Liquid Metal Stream Research Articles

Direct Modeling of the Simultaneous Flow of Compressible Atomizing Gas Jets and a Weakly Compressible Liquid Intermetallic Stream During Gas Atomization

AbstractThe authors have developed a new atomization model enabling direct numerical simulation of the simultaneous flow of compressible atomizing gas jets and a weakly compressible liquid metal stream. It has been used to simulate the atomization of a Ni-50wt.%Al melt stream by argon gas jets in a closed-coupled atomizer. The 2D simulation results show that the presence of the liquid intermetallic stream significantly influences the field variables, particularly the aspiration pressure. At the gas plenum pressure used, the gas nozzles are choked and hence the gas flow upstream of the tip of the liquid delivery tube is not influenced by the presence of the liquid intermetallic stream, whereas the downstream gas flow is affected. Significant differences between model predictions assuming either incompressible or compressible gas are reported. Besides the atomization of liquid intermetallic stream by argon gas, this unified atomization model is available for use to simulate a variety of different twin-fluid atomization processes.

Multiphysics modelling of the spray forming process

An integrated, multiphysics numerical model has been developed through the joint efforts of the University of Oxford (UK), University of Bremen (Germany) and Inasmet (Spain) to simulate the spray forming process. The integrated model consisted of four sub-models: (1) an atomization model simulating the fragmentation of a continuous liquid metal stream into droplet spray during gas atomization; (2) a droplet spray model simulating the droplet spray mass and enthalpy evolution in the gas flow field prior to deposition; (3) a droplet deposition model simulating droplet deposition, splashing and re-deposition behavior and the resulting preform shape and heat flow; and (4) a porosity model simulating the porosity distribution inside a spray formed ring preform. The model has been validated against experiments of the spray forming of large diameter IN718 Ni superalloy rings. The modelled preform shape, surface temperature and final porosity distribution showed good agreement with experimental measurements.

Clean metal nucleated casting

Nucleated casting is a method of casting metal below its liquidus temperature, casting as a semisolid rather than as a liquid. In nucleated casting, a stream of liquid metal is directed into a gas atomizer, which converts it into a spray and accelerates the spray droplets from the atomization zone toward a collection mold. Because of the high surface-to-volume ratio, high relative velocity, and a large temperature difference between the gas and droplets, the droplets lose heat very efficiently to the surrounding gas. Droplet diameter and process parameters such as gas-to-metal flow ratio, process rate, and spray distance have significant impact on the solid fraction of the metal at the collection zone. Small particles lose all of their heat of fusion and are fully solid when they strike; they will likely remelt in the semi-solid pool. Large particles are fully liquid when they strike. Medium-size particles are partially solid. When the particles strike the surface of the pool, a large number of nucleation sites are available for further solidification. Each nucleation site grows until it strikes an adjacent site and all local liquid is consumed and a fine-grained metallurgical structure results. When operated at high gas-to-metal ratio the semisolid will have small ( 75%) fraction liquid, and a mold is required to contain the metal as it continues to solidify. This process is termed nucleated casting. Alloy 718 metal has been cast in a prototype nucleated casting system for demonstrating the feasibility of the new process. Metallographic examination shows that the as-cast material possesses a uniform, equiaxed 0.075 mm (ASTM 4.5) grain structure. Macrosegregation-related defects were not found in the casting or in forgings that were obtained from the ingots.

A power–voltage scaling law for liquid anode X-ray tubes

An analysis is presented of the relative magnitudes of factors affecting the specific loadability (Wmm−2K−1) of liquid metal anode X-ray sources (LIMAX). These factors are to a certain extent analogous to those, such as focus width, focus-anode relative sweep velocity and the anode thermal conductivity, of importance in solid-state (e.g. rotating anode) X-ray tubes.It is shown that in general, convection cooling of the electron window by the liquid metal stream represents the least efficient thermal transport process in LIMAX. As the energy deposited in the electron window through inelastic electron scattering is approximately inversely proportional to electron energy, the power loadability of a LIMAX source operated under constant conditions scales to first approximation with the square of the tube potential. This power–voltage scaling law of liquid metal anode X-ray sources is without analogue in solid-state X-ray sources.A summary is presented of potential applications of high continuous power X-ray sources operating in the high-voltage region of several 100kV.

Liquid metal anode X-ray tubes and their potential for high continuous power operation

A novel type of electron-impact X-ray source is described in which X-rays are produced in a turbulently flowing liquid metal that is separated from the vacuum region of the X-ray source by a thin membrane. Following a summary of the physics of electron and photon transport applicable to the liquid metal anode X-ray (LIMAX), the three diffusion processes responsible for thermal transport in (electron diffusion, heat conduction and turbulent mixing) are briefly discussed and their relative importance is quantitatively assessed.A simple Gaussian model is presented allowing the characteristic ranges of the three diffusion processes to be combined into a mean total diffusion range. The extent to which heat diffuses in the time taken for the liquid metal stream to pass the electron focus permits the loadability (electron beam power density per unit maximum anode temperature rise) of the turbulently flowing liquid metal target to be assessed.A description of an experimental LIMAX facility constructed in these laboratories is given. Preliminary experimental results from the LIMAX facility are presented and satisfactory agreement is found between these results and the predictions of the Gaussian model for the mean total diffusion range. The suitability of the LIMAX proposal for X-ray scatter imaging investigations is briefly considered.

Production of rapidly solidified metal powders by water cooled rotating disc atomisation

In this study production of rapidly solidified metal powders by water cooled rotating disc has been investigated. A rotating disc was cooled by water which was not in contact with the molten metal and the produced powders during atomisation. This method is an effective process to produce rapidly solidified powders and ribbons.Tin, lead, aluminium, zinc, and AA 2014 aluminium alloy powders and ribbons were produced using different types of discs. Parameters such as disc rotation velocity, disc shape, diameter of liquid metal stream, and superheat were investigated. The results showed that increasing disc rotation velocity, decreasing liquid metal stream diameter, and increasing superheat resulted in finer particles as expected.Different types of discs such as flat, flat with fins, and cone with fins were used to investigate the effects of the disc geometry. Performance of the flat type disc with rectangular fins was better than that of the others.Metallographic examinations showed that estimated cooling rates of atomised AA 2014 powders were between 104–105 K s-1 depending on particle size. With the ribbons cooling rates were relatively higher, of the order of 105–106 K s-1.

Spray casting of steel strip: Process analysis

Near-net shape manufacturing (NNSM) of thin steel sections by spray casting eliminates casting as a separate step with attendant improved microstructures and properties and significant energy savings. The process involves atomization of a stream of liquid metal and deposition of droplets in the generated spray on a moving substrate at mass flow rates of 0.25 to 2.5 kg/s. In this paper, NNSM of steel strip by the Osprey spray casting process is investigated by combining numerical simulation and experiments. Critical input parameters for the computation are quantified utilizing existing state-of-the-art mathematical models and specific experiments. Numerical computation of the consolidation of the spray at the substrate during manufacture of thin sections is conducted using bothcontinuum anddiscrete event (“splat solidification”) approaches to predict: (1) variation of strip thickness in the transverse dimension and (2) isotherms and cooling rates across the strip thickness. Predicted geometries of the strip simulated by the continuum model are in good agreement with measurements. Predicted isotherms in narrow strip by the continuum approach are in reasonable agreement with thermocouple measurements for intermediate thicknesses (2 to 5 mm), and the observed microstructure is consistent with predicted cooling rates. The discrete event model predicts significantly higher cooling rates than the continuum model in the basal portion of the strip. This is consistent with the observed grain size in thin strip (<l-mm thick) and in the basal portion of thick strip. Beyond a threshold thickness, however, the discrete event model confirms the formation and persistence of a partially liquid layer at the growing surface of the deposit with an attendant decrease in the cooling rate. The influence of critical parameters on “splat solidification” is analyzed and assessed.

Analysis of the spray deposition process

Net or near net shape products can be manufactured by technologies involving solidification processing, metal forming, paniculate processing, and droplet consolidation. One example of droplet consolidation is spray deposition in the Osprey tm mode. In this process, a stream of liquid metal is atomized by an inert gas to form a spray of molten droplets; these are accelerated towards a substrate where they impinge and consolidate. An integral model for the Osprey tm spray deposition process has been developed using established theoretical principles. Mathematical models describe the interconnected processes of droplet-gas interactions in flight and subsequent droplet consolidation on the substrate. The models predict droplet velocity and temperature as a function of flight distance, the extent of droplet solidification on arrival at the substrate, and temperature distribution in the consolidated material during deposition. This approach demonstrates the utility of modeling studies in order to establish quantitative guidelines for optimization of the process in terms of the evolution of microstructure in droplet consolidation.

Deflection of a stream of liquid metal by means of an alternating magnetic field

When coils carrying high-frequency currents are placed in the neighbourhood of a stream of liquid metal (or other electrically conducting fluid), the magnetic pressure on the liquid surface causes a deflection of the stream. This effect is studied for a two-dimensional stream on the assumptions that the width of the stream is small compared with the scale characterizing the applied magnetic pressure distribution, and that the effect of gravity may be neglected over this scale. The relationship between the angle of deflection of the stream and the power supplied to the perturbing currents is determined. More complex deformations associated with distributed current sources are considered. Experiments are performed in which a thin sheet of mercury is deflected by two antiparallel line currents. The agreement between theory and experiment is reasonable, despite a tendency towards three-dimensionality in the latter. A second configuration is considered in which a thin current-carrying circular jet is deflected by a vertical line current. The path of the deflected jet is calculated. The limitations of the analysis are briefly discussed.

Atomization of Specialty Alloy Powders

This article reviews powder making methods involving the atomization of liquid metal, with particular reference to specialty alloys. The methods are classified as commercial, pilot-scale, and laboratory-scale. Powder characteristics and properties are considered in relation to process parameters; where available, mechanisms of the break-up of the liquid metal stream or sheet are analyzed.

Interactions between nitrogen jets and liquid lead and tin streams

The break-up of a stream of liquid metal by high-velocity gas jets is an important technique for the production of metal powders. For a reasonably complex atomization geometry using four nitrogen jets, high-speed and schlieren photography have been used to gain an understanding of the influence on the disintegration of streams of lead and tin of variables such as metal and gas flow rates and the nature of flow in the gas and metal streams. The actual mechanism of disintegration and the resulting particle size distributions have been discussed by use of detailed experimental information on the velocity distributions in the system of four nitrogen jets. The process of disintegration may be divided into three stages: primary disintegration, secondary disintegration and solidification. The conditions for each of these stages are discussed.

Heat exchange of bundles of rods placed at an angle to an incoming stream of liquid metal

Results are presented of an experimental investigation of the heat exchange between liquid metals (Pr=0.007 and 0.03) and bundles of tubes inclined at anglesψ=30, 60, and 90° to the heat-transfer agent. The experiments were performed in the Pe number range (for the incoming flow) 10–600. Hints for the calculation of the heat-exchange coefficients are given.

Behavior of liquid metal stream in a transverse magnetic field

This is a theoretical study of the effect of a transverse magnetic field on the flow characteristics of a two-dimensional laminar submerged jet stream of an electrically conducting, viscous, incompressible fluid. The analysis may be applicable to liquid metal jet streams for two limiting cases: (i) small magnetic field or (ii) slow motion. In either limiting case, an increase in the intensity of the magnetic field is shown to enhance the extent of velocity decay in the downstream direction.

Graphite-containing refractories for ingot stools for top-pouring killed steel

Graphite-containing inserts made by the semi-dry method have an exact shape and size, exhibit high thermal conductivity and spalling resistance, and are resistant to the erosive and corrosive effect of a stream of liquid metal and to the oxidizing effect of high temperatures. The inserts can be used for multiple ingot casting in two molds, on condition the stream of liquid steel is kept central. The use of graphite-containing inserts manufactured by the technique developed by the authors does not affect the quality of the metal.

Heat-transfer in a tube to sodium-potassium alloy and to mercury

The existing experimental data on heat transfer to liquid metals often do not agree with each other, nor with theoretical values. The reasons for the discrepancies have so far not been clear. From the present work we can draw the conclusion that the main reason for the discrepancies is the thermal resistance that exists at the boundary between the heat-transfer surface and the liquid metal. We have come to this conclusion after measuring the heat-transfer coefficient to liquid metal by two methods: measuring the temperature distribution in the stream of liquid and measuring the temperature of the wall. The temperature distribution in the stream agreed with the distribution calculated from Lyon's equation. It was found that contamination of the sodium-potassium alloy by oxygen leads to a decrease of the heat-transfer coefficient. The results of experiments carried out in the absence of contact thermal resistance at the boundary between the wall and the liquid metal are in agreement with the results of Isakoff and Drew, and also with those of Brown, Amstead, and Short. The present paper contains a description of the experimental apparatus and of the tube section and thermocouple with which the temperature distribution in the stream of liquid metal was measured.

Heat Transfer in Pipes to a Sodium-Potassium Alloy and to Mercury

Experimental data on heat transfer to liquid metals are often conflicting and do not accord with theoretical results. The reasons for these discrepancies have not been clear. The present work shows that the main reason for the disagreement is the thermal resistance between the heated surface and the liquid metal. This conclusion is reached from calculations of the heat transfer coefficient by two methods: from measurements of the temperature distribution in the moving liquid and from measurements of the wall temperature. The former agrees with the distribution calculated from Lyon's equation. It has been found that the presence of oxygen in a sodium-potassium alloy reduces the heat transfer coefficient. The results of experiments with no contact thermal resistance agree with those of Isakoff and Drew and of Brownet al.A description is also given of the experimental apparatus, the test section, and a thermocouple whereby the temperature distribution in a stream of liquid metal can be measured.

Oxygen in liquid open-hearth steel—Oxidation during tapping and ladle filling

A mass of circumstantial evidence is presented to indicate that the main source of alloy losses in open-hearth tapping is oxidation by air, with the steel apparently reacting with an amount of oxygen equivalent to about 30 times its own volume of air. The effect is erratic from heat to heat, depending largely on turbulence and distance of free fall of the stream of liquid metal.