6061-T6 aluminum alloy, including Ni-5wt.%Al, Ni-20 wt.%Al and Ni-30 wt.%Al single layer structure, Ni-5wt.%Al//Ni-5wt.%Al, Ni-5wt.%Al//Ni-20 wt.%Al, Ni-5wt.%Al(Arc)//Al packaging Al2O3(Arc) and Ni-5wt.%Al(Arc)//Al packaging Al2O3(Flame) double layer structures, Ni-5wt.%Al//Zn//Ni-5wt.%Al and Ni-5wt.%Al//Zn//Ni-20 wt.%Al sandwich structures, and we conducted the comparably study for bonding strength, high temperature resistance, erosion resistance, thermal shock resistance, accelerate corrosion and chemical solution tolerance and salt fog corrosion etc, and drew the following conclusion that comprehensive performance of Ni-5wt.%Al//Ni-20 wt.%Al composite coating was the best, so it was used to prepare Ni-Al coating in practice.TWAS droplet breakup form was explosive or mixing form of first breakup and second breakup. Weber number and the atomization gas pressure was approximately linear relationship. At a certain range, increasing the spraying atomization air pressure then increase the droplet velocity, droplet atomization effect was also improved. Droplet solidification layer form and the change of temperature field was uniform. The range of cooling velocity for droplet was around 3.1×107~7.6×107K/s. Through TWAS gas flight dynamics calculated, the calculated and analyzed results showed that the initial acceleration of TWAS particles was rapid in the flame flow, and then gently decreased. As the flight distance accelerated spraying particle was 200 mm, and the particle size range was 5μm~50μm, spraying particles flying velocity could only achieve the spraying flame flow rate of 15%~45%.Main phase of the Ni-5wt.%Al coating was composed of Ni solid solution, in addition to a small amount of Al2O3, Ni3Al4 and Ni O. Main phase of the Ni-20 wt.%Al coating was composed of Ni solid solution, Ni Al and Ni3 Al. And Ni Al and Ni3 Al intermetallic compounds were beneficial for improving the wear property. TEM analysis showed that fined spherical Ni Al3 precipitates and Ni, Al and O amorphous phase were formed in the matrix of Ni solid solutionin in the original state. Ni-20 wt.%Al coating has high bonding strength, hardness, roughness and larger static coefficient friction. The adhesion wear played a vital role in the wear mechanics of Ni-20 wt.%Al coating. Wet friction coefficient was higher than dry friction coefficient after 200 cycles. Surface morphology of Ni-Al coating was observed by using laser scanning confocal microscope. As spraying distance and spraying air pressure decreases, then the surface roughness increases, while as spraying distance decreases, effect of spraying air pressure was weaken.The change of Ni-Al coating of phase composition after 400℃~550℃/4h~48h heat treatment exhibits small. With the increase of annealing temperature and time, the interface diffusion area between the Ni-Al coating and the substrate gradually expands with the formation of Ni Al3 and Ni2Al3 phases, and was effectively controlled by diffusion of aluminum atoms according to the theory of Miedema model. TEM analysis showed that Ni Al3 and Ni2Al3 phases were formed in the matrix after heat treatment. It was the reason that the spraying particles were cooled by the fast solidificaton. Ni Annealing twinning was formed after heat treatment. High temperature diffusion model was established according to S. Arrhenius formula. The grain growth exponent n of diffusion kinetics of the Ni-Al coating, calculated via a high-temperature diffusion model at 400, 480, and 550°C, was between 0.25 and 0.35. This means that it satisfies the cubic law, which was consistent with the general x2∝t1-n theoretical relationship of high-temperature diffusion.Under specific conditions, higher spraying pressure was flexible to produce smaller spraying particles, it also is suitable as a base layer of Ni-Al composite coatings, and lower spraying pressure was relatively to produce relatively larger spraying particles, which is more suitable as the surface layer coating in practice.