Method for producing spherical powders for additive manufacturing and powders obtained thereby

Abstract

A method of producing a spherical powder for use in additive manufacturing, comprises the initial steps of: (a) mixing together a first component comprising at least one source of free carbon, a second component comprising at least one carbide-forming metal or metalloid element, and a third component comprising at least one matrix-forming binder metal; (b) adding water to the first, second and third components; (c) milling the mixture to form a homogeneous slurry having a selected particle size range; and (d) spray-drying the slurry to evaporate water and agglomerate the resultant solid mixture into a spherical powder. Each resultant spherical powder particle comprises the first, second, and third components. The resultant spherical powder can then be used in an additive manufacturing process, wherein a hardmetal comprising a carbide formed from the first and second components encapsulated within a metal matrix formed from the third component is generated in situ from the powder.

Description

[0001] METHOD FOR PRODUCING SPHERICAL POWDERS FOR ADDITIVE MANUFACTURING AND POWDERS OBTAINED THEREBY

[0002] This invention relates, in a first aspect, to an improved method of producing spherical powders for use in additive manufacturing. In a second aspect, the invention relates to spherical powders produced by a method according to the first aspect. In a third aspect, the invention relates to an additive manufacturing process utilising spherical powders according to the second aspect.

[0003] The provision of metals and other materials in powdered form is vital for many materials processing applications, notably including additive manufacturing. The ability reliably and accurately to control physical properties of the powder, such as density and flowability is essential in such applications, and these parameters are directly related to the size and shape (morphology) of the powder particles. Spherical particles, and hence spherical powders, are generally most desirable in this regard.

[0004] The term “additive manufacturing” is used to refer to a variety of processes in which material is accurately deposited, usually under computer control, with the material being added together, typically layer by layer. Such processes can be used to build three-dimensional objects, or to apply coatings to existing objects.

[0005] Laser additive manufacturing is a variant of additive manufacturing, in which a laser beam is used to melt metal powder (or wire) by heating it onto the surface of the object being built or coated. This is also characterised as a directed energy deposition (DED) process. After the material has solidified, the next metal layer can be applied, thereby additively generating a three-dimensional object or coating. The use of laser additive manufacturing to apply a coating to an object is often referred to as laser cladding.

[0006] The application of hard-facing coatings to the surface of an object is a common industrial process used to enhance wear, thermal and corrosion resistance. This typically involves harder or tougher material being applied to a base metal. Hard-facing technologies, such as thermal spraying, and more recently laser additive manufacturing, have been used both in primary manufacturing as well as refurbishing high value wear components, using coating or cladding processes as described above.

[0007] Both thermal spraying and laser additive manufacturing use spherical hardmetal powders in the coating deposition. A hardmetal in this context is defined as a composite material in which a carbide of a first metal or metalloid element is encapsulated within a binder matrix formed from a second metal element. The powder particle size utilised in such applications typically ranges from 10 to 150 pm, and more typically from 15 to 53 pm.

[0008] Currently, laser additive manufacturing uses conventional hardmetal powders developed for thermal spraying. In thermal spraying, no melting is involved and no chemical reaction takes place in the powder during coating deposition. Conventional hardmetal powders therefore contain pre-manufactured carbide and metal binder for the specific coating application. The conventional hardmetal powder manufacturing process involved multiple high energy stages: pre-manufacturing the carbide, grinding it into fine particles (pulverisation) and then agglomerating the carbide and binder matrix into a spherical powder.

[0009] Laser additive manufacturing, or laser cladding, is a fusion process. During coating deposition, a melt-pool is created by the laser beam on the substrate into which the powder is fed. Chemical reactions take place inside the melt-pool within a fraction of a second. When pre-manufactured carbide, for example tungsten carbide, powder is used in the fusion process, the carbide is re-melted during deposition, and this can lead to decomposition taking place to form an unfavourable brittle phase. For this reason, pre-manufactured carbide is undesirable for use in laser additive manufacturing.

[0010] In recent years, in-situ alloying has been attempted in laser additive manufacturing. This involves mixing and feeding elemental metal powders (without free carbon) into the melt-pool. So far, these techniques have been limited to laboratory research, due to challenges associated with powder flowability. While using larger, spheriodised particles helps flowability, the use of large particles also affects the homogeneity of elements in the mixed compound, as well as incomplete chemical reaction of the mixed compounds in the melt-pool during laser processing.

[0011] The genesis of the present invention stems from the realisation that hard carbide phases can be generated in-situ during the laser additive manufacturing process, by feeding both free carbon and elemental metal material into the melt-pool. For example, during the laser process, feeding particles of free carbon, a carbide- forming first metal (or metalloid) and a matrix -forming second metal binder into the melt-pool will lead to a chemical reaction between the constituents. This will result in the in-situ formation of a carbide of the first metal, encapsulated in a matrix of the second metal. The main advantage of this is the ability to tailor the coating composition from the input materials to enhance the coating performance for targeted applications, suppress formation of unfavourable brittle phases and significantly reduce residual stresses within the coating. Examples of carbide-forming first metal (or metalloid) elements suitable for this process include tungsten, titanium, chromium, vanadium, tantalum, hafnium, boron, silicon, molybdenum and zirconium.

[0012] Fine, typically sub-micron, elemental particles are the preferred choice as input materials to ensure homogenous distribution of chemical composition as the hardfacing coating is formed, and thus avoid the incomplete chemical reaction and homogeneity issues encountered in in-situ alloying.

[0013] Forming hard carbide phases will require the presence of free carbon in the mixture. Free carbon is soft and flaky and tends to form semi-solid lumps when compacted. The particle size of the elemental metal materials and carbon will therefore need to be controlled to ensure homogenous distribution of the constituents and to accelerate chemical reaction in the melt-pool. Due to their large surface area, fine particles of elemental materials and carbon tend to clump together and do not flow freely, which can lead to clogging in the dispensing component, such as a nozzle. Free flowing powder is a key requirement for powder-based additive manufacturing processes. The present invention seeks to address these and other issues associated with the prior art techniques described above.

[0014] According to a first aspect of the present invention there is provided a method of producing a spherical powder for use in additive manufacturing, comprising the steps of:

[0015] (a) mixing together a first component comprising at least one source of free carbon (C), a second component comprising at least one carbide-forming metal or metalloid element, and a third component comprising at least one matrix-forming binder metal;

[0016] (b) adding water to the first, second and third components;

[0017] (c) milling the mixture to form a homogeneous slurry having a selected particle size range; and

[0018] (d) spray-drying the slurry to evaporate water and agglomerate the resultant solid mixture into a spherical powder.

[0019] In order to ensure homogeneity, it is generally preferred that step (b) be carried out before step (c), i.e. that water be added to the first, second and third components, preferably in a milling jar, and that the wet mixture then be milled to form the homogeneous slurry. However, in alternative embodiments of the method according to the first aspect of the present invention, the order of these steps (b) and (c) may effectively be reversed, such that the first, second and third components are milled to a desired particle size thereby to form a homogeneous solid mixture, and then the water subsequently added to the homogeneous solid mixture, thereby to form the homogeneous slurry. In such embodiments, the milling step (c) could in theory even be carried out separately in respect of each of the first, second and third components, prior to the mixing step (a). The first, free carbon, component is preferably selected from one or more of graphite, carbon black, and bitumen. The free carbon component preferably constitutes in the range of from 3 to 12% by weight of the solid mixture.

[0020] The second, carbide-forming metal or metalloid element, component is preferably selected from one or more of tungsten (W), titanium (Ti), chromium (Cr), iron (Fe), silicon (Si), Boron (B), molybdenum (Mo), and zirconium (Zr).

[0021] The second, carbide-forming metal or metalloid element, component preferably constitutes in the range of from 50 to 90% by weight of the solid mixture.

[0022] Where the second component comprises tungsten, this preferably constitutes up to 80% by weight of the solid mixture.

[0023] Where the second component comprises titanium, this preferably constitutes up to 60% by weight of the solid mixture.

[0024] Where the second component comprises chromium, this preferably constitutes up to 60% by weight of the solid mixture.

[0025] Where the second component comprises iron, this preferably constitutes up to 60% by weight of the solid mixture.

[0026] Where the second component comprises silicon, this preferably constitutes up to 20% by weight of the solid mixture.

[0027] Where the second component comprises boron, this preferably constitutes up to 20% by weight of the solid mixture.

[0028] Where the second component comprises molybdenum, this preferably constitutes up to 20% by weight of the solid mixture.

[0029] Where the second component comprises zirconium, this preferably constitutes up to 20% by weight of the solid mixture.

[0030] The third, matrix-forming binder metal, component is preferably selected from one or more of cobalt (Co), iron (Fe), aluminium (Al), nickel (Ni), chromium, and copper (Cu). The third, matrix-forming binder metal, component preferably constitutes in the range of from, 5 to 50% by weight of the solid mixture.

[0031] The relative % weight proportions of the second, carbide-forming metal or metalloid element, component and the third, matrix-forming binder metal, component are selected depending on the hardness and toughness required for the particular application of the end product. A higher percentage of carbide will provide a harder coating, but can also result in the coating becoming brittle.

[0032] Most preferably, each of the first, second and third components are present in elemental form.

[0033] In preferred embodiments of the method according to the first aspect of the present invention, the selected particle size range in step (c) is in the range of from 0.1 to 10 pm, and preferably in the range of from 0.1 to 2 pm.

[0034] The solid mixture may optionally further comprise a fourth component comprising a grain refiner. The grain refiner is preferably selected from tantalum (Ta), vanadium (V) and niobium (Nb). Where present, the grain refiner preferably constitutes up to 5% by weight of the solid mixture.

[0035] The water added in step (b) is preferably distilled water. More preferably, the distilled water constitutes in the range of from 15 to 35% by weight of the slurry. The slurry formed in step (c) may optionally further comprise one or more inorganic binder components selected from acrylics, silicates, boric acid, magnesium carbonates, other soluble carbonates, nitrates, oxalates and oxychlorides. These agents acts to prevent flocculation of the mixed powders, and help to bind the powders during the subsequent spray drying process.

[0036] In preferred embodiments of the method according to the first aspect of the present invention, the spray-drying step (d) comprises sub-steps of:

[0037] (i) pumping the slurry into a process chamber via a two-fluid or centrifugal nozzle, to generate finely sprayed droplets; and

[0038] (ii) drying the droplets in the process chamber under hot dry air.

[0039] The hot dry air in step (d)(ii) is preferably maintained at a temperature in the range of from 120 to 300 °C.

[0040] The spray-drying step (d) serves to evaporate the water and agglomerate the mixture into a spherical powder.

[0041] Conventional spray-drying processes in other applications tend to utilise solvent and organic binders. These are undesirable as there are inherent dangers associated with the use of solvents in spray-drying, whilst organic binders are associated with negative environmental impact. The use of distilled water and inorganic binders in the slurry-forming step (c) and the spray-drying step (d) of the present invention therefore provide further benefits in terms of improved safety and reduced environmental impact.

[0042] In preferred embodiments, the method according to the first aspect of the present invention further comprises an additional step of:

[0043] (e) sintering the spherical powder in a furnace.

[0044] The furnace in step (e) is preferably operated at a temperature in the range of from 1100 to 1300 °C. More preferably, the sintering step (e) is carried out in a hydrogen or argon-hydrogen mix gas atmosphere, or in a vacuum, to suppress oxidation of the spherical powder.

[0045] In preferred embodiments, the method according to the first aspect of the present invention further comprises an additional step of:

[0046] (f) screening the spherical powder to select powder for use within a desired particle size distribution.

[0047] The desired particle size distribution is preferably in the range of from 5 to 150 pm, more preferably is in the range of from 15 to 105 pm, and most preferably is in the range of from 15 to 45 pm.

[0048] Powder identified in step (f) as having a particle size below the desired particle size distribution is preferably returned to step (d) to be re-agglomerated.

[0049] Similarly, powder identified in step (f) as having a particle size above the desired particle size distribution is preferably returned to step (c) to be re-milled.

[0050] As hereinbefore described, the present invention is concerned particularly with the provision of spherical powders for use in additive manufacturing. However, it will be appreciated that the spherical powders produced by the above defined method will also be suitable for use in a wide range of other applications, such as metal injection moulding and hot isostatic pressing.

[0051] According to a second aspect of the present invention, there is provided a spherical powder produced by a method according to the first aspect of the present invention as hereinbefore described, wherein each spherical powder particle comprises the first, second, and third components.

[0052] Each said spherical powder particle preferably comprises the first, second, and third components in elemental form. The spherical powders according to the second aspect of the present invention have been found to be free flowing, having a hall flow rate of less than or equal to 30s / 50g, and an apparent density, as an example for tungsten carbide powders, of around 4.5 to 5.5 g / cm3Apparent density will vary depending on the identity of carbide-forming element, the matrix-forming binder element, and their relative % weight proportions. The powders thus match equivalent conventional hardmetal powders and are suitable for use in laser additive manufacturing.

[0053] According to a third aspect of the present invention, there is provided an additive manufacturing process utilising a spherical powder according to the first aspect of the present invention as hereinbefore described, wherein a hardmetal comprising a carbide formed from the first and second components encapsulated within a metal matrix formed from the third component is generated in situ from the powder. The additive manufacturing process according to the third aspect of the invention is preferably a laser additive manufacturing process, or an electron beam additive manufacturing process.

[0054] In such an additive manufacturing process according to the third aspect of the invention, preferably no preliminary carbide-forming step is carried out.

[0055] The production (according to the first aspect of the present invention) of a spherical powder (according to the second aspect of the present invention) for use in a process (according to the third aspect of the present invention) in which the carbide matrix composite is generated in situ, avoids the need for a carbide-forming step to be carried out during the production of the powder. This in turn enables several energy- intensive steps found in the prior art to be removed from the production process, namely: the preliminary carbide-forming step and its associated heating cycle, the grinding (pulverisation) of the carbide into fine particles, the subsequent agglomeration of the carbide and binder metal matrix into a spherical powder. The present invention therefore enables significant energy savings as compared to prior art processes.

[0056] In order that the present invention may be clearly understood, preferred embodiments and applications thereof will now be described in detail, though only by way of example, with reference to the accompanying drawings, in which:

[0057] Figure 1 is a flow chart illustrating a conventional (prior art) manufacturing process for tungsten carbide powder in a cobalt matrix;

[0058] Figure 2 is a flow chart illustrating a hardmetal powder production process, according to a preferred embodiment of the first aspect of the present invention; and Figures 3 and 4 are micrographs showing an example of a spherical powder according to a preferred embodiment of the second aspect of the present invention.

[0059] Referring first to Figure 1 , in a prior art process for manufacturing tungsten carbide - cobalt matrix, a first step (I) of reducing the tungsten ore (wolframite) to extract tungsten is carried out according to the following reaction:

[0060] This reaction is carried out at between 600 to 900 °C over 24 hours, and is thus highly energy intensive.

[0061] A carburisation step (II) is then carried out to form tungsten carbide, according to the following reactions:

[0062] The formation of the brittle tungsten carbide phase W2C is a by-product of the carburisation process (II) and is generally undesirable.

[0063] This reaction is carried out at 1200 °C, and is thus also highly energy intensive.

[0064] The next step (III) is pulverisation, or crushing, to obtain fine tungsten carbide powder. This is then ball milled, in step (IV), together with cobalt and an organic binder.

[0065] Solvent is then added, in step (V), to form a slurry, which is then spray-dried in step (VI) at a temperature of below 300 °C to agglomerate the particles into tungsten carbide - cobalt matrix powder. A final step (VII) of sintering at 1200 °C to densify and bind the tungsten carbide - cobalt matrix powder is then required, before the powder can then be used in thermal spraying, laser cladding, or other powder metallurgy processes.

[0066] The above described prior art process thus includes several energy intensive steps, tends to produce undesirable by-products, and requires the use of organic binders and solvents which are environmentally disadvantageous.

[0067] Referring now to Figure 2, in a method for producing a spherical powder according to a preferred embodiment of the first aspect of the present invention, a preliminary refinement process is carried out to obtain elemental metal powder, by extracting if from the raw ore material of a carbide-forming metal.

[0068] The carbide-forming metal powder is then mixed together in step (a) with a source of free carbon, a matrix-forming binder metal, and an inorganic binder. In the particular embodiment shown in Figure 2, a ball-milling process (c) is included in step (a). In other embodiments however, the milling process (c) may be carried out after the addition of distilled water in step (b). In either step order variant, after carrying out steps (a), (b) and (c) a homogeneous slurry is formed.

[0069] A step (d) of spray-drying the slurry to evaporate the water and agglomerate the resultant solid mixture into a spherical powder is then carried out, at a temperature of below 300 °C. An optional step (e) of sintering the powder at around 1200 °C is then carried out, in order to densify and bind the powder.

[0070] The powder is then ready to be used in laser cladding, directed energy deposition or other metal additive manufacturing processes. The metal carbide - binder metal matrix will be generated in situ in the melt pool during processing. This avoids the need for the energy intensive steps (II) and (III) of the prior art process described in Figure 1 , avoids the formation of brittle carbide phases, and avoids the need to utilise environmentally undesirable organic binders and solvents.

[0071] Referring now to Figures 3 and 4, these show micrographs of an example of a spherical powder according to a preferred embodiment of the second aspect of the present invention. The powder is a spray-dried powder agglomerated from elemental materials produced by the method according to the first aspect of the present invention, as described above with reference to Figure 2.

[0072] Figure 3 shows the powder at 2000x magnification, whilst Figure 4 shows the powder at 300x magnification. Each spherical powder particle comprises the carbide- forming metal, carbon, and the matrix-forming binder metal, in elemental form. In the examples shown in Figures 3 and 4, the carbide-forming metal is tungsten, and the matrix-forming binder metal is cobalt.

[0073] In Figure 3, the tungsten (W) particles can be seen as white in colour, whilst the Carbon (C) particles appear dark grey, and the Cobalt (Co) particles appear light grey.

[0074] Figure 4 shows the spray-dried, pre-sinter powder of a specific formulation, W- 17CO-5C.

[0075] Examples of specific formulations of spherical powders according to the second aspect of the present invention, and produced by a method according to the first aspect of the present invention are further described below:

[0076] Example I

[0077] This example describes the production of a W-Co-C powder, designed to produce a resource-efficient equivalent to conventional WC-17Co powder: 5.2% by weight of carbon, 77.8% by weight of tungsten and 17% by weight of cobalt are mixed together in a milling jar to create a solid mixture.

[0078] 2.0 % of acrylic binder is mixed into distilled water to create a liquid solution.

[0079] The liquid solution is then added to the solid mixture to achieve a solid loading of 75%.

[0080] The mixture is then wet-milled using a ball mill for 24 hours to refine the solid mixture and to form a homogenous slurry.

[0081] The slurry is then spray-dried to evaporate water and agglomerate the solid mixture. This involves feeding the slurry through one inlet of a two fluid nozzle into a commercially available spray-dryer. The inlet air temperature for this process is 220°C and the outlet air temperature is around 90°C.

[0082] The spray-dried spherical powder is then sintered in a furnace using argon-3% hydrogen gas for 2 hours at 1230°C to strengthen the agglomerate particles.

[0083] The agglomerated and sintered powder is separated into size fractions by screening.

[0084] Powder yield for particle size range from 15 to 45 pm is between 18-20%. Apparent density is 3.9-4.2 g / cm3and hall flow rate is approximately 18s / 50g following ASTN B213 standard test method.

[0085] Example

[0086] This example describes the production of a W-Ti-Co-Cr-C powder, to create a tungsten-titanium-chromium multi-carbide in a chromium-cobalt matrix hardfacing coating:

[0087] 8.0% of carbon, 53.0% of tungsten, 16.5% of titanium, 6.5% of chromium, and 16% of cobalt are mixed in a milling jar to create a solid mixture.

[0088] 1.5 % of boric acid as a binder is mixed into distilled water to create a liquid solution.

[0089] The liquid solution is then added to the solid mixture to achieve a solid loading of 75%.

[0090] The mixture is then wet-milled using a ball mill for 24 hours to refine the solid mixture and to form a homogenous slurry.

[0091] The slurry is then spray-dried to evaporate water and agglomerate the solid mixture. This involves is feeding the slurry through one inlet of a two fluid nozzle into a commercially available spray-dryer. The Inlet air temperature for this process is 210°C and the outlet air temperature is around 90°C.

[0092] The spray-dried spherical powder is then sintered in a furnace using argon-3% hydrogen gas for 2 hours at 1230°C to strengthen the agglomerate particles. The agglomerated and sintered powder is separated into size fractions by screening.

[0093] Powder yield for particle size range from 15 to 45 pm is between 16-19%. Apparent density is 3.5-3.8 g / cm3and hall flow rate is approximately 22s / 50g following ASTN B213 standard test method.

Claims

Claims1 . A method of producing a spherical powder for use in additive manufacturing, comprising the steps of:(a) mixing together a first component comprising at least one source of free carbon, a second component comprising at least one carbide-forming metal or metalloid element, and a third component comprising at least one matrix-forming binder metal;(b) adding water to the first, second and third components;(c) milling the mixture to form a homogeneous slurry having a selected particle size range; and(d) spray-drying the slurry to evaporate water and agglomerate the resultant solid mixture into a spherical powder.

2. A method as claimed in claim 1 , wherein:- step (b) is carried out before step (c);- step (b) comprises adding water to the first, second and third components in a milling jar; and- step (c) comprises milling a resultant wet mixture to form the homogeneous slurry.

3. A method as claimed in claim 1 , wherein:- step (c) is carried out before step (b);- step (c) comprises milling the first, second and third components to a desired particle size thereby to form a homogeneous solid mixture; and- step (b) comprises adding water to the homogeneous solid mixture, thereby to form the homogeneous slurry.

4. A method as claimed in any of the preceding claims, wherein the first, free carbon, component is selected from one or more of graphite, carbon black, and bitumen.

5. A method as claimed in any of the preceding claims, wherein the second, carbide-forming metal or metalloid element, component is selected from one or more of tungsten, titanium, chromium, iron, silicon, boron, molybdenum, and zirconium.

6. A method as claimed in any of the preceding claims, wherein the second, carbide-forming metal or metalloid element, component preferably constitutes in the range of from 50 to 90% by weight of the solid mixture.

7. A method as claimed in any of the preceding claims, wherein the third, matrixforming binder metal, component is selected from one or more of cobalt, iron, aluminium, nickel, chromium, and copper.

8. A method as claimed in any of the preceding claims, wherein the third, matrixforming binder metal, component preferably constitutes in the range of from 5 to 50% by weight of the solid mixture.9 A method as claimed in any of the preceding claims, wherein each of the first, second and third components are in elemental form.

10. A method as claimed in any of the preceding claims, wherein the selected particle size range in step (c) is from 0.1 to 10 pm.

11. A method as claimed in any of the preceding claims, wherein the solid mixture further comprises a fourth component comprising a grain refiner.

12. A method as claimed in claim 11 , wherein the grain refiner is selected from tantalum, vanadium and niobium.

13. A method as claimed in any of the preceding claims, wherein the water added in step (b) is distilled water.

14. A method as claimed in any of the preceding claims, wherein the slurry further comprises one or more inorganic binder components selected from acrylics, silicates, boric acid, magnesium carbonates, other soluble carbonates, nitrates, oxalates and oxychlorides.

15. A method as claimed in any of the preceding claims, wherein the spray-drying step (d) comprises sub-steps of:(i) pumping the slurry into a process chamber via a two-fluid or centrifugal nozzle, to generate finely sprayed droplets; and(ii) drying the droplets in the process chamber under hot dry air.

16. A method as claimed in claim 15, wherein the hot dry air in step (d)(ii) is maintained at a temperature in the range of from 120 to 300 °C.

17. A method as claimed in any of the preceding claims, further comprising an additional step of:(e) sintering the spherical powder in a furnace, preferably operated at a temperature in the range of from 1100 to 1300 °C.

18. A method as claimed in claim 17, wherein the sintering step (e) is carried out in a hydrogen or argon-hydrogen mix gas atmosphere, or in a vacuum, to suppress oxidation of the spherical powder.

19. A method as claimed in any of the preceding claims, further comprising an additional step of:(f) screening the spherical powder to select powder for use within a desired particle size distribution.

20. A method as claimed in claim 19, wherein the desired particle size distribution is in the range of from 15 to 105 pm, and preferably is in the range of from 15 to 45 pm.21 . A method as claimed in claim 19 or claim 20, wherein:- powder having a particle size below the desired particle size distribution is returned to step (d) to be re-agglomerated; and / or- powder having a particle size above the desired particle size distribution is returned to step (c) to be re-milled.

22. A spherical powder produced by a method as claimed in any of the preceding claims, wherein each spherical powder particle comprises the first, second, and third components.

23. A spherical powder as claimed in claim 22, wherein each spherical powder particle comprises the first, second, and third components in elemental form.

24. An additive manufacturing process utilising a spherical powder as claimed in claim 22 or claim 23, wherein a hardmetal comprising a carbide formed from the first and second components encapsulated within a metal matrix formed from the third component is generated in situ rom the powder.

25. An additive manufacturing process as claimed in claim 24, wherein no preliminary carbide-forming step is carried out.

26. An additive manufacturing process as claimed in claim 24 or claim 25, being a laser additive manufacturing process, or an electron beam additive manufacturing process.

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