METHOD FOR TREATMENT OF METALLIC POWDER FOR SELECTIVE LASER MELTING

Abstract

Methods are disclosed for treating a base materials in a form of metallic powder made of super alloys based on Ni, Co, Fe or combinations thereof, or made of TiAl alloys, which treated powder can be used for additive manufacturing, such as for Selective Laser Melting of three-dimensional articles.

Claims

1. Method for treating a base material in a form of metallic powder made of super alloys based on Ni, Co, Fe or combinations thereof, or made of TiAI alloys, which treated powder is suitable for additive manufacturing, including Selective Laser Melting (SLM) of three-dimensional articles, the method comprising: determining a chemical composition of the base material for comparison to a calculated target chemical composition with a detailed amount of each element of the powder, which is specified for an SLM manufacturing process; storing and atomizing the powder only under dry and pure protective shielding gas atmosphere, and/or treating the powder by a post gas phase treatment, thereby adding or removing specific elements into or from the powder particles and adjusting content of the added or already existing specific elements to meet the calculated target amount of each element.

2. Method according to claim 1, wherein the base material is any commercially available standard powder and/or an already used, degenerated powder.

3. Method according to claim 1, wherein the post gas phase treatment is at least one selected out of the group consisting of: chemical vapor deposition (CVD), physical vapor deposition (PVD), Fluoride Ion Cleaning (FIC), and gas phase treatment with other Fluor containing compounds, including PTFE, PFA or partly fluorised Silicones.

4. Method according to claim 1, wherein the powder when made of Ni base super alloys is stored and atomized only under dry and pure protective shielding gas atmosphere under at least Argon 4.8.

5. Method according to claim 3, comprising: subjecting the base powder having Al, Ti or combinations thereof to a specific FIC post gas phase treatment for: removing surface contaminations, Al and Ti surface depleting, thereby adjusting a content of Al and Ti, and depositing of metal fluorides, from a group which includes TiF4, on a surface of the powder, wherein dependent on the FIC cycle parameters (p, T, t, gas composition) a controlled amount of said surface metal fluorides is deposited which acts as in-situ flux during the SLM manufacturing process.

6. Method according to claim 1, comprising: applying the post gas phase treatment to deposit second phase particles as a strengthening phase on the powder surfaces, wherein a size of the second phase particles is adjusted to the mechanical properties by tailoring process parameters of a fluorizing process.

7. Method according to claim 6, comprising: precipitating, during said gas phase treatment, finely granulated and distributed carbide, oxide, nitride or carbo-/oxinitrides or intermetallic phases as second phase particles.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The present invention is now to be explained more closely by means of different embodiments and with reference to the attached drawings.

[0033] FIG. 1 shows in a diagram the improvement of flowability by heat treatment up to temperatures of 450 C. for IN738 powder under atmospheric conditions;

[0034] FIG. 2 shows in one embodiment the microstructure of IN738 powder (SEM) after post heat treatment and FIC treatment according to the disclosed method;

[0035] FIG. 3 show in an embodiment EDX results of FIC powder micro-sections;

[0036] FIG. 4 shows an SEM photo of SLM built material MarM247LC with fine and homogeneously distributed carbide precipitations.

DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION

[0037] 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.

[0038] 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 [0039] in a first step the chemical composition of the base material is determined and compared to a calculated target chemical composition with detailed amount of each element of the powder, which is necessary for the following SLM manufacturing process, [0040] the powder is stored and atomized only under dry and pure protective shielding gas atmosphere and [0041] the powder is treated by a post gas phase treatment, thereby adding or removing specific elements into or from the powder particles and adjusting the content of the added or already existing specific elements to meet the calculated target amount of each element according to the first step.

[0042] 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).

[0043] 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.

[0044] In FIG. 1 is shown the improvement of the flowability of commercially available IN738 powder (as delivered), a Ni based superalloy with the following results of EDX analysis (in wt-%):, 2.75 Al, 3.31 Ti, 12.91 Cr, 7.07 Co, 0.52 Nb, 1.57 Mo, 1.00 Ta. 2.22 W and 52.17 Ni by means of a heat treatment in the range from 0-450 C. under atmospheric conditions. In addition, the gas content (typical O.sub.2 and N.sub.2 range) is shown in that temperature range. The partly oxidized/nitrided powder shows an improved flowability.

[0045] According to FIG. 1 the Hausner Index (defined as tapped density/apparent density) decreases with increasing heat temperature (each 1 hour, air). A low Hausner Index means a better flowability. The improvement of flowability is caused by the oxidation layer, which decreases the cohesion power between the particles. Therefore, a powder with a low flowability or a powder with a fine particle size distribution could be improved (higher flowability) without increasing the oxygen content to much (see typical O2 content in FIG. 1).

[0046] 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 TiF.sub.4, 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:

TiF.sub.4+H.sub.2O(g).fwdarw.TiO.sub.2+4HF(g)

M.sub.xO.sub.y+HF(g).fwdarw.MF.sub.n(g)+H.sub.2O(g)

[0047] 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.

[0048] 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.

[0049] FIG. 2 shows the microstructure in SEM (Scanning Electron Microscope) with two different enlargement factors of the powder particles after such FIC treatment. Fine Fluoride particles (TiF.sub.4) could be clearly seen on the particle surface, the Ti content on the surface was increased. In addition, an enrichment of Nb, Ta and C, and a depletion of Al and Ti at least on the surface (achieving a concentration gradient) of the powder particles were investigated.

[0050] 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.

[0051] 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 FIG. 3). In addition, elongated needle like areas enriched in Ti, Nb, Ta could be seen, which would be typical for MC carbides (=HT+FIC+BM).

[0052] In a third embodiment IN738LC powder as delivered was FIC treated in a metal container with TBC powder, for example Y.sub.2O.sub.3 stabilized or pure ZrO.sub.2, on the bottom (=FIC+TBC).

[0053] 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:

[0054] Laser power: 300 W

[0055] Scan speed: 1600 mm/s

[0056] Hatch distance: 0.07 mm

[0057] After cutting, grinding, polishing and etching (electrolytically H.sub.3PO.sub.4) 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] FIG. 4 is a SEM photo of MarM247LC, a well-known commercially available material, after SLM processing. Fine carbide precipitations at dendrite boundaries could be seen.

[0064] 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.

[0065] 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.

[0066] Of course, the invention is not limited to the described exemplary embodiments.