| TA15 titanium alloy is one of the most widely applied structural materials of aerospace and aircraft such as airplanes,vapour generators and missile components,because of their moderate room-temperature and high-temperature strength,excellent creep resistance,good corrosion resistance and welding performances.However,when used as metal shell of space vehicle,the high temperature effect from the aerodynamic heating enables the surface temperature of titanium alloy rapidly rise beyond 600 o C,which leads to the seriously degraded thermal protection performance of alloy.In this paper,TA15 alloy was aluminized and silicon aluminized by pack cementation.The microstructure,composition and composing phases of the alloyed layer were tested and analyzed.Finally,the high temperature oxidation resistance and corrosion mechanisms of the coated titanium alloy were comparatively investigated.The results show that aluminized coatings were successfully prepared on TA15 alloy using pack cementation process at 1000 o C for 4 hours.At the same time,The coating is compat without holes and a single TiAl3 layer with a thickness of about 60?m is obtained.Siliconizing layer was also prepared by packing cementation process under the condition of 1000 o C/4 h.The results show that good quality siliconizing layer can be obtained when the silicon powder content is less than 10%(wt.%).When the silicon powder content is 10%(wt.%),a single Ti Si layer with a thickness of about 15?m is obtained.But 800 oC oxidation experiment showed that siliconizing layer was falling off at 40 hours.This is mainly due to the brittleness of the single siliconizing coating.Obviously,it is bad for its high temperature antioxidant properties.Four different ingredients were designed in the process of Silicon aluminized.The results shows that the relative content of Si element can change the structure of the cosmotic coating.When the content of silicon and aluminum are 1:1,the coating structure is similar to the embedding of silicon coating with the thickness of 18?m,that the TiSi is the principal phase and form a thin TiAl3 phase in the outer layer.Whenthe content of silicon and aluminum are 1:2.The coating has a TiAl3 phase in outer layer.while the inner layer is a TiAl3 phase with a large amount of silicate(Ti5Si3).When the content of silicon and aluminum are 5% Si+25% Al and 2% Si+25% Al,the coating structure is similar to the embedding of aluminium coating.Coating is mainly composed of aluminized layer TiAl3 and its internal is diffuse distribution of silicide(Ti5Si3),at the same time,the more near the substrate the more the amount of silicide.In addition,the more silicon,the thinner the total thickness of the coating.Micro-arc oxidation coating was prepared on aluminized coatings and 5%Si of Silicon aluminized coating.Finally,the coating of microarc oxidation of 5?m on the surface of the packing cementation coating is obtained.High temperature oxidation behavior of packing cementation coating was investigated by isothermal oxidation at 800 oC and 900 oC in air.The oxidation resistance of all kinds of packing cementation coating are much better than that of uncoated TA15 alloy.After oxidation at 800 oC for 100 h,the oxidition weight gain of the TA15 alloy is3.20mg·cm-2 and the oxidition weight gain of the aluminized coatings is1.29mg·cm-2.about 1/3 of TA15 alloy.Among the coatings,the packing cementation coating that its the content of silicon and aluminum are 5%Si+25%Al shows the best oxidation resistance with the lowest weight gain.At the same time,under the condition of 800 oC,micro-arc oxidation coating can effectively improve the high temperature oxidation resistance on the basis of the packing cementation coating.But under the condition of 900 oC,it is just can effectively improve the high temperature oxidation resistance at the beginning. |
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