Study On The Fabrication And Cavitation Erosion Re-Sistance Of Ti-Ni Intermetallic Coating

Cavitation erosion mainly occurs in hydrodynamic systems. The damage of cavitation could reduce the working efficiency and the working life of these components, leading to huge economic losses. The application of materials with excellent cavitation erosion re-sistance is the most effective method for reducing the damage of cavitation erosion. In recent years, TiNi alloys have drawn specific attention of the researchers due to excellent cavitation erosion resistance. However, the wide applications of TiNi alloys are restricted due to high material costs and poor workability. In this paper, the study on the fabrication of TiNi coating with excellent cavitation erosion resistance on the common materials in the open atmosphere was conducted by the author. The current work provides valuable scientific basis and ex-perimental support for the development and application of TiNi coatings.Low temperature high velocity oxy-fuel (LT-HVOF) spraying process with the plume of low temperature and high velocity was used to prepare Ti-Ni coatings. In this paper, TiNi pre-alloy powder and Ni-clad Ti powder were used respectively as raw materials to deposit Ti-Ni coatings by LT-HVOF. The deposition mechanisms and oxidation behaviors of the powders were investigated. The laser melting technology was applied to achieve the alloy-ing of Ti-Ni composite coatings on the different substrates. The analysis methods, such as SEM, TEM, XRD, EDS, microhardness tester, nano-indenter, were used to investigate the microstructure and the basic mechanics performance of the Ti-Ni coatings prepared by dif-ferent processes. Cavitation erosion resistance was examined using a standard ultrasonic cavitation test. The cavitation erosion behaviors and failure mechanisms of TiNi coatings were studied by means of mass loss, erosion morphology observation and micro-hardness et al.TiNi pre-alloy powder was deposited to prepare TiNi coating by LT-HOVF. Most of the particles were deposited onto the substrate in solid state. The severe deformation of the sprayed particles occurred in the process of deposition. The grain refinement at the high de-formed region of the sprayed particles was arisen from dynamic recrystallization of heavily deformed grain during deformation. While the temperature of the plume of LT-HVOF was still high, localized melting occurred on the surfaces of part of the particles, and a few of small particles were melting. The oxidation mainly occurred on the surface of the deposited particles, and the severe oxidation was presented in the region of melting. The as-sprayed coating had a stack structure of deformed particles. The obvious boundaries were observed between the deposited particles.A comparison of cavitation erosion resistance of LT-HVOF sprayed TiNi coating and LPPS sprayed TiNi coating was conducted in this work. LT-HVOF sprayed TiNi coating exhibited poorer cavitation erosion resistance than LPPS sprayed TiNi coating. The presence of oxides and particle boundaries in LT-HVOF sprayed TiNi coating severely impacted the cavitation erosion resistance. The damage of the cavitation erosion mainly occurred on the regions of oxide and particle boundaries. The main failure mechanism of cavitation erosion was the generation and extension of the cracks in the oxides and particle boundaries.In order to reduce the oxygen content of the coating, Ni-clad Ti powder with a certain sizes was used to prepare Ti-Ni coating by LT-HVOF. The powders were deposited in un-melted state, and the external Ni protected the internal Ti from oxidation. The coating pre-sented a stack structure of deformed particles without obvious lamellar structure. The as-sprayed coating kept the original state of powder without changes in chemical composition and phase transition. The oxygen content of the coating was0.65wt.%, slightly higher than the original powder.Laser melting technology was applied to achieve the alloying of the Ti-Ni composite coatings deposited on the316L stainless steel and nickel-aluminum bronze. The rapid fusion and solidification could effectively reduce the oxidation of the TiNi coating. Both of the two TiNi laser melting layer on the different substrate had dense and uniform structures without through crack. Both of the two TiNi coatings consisted of major TiNi matrix phase and mi-nor Ti2Ni dendrites phase. A few of oxides were presented in the TiNi coatings. The oxide in the TiNi coating prepared on the316L stainless steel had a loose and porous structure. The oxide in the TiNi coating prepared on nickel-aluminum bronze mainly existed in the Ti2Ni dendrites phase and had a dense structure. Both of the two TiNi coatings exhibited high hardness and remarkable elasticity. In the distilled water and3.5%NaCl solution, both of the TiNi coatings exhibited excel-lent cavitation erosion resistance. In the distilled water, the cavitation erosion resistances of the two TiNi coatings increased by one order of magnitude, compared to the substrates. The cavitation erosion resistances of TiNi coatings on316L stainless and nickel-aluminum bronze were7.2times and6.5times of WC-10Co-4Cr coatings, respectively. In the3.5%NaCl solution, the effect of corrosion for cavitation erosion was small. The TiNi coatings exhibited more excellent than nickel-aluminum bronze, and the mass loss of the TiNi coat-ings was much less than nickel-aluminum bronze, close to the mass loss in the distilled wa-ter. In the different medium, two TiNi laser melting layer showed a similar failure mecha-nism of cavitation erosion, In the initial period, the damage of cavitation occurred in the weakest oxides. The brittle fracture resulted in the removal of oxides and a small amount mass loss of materials. In the TiNi coating on nickel-aluminum bronze, the oxides existed in the dendrite accompany with Ti2Ni phase. After the removal of the oxides, cracks extended to Ti2Ni phase, leading to the removal of a small amount of Ti2Ni phase. In the later period, the cracks began to generate on the phase boundaries between Ti2Ni phase and TiNi phase and extended to the Ti2Ni phase, resulting in the removal of Ti2Ni phase. The dominant failure mechanism was the generation and extension of the cracks on the phase boundaries. Under the action of stress from the bubble collapses, the stress induced martensite transfor-mation and the reorientation of martensitic plates occurred in the TiNi phase, and dissipated a large amount of energy from the bubble collapses, leading to weaken the damage of the cavitation. However, under the repeat action of the stress, the cracks extended to TiNi phase in a small speed, resulting in the fatigue damage of TiNi phase at last.