Modelling of droplet dynamic and thermal histories during spray forming—II. Effect of process parameters

A computer model has been developed to describe the in-flight dynamic and thermal histories of gas atomised droplets as a function of distance during spray forming. The model has been used to investigate the effects of the dynamic and thermal behaviour of individual gas atomised droplets and the cooling and solidification behaviour of the overall spray. The most influential parameters for a given alloy system, in order of importance, are: (i) droplet diameter and, therefore, the droplet size distribution within the spray; (ii) initial axial gas velocity at the point of atomisation and the subsequent gas velocity decay profile; (iii) melt mass flow rate; (iv) melt superheat at the point of atomisation; and (v) alloy composition. Experimental measurements of gas velocities and droplet size distributions during spray forming allow the spray solid fraction at deposition to be calculated and used in a subsequent computer model of billet heat flow to predict the billet top surface temperatures and solid fractions.     

Modelling of droplet dynamic and thermal histories during spray forming—I. individual droplet behaviour

A computer model has been developed to describe the in-flight dynamic and thermal histories of gas atomised droplets as a function of distance during spray forming. Modifications of the model have been used to evaluate the relative importance of different physical assumptions in controlling droplet cooling and solidification behaviour, and ultimately the evolution of spray formed billet microstructures. Gas atomised droplet dynamic and thermal behaviour is strongly affected by the distribution of droplet diameters in the two phase spray and the momentum transfer between the droplets and the atomising gas, but is relatively unaffected by the details of the droplet solidification mechanism.     

Study of free fall gas atomisation of liquid metals to produce powder

Abstract

A free fall gas atomising unit was designed and constructed to produce metal powder. The effect of focal length, number, and diameter of nozzles and apex angle of atomisers is studied on size and size distribution of the powder, atomisation efficiency, and particle shape by disintegrating the molten stream at various plenum pressures, superheats, and free fall distances. Droplets were found to be solidified and built up around the nozzles and the liquid delivery tube whenever the atomiser was used beyond a particular value of the plenum pressure; this pressure is termed limiting plenum pressure. All the powder collectives produced under a wide range of conditions were found to obey log normal distribution function. The geometric standard deviation was shown to bear a constant value. The mass median size was correlated with the dynamic parameters characterising the atomising field and a correlation is proposed. The atomisation efficiency was correlated with the mass flowrate and velocity of gas.     

Centrifugal atomisation of metallic alloys in inductive plasma onto cooled disc

Abstract

A novel atomisation process for the production of high purity, fine powders of high melting point metallic alloys has been devised. This process associates centrifugal atomisation and inductive plasma techniques. The experimental equipment is described. Results obtained by atomising nickel base superalloy PER3, Ti–0·17%Pd alloy, and pure copper on a cooled copper alloy disc are presented. The effects of process parameters, such as the disc rotation speed and the aerodynamic environment of the disc, on the atomised product characteristics, are investigated. The alloys were melted drop by drop onto a water cooled disc. The atomisation of the dripped alloy can be characterised by three mechanisms: ultraquenched flakes and crusts solidified on the disc; spherical droplets formed by centrifugation; spherical droplets formed by aerodynamic erosion.     

Studies on metal build upon atomisers during free fall gas atomisation of liquid metals

Abstract

In free fall gas atomisation of liquid metals, the atomised droplets mainly move downward, while some of them fly in an upward direction. Under certain conditions the upward moving droplets will deposit on the surface of the gas nozzle and metal delivery tube of an atomiser, which may cause a hindrance to the flow of liquid metal and gas. The effect of atomisation parameters, such as gas pressure, focal length, apex angle and diameter of gas nozzle, on the metal buildup on the atomiser have been studied during the free fall gas atomisation of lead, zinc and aluminium. The plenum pressure of gas at which deposition of atomised droplets on the surface of the gas nozzle and metal delivery tube takes place has been termed as limiting plenum pressure, and the corresponding gas velocity at the impingement point as limiting gas velocity. It has been shown that the limiting plenum pressure is different for different metals in the same atomiser. The limiting plenum pressure has been found to increase with free fall distance or specific gravity of liquid metals, and with a decrease in apex angle of atomiser. A correlation is proposed to determine the limiting gas velocity for free fall gas atomisation of metals.     

Investigation on metal powder production efficiency of new convergent divergent nozzle in close coupled gas atomisation

Abstract

In the present study, a new designed close coupled annular slit type nozzle is discussed for the most efficient powder production and the availability of the finest particle size. Pure tin powder was produced by using nitrogen gas at the Dumlupinar gas atomisation unit. It was found that the finest particle size with the highest efficiency was obtained at the atomising pressure where the maximum aspiration pressure is formed. Above a certain gas/melt flow ratio, the resultant particle sizes cannot be considerably reduced by further increasing atomising pressure. At that point, the applied nozzle system reaches a capacity limit, which also marks the point of the most economic work for the production of fine powders with that system. The main achievement of the nozzle discussed in the present study is that the most efficient powder product is accomplished at relatively low atomisg gas pressures.     

Droplet formation,detachment, and impingement on the molten pool in gas metal arc welding

A mathematical model is developed to describe the globular transfer in gas metal arc welding (GMAW). This work is both theoretical and experimental. Using the volume-of-fluid (VOF) method, the fluid-flow and heat-transfer phenomena are dynamically studied during the following processes: droplet formation and detachment, impingement of a droplet on a solid substrate, impingement of multiple droplets on the molten pool, and solidification after the arc extinguishes. A He-Ne laser, in conjunction with the shadow graphing technique, is used to observe the metal transfer processes. Theoretical predictions and experimental results are in close agreement, suggesting that the theoretical treatment of the model is good.     

Current PM Literature for Engineers and Users

Abstract

Fine tin powders were produced in a pilot plant gas atomiser. Nitrogen gas at 1·56 MPa pressure was used as the atomising agent in a ‘confined design’ nozzle which operated vertically upwards. A range of metal flowrates from 0·864 to 1·425 kg min^?1 was studied at a melt temperature of 450°C. Powders were sized using dry sieving down to 45 μm and wet sieving for smaller sizes. The Sauter mean diameter of the powders varied from 9·01 to 10·28 μm, depending on the rate of production. The size distribution was bimodal (albeit not very well defined) with the peak separation at ～44 μm. In the fine size range, particles were spherical, while those in the coarse range were more elongated or irregular in shape and free of satellites. Comparison of the tin powders with copper powders from another study, AA 2014 aluminium alloy powders, and magnesium and zinc powders from previous work showed that the differences in mean diameter and standard deviation are small among these common metals at a given volumetric production rate. This confirms the overriding importance of liquid metal volume flowrate under fixed gas flow conditions in gas atomisation, while the actual physical properties of the liquid playa secondary role. Although surface tension is secondary to volume flowrate in importance for controlling particle size, the study has shown that a liquid metal with lower surface tension and viscosity than AA 2014 alloy, together with a higher density, yields finer particles. PM/0667A     

Ejection of steel and slag droplets from gas stirred steel melts

Abstract

Liquid ejections from gas stirred melts can be classified into small film and jet droplets caused by bubble bursting and larger splashes resulting from gas channels formed at higher exiting gas velocities. In view of the conditions in ladle metallurgy, experimental investigations were carried out at moderate to low gas flowrates in an arc heated, bottom stirred 150 kg steel melting furnace and an 80 L water tank. Droplets were collected at different heights above the melt level, while gas flowrates, viscosities, surface tensions, and slag layer thickness were varied. The number of steel droplets collected decreased greatly with height (in the range 30-110 mm) and with size (in the range 0·1-1·8 mm). Calculations showed that the entrainment of droplets is strongly influenced by the velocity of upward flowing gases. While at low flowrates typical for secondary metallurgy (0·1 m s ^-1), only droplets <50 μm will be entrained, BOF (basic oxygen furnace) typical flowrates (20-50 m s ^-1) will cause particles up to 500 μm to be carried into the dust removal systems. Higher surface tensions resulted in increased droplet ejection, while higher viscosities led to a decreasing quantity of ejected melt. Slag layers led to a decrease in the ejection of steel droplets and to an increase in ejected slag but they did not completely stop steel ejection, because gas bubbles appear to entrain steel drops when they rise through the slag layer. Bubble bursting in a pure slag system caused large but few slag droplets due to the high viscosity of the slag as compared to the steel melt.     

粉芯丝材电弧喷涂的不稳定性分析

双丝电弧喷涂时,丝材顶端根据被加热状态可分为三个不同区域。其中,丝材顶端最外层被电弧直接加热（Ⅰ区）,这个区域的丝材完全熔化。由于传热效应使得相邻区域（丝材根部方向）的温度升高,从而产生软化区（Ⅱ区）。而与软化区相邻处（丝材根部方向）,传递的热量又软化了丝材并产生持续的变形区（Ⅲ区）,变形是由雾化气体所施加的动力产生的。采用高速摄像系统观察不同运行条件下丝材熔化、金属破碎并粒子形成的过程：Ⅰ区液态金属直接雾化成为很小的液滴,其尺寸是由熔化金属的特性和所施加的雾化气体压力所决定的,软化区是在阴阳极部位的金属薄片的源头。粉芯丝材的金属薄片要比实心丝材的尺寸小。受挤压作用形成的金属薄片形成二次引弧效应,因此增加了电弧喷涂过程的稳定性。本文分析了喷涂参数的影响、填充粉芯对熔化行为的影响、粒子形成以及喷涂过程的不稳定性,并将粉芯丝材和实心丝材的喷涂进行了对比。研究结果有利于提高双丝电弧喷涂模型的精度,并且可以通过优化喷管几何形状来增强金属液滴的雾化效果。     

Hot model experiments of the metal bath spraying effect during the decarburization of Fe-C melts through oxygen top blowing

During recent years decarburization has been steadily gaining importance in converter steel metallurgy at the expense of refining reactions for other slag-forming companion elements. Because decarburization is currently a low-slag operation, the phase contact between gas and metal is critical. With the decrease in the amount of slag foam, more attention must be paid to the spraying of iron droplets during oxygen blowing. The experiments were carried out in a hot model reactor with a 50 kg capacity by oxygen top blowing upon Fe-C melts. The resulting spray of iron droplets was collected with the help of a special droplet sampler in the blowing converter. In the metal droplets a pronounced enhanced decarburization was found in comparison to the metal bath. The amount of metal spray was determined with respect to the oxygen blowing pressure, nozzle diameter, and distance between the lance and bath. Depending on the reactor contents, a high circulation rate of the droplets could be observed. At low blowing rates, FeO-slag is formed and sprayed along with the metal.     

Concept of dynamic foaming index and its application to control of slag foaming in electric arc furnace steelmaking

Abstract

To sustain a foam in steelmaking processes, two basic requirements should be fulfilled, i.e. appropriate physical properties of the slag such as high viscosity, low density, and low superficial tension, and the generation of sufficient reaction gas. To date, foaming indexes have been focused on the physical properties of refining slags. In the present paper a dynamic foaming index (DFI) that involves both above requirements is proposed, using a kinetic model of the electric arc furnace process to calculate the generation rate of reaction gas, mainly CO. When the arc distortion, as affected by electrode submergence in the foam, is compared with the DFI, calculated via the kinetic model, it is observed that both parameters follow very similar trends. This finding indicates the feasibility of knowing the foaming conditions of a heat in advance, or of using the kinetic model online to control the foaming phenomena. Furthermore, experimental results relating to dynamic behaviour of the slag chemistry are well simulated using the kinetic model. To take into account the effect of size distribution of carbon particles injected into the slag to reduce FeO, a Monte Carlo simulation has been integrated into the process simulator, allowing a more realistic prediction of the current steelmaking process.     

Influence of atomising gas pressure on 63A solder powder in supersonic atomisation

Abstract

The atomising gas pressure is one of several important process parameters that affect the characteristics of the powder particles. The work analyses qualitatively the influence of the atomising gas pressure on 63A solder alloy fine powders. Also studied was the mechanism of the effect of the atomising pressure on the base of the air dynamics through the atomising 63A solder alloy experiment with different atomising pressures on the supersonic nozzle. The results indicate that 63A solder alloy fine powders may be attained, which can satisfy the SMT application requirement when the atomising gas pressure is at 0.7 MPa.     

Effect of shield gas composition on surface tension of steel droplets in a gas-metal-arc welding arc

A technique for in-situ measurement of the surface tension of molten steel droplets in a gas-metal-arc welding plasma is described. Surface tension measurements obtained using this method are reported for ER 70S-6 wire, with varying shielding gas compositions, and compared with previously reported results. The in-situ technique is found to produce results consisten with values found in the literature, while incorporating the effects of alloy composition and arc atmosphere.     

Arc Behavior and Droplet Transfer of CWW CO2 Welding

Cable-type welding wire (CWW)CO2 welding is an innovative process arc welding with high quality,high efficiency and energy saving,in which CWW is used as consumable electrode.The CWW is composed of seven wires with a diameter of 1.2 mm.One is in the center,while others uniformly distribute around it.The diameter of twisted wire is up to 3.6 mm,which can increase the deposition rate significantly.With continual wire-feeding and melting of CWW,the formed rotating arc improved welding quality obviously.The arc behavior and droplet transfer were ob-served by the electrical signal waveforms and corresponding synchronous images,based on the high speed digital camera and electrical signal system.The results showed that the shape of welding arc changed from bell arc to beam arc with the increase of welding parameter.The droplet transfer mode changed from repelled transfer,globular transfer to projected transfer in turn.Droplet transfer frequency increased from 18.17 Hz to 119.05 Hz,while the droplet diameter decreased from 1.5 times to 0.3 times of the CWW diameter.     

A novel crucible-less inert gas atomisation method of producing titanium powder for additive manufacturing

A novel low-cost gas atomisation technology producing spherical titanium powder (wire induction heating gas atomisation, WIGA) has been developed for additive manufacturing. Combined with the gas atomisation principle, the characteristics of WIGA were analysed. The effects of the gas pressure, metal temperature and the wire-feeding speed on the particle size of the titanium powder were studied. The results indicated that the decreases in mass median particle diameter (D₅₀) and the increases in efficiency of fine size powders occurred with the increase in gas atomisation pressure and melting temperature and with the decrease of wire-feed speed. The optimum parameters are that the main gas pressure (P₀) is 4.0?MPa, the degree of superheat of the metal melt is 350°C and the wire-feed speed is 50?mm?s^?1. On the condition, the D₅₀ of titanium powder was 40.2?μm and powder morphology was spherical. Satellites rarely existed on the surface of particles.     

Characterization of spray atomization of 3003 aluminum alloy during linear spray atomization and deposition

Linear spray atomization and deposition is an attractive technique to produce near-net-shape deposits, such as aluminum sheet and strip. In the present study, phase Doppler interferometry (PDI) was used in a backscatter mode to characterize, in situ, the droplet size and velocity distributions during linear spray atomization and deposition of a 3003 aluminum alloy. The PDI measurements were obtained along axes corresponding to the direction parallel to the nozzle slit and to the direction perpendicular to the slit. The PDI results delineate the temporal and spatial distribution of the droplet size and velocity during the metal spray. Both point and “line” measurements were obtained and are reported. The line measurements resulted from the integration of measurement made along a line scan obtained in real time (i.e., not ensemble averaged). Postrun analysis of the droplet size distribution using laser diffraction and sieving techniques is also reported. The PDI point measurements revealed that droplet size and velocity distribution were relatively invariant with time. The line measurements of droplet velocity showed that the droplet axial velocity exhibits a bimodal behavior, which becomes more apparent with increasing atomizing gas pressure, a result of droplet recirculation inside the spray chamber. In addition, the peak in the droplet axial velocity distribution increased as atomizing gas pressure increased. The line characterization also showed that the droplet size distribution became more homogeneous with increasing gas pressure, and that the distribution characteristic diameters of droplets decreased consistently with increasing gas pressure. Postrun characterization of the droplet size distribution of the entire metal spray using diffraction and sieving methods indicated that the mass (volume) median diameter D ₅₀ and the Sauter mean diameter (SMD) D ₃₂ decreased with increasing gas pressure in a manner consistent with PDI results.     

Electrocapillary motion of copper and nickel matte droplets on fayalite-based slag surfaces

The electrocapillary motion of Cu₂S and Ni₃S₂ droplets on the surface of fayalite-based slags under 50% Ar-CO atmosphere has been studied for the effects of droplet size, temperature, electric field strength. Cu content m Cu₂S-FeS droplets, Ni content in Ni₃S₂ FeS droplets and slag composition. The copper matte droplets migrate to the anode (positive electrode) while the nickel matte droplets migrate to the cathode (negative electrode). Typical speeds encountered are of the order of 0.05–0.80 cm/s (Cu) and 0.16–0.62 cm^−s (Ni) with droplet diameters between 0.10 and 0.28 cm, applied potentials between 0.17 and 2.0 V/cm (Cu) and 0.75 and 3.3 V cm (Ni) and for temperatures between 1473 and 1573 K. The migration rate appears to be independent of droplet size for droplet diameters between 0.10 and 0.28 cm, but it increases with applied potential field and temperature.The effects of matte and slag contents on the migratory behavior are complex. As the Cu content in the Cu₂S-FeS matte droplet increases above 40% Cu. the migration rates also increase. Below 40% Cu matte grade, the migration rates are not significantly different. As the Ni content in the Ni₃S₂-FeS matte droplet increases, the migration rates decrease. These migration rates are also affected by the slag composition. As the Cu and Ni matte droplets migrate in opposite directions under the influence of the electric field, electrocapillary phenomena may be used to enhance metal recovery in slag cleaning operations using electric furnaces.     

Influence of atomising gas on particle characteristics of Al,Al–1 wt-%Li,Mg, and Sn powders

Abstract

The influence of physical and flow properties of atomising gas on the particle characteristics of gas atomised Al, Al–1 wt-%Li, Mg, and Sn powders was investigated in a pilot plant gas atomiser with IN4/ON18/3·5–4·0 type ‘confined design’ nozzle. In the tests, Al powders were produced under high and low pressures of argon, under air, (N₂ + O₂ ) mixture, nitrogen, and helium; Al–1 wt-%Li binary alloy powders were produced under argon and helium; Mg powders were produced under high and low pressures of argon and helium; and finally Sn powders were produced under argon, nitrogen, and helium. The morphology, size, size distribution, and surface features of the powders used in the present study were examined under SEM together with dry and wet sieving, used for sizing the powders. It was observed that high gas velocities and/or low atomising gas densities not only affect powder particle size, but also shape and surface texture. The oxygen content of the atomising gases also has an influence on the powder particle shape. In this context, powders produced under helium are finer in size owing to efficient secondary breakup; more spherically shaped in their fine size fraction in non-oxidising or difficult to oxidise atomising liquids (such as Sn and Al), because the time to breakup is shorter than that for solidification; and more irregularly shaped in their coarse size fraction in oxidising atomising liquids (such as Mg and Al–Li) owing to oxygen (the time to breakup is longer than that for solidification) compared with other atomising gases such as argon, air, (N₂ +O₂ ) mixtures, and pure nitrogen.     

A Model of Liquid Metal Droplet Vaporization in Arc Heated Gas Streams

A model of simultaneous heat and mass transfer has been constructed to describe vaporization of liquid metal droplets in arc heated gas streams. The major assumptions of the model include 1) plug flow, 2) negligible pressure drop, 3) no droplet-droplet interactions, 4) negligible gas radiation, natural convection, thermal diffusion, diffusion-thermo transport and axial conduction and diffusion, and 5) no interfacial discontinuities in temperature or concentration. The latter assumption is discussed in detail in a separate appendix. The analysis is limited to binary, nonreacting gas mixtures. The mathematical formulation results in five nonlinear, first order differential equations with temperature and compositional dependent properties. The model is applied to the vaporization of sodium and magnesium droplets injected into arc heated argon flowing in a cylindrical reactor. The effect of parameters such as initial droplet radius, reactor diameter, and liquid metal flowrate on the reactor length required to achieve a specified degree of vaporization is calculated.