GB2559950A

GB2559950A - Metal alloy

Info

Publication number: GB2559950A
Application number: GB1700419.3A
Authority: GB
Inventors: Shen Zhijian; Zhong Yuan; Cul Daqing; Wikman Stefan
Original assignee: Individual
Current assignee: Individual
Priority date: 2016-12-19
Filing date: 2017-01-10
Publication date: 2018-08-29
Also published as: GB201621653D0; GB201700419D0

Abstract

An oxide dispersed strengthened (ODS) metal alloy comprising oxide particles dispersed in a metal alloy matrix, wherein the metal alloy matrix comprises an intragranular cellular microstructure, and wherein the oxide particles are dispersed in at least some of the cells of the intragranular cellular microstructure; or a method of additively manufacturing a metal alloy with oxide particles. The metal alloy and oxide may be mixed by a ball mill and the oxide particles may be deposited on the surface. Slowing the scanning speed increases the cell size (fig 5). The ODS metal alloy may be stainless steel. The metal alloy matrix may have a structure of a melt pool (fig 4a and 4b) comprising a plurality of grains each comprising the intragranular structure. The oxide particles may be present in between at least some of the boundaries between adjacent grains as well as within at least some of the cells of the structure. The oxide particles may be metal oxides, preferably Yttrium Y2O3 nanoparticles of 1-200 nm. The design is preferably for heat exchanges resistant to neutron damage and high temperatures for nuclear fusion reactors.

Description

(71) Applicant(s):

Zhijian Shen

Signe Tillischgatan 18, Solna, Sweden

Yuan Zhong

Forskarbacken 7, Stockholm, Sweden

Daqing Cul

Vasavagen 6, SODERTALJE, Sweden

Stefan Wikman

Torres Diagonal Litoral B3, Josep Pia 2, Spain (72) Inventor(s):

Zhijian Shen Yuan Zhong Daqing Cul Stefan Wikman (74) Agent and/or Address for Service:

HGF Limited (London Office)

Document Handling - HGF - (London), 1 City Walk, Leeds, Yorkshire, LS11 9DX, United Kingdom (51) INT CL:

B22F 3/105 (2006.01) B33Y10/00 (2015.01) B82Y 30/00 (2011.01) C22C 38/00 (2006.01) (56) Documents Cited:

EP 2327807 A1

B22F3/00 (2006.01) B33Y 80/00 (2015.01) C22C1/05 (2006.01)

Journal of Materials Science, Vol. 50, 2015, Sallez et al., On ball-milled ODS ferritic steel recrystallization: From as-milled powder particles to consolidated state, pages 2202 to 2217

Metallurgical and Minerals Transactions A, Volume 47A, November 2016, Bergner et al., Alternative Fabrication Routes toward Oxide-DispersionStrengthened Steels and Model Alloys, pages 5313 to 5324

Metallurgical and Materials Transactions A, Vol. 33A, December 2002, Chen et al., The microstructure and recrystallization of flow-formed oxide-dispersionstrengthened ferritic alloy: Part I. Deformation structure, pages 3777 to 3785

Journal of Nuclear Materials, Vol. 428,1-3, September 2012, Nikitina et al., R&D of ferritic-martensitic steel EP450 ODS for fuel pin claddings of prospective fast reactors, pages 117 to 124

Materials Science & Engineering A, Vol. 658, 2016, Chauhan et al., Study of the deformation and damage mechanisms of a 9Cr-ODS steel: Microstructure evolution and fracture characteristics, pages 123 to 134 (continued on next page) (54) Title of the Invention: Metal alloy

Abstract Title: Oxide dispersion strengthened alloy, with oxide dispersed in intragranular cellular microstructure, and ALM method (57) An oxide dispersed strengthened (ODS) metal alloy comprising oxide particles dispersed in a metal alloy matrix, wherein the metal alloy matrix comprises an intragranular cellular microstructure, and wherein the oxide particles are dispersed in at least some of the cells of the intragranular cellular microstructure; or a method of additively manufacturing a metal alloy with oxide particles. The metal alloy and oxide may be mixed by a ball mill and the oxide particles may be deposited on the ' ^x surface. Slowing the scanning speed increases the cell V \ size (fig 5). The ODS metal alloy may be stainless steel. ' \ . '

The metal alloy matrix may have a structure of a melt . ‘ pool (fig 4a and 4b) comprising a plurality of grains each comprising the intragranular structure. The oxide particles may be present in between at least some of the boundaries between adjacent grains as well as within at .--------------:

least some of the cells of the structure. The oxide ^Figure5 particles may be metal oxides, preferably Yttrium Y₂O₃ nanoparticles of 1-200 nm. The design is preferably for heat exchanges resistant to neutron damage and high temperatures for nuclear fusion reactors.

GB 2559950 A continuation (56) Documents Cited:

Mehran University Research Journal of Engineering & Technology, Volume 26, 4, October 2007, Baloch et al, Characterisation of As-Deformed Microstructure of ODS Ni-Base Superalloy and ODS Ferritic Steel Prior to Directional Recrystallisation, pages 345 to 353 (58) Field of Search:

Other: Online: XPSPRNG, XPESP, INSPEC, EPODOC, WPI, PATENT FULLTEXT /4

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FIG. 1

FIG. 2

2/4

FIG. 2a

210

FIG. 3

3/4

FIG. 4a

FIG. 4b

FIG. 4c

4/4

Figure 5

Metal Alloy

BACKGROUND [0001] The present invention relates to an oxide dispersed strengthened (ODS) metal alloy. The present invention also relates to a metal part, and to a method of manufacturing a metal part.

[0002] Thermonuclear fusion promises to be a possible solution to the current energy crisis. This is a process by which nuclei of low atomic weight elements (e.g. hydrogen) combine to form nuclei of higher atomic weight elements (e.g. helium). The reaction produces a large amount of energy. However, so that this reaction can occur, a large amount of energy is required to allow the nuclei to come into sufficiently close contact to overcome their natural repulsion and condense in a plasma state. Although the plasma is confined within the reactor by the magnetic fields rather than the reactor walls, the reactor components must nonetheless be resistant to neutron damage and/or very high temperatures (e.g. in excess of 300 degrees C).

[0003] Similarly, nuclear fission reactors also operate at elevated temperatures while exposed to neutrons. Accordingly, there is a growing need to develop materials that exhibit desirable mechanical properties at the operational conditions of nuclear reactors.

BRIEF DESCRIPTION OF THE DRAWINGS [0004] Embodiments of the invention are further described hereinafter, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic view of an apparatus used to manufacture a metal part according to one example of the present disclosure;

Figure 2 shows views of oxide particles and metal alloy particles used to prepare a metal alloy according to one example of the present disclosure;

Figure 3 is a schematic view of a heat exchanger produced using a metal alloy according to one example of the present disclosure;

Figures 4a to 4c are SEM images showing the hierarchical structure of a metal part formed according to an example of the present disclosure; and

Figures 5a to 5d are SEM images showing the influence on printing parameters on the intragranular cellular structure.

DETAILED DESCRIPTION [0005] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0006] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments.

The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0007] According to one aspect of the present disclosure, there is provided an oxide dispersed strengthened (ODS) metal alloy comprising metal oxide particles dispersed in a metal alloy matrix, wherein the metal alloy matrix comprises an intragranular cellular structure, and wherein the metal oxide particles are dispersed in at least some of the cells of the intragranular cellular microstructure.

[0008] According to another aspect of the present disclosure, there is provided an oxide dispersed strengthened (ODS) metal alloy comprising oxide particles dispersed in a metal alloy matrix, wherein the metal alloy matrix comprises an intragranular cellular microstructure, and wherein the oxide particles are dispersed in at least some of the cells of the intragranular cellular structure and are present in an amount of at least 0.1 weight % of the total weight of the ODS metal alloy.

[0009] The disclosure also provides a metal part comprising the ODS metal alloy described herein.

[0010] According to a further aspect of the disclosure there is provided a method of additive manufacturing a metal part, said method comprising:

a) forming a mixture of metal alloy particles and oxide particles;

b) providing a layer of the mixture;

c) selectively melting the mixture in at least part of the layer using a laser; and repeating steps (b) and (c) a plurality of times, wherein each layer is applied on top of the previous layer.

[0011] In ODS metal alloys, oxide particles are dispersed in a metal alloy matrix. With known methods of ODS manufacture, however, ex-situ oxide particles are introduced as particles that have a tendency to agglomerate within the matrix. Thus, it can be difficult for an even distribution of the oxide particles to be achieved. In the present disclosure, the oxide particles are formed in-situ during a rapid melting and solidification process that ensures that the particles can be distributed more evenly throughout the metal alloy matrix.

[0012] In the present disclosure, a first layer comprising a mixture of metal alloy particles and oxide particles may be applied to a platform. A laser e.g. guided by computer aided design (CAD) software may be used to selectively melt at least part of the layer. A second layer comprising a mixture of metal alloy particles and oxide particles may be applied over the first layer. The second layer may then be selectively melted using a laser, for example, also under the guidance of the CAD software. Thereafter a further layer of the mixture may be applied and the layering and melting steps repeated until a metal part is additively manufactured e.g. according to the CAD design.

[0013] When the laser is directed onto the layer of mixture, it may melt the mixture in the region of the laser to form a melt pool. The melt pool comprises molten oxide and molten alloy. Convection currents established within the melt pool help to ensure that the molten oxide is evenly distributed throughout the molten alloy as finely dispersed melt droplets. As the melt pool cools, the molten alloy solidifies to form crystals or grains within the melt pool, while solidification of oxide melt droplets yields the formation of dispersed oxide particles. Within the grains, an intragranular cellular structure may be formed. Without wishing to be bound by any theory, this intragranular cellular structure may arise as a result of the rapid melting and solidification of the metal alloy during the laser melting step. The inclusion of oxide particles in the cellular structured metal alloy matrix in this manner can improve the mechanical properties of the alloy even at elevated temperatures.

[0014] Because the ODS metal alloy is manufactured from melt pools containing a mixture of molten metal alloy and oxide melt droplets, it is possible to distribute the oxide particles throughout the resulting structure as the additive manufacturing process ensures the presence of oxide melt droplets in each melt pool. This contrasts with prior art methods of forming oxide dispersion strengthened (ODS) alloys (e.g. forging or casting), where the oxide particles may have a tendency to agglomerate within the structure.

Metal Alloy [0015] Any suitable metal alloy may be used to produce the metal part of the present disclosure. In one example, the metal alloy is steel.

[0016] Preferably, the steel may be a stainless steel. In one embodiment, the steel is an alloy of iron and at least one other element selected from carbon, magnesium, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, niobium, phosphorus, sulphur, silicon, manganese, oxygen, nickel and copper. In a preferred embodiment the steel is an alloy of iron and chromium. In a further preferred embodiment, the steel may comprise iron, chromium and carbon. In another embodiment, the steel may comprise iron, chromium, carbon and nickel. In yet another embodiment, the steel may comprise iron, chromium, carbon, nickel and molybdenum. In yet another embodiment, the steel may comprise iron, chromium, carbon, nickel, molybdenum and manganese.

[0017] The steel may comprise iron in an amount of at least 30 wt %, at least 40 wt %, for example, at least 50 wt % iron. The steel may comprise at most 90 wt %, at most 80 wt %, for example, at most 70 wt % iron. In one example, the steel comprises 30 to 90 wt % iron, preferably 40 to 80 wt % iron, more preferably 50 to 70 wt % iron. In a preferred embodiment, the iron is present in an amount of 55 to 67 wt %, for example, 60 to 65 wt %.

[0018] In one example, the steel may comprise chromium. The chromium may be present in an amount of at least 5 wt %, preferably at least 10 wt %, more preferably at least 15 wt % chromium. The steel may comprise at most 30 wt %, preferably at most 25 wt %, more preferably at most 20 wt % chromium. In one example, the steel comprises 5 to 30 wt % chromium, preferably 10 to 25 wt%, more preferably 15 to 20 wt % chromium.

In a preferred example, the steel comprises 16 to 19 wt %, for instance, 17 to 18 wt % (e.g. 18 wt %) chromium.

[0019] The steel may comprise carbon. The carbon may be present in an amount of at least 0.005 wt%, at least 0.01 wt%, for example, at least 0.02 wt%. The carbon may be present in an amount of at most 1 wt %, at most 0.2 wt %, for example, at most 0.1 wt %.

In one example, the steel comprises 0.005 to 1 wt % carbon, preferably 0.01 to 0.2 wt%, more preferably 0.02 to 0.1 wt % carbon. In a preferred example, the steel comprises 0.02 to 0.08 wt %, for instance, 0.03 wt % carbon.

[0020] The steel may comprise nickel. The nickel may be present in an amount of at least 5 wt %, preferably at least 8 wt % nickel. The steel may comprise at most 25 wt %, preferably at most 20 wt % nickel. In one example, the steel comprises 5 to 25 wt % nickel, preferably 8 to 20 wt% nickel. In a preferred example, the steel comprises 10 to 14 wt % nickel.

[0021] The steel may comprise molybdenum. The molybdenum may be present in an amount of at least 0.5 wt %, preferably at least 1 wt %. The molybdenum may be present in an amount of at most 8 wt %, preferably 5 wt %. In one example, the steel comprises 0.5 to 8 wt %, preferably 1 to 5 wt % molybdenum. In a preferred example, the steel comprises 2 to 3 wt % molybdenum.

[0022] The steel may comprise manganese. The manganese may be present in an amount of at least 0.5 wt %, preferably at least 1 wt %. The manganese may be present in an amount of at most 8 wt %, preferably 5 wt %. In one example, the steel comprises 0.5 to 8 wt %, preferably 1 to 5 wt % manganese. In a preferred example, the steel comprises

2 to 3 wt % manganese.

[0023] In one example, the steel comprises the following components in the amounts listed below:

	Broad wt %	Preferred wt %
Carbon	0.005 to 1	0.005 - 0.03
Manganese	0.5-8	1.0-2.0
Phosphorus	0-0.1	0- 0.025
Sulfur	0-0.1	0-0.01
Chromium	5-30	16.0-18.0
Nickel	5-25	10.0-14.0
Molybdenum	0.5-8	2.0-3.0
Nitrogen	0-0.5	0-0.1
Silicon	0-1.5	0-0.75
Iron	Balance	Balance

[0024] In one embodiment, the steel is a stainless steel. The stainless steel may be an 15 austenitic stainless steel, for example, an austenitic chromium-nickel stainless steel containing molybdenum. The stainless steel may be Type 316 or 316L stainless steel.

Oxide Particles [0025] As mentioned above, the metal part comprises oxide particles that are dispersed throughout a metal alloy matrix. The oxide particles may be nanoparticles having a particle size of 200 nm or less. Preferably, the particles have a particle size of less than

150 nm, more preferably less than 100 nm. The particles may have a particle size of greater than 1 nm, preferably greater than 2 nm, more preferably greater than 4 nm. In one embodiment, the particles have a particle size of 1 to 200 nm, preferably 2 to 150 nm, more preferably 4 to 100 nm. In another example, the particles have a particle size of 5 to 80 nm, for example, 10 to 60 nm. Particle size may be calculated from TEM electron microscope images.

[0026] The size of the oxide particles dispersed throughout the metal alloy matrix may differ from the size of the oxide particles that are used to form the mixture of metal alloy particles and oxide particles in step a) of the method of the present disclosure. The size of the oxide particles dispersed throughout the metal alloy matrix may be influenced by, for example, the selective laser melting technique used to disperse the oxide particles in the intragranular cellular structure. As discussed above, the particles may be formed and dispersed as a result of convection currents established in melt pools formed during the selective laser melting process.

[0027] The oxide particles may be metal oxide particles. Any suitable metal oxide may be used. Preferably, the oxide is an oxide of a metal selected from yttrium, zirconium, lanthanum, aluminium, and titanium. In a preferred embodiment, the particles are yttrium oxide particles. In some embodiments, the oxide particles may be non-metal oxide particles, for example, silica and germanium oxide In some embodiments, the oxide particles comprise a mixture of two or more than two metal oxides and/or non-metal oxides. In some embodiments, the oxide particles appear as crystalline particles. In some embodiments, the oxide particles appear as amorphous and partially amorphous particles. Whether oxide particles are crystalline or amorphous or partially amorphous can be determined by electron diffraction on the oxide particles. In some examples, the oxide particles are not silica and/or not germanium.

[0028] The oxide nanoparticles may be present in the metal part in an amount of less than 5 wt%, preferably less than 3 wt%. The oxide nanoparticles may be present in the metal part in an amount of at least 0.1 wt %, preferably at least 0.2 wt %. In one example, the oxide particles are present in an amount of 0.1 to 5 wt %, for example, 0.15 to 3 wt %. In a preferred example, the oxide particles are present in an amount of 0.2 to 2 wt %. In another example, the oxide particles are present in an amount of 0.5 wt % to 2 wt %, for instance, 1 to 2 wt %.

The Metal Part [0029] As mentioned above, the metal part comprises an oxide dispersion strengthened (ODS) metal alloy comprising oxide particles dispersed in a metal alloy matrix, wherein the metal alloy matrix comprises an intragranular cellular microstructure. The intragranular cellular microstructure comprises a cellular network, defined by cell walls formed of the metal alloy. The cell walls may define cells. Oxide particles may be present within at least some of the cells within the intragranular cellular microstructure.

[0030] The cells of the intragranular cellular structure may have an average cell size of 0.1 to 10 Mm, preferably 0.3 to 5 pm, more preferably, 0.4 to 2 pm, for example, 0.5 to 1 pm. The cell size may be determined from electron microscopic images taken of a polished and etched section of the alloy.

[0031] The cells may have a polygonal shape cross-section, for example, the cells may be a substantially square, hexagonal or pentagonal cross-section. Dislocations and associated residual stresses may be concentrated at the cellular boundaries due to segregation of alloying elements, for example, heavy alloying elements e.g. molybdenum that may be present in the metal alloy. This may be due to the de-alloying during the rapid solidification of melt pools in which the element distribution is influenced by the Maragoni convection.

[0032] As described above, the cellular structure may be contained within grains. The grains may be columnar as they may form in the direction of the temperature gradient created during the grain’s formation. The morphology of the intragranular cellular structure may be columnar. The morphology may depend on the growth direction of the columnar grains in which they are contained.

[0033] The grains may be up to 500 microns in size, for example, 10 to 500 microns.

The grains may be 20 to 700 microns, preferably 50 to 100 microns in size. Grain size is determined by electron microscopic images taken on polished sections of the alloy.

[0034] The grains may be contained within melt pools. As described above, the metal part is formed by selectively melting a layer comprising a mixture of metal alloy particles and oxide particles. The mixture in the vicinity of the laser melts to form a melt pool containing oxide melt droplets. Convection currents within the molten melt pool can help distribute the oxide melt droplets evenly within the melt pool. The molten mixture then crystallises to form grains within the melt pool. Because cooling occurs rapidly, the oxide melt droplets may be less likely to agglomerate or settle during the cooling process.

[0035] The layering and selective melting steps are then repeated, with melt pools formed repeatedly adjacent and on top of one another to form the final metal part. The resulting metal part, therefore, is built up of melt pools, each containing grains, which, in turn are formed of an intragranular cellular structure.

[0036] The melt pools may measure from 100 to 500 microns, preferably 150 to 300 microns, for example, 200 microns in width. The melt pools may measure 10 to 200 microns, preferably 50 to 150 microns, for example, 80 to 100 microns in depth, This may be determined by optical microscopic images taken on polished and etched sections of the alloy. The size of the melt pool may depend on the laser spot size and/or laser input energy density. |hmci] [0037] In the present disclosure, the geometry of the melt pools, grains and/or intragranular structure may affect the overall properties of the ODS metal alloy produced. This, in turn, may be influenced by the selective melting (e.g. laser scanning) strategy used to produce the metal part. As described below, therefore, parameters such as laser power, scanning speed, scanning direction, and distance between two scanning lines, and temperature gradients may be controlled to influence the hierarchical structure of the ODS metal alloy produced.

[0038] The oxide dispersion strengthened alloy may have a tensile strength at room temperature of 400 to 900 MPa, preferably 500to 800MPa, for example, 600 to 700 MPa. [0039] Tensile strength may be measured according ASTM E8 standard.

[0040] The metal part may be formed of an ODS metal alloy having a tensile strength at 250 degrees C of 300 to 600 MPa, preferably 400 to 550 MPa, for example, 450 to 500 MPa.

[0041] The metal part may be formed of an ODS metal alloy having a tensile strength at 400 degrees C of 250 to 550 MPa, preferably 300 to 500 MPa, for example, 400 to 450 MPa.

[0042] The metal part may be formed of an ODS metal alloy having a yield strength at room temperature of 300 to 700 MPa, preferably 400 to 600 MPa, for example, 500 to 600 MPa.

[0043] Yield strength may be measured according ASTM E8 standard [0044] The metal part may be formed of an ODS metal alloy having a yield strength at 250 degrees C of 300 to 600 MPa, preferably 400 to 550 MPa, for example, 450 to 500 MPa.

[0045] The metal part may be formed of an ODS metal alloy having a yield strength at 400 degrees C of 250 to 550 MPa, preferably 300 to 500 MPa, for example, 400 to 450 MPa.

[0046] The ODS metal alloy may have a yield strength and tensile strength at room temperature of 400 to 600 MPa and 450 to 700 MPa, respectively. The metal part may have a yield strength and tensile strength at 250 degrees C of 400 to 550 MPa and 450 to

600 MPa, respectively. The metal part may have a yield strength and tensile strength at 400 degrees C of 400 to 450 MPa and 400 to 500 MPa, respectively.

[0047] In one example, the yield/tensile strength of the ODS metal alloy may be up to 500/600 MPa at room temperature, up to 480/500MPa at 250 degrees C and up to 420/450 MPa at 400 degrees C.

[0048] The ODS metal alloy may be resistant to heat fluxes of up to 800 degrees C, for example, up to 700 or 650 degrees C.

[0049] The ODS metal alloy may be resistant to neutron irradiation of £ 1 dpa (e.g but < 1000 dpa (displacement per atom)).

[0050] The metal alloy produced may also be resistant to corrosion. For example, the alloy may be resistant to chloride environment. The Hoy may be reistant to potable water with a chloride concentration of up to 1000mg/l at ambient temperature and 500mg/l at 60 degrees C.

Manufacturing Method [0051] As noted above, the metal part of the present disclosure may be manufactured by an additive manufacturing method comprising:

a) forming a mixture of metal alloy particles and oxide particles;

b) providing a layer of the mixture;

[0052] The metal alloy particles may be formed of any suitable metal alloy (see above). The metal alloy particles may be any suitable size, for example, the particles may have a particle size of between 10 pm and 50 pm, preferably 15-45 pm The morphology of the metal alloy powder may be substantially spherical to increase flowability and free packing density. The metal alloy powder may be produced by any suitable method, for example by gas atomization, or by a plasma rotating electrode process. In some examples, the metal alloy particles are provided as a metal alloy powder.

[0053] The oxide particles may be formed of any of the oxides mentioned above. The oxide particles may be any suitable size, for example, the particles may have a particle size of less than 2 microns, preferably less than 1 microns, for example, 0.02 to 1 microns.

[0054] The oxide particles may be mixed with the metal alloy particles using any suitable method. For example, the particles may be mixed using a ball mill. In the mixing process, the particle size of at least some of the particles may be further reduced. The relative amounts of the oxide particles and metal alloy particles may be controlled to achieve the desired concentration of the oxide particles in the final ODS metal alloy. For example, the amount of oxide particles used may be 1 to 2 weight % of the total weight of oxide particles and metal alloy particles.

[0055] By mixing the particles, the oxide particles may coat or deposit on the surface of the (e.g. larger) metal alloy particles. This may help to ensure even distribution of the oxide throughout the ODS metal alloy structure.

[0056] The particulate mixture (e.g. powder) may be applied as a layer onto a print or building platform. The mixture in at least part of the layer may be selectively melted using a laser. The laser may be guided to melt the mixture according to a predetermined geometry. For example, the predetermined geometry may be governed by a computer aided design (CAD) program.

[0057] Following the completion of one layer, the building platform may be lowered down by a certain amount corresponding to the thickness of newly formed layer (e.g. 10 to 50 microns, preferably 20 microns). A further layer of particulate mixture may then be provided on the top of the completed layer, and the laser may again be guided to melt the particulate mixture according to a predetermined geometry. This procedure may be repeated until the desired product is formed. Parameters such as laser power, scanning speed, scanning direction, and distance between two scanning lines, and temperature gradients may be controlled in order to form the desired product. In one embodiment, a 3dimensional metal part is formed, for example a metal part comprising an ODS metal alloy comprising metal oxide nanoparticles dispersed in a metal alloy matrix.

[0058] Any suitable laser source may be used. Examples include Nd:YAG lasers and CO2 lasers. Preferably, the laser is a pulsed or discontinuous laser. The laser may have a wavelength in the range of 1 pm - 10pm. Preferably, the wavelength is in the infrared or near infrared region.

[0059] The laser may be operated at a power of 5 W - 1000 W.

[0060] The laser may be operated at a scanning speed of 50mm/s - 7000 mm/s, preferably 500 mm/s - 2000 mm/s, more preferably 700 mm/s - 1100 mm/s [0061] The laser may be operated such that the distance between scanning lines is 0.01mm -0.2 mm, preferably 0.05 mm - 0.15 mm [0062] The laser may be used to melt the particulate mixture in a vacuum or in a controlled gaseous atmosphere. The gaseous atmosphere may be inert. The gaseous atmosphere may contain at least one gas selected from nitrogen, argon and hydrogen.

[0063] An advantage of this additive manufacturing technique is that it enables metal parts to be made with complex designs.

[0064] After the metal part has been formed, the metal part may be treated by thermal annealing.

Nuclear Reactor Components [0065] The metal part of the present disclosure may be used for any purpose. However, because of its mechanical strength at high temperatures, the metal part of the present disclosure may be suitable for use in a nuclear reactor, for example, a thermonuclear fusion reactor or a fission reactor. In one embodiment, the metal part is a heat exchanger or heat exchanger component. The heat exchanger may comprise a heat exchanging portion formed of the ODS metal alloy described herein. The heat exchanging portion may define a passageway for the flow of a heat transfer medium. The passageway may comprise a pipe network, for example, a labyrinthine pipe network for the passage of a heat transfer medium.

[0066] The heat exchanger may comprise layers in addition to the heat exchanging portion. For example, while one face (e.g. inner face) of the heat exchanging portion may define a passageway for the flow of a heat transfer medium, another face (e.g. outer face) of the heat exchanging portion may comprise an additional layer. The additional layer may comprise an armour layer and/or a heat transfer layer. Preferably, an armour layer and heat transfer layer are present. In one example, the outer face of the heat exchanging portion is joined to a heat transfer layer. The heat transfer layer may, in turn, be joined to an armour layer. The heat transfer layer may be formed of any suitable material. An example is CuCrZr alloy. The armour layer may be formed of any suitable material. An example is tungsten. Another example may be beryllium.

[0067] Aspects of the present disclosure will now be described by way of example with reference to the accompanying drawings.

[0068] Figure 1 is a schematic view of an apparatus used to manufacture a metal part according to one example of the present disclosure. The apparatus comprises a building platform 10, a collecting platform 12 and a dispersing platform 14. The apparatus also comprises a laser source (not shown) and a coater 16 for applying layers of powder onto the building platform 10.

[0069] In operation, a powder mixture 18 comprising metal alloy powder and oxide powder is loaded onto the dispersing platform 14. The dispersing platform 14 is then raised to slightly (e.g. 10-100 microns) above the level of the upper surface of the building platform 10. At the start of the process, this may be the upper surface of the building platform 10 itself. Subsequently, as the manufacturing process proceeds, this may be the upper surface of any powder layer fused on the building platform (10).

[0070] When the dispersing platform 14 is above level with the building platform 10, the coater 16 is used to distribute the powder 18 as a layer over the surface of the building platform 10. A laser 20 is then guided over the layer using CAD software to selectively melt a portion of the layer. The molten powder forms a melt pool 22. Turning off the laser 20, allows the molten metal alloy powder to crystallise as grains. The grains form along the temperature gradient shown by arrow 22. The size of the melt pool may depend on the laser spot size and/or laser input energy density.

[0071] Once all the desired regions of a particular layer have been selectively melted with the laser 20, the building platform 10 may be lowered and the dispersing platform 14 raised to apply a further layer of powder onto the platform. The laser melting process is then repeated to manufacture a metal part of the desired shape. The part will be built from melt pools stacked on top of one another.

[0072] Excess un-melted powder may be removed from the building platform by altering the relative positions of the building 10 and collecting 12 platforms. This may be transferred to the dispersing platform for re-use.

[0073] Figure 2 is a schematic diagram of the oxide particles and metal alloy particles in the powder mixture 18. The oxide particles 100 are deposited over the surface of the metal alloy particles 120. The powder mixture 18 may be formed by mixing and grinding a metal alloy (e.g. SS316L) powder together with oxide particles (e.g. Y2O3 powder) in a ball mill. The metal alloy powder may have a particle size of 10 to 45 microns, while the oxide particles may have a particle size of 800 nm or less.

[0074] Figure 2c is an SEM photograph of a powder mixture 18 formed from SS316L powder and Y2O3 powder. The smaller Y2O3 particles can be seen deposited on the surface of the SS316L powder particles.

[0075] Figure 3 is a heat exchanger 200 comprising an ODS metal alloy according to the present disclosure. The heat exchanger 200 comprises a heat exchanging portion 210 formed from an ODS metal alloy according to the present disclosure. The heat exchanging portion 210 defines a passageway 212 for the passage of a heat transfer medium (not shown). The passageway may be convoluted or labyrinthine.

[0076] The heat exchanger 200 may also comprise a heat conducting layer 214 and an armour layer 216. These layers may be joined to the heat exchanging surface 210. For example, an internal face of the heat exchanging portion 210 may define the passageway 212 for the heat transfer medium. An external face of the heat exchanging portion 210 may be coupled (e.g. by hot isotactic pressing) to the heat conducting layer 214. An armour layer 216 may be coupled (e.g. by hot isotactic pressing) to the heat conducing layer 214.

[0077] The armour layer 216 may be formed of tungsten, while the heat conducting layer 214 may be formed of an alloy of CuCrZr.

Example 1.

[0078] A Y2O3 ODS SS316L with a hierarchical structure was produced by selective laser melting.

[0079] SS316L stainless steel powders with a size range (10 to 45 microns) and Y2O3 (800nm) were mixed and ball milled to form an ODS SS316L powder mixture. The concentration of Y2O3 in the powder mixture was 1 wt%.

[0080] The stainless steel powders were melted by a Nd:YAG laser (wavelength 1064 nm) controlled by a scanner guided by a pre-set CAD program under a laser energy of 195 W. The laser beam was scanned with a speed of 800 mm/s and a hatch line distance of 0.08mm. The laser beam was rotated at an angle of 45° in neighbouring layer.

A dense block of ODS SS316L (40*10*10 mm) was produced. Figure 4 shows SEM images of the structure of the block. Figure 4a shows the melt pools in the structure.

Figure 4b shows the grains within each melt pool and Figure 4c shows the oxide particles as black dots distributed within the intragranular cellular structure of the grains.

Example 2 [0081] A Y2O3 ODS SS316L with a hierarchical structure was produced by selective laser melting.

[0082] SS316L stainless steel powders with a size range (10 to 45 microns) and Y2O3 (800nm) were mixed and ball milled to form an ODS SS316L powder mixture. The concentration of Y2O3 in the powder mixture was 1 wt%.

[0083] The stainless steel powders were melted by a Nd:YAG laser (wavelength 1064 nm) controlled by a scanner guided by a pre-set CAD program under a laser energy of 195 W. The laser beam was scanned with different speeds of 7000/4250/850/283 mm/s and hatch line distances of 0.01/0.02/0.1/0.3 mm. The laser beam was rotated at an angle of 45° in neighbouring layer.

Figure 5 shows how the size of the cells in the intragranular cellular structure can be varied by controlling the process parameters of the scanning step. Figure 5a shows cells formed at scanning speeds of 7000mm/s and hatch line distances of 0.01 mm. Figure 5b shows cells formed at scanning speeds of 4250 mm/s and hatch line distances of 0.02 mm.

Figure 5c shows cells formed at scanning speeds of 850 mm/s and hatch line distances of 0.1 mm. Figure 5d shows cells formed at scanning speeds of 283 mm/s and hatch line distances of 0.3 mm. Slowing the scanning speed increases the cell size

Claims

1. An oxide dispersed strengthened (ODS) metal alloy comprising metal oxide particles dispersed in a metal alloy matrix, wherein the metal alloy matrix comprises an intragranular cellular microstructure, and wherein the metal oxide particles are dispersed in at least some of the cells of the intragranular cellular microstructure.

2. An oxide dispersed strengthened (ODS) metal alloy comprising oxide particles dispersed in a metal alloy matrix, wherein the metal alloy matrix comprises an intragranular cellular structure, and wherein the oxide particles are dispersed in at least some of the cells of the intragranular cellular structure and are present in an amount of at least 0.1 weight % of the total weight of the ODS metal alloy.

3. A metal alloy according to claim 1 or 2, wherein the ODS metal alloy is an ODS stainless steel.

4. A metal alloy according to claim 3, wherein the stainless steel is an alloy of iron and at least one other element selected from carbon, magnesium, nickel, chromium, molybdenum, boron, titanium, vanadium, tungsten, cobalt, niobium, phosphorus, sulphur, silicon, manganese, oxygen, nickel and copper.

5. A metal alloy according to any preceding claim, wherein the metal alloy matrix has a structure comprising melt pools, wherein each melt pool comprises a plurality of grains, and wherein each grain comprises the intragranular cellular structure.

6. A metal alloy according to claim 5, wherein at least some of the grains extend beyond the boundary between two adjacent melt pools.

7. A metal alloy according to claim 5 or 6, wherein the oxide particles are present in between at least some of the boundaries between adjacent grains as well as within at least some of the cells of the intragranular cellular structure.

8. A metal alloy according to any preceding claim, wherein the oxide particles comprise an oxide of at least one element selected from yttrium, zirconium, lanthanum, aluminium,andtitanium .

9. A metal alloy according to any preceding claim, wherein the oxide particles are Y2O3 nanoparticles.

10. A metal alloy according to any preceding claim, wherein the oxide particles have a diameter of 1 to 200 nm.

11. A metal alloy according to any preceding claim, wherein the oxide particles are present in an amount of up to 2 wt% of the total weight of the oxideparticles and the metal alloy matrix.

12. A metal part comprising a metal alloy as claimed in any one of the preceding claims.

13. A metal part according to claim 12, which is a component of a nuclear reactor.

14. A metal part according to claim 12 or 13, which is a heat exchanger.

15. A metal part according to claim 14, wherein the heat exchanger comprises a heat exchanging portion comprising the metal alloy defined in any one of claim 1 to 10, wherein the heat exchanging portion defines a fluid pathway for the passage of a heat transfer medium.

16. A method of additive manufacturing a metal part, said method comprising:

d) forming a mixture of metal alloy particles and oxide particles;

e) providing a layer of the mixture;

f) selectively melting the mixture in at least part of the layer using a laser; and repeating steps (b) and (c) a plurality of times, wherein each layer is applied on top of the previous layer.

17. The method according to claim 16, wherein oxide particles are deposited on the surface of at least some of the metal alloy particles.

18. The method according to claim 16 or 17, wherein the metal alloy particles and oxide particles are mixed using a ball mill.

19. The method according to any one of claims 16 to 18, wherein the laser is guided by computer-aided design (CAD) software.

20. The method according to any one of claims 16 to 19, wherein the metal part is a metal part according to any one of claims 12 to 15.

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