Method of refurbishing a powder and related apparatus

Abstract

Powder 1 for powder bed fusion additive manufacturing which comprises a superficial surface oxide is refurbished by subjecting the powder particles 1 to a fluidized bed 2 where the powder particles 1

Description

The present invention relates to a method of refurbishing a powder for powder bed fusion additive manufacturing and / or laser metal deposition approaches. The refurbishment particularly relates to a rejuvenating treatment of used powder fractions for the additive manufacturing industry in order to reduce scrap rates and allow for a sustainable recycling of the related high-performance powder materials. Additive manufacturing (AM) or 3D-printing of high-performance components comprise e.g. powder bed fusion methods, such as selective laser melting (SLM) or laser powder bed fusion (LPBF), selective laser sintering (SLS) and electron beam melting (EBM). Preferably the mentioned powder relates to nickel-or cobalt-based superalloy powders and, hence, to the feedstock of high-performance, components, like components applied in the flow path hardware of gas turbines, components for automotive, aviation and space applications. AM, particularly powder-bed methods, have also proven to be useful and advantageous in the fabrication of prototypes or complex components, such as components with filigree structure or functionally cooled components. The AM techniques mentioned herein are known and stands out for its short chain of process steps which enables material economization and a favorably low component lead time. Further additive manufacturing approaches relate to „Directed Energy Deposition (DED)", such as laser cladding, electron beam or plasma welding, metal inkjet molding (MIM), so-called sheet lamination methods, or even thermal spraying (VPS, LPPS) methods. Related machine hardware or setups for such methods usually comprise a manufacturing or build platform on which the component is built layer-by-layer after the feeding of a layer of base material which may then be melted, e.g. by an energy beam, such as a laser, and subsequently solidified. The layer thickness is determined by a recoater that moves, e.g. automatically, over the powder bed and removes excess material from a manufacturing plane or build space. Typical layer thicknesses amount to between 20 pm and 40 pm. During the manufacture, said energy beam scans over the surface and melts the powder on selected areas which may be predetermined by a CAD-file according to the geometry of the component to be manufactured. Said scanning or irradiation is preferably carried out in a computer-assisted way, such as Computer-Aided-Manufacturing (CAM) instructions, which may be present in the form of a dataset. Most of the powder used in powder bed fusion is filling the process chamber but is not actively involved in the component creation. Thus, the expensive raw power constitutes a very costly "floating" asset. Once the raw powder product is crated, the (un-used) powder is inherently involved in a powder rejuvenation process or recycling loop. This process may further contain several sieving steps to remove e.g. splatter and agglomerated powder particles. Not only during the printing process, when the powder is actually subjected to manufacturing conditions at elevated temperatures, the powder takes up oxygen from an oxygen-containing gas environment during the printing process due to the heat and (residual) traces of oxygen contained in the so-called shielding gas. This accumulation of oxygen in form of oxides on the powder particle surface likely leads to severe degradation of the material's mechanical properties in the as-printed form. A skilled person is aware that already slight variations (excess amounts) of oxygen - as compared to a standard specification - can strongly influence rigidity, creep resistance, oxidation resistance and further properties, and hence render the fabricated component useless for its intended operation. The problem of accumulated oxide on the particle surfaces has not been solved yet. To date, conventional powder rejuvenation or recycling methodologies of deployed in powder bed fusion technology are not capable of removing superficial oxides on the powder particle surface. Typically, an amount or share of around 500 ppm is roughly regarded as "scrap limit", i.e. the amount of oxygen that can at most be tolerated in the additive processing of nickel-base and / or cobalt-base superalloy powders without overly affecting the mentioned solid material properties. It is, thus, an object of the present invention to provide means that solve the above-mentioned problem and which particularly enable a true refurbishment of high-performance selective laser melting and laser metal deposition powders, meaning a reliable reduction of superficial oxides. With this, additive manufacturing industry is given the possibility to maintain much greater floating material stocks and thereby to guarantee the specified material properties throughout. Furthermore, the present invention provides a valuable solution to sustain high-performance powder materials in the industrial recycling loops and to significantly reduce scrap and raw material waste in the related industry. The mentioned object is achieved by the subject-matters of the independent claims. Advantageous embodiments are subjectmatter of the dependent claims. An aspect of the present invention relates to a method of refurbishing a powder, like a powder refurbishment or rejuvenating treatment, suitable for powder bed fusion and / or laser metal deposition manufacturing, for instance. The method comprises providing a powder suitable for a powder bed fusion process, wherein the powder particles comprise a superficial oxide. E.g., the powder particles have at least partly reacted with oxygen previously, such as when used in a related additive manufacturing process and / or when subjected to a non-inert gas environment, like at elevated temperatures, for instance. A significant uptake of oxygen at the surfaces of at least parts of a fresh powder fraction may occur already once a new powder package is opened or crated under a non-inert gas environment. In this context, it is not even necessary that a fresh powder fraction is indeed subjected to the manufacturing conditions, i.e. direct or indirect laser irradiation and / or elevated temperatures of several 100° C. Instead, an oxygen-containing ambient gas may be sufficient to let exposed particles oxidize. The method further comprises subjecting the powder particles to a fluidized bed or applying a fluidized bed to the powder particles, wherein the powder particles or their related surfaces are treated, particularly pickled, with hydrogen fluoride (HF) being introduced into the fluidized bed of a reaction chamber as a reaction and / or carrier gas. This pickling action potentially removes the typical superficial oxides from the surfaces of the powder particles. As an advantage, powder that would otherwise need to be scrapped because of violation of the oxygen level or specification, can evidently be salvaged and recycled, thereby bringing huge cost advantages for the additive manufacturing operators. Apparently, the inventive solution further brings about huge merits in terms of material sustainability. Waste of used powder can significantly be reduced without any implication on the material properties of the as-manufactured parts. At least the used powders can be recycled far more often to the benefit of the whole additive manufacturing process chain. Consequently and advantageously, the invention reduces dependency of (scarce) raw materials, especially when high-performance base material powder stocks are concerned. In an embodiment, the reaction gas is moving fast as compared to the heavier powder particles in the fluidized bed, thereby effectively leading to a floating of the powder particles in the bed's gas stream. The core advantage of the application of a fluidized bed is that an instant and uniform contact of the powder particle surface with the reaction gas is achieved. Therefore, a rapid surface reaction on the whole particle surface is promoted at the benefit of a reliable removal of the reaction products at the surfaces. In an embodiment, the powder is made of a nickel-based or cobalt-based superalloy. In the alternative, a steel-based alloy, titanium-based alloys or other high-performance alloys, such as those with alloy constituents of rare earth or scarce or precious material constituents are predestined for the presented powder refurbishment strategy. In an embodiment, the superficial oxide corresponds to an oxygen amount of at least 500 ppm, roughly indicating an oxygen scrap limit in the additive processing of superalloys by powder bed fusion approaches. In an embodiment, a mixture of hydrogen fluoride, and argon and / or hydrogen is used as reaction gas in the fluidized bed, wherein an acquired solution of hydrogen fluoride between 1% and 50% is used, preferably between 5% and 40%, particularly between 5% and 25%, or between 10% and 25%. The related percentage is preferably related to volume-related percentages. The percentages may further relate to an evaporation or gasification of the related HF solution as a fluidizing liquid. The above-mentioned concentrations of the HF reactive agent turn out to be generally advantageous and suitable to further tailor and adapt the concentration towards the best outcome in terms of the deoxidizing effect. Thereby, preferably a good trade-off between a reliable oxygen reduction and poor contamination of further agents is achieved. In an embodiment, the general chemical reaction "HF + MxOy => MxFy + H2O" is promoted by the present invention in the fluidized bed. In a further embodiment, the method advantageously applies an elevated temperature between 800°C and 1200°C, in particular between 900°C and 1100°C, most preferably between 950°C and 1050°C, to the hydrogen fluoride treatment. This feature expediently allows to thermally support the HF reaction, such as in a suitable reaction chamber. In an embodiment, a pressure of up to two bar is applied to the hydrogen fluoride reaction and / or the fluidized bed treatment. Such a pressure may further push the efficiency of the oxygen reduction and reliability of the proposed treatment in general. Also, an ultrasonic and / or vibratory agitation may be applied according to another embodiment in addition to the described treatment of the powder particles in the fluidized bed. Likewise, by this embodiment, powder movement inside the fluidized bed reactor may advantageously be enhanced. Furthermore, the fluidized bed may be embodied as a circulating bed, wherein powder particles which have already been treated with hydrogen fluoride and hence separated from the reaction chamber (cf. details of a particular embodiment described further below), are subjected to the fluidized bed anew, i.e., for a second cleaning run. This measure may as well improve efficiency and output of the refurbishment. A content of the fluidized bed is, according to an embodiment of the invention, passed through a cyclone after the treatment, in order to separate the cleaned (deoxidized) powder particles from the reaction chamber. Apparently, application of a cyclone allows for an easy separation of solid powder particles from the fluid. Preferably, in the given context, the reaction and / or carrier gas is reintroduced into the fluidized bed or the reaction chamber, so to form a circulating fluid. Possibly, this may go along with a certain addition of new HF reaction gas to compensate for the consumption of the agent during the (previous) reaction. At or close to the end of the process, the powder is (sufficiently) free of surface oxide and the (filtered) reaction gas passed through a base or lie containing a scrubber in order to neutralize the hydrogen fluoride. This measure may be necessary according to occupational safety requirements as well as for reliability and reproducibility reasons. In an embodiment, the presented refurbishment method is part of a powder recycling or powder rejuvenation methodology for powder bed fusion approaches, wherein the recycling further comprises sieving of the powder particles to remove e.g., spatter and / or other weld agglomerates from the powder. A further aspect of the present invention relates to an apparatus for carrying out the method as described, wherein the apparatus comprises a (preferably heatable) reaction chamber with the gas distributing plate, such as at the bottom of said reaction chamber through which the reaction gas can be pumped. The reaction chamber is expediently suitable to generate the fluidized bed and to sustain hydrogen fluoride as reaction gas. In an embodiment of the apparatus, the gas distributing plate comprises holes with diameters smaller than an average diameter of the powder particles, preferably smaller than 30 pm. Advantages and embodiments relating to the described method are valid and pertain likewise to the apparatus and vice versa . Further, features and advantageous embodiments become apparent from the following description of the exemplary embodiment in connection with the Figure. Figure 1 shows a schematic overview of an inventive apparatus being suitable to form a fluidized bed and to apply a hydrogen fluoride gas for powder refurbishment. Like elements, elements of the same kind and identically acting elements may be provided with the same reference numerals in the figure. The Figure is not necessarily depicted true to scale and may be scaled up or down to allow for a better understanding of the illustrated principles. Rather, the described Figure shall be construed in a broad sense and as a qualitative base which allows a person skilled in the art to apply the presented teaching in a versatile way. The term „and / or" as used herein shall mean that each of the listed elements may be taken alone or in conjunction with two or more of further listed elements. Figure 1 shows a schematic sectional view of an inventive apparatus 10. The apparatus 10 is suitable to generate a fluidized bed 2 in a reaction chamber 3. The apparatus 10 further comprises a gas distributing plate 6 through which a reaction gas and / or a carrier or fluidizing gas can be introduced into the reaction chamber 3. The apparatus 10 is particularly configured to carry out the inventive method of refurbishing a powder 1 for powder bed fusion additive manufacturing approaches, preferably industrialized additive manufacturing approaches, wherein a huge amount of high-performance material feedstock needs to be held available. The method comprises the steps of providing a powder suitable for a powder bed fusion process, wherein powder particles of the powder comprise a superficial oxide (MxOy) . The method further comprises subjecting the powder particles 1 to a fluidized bed 2 wherein the powder particles 1 are treated with hydrogen fluoride (HF) which is introduced into the fluidized bed 2 as a reaction gas to remove the superficial oxides from the powder particles 1. Evidently, scrap of costly base powder portions may be reduced by the refurbishment capabilities of the invention. This merit particularly applies for high-performance powders, such as nickel-based or cobalt-based superalloy powders, for the additive manufacture of high-performance materials, such as gas turbine components. The gas distributing plate 6 comprises holes with diameters smaller than an average diameter of the powder particles 1. Hence, the holes have anyway diameters of preferably below 30 pm, considering main particle fractions of the powder for powder bed fusion processes may range between 10 pm and 45 pm. In the depicted exemplary embodiment of Figure 1, the apparatus 10 is configured as a so-called circulating fluidized bed apparatus, wherein powder particles 1 which have already been treated with hydrogen fluoride HF as reaction gas and have already been separated from a reaction chamber 3, may be subjected to the fluidized bed 2 anew (cf. arrows pointing clockwise on the right). The HF-containing reaction gas may further function as a fluidizing or carrier gas in the context. In the alternative, a further carrier gas or gas fraction may be introduced. For instance, the reactive agent HF may be mixed with argon (Ar) and / or pure hydrogen (H;), and this mixture may form the circulated gas, possibly having a reactive and a carrying and / or in fluidizing function. To this effect, a carrier gas inlet 16 and a reaction gas inlet 17 are provided, shown at the bottom left in Figure 1. Moreover, a carrier gas outlet 12 is shown. Through this outlet 12, the carrier gas may be separated, e.g. be filtered in a filter unit 13 and, by means of pump 14, integrated again into the cycle through the plate 6. Further, there may be a vent or ventilation device 15 through which parts of the related gas may be let out or its pressure regulated. The reaction chamber 3 per se may be formed by a retort 8 which is equipped with heating elements 7, for which the chamber or retort may be heated to elevated temperatures and the related reaction thermally facilitated. There may further be a thermal insulation around the chamber 3 and the heating elements 7. The heating equipment is preferably configured to apply temperatures of at least up to 1200°C. Accordingly, in order to thermally support the HF reaction, the method advantageously applies an elevated temperature between 800°C and 1200°C, in particular between 900°C and 1100°C, most preferably between 950°C and 1050°C, to the hydrogen fluoride treatment. Above the reaction chamber 3, there is an extraction tube 18. The extraction tube 18 has a greater diameter as compared to the gas outlet 12. A certain share of the fluid may be extracted from the chamber 3 and then passed through a cyclone 4. Hence, a mixture of the powder particles 1 and the HF reaction gas is, preferably, passed through the cyclone 4 after the treatment in order to separate the cleaned powder particles 1 from fluid stream. According to its circulating nature, the remainder of the reaction gas is reintroduced into the fluidized bed 2 in the reaction chamber 3. Further, a powder hopper 19 is shown at the top which may serve the purpose of adding new powder particles to the process which need a related refurbishment and oxygen reduction. After passing the cyclone 4, the related gas or gas mixture is led through a filtering unit or scrubber 5 through which the poisonous or reactive agents in the gases may be neutralized or cleaned, such as by a lie or a base. Moreover, a fan 11 may be contemplated to control gas flow for the circulation, for example. In other words, the present invention deploys a combination of fluidized powder bed and fluoride ion cleaning. The main advantage is that an instant and uniform contact of the powder particle surface with the reaction gas can be achieved. The reaction gas is thereby able to move fast in relation to the powder particles which effectively enables floating of the powder particles in the gas fluidized stream. A rapid surface reaction on the whole particle surface is therefore promoted and the removal of the oxide reaction products supported. By using a mix of Ar and HF, the fluoride may be taken at a concentration of 1 to 50 vol%, preferably 5 to 25 vol % HF. Instead of Ar, H2 could as well be used in the alternative but preferably with the same percentage or concentration of HF. In general, the invention preferably promotes the following chemical reaction: HF + MxOy => MxFy + H20, with the following more particular reactions for nickel-based superalloys: 6HF + A12O3 <=> 2A1F3 + 3H20 4HF + TiO2 <=> TiF4 + 2H20 6HF + Cr2O3 <=> 2CrF3 + 3H20 4HF + NiO2 <=> NiF4 + 2H20 6HF + Co203 <=> 2CoF3 + 3H20 In addition, application of pressure in the chamber 3 of up to 2 bar can support the related reactions. Also, ultrasonic agitation or vibratory agitation might be implemented in the reaction chamber 3 with the related hardware which will advantageously further enhance powder movement inside the reactor setup 10 (ultrasonic or vibrational excitation means not explicitly indicated in Figure 1). Typical oxide products on the surface of nickel-base alloy powder particles can therefore be effectively removed. As mentioned above, the fluidized gas is passed through a cyclone 4 after the reaction in the chamber 3 to separate and collect the cleaned powder particles 1. The gas stream might also be re-introduced into the retort, eventually with a little add-on of HF reactant to compensate for its consumption during the reaction. At the same time, the collected powder 1 might be reintroduced into the reaction chamber as well for a second cleaning run. At the end of the process, when the powder is completely free of surface oxide, the reaction gas might be fed through a NaOH containing scrubber to neutralize the HF or through similar means.

Claims

1. A method of refurbishing a powder (1) for powder bed fusion additive manufacturing comprising the steps of: - providing a powder suitable for a powder bed fusion process, wherein powder particles of the powder comprise a superficial oxide (MxOy) , and- subjecting the powder particles (1) to a fluidized bed (2), wherein the powder particles (1) are treated with hydrogen fluoride (HF) which is introduced into the fluidized bed (2) as a reaction gas to remove the superficial oxides (MxOy) from the powder particles (1).

2. The method according to claim 1, wherein the provided powder (1) is made of a nickel-based or cobalt-based superalloy.

3. The method according to claim 1 or 2, wherein the powder particles (1) have at least partly reacted with oxygen (O2) , such as when used in a related additive manufacturing process and / or subjected to a non-inert gas environment.

4. The method according to one of the previous claims, wherein the superficial oxide (MxOy) corresponds to an oxygen amount of at least 500 ppm.

5. The method according to one of the previous claims, wherein a mixture of hydrogen fluoride (HF), and argon (Ar) or hydrogen (H2) are used as reaction gas in the fluidized bed (2), wherein an aqueous solution of hydrogen fluoride between (HF) 1 and 50 % is used, and wherein the chemical reaction:HF + MxOy => MxFy + H20) is promoted.

6. The method according to one of the previous claims, wherein an elevated temperature between 800°C and 1200°C, in particular between 900°C and 1100°C, preferably between 950°C and 1050°C, is applied to the hydrogen fluoride treatment.

7. The method according to one of the previous claims, wherein a pressure of up to 2 bar is applied to the hydrogen fluoride (HF) treatment.

8. The method according to one of the previous claims, wherein an ultrasonic and / or vibration agitation is applied in addition to the treatment of the powder particles (1) in the fluidized bed (2) .

9. The method according to one of the previous claims, wherein powder particles (1) which have already been treated with hydrogen fluoride (HF) and separated from a reaction chamber (3), are subjected to the fluidized bed (2) anew.

10. The method according to one of the previous claims, wherein a content (1, HF, Ar) of the fluidized bed (2) is passed through a cyclone (4) after the treatment in order to separate the cleaned powder particles (1) from a reaction chamber (3), and wherein the reaction gas is reintroduced into the fluidized bed (2).

11. The method according to one of the previous claims, wherein, when the powder (1) is free of surface oxides (MxOy) , the reaction gas is passed through a base or lye containing scrubber (5) to neutralize the hydrogen fluoride (HF) .

12. The method according to one of the previous claims, being part of a powder recycling methodology for a powder bed fusion process, and wherein the recycling further comprises sieving of the powder particles to remove spatter and / or weld agglomerates from the powder (1).

13. An apparatus (10) for carrying out the method according to one of the previous claims, the apparatus (10) comprising a reaction chamber (3) with a gas distributing plate (6) through which the reaction gas can be pumped, wherein the reaction chamber (3) is suitable to generate the fluidizedbed (3) and to sustain hydrogen fluoride (HF) as a reaction gas .

14. The apparatus according to claim 13, wherein the gas5 distributing plate (6) comprises holes with diameters smaller than an average diameter of the powder particles (1), preferably smaller than 30 pm.16

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