Research On Densification Of Ti Coatings Prepared By Low Temperature High Velocity Oxygen Fuel

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 Q235steel substrate and316L 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 was1/15and1/20, the viscosity coefficient of N2in Ti composite coatings was the lowest one, followed by the composite coatings with the volume ratio of WC-12Co/Ti1/5. 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: Q235substrate> WC-12Co/Ti coated Q235steel> G1coated Q235steel> pure Ti coated Q235steel> G2, G3and G4coated Q235steel> 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:Q235steel<WC-12Co/Ti and pure Ti coated Q235steel<G1coated Q235steel<G2, G3and G4coated Q235steel. 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 was1/5for 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 was1/15for G/Ti composite coatings. The optimized coating had excellent gas tightness and good corrosion resistance with low porosity (less than1%) and low embedment of glass content (less than0.5%).