Study On Salt Bath Boronizing Process Of TC11 Titanium Alloy

TC11 is anα+βtitanium alloy,because of the superior high temperature strength properties,widely used in the manufacture of compressor discs,blades,annular parts,drums,fasteners and other parts of aero-engines.Surface boronizing treatment of TC11 titanium alloy can effectively increase its surface wear resistance and improve the service life of the titanium alloy component under the wearing condition.Boronizing experiments were carried out on the surface of TC11 titanium alloy by molten salt method.When Cl-was used as activator,the boronizing layer was thinner,the serrated structure is sparse and coarse,bending and irregular.When CO₃^2-was used as activator,the boronizing layer was thicker,the serrated structure was sharp,slender and dense,so,CO₃^2-as an molten salt boronizing activator is better.using CO₃^2-as an activator,optimized the molten salt method boronizing agent6%K₂CO₃+10%Al+84%Na₂B₄O₇ by orthogonal experiment.Carried out the single boronizing,Ni-B and Ni-Mo-B Compound infiltration experiments on the surface of TC11 titanium alloy by optimized boronizing agent,the effect of boronizing process（temperature,time）on boronized structure was studied.The results show that no obvious boronizing layer can be formed on the alloy surface below 950℃.When the temperature is higher than 950℃,the thickness of boronizing layer increases with the increase of boronizing temperature and holding time.After electrodeposited Ni,Ni-Mo on the surface of TC11 titanium alloy,carried out the diffusion heat treatment at 1000℃for 4h,the Ni diffusion layer can reach 16.39μm,and the Ni-Mo diffusion layer can reach 13.53μm.The thickness of Ni-B and Ni-Mo-B composite layers increased significantly based on Ni diffusion layer and Ni-Mo diffusion layer alloy compared with single boronizing.when the boronizing temperature is 1050℃,holding 15h,the thickness of the single boronized dense layer reach 8.50μm,Ni-B layer can reach 24.00μm and Ni-Mo-B can reach 28.66μm.The Ni and Mo elements can accelerate boronizing process,and the Ni-Mo composite permeability is better than that of single Ni.Phase analysis of single boronizing layer,Ni-B and Ni-Mo-B composite layers indicate that the single boronized structure consists of outer dense layer TiB₂ and inner serrated TiB,Ni-B composite layer consists of TiB₂ and Ti₃Ni₄ intermetallic compound with low hardness and brittleness and Ni-Mo-B composite layer consists of TiB₂,Ni₃B and TiMoB₄ Phase.The Ni alloy element weakens the preferential diffusion ability of B atom in Ti（010）crystal,which is the main reason why Ni-B and Ni-Mo-B composite layer can not form serrated TiB structure.The microhardness analysis of single boronizing layer,Ni-B and Ni-Mo-B composite layers showed that the Ni element reduced the microhardness of the boronized layer and improved the toughness of the boronized layer.Compared with the single boronized layer（1400HV）,the microhardness of the Ni-B composite layer（1004HV）and the Ni-Mo-B composite layer（1169HV）are significantly reduced.and the hardness gradient from the surface of the layer of three kind of nfiltration layer to the substrate are reduced gradually to base metal.