Atomization Mechanism And Flow Field Distribution Of TC4 Alloy Powder Prepared By EIGA

With its significant advantages of high eficiency,high speed and being suitable for processing complex parts,additive manufacture(3D printing)has become the focus of rapid prototyping technology at home and abroad in recent years.In recent years,the hotspot of 3D printing technology research and development has gradually shifted to the metal material 3D printing technology where the metal material printing equipment has gradually been applied.For the metal additive manufacture,the powder is a dominant material which has a great influence on the final product;meanwhile,powder is the bottleneck of restricting the development of the entire metal 3D printing.The amount of titanium and its alloys can represent a country’s aerospace level.The working temperature of TC4 alloy can reach 400℃,widely applied in the field of aerospace,mainly in the preparation of engine fan,compressor disk,blade aircraft structural beam and other important tensional components.However,since the local TC4 powder have not reached the standard,China mainly relies on importing 3D printing powder.At present,the preparation of 3D printing titanium alloy powder mainly depends on mechanical polishing,chemical vapor deposition and atomization.As liquid metal doesn’t contact with the crucible and directly flows into the atomization zone,the induction Electrode Inert Gas Atomization(EIGA)powder has the following advantages,including high purity,good performance,a high degree of sphere,narrow particle size distribution and high-cooling rate and little environmental pollution.Therefore,EIGA has become the main method of preparing atomized powder of titanium alloy.3D printing TC4 titanium alloy powder has been prepared by the self-designed rotating electrode induction melting gas atomization equipment in this paper.In this paper,the software Fluent 6.3 is adopted to simulate the flow field of rotating electrode vacuum atomization in different pressure,combined with the breakup and solidification mechanism of droplet to do a deep research of the breakup and solidification of metal droplet in the process of atomization.The self-designed rotating electrode induction melting gas atomization equipment is applied to produce 3D printing TC4 alloy powder which has low oxygen and a high degree of sphere.XRD,optical microscope,scanning electron microscopy(SEM),hall flowmeter and laser granularity analyzer are also utilized to measure components,microstructure,superficial morphology,fluidity,apparent density and particle size distribution;the effect of atomization melting power on the particle size distribution,surface morphology and microstructure of TC4 alloy powder has been studied as well.According to the study,it can draw a conclusion that:(1)The basic characteristics of the gas flow field of the rotating electrode were explored by the simulation of the vacuum atomization flow field of the rotating electrode vacuum atomization.It can be found that the sucked pressure around the recirculation zone was crucial to the breakup process of metal droplet.The sucked pressure was related to pressure of atomization and structure of nozzle;the droplet size was calculated by simulated data,and the results of particle size range was 10～295μm which appropriately matched the practical particle size range(0～300μm).(2)Under this experimental condition,TC4 alloy powder prepared by rotating electrode induction melting gas atomization equipment had an oxygen content range from 0.086%to 0.108%,a high degree of sphere,good fluidity,narrow particle size distribution.Also,owing to a significant gap between the velocity of droplet(3.43m/s)and that of the counterpart in atomizing chamber(200 m/s),the secondary breakup was more adequate,so the particle size distribution was resembled normal distribution.(3)In the atomizing process,metal droplet following the code of Weber experienced different module of breakup where the critical value of Weber number was 10.7.The mutual conversion took place in the breakup due to the physical property of melts,flow field characteristics and other factors.The module of breakup includes:1.dumb-bell crush;2.bags-like crush;3.extended crush;4.explosive crush.(4)The major nucleation of metal droplet was heterogeneous nucleation.Thanks to"recalescence phenomenon" and itself,large droplet arouses heterogeneous nucleation,small droplet occure heterogeneous nucleation and its oxides become the core of heterogeneous nucleation.The growth of crystal nucleus was accordance with the rapid solidification theory.The major microstructure of TC4 alloy powder was cellular crystal.(5)With the increase of the melting power and energy input,the superheat of alloy droplets rose,resulting in expansion of liquid droplet volume,increase of molecular separation and weakened adsorption forces on the surface of the liquid droplet molecule.At the same time,the viscosity and surface tension of titanium alloy melts gradually declined,which contributed to the breakup of titanium alloy droplets in atomizing process.The average particle size of TC4 alloy powder gradually dropped to:141.8μm,126.3μm,124.4μm,120.5μm,103.5μm,reaching the minimum in 65MPa;with the increase of the melting power,the spherical degree and the phase of TC4 powder almost remained stable,while the oxygen content increased slightly.