| Titanium alloy has comprehensive excellent properties which was a key material in aerospace,biomedicine,military industry,national defense,marine,chemical and other fields.However,the traditional melt-forged titanium alloy is expensive due to the low material utilization for the preparation of complex-shaped parts,which limits its wide application.Gelcasting technology has unique advantages in the preparation of large-size and complex-shaped titanium parts.However,due to the active chemical activity of titanium powder,it easily reacts with the organic gel system and adsorbs interstitial oxygen impurities during the forming and sintering process,resulting in high carbon and oxygen content,low sintering density and poor performance of the gelcasting titanium alloy.In this paper,the goal is to prepare high-strength,high-plasticity,large-size and complex-shaped gelcasting TC4 titanium parts.A series of studies were carried out on the control of low gaps and sintering densification of titanium powder.Research the gelcasting of organic coated to isolate the interaction between gel organic matter and titanium powder.Based on the principle of chlorination deoxidation,an anaerobic passivation technology for titanium powder is proposed,which forms a Ti-O-Cl-Sn passivation layer on the surface of the titanium powder through a gas-solid reaction.It plays the role of surface deoxidation and activation sintering.Further,the use of anaerobic passivated spherical titanium powder for gekcasting can realize the preparation of large-size and complex-shaped TC4 titanium alloy products with low carbon and oxygen residues,high density,and high performance.In order to reduce costs,the gelcasting process of hydrogenated dehydrogenated titanium powder has been studied.A low-oxygen gel system has been developed,which lays the foundation for the low-cost,high-performance,large-size and complex-shaped TC4 titanium components developments.The main research results obtained in this paper are as follows:(1)In order to isolate the interaction between the titanium powder and the gel system,organic coating titanium powder was used,and the titanium alloy gelcasting process was improved.Based on HEMA gel system,using 0.3 wt.%PW/0.3 wt.%SA organically coated spherical TC4 powder for gelcasting,a low-viscosity suspension slurry with a solid content of 60 vol.%can be obtained,which can be molded and debinding.After sintering,a titanium alloy with higher comprehensive performance was obtained.The carbon and oxygen content of the gel injection molded sintered titanium alloy decreased from 0.41 wt.%,3507 ppm to 0.23 wt.%,2879 ppm,respectively.The relative density of the titanium alloy reached 97%,the tensile strength was 1029 MPa,and the yield strength was 967 MPa with elongation of 6.4%.Without organic coating,the titanium alloy was brittle fracture,and the elongation was only 0.9%.It can be seen that organic matter can isolate the interaction between titanium powder and the gel system to a certain extent,and obtain a gelcasting TC4 titanium alloy with certain properties.(2)In order to reduce the oxygen content of the titanium powder,SnCl2 was used for chlorination deoxidation.The titanium powder with different initial oxygen content was mixed with SnCl2,after pressed and sintered,and it was found that the oxygen content in the system would be greatly reduced.After adding SnCl2 with a Sn content of 5 wt.%,the oxygen content in the sintered titanium alloy was reduced from 5727 ppm to 4323 ppm,10643 ppm to 7391 ppm,and 4796 ppm to 3426 ppm.Different from the traditional principle of high-temperature oxygen solidification of active metals such as Re,Ca,Mg,SnCl2 reacts with the surface oxides of the substrate Ti powder at low temperature(500?600?)to form TiOxCly and sublimation escape.Oxygen removal reducesd the oxygen content of titanium powder and achieves chlorination deoxidation.At the same time,the Sn element in the system can play a role in promoting sintering and grain refinement.(3)In order to take into account the beneficial effects of Sn and Cl,based on the SnCl4(gas)-Ti(solid)gas-solid surface reaction,the titanium powder anaerobic passivation technology was proposed to form a uniform Ti-O-Cl-Sn on the surface of the titanium powder The passivation layer can not only break the dense oxide film on the surface of the titanium powder,but also play a deoxidizing effect in the subsequent sintering process.Studies have found that when the fluidization temperature is higher than 500?,Cl atoms have already begun to deoxidize during the fluidization process,and the remaining elements form a Ti-Sn solid solution layer.During the deoxidation process,the oxidation-reduction reaction of titanium atoms and titanium ions is carried out.Using 350? fluidization for 60 min,a thin and uniform passivation layer can be obtained on the surface of the titanium powder,and SnCl4 anaerobic passivation titanium powder can be prepared,which can be applied to subsequent titanium alloy gelcasting.(4)In order to obtain high-strength and high-plasticity gelcasting TC4 titanium alloy,SnCl4 anaerobic passivated spherical titanium alloy powder was used for gelcasting.A high-fluidity titanium powder slurry with a solid phase volume fraction of 60 vol.%was prepared based on HEMA system.After forming and sintering,the oxygen and carbon content in the titanium alloy was greatly reduced.The oxygen content was reduced from 3507 ppm to 2558 ppm,and the carbon content was reduced from 0.41 wt.%to 0.17 wt.%.During the sintering process,the anaerobic passivation layer on the titanium powder surface plays a role of deoxidation and carbon control,which effectively isolates the interaction between the titanium powder and the gel system.The passivation layer isolated and reduced oxygen while further chlorinating and deoxidizing,forming gaseous TiClxOy and COn substances from the system.After sintering at 1320?,the density of the titanium alloy reached 98.8%and showed excellent mechanical properties.The tensile strength was 1049 MPa,the yield strength was 950 MPa,and the elongation was 12.3%.(5)In order to reduce the cost,the HDH titanium powder with anaerobic passivation was used as the raw material,and the gelcasting was carried out based on the HEMA gel system.Although oxygen and carbon can be controlled to a certain extent,the effect was limited and the ideal performance cannot be achieved.Therefore,the gel system was improved,and a low-oxygen Ip/PVB binary gel system was developed.Using low-cost HDH-TC4 powder as raw material,gelcasting was performed based on the Ip/PVB binary gel system,and cold isostatic pressing was used to further eliminate the pores of the green body.After debinding and sintering,a titanium alloy with a relative density of 98.9%was obtained.The impact of the Ip/PVB system on the carbon and oxygen residues of the gelcasting titanium alloy was greatly reduced,and the carbon and oxygen contents were 0.14 wt.%and 3563 ppm,respectively.The tensile strength of the titanium alloy was 1062 MPa,the yield strength was 987 MPa,and the elongation was 8.8%,which lays the foundation for the preparation of low-cost,high-performance,large-size,and complex-shaped gelcasting titanium parts. |
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