DE102011116212A1 - Heat treatments of metal mixtures formed by ALM to form superalloys. - Google Patents

Abstract

A method of forming an article comprising: (i) forming a layer of a mixture of at least two different metal powders selected so that when combined, they are chemically present in the superalloy portions that are gamma 'phase contains (ii) locally melting the powders without diffusion to define the shape of part of the article so that the materials of the different metal powders remain essentially chemically separate areas of formation with a different chemical composition, (iii) repeating steps (i) and ( ii) until the derived article is formed, and (iv) heat treating the finished article so that at least one of the different separated materials to form a gamma'-phase superalloy diffuses with the other of the different separated materials.

Description

This invention relates to the processing of metal powders, e.g. By a combination of "additive layer manufacturing (ALM)" (thermal build-up) (eg laser or electron beam) followed by heat treatments, which then form a superalloy and in particular superalloys which have a gamma ' Phase included.

background

Superalloys are alloys that are solidified not only by the nature of their matrix and chemistry, but also by the presence of special solidification phases, usually precipitates. For a more detailed description of superalloys cf. "Superalloys: A Technical Guide", ASM International, ISBN 0-87170-749-7 and "The Superalloys: Fundamentals and Applications", Cambridge University Press, ISBN 10 0-521-85904-2 , These are alloys that have recently been developed for use in rocket and jet engines.

A superalloy is generally defined as an alloy having excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Some of the most useful superalloys form secondary phase precipitates, such as Gamma prime precipitates, and these gamma excretions often include titanium and aluminum. These superalloys present many processing challenges that are often presented in a diagram, as in the US Pat 1 is shown. The inventor has often found that these superalloys to the left of the "weldability" line, such. Inconel 718 can be processed to a high temperature by ALM without heating as a whole, whereas this is not the case for the superalloys to the right of the "weldability" line.

It is known that functional metal parts can be made by ALM from various pure metals and alloys. In the past, a so-called "liquid-phase sintering" process has been used to form mechanically hard parts, such as the like. As forms, from a multi-component metal powder of a manufacturer, such. DirectMetal 20 and DirectSteel products from EOS GmbH. Liquid phase sintering is a process in which one component of the lower melting powder is melted by the laser, but the other components with a higher melting point remain solid. DirectMetal is described by its manufacturers as a "bronze based matrix containing nickel" with a residual porosity of 8%, with no heat treatment required or described. DirectSteel H20 is described as a "steel-based multi-component metal powder" which, when laser sintered, forms a steel alloy with a density of more than 99.5%, which provides 5 to 10% improved tensile strength and 10 to 15% has% improved yield strength after a heat treatment.

Recently, in the course of more suitable lasers, complete melting of many homogeneous metal powders has become common. The powder source for such parts is chemically homogeneous for the resulting alloy and is made by melting a chemically homogeneous rod stock or elements and has the composition required for the finished part.

In the best ALM process of the prior art, if desired, the powder is completely melted by the selective application of a source of energy (typically a laser or an electron beam) and then solidifies to form, layer by layer, a metal part of maximum density is generated, which corresponds to a sliced design file. Since the powder is completely melted and many metals and alloys have a high coefficient of thermal expansion, the part as produced has significant internal stresses and the part is preserved to maintain dimensional accuracy by a mounting plate or fixtures and fixtures during assembly and during a subsequent heat treatment to substantially completely eliminate these stresses prior to removal from the mounting plate, fixtures or jigs. In addition, many metals undergo a phase change upon cooling from the liquid, resulting in additional stresses.

However, for some alloys - especially nickel base superalloys - the internal stress is sufficient to cause the part to crack either during the ALM process or during the subsequent stress relief treatment. For example, a significant portion of nickel superalloys containing gamma prime alloying elements exhibit this behavior and it is known that these superalloys are difficult to process by conventional methods and are often classified as "non-weldable". These alloys are also of significant commercial interest, as they are suitable for very high temperature applications - e.g. B. in combustion components in engines used and often can only be cast and are difficult or impossible to repair.

The solution of the prior art is to add heat to the area of the melting metal in order to minimize thermal imbalance of the solid metal that has already formed and solidified. In the case of metals with a lower melting temperature, such. Titanium, this may be a pragmatic solution in practice, but for the superalloys of interest, particularly nickel-based superalloys, it is either practically or economically disadvantageous to heat the part due to the high temperatures and time required; for which the temperature must be applied, and controlled cooling to obtain sufficient stress reduction and cracking.

Bourell, 5,296,062, describes the use of a "powder comprising particles of a first material coated with a second material, the second material having a lower softening temperature than the first material."

Bampton describes in US 5,745,834 various methods for "selective laser bonding and transition liquid sintering of mixed powders". These include a method in which 3% boron (a known melting point depressant) is added to a top layer of a Haynes 230 superalloy powder corresponding to 15% of the total layer thickness, and preheating is applied to a temperature immediately below that Melting point of the layer with added boron is. A laser is then used to selectively melt the layer with the boron as a melting point depressant. This liquid metal penetrates into the 85% layer thickness below it and the boron diffuses from the liquid phase into the solid powder to produce "a segment of the nearly maximum density component". Bampton emphasizes that it is difficult to perform processing at such a high (preheat) temperature with a conventional equipment and that there is a significant temperature gradient from the laser melted spot and material mass, typically greater than 100 ° C, and thereby residual stresses be generated. Bampton also describes a process in which polymer powder is mixed with the metal and a laser layer process is performed. The binder is burned out, leaving a porous metal powder sintered together as a solid. This porous sintered part is then densified either by encapsulation and then hot isostatic pressing or infiltration by a lower melting point liquid metal. Both methods have significant disadvantages. The article that has been HIPed will have a significant shrinkage and this process is expensive. In the case of introducing liquid metal, the solid part does not make up 100% of the desired alloy and therefore does not have the desired mechanical properties.

Bampton then describes a mixture of three powder constituents comprising the desired parent metal, the same parent metal with a melting point depressant, and a polymer binder, which layers are "locally laser melted the polymer constituent of the powder which rapidly re-solidifies so that the metal particles of the polymer Powder tied with connective constrictions or bridges, "be built. The binder is then removed in an oven to produce a low strength part which, during a transient liquid sintering process (generally), provides temporary support to e.g. B. a ceramic powder needed.

It is also known ( WO / 0211928 ), in all metal powder (no polymer) additives for melting point depression, such. As boron or carbon, to include. Further, the additives may be in a small percentage of the total powder but, when used as separate powders, may be locally present in much higher percentages, thereby initiating melting, wetting and bonding of the powder to form an article. which has substantially the maximum density.

Describes accordingly US2004 / 0182201 a method in which "graphite is also used in sintering an iron-based powder to lower the melting point of the composition to be sintered""graphite powder is particularly effective in improving the wettability during melting or micro-cracks during the solidification of To reduce high density sections. "

In Hede, WO 02/092264 , a recent (November 2002) process and powder for selective laser sintering with metal powders and lasers (and all known free-form processes) are described as not being suitable for producing a material of maximum density, leaving a porosity of 5 to 30%, so that infiltration with a low melting point material is required.

Hede describes experiments with tool steels that result in "a martensitic layer with a high hardness and internal stresses that make it difficult (impossible) to smooth the deposited layer with a scraper before the next layer is applied" and "a large one Risk of cracking exists ".

Hede then describes the use of iron and copper based precipitation hardening alloys which "yield a soft material just after laser sintering ... where the desired hardness could be achieved instead by precipitation hardening ... after laser sintering." Ferroalloys, and more particularly Maraging steel and a 17-4PH stainless steel are described. While the martensite hardening steel 18NiMAR250 has been tested, Hede continues to speculatively broadly include 17-4PH as a sample material subject to precipitation hardening.

At the present time (2011), a 17-4PH stainless steel alloy is one of the most widely used metal powders in laser-selective laser sintering, and has been the case for many years. The applicants use these in their company on a daily basis. Already in 2006, it was often described (eg by EOS GmbH) as "precipitation-hardening", but it was found by the applicants that a 17-4PH alloy powder actually does not undergo precipitation hardening after processing in the commercial EOS M270 machine and the 17-4 powder is no longer marketed as precipitation hardening powder material. There is clearly a new field in which extensive speculation can not serve as a good guide to the performance of materials in selective laser processing and post-build heat treatments.

Tegal, DE 10039143 describes in [003] the problem to be solved as a high degree of porosity when metallic components are prepared from conventional powder mixtures. It describes a density of about 90% of the theoretical density with a steel powder and with laser sintered parts made of bronze remains a residual porosity of about 30%.

The disclosure describes the laser sintering of a powder material which comprises a mixture of at least two powder elements and which is characterized in that the powder mixture is formed by iron powder as the main component and by further powder alloying elements which are in an elemental, pre-alloyed or partially alloyed form, and that powder alloy results from these powder elements in the course of the laser sintering process.

Tegal further emphasizes this by saying that during the laser sintering process, the powder alloy components are converted within milliseconds to a powder alloy that makes up the component. Any subsequent treatments are described as homogenization, flash annealing, heat treatment, reduction of internal defects, and surface quality improvement.

It should be noted that the prior art has considerably advanced since the filing of this disclosure in 2000, and with a present-generation plant (which effects complete melting instead of sintering the metal powder) has porosity to the extent that Tegal has been described, is no longer a problem.

• (i) Bilden einer Schicht eines Gemischs aus mindestens zwei unterschiedlichen Metallpulvern, die so ausgewählt sind, dass sie dann, wenn sie vereinigt werden, chemisch in den Anteilen einer Superlegierung vorliegen, die eine gamma'-Phase enthält,
• (ii) lokales Schmelzen der Pulver ohne Diffusion zur Festlegung der Gestalt eines Teils des Gegenstands, so dass die Materialien der unterschiedlichen Metallpulver im Wesentlichen chemisch getrennte Bildungsbereiche mit einer unterschiedlichen chemischen Zusammensetzung bleiben,
• (iii) Wiederholen der Schritte (i) und (ii), bis der abgeleitete Gegenstand ausgebildet ist, und
• (iv) Wärmebehandeln des fertigen Gegenstands, so dass mindestens eines der unterschiedlichen getrennten Materialien zur Bildung einer gamma'-Phase-enthaltenden Superlegierung mit dem anderen der unterschiedlichen getrennten Materialien diffundiert.

In one aspect, the invention consists of
a method of forming an article, comprising:

• (i) forming a layer of a mixture of at least two different metal powders selected so that when combined they are chemically present in the proportions of a superalloy containing a gamma prime phase,
• (ii) locally melting the powders without diffusion to define the shape of a portion of the article such that the materials of the different metal powders remain substantially chemically distinct regions of formation having a different chemical composition,
• (iii) repeating steps (i) and (ii) until the derived article is formed, and
• (iv) heat treating the finished article such that at least one of the different discrete materials diffuses to form a gamma prime phase-containing superalloy with the other of the different discrete materials.

Consequently, in the processes and materials we describe here, in contrast to e.g. For example, Tegal does not contain a coated powder, polymers, or they do not require the formation of a porous state or "green" state prior to subsequent heat treatments. We do not try to solve a porosity problem resulting from sintering (not melting) the metal powder. We produce a superalloy and not a "soft" material or a precipitation-hardening iron or copper based alloy that is subsequently precipitation hardened. We do not use a means of lowering the melting point or use such, such. Boron or graphite (carbon) to obtain the process of the invention. We also do not form the alloy during the laser "sintering" process - we do not explicitly form the alloy during this because it would lead to failure because the alloy to be formed will crack in the laser process. We form the alloy during a subsequent heat treatment cycle.

The invention also consists of a metal powder for layer processing which chemically comprises the elements of a gamma prime-forming superalloy minus a substantial portion of one of its gamma-forming alloying elements. The alloy may generally be a nickel based alloy and the gamma prime forming element may be generally aluminum. Known gamma-hardened superalloys include: Inconel 713C, Inconel 100, MAR M247, MAR M200, Inconel 738, RR1000, UDIMET 500, Inconel 939, Unimet 720, MAR M002, CMSX-4, Haynes 282, Rene 41.

This metal powder may be alone or in admixture comprising this metal powder mixed with a second powder chemically comprising its gamma-forming elements so that together they form a non-homogeneous physical mixture of powders. This mixture of powders can then chemically correspond to a recognized nickel superalloy. The second powder may contain aluminum as well as titanium and may suitably be titanium aluminide (TiAl, TiAl3, Ti3Al).

Applicants have recognized that by using a locally acting laser to fuse together the desired portions of the material to form the shape of a layer of the article, they form an effective matrix in the form of the intended article of one powder of the component of the mixture and holding the other Powder component can build in a substantially chemically separate manner. Thus, by selecting the powder forming the matrix, i. that is, the bulk of the powder, such that it is a powder that does not have high internal stresses that induce cracking, form an article that is subject to removal either from the baseplate during and / or during the ALM assembly first heat treatment to relax the article is not likely to crack.

Thus, in a preferred embodiment, the mixture of powders comprises a powder component which makes up over 50% of the mixture and thus forms the matrix bulk of the article. Preferably, the component comprises over 60% by weight of the mixture. In some embodiments, the component may be nickel, or in other embodiments, the major component may be nickel and chromium. In further embodiments, the main component may comprise or consist of iron.

In a preferred embodiment, the diffusion takes place in step (iv) by a solid state diffusion.

As mentioned above, the method may also include relaxing the article by a heat treatment prior to step (iv).

In a particularly preferred embodiment, the formation of a superalloy is provided which contains an additive x at a concentration C. The method may include mixing two powders A and B, wherein A is the intended major constituent of the superalloy, having a concentration of an additive x, where x = (C _x ) _A , and is selected to allow processing without cracking and wherein the powder B is a minor component of the intended alloy with a concentration of x = (C _x ) _B ; mixing together the powders A and B such that B is a fraction of f of the total, such that C _x = f (C _x ) _B + (1-f) * (C _x ) _A , and where (C _x ) _B > (C _x ) _A is included.

Each of the powders may be locally melted during step (ii). The powders may be selected from materials that are less susceptible to stress cracking than the intended alloy. The proposed alloy may be a nickel-base superalloy. The alloy may comprise an additive x, which may be aluminum or titanium or both. X can form more than 4% by weight of the powder.

In another aspect, the invention consists of selecting and mixing two or more powder compositions which together chemically give the proportions of a desired superalloy, and performing a lamination process with that mixture so that they completely melt to form a substantially dense metal mixture which is not the desired superalloy and which is characterized by having a sufficiently low stress such that it does not crack during construction or subsequent heat treatments and heat treating the metal mixture to form the desired superalloy without cracking.

Although the invention has been defined above, it should be noted that it represents any combination of the invention as described above or in the following description Features includes. The invention will now be described by way of example with reference to the accompanying drawings, in which:

1 is a known diagram indicating the weldability of nickel alloys with aluminum and / or titanium additions, and

2 a flow chart of an embodiment of the invention is.

1 is a diagram that out "Nickel Based Superalloy Welding Practices for Industrial Gas Turbine Applications" by MB Henderson et al. is taken and designed according to a diagram that in G. Cam and M. Kocak, "Progress in Joining Advanced Materials", International Materials Reviews, 43, No. 1 (1988) is shown. Corresponding diagrams are widely used in the literature concerning the welding and strain age cracking of alloys containing gamma prime precipitates.

Now, the inventor believes that this diagram (and conclusions drawn therefrom) provides good guidance as to which superalloys are thermally ALM processable "crack-free" by the prior art. For the purposes of this analysis, a thermal based ALM may be considered as a type of welding. It is believed that if the aluminum and titanium total concentration of a given alloy exceeds a threshold, which is often assumed to be 4% by weight, the alloy is "difficult" to weld and it becomes increasingly difficult to weld; the more the percentage increases.

2 is the process flow of the embodiment of the invention. It should be noted that Alloy A and Alloy B need not be recognized alloys because the metal powder feedstock for the ALM process can be made from elemental materials in a contract manufacturing facility, and any composition can therefore be ordered at no extra cost or delay become.

The 3 and 4 are micrographs of a sample of an embodiment of the invention immediately after the ALM or a sample after the ALM and a subsequent heat treatment.

• 1] Das Auswählen und das Mischen von zwei oder mehr Pulverzusammensetzungen, die chemisch zusammen die Anteile der gewünschten Superlegierung ergeben
• 2] Schichtaufbauverarbeitung dieses Gemischs von Pulvern, wobei sie vollständig schmelzen, um ein Metall-„Gemisch” mit einer im wesentlichen Dichte zu bilden, das nicht die gewünschte Superlegierung ist und dadurch gekennzeichnet ist, dass es eine ausreichend geringe Spannung aufweist, so dass es während des Aufbaus oder nachfolgender Wärmebehandlungen keiner Rissbildung unterliegt
• 3] Wärmebehandlungen des Metallgemischs zur Bildung der gewünschten Superlegierung ohne Rissbildung

Consequently, the inventor has recognized the following new possibilities:

• 1] Selecting and mixing two or more powder compositions that together chemically give the proportions of the desired superalloy
• 2] Stratification processing of this mixture of powders, where they melt completely to form a metal "mixture" having a substantially density which is not the desired superalloy and characterized by having a sufficiently low stress such that it has is not cracked during construction or subsequent heat treatments
• 3] Heat treatments of the metal mixture to form the desired superalloy without cracking

It should be noted that the stresses generated by a thermal ALM powder bed process close to room temperature are such as to require a substantial baseplate typically weighing 20 kg to withstand mechanical relaxation caused by stress, which has been generated by the structure is caused. This significantly complicates further thermal processing as it contributes significantly to thermal mass. Therefore, a method is desired which allows the part to be removed from a base plate or jigs and jigs without cracking before high-temperature heat treatments and complicated heat treatments.

In a specific embodiment, the powder A has a chemical element composition which corresponds approximately to an "easily processed" alloy, which is generally the main constituent of the powder mixture. The powder B, on the other hand, has a chemical composition of elements such that, when mixed with the powder A in the proper ratio, a total chemical composition results from its elements corresponding to that of the desired final superalloy. Therefore, if we want to make parts of a superalloy containing an additive element x at a concentration C _x , we would mix two powders A and B. The powder A, in which it is the main component would be a lower concentration of the element x, (C _{x) A,} have, so it could be processed without cracking. This will hereinafter be referred to as "bulk powder". The powder B, which is the minor ingredient, would have a higher concentration of the element x, (C _x ) _B , and would be mixed with the powder A to be present in a proportion f of the total amount, such that: C _x = f * (C _x ) _B + (1-f) * (C _x ) _A Equation 1 The powder B will hereinafter be referred to as the "dopant powder".

During ALM processing, both powder A and powder B are completely melted, but because of the short time they are in the liquid phase, would be present as substantially separate regions having different chemical compositions.

If several additional elements are used in an alloy, then the dopant powder may have the appropriate concentrations of each of them. Alternatively, further dopant powders can be added, wherein in each case another element is introduced. It is clear that it might sometimes be advantageous to incorporate the total amount of a particular additive element in a dopant powder. Furthermore, as a limiting case, a dopant powder could be a pure additive element.

The resulting structure in the part formed by ALM will be a structure of isolated islands of material having the approximate composition of the dopant powder surrounded by a matrix of material having the approximate composition of the bulk powder of the powder. Since the mechanical properties and internal stresses are dominated by the powder bulk, the resulting material can be built and heat treated without cracking by ALM. It should be noted that the part as it is built up is completely molten, has a substantial density, and chemically corresponds to the desired superalloy, but is not present in the microstructure of the desired superalloy. The microstructure is generated by a separate second stage process by a heat treatment.

It is known that powder mixtures sometimes occur in an ALM z. B. for the production of molds are produced by means of laser sintering - in such a method is direct metal laser sintering. It should be noted that such sintering processes do not completely melt the material, the material does not have the maximum density and heat treatments are not used to form a high performance superalloy - the use of the term "direct" indicates that further heat treatments are not required to form the desired material.

In the invention, a subsequent heat treatment (single-stage or multi-stage) is then employed to cause the additive x from the dopant islands, which have a high concentration, to diffuse out into the lower concentration composition, resulting in a superalloy with the required microstructure z. B. the characteristic excretion of the finished alloy and in particular the gamma'-precipitates leads.

A suitable heat treatment may include solution and aging steps. First, the gamma prime precipitates, topologically close packed phases and carbides are dissolved in the gamma matrix and then aged to form the precipitates and carbides in the desired shapes and configurations. In this way, a part of the high-performance superalloy is obtained without cracking.

In a preferred embodiment for the addition of both aluminum and titanium, it is convenient to add an appropriate amount of one of the titanium aluminide intermetallic compounds. An experiment was conducted to prove the principle that a bulk powder made of the nickel superalloy C263 was used. This 4 wt .-% TiAl ₃ powder were added. C263 is a gamma-containing alloy with moderate concentrations of aluminum and titanium. It is generally considered to be a weldable alloy and can be processed without cracking by ALM. The addition of 4 wt .-% TiAl ₃ brings the total titanium / aluminum concentration in the range of difficult to weld alloys such. C1023, which are subject to cracking when processed by ALM processes. Photomicrography in the 3 shows a sample of this material immediately after the ALM process. Separate dark areas (such as the one near the center of the image) that contain high concentrations (electron diffraction spectroscopy analysis) of titanium and aluminum are visible throughout the material. This shows that these elements do not significantly diffuse during the very short melting period provided by the ALM process. Photomicrography in the 2 shows another sample of the material which has undergone a solution heat treatment subsequent to the ALM process. There are no separate areas with a high aluminum / titanium concentration visible. This shows that these elements can actually be successfully distributed in bulk by solid-state diffusion during such heat treatment. Further, despite the high levels of gamma prime-forming elements present, there was no evidence of cracking.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 5745834 [0010]
• WO 02/11928 [0012]
• US 2004/0182201 [0013]
• WO 02/092264 [0014]
• DE 10039143 [0018]

Cited non-patent literature

• "Superalloys: A Technical Guide", ASM International, ISBN 0-87170-749-7 [0002]
• "The Superalloys: Fundamentals and Applications", Cambridge University Press, ISBN 10 0-521-85904-2 [0002]
• "Nickel Based Superalloy Welding Practices for Industrial Gas Turbine Applications" by MB Henderson et al. [0036]
• G. Cam and M. Kocak, "Progress in Joining Advanced Materials", International Materials Reviews, 43, No. 1 (1988) [0036]

Claims (19)

A method of forming an article, comprising: (i) forming a layer of a mixture of at least two different metal powders selected so that when combined they are chemically present in the proportions of a superalloy containing a gamma prime phase, (ii) locally melting the powders without diffusion to define the shape of a portion of the article such that the materials of the different metal powders remain substantially chemically distinct regions of formation having a different chemical composition, (iii) repeating steps (i) and (ii) until the derived article is formed, and (iv) heat treating the finished article such that at least one of the different discrete materials diffuses to form a gamma prime phase-containing superalloy with the other of the different discrete materials. The method of claim 1, wherein the diffusion takes place by means of solid-state diffusion. A method according to claim 1 or claim 2, wherein the article is relaxed by a heat treatment prior to step (iv). A method according to claim 1 or claim 2, wherein the article is formed on a base plate and removed therefrom prior to step (iv). A method according to claim 1 or claim 2, wherein it is intended to form a superalloy containing an additive X at a concentration C, comprising: mixing two powders A and B, wherein A is the intended main constituent of the superalloy, which is a Concentration of x = (C _x ) _A , and is selected so that processing without cracking is possible, and wherein the powder B is a minor constituent of the intended alloy with a concentration of x = (C _x ) _B ; Mixing together the powders A and B such that B is such proportion of the total that C _X = f (C _X ) _B + (1-f) (C _X ) _A , and where (C _X ) _B > (C _X ) is _A Method according to one of claims 1 to 5, wherein each powder is locally melted during step (ii). A method of forming an article wherein the powders are of materials that are less susceptible to stress cracking than the intended superalloy. A method according to any one of the preceding claims wherein the envisaged superalloy is a nickel base superalloy. Method according to one of the preceding claims, wherein the alloy comprises an additive x. The method of claim 9, wherein x is aluminum or titanium or both. Process according to claim 10, wherein x represents more than 3.5% by weight or preferably 4% by weight of the powders. A method according to any one of the preceding claims, wherein the article is part of a larger article. A method according to any one of the preceding claims, wherein more than 50% by weight of the mixture of powders of metals or alloys are formed which do not crack during ALM processing. The method of claim 13, wherein more than 50% of the powder is Ni, Cr or Fe or a mixture or alloy of at least two thereof. A method of forming a superalloy, comprising: (i) selecting and mixing two or more powder components which together chemically give the proportions of the desired superalloy, (ii) performing an ALM processing on the mixture, thereby obtaining a "mixture" of substantial density in the form of an article which is not the desired superalloy but which is characterized by having a sufficiently low stress such that it is not cracked during construction or subsequent heat treatment, and (iii) heat treating the article to form the desired superalloy without cracking. A layered metal powder which chemically comprises the constituent components of a gamma prime superalloy, but wherein at least one of the constituent components is substantially depleted in relative proportion which would be present in the superalloy. A non-homogeneous metal powder mixture comprising a metal powder according to claim 16 and a secondary powder containing the elemental constituent (s) depleted in the first powder. A method according to any one of claims 1 to 15, wherein the powder of claim 16 or the powder mixture of claim 17 is used. A non-homogeneous metal powder mixture according to claim 17 comprising a secondary powder containing the elements aluminum and / or titanium.

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