Preparation Of Near-spherical Titanium Powders Via Jet Milling And Its Applications

Titanium alloys have the advantages of low specific weight,high specific strength,corrosion resistance,and good heat resistance.These advantages make titanium alloys as a key indispensable material in high technology fields such as aerospace,biomedical,marine industry,and petrochemical.However,due to the characteristics of high melting point,high activity and difficult processing,titanium alloys have greater difficulties in melting,extraction as well as processing,resulting in higher production costs.Both additive manufacturing(AM)and powder sintering can achieve near-net forming of materials,which can significantly improve the utilization of raw materials and can solve the high cost challenges faced by complex structured titanium parts in the processing aspects.The AM process usually requires spherical titanium powder with excellent flowability as raw material,while the powder sintering process requires fine powder with low oxygen content.However,the preparation of titanium powder required for these processes is complicated and expensive(>1000 CNY/kg for spherical titanium powder),which greatly limits the development and application of powder metallurgy titanium(PM Ti).To this end,the study developed a low-cost titanium powder modification preparation technology based on the principle of jet milling.The low-cost irregularly shaped hydride-dehydrate titanium(HDH Ti)powder is used as raw material(about 150 CNY/kg),and the powder particles are driven to impact with each other by jet milling to achieve the effect of shaping and modification.Thus achieving the improvement of sphericity and flowability,as well as size classification and effective passivation.The produced titanium powder is suitable for AM and powder sintering processes,and achieves effective utilization in different particle size,which provides an attractive way for the cost reduction of near-net forming of titanium powder.The main research contents and results are as follows.(1)The modified titanium powder technology of jet milling was developed to produce near spherical titanium powder(D50=40.1 μm)with an oxygen content of 0.17 wt.%and a fluidity of 29 s/50g.The preparation cost was reduced by more than 60%compared with gas atomized spherical powder.The modified titanium powder(D50=10.9 μm)with good dispersion,concentrated particle size,specific surface area(216.6 m2/kg)and oxygen content of 0.22 wt.%was also obtained.The highest sphericity of the powder was obtained with milling time of 6 min for modification.Based on CFD-DEM simulation,the relationship between collision stress,energy,milling pressure and powder characteristics was analyzed.The maximum stress of powder collision was obtained,which proved that powder microstructure changes were closely related to the collision stress.It is concluded that when the milling pressure is 0.6 MPa,the energy efficiency ratio of grinding can be maximized while the powder sphericity is significantly improved.(2)The microstructure and composition distribution of the atomic-scale oxide film on the surface of modified titanium powder were revealed,and the distribution characteristics of oxygen in titanium powder were obtained as TiO2,TiO2-x→TiO→Ti[O].The modified powder was considered to have good antioxidant ability by comparison of spherical powder.The characterization by atomic-scale analysis of the oxide film showed that the formation of stable oxide(TiO2)on the powder surface,as well as the increase in oxide film thickness and densities,were the main factors for the excellent passivation properties of the modified powder.The kinetic mechanism shows that controlling the thermal stability of the oxide film can help improve the oxidation resistance of the powder,which is conducive to controlling the oxygen content of the powder and improving the mechanical properties of the subsequent formed parts.(3)The application of jet milled titanium powder in additive manufacturing and powder sintering was investigated to achieve efficient utilization of the modified powder.It was shown that the laser absorption of the near spherical powder could reach more than 70%.The near spherical powder is highly applicable to LPBF forming with different printing parameters,and the densities of the printed parts reach more than 98%,and the tensile strength and elongation reach 731.5 MPa and 20.5%,respectively.The tensile strength and elongation of the modified powder sintered parts can reach 660.3 MPa and 17.3%,respectively.It is concluded that the formation of α’ martensite with fine grains and large number of b=1/3[1213]dislocations and twins such as {1011}<1012>is the reason for the good mechanical properties of the printed parts.The mechanical properties of the powder sintered parts are mainly due to oxygen content(0.22 wt.%)and the high density(93.8%).The PM Ti with bimodal microstructure were obtained with tensile strength and elongation reach up to 779.8 MPa and 22.5%,respectively,which achieved the synergistic increase of strength and plasticity of PM Ti products.