WO2022067395A1 - Boron nitride nanotube modified metal powder for additive manufacturing - Google Patents

Abstract

A method of forming a metal component by additive manufacturing, the method comprising: providing a metal powder feedstock comprising metal particles having one or more boron nitride nanotubes attached thereto; and processing the metal powder feedstock by additive manufacturing to form the metal component; wherein the so formed metal component has one or both of the boron nitride nanotubes and metal-boride/nitride substantially uniformly distributed therethrough.

Description

BORON NITRIDE NANOTUBE MODIFIED METAL POWDER FOR ADDITIVE MANUFACTURING

FIELD OF THE INVENTION

5 The present invention relates generally to additive manufacturing, and in particular to methods of forming metal components by additive manufacturing, metal powder feedstock for additive manufacturing, and an additive manufactured metal component.

BACKGROUND ART

10

Additive manufacturing includes a collection of manufacturing processes that affords layer- by-layer growth of three-dimensional physical objects from a digital design. The inherent simplicity of additive manufacturing can efficiently afford production of objects having complex geometries.

15

Additive manufacturing has been successfully implemented for the production of highly complex geometrical shapes out of a variety of materials, including thermoplastic polymers, ceramics, and glass. However, additive manufacturing of metals is mostly confined to a limited selection of weldable metals, while additive manufacturing of non-weldable metals

20 remains challenging.

In some additive manufacturing procedures for metals, each successive layer of metal bonds through a molten sate to a preceding layer during processing. The fast melting and solidification dynamics that characterise additive manufacturing generally lead to formation

25 of intolerable microstructures characterised by coarse columnar grain structures and periodic cracks. Other additive manufacturing procedures involve the initial layer-by-layer production of a three-dimensional object made of metal particles joined by a binder, followed by sintering the metal particles to consolidate into the final component. During sintering, uncontrolled diffusion-driven grain growth can produce undesired microstructures

30 characterised by coarse grains similar to those observed through fast melting and solidification. Formation of such intolerable microstructures can cause undesired anisotropy

in the mechanical properties of the manufactured component, with consequent impairment of grain size -dependent mechanical properties such as specific strength, fatigue life, and fracture toughness.

Attempts to address the above limitations have mostly focused on modifying additive manufacturing parameters (e.g. printing speed, power of the heat source, mass feeding rate of powder), introducing external fields (e.g. ultrasounds), or inducing plastic deformation (e.g. inter-pass rolling) in an attempt to temper the conditions that promote columnar growth, with no significant success.

There remains therefore an opportunity to address or ameliorate one or more problems associated with existing additive manufacturing processes for metals.

SUMMARY OF THE INVENTION

The present invention provides a method of forming a metal component by additive manufacturing, the method comprising: providing a metal powder feedstock comprising metal particles having one or more boron nitride nanotubes attached thereto, and processing the metal powder feedstock by additive manufacturing to form the metal component, wherein the so formed metal component has one or both of the boron nitride nanotubes and metal boride/nitride substantially uniformly distributed therethrough.

It has been surprisingly found that when boron nitride nanotubes (BNNTs) are attached to the metal particles in the metal feedstock, the BNNTs can effectively act as grain inoculants to control the solidification dynamics of the melt pool or the diffusion-driven grain growth during sintering. As a result, grain growth can advantageously be restricted during additive manufacturing. Specifically, it is believed that during manufacturing the presence of BNNTs attached to the metal particles in the feedstock can advantageously change the dominant grain formation mode from directional columnar grain growth to non-directional grain

growth.

Without wishing to be limited by theory, it is believed the BNNTs attached to the metal particles in the feedstock modify the forming micro-structure of the cooling or sintering metal by promoting heterogeneous nucleation during solidification of the melt pool or during consolidation. As a result, the cooling or sintering metal crystallises to a micro-structure characterised by a fine and homogeneous dispersion of substantially crack-free equiaxed grains (i.e. grains of roughly equal length, width and height). Such fine-grained microstructure is believed to minimise the anisotropic nature of grain-size dependent mechanical properties which is typically conferred by micro-structures having grains elongated along a preferential direction. That in turn has been found to impart to the additive manufactured component improved specific strength, fatigue life, fracture toughness and hot-strength relative to an additive manufactured component obtained using a metal powder feedstock in which the BNNTs are not attached to the metal particles (e.g. by presenting as isolated aggregates loosely dispersed in the interstices of the metal powder), and comparable to metal components formed using conventional mechanical manufacturing.

If the BNNTs are not attached to the metal particles, the effectiveness of the BNNTs on grain refinement can be significantly discounted during additive manufacturing. For instance, if the BNNTs present as aggregates loosely dispersed throughout the feedstock, they may either be undesirably removed from the feedstock during processing, or may not afford effective grain refinement in the cooling or consolidating metal.

The proposed method can advantageously be employed in a wide range of additive manufacturing processes. The additive manufacturing may be a powder bed-based process. Examples of suitable processes in that regard include powder bed-based processes such as selective laser sintering (SLS), selective laser melting (SLM), laser powder bed fusion (LPBF), direct metal laser sintering (DMLS), direct metal laser melting (DMLM), binder jet printing (BJP), electron beam melting (EBM) and blown powder processes named laser engineered net shaping (LENS), direct laser forming (DLF), and directed energy deposition (DED).

Conventional additive manufacturing procedures for metals can be inherently confined to metals that are known to be weldable due to the similar melting/cooling dynamics induced during both additive manufacturing and welding. However, presence of BNNTs attached to the metal particles in the feedstock ensures that grain refinement can be promoted also on notoriously un-weldable metals, for example high-performance engineering alloys. Similar limitations affect additive manufacturing processes that rely on sintering of metal particles joined by a binder. Accordingly, the nucleating ability of BNNTs attached to the metal particles can advantageously be exploited for a large variety of metals.

The metal powder feedstock used in accordance with the invention may comprise metal particles of titanium, aluminium, magnesium, nickel, iron, copper, gold, silver, bismuth, manganese, zinc, chromium, molybdenum, platinum, zirconium, iridium, yttrium, vanadium, niobium, tantalum, tungsten, cobalt, tin, lead, gallium, or an alloy of one or more thereof.

There is also provided a metal component manufactured according to the method described herein.

Further, the invention provides a metal powder feedstock for additive manufacturing, the feedstock comprising metal particles having one or more boron nitride nanotubes (BNNTs) attached thereto.

The feedstock in accordance with the present invention can advantageously promote in situ grain refinement and mechanical reinforcement of the metal during additive manufacturing. The feedstock is therefore particularly useful in the production of additive manufactured metal components having complex shape and being characterised by specific strength, fatigue life and fracture toughness comparable to those of metal formed using conventional mechanical manufacturing procedures.

Since the BNNTs are attached to the metal particles, the resulting metal component

advantageously has one or both of metal boride/nitride and the BNNTs substantially uniformly distributed therethrough. Additive manufactured metal components comprising one or both of metal boride/nitride and BNNTs substantially uniformly distributed therethrough are believed to be unique on their own right.

Accordingly, the present invention also provides an additive manufactured metal component comprising one or both of metal boride/nitride and boron nitride nanotubes (BNNTs) substantially uniformly distributed therethrough. If the BNNTs are not substantially uniformly distributed therethrough, the manufactured metal component may display poor mechanical properties. For instance, if the BNNTs are not substantially uniformly distributed throughout the metal component, they may present as aggregates. Aggregates are undesired as they can impair the mechanical properties of the component by generating localised porosity within the metal structure.

Accordingly, some aspects of the present invention relate to an additive manufactured metal component comprising one or both of boron nitride nanotubes and metal-boride/nitride substantially uniformly distributed therethrough, wherein the metal component has a microstructure that exhibits an average grain size at least 20% smaller than that of the microstructure of the metal component absent the one or both of boron nitride nanotubes and metal-boride/nitride.

Another aspect of the present invention relates to an additive manufactured metal component comprising one or both of boron nitride nanotubes and metal-boride/nitride substantially uniformly distributed therethrough, wherein the metal component exhibits a yield strength at least 10% higher than that of the metal component absent the one or both of boron nitride nanotubes and metal-boride/nitride.

An additional aspect relates to an additive manufactured metal component comprising one or both of boron nitride nanotubes and metal-boride/nitride substantially uniformly distributed therethrough, wherein the metal component exhibits a compressive yield strength at least 10% higher than that of the metal component absent the one or both of boron nitride

nanotubes and metal -boride/nitride.

In a further aspect, the invention provides an additive manufactured metal component comprising one or both of boron nitride nanotubes and metal-boride substantially uniformly distributed therethrough, wherein the metal component exhibits an ultimate strength at least 10% higher than that of the metal component absent the one or both of boron nitride nanotubes and metal -boride/nitride.

Further aspects and embodiments of the invention are outlined below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be now described with reference to the following nonlimiting drawings, in which:

Figure 1 shows scanning electron microscope (SEM) images of a) purified boron nitride nanotubes, b) conventional Ti-6A1-4V feedstock powder, and c) a surface portion of Ti-6A1- 4V powder having BNNTs attached thereto,

Figure 2 shows a) a schematic overview of a direct laser deposition (DLD) process, whereby the blown powder is melted by direct exposure to an energy source (laser or electron beam) and fused onto the previous (underlying) layer of metal during solidification, and b) additive manufactured Ti-6A1-4V alloy rod obtained using feedstock with (top portion) and without (bottom portion) BNNTs attached to the metal particles,

Figure 3 shows optical and backscattered SEM images revealing the coarse columnar grain structure of a) as-fabricated Ti-6A1-4V alloy obtained using conventional feedstock, and obtained using a feedstock comprising metal particles having b) 0.1 wt.% (0.1BNNT) and c), d) 0.4 wt.% (0.4BNNT) BNNTs attached thereto,

Figure 4 shows a) prior P grain size and b) a lath thickness a function of BNNT fraction, obtained from the images shown in Figure 3,

Figure 5 shows true-stress strain curves obtained during cylindrical compression testing obtained from as-fabricated Ti-6A1-4V alloy obtained using conventional feedstock (Ti-6A1- 4V), and obtained using a feedstock comprising metal particles having 0. 1 wt.% (0. 1BNNT) and 0.4 wt.% BNNTs (0.4BNNT) attached thereto,

Figure 6 shows SEM fractograph of the AM-fabricated Ti6-Al-4V alloy with a) and without BNNT modification b). The white and black arrows refer to the dimple tips and TiB nanowhiskers, respectively,

Figure 7 shows weakened a-b) a and c-d) texture of Ti-6A1-4V alloy fabricated using feedstock particles having BNNTs attached thereto. The inverse pole figure (IPF) images were plotted using +electron backscatter diffraction (EBSD) data,

Figure 8 shows a-b) atom probe tomography (APT) B, N distribution maps and composition profile of nano-whiskers, c) bright field TEM image and d) SAD pattern of TiB nanowhiskers formed during fabrication of a Ti-6A1-4V alloy component using feedstock particles having BNNTs attached thereto,

Figure 9 shows a) the schematic evolution of BNNTs in Ti-6A1-4V during advanced manufacturing and b) and its effect on grain refinement in comparison with conventional casting in the presence of B, and

Figure 10 shows the binary Ti-B phase diagram highlighting (left hand side) compositional pathways of conventional hypoeutectic precipitation (straight vertical arrow) and local hypereutectic precipitation (curved arrow).

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method of forming a metal component by additive manufacturing.

The expression "additive manufacturing" is used herein to indicate a process of joining materials to make physical objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies. Additive manufacturing is a collection of additive processes where successive layers of material are laid down in different shapes using a heat source. This is in comparison to traditional machining techniques, which mostly rely on the removal of material by methods such as cutting, machining, or milling. Additive manufacturing is sometimes known as “3D printing”, “additive layer manufacturing” (ALM) or “rapid prototyping”.

Additive manufacturing techniques suitable for use in the invention embrace all those known to a skilled person for the additive manufacturing of metal components that require the use of a metal powder feedstock.

For example, suitable additive manufacturing techniques may encompass those in which a heat source (such as a laser beam) is used to form a spatially confined melt pool on either a metallic substrate or on a previously deposited metal layer. A metal powder feedstock is fed into the melt pool using a carrier gas, where it melts to form a deposit that is fusion bonded to the substrate or on the previously deposited metal layer.

Other suitable additive manufacturing techniques may embrace those in which a metal powder feedstock is first spread on a metallic substrate or on a previously deposited metal layer. The feedstock is then locally melted using a heat source (such as a laser or electron beam) under a process gas, causing the feedstock particulate to coalesce and fuse to the substrate or on the previously deposited metal layer.

Further suitable additive manufacturing techniques may include processes in which a print

head deposits a liquid binding agent onto selected areas of subsequent layers of metal particles, thereby building a three-dimensional obj ect of metal particles joined by the binder. The object is subsequently consolidated by sintering to form the final metal article.

Accordingly, the additive manufacturing suitable in the present invention may comprise powder bed-based processes such as selective laser sintering (SLS), selective laser melting (SLM), laser powder bed fusion (LPBF), direct metal laser sintering (DMLS), direct metal laser melting (DMLM), binder jet printing (BJP), electron beam melting (EBM) and blown powder processes named laser engineered net shaping (LENS), direct laser forming (DLF), or directed energy deposition (DED).

A skilled person would be aware of specific devices and operational parameters for the additive manufacturing of a metal component given a metal feedstock of the kind described herein.

The metal powder feedstock used in the method of the invention comprises metal particles.

The term "metal" is used herein in accordance to its broadest meaning to encompass materials that (i) comprise at least one metal element, and (ii) can be processed by additive manufacturing. As such, the term will be understood to include elemental metals as well as metal alloys. By "metal alloy" is meant herein a combination comprising either (i) two or more metal elements or (ii) one metal element and one or more non-metal alloying element(s).

Accordingly, in some embodiments the metal powder feedstock used in accordance with the invention comprises metal particles of titanium, aluminium, magnesium, nickel, iron, copper, gold, silver, bismuth, manganese, zinc, chromium, molybdenum, platinum, zirconium, iridium, yttrium, vanadium, niobium, tantalum, tungsten, cobalt, tin, lead, gallium, or an alloy of one or more thereof.

In some embodiments, the metal powder feedstock comprises titanium particles. The

titanium may be in the form of pure titanium or titanium alloy. In its pure form, the titanium may encompass commercially pure titanium, for example 98 to 99.5% titanium.

The expression “titanium alloy” used herein encompasses alloys in which the major element is titanium. The expression may also encompass proprietary titanium alloy systems as well as titanium alloys based on titanium powder metallurgy and titanium compounds, and may also encompass alloy systems such as titanium gum metal. The expression may encompass, for example, alpha titanium alloys, near alpha titanium alloys, alpha-beta titanium alloys, beta titanium alloys and titanium alloys strengthened by small additions of oxygen, nitrogen, carbon and iron. Examples of typical titanium alloys used herein include, for example, Ti- 1.50, Ti-0.2O, Ti-0.3O, Ti-0.20-0.2Pd, Ti-3A1-2.5V, Ti-6A1-4V, Ti-6A1-4V ELI (Extra Low Interstitials) and Ti-6Al-4V-0.06Pd. In some embodiments, the titanium alloy is an alpha titanium alloy (e.g. Ti-5Al-2Sn-ELI, Ti-8Al-lMo-lV). In some embodiments, the titanium alloy is a near-alpha titanium alloy (e.g. Ti-6Al-2Sn-4Zr-2Mo, Ti-5Al-5Sn-2Zr- 2Mo, IMI 685, Ti 1100). In some embodiments, the titanium alloy is an alpha-beta titanium alloy (e.g. Ti-6A1-4V, Ti-6A1-4V ELI, Ti-6Al-6V-2Sn, Ti-6Al-7Nb). In some embodiments, the titanium alloy is a beta-near beta titanium alloy (e.g. Ti-10V-2Fe-3Al, Ti-29Nb-13Ta- 4.6Zr, Ti-13V-l lCr-3Al, Ti-8Mo-8V-2Fe-3Al, Beta C, Ti-15-3).

In some embodiments, the metal powder feedstock comprises particles of an alloy selected from 1000, 2000, 3000, 4000, 5000, 6000, 7000 and 8000 series aluminium alloy.

In some embodiments, the metal powder feedstock comprises particles of AlSilOMg, Ti- 6A1-4V, or Inconel 718.

In some embodiments, the metal powder feedstock comprises particles of high- strengthening-phase nickel superalloy.

The metal particles of the feedstock may be of any size provided they can be sintered/melted under typical operational conditions (e.g. pressure, temperature) of additive manufacturing.

In some embodiments, the metal particles have an average particle size of less than about 500 pm, less than about 250 pm, less than about 100 pm, less than about 50 pm, less than about 25 pm, or less than about 10 pm.

Typically, the average particle size range of the metal particles is selected in accordance with the type and conditions of equipment used for powder additive manufacturing. For example, the average particle size range of the metal particles can range from about 5 pm to about 20 pm, from about 45 pm to about 150 pm, from about 10 pm to about 60 pm, from about 45 pm to about 105 pm, from about 15 pm to about 45 pm, from about 20 pm to about 63 pm, or from about 25 pm to about 75 pm.

As used herein, the expression “average particle size” refers to the average size of particulate matter measured using optical or SEM images following a standard linear measurement.

The metal particles of the feedstock may be made by any means known to a skilled person. For example, the metal particles can be produced by solid-state reduction, atomization, electrolysis, mechanical shearing, or chemical processes. Those processes may include, for example, rotational granulation, fluidized bed granulation, high shear granulation, crushing granulation, melt granulation, spray granulation, and micro-emulsion granulation.

In the present invention, at least a fraction of the metal particles in the feedstock have one or more boron nitride nanotubes (BNNTs) attached thereto.

BNNTs are known in the art to be tubular structures comprising co-axial hexagonal boron nitride (7z-BN) networks. BNNTs useful for use in the invention may be obtained by any procedure known to a skilled person. For example, the BNNTs may be produced by arcdischarge, laser vaporization, B/N substitution from carbon nanotube templates, chemical vapor deposition (CVD) using borazine, induction heating boron oxide CVD (BOCVD), high-temperature ball milling, plasma-enhanced pulsed-laser deposition (PE-PLD), or combustion of Fe4N/B powders.

Provided the BNNTs can be attached to the metal particles and function as grain inoculants during additive manufacturing, the BNNTs may have any dimension. Typically, the dimensions of BNNTs are defined in terms of diameter and length of the BNNTs.

In some embodiments, the BNNTs have a diameter of less than about 250 nm, less than about 200 nm, less than about 150 nm, less than about 100 nm, less than about 50 nm, or less than about 25 nm. For example, the BNNTs may have a diameter of from about 5 nm to about 250 nm, from about 5 nm to about 200 nm, from about 5 nm to about 150 nm, from about 5 nm to about 100 nm, from about 5 nm to about 50 nm, or from about 5 nm to about 25 nm. In some embodiments, the BNNTs have a diameter of from about 30 nm to about 100 nm.

In some embodiments, the BNNTs may have a length of at least about 100 nm, at least about 250 nm, at least about 500 nm, at least about 750 nm, at least about 1 pm, at least about 5 pm, or at least about 10 pm.

In some embodiments, the BNNTs have a diameter of from about 5 nm to about 250 nm and a length of from about 100 nm to about 2 pm. For example, the BNNTs may have a diameter of from about 10 nm to about 200 nm and a length of from about 100 nm to 1 pm, or a diameter of from about 10 nm to about 100 nm and a length of from about 100 nm to 1 pm.

Examples of BNNTs suitable for use in the invention are shown in Figure 1(a). The Figure shows BNNTs having a bamboo-like structure with a diameter of 30-100 nm and a length up to a several micrometres.

By the one or more BNNTs being "attached" to the metal particles is meant that the one or more BNNTs is/are physically coupled to at least a portion of the surface of the metal particles. The coupling interaction may be of any form that impedes movement during additive manufacturing of the BNNTs relative to the metal particles, that being in contrast with the BNNTs simply presenting as isolated particles or aggregates freely distributed in the interstices of the metal powder. The BNNTs may be attached to the metal particles by

way of physical bonding, for example through Van Der Waals forces, liquid binding agents, or by being chemically grown (e.g. via a chemical vapour deposition process) on the surface of the metal particles.

Provided they are attached to the metal particles and they effectively act as grain refining agents, the BNNTs may be present as individual BNNTs and/or aggregates.

By the BNNTs being attached to the metal particles, the BNNTs become inherently trapped within the melt pool or located at the sintering-induced inter-particle necks during additive manufacturing. That ensures the BNNTs can effectively perform the intended grain refinement during cooling or consolidation. That also ensures that the so formed metal component has one or both of the boron nitride nanotubes and metal-boride/nitride substantially uniformly distributed therethrough. If the BNNTs are not attached to the metal particles, for example by being merely distributed as free particles or aggregates in the interstices of the feedstock metal powder, laser induced plasma or carrier gases typically used during additive manufacturing can easily blow them away from the melt pool, preventing effective grain refinement. Also, if BNNTs are merely provided as free particles or aggregates distributed in the interstices of the feedstock metal powder, their contribution to grain refinement is limited even if they remain trapped in the melt pool or located at the sintering-induced inter-particle necks due to the number density or size effect. Rather, in that state they may impair the overall mechanical characteristics of the manufactured component by creating local defects.

Provided grain refinement is achieved, any fraction of the metal particles in the feedstock processed by additive manufacturing may have one or more BNNTs attached thereto. In the context of the invention, the feedstock may therefore be qualified by the 1 % fraction of metal particles that have one or more BNNTs attached thereto. It will be understood that such 1% value represents a fraction of the particles that enter into the melt pool or consolidate during subsequent sintering. For instance, in the context of the invention at least 1% of the metal particles in the feedstock may have one or more BNNTs attached thereto. In those instances, at least one in every 100 metal particles that enter into the melt pool or consolidate during

subsequent sintering has one or more BNNTs attached to it.

Accordingly, in some embodiments at least about 1% of the metal particles in the feedstock have one or more BNNTs attached thereto. In some embodiments, at least about 2%, at least about 5%, at least about 10%, at least about 25%, at least about 40%, at least about 50%, at least about 60%, at least about 75%, or at least about 90% of the metal particles in the feedstock have one or more BNNTs attached thereto. In some embodiments, from about 40% to about 100% of the metal particles in the feedstock have one or more BNNTs attached thereto. In some embodiments, from about 50% to about 100% of the metal particles in the feedstock have one or more BNNTs attached thereto, from about 60% to about 100% of the metal particles in the feedstock have one or more BNNTs attached thereto, from about 70% to about 100% of the metal particles in the feedstock have one or more BNNTs attached thereto, or from about 80% to about 100% of the metal particles in the feedstock have one or more BNNTs attached thereto. In some embodiments, about 100% of the metal particles in the feedstock have one or more BNNTs attached thereto.

The BNNTs may be attached to the metal particles according to any procedure that would result in the BNNTs being physically attached to at least a portion of the surface of the metal particles. For example, the BNNTs may be attached to the metal particles by way of dry or wet mixing techniques, or by way of a binder.

In one embodiment, the BNNTs are attached to the metal particles by way of a wet mixing technique. For example, the BNNTs may be attached to the metal particles by mixing the metal particles with a suspension of the BNNTs in a liquid medium, followed by removal of the liquid medium. The liquid medium may be any liquid medium into which the BNNTs can be suspended and that can be readily removed to provide metal particles having the BNNTs attached thereto. For example, the liquid medium may be a volatile liquid medium, such as ethanol or acetone. Removal of the liquid medium may be achieved by any means known to a skilled person that would result in the liquid medium being removed leaving the BNNTs attached to the metal particles. For example, the liquid medium may be removed by evaporation under atmospheric conditions, vacuum drying. Heat may also be used to assist

with the removal of the liquid medium (e.g. evaporation under atmospheric conditions in the presence of heat, or vacuum drying in the presence of heat).

In some embodiments, the BNNTs are attached to the metal particles by way of a binder. Examples of suitable binders include organic cross-linking agents such as butyral resins, polyvenyls, poly siloxanes, polyacrylic acids (PAA), acrysol and aqueous solutions of starch, dextran and maltodexterins and inorganic solution such as silica colloids. In a typical procedure, the binding agent can be either entirely carried by jetted liquid (In-Liquid binding) or printed and interacts with dry glue particles embedded in the powder bed (InBed binding).

Provided the BNNTs are attached to the metal particles and perform the intended grain refinement function, the amount of BNNTs in the overall metal powder feedstock is not particularly limited. For example, the metal powder feedstock may contain at least about 0.01 wt% of BNNTs relative to the weight of the metal particles. In some embodiments, the metal powder feedstock contains at least about 0.015 wt%, at least about 0.05 wt%, at least about 0.1 wt%, at least about 0.5 wt%, at least about 1 wt%, at least about 5 wt%, or at least about 10 wt% of BNNTs relative to the weight of the metal particles. In some embodiments, the metal powder feedstock contains from about 0.1 wt% to about 0.4 wt% of BNNTs relative to the weight of the metal particles.

The metal component obtained by the additive manufacturing contains one or both of the boron nitride nanotubes and metal-boride/nitride. That is, in addition or in alternative to the BNNTs, the metal component may contain metal-boride/nitride.

As used herein, the expression "metal-boride/nitride" means "metal-boride and/or metal nitride", in the sense of "at least one of metal-boride and metal -nitride". As a skilled person can appreciate, at least one of metal-boride and metal-nitride may form when the feedstock is processed, depending on factors such as the solubility of each of B and N in the metal, the content of BNNTs in the feedstock, and process conditions such as time and temperature. In general, an increase of one or more of (1) the solubility of B and/or N in the metal, (2) the

amount of BNNTs in the feedstock, (3) the processing time, and (4) the processing temperature increase will lead to a higher likelihood that at least one of metal-boride and metal-nitride would form during additive manufacturing.

It is believed BNNTs can tolerate the heat involved with melting of the metal particles and remain in an undissolved state during the entirety of the printing procedure. That is, upon formation of the melt pool at least a majority fraction of the BNNTs remains trapped in the melt pool, where they quickly diffuse to provide a homogeneous distribution of BNNTs throughout the volume of the melt pool. As the melt pool cools, the BNNTs perform their inoculant function and remain trapped within the crystallised metal providing a substantially uniformly distribution of BNNTs throughout the solidified metal. In some instances, and typically within the last printed layer of metal, a fraction of BNNTs may remain undissolved and retained in a solidifying layer surrounding the melt pool. Depending on the nature of the metal element(s) in the metal particles, this may trigger diffusion-controlled solid-state reactions between the retained BNNTs and the metal matrix resulting in in-situ transformation of the BNNTs into crystalline precipitates of metal-boride/nitride.

It is believed that formation of metal-boride/nitride may be facilitated by out-diffusion of boron and nitrogen from the BNNTs into the metal matrix surrounding the melt pool, against inner diffusion of metal element(s) into the BNNTs. For example, when the metal particles are titanium particles or titanium alloy particles, precipitation of TiB nanowires/whiskers has been observed to occur in the solidifying layer surrounding the melt pool. Advantageously, the crystalline precipitates of metal-boride/nitride (when present) are believed to provide grain refinement of the cooling melt in synergy with the BNNTs. According to similar mechanisms, it is believed that the presence of BNNTs and metal- boride/nitride (when either one is present) in the metal component restricts the grain growth of metals also during layer-by-layer deposition, heat treatment, or sintering.

As a net effect, it is believed the combined influence of substantially uniformly distributed nanoscale BNNTs, constitutional supercooling and in-situ formed metal-boride/nitride precipitates (when present) advantageously boost the columnar-to-equiaxed transition

(CET) in the dynamics of grain formation during cooling of the melt pool, giving rise to a fine equiaxed grain structure throughout the entire build. By providing a high density of low energy-barrier heterogeneous nucleation sites ahead of the solidification front, the critical amount of undercooling needed to induce equiaxed growth can be decreased. That allows for a fine equiaxed grain structure that accommodates strain and prevents cracking under otherwise identical solidification conditions.

An example of such a grain refining process is schematically depicted in the schematic of Figure 9, in the case of additive manufacturing performed using a feedstock powder comprising Ti-6A1-4V alloy particles having BNNTs attached thereto. The refining process starts with partial dissolution of the BNNTs upon the first layer deposition followed by consequent boron enrichment in the melt near BNNTs. With a succeeding layer deposition, a unique combination of well-dispersed nanoscale TiB precipitates and increased constitutional supercooling can significantly boost heterogeneous nucleation for CET in the melt and result in an grain refinement in the resulting Ti-6A1-4V alloy component with only a trace amount of BNNT addition.

One or both of the BNNTs and the metal-boride/nitride (when either is present) is/are substantially uniformly distributed throughout the metal component. By being "substantially uniformly" distributed, each of the BNNT and the metal-boride/nitride (when either is present) are distributed throughout the volume of the metal component in a substantially equidistant arrangement, such that at least one of the BNNTs and the metal-boride/nitride (when either is present) can be found in any one cubic millimetre volume of the metal component.

Advantageously, the metal component has a micro-structure that exhibits an average grain size smaller relative to that of an additive manufactured component obtained using a metal powder feedstock in which, for example, the BNNTs are not attached to the metal particles (e.g. by presenting freely distributed in the interstices of the metal powder), or an additive manufactured component formed using conventional mechanical manufacturing. As a result, the grain size-dependent mechanical properties (e.g. specific strength, fatigue resistance,

fracture toughness and hot-strength) of the metal component in accordance to the present invention can be advantageously superior to those of a corresponding metal component obtained using a feedstock in which the BNNTs are not attached to the metal particles, or a feedstock with no BNNTs.

The average grain size in the microstructure of the metal component obtained with the method of the invention may be any average size, provided it is smaller than the average grain size of a corresponding metal component obtained using a feedstock with no BNNTs. For example, the metal component may be characterised by a micro-structure that exhibits an average grain size at least 20% smaller than that of the micro-structure of the metal component obtained using a feedstock with no BNNTs.

As used herein, the expression "average grain size" refers to the average grain size of the micro-structure of the metal component measured using optical imaging of polished cross- sectional surfaces of the component following the Circular Intercept Procedures of ASTM standard El 12-13.

In some embodiments, the metal component has a micro-structure that exhibits grains having an average grain size of less than 500 pm, less than 100 pm, less than 50 pm, less than 10 pm, less than 5 pm, or less than 1 pm. In some embodiments, the metal component has a micro-structure that exhibits grains having an average grain size of from 5 pm to 50 pm.

In some embodiments, the metal component has a micro-structure that exhibits equiaxed grains with a mean aspect ratio below 5. For example, the metal component may have a micro-structure that exhibits equiaxed grains with a mean aspect ratio below 4, below 3, or below 2. In some embodiments, the additive manufactured metal component may have a micro-structure that exhibits equiaxed grains with a mean aspect ratio of about 1.

It is not excluded that the metal component may undergo additional processing steps to achieve further micro-structural refinement and improvement of its mechanical properties. Accordingly, in some embodiments the method further comprises a post-processing step

performed on the metal component. The post-processing step may be, for example, selected from one or more of heat treatment, thermomechanical processing, hot isostatic pressing (HIP), machine, deformation, remelting, and sintering. This may afford further refinement of the component micro-structure and improve the overall mechanical properties of the component. Such post-processing steps may be useful in the production of, for example, high performance metal components.

The present invention also provides a metal component obtained in accordance with the method described herein.

It is also believed that the metal powder feedstock used in the method of the invention is unique in its own right. Accordingly, the present invention also provides a metal powder feedstock for additive manufacturing, the feedstock comprising metal particles having one or more boron nitride nanotubes (BNNTs) attached thereto.

By the feedstock being "for additive manufacturing" will be taken to imply that any unit volume of the feedstock ultimately intended for processing would include metal particles having one or more BNNTs attached thereto. That is, whether the metal particles having one or more BNNTs attached thereto are homogeneously distributed throughout the volume of the feedstock is not particularly relevant, provided that the feedstock can be provided in a form such that, when processed, it contains metal particles having one or more BNNTs attached thereto.

The metal particles and BNNTs are as described herein. In addition, the metal powder feedstock may contain the metal particles and the BNNTs in the amounts described herein.

The metal powder feedstock advantageously enables additive manufacturing of components made of a wide variety of metals, including un-weldable metals, without compromising the mechanical properties of the native metal. In particular, the feedstock of the invention can promote grain refinement dynamics of the type described herein.

Additive manufactured metal components comprising BNNTs substantially uniformly distributed therethrough are also believed to be unique on their own right. Such metal components can advantageously present a micro-structure exhibiting finer grains, and therefore improved mechanical characteristics, relative to corresponding metal components in which the BNNTs are either not substantially uniformly distributed therethrough, or not present at all. For instance, if the BNNTs are not substantially uniformly distributed throughout the metal component, they may present as aggregates. Aggregates are undesired as they can impair the mechanical properties of the component by generating localised porosity within the metal structure.

Accordingly, the present invention also provides an additive manufactured metal component comprising one or both of metal boride/nitride and boron nitride nanotubes (BNNTs) substantially uniformly distributed therethrough.

An aspect of the present invention also relates to an additive manufactured metal component comprising one or both of boron nitride nanotubes (BNNTs) and metal-boride/nitride substantially uniformly distributed therethrough, wherein the metal component has a microstructure that exhibits an average grain size at least 20% smaller than that of the microstructure of the metal component absent the one or both of BNNTs and metal-boride/nitride.

Smaller average grain size in the metal component can advantageously impart improved specific strength, fatigue life, fracture toughness and hot-strength relative to an additive manufactured component which microstructure exhibits grains of larger average size.

In some embodiments, the metal component has a micro-structure that exhibits an average grain size at least about 25%, at least about 40%, at least about 50%, at least about 60%, at least about 75%, at least about 100% smaller than that of the micro-structure of the metal component absent the one or both of BNNTs and metal-boride/nitride.

In another aspect, the invention relates to an additive manufactured metal component comprising one or both of boron nitride nanotubes (BNNTs) and metal-boride/nitride

substantially uniformly distributed therethrough, wherein the metal component exhibits a yield strength at least 10% higher than that of the metal component absent the one or both of BNNTs and metal-boride/nitride.

In some embodiments, the additive manufactured the metal component exhibits a yield strength at least about 15% higher, at least about 20% higher, at least about 30% higher, at least about 40% higher, at least about 50% higher, at least about 60% higher, at least about 70% higher, at least about 80% higher, at least about 90% higher, or at least about 100% higher than that of the metal component absent the one or both of boron nitride nanotubes and metal-boride/nitride.

An additional aspect relates to an additive manufactured metal component comprising one or both of boron nitride nanotubes (BNNTs) and metal-boride/nitride substantially uniformly distributed therethrough, wherein the metal component exhibits a compressive yield strength at least 10% higher than that of the metal component absent the one or both of BNNTs and metal-boride/nitride.

In some embodiments, the additive manufactured the metal component exhibits a compressive yield strength at least about 15% higher, at least about 20% higher, at least about 30% higher, at least about 40% higher, at least about 50% higher, at least about 60% higher, at least about 70% higher, at least about 80% higher, at least about 90% higher, or at least about 100% higher than that of the metal component absent the one or both of boron nitride nanotubes and metal-boride/nitride.

In a further aspect, the invention provides an additive manufactured metal component comprising one or both of boron nitride nanotubes (BNNTs) and metal-boride/nitride substantially uniformly distributed therethrough, wherein the metal component exhibits an ultimate strength at least 10% higher than that of the metal component absent the one or both of BNNTs and metal-boride/nitride.

In some embodiments, the additive manufactured the metal component exhibits an ultimate

strength at least about 15% higher, at least about 20% higher, at least about 30% higher, at least about 40% higher, at least about 50% higher, at least about 60% higher, at least about 70% higher, at least about 80% higher, at least about 90% higher, or at least about 100% higher than that of the metal component absent the one or both of boron nitride nanotubes and metal-boride/nitride.

The additive manufactured metal component may be made of a metal of the kind described herein. Also, the BNNTs and the metal-boride/nitride (when either is present) contained in the additive manufactured metal component may be BNNTs and metal-boride/nitride of the kind described herein.

In some embodiments, the additive manufactured metal component has a micro-structure that exhibits equiaxed grains with a mean aspect ratio below 5. For example, the additive manufactured metal component may have a micro-structure that exhibits equiaxed grains with a mean aspect ratio below 4, below 3, below 2, below 1.5. In some embodiments, the additive manufactured metal component may have a micro-structure that exhibits equiaxed grains with a mean aspect ratio of about 1.

Typically, the additive manufactured metal component exhibits a yield strength at least 10% higher than that of the metal component absent the one or both of boron nitride nanotubes and metal-boride/nitride. In some embodiments, the additive manufactured metal component exhibits a yield strength at least 20%, at least 40%, or at least 60% higher than that of the metal component absent the one or both of boron nitride nanotubes and metal-boride/nitride.

Values and ranges of "compressive strength" described herein refer to uniaxial compressive strength measured on cylindrical specimens (5 mm in diameter and 7.5 mm in height) which are machined from as-printed cubic samples with the compression axis parallel to the build direction. The uniaxial compression tests are carried out at room temperature at a strain rate of 10 ’ s ¹ using an Instron 8801 universal testing machine equipped with a 100 kN load cell and a non-contact video extensometer. The “strength” also applies to yield strength obtained by other testing methods such as tensile testing.

The proposed invention can find application in a large number of manufacturing fields, including architecture, engineering, construction, industrial design, automotive, aerospace, military, engineering, civil engineering, dental and medical industries, biotech (human tissue replacement), fashion, footwearjewellery, eyewear, and the likes fields.

The invention is also described with reference to the following non-limiting examples.

EXAMPLES

EXAMPLE 1 - Feedstock preparation

BNNTs were synthesised via ball milling and annealing boron powder in a nitrogen atmosphere using a specially designed rotating furnace, in accordance to procedures described in Chen, et al. " Synthesis of boron nitride nanotubes at low temperatures using reactive ball milling ", Chemical Physics Letters, Volume 299, page 260-264 (1999), the contents of which are incorporated herein in their entirety. The as-synthesized BNNTS have a bamboo-like structure with a diameter of 30-100 nm and a length up to a few micrometres (Figure 1(a))

Gas atomised Ti-6A1-4V (Grade 23) powder was supplied by Advanced Powders & Coatings (AP&C, Canada), having a size range of 45-106 pm (Figure 1(b)).

The BNNTs were suspended in ethanol, and the suspension mixed with the alloy powder according to 0. 1 and 0.4 wt. % BNNTs relative to the weight of the alloy powder. The ethanol was subsequently dried, leaving the BNNTs randomly dispersed and attached on the surface of the alloy particles (Figure 1(c)).

EXAMPLE 2 - Additive manufacturing

The Ti-6A1-4V-BNNT feedstock obtained using the procedure described in Example 1 were

subjected to 3D printing using an Optomec LENS MR-7 coaxial blown powder laser deposition facility equipped with a 1 kW IPG fibre laser. The feedstock was loaded in a hopper and transported by ultra-high purity argon (99.999%) carrier gas to the focused laser beam (see schematic of Figure 2(a)). Cylinder and cuboid samples were printed on a Ti-6A1- 4V substrate for each composition (Figure 2(b), showing a cylinder sample), using parallel tracks and random contouring (parameters are shown in Table 1). Deposition occurred in a controlled Ar atmosphere with an oxygen content of about 1 ppm. The specimens were cut off from the substrate for microstructural examination and mechanical testing.

For sake of easy comparison, the Ti-6A1-4V alloy and Ti-6A1-4V/BNNT composites were printed by LENS using a same set of parameters.

Table 1 - LENS processing parameters for the Ti-6A1-4V alloy and Ti-6A1-4V/BNNT composites

Laser Scanning Laser Beam Powder Mass Hatch Layer Working

Power Velocity Diameter(mm) Feed Rate Distance Thickness Distance

(W) (mm/min) (g/min) (mm) (mm) (mm)

250 750 0.3 -6.24 0.38 -0.25 9.5

EXAMPLE 3 - Microstructure characterization

Phase determination was conducted using a laboratory PANalytical PRO MRD (XL) X-ray diffractometer (XRD) with Cu Ka radiation in point focus. Microstructural characterization was performed using a combination of optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Atom Probe Tomography (APT) techniques.

SEM analysis was conducted using a JEOL JSM 7800F field emission gun scanning electron microscope (FEG-SEM) equipped with an electron backscatter diffraction (EBSD) detector fully automated by the AZtecHKL software (Oxford Instruments). The samples for SEM, XRD analysis and hardness measurement were prepared following a standard grinding/polishing procedure including final polishing with a colloidal silica suspension

(OPS, Struers, Denmark) with 10% hydrogen peroxide (H2O2) the content of which is incorporated herein in its entirety. Some specimens were further etched in Kroll’s reagent (i.e. 2% HF, 6% HNO3 and 92% H2O) for optical and SEM observation.

Electron Backscatter Diffraction (EBSD) data was collected at 20kV using a step size of 0. 1- 0.3 pm. Based on the Burgers orientation relationship (i.e. {110}^ || {0001}_a and (lll) j H (1120)_a) during the P^a phase transformation in titanium alloys, the parent grain orientation was reconstructed by an automatic computer program ARPGE using the EBSD data measured from the transformed daughter a/ a' microstructures. The as-measured and reconstructed EBSD data were analysed and plotted using the HKL CHANNELS software. The P grain size was measured using optical images following the Circular Intercept Procedures in ASTM standard El 12-13. The dimension of TiB whiskers was determined using a software ImageJ.

TEM imaging and diffraction was performed on a JEOL JEM 21 OOF high resolution FEG transmission electron microscope (TEM) equipped with a JEOL JED-2300T energy dispersive X-ray spectrometer (EDS). TEM thin foils with the size of 6x8 pm2 were prepared by focused ion beam (FIB) milling using an FEI dual-beam Quanta 3D FIB machine. APT studies were carried out on a Local Electrode Atom Probe (LEAP) Cameca 5000 XR microscope in voltage mode.

The APT sample tips were lifted out from the area of interest using a Focused Ion Beam (FIB) on a FEI Quanta 3D FEG SEM. The APT data were acquired under ultrahigh vacuum at a base temperature of 50 K, with a pulse fraction (ratio of pulse voltage to steady state direct-current imaging voltage) of 15% and a pulse repetition rate of 200 kHz. The APT data were then reconstructed and analysed using Cameca IVAS 3.8.2 software.

Mechanical evaluation

Cylindrical specimens (5 mm in diameter and 7.5 mm in height) were machined from the as printed cubes with the compression axis parallel to the build direction. Room temperature

uniaxial compression tests were carried out at a strain rate of 10 ’ s ¹ using an Instron 8801 universal testing machine equipped with a 100 kN load cell and a non-contact video extensometer.

Vickers hardness was measured using a Struers DuraScan hardness tester at a load of 10 kg and 15 s dwell time, with 6-8 measurements conducted for each material.

EXAMPLE 4 - Microstructural Refinement

Microstructure analysis using optical and scanning electron microscope (SEM) performed on the samples manufacture in accordance with the procedure described in Example 2 revealed a dense structure mostly free of pores or unmelted powder for the as-built deposits without and with BNNT addition (Figures 3(a)-3(d)), indicative of great printability for both materials.

The as-fabricated comparative Ti-6A1-4V alloy (no BNNTs in the feedstock) displays a coarse columnar prior-P grain morphology with Widmanstatten-type a+ lamellae in the grain interior, which is commonly seen in additively manufactured titanium alloys. The prior-P grains have an average width of 337± 10 pm and grow epitaxially along the build direction to a few millimetres in length.

When 0. 1 wt% and 0.4 wt% BNNTs are attached to the Ti-6A1-4V alloy particles in the feedstock (hereafter named 0.1BNNT and 0.4BNNT, (Figures 4(a), 4(b)), LENS printing provides metal components exhibiting uniform and highly refined equiaxed prior-P grains of 33±4 pm and 9.0±0.2 pm, respectively. These refined prior-P grains restrict the growth of a laths in the grain interior during P^a transformation, resulting in a remarkably reduced a lath aspect ratio of about 2.1, compared to >10 in the unmodified Ti-6A1-4V alloy.

A closer inspection reveals a nano-whisker network predominately dispersed at prior-P grain boundaries (“GBs” in Figures 3(a)-3(d)). The whiskers are about 60 nm in diameter/thickness which are comparable in size to the input BNNTs.

Similar types of prior-p GBs also appear in cast and additively manufactured titanium and its alloys where boron was used as a micro-alloying element. Nevertheless, none of those samples shows uniform near-equiaxed grain structure, and the extent of their grain refinement was rather limited, with about 100 pm in boron modified casting and about 300 pm in wire -arc additive manufacturing (WAAM) and at least tens of microns for LENS/DLD process.

When the weight percent of additives is considered, the method of the present invention can afford much greater effectiveness in grain refinement of titanium alloys. For instance, a trace amount of merely 0.4 wt.% BNNTs attached to Ti-6A1-4V alloy particles in the feedstock is seen to provide dramatic refinement of prior-P grains by at least an order of magnitude (to about 9 pm).

EXAMPLE 5 - Mechanical Reinforcement

Due to the remarkable grain refinement, the use of BNNTs attached to the metal particles in the feedstock leads to remarkable strength enhancement with slight-to-moderate reduction in plasticity (as shown in Figure 5, and in the data of Table 2).

Table 2 - Mechanical properties of the as-printed Ti-6A1-4V alloy and Ti-6A1-4V/BNNT composites

Maximum Maximum Vickers Hardness

Composition GY02 Compressive Compressive (HV)

Strength (MPa) Strain (%)

Ti-6AI-4V 921 : 18 1 198 : 5 24.2 : 1 . 1 342.4 : 12

0.1 BNNT 1199±25 1393±39 18.8±0.1 400.2±1.5

0.4 BNNT 1407±2 1678±12 14.3±1.0 433.8±1.7

The Ti-6A1-4V/O.4%BNNT composite reveals a compressive yield strength (0.2% proof stress) of about 1407 MPa and an ultimate strength of about 1678 MPa, which are about 50% and 40% greater than the as-built Ti-6A1-4V, respectively. This also differs from most boron-modified Ti-6A1-4V alloys fabricated by casting, WAAM and LENS, where the

extent of strengthening is significantly less (up to 15% increase in yield strength with the addition of 0.5 wt.% B).

The significant mechanical reinforcement induced by the use of trace amount of BNNTs attached to the metal particles suggests alternative strengthening mechanisms. Despite this noticeable strength enhancement, an appreciable compressive plasticity over 15% is still maintained for the 0.4% BNNT composite, evidenced by a dimpled fracture surface shown in the cross-sectional images of Figures 6(a) and 6(b).

EXAMPLE 6 - Texture weakening

Quantitative examination of the metal components produced by the procedure described in Example 2 was conducted using electron backscatter diffraction (EBSD) mapping. EBSD revealed significant weakening of texture for a and P phases in the Ti-6A1-4V/BNNT composites (Figures 7(a)-7(d)). EBSD results further confirms the CET of coarse columnar prior- grains induced by 0.4 wt.% BNNT modification, where the refined equiaxed grains are mostly enclosed by high angle grain boundaries (HAGBs) with a mean aspect ratio below 1.5. With an increase in BNNT fraction, there is merely a slight variation in a lath width and P phase fraction, indicative of much less influence of BNNT on their formation via solid- state phase transformation.

Accompanied by the CET and grain refinement, the texture maxima of a and P were reduced from 2.98 and 3.75 to 1.26 and 1.86, discretely in the inverse pole figure (IPF) plotted along the build direction (BD). The weakened prior-P grain texture is a clear implication that random and abundant nucleation of prior-P grains appears in the stage of solidification, which further renders a variant selection during P^a transformation upon subsequent cooling.

EXAMPLE 7 - Microstructural evolution of BNNTs during titanium additive manufacturing and its effect on grain refinement

To identify the nature of these nanowires/whiskers, site-specific thin needles/foils were prepared using focused ion beam (FIB) milling and in-situ lift-out method from each sitespecific area and subjected to examinations using atom probe tomography (APT) and transmission electron microscopy (TEM).

APT analysis on the BNNT-like nanowires revealed a Ti:B atomic ratio close to 1 : 1 without detectable N enrichment (Figures 8(a) and 8(b)). TEM and associated selected area diffraction (SAD) analysis on nano-whiskers near prior-P GBs revealed faceted whisker morphology and an orthorhombic crystal structure (Space Group 62, Pnma) with lattice parameters of a = 0.612, b = 0.306, c = 0.456 nm (Figures 8(c) and 8(d)).

These results confirm the absence of any remnant BNNTs and their in-situ transformation into TiB in the course of LENS deposition.

The combined influence of finely dispersed nanoscale BNNT, constitutional supercooling and in-situ formed TiB enables columnar-to-equiaxed transition, giving rise to a fine equiaxed grain structure throughout the entire build (as shown in Figures 3-4). This grain refining process is schematically depicted in Figure 9. It starts with BNNT partial dissolution upon the first layer deposition followed by consequent boron enrichment in the melt near BNNTs. The rapid but localised increase in boron renders primary precipitation of coarse TiB in the vicinity of partially dissolved BNNT. This facilitates the initial stage of refinement to yield a fine columnar prior-P grain structure with fine TiB whiskers formed at the last-solidified inter-grain (or interdendritic) region via eutectic solidification. With a succeeding layer deposition, a unique combination of well-dispersed nanoscale TiB and increased constitutional supercooling can significantly boost heterogeneous nucleation for CET in the melt and result in an unparalleled grain refinement in the AM Ti-6A1-4V alloy with only a trace amount of BNNT addition.

This mechanism is different from the previous works which use eutectic micro-alloying elements such as B, C and Si in casting/WAAM of titanium alloys and far more effective than the recent B modified LENS printing of Ti-6A1-4V alloys. For instance, in a cast B-

modified titanium alloys, a relatively homogenous liquid phase is usually formed due to long incubation time. It is not possible to form TiB as a primary/leading phase with a hypoeutectic alloy composition (Figure 10). Heterogeneous nucleation is mainly achieved by the development of constitutional undercooling ahead of an advancing solid-liquid interface and this only leads to limited refinement of about 100 pm.

This is also true for WAAM process of B/Si modified titanium alloy using pre-alloyed wires, where coarse P grains form first prior to TiB precipitation via eutectic solidification mostly in the inter-dendritic regions and grain interiors. More remarkable grain refinement can be achieved when a mixture of micron-sized B or TiC incubation particles and titanium powder are used in LENS processing.

However, the relatively larger particle size results in much lower nuclei density and longer dissolution time in the melt. Columnar grains are rarely completely eliminated even at high loadings of boron (1.0 wt.%), leading to a mixed grain structure or a non-uniform microstructure with alternating columnar and equiaxed grains. In comparison, the current addition of 0.4 wt.% BNNT attached to the metal particles in the feedstock powder provides over 2x l0⁷ nano-whiskers/nanotube in one cubic millimetre of melt, leading to an unprecedented level of microstructural control at a given amount of incubate addition. Also due to the size effect, this type of refiners can be also applicable in other additive manufacturing techniques with faster solidification rate and shorter energy-material interaction time (e.g. powder-bed fusion) which cannot fully dissolve coarse particles.

EXAMPLE 8 - Mechanical reinforcement and mechanism

The remarkable strength enhancement obtained by having BNNTs attached to the metal particles of the feedstock can be closely related to the unique microstructure fabricated in the BNNT modified alloy. Here the refined grain size and a lath aspect ratio only contributes to limited strength enhancement as they only reduce the dislocation slip length in a laths. Nitrogen is common a-Ti stabilizers, which has the most pronounced solid solution hardening effect over other interstitial elements (e.g. O, and C).

Bulk chemical analysis revealed a nitrogen content of 0.09 wt.% in the 0.4 BNNT composite material, compared with 0.01 wt.% N for the Ti-6A1-4V deposit (Table 3). The solution N may offer a considerable yield strength enhancement of about 200 MPa in titanium. Besides, supersaturated boron solute may also contribute to the strengthening of titanium if it was not completely rejected to the P GBs. In addition, rapid solidification in the LENS process also restricts the growth and coarsening of TiB nano-whiskers during solidification and the layer- by-layer deposition.

Table 3 - Chemical composition of the as-built Ti6A14V alloy with and without BNNT modification

Sample Ti Al V Fe B N O C

Ti-6A1-4V Bal 6.24 3.99 0.27 <0.01 0.01 0.11 0.02

0.1BNNT Bal 6.24 3.97 0.28 0.03 0.03 0.13 0.02

0.4BNNT Bal 6.34 4.00 0.30 0.16 0.09 0.13 0.03

According to the theoretical model of particle strengthening, these high aspect-ratio TiB nano-whiskers offer greater strengthening relative to the spherical particles (e.g. W) or coarse TiB whiskers at the same volume fraction. The reinforcing TiB nano-whiskers also has greater benefits to toughness over their coarser counterparts and their alignment at GBs can lead to less reduction in ductility when compared to randomly dispersed TiB. Given the exceptional strength, the current composite has great potentials in making structural -efficient titanium parts. The presence of TiB nano-whiskers may also provide profound thermal stability and hot-strength at elevated temperatures, which is particularly attractive to the aerospace sector.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word ‘comprise’, and variations such as ‘comprises’ and ‘comprising’, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims

- 33 - THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS

1. A method of forming a metal component by additive manufacturing, the method comprising: providing a metal powder feedstock comprising metal particles having one or more boron nitride nanotubes attached thereto; and processing the metal powder feedstock by additive manufacturing to form the metal component; wherein the so formed metal component has one or both of the boron nitride nanotubes and metal-boride/nitride substantially uniformly distributed therethrough.

2. The method of claim 1, wherein at least 1% of the metal particles have one or more boron nitride nanotubes attached thereto.

3. The method of claim 1 or 2, wherein the boron nitride nanotubes are attached to the metal particles by mixing the metal particles with a suspension of the boron nitride nanotubes in a liquid medium, followed by removal of the liquid medium.

4. The method of any one of claims 1-3, wherein processing the metal powder feedstock by additive manufacturing comprises selective laser sintering (SLS), selective laser melting (SLM), laser powder bed fusion (LPBF), direct metal laser sintering (DMLS), direct metal laser melting (DMLM), binder jet printing (BJP), electron beam melting (EBM), laser engineered net shaping (LENS), direct laser forming (DLF), or directed energy deposition (DED).

5. The method of any one of claims 1-4, wherein the metal particles have average size of less than about 500 pm.

6. The method of any one of claims 1-5, wherein the metal powder feedstock comprises metal particles of titanium, aluminium, magnesium, nickel, iron, copper, gold, silver, bismuth, manganese, zinc, chromium, molybdenum, platinum, zirconium, iridium, yttrium, - 34 - vanadium, niobium, tantalum, tungsten, cobalt, tin, lead, gallium, or an alloy of one or more thereof.

7. The method of claim 6, wherein the metal powder feedstock comprises metal particles of Ti-6A1-4V alloy.

8. The method of any one of claims 1-7, wherein the boron nitride nanotubes have a diameter of about 30-100 nm.

9. The method of any one of claims 1-8, wherein the boron nitride nanotubes in the feedstock are present in an amount of at least 0.01 wt% relative to the total weight of the metal particles.

10. A metal component obtained according to the method of any one of claims 1-9.

11. A metal powder feedstock for additive manufacturing, the feedstock comprising metal particles having one or more boron nitride nanotubes attached thereto.

12. The metal powder feedstock of claim 11, wherein at least 1% of the metal particles have one or more boron nitride nanotubes attached thereto.

13. The metal powder feedstock of claim 11 or 12, wherein the metal particles have average size of less than about 500 pm.

14. The metal powder feedstock of any one of claims 11-13, comprising metal particles of titanium, aluminium, magnesium, nickel, iron, copper, gold, silver, bismuth, manganese, zinc, chromium, molybdenum, platinum, zirconium, iridium, yttrium, vanadium, niobium, tantalum, tungsten, cobalt, tin, lead, gallium, or an alloy of one or more thereof.

15. The metal powder feedstock of claim 14, comprising metal particles of Ti-6A1-4V.

16. The method of any one of claims 11-15, wherein the boron nitride nanotubes have a diameter of 30-100 nm.

17. The metal powder feedstock of any one of claims 11-16, wherein the boron nitride nanotubes are present in an amount of at least 0.01 wt% relative to the total weight of the metal particles.

18. An additive manufactured metal component comprising one or both of boron nitride nanotubes and metal-boride/nitride substantially uniformly distributed therethrough, wherein the metal component has a micro-structure that exhibits an average grain size at least 20% smaller than that of the micro-structure of the metal component absent the one or both of boron nitride nanotubes and metal-boride/nitride.

19. An additive manufactured metal component comprising one or both of boron nitride nanotubes and metal-boride/nitride substantially uniformly distributed therethrough, wherein the metal component exhibits a yield strength at least 10% higher than that of the metal component absent the one or both of boron nitride nanotubes and metal-boride/nitride.

20. An additive manufactured metal component comprising one or both of boron nitride nanotubes and metal-boride/nitride substantially uniformly distributed therethrough, wherein the metal component exhibits a compressive yield strength at least 10% higher than that of the metal component absent the one or both of boron nitride nanotubes and metal- boride/nitride.

21. An additive manufactured metal component comprising one or both of boron nitride nanotubes and metal-boride/nitride substantially uniformly distributed therethrough, wherein the metal component exhibits an ultimate strength at least 10% higher than that of the metal component absent the one or both of boron nitride nanotubes and metal- boride/nitride.

22. The additive manufactured metal component of any one of claims 18-21, having a micro-structure that exhibits equiaxed grains with a mean aspect ratio below 5.