Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F9/00—Making metallic powder or suspensions thereof
  • B22F9/02—Making metallic powder or suspensions thereof using physical processes
  • B22F9/06—Making metallic powder or suspensions thereof using physical processes starting from liquid material
  • B22F9/08—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying
  • B22F9/082—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying atomising using a fluid
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F9/00—Making metallic powder or suspensions thereof
  • B22F9/02—Making metallic powder or suspensions thereof using physical processes
  • B22F9/06—Making metallic powder or suspensions thereof using physical processes starting from liquid material
  • B22F9/08—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying
  • B22F9/082—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying atomising using a fluid
  • B22F2009/0824—Making metallic powder or suspensions thereof using physical processes starting from liquid material by casting, e.g. through sieves or in water, by atomising or spraying atomising using a fluid with a specific atomising fluid
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
  • B22F2999/00—Aspects linked to processes or compositions used in powder metallurgy

Definitions

• the invention pertains to a method for producing metal powders according to claim 1.
• Additive manufacturing is a rapidly growing market for metallic powders. It essentially consists of the distribution of a thin layer of a metal powder from a reservoir followed by fusion/sintering of the powder using an energy source (e. g. laser or electron beam). This is repeated building up the final product layer by layer. Powder size and morphology are critical characteristics that significantly impact the product quality. Generally speaking fine powders with a uniform particle size are preferred as this enables greater consistency as individual layers are deposited. Smaller particle sizes allow for thinner layers which determine the resolution of the component detail. From W. J. Sames, F. A. List, S. Pannala, R. R. Dehoff and S. S. Babu (2016: The metallurgy and processing science and metal additive manufacturing, International Material Reviews ) an overview over different methods of additive manufacturing can be derived.
• Powder raw materials for these additive manufacturing processes are produced using a variety of atomisation processes.
• a description of the main methods is available from J. Davis, R. Bowerman and R. Trepleton (2015, 59 (3): Introduction to the additive manufacturing powder metallurgy supply chain; Johnson Matthey Technology Review, 243-256 ) common routes of powder production are known.
• These routes include water atomisation, gas atomisation, plasma atomisation the hydride-dehydride process and the TiROTM process.
• the melting process has a number of variations but generally, when atomizing using water, the feedstock is first melted in a furnace before being transferred to a tundish (a crucible that regulates the flow rate of the melt into the atomizer). The liquid alloy enters the atomisation chamber from above, in which it is free to fall through the chamber. Water jets are symmetrically positioned around the stream of liquid metal, atomizing and solidifying the particles. The final powder exits at the bottom of the chamber, where it is collected. Additional processing steps are then required to dry the powder. Metal powder produced in this way is typically highly irregular in morphology, which reduces both packing properties and flow properties. Water atomisation is the main method for producing iron and steel powders and typically feeds into the press-and-sintered industries rather than the additive manufacturing industry.
• Gas atomisation processes mimic water atomisation, with the differentiator being the use of gas instead of water during the processing.
• Air can be used as the atomizing medium, but it's more preferred that an inert gas (nitrogen or argon) will be used to reduce the risk of oxidation and contamination of the metal.
• the process of melting the metal ingots can be the same as described for water atomisation, however for powders produced for high end applications such as aerospace, the need to control interstitial elements has led to increased use of vacuum induction melting furnaces. In this case the molten stream of liquid enters the atomisation chamber directly from the furnace rather than through a tundish. The stream of liquid metal is atomized by high pressure jets of gas.
• the metal droplets Due to the lower heat capacity of the gas in comparison with water the metal droplets have an increased solidification time which results in comparatively more spherical powder particles. Whilst it is not possible to have complete control over the particle size of atomized powder, the distribution can be influenced by varying the ratio of the gas to melt flow rate.
• the feedstock used in the process can either be in wire form such as the method by AP & C Advanced Powders and Coatings Inc, Canada, or in powder form such as the method used by Tekna Plasma Systems Inc, Canada.
• the spool of wire or powder feedstock is fed into the atomisation chamber where it is simultaneously melted and atomized by coaxial plasma torches and gas jets.
• Plasma rotating electrode process is a variation of plasma atomisation whereby a bar of rotating feedstock is used instead of a wire feed. As the rotating bar enters the atomisation chamber plasma torches melt the end of the bar, ejecting material from its surface. The metal solidifies before hitting the walls of the chamber.
• Oxygen content of the powders is a key quality parameter.
• Surface oxidation can adversely impact the shape and size distribution of powders produced during atomisation and residual oxygen content can lead to embrittlement of welds during the laser processing of powders. This can result in the final product failing to meet mechanical specifications and consequently there is significant pressure to reduce the oxygen content of the precursor powders.
• controlled atmospheres can be used to avoid oxidation during melting, or to prevent oxidation after liquid metal has been transferred to a tundish or indeed there is potential to use such atmospheres to avoid oxidation during atomizing of a liquid metal stream by gas jets.
• EP 2 050 528 B1 describes the use of gaseous boranes or silanes as dopants in Ar or N2 as a method to avoid such oxidation. These are pyrophoric gases that demand specialist handling and infrastructure and hence their application as described in EP 2 050 528 B1 is not practically viable due to the necessary investment and precautionary measures that would be required in a foundry environment. Furthermore, boranes are highly toxic at concentrations much lower than those proposed for the application described in EP 2 050 528 B1 .
• CN1174108 discloses an atomisation process for producing a metal powder.
• a reducing atomisation atmosphere by adding one or more of hydrogen, carbon monoxide, methane, ethane, propane and the like to the atmosphere.
• the reducing atmosphere will react with the oxygen present in the atomisation atmosphere, thereby reducing oxidation of the powder.
• Carbon dusting is a catastrophic form of corrosion associated with carburization of metals and alloys exposed to carbonaceous atmospheres at carbon activities > 1. Dusting has potential to rapidly destroy metallic components of the atomizing system.
• a metal powder is manufactured by atomisation of a liquid metal with a gas or a liquid.
• the liquid metal is exposed to a controlled gas atmosphere which comprises at a mixture of inert gas (at least 50% by volume) as well as carbon monoxide (CO) and carbon dioxide (CO2).
• the invention relates to a safe method for minimizing or avoiding oxidation during liquid metal atomisation.
• Pyrophoric gases such as silane or borane can be avoided.
• the explosion hazards associated with silanes and boranes can be completely avoided.
• the application of the invention will be considerably safer, less expensive and more effective.
• Silane of borane utilization will result in contamination of the metal powders with SiO2, B2O3 or BN dependent upon the choice of inert gas. Further, the infrastructure associated with the atomizing process would be essentially unchanged. There would be no requirement to modify electrical equipment to be spark free or explosion proof as there would if pyrophoric gases were employed.
• inert gas e. g. inert gas of 99.999% purity
• inert gas often has a high enough (ppm level) oxygen content to dramatically lower surface tension of liquid metals and to lead to surface oxidation. Adding CO can avoid these undesirable consequences.
• the inventive addition of CO and CO2 is safer than just adding only CO.
• concentration of CO required to avoid oxidation can be reduced because adding CO and CO2 to the inert gas allows the oxygen potential to be tailored to avoid oxidation of the metal alloy system being processed.
• a gas mixture of CO and CO2 defines a specific oxygen potential at a given temperature.
• PCO2 the equilibrium constant
• PO2 the Gibbs Energy change for that reaction at the specified temperature.
• the ratio of CO:CO2 could for example be between 2:1 and 50:1 in order to achieve the same practical impact as doping with CO alone, but avoiding the above-mentioned safety issues related to CO.
• the controlled atmosphere contains an inert gas, CO, CO2 and SO2. Such an atmosphere would allow to control both oxygen and sulphur partial pressures which can be used to moderate liquid metal surface tension.
• the liquid metal contained in the crucible/tundish or furnace is exposed to a controlled atmosphere containing the inert gas, CO and CO2. Additional tensioactive species or microalloying elements which lower the surface tension could be added to the liquid metal in concentrations which are capable of reducing surface tension but having no or negligible impact on other physical or chemical properties of the metal.
• Atomisation of a liquid metal results in a huge increase in the surface to volume ratio of the metal being processed. This requires that energy be provided to generate the increased surface energy of the product phase. If the surface tension of the liquid is reduced by equilibration with a gas atmosphere capable of introducing tensioactive species and/or adding microalloying elements to the liquid metal it is possible to lower the average particle size generated for given atomisation conditions. Alternatively it is possible to retain a desired mean particle size but to do so by means of lower momentum or lower pressure gas (or water) jets.
• argon is used as inert gas.
• the inert gas has a purity of at least 95% by volume, preferably at least 99% by volume or at least 99.5% by volume.
• liquid metal is exposed to the controlled gas atmosphere during the atomisation step.
• the inventive method comprises a step of melting a metal to get the liquid metal.
• a metal to get the liquid metal.
• the controlled atmosphere consists of Ar, CO and CO2.
• micro-alloy the respective metal with microalloying elements which have been mentioned above or with group V and Vi elements which are known to be surface active in liquid metal solutions.
• the alloying can take place according to the manufacturing route of the metal. This can be a metallurgical ladle or in the crucible or tundish before casting the metal into the atomisation chamber.

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Cited By (3)

Citations (4)

• 2018
  • 2018-02-26 EP EP18020079.2A patent/EP3530385A1/fr not_active Withdrawn
• 2019
  • 2019-02-04 WO PCT/EP2019/025036 patent/WO2019161971A1/fr active Application Filing

Patent Citations (4)

Non-Patent Citations (1)

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