Research on densification of Ti coatings prepared by Low Temperature High Velocity Oxygen Fuel
Dai Hongliang
Abstract
Titanium had found its wide applications in aviation, aerospace, ship and marine engineering, petrochemical, metallurgy, light industry machinery, medical and other fields for its high specific strength and good corrosion resistance. However, the application of titanium is greatly restricted due to the chemical activity, high production cost and difficulty in subsequent process. By changing the structure of spray gun and injecting cooling reagent into the combustion chamber, low temperature HVOF (LT-HVOF) process was achieved, characterized by the high speed and low temperature plume. Titanium coating with excellent performance was able to be deposited by LT-HVOF in the atmospheric environment, which would not only reduce the production cost enormously, but also have broaded application prospects.
In this paper, Ti and its composite coatings were deposited by the newly developed LT-HVOF process with uniform mixture of titanium powder with spherical glass powder(G)， WC-12Co powder and WC powder with various particle sizes as feedstocks respectively on the Q235 steel substrate and 316L porous stainless steel substrate.Microstructure, gas tightness and corrosion resistance of titanium and its composite coatings were studied by means of OP， SEM, EDS and etc.
The results showed both pure titanium and its composite coatings were composed by loose surface and relatively dense interior zone, in which the unmelted Ti particles and the embedding of added particles were observed. The porosity of coatings could be all greatly decreased by adding spherical glass powder and WC-12Co power in the ratio range, but not the WC power with various particles. The amount of the embedded glass powder was not increased obviously with the increase of glass powder. But the embedment of WC-12Co was significantly increased with the increasing WC-12Co
content; As for the WC powder, the embedment of WC particles was decreased remarkably by reducing the content of WC powder and increasing the diameter of particles.
Gas tightness test for Ti and its composite coatings showed that when the volume ratio of G/Ti was 1/15 and 1/20, the viscosity coefficient of N2 in Ti composite coatings was the lowest one, followed by the composite coatings with the volume ratio of WC-12Co/Ti 1/5. Adding glass powder and WC-12Co powder both decreased the viscous permeability coefficient of titanium composite coatings and improved the coating density. This was probably owing to the added hard particles, which densify Ti coatings for the shot peening effect upon Ti coatings, and then make pores and cracks
narrowed or even closed.
Corrosion resistance of titanium and its composite coatings was studied. The electrochemical test results showed that the corrosion current density was ranked as:Q235 substrate > WC-12Co/Ti coated Q235 steel > Gl coated Q235 steel > pure Ti coated Q235 steel> G2, G3 and G4 coated Q235 steel > titanium plate, which indicated that Ti and its composite were beneficial for the corrosion resistance of the substrate.Neutral salt spray corrosion results showed that the salt spray corrosion resistance was ranked as: Q235 steel < WC-12Co/Ti and pure Ti coated Q235 steel < Gl coated Q235 steel < G2, G3 and G4 coated Q235 steel. The corrosion of Ti and its composite coated substrates was governed by the diffusion of corrosive media into the interface through the holes, cracks, and other defects in the coatings, and finally leading to the corrosion of the substrate. The galvanic corrosion of WC-12Co and Ti aggravated the corrosion of coatings, which promote the emergence of the corrosion porosity of the coatings, led to the diffusion of corrosive media into the substrate and accelerated the corrosion rate of the substrate.
In conclusion, Ti composite coatings exhibited better microstructure and gas tightness by comparing with the pure Ti coating; All the G/Ti composite coatings possessed better corrosion resistance than the pure Ti coating, except the volume proportion was 1/5 for G/Ti composite coatings; However the corrosion resistance of the pure Ti coating was better than the WC-12Co/Ti composite coatings. The deposition efficient of the coating was highest when the volume proportion was 1/15 for G/Ti composite coatings. The optimized coating had excellent gas tightness and good
corrosion resistance with low porosity (less than 1%) and low embedment of glass content (less than 0.5%).
Keywords: Low Temperature High Velocity Oxygen Fuel (LT- HVOF); titanium and its composite coatings; gas tightness; corrosion resistance
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