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Title:
POWDER DELIVERY ASSEMBLY
Document Type and Number:
WIPO Patent Application WO/2018/134605
Kind Code:
A1
Abstract:
A powder delivery assembly (12) comprising a central bore (100) with an outlet through which, in use, a heating beam, such as a laser, and optionally a flow or gas, is arranged to pass along a beam axis (18) coaxial with the central bore, and a plurality of powder delivery nozzles (22), each powder delivery nozzle (22) comprising a bore and an outlet through which bore a stream of gas-powder flows, in use, along a respective powder trajectory (102) which is coaxial with the respective powder delivery nozzle bore, wherein the powder delivery nozzles (22) are arranged such that their respective powder trajectories (102) substantially intersect (32) the beam axis (18) downstream of the central bore's (100) outlet. The angle (104) between the powder trajectories (102) and the beam axis (18) is preferably 42-degrees, which improves powder deposition and confinement at the melt pool. The shape, materials and surface of the powder delivery assembly (12) is/are configured to minimise absorption of heat radiated from the melt pool (32), and to improve, by streamlining, the flow of gases away from the melt pool (32).

Inventors:
FEARON EAMONN (GB)
Application Number:
PCT/GB2018/050154
Publication Date:
July 26, 2018
Filing Date:
January 19, 2018
Export Citation:
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Assignee:
ADVANCED LASER TECH LTD (GB)
International Classes:
B23K26/342; B23K26/14; B23K26/144; B29C64/153; B29C64/209; B33Y30/00
Domestic Patent References:
WO2000066895A22000-11-09
Foreign References:
US6046426A2000-04-04
US20060003095A12006-01-05
GB201620735A2016-12-06
Attorney, Agent or Firm:
HUTCHINSON IP LTD (GB)
Download PDF:
Claims:
CLAIMS

1. A powder delivery assembly comprising:

a central bore with an outlet through which, in use, a heating beam is arranged to pass along a beam axis coaxial with the central bore, and

a plurality of powder delivery nozzles, each powder delivery nozzle comprising a bore and an outlet through which bore a stream of gas-powder flows, in use, along a respective powder trajectory which is coaxial with the respective powder delivery nozzle bore, wherein

the powder delivery nozzles are arranged such that their respective powder trajectories substantially intersect the beam axis downstream of the central bore's outlet.

2. The powder delivery assembly of claim 1, wherein the powder delivery nozzles are arranged radially around the beam axis.

3. The powder delivery assembly of claim 2, wherein the powder delivery nozzles are equispaced radially around the beam axis.

4. The powder delivery assembly of any preceding claim, comprising four powder delivery nozzles arranged radially at 90-degrees intervals around the beam axis.

5. The powder delivery assembly of any preceding claim, wherein the axes of the powder feed nozzles' bores are arranged at, or substantially at, between 40 and 44-degrees to a plane transverse to the beam axis.

6. The powder delivery assembly of claim 5, wherein the axes of the powder feed nozzles' bores are arranged at, or substantially at, 42-degrees to a plane transverse to the beam axis.

7. The powder delivery assembly of claim 5 or claim 6, wherein bore axes of the powder delivery nozzles are arranged at an angle of between substantially 40-42 degrees to the beam axis and intersect the beam axis at a point substantially 15mm downstream the outlet of the central bore.

8. The powder delivery assembly of any preceding claim, wherein the central bore and each powder delivery nozzle's bore is between about 1.5-2mm internal diameter.

9. The powder delivery assembly of any preceding claim, further comprising a gas feed, which feeds a supply of gas through the central bore.

10. The powder delivery assembly of any preceding claim, wherein the heating beam comprises a laser beam.

11. The powder delivery assembly of any preceding claim, further comprising a powder splitter upstream of the powder delivery nozzles, the powder splitter being configured, in use, to separate a feed of gas-powder into a respective number of similar or substantially identical gas- powder flow streams, which flow into each of the powder delivery nozzles.

12. The powder delivery assembly of any preceding claim, wherein the powder delivery assembly is at least partially manufactured of, or coated with, a material that reflects thermal radiation.

13. The powder delivery assembly of claim 12, wherein the thermal radiation comprises infrared light.

14. The powder delivery assembly of any preceding claim, wherein the powder delivery assembly is at least partially manufactured of, or coated with, a material with a high thermal conductivity.

15. The powder delivery assembly of any claims 12 to 14, wherein the material comprises any one or more of the group consisting of: copper; copper alloy; aluminium; aluminium alloy; gold; and silver.

16. The powder delivery assembly of any of claims 12 to 15, wherein parts of the surface of the powder delivery assembly, which are exposed, in use, to thermal radiation are polished to a smooth and/or a mirror finish.

17. The powder delivery assembly of any preceding claim, wherein the shape of the powder delivery assembly is streamlined.

18. The powder delivery assembly of claim 17, wherein the streamlining is configured, in use, such that hot gas convected from the area heated by the heating beam is directed smoothly over and/or past the nozzle assembly.

19. The powder delivery assembly of any preceding claim, wherein the shape of the powder delivery assembly presents few or no planes perpendicular to thermal radiation emitted from a heated surface created by the heating beam.

20. The powder delivery assembly of any preceding claim, wherein the shape of the parts of the powder delivery assembly are tapered, such that the parts nearest to the intersection point have a smaller cross-section than parts further away from the intersection point.

21. The powder delivery assembly of claim 20, wherein the cross-sectional area of the components increases with distance from the intersection point.

The powder delivery assembly of any preceding claim, wherein the central bore is formed as part of a beam nozzle.

23. The powder delivery assembly of any of claims 17 to 22, wherein any one or more of the beam nozzle and powder delivery nozzles have an external surface profile, which is substantially truncated-elliptical paraboloidal.

24. The powder delivery assembly of any of claims 17 to 22, wherein any one or more of the beam nozzle and powder delivery nozzles have an external surface profile, which is substantially truncated-conical.

25. The powder delivery assembly of any preceding claim, wherein the powder delivery nozzles are supported by ribs extending from a main body part of the powder delivery assembly.

26. The powder delivery assembly of claim 24, wherein the cross-section of the ribs is streamlined.

27. The powder delivery assembly of claim 24 or claim 25, wherein the main body comprises curved surfaces extending between adjacent ribs.

28. The powder delivery assembly of any of claims 22 to 26, wherein the main body comprises curved surfaces extending between the ribs and the beam nozzle.

Description:
POWDER DELIVERY ASSEM BLY

This invention relates to a powder delivery assembly, and in particular, but without limitation, to a powder delivery assembly comprising a plurality of outlets arranged to provide converging streams of powder; and/or a thermally-improved powder delivery nozzle.

A powder delivery nozzle or assembly, such as that used in a laser-assisted blown powder deposition system, has the ability to deliver a gas-fluidised (usually by an inert gas) powder stream to a melt pool on a surface. The melt pool is typically created by a defocussed laser beam, and by adding powder to the melt pool and subsequently allowing it to solidify, it is possible to build parts or components layer-wise using techniques that are generally well-understood in the field of additive manufacturing. Blown powder additive manufacturing techniques are in widespread use nowadays for manufacturing coatings or parts out of metals.

In the blown powder deposition method, a laser/powder delivery nozzle assembly as described herein is moved relative to the surface, depositing a layer of solidified material in its wake in a manner similar to that of welding with a filler material. The height/thickness of the deposited layer thus produced typically increases with increased powder flow rate (or vice-versa) - the more material that is added per unit of time for a given traverse speed, the thicker the deposited layer. Additionally or alternatively, the height/thickness of the deposited layer thus produced typically increases with decreased relative traverse speed (or vice-versa) - the slower the assembly moves over the substrate, the longer the assembly resides over any given point, thereby increasing the amount of powder deposited, per unit length of relative traverse.

In known laser/powder delivery nozzle assemblies, there is a nozzle that delivers the flow of powder to the melt pool. Whilst the known assembly is moving at constant speed or in a constant direction, the deposition rate is relatively uniform. However, when the assembly accelerates (speeds up/slows down/changes direction), the deposition rate per unit length travelled can be affected due to finite differences in the speed/direction of the nozzle relative to laser/melt pool during such acceleration. This phenomenon can lead to increased layer thickness at corners or other non- uniformities adversely affecting the build part. Whilst these errors/variations can be designed-out in a control algorithm for the assembly, for example, by varying the flow rate or traverse speed to minimise these effects, due to the somewhat erratic an unpredictable nature of the deposition process, this is not possible in many cases.

A need therefore exists for a solution to one or more of the above problems, and/or for an improved and/or alternative laser/powder delivery nozzle assembly.

Various aspects of the invention are set forth in this disclosure.

According to an aspect of the invention, there is provided a powder delivery nozzle that can, in certain cases, achieve consistent delivery of powder per unit length traversed- independent of relative part movement.

According to another aspect of the invention, there is provided a powder delivery nozzle that can, in certain cases, control of the height of the deposited layer to achieve consistent delivery of powder per unit length traversed- independent of relative part movement.

Another aspect of the invention provides an improved powder delivery nozzle for blown powder laser additive manufacture for delivering a mixture of powder particles and carrier gas (collectively a "powder stream") to a laser-generated melt pool while dynamically controlling the amount of powder delivered to the melt pool region and hence the height of the deposited layer.

According to another aspect of the invention, there is provided a powder delivery system comprising a central aperture through which, in use, a heating beam is arranged to pass along a beam axis, and a plurality of powder delivery nozzles, each having an outlet through which a stream of powder flows along a respective powder trajectory, the powder delivery nozzles being arranged such that their respective powder trajectories substantially intersect the beam axis.

A yet further aspect of the invention provides a powder delivery assembly comprising: a central bore with an outlet through which, in use, a heating beam is arranged to pass along a beam axis coaxial with the central bore, and a plurality of powder delivery nozzles, each powder delivery nozzle comprising a bore and an outlet through which bore a stream of gas-powder flows, in use, along a respective powder trajectory which is coaxial with the respective powder delivery nozzle bore, wherein the powder delivery nozzles are arranged such that their respective powder trajectories substantially intersect the beam axis downstream of the central bore's outlet.

The powder delivery system or assembly of the invention may incorporate various improvements over existing powder delivery assemblies, such as: an improved angle for introduction of the powder streams to the melt pool, design features to reduce heating of the nozzle from the melt pool and hence improve powder stream geometry and associated laser coupling efficiencies.

Suitably, the powder delivery nozzles are equispaced around the central aperture, that is to say, with substantially equal angular separations about the beam axis.

Suitably, three or more powder delivery nozzles are provided. Most preferably, there are four powder delivery nozzles arranged at 90-degrees around the beam axis.

The powder feed nozzles are suitably arranged at, or substantially at, between 40 and 44- degrees to a plane transverse to the beam axis. Preferably, the powder feed nozzles are arranged at, or substantially at, 42-degrees to a plane transverse to the beam axis. Surprisingly, it has been found that arranging the powder feed nozzles at 42-degrees to a plane transverse to the beam axis, the process can be optimised. In particular, it has been found that a 42-degree incident angle causes the powder to impinge on the melt pool at an angle which allows the gas being fed along the laser beam axis to compress the melt pool redirect divergent lower velocity powder particles comprising the upper part of each powder stream back into the main body of each powder stream, both increasing the density and increasing the definition of the upper boundary of the combined powder streams at their intersection with the melt pool, and thus height-limiting the resultant layer produced it. This means that the laser is able to heat the melt pool more efficiently, and because the beam-axis gas flow assists in constraining the upper vertical divergence of powder streams powered in the melt pool, rather than having to reply on relatively high opposing gas flows from the powder delivery nozzles themselves, the gas flow rates through the powered delivery nozzles can be reduced also. In practice, this can lead to a reduction in laser power from, say 1.2kW (in a conventional system) to about 300- 400W (in the invention), and a reduction in gas flow from about 0.75 l/s to about 0.1-0.2 l/s - whilst obtaining the same, or substantially the same melt pool characteristics. The use of such a low incident angle (i.e. ~42-degrres, as opposed to >45-degrees in known systems) is contraindicated: accepted wisdom in the art suggests that higher incident angles (>45-degrees) lead to more compression of the powder down onto and into the melt pool and reduces powder wastage. However, by reducing the incident angle, the invention is able to utilise the effect of the axial gas flow to achieve similar or better melt pool powder stream containment, and because the interaction zone between the laser and the melt pool is thus significantly reduced in volume, further reducing the attenuation of the incident laser beam is minimised compared to those systems feeding powder at higher incident angles. Further, this design results in most of those powder particles which have not been incorporated into the melt pool also bypassing the beam itself, allowing recovery and reuse of those powder particles.

Most preferably, a plurality of tubes or pipes of typical bore 1.5-2mm, carrying a "powder stream" arranged with radial symmetry around the beam axis are provided, which have their long axes at an angle of 40-42 degrees to the radial plane such that these long axes converge to the beam axis at a point typically 15mm from the opening of central aperture.

The heating beam suitably comprises a laser beam.

A possible advantage of the invention is that because the powder is fed into the heating (laser) beam from different angles, the powder delivery assembly is effectively rendered more omnidirectional. In other words, the powder delivery assembly of the invention can be less sensitive, or insensitive to the traverse direction/acceleration with regard to powder delivery consistency than known powder delivery nozzles.

Suitably, and to assist in achieving this end, a powder splitter is suitably provided upstream of the nozzles to separate a feed of powder into a respective number of similar or substantially identical flow streams. An example of such a splitter is described in our co-pending patent application: GB 1620735.9 (6 December 2016). Suitably, the powder delivery assembly comprises design features that minimise the effects (upon both the nozzle and the process) of incident thermal radiation and convected hot gases emitted from the laser-heated melt-pool. These design features, include, but are not limited to, any one or more of the group comprising:

At least partially manufacturing the nozzle of, or coating it with, a material that reflects thermal radiation, such as infrared light. The thermal radiation/infrared reflective material, where provided, may be copper or a copper alloy; aluminium or aluminium alloy or other materials with relativity high reflectivity to infrared. Additionally or alternatively, parts of the surface of the nozzle which are exposed to thermal radiation, in use, may be polished, for example, to a smooth and/or mirror finish so as to facilitate reflection of thermal radiation.

The body of the system being designed such that hot gas convected from the area heated by the laser is directed past the nozzle body due to streamlining. Streamlining in this way may allow the hot gasses produced, in use, by the melt pool (i.e. "convective gas streams") to pass freely around and over the nozzle, thus reducing or minimising convective heat transfer into the nozzle. Streamlining may also have the advantage of reducing or preventing derangement of the powder streams incident to the melt pool as a result of these convective gas streams;

Designing the nozzle such that no plane, or as few as possible planes, of the nozzle body is/are perpendicular to thermal radiation emitted spherically/hemi-spherically (i.e. radially outwardly in any direction) from a melt pool surface created by, for example, a laser beam passing through the system;

Manufacturing the nozzle from, and/or coating the nozzle with, a material with high thermal conductivity. The high thermal conductivity material may be copper or its alloys, aluminium or its alloys or other materials chosen for high thermal conductivity; and

The nozzle being manufactured such that the components of the nozzle closest to the melt pool are minimised in cross-section. In addition, each component can increase in cross-section and material bulk in the direction of heat flow away from that part closest to the melt pool, in order to facilitate rapid cooling. An exemplary embodiment of the invention is shown in the accompanying drawings in which:

Figure 1 is a side view of a powder feed comprising a powder delivery assembly in accordance with the invention;

Figure 2 is a view from below of the powder feed of Figure 1; and

Figure 3 is a cross-section of Figure 2;

Figures 4 and 5 are views of the powder feed showing its streamlined design for improved bypass of hot gas convective flow and presentation of oblique surfaces to incident thermal radiation;

Figure 6 is a side view showing different areas of the powder feed;

Figures 7, 8 and 9 are, respectively, perspective, side and bottom views of an alternative powder feed assembly in accordance with the invention;

Figure 9 is an illustration of the effect of the streamlining of the powder delivery assembly shown in Figures 1 to 6;

Figure 10 is an illustration of the effect of the streamlining of the powder delivery assembly shown in Figures 7 to 9; and

Figure 11 is an illustration of the radiative heating effects on a powder delivery assembly shown in Figures 7 to 9.

Referring to Figures 1 to 6 the drawings, a powder feed 10 comprises a powder delivery assembly 12 according to the invention.

In Figure 1, the powder feed 10 is mounted substantially vertically and has a central tube 14 through which, in use, a focussed laser beam 16 passes. The laser beam 16 has a beam axis 18, which passes through an aperture 20 at the tip of the central tube 14.

The powder delivery assembly 12 is formed by the central tube 14, and four radially symmetric powder feed nozzles 22, which have a bore diameter of about 1.5mm to 2mm. This particular dimension has been usefully found to minimise the width of the powder stream emitted from each of the nozzles 22, and hence increase catchment efficiency for relatively small melt pool diameters. The powder feed nozzles 22 are arranged at 42 degrees to a plane transverse to the beam axis 18. The (substantially) 42-degree angle has been found, empirically, to provide optimum results in limiting the layer thickness formed by the system.

The powder feed nozzles 22 are fed, via powder feed tubes 26, which connect to respective outlets of a 4-way powder splitter 28, by a stream of powder-gas mixture, which has a laminar-flow powder gas mix fed through them. This results in a gas/powder stream delivered by each nozzle 22 along respective powder stream trajectories 30, which intersect the beam axis 18. The intersection point 32 is roughly, in use, the location of the melt pool formed by the system.

As the powder-gas stream exits each respective powder nozzle 22, the carrier gas expands into the surrounding atmosphere, and, due to carrier gas stratification of differently-sized powder particles (in addition to the Magnus effect for irregular particles), powder flaring from the top of the powder stream consists of smaller particles than the bulk flow. This, coupled with the fact that particles traveling off-axis from the main stream (in the laminar flow regime), necessarily have lower particle velocities, results in the fact that powder flaring from the top of the powder stream has a lower momentum than that in the main part of the powder stream.

An inert gas, such as argon, is fed through the orifice 20 (in this case, having a 2mm diameter) at the tip of the central tube 14, which is, of course, coaxial with the laser beam. The inert gas, upon exiting the orifice 20, diverges due to gas expansion, and this inert gas stream is thus directed towards the area where the powder-gas streams emitted from the powder feed nozzles 22 will intersect 32 at the position of the melt pool.

The diverging inert gas flow referred to in the preceding paragraph is used to redirect the off- axis powder particles back into the main powder stream.

A 'balance point' between the powder/gas mix flow rates and the coaxial gas flow rate can be obtained (dependent upon the density and size distribution of the powder), which will result in a refinement of the top boundary of the individual powder streams as they combine into a powder 'cloud'. In order to achieve this, it is often necessary to use a powder feeder which is capable of varying the assist gas flow rate independently of the powder mass flow rate and a powder stream splitter assembly to divide the output of the powder feeder into four equal gas/powder streams.

The refinement of the top of the powder cloud boundary causes an abrupt reduction in powder catchment efficiency in a vertical plane at a fixed position relative to the powder feed nozzle 30. In other words, a deposited layer will build to the top of this interface, but cannot build any higher due to a lack of available powder. This means that the height of the deposited layer is limited by the physical position of the powder feed nozzle and not by the other deposition parameters, allowing the layer height to be controlled by the programmed incremental step height per layer rather than vice versa.

This type of powder delivery nozzle is therefore referred to as a "layer height-restricting powder delivery nozzle" and was first described in Fearon E, 'Laser Free Form Fabrication applied to the manufacture of metallic components' PhD thesis, University of Liverpool 2002.

In this disclosure, the "layer height-restricting nozzle" is modified slightly, such that no plane of the nozzle body 40 is directly incident to any thermal radiation which may be emitted in a hemispherical manner from the melt pool surface created by the laser beam passing through the nozzle 10.

This measure of avoidance of absorption of incident radiation is enhanced by making the nozzle of, or coating it with, an infrared reflective material such as copper or its alloys, aluminium or its alloys or other materials with relativity high reflectivity to infrared.

Additionally or alternatively, the body of the nozzle may be designed such that hot gas convected from the area heated by the laser is directed past the nozzle body due to streamlining, as indicated by 42 in the Figures. Possible effects upon the nozzle and process of these design features are such that:

Radiative or convective heating of the nozzle and therefore its cooling requirements are reduced in comparison to a powder feed nozzle of this class without such refinements. This allows more efficient temperature stabilization of the nozzle using water cooling methods Both the powder cloud produced by the confluence of individual low velocity powder streams and the coaxial gas flow described as a function of the basic nozzle design are affected by temperature changes to the surrounding environment. Derangement of these gas flows reduces the effectiveness of both the layer height restriction mechanism and deviates a proportion of particles in the powder streams away from the centre of the melt pool.

Temperature stabilization of the nozzle system described above has corresponding benefits to the stabilization of the powder cloud. Stabilization of the powder cloud maintains both consistency of layer height control by the method described above and the proportion of powder stream directed to the centre of the melt pool.

Referring now to Figures 7 to 9 of the drawings, an alternative embodiment of the previously- described powder delivery assembly is shown: noting that only the "nozzle" part is shown for clarity, and not the powder feed system as well, as in some of the previous Figures.

The powder delivery assembly shown in Figures 7 to 9 has essentially the same configuration as that described previously, albeit with some refinements. For ease of reference, identical features to those described previously have been called out in Figures 7 to 9 with the same reference signs as the corresponding features of Figures 1 to 6.

The powder delivery assembly 12 has a central nozzle 100 and four powder delivery nozzles 22 arranged radially at 90-degree intervals around it. Each nozzle 100, 22 is part-conical with its narrow tip end pointing towards the melt pool (not shown) or intersection point 32. The tip 110 of each nozzle 100, 22 is rounded so as to present a rounded surface to the radiated heat emitted from the intersection point 32, thus reflecting radiated heat away; and also streamline the nozzles 100, 22 relative to gases moving away from the intersection point 32.

The powder delivery nozzles 22 each have a central bore whose axes 102 intersect the beam axis 18 at the intersection point. The angle 104 between the delivery nozzles' central bore axes 114 and a normal to the beam axis 18 is 42-degrees - for the reasons previously stated. Each powder delivery nozzle 22 is supported on a respective rib 106, whose cross-sections are part-elliptical so as to present a rounded surface to the radiated heat emitted from the intersection point 32, thus reflecting radiated heat away; and also streamline the ribs 106 relative to gases moving away from the intersection point 32.

The ribs 104 are formed integrally with a main body part 106 of the powder delivery nozzle

12, which has a rounded profile 108 which tapers lengthwise to merges smoothly into the form of the central nozzle 100, and laterally between the ribs 106. This configuration also presents a rounded surface to the radiated heat emitted from the intersection point 32, thus reflecting radiated heat away; and also streamlines the main body 106 and the ribs 106 relative to gases moving away from the intersection point 32.

The effect of the shape of the powder delivery assembly 12 can be seen in Figures 10 to 12 of the drawings. Figures 10 and 11 show streamlines for gases moving away from a region heated by the laser beam, namely, from the intersection point 32 previously described, for the powder delivery assembly shown in Figures 1 to 6, and in Figures 7 to 9, respectively. As can be seen, as the surface is heated at the intersection point 32, hot gases flow away from that point 32. Due to the streamlining of the powder delivery assembly, the gasses are able to flow smoothly around and over the various parts of the assembly 12 without creating eddies or turbulent flow. This not only facilitates getting rid of hot gas away from the melt pool (as there are no eddies or reverse-flow effects to impede movement of gas away from the melt pool), but also the smooth gas flow is able to more evenly distribute its heat, thereby avoiding "hot spots" being formed on the surface, or in the bulk of, the powder delivery assembly 12.

Finally, referring to Figure 13 of the drawings, this shows the heat profile of the powder delivery assembly 12 caused by radiated heat from the melt pool 32. Because the powder delivery assembly 12 of the invention does not have surfaces that are normal/perpendicular to the intersection point 32, radiated heat is more effective reflected off the surfaces of the powder delivery assembly and away from the melt pool 32. The improved thermal and powder delivery characteristics of the invention mean that much lower laser powers can be used to obtain the same or similar effects to known powder delivery nozzles. Also, as the powder delivery assembly of the invention is inherently less susceptible to being heated convectively and/or radiatively, the cooling requirements for the invention are much reduced, compared with known powder delivery assemblies.

The invention is not restricted to the details of the described embodiment, which is exemplary only, and that various modifications to it are possible without departing from the scope of the invention.




 
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