Method of preparing a powder and process for additive manufacturing

Abstract

A method of preparing a metal matrix composite powder for additive manufacturing, the method comprising: preparing a cored wire comprising: a metal shell and a ceramic contained within the metal shell; and using the cored wire as an electrode in an atomisation process to prepare a metal matrix composite powder.

Description

[0001] A Method

[0002] Cross reference to related applications

[0003] This application claims priority from GB2415836.2 filed on 28 October 2024, the contents of which are hereby incorporated by reference.

[0004] Background

[0005] The present invention relates to a powder for additive manufacturing (AM) and in particular to a metal matrix powder for additive manufacturing and a method of preparing such a metal matrix composite powder for additive manufacturing. The invention further relates to a method of manufacturing a metal matrix composite component and an AM component.

[0006] Metal matrix composites (MMCs) which contain a combination of a metal matrix in which ceramic particles are embedded are attractive for manufacturing components as they can provide a unique balance of properties. For example, in aluminium MMC’s a high level of specific strength is achievable through increased stiffening by the ceramic particles with little density change of the metal.

[0007] In traditional methods of MMC production such as casting or forming, it is hard to get an even distribution of ceramic in the metal matrix leadingto inhomogeneity in physical properties. Also, the amount of machining required to get to the final product form has significant expense.

[0008] Because most traditional MMCs contain very hard phases such as SiC, TiB2or other boride, carbide, nitride and oxide phases, they are difficult to machine and require specialized and expensive tooling. It is therefore desirable to develop MMCs that contain phases that are less hard and cause reduced tool wear. This invention also describes a special case of more machinable MMCs in which the ceramic phase is a relatively softer MAX phase.

[0009] It is desirable to provide a method of manufacturing MMC components with even distribution of ceramic and in which the amount of post-machining required to get to the final product is low. The present invention provides a method of preparing a metal matrix composite powder for additive manufacturing, the method comprising: preparing a cored wire comprising: a metal shell and a ceramic contained within the metal shell; and usingthe cored wire as an electrode in an atomisation process to prepare a metal matrix composite powder. This method has the advantage of resulting in highly spherical powder particles comprised or a matrix of metal with embedded ceramic particles. The process enables high ceramic fractions in the MMC powder and eliminates the need for ball milling, which is a time consuming and relatively expensive process. In additive manufacturing the powder of the present invention flows easily and results in very good homogeneity of ceramic distribution. Moreover, the near net shape nature of the additive manufacturing process means there is little or no machining required which results in a significant cost reduction.

[0010] In an embodiment the ceramic powder is TiB2or SiC or AIN ceramic powder. These powders are suitable for strengthening MMCs.

[0011] In an embodiment the ceramic powder is a MAX phase ceramic powder. Such powder is suitable for strengthening MMCs and also has the advantage of resulting in a component which is easily machinable.

[0012] In an embodiment the MAX phase of the MAX phase ceramic has a layered hexagonal structure, preferably a layered hexagonal crystallographic structure with weak interlayer bonding and / or wherein the layered hexagonal crystallographic structure is a space group P63 / mmc. Such structures machine particularly well.

[0013] In an embodiment the MAX phase of the MAX phase ceramic has a general formula Mn+iAXn, where M is one or more metals, A is one or more A-group element and X is C and / or N and / or B. Preferably M is one or more of: Ti, Sc, V, Cr, Zr, Nb, Hf, Mn, Mo and / or A is one or more of: Cd, Si, Sn, In, Ga, Tl, Al, Ga, Sn, Pb, Zn, Au. Such MAX phases are particularly suited to the task.

[0014] In an embodiment the MAX phase of the MAX phase ceramic is Ti3SiC2. In another embodiment the MAX phase of the MAX phase ceramic is Al4SiC4and Al8SiC7. In an embodiment the ceramic powder has a d10of 5 pm or less and a d9o of 10 pm or less. This distribution is well suited for a ceramic phase in a MMC.

[0015] The method of any of the preceding claims, wherein the cored wire further comprises a metal component, wherein the metal component is contained within the shell. This has the advantage that the dimensions of the metal shell are not soley determinative of the final composition of the powder, meaning that the dimensions of the metal shell can be chosen substantially independent of the desired final composition and appropriate for the physical requirements of the shell.

[0016] In an embodiment the metal component includes a metal powder. This is a form of component easy to insert into the metal shell and easily mixable with the ceramic.

[0017] In an embodiment the metal component includes a metal wire. A wire is easily insertable into the metal shell.

[0018] In an embodiment the metal shell and optionally any metal component is non-ferrous. Such metals benefit in particular from having ceramic particles distributed in them to form a MMC.

[0019] In an embodiment the metal shell and optionally any metal component is one or more of: nickel and nickel alloys, steels including stainless steels, titanium and titanium alloys, aluminium and aluminium alloys, cobalt and cobalt alloys, magnesium and magnesium alloys, copper and copper alloys, refractory metals and refractory metal alloys, platinum group metals and their alloys, vanadium metals and their alloys. These metals and alloys benefit in particular to having ceramic particles distributed in them to form a MMC.

[0020] In an embodiment the metal shell is formed of an elemental metal or an alloy of an element which is 98% or more pure. This allows greater flexibility in tuning the composition of the metal matrix in the final product by adding master alloy powder or wire into the cored wire electrode

[0021] In an embodiment the volume of ceramic powder is at least 5% of the total volume of the sum of metal powder and ceramic powder, preferably wherein the volume of ceramic powder is at least 10% of the total volume of the sum of metal powder and ceramic powder, optionally wherein the volume of ceramic powder is 50% or less of the total volume of the sum of metal powder and ceramic powder. Such large quantities of ceramic in MMCs are difficult to manufacture in any other way.

[0022] In an embodiment the atomisation process is induction gas atomisation. The powder resulting from such a process ha high spheroidicity meaning it is a powder easy to handle and flows uniformly.

[0023] In an embodiment the atomisation process occurs at a temperature above the melting point of any metal in the cored wire and below the melting point of the ceramic powder. This ensures even distribution of the ceramic in the metal matrix.

[0024] An embodiment of the present invention provides a method of manufacturing a metal matrix composite component using the metal matrix composite powder produced by the method of any preceding claim in a metal additive manufacturing process. This results in a product with high uniformity and evenly distributed ceramic particles.

[0025] The invention will now be described, byway of non-limiting example only, with reference to the following drawings, in which:

[0026] Figure 1 shows schematically the size ratio of ceramic particles to metal particles in metal matrix composite (MMC) powder - the ceramic particles are required to be substantially smaller than the metal particles, approximately one order of magnitude.

[0027] Figure 2 is a schematic of a cored wire.

[0028] Figure 3 shows the caking effect which results when blended powders of separate metal particles and ceramic particles are used in a powder bed fusion additive manufacturing machine. Caking leads to uneven powder spreading and subsequently defects in the additively manufactured component.

[0029] Figure 4 illustrates ball milling - a process for preparing composite ceramic-alloy powder particles by ball collisions. During this process, the starting mixture of ceramic and metal powders undergoes plastic deformation, fracturing and welding, resulting in an even dispersion of alloy and ceramic. Figure 5 shows ball-milled powders of aluminium and Al2O3. Ball milling deforms composite powder, therefore after the process the powder shape is irregular and far from spherical. This irregular powder shape inhibits powder flow, leading to caking.

[0030] Figure 6 shows the schematic of electrode inert gas atomisation (EIGA).

[0031] Description of the problem

[0032] The traditional methods of making a powder for production of MMC materials are the following processes:

[0033] • Powder blending: mixing a ceramic and metallic powder where the ceramic powder is in contact with the metallic powders but not inside the metallic powder

[0034] • Ball milling: Mixing metallic constituents and ceramic constituents in a vessel with ball milling media to form a powder which contains ceramic phase within the metal particles

[0035] • In-situ ceramic formation: selecting an alloy composition which precipitates a ceramic phase during solidification

[0036] • Milling, pressing and electrode atomisation: conducting ball milling as above, then turning the powder into a rod using cold isostatic pressing, then atomising the rod using electrode inert gas atomisation (EIGA)

[0037] Powder blending is the least expensive method to produce a MMC powder. However, it has a number of drawbacks when applied in additive manufacturing processes. First, the ceramic particles are required to be substantially smaller than the metal particles, approximately one order of magnitude smaller, as schematically shown in Figure 1 . The fine ceramic powder significantly decreases the flowability of the powder. As the fraction of ceramic increases the flowability decreases, further increasing the difficulty to additively manufacture components in an MMC with a high volume faction of ceramic particles. Second, the mixed blended powder is agitated throughout use in additive manufacturing. As there is no strong bonding between the ceramic and metal powders and as there is a size difference between them, it is possible that the agitation of the powder may lead to separation over time locally changing the actual ceramic content in the blend relative to the target content.

[0038] Figure 3 shows a powder bed fusion additive manufacturing machine in which components are built up by spreading a layer of powder out and fusing particles of powder together using a laser in areas where the component will end up, before removing the layer of remaining unfused powder and providing a further layer of powder over the powder bed as well as the fused part of the formed component. Areas where this layer should be present are then fused with the laser before removing the remaining powder and continuing with the process to build up the next layer. Figure 3 shows a caking effect which results when blended powders of separate metal particles and ceramic particles are used in such a process. Not only is the powder difficult spread over the powder bed (due to the difference in sizes between the metal particles and the ceramic particles and due to the usual non-spherical shape of the ceramic particles), but also separation between the metal particles and the ceramic particles (seen as waves in Figure 3) appear. The uneven ceramic distribution in the powder layer is then replicated in the component and the caking resulting from poor flowability hampers efficient manufacturing.

[0039] Ball milling is a process that includes plastic deformation, welding and fracturing mechanisms. In this technique metal and ceramic powder particles are prepared by ball collisions. From that very fine particle sizes and uniform distribution of the ceramic particles can be obtained (see Figure 4). However, it has several drawbacks when applied in additive manufacturing processes: first, it is difficult to control the resulting powder morphology. The method deforms metal ceramic composite powder, therefore after the process the powder shape is irregular and far from spherical. This irregular powder shape inhibits powder flow significantly. See Figure 5 of Al2O3composites formed in this way. Second, the high amount of energy and time required for the ball milling process makes it very expensive. Also, the lack of control over resulting powder size makes it difficult to achieve a high yield for powders within a targeted size distribution. The description above refers to high energy ball milling (HEBM).

[0040] In-situ formation: in some instances, an alloy composition can be defined such that the chemistry is completely liquid during melting and during a solidification process the phase transformations in the alloy result in a metallic material with a low ceramic fraction. This can be used to form MMC’s with a low fraction of the ceramic phase. Higher fractions result in a viscous melt which flows only slowly through the nozzle at the bottom of the atomisation crucible. Ceramic particles may also accumulate at the tip of the nozzle, blocking it shut. This limits the process to low ceramic volume factions and only to certain metal systems.

[0041] Milling, pressing and electrode atomisation: Some of the present inventors have previously described a fourth, new method for making MMC powders with a high ceramic fraction and good flowability, suitable for AM. In this method, metal matrix composite particles are formed, each of which comprise a metal matrix in which a plurality of ceramic particles are embedded and in which the particles are substantially spherical in morphology. Such a powder addresses deficiencies in flowability (due to the spherical shape) and homogeneity because the particles are formed to have a metal matrix and a plurality of ceramic particles embedded therein. Even uneven powder coverage across the layer of powder on the powder bed during additive manufacturing does not result in inhomogeneous distribution of ceramic particles contrary to the powder blending described above and shown in Figure 1 and Figure 3. This is because any separation due to size differences between particles does not lead to segregation of metal and ceramic because the metal and ceramic are present in each particle. The advantages can be summarized as follows:

[0042] • Spherical powders are formed so that powder flow is optimized for additive manufacturing processes

[0043] • The ceramic is completely embedded in the metal particle so that it cannot separate, meaning the fraction of ceramic in the powder is controlled through powder processing by additive manufacturing

[0044] • A high faction of ceramic particles can be included because the process starts from a blended ceramic / powder mixture

[0045] The inventors have been able to produce such powder using electrode induction gas atomization of an electrode which is prepared by mixing a metal powder with a ceramic powder, binding the mixed metal and ceramic powders together to form an electrode and using that electrode in an electrode induction gas atomization process to prepare the metal matrix composite powder (see for example GB patent application number 2405539.4). Electrode induction gas atomization (EIGA) automatically results in nearly spherical particles being produced. The nature of the electrode naturally results in powders which are comprised of a metal matrix in which ceramic particles are embedded.

[0046] This final method produces high-quality MMC powder, but is time consuming and somewhat expensive due to the number of steps involved. It is in practice also difficult to produce MMCs with very high ceramic fractions (e.g. >50 vol%), which tend to be highly viscous in the molten state and hard to form into a molten stream of material that can be gas-atomised. The viscosity can be reduced by increasing the temperature of the melt, but EIGA rigs do not usually deploy induction coils that are powerful enough to achieve high enough temperatures.

[0047] Cored wire atomisation (CWA): This method is the subject of the present invention and addresses the shortcomings of the Milling, pressing and electrode atomisation (MPEA) route described above, as it enables equivalent or even higher ceramic fractions in the MMC powder than MPEA. It also eliminates the need for ball milling, which is a time consuming and relatively expensive process. At the same time, CWA retains all the advantages of the MPEA route. It produces spherical MMC powders which flow well and are suitable for additive manufacturing processes. The ceramic is completely embedded in the metal particles so that it cannot separate, resulting in a homogeneous ceramic dispersion in the microstructure of the item made by AM from CWA powder.

[0048] The inventors have invented a way to produce such powder by first producing a cored wire consisting of a metal shell (e.g. a tube of metal, for example a tube of an alloy material), filled with a ceramic powder. Optionally, a metal component, such as metal / alloy powders and / or solid metal / alloy such as wires are also provided in the metal shell. Such a cored wire is schematically shown in Figure 2.

[0049] The mixture of constituents of the cored wire is chosen such as to achieve the desired MMC powder composition and distribution of ceramic particles when the cored wire is melted, for example in an EIGA process. The cored wire may be manufactured by means of existing and widespread methods of producing flux cored wires for welding.

[0050] The wire is then atomised, preferably using the electrode inert gas atomisation (EIGA) process. In EIGA, the cored wire is first heated up using an induction coil to a temperature sufficient to liquefy the non-ceramic constituents of the wire in a chamber filled with inert gas. The liquid stream is then atomised into MMC powder particles using a high-velocity stream of inert gas. In an alternative embodiment, plasma atomisation is used. In plasma atomisation argon plasma torches are used to melt the electrode (perhaps in the form of a wire spool) which is fed at a constant rate into the plasma torches to produce fine droplets of metal in which the ceramic particles are embedded.

[0051] Using the wire directly in a process such as wire arc additive manufacturing (WAAM) is distinctly different from using alloy or MMC powder to produce items using methods such as LPBF as the latter can be used to make items with a much finer resolution. For example, it is not obvious that the cored wire as disclosed by CN108000004B can be atomised into powder and used for LPBF.

[0052] Additionally, ultrasonic atomisation such as disclosed in Jager, S., et al. "Potentials of Ultrasonically Atomized Cored Wires for Powder Metallurgy and Additive

[0053] Manufacturing." HTM Journal of Heat Treatment and Materials 78.3 (2023): 181-192 is a method that can currently produce only small amounts of powder and is therefore not suitable for industrial use. It also does not use ceramic powders to produce MMC powders.

[0054] Description of the solution

[0055] Possible materials

[0056] The process is suitable for use with a wide variety of metals which can be used in additive manufacturing. A non-exhaustive list includes nickel and nickel alloys, steels including stainless steels, titanium and titanium alloys, aluminium and aluminium alloys, cobalt and cobalt alloys, magnesium and magnesium alloys, copper and copper alloys, refractory metals and refractory metal alloys, platinum group metals and their alloys, vanadium metals and their alloys. In this context alloy means a mixture of the main metal with up to 70 wt% of alloying elements. EIGA is a flexible process by which one can produce powders of all the above alloy systems suitable for powder metallurgy processes. EIGA is particularly well-suited to alloys that cannot be produced via crucible gas atomization due to the limitations of the refractory ceramics used in crucibles (e.g. refractory alloys) or the difficulty achieving a homogeneous and low- viscosity melt (e.g. metal matrix composites as described in this invention).

[0057] A particularly promising metal for additive manufacturing is aluminium and aluminium alloys. In particular, an aluminium alloy consisting of 0-10% in sum of silicon, copper, iron, manganese, zinc, titanium, magnesium, lithium, zirconium, beryllium, scandium, sodium, cerium, yttrium, lithium, calcium, silver and the balance being aluminium and inevitable impurities.

[0058] Ceramics which have higher melting point than the metal with which they are combined are suitable as the ceramic. The melting point of the ceramic (both measured in Kelvin) is preferably at least 20% higher than that of the metal matrix. It is also preferable that the reactivity of the ceramic with the metal is low. Low reactivity means the ceramic particles will largely retain their original phase composition and fraction after EIGA and the additive manufacturing process - a small degree of reactivity however is acceptable. For example, up to 20 volume percent of the original ceramic phase may react with the metal matrix. The degree of reaction can be measured by scanning electron microscopy (SEM) techniques and image processing. One example of this is to take a suitably flat and polished sample of the additively manufactured material described by this invention and acquiring EDX and EBSD scans of at least 100 microns by 100 microns. One can then use standard SEM software for automatic phase analysis that makes use of composition data from EDX and crystallographic data from EBSD and matches it to an internal database to identify the phases in the scan. Note that the ceramic phase may partly dissolve in the metal matrix during atomisation and additive manufacturing but reprecipitate as fine particles of the same phase during solidification and heat treatment. This is not considered as reactivity.

[0059] A particularly promising ceramic powder is titanium diboride (TiB2) which is particularly suitable as a ceramic in a metal matrix composite. In particular a combination of titanium diboride and aluminium or aluminium alloy forms a suitable MMC powder. Another particularly promising ceramic powder is AIN which is particularly suitable as a ceramic in a metal matrix composite. In particular a combination of AIN and aluminium or aluminium alloy forms a suitable MMC powder.

[0060] Another particularly promising ceramic powder is SiC which is particularly suitable as a ceramic in a metal matrix composite. In particular a combination of SiC and aluminium or aluminium alloy forms a suitable MMC powder.

[0061] TiB2, TiC, AIN and SiC have the disadvantage that they are very hard and so result in low machinability of the produced article. This is acceptable in the case where the product can be additively manufactured to the exact final dimensions or very close thereto. However, where this is not possible there are advantages in using a ceramic which results in better machinability of the 3D printed MMC product. Particularly promising ceramic powders are MAX phases. MAX phases are characterized by their high ratio of specific stiffness to hardness and a layered hexagonal structure (space group P63 / mmc) with weak bonds between the layers, which can easily delaminate. MAX phases typically consist of at least 3 elements, 2 of which are transition metals or metalloids, while the third is typically either C, N or B (or a mixture thereof). A general formula is often Mn+iAXn, where M is one or more metals, particularly an early transition metal from group 3 to group 7 of the periodic table and A is one or more A-group (mostly 11 IA and IVA or groups 13 and 14) element and X is C and / or N and / or B. Known examples of M are Ti, Sc, V, Cr, Zr, Nb, Hf, Mn, Mo but also Al. Known examples of A are Cd, Si, Sn, In, Ga, Tl, Al, Ga, Sn, Pb, Zn, Au, S, P. In an embodiment n=1 -7. Note that these rules cover most, though not all MAX phases - again, their key characteristic is the layered hexagonal crystallographic structure (typically space group P63 / mmc) with weak interlayer bonding.

[0062] The low hardness of MAX phases and their ability to delaminate makes them much easier to machine than ceramics typically used in MMCs (TiB2, SiC, TiC). MAX phases are preferred for making machinable MMCs.

[0063] A particularly promising ceramic powder is MAX phase Ti3SiC2which is particularly suitable as a ceramic in a metal matrix composite. In particular a combination of Ti3SiC2and aluminium or aluminium alloy or a steel forms a suitable MMC powder. Another particularly promising ceramic powder are Al4SiC4and Al8SiC7which are particularly suitable as ceramics in a metal matrix composite. In particular, a combination of Al4SiC4and Al8SiC7and steel or aluminium or aluminium alloy forms a suitable MMC powder.

[0064] Other examples of MAX phases are included in the following table:

[0065] Cored wire preparation

[0066] After selecting the composition of the target MMC and its ceramic reinforcement, the cored wire MMC electrode must be prepared. Working within the set of possible materials, the first step is to select and procure a metal / alloy strip approximately 0.1 to 5 mm thick, of width appropriate for the target dimensions of the cored wire. The strip material may be a steel, aluminium, nickel, copper or other metal or alloy. The strip must be ductile enough to enable its processing into a hollow wire in one of the next steps. Lightly alloyed strips (> 98 wt%) may be preferred as they allow greater flexibility in tuning the composition of the metal matrix in the final product by adding master alloy powder or wire into the cored wire electrode.

[0067] The second step is to select the filler for the cored wire electrode. The filler is composed of ceramic powder of a size which will fit in the metal shell made from the strip material, for example of a size between -500 nm and 0.5 mm. In an example, the ceramic powder has a dio of 5 pm or less and a d9o of 10 pm or less. Size refers to the desired mean particle diameter (equivalent spherical diameter) in the finished product. The size distribution is usually measured by different techniques such as laser diffraction analysis (according to ASTM B822), dynamic light scattering (according to ASTM B822), and electron microscopy.

[0068] Optionally, metal / alloy powder of arbitrary size but substantially smaller than the inner diameter of the cored wire can be added to adjust the composition of the alloy in the final product. The mean particle diameter of the alloy powder is not so critical. This is because the alloy melts and is reshaped into a different particle size distribution during the electrode induction gas atomization process. However, if the metal powder is too large, this can result in non-uniform mixing of the molten metal and ceramic during atomisation, leading to the inhomogeneity of the final atomised powder. The metal powder can have a broad size distribution, but should preferably have a d10diameter of at least 10 pm and a d9o diameter of at most 2000 pm measured by ASTM B822. If metal / alloy powder is used it is preferably to be blended with the ceramic powder before using it to form cored wire to ensure homogeneity of the resultant atomised powder. The blending may be done by any suitable or desirable method, preferably drum blending, although low energy ball milling may be used in cases where ensuring homogeneity is critical.

[0069] Optionally, a metal component, for example solid metal / alloy, such as alloy wire with a diameter smaller than the inner diameter of the cored wire, can be added instead of or in addition to the alloy powder for the same purpose. The optional metal component (alloy powder or wire) is selected based on the desired composition of the target MMC.

[0070] The third step is to produce cored wire. This can be done using any number of standard manufacturing processes currently used to produce seamed or seamless flux cored wires for welding. The process generally follows these steps: start with alloy strip, roll it into a U profile, fill it with ceramic powders and, optionally, metal component (alloy powders or solid metal, such as wires), roll the profile nearly closed, lubricate the outer surface, conduct a wire drawing step to fully enclose the core and reduce the diameter of the cored wire to the desired value, remove the lubricant, optionally oil or otherwise coat the surface to prevent corrosion, and finally wind the cored wire onto a spool. Atomisation

[0071] Once prepared, the cored wire described above is atomised. Preferably, this is done using the cored wire as an electrode in electrode inert gas atomisation (EIGA), plasma atomisation (PA) or a similar method. The parameters and in particular the current, voltage, feed rate, and axisymmetric rotation speed used in the electrode induction gas atomization process (see Figure 6 for the schematic of the process) are adjusted dependent upon the unique composition of the cored wire, as well as the electrode geometry, e.g. diameter, wall thickness and the presence and features of any metal component (e.g. alloy powder or wire). These parameters will determine the formation of a melt film and subsequent stability of the molten metal stream or droplets that fall from the electrode tip into the inert gas nozzle to form spherical-shaped fine powder. The current and voltage values are determined using the following rules where the electrode temperature can be increased by increasing current or voltage. If the electrode is made too hot, this can result in the vaporisation of the metal and / or the generation of high thermal gradients from the melt tip into the body of the electrode resulting in melt tip discontinuation and eventual tip detachment. However, if the electrode is not hot enough, the flow of metal stream is poor caused by a partially formed melt tip. Poor flow may result in a preferential melting and atomisation of the metal components of the cored wire and the agglomeration of ceramic reinforcement in the tip. In these conditions, obtained atomised powder particles have reduced ceramic content while ceramic atomises as agglomerates of much larger size. These agglomerates are preferably sieved out for the powder to conform to the AM process requirements. As a result, an MMC powder with reduced ceramic fraction is produced. This is not desirable.

[0072] Although the EIGA process parameters are dependent on the setup (shape of induction coils, gas nozzle, and other key parts which tend to vary between machines) general conditions are necessary for obtaining powders suitable for AM. These include atomising the cored wire with diameters up to 10 mm, but preferably between 3 and 7 mm in diameter using an inert gas, preferably argon, forthe melting and atomisation processes. The particles atomized in the EIGA process can be sorted (e.g. sieved) to select a suitable (predetermined range) size for laser powder bed fusion additive manufacturing. A suitable size distribution is such that its d10is larger than 20 pm and d9o is smaller than 85 pm as measured by ASTM B822.

[0073] The MMC powder is preferably suitable for additive manufacturing (AM), with a narrow, microscopic size distribution, lack of impurities, good circularity and flowability. The preferred powder morphology and size distribution depends on the exact AM method used. The preferred AM method in this invention is laser powder bed fusion (LPBF), for which the preferred size distribution is discussed above. In an embodiment, when analysing the cumulative distribution function of particle diameters, the tenth percentile diameter (d10) is preferably between 10 pm and 35 pm and the ninetieth percentile diameter (d9o) is preferably between 50 pm and 100 pm. The diameter is measured using laser diffraction either from the volume equivalent of a sphere or the area equivalent of a circle. The average aspect ratio of the powder, defined in the usual way as the ratio of the longest Feret diameter of a particle to the shortest one, is preferably below 2. The average circularity of the powder is preferably between 0.8 and 1 and is defined by the formula 4nA / P2where A is the projected area of the powder particle and P is the overall perimeter of the projection, according to ISO9276-6. Aspect ratio and circularity are measured on images taken using optical microscopy or scanning electron microscopy on a sample of powder deposited on a flat surface. The sample should be thin enough to form a single layer of particles in which particles have limited overlap and minimal vertical stacking. The preferred flow properties of the powder as measured by powder flow results from Carney and Hall flow methods should be less than 11 s / 50 g and 35 s / 50 g (the time it takes for 50 g of powder to pass through the funnel with a circular orifice diameter D = 0.508 cm and 0.254 cm respectively), respectively.

[0074] Additive manufacturing

[0075] The mixed powder can then be manufactured into an item using an additive manufacturing process. This involves depositing and melting a layer of powder, allowing it to solidify, depositing and melting another layer of powder, allowing it to solidify and repeating the process, adjusting the cross-section and thickness of the layer as well as the laser scan parameters as necessary, to produce the target item.

[0076] Alternatively, the additive manufacturing process can involve depositing multiple layers of powder mixed with a binding agent as in binder jetting or bound metal deposition. The layers can again differ in cross-sections, thicknesses and the parameters used for deposition. This route needs to end with a sintering step that volatilises the binder phase and allows the MMC powder particles to sinter.

[0077] The additive manufacturing process is preferably a mature process such as directed energy deposition (DED) or laser powder bed fusion (LPBF). In an embodiment the additive manufacturing process may be metal injection moulding in which the powder is mixed with a binder material to create a feedstock which is then used in an injection moulding process to be formed into the final shape of the product. After moulding, the product undergoes processing to remove the binder (e.g. using a solvent or thermal treatment or a catalytic process) and fuse the metal particles together (e.g. through diffusion bonding).

[0078] Unexpectedly it has been found that the addition of ceramic particles, which remain solid in the melt of the matrix alloy, increase the viscosity of the melt pool, which results in a stable, viscous, less turbulent melt pool, which leads to a wider processing window, as well as reduced defect density and as-built roughness. The stable melt pool also contributes to improved feature fidelity compared to using matrix alloy powder alone. This increased feature fidelity is essential in printing complex, intricate geometries. Ceramic particles with good wettability also provide desirable properties by acting as heterogeneous nucleation sites in the melt, resulting in finer grains. Fine grain size is beneficial because it increases ductility and strength and reduces the propensity to hot cracking.

[0079] The preferred method of AM in this invention is laser powder bed fusion (LPBF). The preferred LPBF process parameters depend on the particular combination of matrix material and ceramic reinforcement. In an embodiment, the process preferably should result in a printed article with an optical density >98%. Optical density is measured by first acquiring an optical or scanning electron micrograph of a polished part of the cross-section of the article, then using image analysis techniques to quantify the area associated to porosity and finally dividing it by the total measured are to derive optical density.

Claims

Claims1 . A method of preparing a metal matrix composite powder for additive manufacturing, the method comprising: preparing a cored wire comprising: a metal shell and a ceramic contained within the metal shell; and using the cored wire as an electrode in an atomisation process to prepare a metal matrix composite powder.

2. The method of claim 1 , wherein the ceramic is a ceramic powder.

3. The method of claim 1 or 2, wherein the ceramic powder is TiB2or SiC or AIN ceramic powder.

4. The method of claim 1 or 2, wherein the ceramic powder is a MAX phase ceramic powder.

5. The method of claim 4, wherein the MAX phase of the MAX phase ceramic has a layered hexagonal structure, preferably a layered hexagonal crystallographic structure with weak interlayer bonding and / or wherein the layered hexagonal crystallographic structure is a space group P63 / mmc.

6. The method of claim 4 or 5, wherein the MAX phase of the MAX phase ceramic has a general formula Mn+iAXn, where M is one or more metals, A is one or more A-group element and X is C and / or N and / or B.

7. The method of claim 6, wherein M is one or more of: Ti, Sc, V, Cr, Zr, Nb, Hf, Mn, Mo and / or A is one or more of: Cd, Si, Sn, In, Ga, Tl, Al, Ga, Sn, Pb, Zn, Au.

8. The method of any of claims 4-7, wherein the MAX phase of the MAX phase ceramic is Ti3SiC2.

9. The method of any of claims 4-8, wherein the MAX phase of the MAX phase ceramic is Al4SiC4and Al8SiC7.

10. The method of any of the preceding claims, wherein the ceramic powder has a d10of 5 pm or less and a d9o of 10 pm or less.11 . The method of any of the preceding claims, wherein the cored wire further comprises a metal component, wherein the metal component is contained within the shell.

12. The method of claim 11 , wherein the metal component includes a metal powder.

13. The method of claim 11 or 12, wherein the metal component includes a metal wire.

14. The method of any of the preceding claims, wherein the metal shell and optionally any metal component is non-ferrous.

15. The method of any preceding claim, wherein the metal shell and optionally any metal component is one or more of: nickel and nickel alloys, steels including stainless steels, titanium and titanium alloys, aluminium and aluminium alloys, cobalt and cobalt alloys, magnesium and magnesium alloys, copper and copper alloys, refractory metals and refractory metal alloys, platinum group metals and their alloys, vanadium metals and their alloys.

16. The method of any preceding claim, wherein the metal shell is formed of an elemental metal or an alloy of an element which is 98% or more pure.

17. The method of any preceding claim, wherein the cored wire is greater than 70% by weight aluminium.

18. The method of any preceding claim, wherein the metal shell is AI6061 alloy, preferably with a composition of 0.80-1 .20 wt% Mg, 0.40-0.80 wt% Si, 0.15-0.40 wt% Cu and 0.04-0.35 wt% Cr with optional additions of Fe up to 0.70 wt%, Zn up to 0.25 wt%, Ti up to 0.15 wt% and Mn up to 0.015 wt% with other elements present in an amount of 0.05 wt% or less each with a total sum of other elements of 0.15 wt% and remainder aluminium and incidental impurities.

19. The method of any of the preceding claims, wherein the volume of ceramic powder is at least 5% of the total volume of the cored wire, preferably wherein the volume of ceramic powder is at least 10% of the total volume of the cored wire, optionally wherein the volume of ceramic powder is 50% or less of the total volume of the cored wire.

20. The method of any preceding claim, wherein the atomisation process is induction gas atomisation.21 . The method of any preceding claim, wherein the atomisation process occurs at a temperature above the melting point of any metal in the cored wire and below the melting point of the ceramic powder.

22. A method of manufacturing a metal matrix composite component using the metal matrix composite powder produced by the method of any preceding claim in a metal additive manufacturing process.

23. The method of claim 22, wherein the metal additive manufacturing process is a selective laser melting process.

24. The method of claim 23, wherein the metal additive manufacturing process is laser powder bed fusion.

25. The method of claim 22, wherein the metal additive process is directed energy deposition.

26. The method of claim 22, wherein the metal additive process is metal injection moulding.

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