Patent Grant
11780012
The present invention relates to the reduction or elimination of small, weld-attached satellite particles on the surface of gas atomized powders that are formed by contact of pre-solidified particles, typically smaller, with molten or partially solidified droplets in the spray chamber during gas atomization (GA). Non-limiting examples are embodied in several non-limiting apparatuses which were developed using a system-agnostic methodology. These disclosed exemplary apparatuses are useful for producing precision powder feedstocks that are used across the powder industry, and are able to meet the strict requirements of powder quality, primarily flowability, necessary for metal additive manufacturing (AM). A description of this method is also disclosed that is capable of analyzing an individual GA system and developing an appropriate satellite reduction apparatus and strategy based on operational and capital expense considerations.
Additive manufacturing (AM) is an extremely active area of materials and manufacturing sciences research due to its promise to open up considerable design space and to precipitate a revolution in complex shape manufacturing. While the technical barriers for AM of polymeric materials are quickly being overcome, additive manufacturing of metallic alloys remains challenging. Material feedstocks for metal AM range from sheet to wire and powder and methods for consolidation by AM vary in at least as many ways. Fabrication methods based on metal powder exhibit the most flexibility and have been widely adopted into many industries. A spherical powder shape is preferred in high quality metallic feedstock powders for the powder bed fusion (PBF) types of AM to enhance flowability, layer spreading, and loose powder packing, where either a high-power laser or an electron beam are the heat source for the highly localized melting and solidification processes that are used to add to the AM “build.” Directed energy deposition (DED) processes for AM that typically use a gas-borne powder feeder and laser melting are more tolerant of fragmented powder shapes, but smooth spherical powder also is preferred for DED to maintain a constant powder feed (i.e. mass flow rate) into the highly localized fusion zone.
As is well-known to those skilled in this technical area, PBF methods use either a laser or electron beam to melt locally and solidify a sequence of thin layers of feedstock powders together as they are spread across the build area, thereby incrementally building three-dimensional (3D) objects. The heat source is applied to particles contained within each incrementally added powder layer of a powder bed, which gradually indexes down before each layer of fresh powder is spread over the build area.
While some defects that occur during a build are build parameter or alloy design related and can be minimized or healed by post-processing, e.g., hot isostatic pressing (HIP) and/or annealing, many defects related to porosity have their origin in other “quality” attributes of powder feedstock and cannot be truly eliminated by these methods. Limits on fatigue strength and fracture toughness due to residual voids in the build are probably the most important type of microstructural defect that must be avoided for wide acceptance of critical parts made by AM, especially parts for high temperature applications [1, 2, 3, 4 of Bibliography, infra]. Thus, it is typically total void volume, void size distribution, and void shape that are characterized in detailed studies of AM build samples in an attempt to recognize an optimum “minimum void” condition [3 of Bibliography, infra]. One type of problematic (larger) porosity in AM builds can result from powder that has attached “satellites” or projections [5 of Bibliography, infra] that prevent smooth flowability and impede uniform powder layer formation during spreading of successive powder layers.
Gas atomization has been identified as the leading production method for AM powders with sufficient capacity to meet the high market demand and the potential to meet tight requirements for AM metal powder feedstocks at a low cost of production, if sufficient process improvements can be made. Current commercial GA practices include free fall (FF-GA), which inherently lacks the means to improve particle size control, and close-coupled (CC-GA), which often operates within a limited (conventional/historical) set of parameters. Currently, a lack of fundamental gas atomization knowledge that can be implemented in CC-GA process control significantly limits powder manufacturers that employ CC-GA gas-dies from increasing their process yields and improving powder quality characteristics that are most useful for AM.
Extensive development of a discrete jet version of CC-GA, often termed high pressure gas atomization (HPGA), has been spearheaded by the inventors' group for several decades as illustrated by U.S. Pat. No. 5,125,574 issued Jun. 30, 1992; U.S. Pat. No. 5,228,620 issued Jul. 20, 1993; U.S. Pat. No. 6,142,382 issued Nov. 7, 2000; and U.S. Pat. No. 9,981,315 B2 issued May 29, 2018, each incorporated by reference herein. A schematic representation of a typical atomization apparatus 10 is illustrated in
Within the above described method of producing metal powders by gas atomization, including both FF-GA and CC-GA, several mechanisms exist which contribute to the attachment of small ‘satellite’ particles S to the primary (larger) powders P and decrease in the quality of the resultant powders. These mechanisms can be divided into two broad classifications which are defined by the time/location and temperature required for attachment, as illustrated in
The expanding gas jets used to break up the primary melt create a low pressure region near the gas die 14 (denoted by ‘L’ near the top of
The present invention resulted from the inventors' effort to increase the yield of metallic powders in the size range and quality necessary for AM processing by optimizing the system design and process controls during CC-GA with the intent of reducing or eliminating recirculated fines which lead to satelliting while maintaining close control of particle size and yields. To this end, the method of optimization and subsequent embodiments pursuant to the invention includes:
As can be seen, the inventors have identified there is room for improvement in this technical art.
A primary object, feature, or advantage of the present invention is to provide methods, apparatus, or systems which solve or improve over problems or deficiencies in the state of this technical art.
Further objects, features, or advantages of the present invention is to provide methods, apparatus, or systems which:
This present invention relates to the reduction of small satellite particles on powders produced via gas atomization. More particularly, this invention relates to several apparatuses which were developed using an atomization system-agnostic methodology for the production of reduced or satellite-free powders, which are critical for improving powder flowability and compaction in many powder metallurgy (PM) applications, including additive manufacturing (AM) and HIP consolidation.
In one aspect of the invention, droplet cooling and solidification models incorporated into a robust Lagrangian particle tracking simulation allow for the determination of the critical regions of the atomization chamber in which droplets are between fully liquid and fully solid and are at the greatest risk for satellite formation. Different active and passive techniques can then be applied in these critical areas to inhibit recirculated ultrafine (typically dia.<20 μm) particulate from entering these regions within the atomizer. This invention differs from current state-of-the-art due to the atomizer-agnostic ability to pinpoint critical regions of interest inside of any atomization system and to optimize the application of the appropriate suppression apparatus based on capital expense vs. operational requirements necessary for a given design. Multiple non-limiting apparatuses for satellite suppression are disclosed based upon implementation of this technique under different atomization and industrial requirements.
These and other objects, features, advantages, and aspects of the invention will become more apparent with reference to the accompanying specification and claims.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The appended drawings include figures and illustrations which are referred to in this disclosure and are summarized as follows:
For a better understanding of the various aspects of the invention, several examples of how those aspects can be practiced are set forth now in detail. It is to be understood that these are exemplary; they are neither inclusive nor exclusive of all forms and embodiments the aspects of the invention can take. For example, variations obvious to those skilled in this technical art are a part of the invention and its aspects.
Some of the examples are discussed in the context of specific GA set-ups or operation, or specific AM applications. As will be appreciated by those skilled in this art, at least certain aspects of the invention can be applied in analogous ways to other such set-ups, operations, or applications.
As will be appreciated by those skilled in this technical art, the exemplary embodiments provide details regarding at least the following apparatus and method aspects of the invention in terms of suppression of satellites:
The following references are each incorporated by reference herein and provide background information for aspects of the invention:
The following are prior art publications or patents, each incorporated by reference herein, that provide background information about others' work in this technical field.
The following are patents, each incorporated by reference herein, that provide background information about certain techniques or components that are used in exemplary embodiments according to the invention.
At a general level, methods, apparatus, and systems according to at least certain aspects of the invention can be made and used as follows.
The broad applicability of at least certain aspects of the present invention derives from the ability to determine the critical location where external satellite formation occurs and where conditions exist that promote external satellite formation for any gas atomization system or design and allows for the rapid assessment of the effectiveness of various satellite reduction strategies, including but not limited to several novel embodiments detailed herein. Aspects of this invention can be utilized during initial atomization system design in order to evaluate effective chamber geometries and enabling strategies which reduce/eliminate satelliting, or can be retrofit to existing systems and allows for economic evaluation of effectiveness based on initial capital expenditures versus increased operating requirements/expenses.
1. Computational Fluid Dynamics (CFD) Methodology
One aspect according to the invention utilizes CFD simulation software to track metal particulate recirculation in an atomization spray chamber.
Once the model 32 is designed, the parameters for a given GA set up can be entered. See 35, and the model run to simulate the GA operation. See 36. The simulations can be used to evaluate conditions relevant to creation of satelliting (per
As will be appreciated, the modelling can take different forms and embodiments. For example, non-limiting examples of some features of the modelling are illustrated in
As such, a computational baseline for critical sources/locations of satelliting behavior can be created. Parameters about the powderization process (e.g. type of material, size and characteristics of the gas atomization chamber system, atomization pressure, etc.) are programmed into the model. Analysis of thermochemical properties of the alloy, including liquidus and solidus temperatures, are compared with simulation model results to identify areas in which to address satelliting. The modelling can reveal locations in need of process enhancement, including interventions, regarding deterring satellite formation. For example, the modeling can reveal such things as (a) the extent of predicted recirculation of fines and (b) average solidification depth.
Intervention techniques can be designed based on the results of the simulation. They can include active and passive techniques, or combinations of the same, including to reduce fines recirculation. The results are economical process optimization to produce powder feedstock for AM or other uses by promoting highly spherical gas atomized powders with minimal satellite content.
2. Atomization Chamber Interventions to Mitigate Satelliting
In one aspect according to the invention, the atomization chamber includes passive, active, or combined passive and active anti-satelliting apparatus or techniques, sometimes called herein “interventions”. The unifying concept is that each in some manner disrupts, modifies, or diverts the fine powder recirculation away from critical satelliting regions in the chamber in a manner to mitigate satellite formation there.
Technical problems in the state of the art have been summarized supra. There are a number of ways to convert metal-based feedstock to metal powder. See, for example, Dawes et al., Johnson Matthey Technol. Rev, 2015, 59, (3), 243-256, incorporated by reference herein. However, there are a variety of competing factors involved. Some of them are antagonistic with one another. Balancing of cost, complexity, end powder requirements, ability to adapt/vary to different processing set-ups and requirements are some of those factors. Gas atomization (GA) is one powder conversion technique, but with GA itself there are many competing factors including those listed above. There is considerable variability in set-ups, size/scale, and effectiveness for certain powder requirements. One particular issue is satelliting, which itself is dependent on a variety of different factors. This is recognized in Dawes et al., Introduction to the Additive Manufacturing Powder Metallurgy Supply Chain. Johnson Matthey Technol. Rev, 2015, 59, (3), 243-256 as a significant issue with GA. Thus, technical problems in the state of the art relating to effective control of satelliting in GA is neither trivial nor predictable.
The present invention pertains to solutions to the technical problems in the state of the art in several aspects.
In a first aspect, solutions according to aspects of the invention addresses satelliting caused by recirculation of fines and ultra-fines during GA processing with interventions against it or at selected locations of the atomization or spray chamber of the GA set-up. Such interventions can be passive, active, or a combination of both.
At a general level, passive apparatus/techniques can include, but are not necessarily limited to:
While such apparatus/techniques may add somewhat to the cost or complexity of a system set-up or operational expenses, the benefits of effective satellite mitigation can be substantial. For example, intervention passive, active, or in combination along the primary axial gas flow from the gas die in GA can influence satellite-causing flow eddies along the primary axial gas flow to deter ultra-fines moving to crucial regions that result in satelliting. Another example is intervention by interposing in the recirculation of fines that tends to occur in GA from the primary axial gas flow back up to low pressure regions at or near the gas die. One example is a filter that blocks such particle from re-entering the axial flow. Another is sheathing or walling off such return with secondary gas flows.
3. Combination of Prediction Method and Intervention Apparatus/Technique
Another aspect according to the invention is the combination of both the computerized modelling and an intervention apparatus/technique as individually described above. This promotes optimization of satellite mitigation by generating a rapid evaluation of any given GA set up via acquisition of the required data and use of the relevant modelling methodology (with these input parameters) to identify the critical regions. This promotes optimization of satellite mitigation by enabling selection of an intervention apparatus or technique that is deemed most effective for mitigation, cost, and practicality for a given application.
As discussed above, method 30 can inform both design and operation of GA set ups to reduce satelliting. In one aspect computer-based simulation modelling, using insights and discoveries of the inventors, allows for rapid evaluation of almost any GA set up and with efficient use of computer processing resources.
In one aspect according to the invention, effectiveness of intervention can be improved by predicting the critical area(s) for intervention in a given GA atomization chamber. At a general level, effective predictions are accomplished by combining:
Based on the foregoing, critical areas for satelliting mitigation intervention can be predicted. The prediction can be used to select an intervention, or in some cases, beneficial modification of processing parameters with no additional intervention.
In addition, the foregoing can also be used to evaluate/predict the magnitude of any satelliting issues for the given feedstock and given GA set-up with or without an intervention.
Then, the result of evaluation based on the simulations from the modelling allows the designer the option to select satelliting interventions as needed or desired.
As such, the generalized embodiments provide for individual improvements of satellite mitigation, as well as combined interventions and symbiotic improvement, according to need. As will be appreciated by those skilled in the art, the number of parameters and factors that are involved in GA are many (e.g. feedstock, pour tube orifice size, pour temperature, chamber diameter/length/form factor, gas die pressure, gas die jet area, gas mass flow, melt mass flow, gas/metal ratio, gas jet apex angle, average particle diameter, etc.). It is an inherently complex and unpredictable technology. Variation in one of these parameters can affect, sometimes adversely, one or more others. The inventors' specific modeling methodology combines specific method steps and features that allow practical and effective critical-area-predictions and intervention strategies for any GA set-up and operating parameters.
Similarly, complexity and variability of these numerous GA factors makes introduction of additional structural barriers or influences into the atomization chamber during operation inherently difficult. Adding a surface, structure, filter, or air flow can affect, sometimes adversely, the GA operation. The inventors' interventions thus may have counter-intuitive aspects.
For further understanding of the generalized aspects of the invention, specific examples of how they can be made and used will now be set forth. Reference will frequently be made to the figures in the appended Drawings. For a better understanding of examples of possible anti-satelliting interventions, an example of the methodology that can be used to assist in placement and/or estimating efficacy of any such intervention for a given GA set-up is discussed first.
1. Method of Determining Critical Areas for Intervention
With particular reference to
With reference to
Background information on CFD modeling and simulations is discussed at U.S. Pat. No. 8,756,040, incorporated by reference herein; openFOAM™ as CFD simulation software at U.S. Pat. No. 8,775,220, incorporated by reference herein; and Lagrangian particle tracking modelling at US 2018/0260499 A1, incorporated by reference herein.
For the Lagrangian phase, the droplet position, momentum, and temperature are governed by the following three equations,
where Xd is the droplet position at a given time, ud is the droplet velocity, Ug is the gas velocity. τd stands for the drag relaxation time scale and is calculated using the standard empirical correlations for a rigid sphere particle. The energy equation for droplets, Eqn. 3, includes both convection and radiation heat transfer, in which Td and Tg are the droplet and gas phase temperature respectively. h stands for the heat convection coefficient and is calculated using conventional Ranz-Marshall correlations. ∈ is the emissivity, σ is the Stefan-Boltzmann constant, and G is the local irradiation, which is solved with the conventional P1 model. P1 model is a known regression-type statistical model that predicts probability of presence or absence of a relationship between pairs of data (see, e.g., P1 is the simplest model to compute irradiation. Other options are more computationally expensive. A P1 regression-type statistical model for radiation is well known in the literature and has been cited for decades). We also consider partial solidification in the model. cp,l and cp,s stand for the heat capacity of liquid and solid metal, respectively, Hf is the latent heat of fusion, and f is the solid fraction in a droplet, which is calculated with Scheil's solidification theory [see, e.g., etheses.whiterose.ac.uk/14831/1/366334.pdf; and a fundamental reference at:
E. Scheil, “Bemerkungen Zur Schichtkristallbildung,” Z. Für Met., 34(3), 1942, pp. 70-72, both incorporated by reference herein]
as,
where TM is a reference temperature depended on the metal species, Tl is the liquidus temperature, fr is the solid fraction at the end of recoalescence. The atomization and breakup processes near the nozzle can also be modeled in this code, however verification and validation of the available models are needed for simulation of liquid metal droplet breakup (with, e.g., high melting temperature and surface energy) since they were developed for liquid fuel sprays. Therefore, in the example simulations given below, the particles were injected with prescribed size distribution near the edge of the atomization gas die, which was obtained from one of the experimental measurements. The method is illustrated in
The governing equations of the gas phase include the transport equation of mass, momentum and total energy of the gas:
where ρg is the gas density, Ug is the gas velocity, p is the pressure, g is gravity acceleration, τg is viscous stress tensor, total energy Eg=eg+½|−U{umlaut over (g)}|2 with eg being internal energy, and q is heat flux. The term, Spg, Mpg, and Hpg are the mass, momentum, and energy source terms due to the coupling with the spray. The gas thermal properties are calculated by using JANAF (see janaf.nist.gov from National Institute of Standards and Technology) thermochemical tables, and viscosity is calculated using Sutherland model.
Due to the high computation cost of full 3D simulations, a 2D axisymmetric “wedge-shaped” computational domain with a 4° angle is used in the simulations of the atomization spray chamber, as illustrated in
This example is discussed with particular reference to
Historical efforts in metal powder process improvements, especially for CC-GA atomization gas dies, were largely applicable for producing small powders (<45 μm), useful for many traditional powder metallurgical processes. This technique utilizes high atomization gas pressures and large gas jet apex angles in order to cause rapid disintegration of the melt to particles that quickly cool and solidify within the atomization chamber. The use of a narrow (i.e. 1 ft) atomization spray chamber for these processes was advantageous to inhibit recirculation and thereby suppress fine powder recirculation and gain control over external satelliting mechanisms. As can be noted from
The basis for this example was built on several decades of experience with gas atomization of a wide variety of metals and alloys, but was focused on atomization of a pre-alloyed nickel-based superalloy utilizing ultra-high purity argon as the atomizing gas and relied upon a gas die with 36-jets sized 0.0635-inches in diameter and an apex angle of 20°. Low pressures (down to ˜40 psi) were utilized in order to enhance the production of larger particles most suitable for AM applications. The specific benefit of the narrow spray chamber design (that minimized upward recirculation volume) for satellite suppression appeared to be amplified significantly by moving from traditionally high atomization gas pressures to very low pressures and this was demonstrated by the ability to maintain spherical smoothness of the resulting powder as shown in
As noted,
Example 2 sets forth the baseline analysis of the critical sources/locations of satelliting behavior in a Pilot Scale Atomizer utilizing a 2 foot atomization chamber 16. The increase in chamber diameter provides a longer path length for the large particles to solidify before any potential impact with the chamber wall and is intended to limit the formation of ‘flakes’ and ‘needles’ in the resultant powder collected. In contrast to Example 1 however, the increase in chamber diameter and necessity to include a 2 foot to 1 foot reducer prior to the powder collection system creates a ‘bottleneck’ at the exit of the chamber 16 which creates a pressure differential in the system and encourages recirculation of process gas and ultrafine (typically dia.<20 μm) particulate up the chamber walls. Many commercial-scale systems utilize similar reducers within the atomization spray chamber which create similar bottleneck-recirculation driven flows.
The basis for this application of the CFD analysis methodology was focused on typical operational conditions for pure nickel atomization utilizing ultra-high purity argon as the atomizing gas and relied upon a gas die with 30-jets sized 0.082-inches in diameter at a 14° apex angle. Liquid metal temperature was assumed to exit from the melt transport tube at 1878 K at a flow rate of 0.164 kg/s which is typical for this gas die configuration and selected metal. Critical satelliting regions were investigated for three different atomization pressures, as shown in
The Tp represents the average particle temperature. Knowing the thermophysical properties of this material (nickel), we can identify the solidus temperature Ts and probe the simulation results to determine regions above this critical temperature where satellite attachment could occur. As noted,
As can be seen from the foregoing, the spray chamber modeling methodology (e.g.
As discussed above, the modeling can predict the critical regions in the chamber 16. One way is showing where the average solidus temperature would likely be for a given set-up and operating parameters. As discussed with respect to
But further, it can allow optimization of effective interventions during initial design of a system. The computationally-efficient simulations allow the variables to be easily and quickly adjusted to allow comparison between interventions during retrofit design. The method can be used to compare different set-ups and operating parameters with the inclusion of possible interventions to predict whether the intervention will be effective, before any funds are spent on installation of an intervention and on GA trials. For example, a proposed gas halo can be simulated in terms of its placement and operating parameters and in the context of a given GA set-up and operating parameters. The modelling can provide information by which the designer can predict if the proposed gas halo will be effective.
As will be appreciated by those skilled in this technical art, the magnitude of satelliting reduction that is needed or desired can vary. For example, in some applications, any predicted or actual reduction of satelliting compared to operating without an invention will be considered satisfactory. In other applications, a significant reduction of predicted or actual satelliting will be considered satisfactory. For purposes of the invention, the term “effective” regarding reduction of predicted or actual satelliting will mean any predicted or actual reduction of satelliting compared to operating without the proposed intervention(s). But as shown herein, aspects of the invention can result in very significant satellite suppression over operating without the proposed intervention. In at least one example, the reduction can be quantified by a significant difference in various types of powder flowability measurements, e.g., Hall flowmeter, for powders that have dia.>45 μm.
There are several ways to define efficacy of this invention.
One such method is through analysis of high-resolution micrographs of the powders using image analysis tools and actually performing a count of the average number of satellites per particle. Advances in machine learning are making this task much easier and informative way of quantifying the effectiveness of these strategies.
Additionally, powder rheometry/powder flow testing can effectively measure the flowability of a powder and is an external test which can be applied to qualitatively access the improvements from anti-satelliting strategies. These tests can be affected, however, by humidity in the air, oxide formation on the surface of the powders, or even static ‘cling’ making interpretation of these results a bit difficult at times.
The satellites S can be counted (manually or digitally) for a given number of particles P in a sample. Then, all the samples will give one statistical value, say X % content is satellited. The X % with apparatus and without apparatus are compared. The relative change in percentage could be the reduction or increase per use of the invention properly or wrongfully, or the lack of use.
As can be further seen, the methodology allows for a highly flexible and effective way to characterize any GA set-up in terms of critical satelliting regions. The simulations can be generated for different operating conditions and GA setups. It allows for side-by-side visualizations/comparisons. It can efficiently and automatically or at least semi-automatically predict the critical region(s) for a set-up. Thus, the prediction(s) are effective to help optimize an intervention.
As will be appreciated by those skilled in the art, the foregoing method meets or exceeds at least one or more of the stated objectives of this aspect of the invention. The precise steps of the methodology can vary.
2. Atomization Chamber Interventions to Mitigate Satelliting
Whether or not the method of predicting critical regions is utilized, examples of several specific interventions are now described. Some are passive. Some are active. Some combine passive and active. These examples will be discussed with particular reference to
The atomization apparatus 10 commonly used by the applicants in proof of concept of at least this exemplary embodiment includes a single bottom-pour crucible capable of shorter duration batch runs when compared with more commonly used industrial ‘tilt-pour’ melting systems with a heated tundish. A tundish is a smaller reservoir with a small open bottom orifice and a large top receiver opening into which a large batch of molten metal is slowly poured. The open bottom orifice is mated to the ceramic pour tube which delivers the molten liquid to the atomization gas die 14. Due to the low quantity of material typically which can be run in a bottom pour crucible, in this embodiment of the invention, unlike many industrial atomizers which utilize a water jacket to keep the chamber walls sufficiently cool, the applicants utilized additional ‘gas halos’ 40 downstream in order to increase the rate of particle cooling, ensure chamber walls are maintained sufficiently cool, and provide the added benefit of allowing addition of different gas species and compounds to the spray chamber atmosphere to control the surface chemistry on the particles. Furthermore, the careful placement of these halos 40 can help to control the recirculation of fines within the spray chamber 16 and thereby reduce the external satelliting effect. Alternatively, incorrect placement of the halos 40 can result in a worsening of any constriction at the spray chamber exit and cause increased satelliting from recirculated powders. For example, adding too many halos or too much flow of supplementary gas into the spray chamber can overwhelm the exit capacity of the chamber 16 and add to the pressure driven flow dynamics inside the chamber. In general, supplementary halos 40 should be a) placed so as to divert the full chamber recirculation flows and to limit the upward transport of solidified ultrafine powders that are available for satelliting and b) placed such that a gas “curtain” is able to redirect secondary powder flows away from the critical satelliting regions.
Essentially, in an industrial tilt-pour system, hundreds of pounds of material are melted and poured into a smaller crucible—the tundish—which is connected to the ceramic pour tube that delivers the molten metal to the atomization gas die. This allows for industrial practitioners to run for many hours, even swapping the large metal bath in the middle of the run, or continually feeding this molten pool for continuous operation. Thus it is necessary for industrial practitioners to supply additional means of removing excess heat (the water jacket). A bottom pour crucible is a batch process with a limited volume of material which can be atomized and thus a limited duration of the run. Typically, the gas halos 40 are sufficient and beneficial to cool the atomizer components and prevent over-heating, as described later herein.
Generally, these passivation processes are known or in published literature or patents (including by the present authors). Gas species can include tightly controlled oxygen concentrations (ppm level), fluorinated compounds such as SF6 or 3Ms Novec 612, etc. This ‘reaction gas’ could be included in COANDA flow, another intervention option, discussed infra.
As will be appreciated, the correct placement can be informed by simulation results from the predicting method based on the computer-assisted modeling as previously described. Here, again, the effectiveness of placement means that it is at least approximately at or near to a position in the modelled chamber. As will be appreciated by those skilled in the art, to be effective the placement can be approximate. But as will be further understood, modeling or evaluation of modelling can allow for quite specific dimensional resolution for a given chamber. For example, a critical zone upper and lower boundaries predicted by average liquidus and average solidus temperatures Tl and Ts respectively can be typically resolved to within plus or minus 6 inch(es) in the scale of chambers in the specific exemplary embodiments, which are between 1 ft. and 2 ft. diameter and between 8-10 ft. in height. Similar resolution is envisioned to be likely for larger scale chambers. But, further, effective placement refers just to predicting where in a chamber 16 an intervention should be placed. The modelling allows variation in input parameters to then predict the effectiveness of the invention relative to satelliting, which has been discussed earlier. Actual empirical testing can then be used to test actual effectiveness of anti-satelliting. Furthermore, once an intervention is positioned, whether in simulation or actual testing, further simulation or empirical testing of the invention in different positions from that original one can be used to optimize effectiveness of the invention.
Resolution is dictated both by grid size of the simulation and the accuracy of the heat transfer and solidification models used. Grid size is only limited by the total number of grid points that can be accommodated by the computational resources and time allotted to solve the problem. Using either a high-performance desktop system or high performance computing resources, the 2D axisymmetric case is not too computationally expensive to run and so grid resolution can be quite fine.
Ultimately, the question then becomes accuracy of the heat transfer and solidification models to predict the critical region successfully. For well-known materials (pure metals, well studied alloys, etc.) these models can be robust and provide a very reliable approximation of the critical satelliting region. For novel alloys with less thermophysical information known, these become less accurate overall. Thus, one skilled in the area of heat and mass transfer and thermodynamics should be able to make an informed decision as to the accuracy of the results and the appropriate ‘safety factor’ to place on the calculated critical satelliting region. The basis for proof-of-concept testing for this embodiment was focused on analyzing the effect of using multiple gas halos, ‘traditionally’ placed inside of the atomization chamber for the purpose of chamber cooling and powder passivation, on increasing or decreasing satelliting behavior for pure nickel atomization utilizing ultra-high purity argon as the atomizing gas and relied upon a gas die with 30-jets sized 0.082-inches in diameter at a 14° apex angle.
For both purposes of experimental investigation and CFD simulation of satelliting behavior due to the use of gas halos 40, atomization gas was supplied at 158 PSI. Liquid metal temperature was assumed to initiate at 1878 K at a flow rate of 0.164 kg/s which are typical for this gas die configuration and selected alloy. Two cases utilizing gas halos were explored utilizing the critical region predicting method discussed supra with experimental results provided for one of the prediction cases. First, a CFD study (per the critical region predicting method discussed supra) utilizing a single halo at various chamber heights was used to optimize the position for increased satellite reduction. Then, utilizing a series of four gas halos 40A, B, C, and D (as illustrated in
As discussed above, the authors have previously used gas halos for introduction of chemically passivating gas species, to increase the particle cooling rate, and for cooling of the atomization chamber walls. The current effort demonstrates the first effort to utilize these supplemental gas halos for the purpose of mitigating satelliting on the powders. As can be seen from the modeling results, the ‘traditional’ configuration to maximize chamber and particle cooling actually yielded unfavorable results for satelliting, and the correct placement of a single halo actually proves to be a better implementation of this strategy. We include both cases in this document to illustrate the utility of the modeling approach coupled with the experimental validation. As noted,
Gas halos are essentially ring-shaped plenums 44 operatively connected to a pressurized fluid (gas phase) source 46 with multiple radially-inward pointing outlets or nozzles 42 around the ring which are oriented according to a specified angle relative to the direction of flow 41. The fluid source pressure can be controlled to modify the overall gas flow rate. The design of these gas halos can be as simple as bending a copper tube into a fixed diameter circle and drilling holes at a fixed angle around the entire length of the tubing and mounting it at the appropriate height inside the atomization spray chamber. Alternatively, the gas halos can consist of unique chamber sections with integrated gas plenums and tapped holes about the entire inner circumference of the gas plenum. Discrete gas jets at fixed angles can then be mounted to each tapped whole and can be designed with jets based upon the jet orifice size, spray angle, or other desired features. In general, the halos can be designed regarding number, orifice size, and nature so as to influence the “spray pattern” and coverage both individually and collectively. Examples of gas halos to inject reactive gases into a GA chamber are described at US Published Patent Application 2018/0133793A1, incorporated by reference herein, and at
In contrast, the reactive gas injectors in the above-cited patents inject the reacting gas straight towards the center of the chamber to ensure maximum mixing of the reaction gas with the powders to passivate the surface. The halos used here use an angle of the gas jets either coflowing with the process stream or at a 45° angle inward.
As noted, this combination of halos 40 not only influence satellite mitigation at the critical region inside the chamber, but can also supplement cooling in other parts of the chamber. Such cooling can have at least the following benefits: (a) influencing particles to stay in the primary gas stream from the gas die to deter upward movement or recirculation to the critical region; (b) promote complete solidification of the droplets before collision with the chamber walls or the bottom. As will be appreciated by those skilled in the art, the specific form factor, number of openings, directions of jets (e.g. angle to a lateral plane through the chamber), gas source, and pressure can vary according to need. Typically, the design would at least be effective to promote droplet cooling and solidification.
CFD modeling of a single gas ‘halo’ 40 was investigated for two different positions (1 foot and 2 foot) downstream from the atomization region (illustrated by halo ‘Beta’ in
As can be seen from the results of the CFD study in
As discussed above with reference to
But, importantly, proof-of-concept of the two position modelling can be extrapolated. For example, three, four, five, or more alternative halo placements can be modelled and the results compared. Theoretically, the number of placement options is unlimited. Using many possible placements, with differences just inches or factions of an inch apart, would allow much more minute resolution of analysis. Of course, there are also practical limitations on how many choices would be modeled and compared. Similarly, more choices and variability between choices could be modelled for the other variables of such systems. As mentioned, non-limiting examples include halo orifice diameter and direction, gas pressure and rate, gas source and characteristics. And, then, chamber and operating parameters can be varied. The modelling would allow relatively easy, efficient, and economical variation of many variables.
As such, from cruder comparisons between a limited number of choices, to much more refined resolution between many more choices, this aspect of the invention is shown to promote one or more objects of the invention, including identifying critical regions of almost any GA set up, and then help in selection of an intervention for the purposes of the invention.
CFD modeling of a case where four gas ‘halos’ 40 was investigated utilizing typical experimental heights and flows (illustrated in
The proof-of-concept of
Example 4 details a hybrid apparatus consisting of both active and passive methods of satellite reduction. This apparatus consists of a combination of (a) a Coanda-type surface 50 installed in the inside wall of the atomization chamber 16 in the critical region and (b) pressurized gas 54 injected so that it exits on the inner-facing Coanda surface 56/57 (see
The Coanda effect is the tendency of a fluid jet emerging from an orifice to follow and adjacent flat or curved surface and to entrain fluid from the surroundings so that region of lower pressure develops. Examples of a Coanda surface with gas injection can be seen at U.S. Pat. No. 10,364,984 B2 and US 2013/0053796 A1, both incorporated by reference herein.
Specifics about the Coanda surface and gas injection used in this example are:
The filter 60 of is shown also at
As can be seen in
In addition, the filter media 61 is interposed in the circulation pathway coming up from the bottom of the chamber 16 along the chamber walls. The media 61 would basically be a ring of 6-12 inches in diameter, and 0.005-0.25 inch thickness, placed at or above the Coanda device 50 and essentially intercepting at least most of the recirculation flow along the chamber wall. The filter characteristics are selected to balance a meaningful removal rating of the types of fines or ultra-fines that cause satelliting from the recirculation, but without substantially or negatively affecting yield from the GA by creating an undesired vacuum pressure condition inside of the atomization zone.
US Published Patent Application 2002/0125591 A1 issued Sep. 12, 2002 and US Published Patent Application 2008/0271568 A1 issued Nov. 6, 2008, each incorporated by reference here and patents by Dunkley (2008) and Praxair (2000), regard use of re-circulation gas for satellite suppression and recycle of process gas, respectively. These patents discuss commonly used methods and require either a blower or compressor to use the (Clean) recirculated process gas. Contrary to prior art, the uniqueness of the vacuum created from the atomization gas die and Coanda flow device, according to embodiments of the present invention, are sufficient to drive process gas recirculation.
Several examples of filter media 61 and its characteristics are indicated in the table supra. As will be appreciated by those skilled in the art however, other filter media and set-ups that achieve desired or needed results are possible. For example, woven wire mesh, sintered powder metal filters, or similar. Different materials of construction and methods of fabrication of these materials are also included. A principal function of the filter media is to have an effective removal rating of the types of recirculating particles that can cause satelliting. By effective removal rating is meant at least removal of more such particles than without a filter.
As noted, this combination of Coanda device and filtering not only influence satellite mitigation at the critical region inside the chamber, but can also supplement this anti-satelliting intervention with physical removal of particles that can cause satelliting. As will be appreciated by those skilled in the art, the specifics of the Coanda device and the filter can vary according to desire or need. Typically, the design would at least be effective to reduce satelliting.
One of the critical concepts is to not be restrictive of recirculation flow. Both the Coanda-flow AND the primary atomization gas create negative pressure regions which can drive particulate flow into the critical satelliting region. IF the filter restricts too much flow, then negative consequences may result and actually realize increased satelliting behavior.
With reference to
The example of chamber diameter has been previously discussed in the context of prediction of a critical area for placement of an intervention, in particular, a gas halo or halos. But that was in the context of a specific type and set of GA set ups.
The present example indicates that, similarly, varying one or more of the typical physical characteristics of any GA set up can reveal predictions via the modeling methodology that can be beneficial for GA operation, including anti-satelliting. Even if such insights provide minor or even marginal improvements to anti-satelliting, it can still be highly beneficial. Even relatively small improvements to any of flowability, powder morphology, or other end product powder characteristics, as a result of an aspect or aspects of the invention can be significant.
In the example of chamber height, the following considerations are important. 1) Increased chamber height increases the global potential for powder cooling available in the chamber. 2) Increasing chamber height increases the number of localized recirculation eddy-flows and required number of turbulent flow pathways necessary in order to successfully navigate the global recirculation necessary to reach the critical satelliting region. Additionally, increased chamber height can serve to reduce the influence of the ‘bottleneck’ phenomena due to flow restrictions common in industrial atomizers due to the long Height:Diameter ratios. Of note, is that chamber height generally adds complexity to the setup and operation of an atomizer and may greatly increase the capital costs associated with a new system. Optimization of this chamber height is critical to obtaining the greatest benefit in minimizing recirculation while minimizing the costs and complexity of implementation. Other factors that could be used as variables are listed below:
With reference to
Example 6 relates to satellite suppression via external clean process gas recirculation and Coanda-driven gas sheath flows. It is similar to Example 4 of
This example is a modification of the Coanda/filter hybrid concept and simulation results should be similar in nature. The big difference in the application is the reliability of the process for an industrial adoption scenario. Either, the driven gas is cleaned internal to the atomization chamber due to the use of filters, or, the driven gas is cleaned via regular processes and recycled externally. The advantage of external gas recirculation is not having to worry about plugging of screens or filters and loss of effectiveness at different times during an atomization. As diagrammatically illustrated in
To further describe, in this example, a moderate pressure gas source is used to direct gas onto the Coanda surface 56 to generate the anti-satelliting sheath flow. The typical GA set up creates low pressure around the gas die 14 (see the topmost “V” symbols in
Additionally, in this example the gas for Coanda sheath flow can be advantageously taken from sources available in conventional GA set-ups. Here the sources are from process gas used for the GA process and prior to a wet scrubber that generates its own pressure flow and is used in many typical GA set-ups. In this example, no additional sources are needed. The only major structural modification is (as shown in
As will be appreciated, the powder-free gas via plenum 76 could be used alone (without the Coanda device 50 and its gas injection. The modelling technique according to the invention could be used to evaluate the same.
As noted, this combination of Coanda device and upper plenum gas flow to the Coanda device not only influence satellite mitigation at the critical region inside the chamber, but can work together to reduce satelliting. As will be appreciated by those skilled in the art, the specifics of the Coanda device, the upper plenum, and their cooperation can vary according to desire or need. Typically, the design would at least be effective to reduce satelliting.
With reference to
Example 7 relates to satellite suppression in a Pilot Scale Atomizer with 2 foot diameter chamber, as described supra, but utilizing one or more passive baffles 80.
By passive baffle, it is meant that a physical structure or apparatus that alters gas flow is installed or built-into the interior of the atomization chamber 16.
One implementation of the baffle concept is to combine the benefit of the 1-foot chamber (illustrated above) while mitigating the risk of flake formation due to collision of un-solidified droplets with the chamber walls. Essentially, a 1-foot diameter tube can be mounted concentric to the atomization gas die 14 at the top of the chamber 16 in order to create a “narrow-chamber” baffle section 80. The length can be determined via analysis of the spray pattern 17 and selecting a length such that the spray will not impact the narrow baffle, thus eliminating the formation of flakes while minimizing localized recirculation inside of the baffle and preventing global recirculation of fines into the critical satelliting region. As can be appreciated, multiple baffles 80 of increasing diameter could be suspended with lengths corresponding to the greatest extent as not to impact the spray, if additional protection is needed further downstream as determined from the CAD analysis of critical satelliting regions inside of the atomization spray chamber.
In another implementation, rather than fixing the baffle directly to the chamber roof and creating a localized vacuum at the top of the baffle section, the baffle can be affixed to a clean recirculated gas supply plenum, similar to Example 6, in order to further reduce the chance of local recirculation driven satellite formation inside of the baffle.
As will be appreciated by those skilled in this technical area, air flow baffles are used in a wide variety of industries and applications to alter air or gas phase flow. One example is in commercial or residential refrigerators. Another is commercial or residential heating and air conditioning. In many of these uses, the baffles are aerodynamically engineered and emplaced to direct flow, including to set up air sheaths or curtains within an enclosed space.
Similarly, in this example, these completely passive structures can be used to influence air sheaths or curtains at least similar to those created by the gas halos, Coanda devices, or plenums in the examples supra. For example, the baffle 80 of
Dimensional features of the baffle 80 of
As noted, this passive physical baffle 80 is designed to influence satellite mitigation at the critical region inside the chamber. As will be appreciated by those skilled in the art, the specifics of the baffle can vary according to desire or need. Typically, the design would at least be effective to reduce satelliting.
Prototyping, vol. 6, no. 4, pp. 215-223, 2011.
Handbook - Ninth Edition, Metals Park, Oh, American
As mentioned, the invention can take many forms and embodiments. The exemplary embodiments are just a few. For example, variations obvious to those skilled in the art will be included within the invention.
A few additional examples are as follows:
1. GA Set-Ups
As discussed supra, the methodology of modeling for critical regions and effectiveness of interventions is described primarily in the context of the Pilot GA set up in a CC-HPGA of relatively small scale (e.g. ˜1 or 2 foot chamber diameter) with processing parameters mentioned in those examples. But as appreciated, the methodologies and the intervention apparatus/methods can be applied similarly in analogous ways to other scales of such set-ups or to other GA set ups.
2. Modelling
As discussed supra, the methodology of modeling for critical regions and effectiveness of interventions is described primarily in the context of specific modelling selections, programs, and algorithms. But as appreciated, variations of the methodology can be applied similarly in analogous ways with other versions.
3. Interventions
As discussed supra, the interventions are described primarily in the context of the Pilot GA set up in a CC-HPGA of relatively small scale (e.g. ˜1 or 2 foot chamber diameter) with processing parameters mentioned in those examples, and gas halos, Coanda devices with gas injection, plenums with gas injection, or baffles. But as appreciated, the intervention apparatus/methods can be applied similarly in analogous ways to other scales of such set-ups or to other GA set ups, or different configurations for control of gas flow.
4. Filtering
As discussed supra, filtering to remove or decrease particles capable of satelliting as a part of or addition to other interventions are described primarily in the context of the filtering media of specific Example 4, supra. But as appreciated, the filtering apparatus/methods can be applied similarly in analogous ways by different filtering apparatus/methods.
5. Combinations of Features
As discussed supra, the methodology of modeling for critical regions and effectiveness of interventions can be used apart from the interventions and vice versa. They can be used together. The interventions can be used individually or in various combinations.
This application claims the benefit of Provisional Application U.S. Ser. No. 62/705,347 filed on Jun. 23, 2020, all of which is herein incorporated by reference in its entirety.
This invention was made with Government support under the U.S. Department of Energy (DOE) Contract Number DE-AC02-07CH11358. The government has certain rights in the invention.
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