Additive Manufacturing

Abstract

A method of additive manufacturing is disclosed, comprising using a powder comprising a first particulate component (1) with a first mean particle diameter, and a second particulate component (2) with a second mean particle diameter. The first mean particle diameter is at least twice the second mean particle diameter. The particles (2) of the second component are bonded to the particles (1) of the first component, and the first and second components comprise different materials. The powder is deposited.

Claims

1. An additive manufacturing tool comprising; a first powder holder; a second powder holder; a blender operable to blend a first powder material from the first powder holder and a second powder material from the second powder holder to form a feedstock powder wherein: i) the feedstock powder comprises a first particulate component with a first mean particle diameter, and a second particulate component with a second mean particle diameter, wherein the first mean particle diameter is at least twice the second mean particle diameter, and the particles of the second component are bonded to the particles of the first component with a binder material to form a flowable power, and ii) the blender comprises a hinder dispenser for adding the binder material to the powder, and a dryer for drying the powder to form the feedstock powder; and a dispenser, wherein the dispenser is operable to dispense the feedstock powder.

2. The additive manufacturing tool according to claim 1, wherein the tool is configured to form feedstock powder as it is required by the dispenser.

3. The additive manufacturing tool according to claim 1, wherein the second powder holder comprises at least one container, and the blender is configured to blend material from the first powder holder and one or more selected containers of the second powder holder in variable proportions to form a feedstock powder with selectable proportions of different materials.

4. The additive manufacturing tool according to claim 1, further comprising a directed heat source operable to sinter or melt the feedstock powder to form a part.

5. The additive manufacturing tool according to claim 4, wherein the dispenser is operable to deposit successive layers of the feedstock powder and the directed heat source is operable to produce a solid part by selectively sintering or melting regions of each successive layer, and the tool is operable to vary the composition of the feedstock powder so that different layers have a different composition so as to produce a part with a first region having a first material composition, and a second region having a different material composition, by varying the proportions of the materials in the feedstock powder during production of the part.

6. The additive manufacturing tool according to claim 1, further comprising a cold spray deposition device operable to form a part by impacting the feedstock powder on a surface.

7. The additive manufacturing tool according to claim 1, wherein the blender is operable to bond particles of the first powder with particles of the second powder, thereby forming the feedstock powder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

[0063] FIG. 1 is a schematic of a particle of a powder according to an embodiment of the first aspect of the invention;

[0064] FIG. 2 is a scanning electron micrograph (SEM) of a powder according to an embodiment of the first aspect;

[0065] FIG. 3 is a graph showing particle size distribution for a powder according to an embodiment of the first aspect;

[0066] FIG. 4 is a set of four SEMs showing two different regions of a part according to a first embodiment of a fifth aspect of the invention, each region being shown at two different levels of magnification;

[0067] FIG. 5 is an SEM of a partially transformed particle of the second component in the part according to the first embodiment of the fifth aspect,

[0068] FIG. 6 is an SEM produced using back scattered electrons (BSE) showing the formation of TiB needles in the part according to the first embodiment of the fifth aspect;

[0069] FIG. 7 is a pair of SEMs, at different magnifications, of a pull-out particle of the part according to the first embodiment of the fifth aspect, showing basket weave type microstructure;

[0070] FIG. 8 is a set of four SEMs of a part according to a second embodiment of the fifth aspect of the invention, showing microstructural features of the part; and

[0071] FIG. 9 is a schematic diagram of a additive manufacturing tool according to an embodiment of the third aspect of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

[0072] Referring to FIG. 1, a particle 100 of a powder according to an embodiment of the first aspect of the invention is shown. The particle 100 comprises a first particle 1, to which are bonded a plurality of smaller particles 2. To put it another way, the first particle 1 is satellited by a plurality of smaller particles 2. The particle 100 is thereby a satellited particle, consisting of a first component 1 and a second component 2. The first component of the particle 100 is the first particle 1, and the second component is the plurality of smaller particles 2 bonded to the first component. The material of the larger particle 1 is different from that of the smaller particle 2. In one example embodiment the larger particle 1 may comprise a metal such as titanium (or an alloy thereof), and the smaller particles 2 may comprise a ceramic such as titanium dihoride. The larger particle 1 may be formed from Ti-6Al-4V, and the smaller particle 2 may be TiB.sub.2.

[0073] Alternatively, the larger particle 1 may comprise a metal such as aluminium, and the smaller particles 2 may comprise an alloying additive, such as copper. Any materials that are suitable for additive manufacturing from a powder form may be combined in this way, and it will he understood that the invention is not limited to metals. For instance, the larger particle 1 may comprise a plastics material.

[0074] Combining materials in this way, by bonding (or satelliting) smaller particles 2 comprising a second material to larger particles I comprising a first material, enables particles with different sizes and properties to be combined without compromising homogeneity or flowability of the resulting powder.

[0075] Referring to FIG. 2, the SEM shows a powder 200 comprising particles of the type schematically represented in FIG. 1. In FIG. 2 the larger Ti-6Al-4V powder particles 1 can be seen surrounded by the much smaller TiB.sub.2 particles 2, which appear lighter in colour. The first component of the powder in this embodiment are relatively large. Ti-6Al-4V particles 1, which have been satellited with the second component 2, consisting of TiB.sub.2 particles with a relatively small diameter. In the example of FIG. 2, the TiB.sub.2 particles 2 were bonded to the titanium alloy particles 1 using a binder material. In this case, the binder material was 2.7 volume % poly vinyl alcohol (PVA) solution in water which was sprayed as an aerosol on a pre-blended mixture of the powder.

[0076] In the example embodiment of FIG. 2, having wet the powder with the PVA solution, it was thoroughly mixed to avoid agglomeration of the mixed powder. Following a drying process, it was found that the first and second component 1,2 adhere together sufficiently well that they remain substantially adhered during powder feeding associated with additive manufacturing processes, and will remain adhered during a transportation process (e.g. road/rail/air).

[0077] The second component 2 of the powder 200 may be bonded to the first component 1 using any suitable technique. For example, other binder materials could be used, or the first and second component 1,2 could be bonded together without the use of a binding agent, for example by cold welding, intermolecular forces and/or electrostatic attraction. Note that the powder 200 need not consist wholly of satellited particles. In some embodiments the powder 200 may comprise a mix of satellited particles and not satellited particles.

[0078] FIG. 3 shows the particle size distribution of the powder 200 of FIG. 2 as a fraction of the volume of the powder, obtained by X ray diffraction. A first size distribution 3 can be identified, associated with the first component 1 of the powder 200, and a second size distribution 4 can be identified, associated with the second component 2 of the powder 200. In this example, the first component 1 (Ti-6Al-4V) has a mean particle diameter in the region of 100 m, and the second component 2 (TiB.sub.2) has a mean particle size in the region of 10 m. The ratio of the mean particle diameter of the first and second component 1, 2 is thereby approximately ten,

[0079] The applicant has found, in contrast to mixed unsatellited powders, that powders according to embodiments of the invention maintain sufficient spherocity to exhibit acceptable levels of flowability, packing density and homogeneity in use. Sufficient packing density for additive manufacturing of dense parts can be achieved.

[0080] Referring to FIG. 4, the microstructure of a part according to a first embodiment of the fifth aspect of the invention is shown. In this case, the part was produced by blown powder deposition, in which a powder according to the first aspect was blown through the region of a laser beam, so as to melt the powder to form a solid part. A 2 kW Ytterbium-doped, CW fibre laser operating at 1.07 m wavelength coupled with a beam delivery system comprising (125 mm collimating lens and a 200 mm focussing lens was used (Precitec YC 50 cladding head). The laser beam was de-focussed to give a Gaussian beam profile with a circular spot size of 3.1 mm for the feedstock processing. The laser system was mounted on a 4 axis computer numerically controlled (CNC) table to traverse the work piece while the laser beam is kept stationary. The powder feedstock was steadily delivered by a Model 1264 powder feeder (Praxair Surface Technologies) into the 3.1 mm circular diameter melt pool through a side feeding nozzle inclined at angle of 23 to the laser beam axis. Ti-6Al-4V rectangular plates with dimensions 1801005 mm were used as substrate and this was grit-blasted and cleaned with acetone prior to the deposition process. A flexible chamber was used to isolate the deposition work space and the work space was flushed with argon, Ar, for 10 minutes prior to the start of deposition and continuously flushed during the experiment at 30 litres.Math.min.sup.1 flow rate.

[0081] It will be appreciated that the specifics of the additive manufacturing process are merely illustrative, and that any suitable additive manufacturing process may be used to form a part using a powder according to the invention.

[0082] FIG. 4 shows the top and side sections of the TiB.sub.2/Ti-6Al-4V composite bead produced by the blown powder process using laser power of 1400 W, 200 mm/min traverse speed, and powder flow rate of 10 g/min. Partially melted Ti-6Al-4V particles 5 can be seen in the composite matrix as shown in FIG. 4(a) and these particles 5 were observed to have an acicular a (transformed martensitic structure. Significant numbers of partially melted and partially transformed TiB.sub.2 particles 6 were observed in the composite bead periphery region as shown in FIG. 4(b). The dark grey TiB.sub.2 particles 6 were observed to possess light grey edges indicative of solid state transformation due to boron diffusion during processing, and tiny rods characterised as TiB whiskers were found to grow from the TiB.sub.2 particles. FIG. 5 shows a SEM image of a partially transformed TiB.sub.2 particle 6 with a light grey edge and a radial array of light grey TiB whiskers 7 growing from the particle surface in all directions into the composite matrix. The growth directions may correspond with paths along which boron atoms diffuse out of the particle surface as it experiences laser irradiation. The TiB.sub.2 particles 6 are decomposed into TiB plates or whiskers 7 either by partial or complete particle melting or solid state boron diffusion into the molten Ti pool.

[0083] Owing to the Gaussian distribution of laser beam energy, a near complete transformation of the TiB.sub.2 particles 6 present in the central region of the composite bead is to be expected, consistent with the high aspect ratio whiskers 7a that can be observed in the central region of the composite bead that is shown in FIG. 6. The central region was found to possess both short whiskers (<3 m length) and long whiskers 7a (70 m) while the majority of the whiskers were of length 40 m.

[0084] The TiB whiskers 7 were observed to be randomly oriented and interlinked in the composite bead which can be attributed to the growth of TiB needles 7 in all directions from the evenly distributed TiB.sub.2 particles 6 in the feedstock.

[0085] Some of the interwoven whiskers 7 were observed to be hollow, and these may be filled with Ti. Such filled, hollow whiskers 7 may be advantageous to improve hardness, fracture toughness and wear resistance.

[0086] A partially melted particle pull out is shown in FIG. 7, on which can be seen a basket-weave microstructure, with a tight 3D network of TiB whiskers 7, randomly interwoven. The exposure of this hemispherical pocket with a basket-weave network of TiB whiskers 7 suggests that the entire composite part consists of matrix reinforced by interwoven high aspect ratio TiB whiskers 7. The interwoven and random orientation of the TiB whiskers is again at least partly attributable to the uniform distribution of small TiB.sub.2 particles throughout the part, which have decomposed into uniformly tangled TiB whiskers 7. This random orientation is likely to increase the degree of isotropy of the part.

[0087] Referring to FIG. 8, a number of views of a second embodiment of a part according to the fifth aspect of the invention is shown. This part was produced using an additive layer manufacturing process, employing selective laser melting (using a Realizer SLM50). The SL50 is equipped with a 100 W Ytterbium-doped, fibre laser (IPG Laser), operating at 1.07 m wavelength and delivers a 15 m diameter circular spot at focus. Due to the manner in which powder is deposited in this technique (using a re-coater mechanism) flowability and packing density are important factors in ensuring good build quality which is enhanced by the method of material preparation.

[0088] This second embodiment of the fifth aspect of the invention was manufactured from feedstock powder 200 according to the first aspect of the invention. The second component 2 of the feedstock powder 200 was again a TiB.sub.2 powder with a mean particle diameter of approximately 10 m. The first component 1 was a Ti-6Al-4V powder with a particle size range of 15-45 m. Consolidated single scan vector walls were first realised and 555 mm cubes were built on a Ti-6Al-4V working platform using a cross hatching technique with a zigzag scan vector strategy.

[0089] The cubes were built on a 70 mm diameter Ti-6Al-4V platform which was maintained at 200 C. preheating temperature, in an argon flushed chamber. A maximum output laser power of 100 W was employed, and a powder bed layer thickness of 25 m was used.

[0090] FIG. 8(a) is a low magnification micrograph of the x-z plane of SLM block processed with laser power of 100 W, scanning speed of 1200 mm/min and hatch spacing of 0.2 mm, which shows a wavy morphology indicative of the cross hatching technique with the zigzag scan vector strategy employed to produce the composite block. At higher magnification (FIG. 8(c& d)), TiB whiskers 7, which were the product of TiB.sub.2 reactive decomposition, were observed in the composite matrix. The whiskers 7 were randomly oriented in the composite matrix with lower aspect ratio when compared to aspect ratio of whiskers 7 in composite obtained from blown powder process. The composite matrix is dominated by short whiskers (<10 m) with some few whiskers observed to be of length 20-25 m. The maximum length of most TiB whiskers is limited by the thickness of the successive powder layers used to build up the part. In this embodiment, the TiB whiskers were limited to up to about 25 m lengths, since, the powder slice layer thickness used was 25 m. The TiB whiskers were interlinked and it is anticipated that the whiskers would be interwoven into a basket-weave type of microstructure as observed in the blown powder composite sample shown in FIGS. 4 to 7.

[0091] The hardness values of sample parts produced by both SLM additive layer manufacturing and by blown powder additive manufacturing were assessed. Vickers hardness tests were conducted using a load of 300 gf (2.94N) and a loading time of 15 s. It was found that the beads produced by blown powder additive manufacturing onto a Ti-6Al-4V substrate (illustrated m FIGS. 4 to 7) have varying hardness, depending on the location within the bead. A top region of the bead was found to have a somewhat variable mean hardness value in the range 490-590 HV.sub.0-3. Within the bead the hardness was more uniform, being 440-480 HV.sub.0-3. A transition of hardness to values of less than 400 HV.sub.0-3 was observed as indents were made in the heat affected zone in the Ti-6Al-4V substrate under the bead. The heat affected zone was less than 0.5 mm deep. It is thought that the variable hardness in the top region may be due to particles of partially transformed TiB.sub.2.

[0092] Hardness values were evaluated in the same way for parts produced by SLM, and the mean hardness was found to vary in the range 440-503 HV.sub.0-3. Some dependence on the process parameters for both manufacturing processes were found. In the blown powder process (a part from which is shown in FIG. 8), higher hardness was associated with increasing laser power. In the SLM process, higher hardness was associated with reducing scanning speed. Both these process parameters affect the volumetric energy density received by the powder during processing, with increasing levels of volumetric energy density tending to lead to increased hardness.

[0093] FIG. 9 shows a schematic of an additive manufacturing tool 300 according to an embodiment of the third aspect of the invention. The tool 300 comprises a first powder holder 11, a second powder holder 15, a blender 30, and a dispenser and directed heat source 40.

[0094] The first powder holder 11 is for storing a powder consisting of the first particulate component. The second powder holder 15 is for storing at least one powder for use as the second particulate component, as required by the tool. In this embodiment the second powder holder 15 comprises a first container 1 and a second container 13. The first and second containers 12, 13 may be used to store different powders, so that the composition of the second component 2 of the feedstock to the blender 30 can be varied by the tool 300. The first and second powder holders 11, 15 are configured to dispense the respective powders stored therein to the blender 30. Any suitable arrangement can be used to achieve this, such as a screw type dispenser.

[0095] The powders 21, 22, 23 dispensed from the first and second powder holders 11, 15 are received by the blender 30, which is operable to blend the powders together so that they bond, so as to form a satellited powder 200. Preferably, the blender 30 may comprise means for adding a binding agent, and drying the blended and bonded powder 200. The tool 300 is operable to produce a powder 200 according to the first aspect of the invention as the output of the blender 30. The blender 30 comprises a dispenser for transferring the powder 200 to the dispenser and directed heat source 40. The directed heat source may comprise any suitable heat source, such as a laser or electron beam. The dispenser and directed heat source may be configured to deposit successive layers of the powder 200, and to produce a solid part 50 by selectively sintering or melting regions of each successive layer. The dispenser and directed heat source alternatively or additionally may be configured for blown powder additive manufacturing, in which the powder 200 is blown through a region that is heated by the directed heat source, such that the powder 200 melts and is deposited, thereby forming a part 50.

[0096] The tool 300 be operable to produce powder 200 in relatively small quantities, as required by the dispenser and directed heat source 40. The composition of the powder 200 may be readily be varied between batches, for instance allowing different layers of powder 200 to have a different composition, thereby enabling parts to be produced comprising functionally graded materials. Alternatively, the composition of the powder may be varied between producing parts, so that a material composition of each part produced by the tool 300 can conveniently be selected, without the need to procure a different feedstock powder.

[0097] The various elements of the tool 300 may be housed within a single enclosure, or may be separated into functional modules that are combined to provide the functionality of the tool 300.

[0098] Although example embodiments have been discussed in detail in relation to examples in which a titanium based MMC part is produced, the invention is not so restricted. Powders suitable for additive manufacturing comprising any combination of materials can be produced according to various embodiments of the invention.

[0099] Embodiments of the invention provide a significant enhancement to additive manufacturing processes, and overcome a number of problems in additive manufacture. For instance, enhancements of around 30% in the hardness of Ti-6Al-4V can be realised according to embodiments of the invention.

[0100] Embodiments of the invention facilitate greatly improved flexibility in additive layer manufacturing, enabling small batches of material with tailored material composition to be readily prepared, potentially in situ with the tool used to deposit the material to form a part by additive manufacturing.

[0101] Various other changes will be apparent to the skilled person. Any such variations are within the scope of the invention, as defined by the appended claims.