The present invention relates to metallic powder which is used for additive manufacturing processes, especially selective laser melting (SLM). More specifically, the invention refers to a method for treating powder made of Ni-, Co-, Fe-base super alloys or TiAl alloys which is used for manufacturing of three-dimensional articles, for example components for gas turbines, like blades or vanes. Said method can be applied for manufacturing of new powder, for a post conditioning of metallic powder or for recycling/refreshing of already used metallic powder.
There exists a demand in current state of the art for improvement of metallic SLM powder treatments because of the following limiting shortcomings:
To summarize it, quality deviations of commercially available SLM powders, together with the fact that commercially available super alloys, for example super alloys based on Ni, Co, Fe or combinations thereof, or commercially available TiAI alloys have to be specifically modified/adapted for the successful application within the SLM processing and high costs resulting from frequent SLM powder replacement in order to reach a specified SLM article quality, lead to a strong demand for improvement of existing SLM powder manufacturing, powder post processing and powder recycling.
It is an object of the present invention to provide an effective, simple and cost-efficient method for improvement of SLM powder manufacturing, powder post-processing and powder recycling to overcome the described shortcomings of the prior art methods.
These and other objects are obtained by a method according to independent claim 1.
The method refers in general to treating of SLM powder particles by means of gas phase conditioning.
The disclosed method for treating a base material in form of metallic powder, wherein said powder is made of super alloys based on Ni, Co, Fe or combinations thereof or made of TiAI alloys and wherein the treated powder is then used for additive manufacturing, especially for Selective Laser Melting (SLM) of three-dimensional articles, is characterized in that
The method according to claim 1 has the advantage that it allows easily to modify commercial standard alloys in a short time and with relative low costs. A reproducible manufacturing of components with SLM powders could be ensured. With the storage/atomization of the powder under the mentioned conditions an uncontrolled adsorption/contamination of the powder, for example by N2, O2, H2O can be avoided. This is important for the following correct fluorination of the powder in the gas phase. Standard alloys formulations could be adjusted by post processing and yielding particles with a defined compositional gradient. Different SLM powder exhibiting a chemical gradient in contrast to homogeneous composition, that means powder fractions deviating from the alloy specification, could be used, but finally yields in a similar overall alloy composition during the following SLM processing. In addition, it allows manufacturing derivatives of standard alloys in small batches with low cost impact.
It is an advantage that commercially available standard powder (that means new, so far not used powder) and/or already used and therefore degenerated aged powder could be used as the base material. Therefore the method is applicable for new powder for SLM manufacturing of three-dimensional articles, but also for post conditioning and for recycling of metal powder for SLM processes.
In preferred embodiments, the post gas phase treatment is at least one selected out of the group of chemical vapor deposition (CVD), physical vapor deposition (PVD), Fluoride Ion Cleaning (FIC) or gas phase treatment with other Fluor containing compounds, preferable Polytetrafluorethylene (PTFE), Polyfluoroalkoxy (PFA) or partly fluorised Silicones. With the application of Fluor containing compounds by gas phase treatment very thin film of flux on each powder particle is applied. These films liberate in-situ Fluor by pyrolization (laser energy input), which in-situ removes potential oxide/nitride films, but additionally confer a hydrophobicity to the metallic powder particles during storage. Such a water repellant surface is less prone to physical water adsorption in humid air and dry faster under heat treatment, e.g. within the SLM process chamber or within a pre-het treatment before application in the SLM process.
For treatment of base powder comprising Al, Ti or combinations thereof a most preferred embodiment is to subject said powder to a specific FIC gas phase treatment not only as already known from the prior art for removing surface contaminations and for Al and Ti surface depleting, but according to the invention for adjusting the content of Al and Ti and for depositing of metal fluorides, especially TiF4, on the surface of the powder, wherein dependent on the FIC cycle parameters a controlled amount of said surface metal fluorides is deposited which act as in-situ flux during the following SLM process. During laser melting this Fluor containing phase removes potential humidity and any resulting oxide phases which might have formed during SLM processing:
TiF4+H2O(g)→TiO2+4HF(g)
MxOy+HF(g)→MFn(g)+H2O(g)
The change of SLM powder composition including potential local material inhomogeneity by formation of material inclusions which would be otherwise created by commercial flux product additions is avoided. Due to the low amount of fluorides, the volatility of conjugated metal fluorides formed, no or very limited Fluor containing residues are expected within the built SLM article.
In an embodiment of the invention powder which is made of difficult to process Ni base super alloys (alloys, which tend to crack during processing or subsequent heat treatment, typically a function of Al+Ti content) is stored and atomized only under dry and pure protective shielding gas atmosphere under at least Argon 4.8. This has the advantage that alloys free of nitride phases are processed.
It is an advantage when second phase particles as a strengthening phase are applied with the disclosed gas phase treatment on the powder surfaces, especially when the size of the second phase particles is adjusted to the need of the mechanical properties by tailoring the process parameters. As a preferred embodiment finely granulated and distributed carbide, oxide, nitride or carbo-/oxinitrides or intermetallic phases are precipitated as second phase particles during said gas phase treatment. This improves the properties of the manufactured component.
The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.
The present invention provides an effective, simple and cost-efficient method for improvement of SLM powder manufacturing, powder post-processing and powder recycling to overcome the described shortcomings of the prior art methods. More specifically, the method refers in general to the treating of SLM powder particles by means of gas phase conditioning.
The disclosed method for treating a base material in form of metallic powder, wherein said powder is made of super alloys based on Ni, Co, Fe or combinations thereof or made of TiAI alloys and wherein the treated powder is then used for additive manufacturing, especially for Selective Laser Melting (SLM) of three-dimensional articles, is characterized in that
The detailed determination of the amount of the elements (first step of the method) could be done by any method according to the state of the art, for example by EDX (Energy Dispersive X-ray Spectroscopy).
The mentioned post gas phase treatment is preferably at least one selected out of the group of chemical vapor deposition (CVD), physical vapor deposition (PVD), Fluoride Ion Cleaning (FIC) or gas phase treatment with other Fluor containing compounds, preferable Polytetrafluorethylene (PTFE), Perfluoroalkoxy (PFA) or partly fluorised Silicones. In addition, a post heat treatment under atmospheric conditions for improvement of the flowability of the powder is also possible.
In
According to
For treatment of base powder comprising Al, Ti or combinations thereof a most preferred embodiment is to subject said powder to a specific FIC gas phase treatment not only as already known from the prior art for removing surface contaminations and for A1 and Ti surface depleting, but according to the invention for adjusting the content of A1 and Ti and for depositing of metal fluorides, especially TiF4, on the surface of the powder, wherein dependent on the FIC cycle parameters a controlled amount of said surface metal fluorides is deposited which also act as in-situ flux during the following SLM process. During laser melting this Fluor containing phase removes potential humidity and any resulting oxide/nitride phases which might have formed during SLM processing:
TiF4+H2O(g)→TiO2+4HF(g)
MxOy+HF(g)→MFn(g)+H2O(g)
The change of SLM powder composition which would be otherwise created by commercial flux product additions is avoided. Due to the low amount of fluorides, the volatility of conjugated metal fluorides formed, no or very limited Fluor containing residues are expected within the built SLM article.
In a first embodiment, commercially available IN738 powder, stored in a small welded metal box (steel), was post heat treated at 500° C./1 h/Air and then a FIC cleaning with special parameters (p, T, t, gas composition) was done (=HT+FIC). The heat treatment results in at least partly oxidized powder and with the following FIC the “oxide/nitride skin” (including any other surface contaminations) is removed. The used specific FIC process regime results in a partial fluorisation of the Ni powder without unwanted secondary effects.
The latter one was also the result of comparison of the microstructure of the powder after heat treatment and after heat treatment plus FIC treatment. As result of the strong attack of the surface region of the powder which was FIC treated there was a depletion of Al and Ti.
In a second embodiment IN738LC powder from a different supplier was heat treated under atmospheric conditions and then FIC treated and ball milled (BM). SEM and EDX (Energy Dispersive X-ray Spectroscopy) investigations show also a depletion of Al and Ti in the surface region, in the center were observed gamma prime particles (see
In a third embodiment IN738LC powder as delivered was FIC treated in a metal container with TBC powder, for example Y2O3 stabilized or pure ZrO2, on the bottom (=FIC+TBC).
With such variably treated powder a SLM processing (single layer processing, small grooves with width of 1 cm and depth of 80 pm) was done with the following parameters:
Laser power: 300 W
Scan speed: 1600 mm/s
Hatch distance: 0.07 mm
After cutting, grinding, polishing and etching (electrolytically H3PO4) of the SLM processed probes they were inspected by light microscopy and SEM of surface and microsections. The surface under the light microscope of the different probes showed no significant differences. SEM tests showed that the amount of surface oxides is varying according to the gas phase treatment. The FIC+TBC embodiment shows small and less oxides than the other ones and mostly dense oxide precipitations. In addition, no cracks were detected within the metallurgically investigated probes. This treatment seems to be the best one.
Dependent on the post gas phase treatment parameters (p, T, t, gas composition) there was detected a depletion of Ti, Al in the outer area and an enrichment of Ti and also some Nb, Ta, C on the surface. This has an influence on the weldability of the material as well as on formation of the oxides (amount, position) during the welding.
The disclosed method allows easily modifying commercial standard alloys in a short time and with relative low costs. A reproducible manufacturing of components with SLM powders could be ensured. Standard alloys formulations could be adjusted by post processing and yielding particles with a defined compositional gradient. Different SLM powder exhibiting a chemical gradient in contrast to homogeneous composition, that means powder fractions deviating from the alloy specification, could be used, but finally yields in a similar overall alloy composition during the following SLM processing. In addition, it allows manufacturing derivatives of standard alloys in small batches with low cost impact.
Both commercially available standard powder (that means new, so far not used powder) and already used and therefore degenerated aged powder could be used as the base material. Therefore the method is applicable for new powder for SLM manufacturing of three-dimensional articles, but also for post conditioning and for recycling of metal powder for SLM processes.
In an embodiment of the invention powder which is made of difficult to process Ni base superalloys is stored and atomized only under dry and pure protective shielding gas atmosphere under at least Argon 4.8. This has the advantage that alloys free of nitride phases are processed.
It is an advantage when second phase particles as a strengthening phase are applied with the disclosed gas phase treatment on the powder surfaces, especially when the size of the second phase particles is adjusted to the need of the mechanical properties by tailoring the process parameters. As a preferred embodiment finely granulated and distributed carbide, oxide, nitride or carbo-/oxinitrides or intermetallic phases are precipitated as second phase particles during said gas phase treatment. This improves the properties of the manufactured component.
In addition, in a further embodiment of the invention, the powder is subjected to a fluorised Silicone gas post treatment to adjust the Si content which is critical for the weldability of Ni base superalloy powder.
The adjustment of Si content should be on the lowest acceptable level for the Ni base super alloy composition. Preferentially, the Ni base alloy powder to be used for fluorination shall be free of Si. The necessary Si concentration is reached by post gas treatment.
Of course, the invention is not limited to the described exemplary embodiments.
Number | Date | Country | Kind |
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15188304.8 | Oct 2015 | EP | regional |