| 3D printing technology is widely used in manufacturing titanium alloy parts,which has the characteristics of short-cycle,individualization,near-net shape and so on,and titanium powder is the key raw material for 3D printing.The quality of titanium powder affects the properties of the products.Therefore the metal powder for 3D printing should have the characteristics of good sphericity,low oxygen content and fine particle size.The traditional methods for preparing titanium powder have some disadvantages,such as low yield,low atomization efficiency and large energy consumption,which lead to high cost for titanium powder preparation.Hence,powder preparation,especially for the titanium powder with high-quality and cost-efficient is important for the development of 3D printing technology.Therefore,the aim for this research was to develop a high-quality,and cost-efficient technology for producing spherical titanium powder.In this paper,the 'titanium wire high frequency induction heating gas atomization' method,which combined non-crucible melting technology with gas atomization,was proposed and the apparatus of such method was designed and manufactured to fabricate high quality and low cost spherical titanium powder.Based on the combination of numerical simulation and experimental methods,the change rule of high frequency induction melting process of titanium wire and the mechanism of close-coupled and non-confined atomization were analyzed systematically.Therefore,on the basis of successful establishment of the apparatus for titanium wire high frequency induction heating gas atomization,the low-cost and high quality spherical titanium powder was prepared.Moreover,the titanium powder producing process was studied and the characteristics of the powder was investigated.The main achievements of this research are listed as below:On account of the principle of high frequency induction heating and the physical property of titanium,the titanium wire was melted by high frequency induction by the non-crucible means.Titanium wire high frequency induction melting model was coupled with multi-physics fields,including electromagnetic fields,temperature fields,and flow fields,was established,and the changing law of multiphysics fields during high frequency induction melting process of titanium wire has been analyzed by COMSOL Multiphysics software.The results indicated that the magnetic field intensity decreased from bottom to top in tapered coil,and the titanium wire could be melted from bottom to top step by step during the heating process.Thus,the 300° taper coil was then chosen as induction heater.When the current frequency increased,the heating rate increased and the temperature difference between titanium surface and core also increased.The titanium wire acquired the best heating efficiency when the current frequency was 300kHz.The dynamic process of titanium wire high frequency induction melting has been simulated by level set method and the melting mechanism was revealed.It was found that the titanium melting process was affected by the wire feed speed.When the wire feed speed increased,the melt morphology transformed from droplet to liquid flow.When the feed speed was 0.01 m/s,the titanium wire with diameter of 3mm was melted to form droplets in 0.7 s,attributed to the gravity and surface tension.When the feed speed was 0.03 m/s,the titanium wire was melted to form a continuous flow because the melting amount of titanium wire increased per unit time.The liquid forming time decreased with the increase of current loading and frequency.The variation of the atomization flow in the close-coupled and non-confined atomization nozzle and the atomization mechanism have been investigated.The atomization flow in different jet angle nozzles was analyzed by the computational fluid dynamics software Fluent.The results showed that the aspiration pressure and the stagnation point pressure increased with the increasing atomization jet flow angle.The liquid flew into the atomization zone by suction under no diversion condition.The nozzle had the highest efficiency for atomization under non-confined condition when the jet angle was 35°.Besides,with the increasing atomization pressure,the highest gas velocity in the gas field increased,and the structure of flow field changed from opened-vortex to closed-vortex structure when the gas inlet pressure was no less than 16atm.The mechanism of gas-liquid flow breakup was clarified.The results indicated that the radical pressure gradient and reverse dynamic pressure existed in the center hole of the atomization nozzle.When the metal melt entered in the atomization zone,the metal melt completed primary breakup process and formed liquid film by the action of two directional pressure.The first broken droplets passed across the front of the stagnation point and the secondary disintegration of droplet was carried out by stripping and thinning.The apparatus for 'titanium wire high frequency induction heating gas atomization'method has been designed and manufactured.The equipment mainly included delivery system,smelting system and atomization system.The delivery system was consisted of straightening equipment and feeder equipment,which realized automatic control of continuous straightening and conveying of titanium wire.The smelting system was consisted of a high frequency induction heating equipment and the titanium wire was melted continuously by controlling the conveyor.The atomization system included Laval nozzle which obtained supersonic flow through Laval nozzle.A close-coupled and non-confined supersonic atomization nozzle structure was designed to make the atomization process pollution-free and efficient.The titanium powder was prepared by the independently developed titanium wire high frequency induction heating gas atomization apparatus,and the influence of atomization parameters on the properties of the titanium powder has been analyzed.After optimization,a reasonable and economic atomization process was established as following:the diameter of titanium wire was 3mm,the atomization pressure was 40 atm,the supply current was 40 A,the current frequency was 300 kHz and the feeding speed was 0.03 m/s.Under such conditions,the average grain size of the produced titanium powder was 41.8 ?m,with a fine powder(?45?m)yielding rate of 57.7%.The produced powder had a spherical shape and the sphericity of the powder was near 100%.Moreover,the powder has very few satellite particles on the surface.Compared with electrode induction melting gas atomization technology,the fine powder yielding rate has been increased by 24.7%. |
PDF Full Text Request