Fabrication Of Coated Titanium Composite Powders And Their Application On Near NET Forming

Powder metallurgy technology with the advantages of low energy consumption,high raw material utilization rate and near net forming,could overcome the problems of high processing costs and difficult preparation of complex structures by traditional cast titanium alloys and titanium matrix composites.Therefore,it has been widely used for the preparation of titanium alloys and titanium matrix composites.However,some problems including uneven mixtures,agglomeration,and contaminations were inevitable during the fabrication process of titanium alloys and titanium matrix composites.This seriously degrads the microstructure homogeneity and mechanical properties,leading limitation on their engineering applications.This dissertation proposes a method of synthesizing coated titanium composite powder to solve the problems of uneven mixing of reinforcing phase and titanium powder in in alloy powder raw materials,and adopts appropriate powder metallurgy technology to prepare titanium alloy and titanium matrix composites,and investigate the effet of coated titanium conposites powder on the density,microstructure and mechanical property of titanium alloys and titanium matrix composites.In this paper,nickel-coated titanium composoite powders with different Ni contents were successfully fabricated by electroless plating and a Ti-Ni alloy was prepared using press-and-sinter powder metallurgy.The Ni particles uniformly coated on Ti powder were nano-sized and therefore started to diffuse into Ti at the very early stage of sintering.Comparing with the titanium-nickel alloys fabricated by mixed powders,alloys prepared by nickel-coated titanium composite powders had high density,low oxygen content and equiaxed ?-Ti grains were prepared.Besides,the alloys prepared with nickel-coated titanium composite powders had higher yield tensile strength and ductility.The linear shrinkage response is good for accurately controlling the dimension of the sintered parts.The coated Ni content was optimized at 3.45 wt pct,beyond which the excessive brittle eutectoids were formed and largely degraded the tensile ductility.On the basis of the successful preparation of nickel-coated titanium composites,by changing the electroless plating process process,it was found that the introduction of boron can refining grain and increase the sintered density and tensile properties of the titanium alloy.The prepared nickel-coated titanium composite powders were used in-situ synthesis of carbon nanotubes-coated titanium composite powder with fluidized bed-chemical vapor deposition under different synthesis temperatures,time and velocity ratio of acetylene gas.Under optimized process conditions,carbon nanotubes with high crystallinity,good morphology and uniform distribution were obtained.Further,a titanium matrix composites was fabricated by spark plasma sintering process at different sintering temperatures.With the increase of the sintering temperature,the density and compressive yield strength of titanium matrix composites were accordingly improved.When sintering at 800°C,the compressive yield strength of titanium matrix composites was 865 MPa,about 25% higher than that of pure titanium prepared under the same conditions,owing to the reinforcement of the carbon nanotubes.Due to the strong reaction between the carbon nanotubes and the titanium matrix,the plasticity of titanium matrix composites reduced when the sintering temperature exceeded 900°C.