WO2008003942A2

WO2008003942A2 - Powder delivery apparatus

Info

Publication number: WO2008003942A2
Application number: PCT/GB2007/002465
Authority: WO
Inventors: Lin Li; Wei Wang; Waheed Ul Haq Syed
Original assignee: University of Manchester
Current assignee: University of Manchester
Priority date: 2006-07-01
Filing date: 2007-07-02
Publication date: 2008-01-10
Anticipated expiration: 2009-01-01
Also published as: WO2008003942A3; GB0613180D0

Abstract

A direct radiation deposition apparatus comprising a feed conduit and a nozzle arranged to deliver powder to a substrate, and a radiation source arranged to simultaneously direct a radiation beam onto the substrate and thereby melt the powder, wherein the apparatus further comprises an actuator arranged to apply a vibration to the nozzle which causes powder to fall from the nozzle onto the substrate.

Description

Powder Delivery

The present invention relates to powder delivery, for example for use in direct laser deposition and laser surface cladding or alloying.

Direct laser deposition (DLD) is a well known technique which may be used for example to form 3 dimensional metal components. Powdered metal is delivered onto a substrate upon which a high power laser beam is incident. The powdered metal melts to form a pool of molten metal known as a melt pool. The substrate is moved relative to the laser beam spot along a predetermined path, whilst powdered metal continues to be delivered onto the substrate. Due to the movement of the substrate, a trail of melted metal is formed which subsequently cools and solidifies. The predetermined path along which the substrate is moved is arranged such that the trail of solidified material builds a desired component through multiple layer depositions.

Direct laser deposition may be used for example to form components of aircraft or other vehicles. Laser surface cladding refers to laser deposition of a single layer or two layers of material over a substrate surface, with minimum dilution or contamination by the substrate material. Laser surface cladding is used to modify the surface properties of the substrate. Laser surface alloying refers to the modification of surface properties of a substrate, by mixing an applied material with the substrate to form a new material that is different from the substrate and the applied material.

Powder is delivered onto the laser beam spot by a powder feeder through a conduit or conduits and a set of nozzles. The powder is carried to the nozzles, and through the nozzles, by a carrier gas. This technique is known as blown powder DLD. The pressure of the carrier gas and the diameters of the nozzles may be selected to provide a desired rate of powder delivery. A problem associated with blown powder DLD is that a significant proportion of powder which passes through the nozzles is not delivered to the laser beam spot. This powder does not melt, and does not contribute to building of the component. The powder can be expensive, especially for applications using special alloys, and wastage of the powder in this way adds significantly to the cost of building components using direct laser deposition. In addition, the work environment when using the blown powder DLD technique is messy, especially when the powder size is small or part of the powder size is small (for example < 30 μm). A further problem caused by the blown powder DLD technique is that un-melted or semi-melted powders can stick to the surface of a deposited track and cause the roughening of the component. A subsequent surface finishing process is thus normally required.

It is an object of the present invention to overcome or mitigate at least one of the above disadvantages.

According to a first aspect of the invention there is provided a direct radiation deposition apparatus comprising a feed conduit and a nozzle arranged to deliver powder to a substrate, and a radiation source arranged to simultaneously direct a radiation beam onto the substrate and thereby melt the powder, wherein the apparatus further comprises an actuator arranged to apply a vibration to the nozzle which causes powder to fall from the nozzle onto the substrate.

Since a carrier gas is not used to deliver the powder, problems associated with the use of the carrier gas do not arise.

Preferably, the apparatus further comprises a pressure isolation apparatus, arranged such that in use it provides some isolation of the nozzle from pressure arising from the weight of powder to be delivered to the substrate.

Preferably, the degree of pressure isolation provided by the pressure isolation apparatus is sufficient that jamming of powder in the nozzle does not occur when vibration is being applied to the nozzle.

The pressure isolation apparatus may comprise a chamber which is located upstream of the nozzle, and a powder delivery conduit which is arranged to deliver powder to the chamber, the powder delivery conduit and the chamber being configured such that in use the volume of powder present in the chamber remains substantially constant.

Preferably, the powder delivery conduit extends into the chamber, such that a powder delivering opening at a lower end of the powder delivery conduit defines a self-regulating level for the powder.

Preferably, the powder delivery conduit comprises an aperture. Preferably, the apparatus further comprises a member which is moveable into and out of contact with an opening at the bottom of the chamber, thereby defining the nozzle from which the powder falls onto the substrate.

The pressure isolation apparatus may comprise an aperture provided in the feed conduit.

Preferably, the diameter of the aperture is such that powder passes freely through the aperture when the vibration is applied to the nozzle and, and does not pass through the aperture when the vibration is switched off.

Preferably, the diameter of the aperture is between 1 and 6 mm.

The pressure isolation apparatus may comprise providing at least part of the feed conduit at an angle relative to the vertical.

The pressure isolation apparatus may comprise an offset of the axis of the feed conduit relative to an entrance of the nozzle, such that pressure exerted by powder in the feed conduit is not directed vertically downwards onto the entrance of the nozzle.

Preferably, an entrance of the nozzle is sloped, the slope having an angle relative to the horizontal which is such that pressure transmitted to the nozzle entrance by powder in the feed conduit is not sufficient to cause jamming of the powder at the nozzle entrance.

Preferably, an entrance of the nozzle tapers outwardly.

Preferably, the actuator is arranged to apply an ultrasonic vibration to the nozzle.

Preferably, the apparatus comprises two or more nozzles.

The radiation source may be an electron beam source and the radiation beam an electron beam.

The radiation source may be a laser, and the radiation beam a laser beam.

The apparatus may include optics arranged to direct the laser beam perpendicularly onto the substrate, and the one or more nozzles are arranged to direct powder onto the substrate in a non- vertical direction.

The apparatus may include optics arranged to direct the laser beam onto the substrate in a non-perpendicular direction, and the nozzle is arranged to direct powder onto the substrate in a vertical direction, the optics being arranged to provide the laser beam with a non-circular cross-section, such that the beam spot formed by the laser on the substrate is circular.

The laser may be one of a plurality of lasers, arranged to generate laser beams which are directed onto the substrate from different directions but with substantially the same angle of incidence.

The nozzle may be located in a vacuum chamber.

More than one pressure apparatus may be combined in a single direct radiation deposition apparatus.

According to a second aspect of the invention there is provided a powder delivery apparatus comprising a nozzle arranged to deliver powder and a feed conduit arranged to deliver powder to the nozzle, wherein the apparatus further comprises a pressure isolation apparatus which in use provides some isolation of the nozzle from pressure arising from the weight of powder to be delivered by the apparatus.

Preferably, the apparatus further comprises a member which is moveable into and out of contact with an opening at the bottom of the chamber, thereby defining the nozzle from which the powder is delivered.

Preferably, the diameter of the aperture is such that powder passes freely through the aperture when the vibration is applied to the nozzle and, and does not pass through the aperture when the vibration is switched off. The pressure isolation apparatus may comprise an offset of the axis of the feed conduit relative to an entrance of the nozzle, such that pressure exerted by powder in the feed conduit is not directed vertically downwards onto the entrance of the nozzle.

More than one pressure apparatus may be combined in a single powder delivery apparatus.

According to a third aspect of the invention there is provided a method of direct radiation deposition comprising directing a radiation beam onto a substrate, and simultaneously delivering powder to the location at which the radiation beam is incident upon the substrate such that the powder is melted by the radiation beam, wherein the powder is delivered to the substrate from a nozzle by applying a vibration to the nozzle such that the powder falls from the nozzle onto the substrate.

The pressure isolation apparatus may comprise a chamber which is located upstream of the nozzle, and a powder delivery conduit which is arranged to deliver powder to the chamber, the volume of powder present in the chamber remaining substantially constant.

The nozzle and the substrate may be in a vacuum.

The radiation source is may be an electron beam source, and the radiation beam an electron beam.

Preferably, the flow of powder onto the substrate is interrupted by interrupting the vibration of the nozzle. Specific embodiments of the invention will now be described by way of example only, with reference to the accompanying figures, in which:

Figure 1 is a schematic cross-sectional view of a direct laser deposition apparatus which embodies the invention;

Figure 2 is a more detailed view of a powder feeding head of the apparatus of figure 1;

Figure 3 is a schematic cross-sectional view of an alternative embodiment of the invention;

Figure 4 is a schematic cross-sectional view of a further alternative embodiment of the invention;

Figure 5 is a more detailed view of part of the apparatus of figure 4; and

Figures 6 to 9 are more detailed views of the powder feeding head of the apparatus of figure 3.

Figure 1 shows schematically in cross-section a direct laser deposition apparatus which embodies the invention. The apparatus comprises a powder hopper 2 arranged to feed powder into a funnel 4 which in turn delivers the powder onto a substrate 6. The apparatus further comprises a laser 8 arranged to generate a radiation beam 9 which is incident upon the substrate 6 at the same location as the powder. The laser 8 may for example be a high power diode laser, CO₂ laser, a fibre laser or some other laser capable of generating a high power radiation beam 9 (the power of the radiation beam should be sufficiently high that it can melt the powder). The laser may operate in a pulsed mode or a continuous wave mode.

The powder hopper 2, funnel 4 and associated components are hereafter referred to collectively as a powder feeding head.

The powder hopper 2 is filled with a powder which is suitable for direct laser deposition. Examples of powders which could be used include 316L steel powder, copper, titanium alloy or nickel alloy powder, although it will be appreciated that other suitable powders may be used. The powder hopper 2 is provided with an opening 10 at its centre, the opening being positioned to allow the beam of radiation 9 to pass uninterrupted through the powder hopper. Powder delivery conduits 12 extend downwardly from the powder hopper 2. Although only two powder delivery conduits 12 are shown in figure 1, three or four powder delivery conduits are provided. The powder delivery conduits are equally spaced around the funnel 4. The powder delivery conduits 12 may have a constant diameter, as shown. Alternatively, they may be tapered or may be provided with apertures. The apertures may be dimensioned so as to control the flow of powder, for example using a configuration equivalent to that described below in relation to figure 5.

A piezoelectric actuator 16 is connected via a connector 18 to the powder hopper 2. Vibrations generated by the piezoelectric actuator 16 cause the powder hopper 2 to vibrate, and also cause each of the powder delivery conduits 12 to vibrate.

The piezoelectric actuator 16 is powered by a high voltage amplifier 22 which is connected to a frequency generator 24. The frequency generator 24 generates a waveform which is amplified by the high voltage amplifier 22 and is passed to the piezoelectric actuator 16, so that the piezoelectric actuator moves through a motion which corresponds with the waveform. In this particular instance the movement is in the direction indicated by the double headed arrow 26. However, the movement may be in other directions.

An oscilloscope 28 may be used to monitor the waveform generated by the frequency generator 24. The frequency generator may be controlled by a microprocessor (not shown). The oscilloscope 28 may be replaced with any other waveform indicator, for example a display device of a computer which incorporates the microprocessor.

The funnel 4 has a gradient which is sufficiently steep, and a surface which has sufficiently low friction, that powder which is incident upon it does not adhere to the funnel but instead passes downwards and out of the funnel and onto the substrate 6.

Optics 32 are positioned between the laser 8 and the substrate 6, and are arranged to direct the laser beam 9 onto the substrate with a predetermined beam spot size. This may be achieved for example by arranging that the beam waist of the laser beam 9 coincides with the substrate 6. Alternatively, the optics 32 may be arranged such that the beam spot size is larger than the beam waist. Although the optics 32 are shown as a pair of convex lenses, this is intended to be schematic and it will be appreciated that any suitable optics may be used. A tube 35 extends through the middle of the powder feeding head. The tube allows the laser beam to pass unimpeded onto the substrate 6, and is hereafter referred to as the beam tube 35. The beam tube 35 includes an opening 37 at its lowermost end which is sufficiently big to allow the laser beam 9 to pass unimpeded. The beam tube is tapered at its lowermost end, and is moveable in the z-direction (e.g. vertically). The beam tube 35 can thus be brought into contact with the funnel 4, thereby preventing powder from flowing out of the funnel. Movement of the beam tube 35 in the z-direction provides control of the size of a gap between the beam tube and the funnel 4. This gap is a nozzle through which powder is delivered to the substrate. The powder delivery conduits 12 and the funnel 4 may together be considered to be a feed conduit, which feeds powder to the nozzle.

In use, the powder hopper 2 is filled with a powder 3 which is to be used for direct laser deposition, for example 316L steel powder, or inconel powder. The laser 8 is switched on so that the laser beam 9 is incident upon the substrate 6. The frequency generator 24 and amplifier 22 are switched on such that the piezoelectric actuator 16 vibrates. This causes powder to fall from the powder delivery conduits 12, and pass down the funnel 4 onto the substrate 6.

The rate at which the powder falls from the powder delivery conduits 12 is greater than the rate at which powder is delivered to the substrate 6 from the funnel 4. This means that once the frequency generator 24 and amplifier 22 have been switched on, the level of powder fed into the funnel 4 from powder delivery conduits 12 rises quickly up to the bottom of the powder delivery conduits 12 (as shown in figure 1). The level of powder does not extend beyond the bottom surface of powder delivery conduits 12, since lowermost ends of the powder delivery conduits 12 are blocked by the powder when the powder level is up to the nozzle opening. The level of powder in the funnel 4 is thus self-regulating. The part of the funnel 4 within which the powder resides may be thought of as a chamber, which holds the powder prior to it passing through the nozzle defined at the bottom of the funnel. This chamber helps to provide a steady and continuous flow of powder from the funnel 4 onto the substrate.

The chamber and the powder delivery conduits may together be considered to be an example of a pressure isolation apparatus, since they provide some isolation of the nozzle from pressure arising from powder in the hopper 2. The powder delivery conduits are dimensioned to ensure that the amount of pressure exerted on the powder in the funnel 4 is not sufficient to cause jamming of powder in the funnel 4 to occur. The powder delivery conduits are sufficiently narrow that the majority of the pressure generated by the weight of the powder in the hopper 2 is not transmitted down through the powder delivery conduits to the powder in the funnel 4. In addition, the weight of powder present in the powder delivery conduits is substantially lower than the weight of the powder in the hopper 2. The powder delivery conduits may for example have an inner diameter of between 3.5 and 4mm (this is effective for a range of powders having particle sizes over a range of at least 50-150 micrometers). Powder delivery conduits having an inner diameter of between 3.5 and 4mm are effective for all powder particle sizes that are used in commercial laser deposition processes. It may also be possible to dispense particles which have particle sizes less than 50 micrometers (for example as little as 10 micrometers) or more than 150 micrometers (for example as much as 300 microns) using powder delivery conduits with an inner diameter of between 3.5 and 4mm.

Since the funnel 4 has sloping sides, the majority of pressure arising from the weight of powder in the funnel is exerted on sidewalls of the funnel. Some pressure pushes the powder downwards and out of the funnel.

The mechanism via which the powder is delivered from the hopper 2 to the substrate may be considered to be a combination of vibration and gravity.

Upon being incident on the substrate 6 the powder is heated by the laser beam 9 and is thereby melted. The substrate is moved beneath the funnel 4 and laser beam 9, such that a trail of melted material is formed which subsequently cools and solidifies. The movement of the substrate 6 is arranged such that the trail of solidified material builds a layer of a desired component. The component may for example be a metal component, formed using metallic powder.

During building of the component it may be desired to interrupt the supply of powder, for example if the component does not require material at a particular location. This is done by interrupting the signal to piezoelectric actuator 16, and thereby interrupting the vibration of the powder feeding head. This stops the flow of powder from the powder delivery conduits 12, and thereby stops the flow of powder onto the substrate 6. As previously mentioned, movement of the beam tube 35 in the z-direction provides control of the size of a gap between the beam tube and the funnel 4. Increasing the size of this gap will increase the rate of delivery of powder to the substrate 6, whereas reducing the size of the gap will decrease the rate of delivery. Therefore, the rate of delivery of powder to the substrate may be controlled by controlling the position of the beam tube 35.

The frequency generator 24 may be arranged to generate a waveform at a frequency which is ultrasonic, so that the piezoelectric actuator 16 vibrates at an ultrasonic frequency. The term ultrasonic frequency is generally understood to mean frequencies of 18kHz or higher. The frequency generator 24 may be configured to generate a waveform at a sonic frequency, so that the piezoelectric actuator 16 vibrates at a sonic frequency. However such vibration may be noisy, and an ultrasonic frequency may be preferred in order to minimise noise.

A stream of inert gas may be directed onto the substrate in a ring which surrounds the laser beam spot. This helps to keep the powder in the vicinity of the laser beam spot, and to prevent unwanted contamination (e.g. dust particles) from entering the laser beam spot. The gas is not carrier gas which carries the powder onto the substrate.

Water cooling may be used to cool the optics 32.

A more detailed illustration of the powder hopper 2, funnel 4 and associated components (referred to hereafter as a powder feeding head) is shown in figure 2. Figure 2a is a perspective view of the powder feeding head, and figure 2b is a partial cross-section of the powder feeding head.

The powder hopper 2 is connected to four powder delivery conduits 12 (only two of which are visible). Lowermost ends of the powder delivery conduits 12 open into the funnel 4. Powder (not shown) may thus pass from the powder hopper 2, through the powder delivery conduits 12, into the funnel 4, and from a bottom end of the funnel onto a substrate (not shown).

The beam tube 35 is provided within the powder feeding head. Optics (not shown) may be provided within the beam tube 35, or may be provided some distance above the protective screen. A lowermost end 40 of the beam tube had a frusto- conical shape which is provided with an opening 37. The beam tube 35 is connected to the hopper 2 via a thread 48 which allows the beam tube 35 to be screwed up or down vertically. This allows the gap between the beam tube 35 and funnel 4 to be adjusted by screwing the beam tube 35 up or down.

The piezo electric actuator 16 is connected to an outer wall of the powder feeding head. The vibration of the piezo electric actuator 16 thereby causes the entire powder head to vibrate.

An additional funnel 42 is provided outside of the funnel 4 (referred to here as the powder feeding funnel 4). This additional funnel is referred to here as the gas delivery funnel 42. The gas delivery funnel 42 defines a conical space 44 between the gas delivery funnel and the powder feeding funnel 4, through which gas may flow. Gas ports 46 are provided at an upper end of the gas delivery funnel, allowing gas to be passed into the conical space 44 such that it flows downwards through the space and out of an annular opening at the bottom of the gas delivery funnel 42. This opening is adjacent to the point at which the powder leaves the powder feeding funnel 4.

A locking ring 47 is used to connect the gas delivery funnel 42 to the powder feeding head.

In use, the powder hopper 2 is filled with the powder which is to be used for direct laser deposition. Powder passes downwards through the powder hopper 2 into the powder delivery conduits 12. The powder travels downwards along the powder delivery conduits 12 and out of openings at the bottom of the powder delivery conduits into the powder feeding funnel 4. Provided that a gap is present between the beam tube 35 and the funnel 4, the powder then passes downwards through the funnel and out of the bottom of the funnel, falling as a stream of powder onto a substrate. Gas provided from the gap 44 is directed towards the substrate, forming a ring of gas on the substrate which surrounds the delivered powder. This helps to keep powder focussed in the vicinity of a laser beam spot during direct laser deposition, and helps to prevent unwanted contamination (e.g. dust particles) from entering the laser beam spot. In addition, the gas (which is inert) prevents the melted powder from being oxidized. An alternative embodiment of the invention is illustrated schematically in Figure 3. Parts of the embodiment of the invention shown in Figure 3 correspond with the embodiment shown in Figure 1. Where this is the case, corresponding reference numerals will be used.

Referring to Figure 3, a direct laser deposition apparatus according to an alternative embodiment of the invention comprises a plurality of feed conduits 50 and nozzles 52 arranged to direct powder onto a particular location on a substrate 6, and a laser 8 arranged to direct a laser beam onto the same location. The feed conduits 50 are connected to a powder hopper (not. shown). The feed conduits 50 are connected via a connector 18 to a piezoelectric actuator 16. Although the connector 18 is shown as only being connected to the nearest feed conduit 50, it is in fact connected to the other feed conduit shown in the figure (this connection is not shown in order to avoid complicating the figure). A frequency generator 24 generates a waveform which is amplified by an amplifier 22 and passes to the piezoelectric actuator 16, thereby causing the piezoelectric actuator to vibrate. An oscilloscope 28 or other display device may be used to display the waveform applied to the piezoelectric actuator 16. Optics 32 are used to condition the laser beam 9 prior to it being incident upon the substrate 6. A screen 34 is provided around the optics 32.

In use, a vibration is applied to the feed conduits 50 by the piezoelectric actuator 16, and this causes powder to fall from the nozzles 52 onto the substrate 6. It is a combination of the vibration of the nozzles 52 and gravity which causes the powder to fall from the nozzles. The substrate 6 is moved in order to form a trail of melted material which subsequently cools and solidifies, thereby building a desired component.

The embodiment of the invention shown in Figure 3 differs from that shown in Figure 1 primarily in that the nozzles 52 are used to pass powder directly onto the substrate 6 rather than passing the powder via a funnel 4. An advantage of doing this is that it allows the supply of powder to the substrate 6 to be interrupted easily. In order to interrupt the supply of powder to the substrate 6, the vibration of the piezoelectric actuator 16 is stopped. The supply of powder to the substrate 6 is thereby immediately interrupted (powder will not flow from the nozzles 52 in the absence of vibration). The nozzles 52 are disposed at an angle relative to the substrate 6. The laser beam 9 is perpendicular to the substrate 6. Since the feed conduits 50 are arranged at an angle relative to the substrate 6, the powder delivered by a given feed conduit may not be delivered in a manner which allows omni-directional operation of the apparatus. The term omni-directional is intended to mean that the build up of material will take place at the same rate, irrespective of the direction of movement of the substrate 6. In other words, in terms of the Cartesian coordinates marked on Figure 3, movement of the substrate in the x-direction should cause the same amount of material to be built up on the substrate as movement of the substrate in the y- direction.

Providing the apparatus with a plurality of nozzles 52, and associated feed conduits 50, helps to provide omni-directional operation. Although only two feed conduits 50 are shown in Figure 3, it will be appreciated that more feed conduits may be used. For example, in addition to the illustrated feed conduits 50 which lie in the xz plane, feed conduits may also be provided which lie in the yz plane. Where this is done, the powder is incident upon the substrate 6 in the same manner, irrespective of whether the substrate is being moved in the x or y directions (in the positive or negative direction).

A further alternative embodiment of the invention is shown schematically in Figure 4. Components of Figure 4 which correspond to those shown in Figures 1 and 3 are given corresponding reference numerals.

Referring to Figure 4, a direct laser deposition apparatus comprises a feed conduit 60 and nozzle 62 arranged to direct powder onto a particular location on a substrate 6. The apparatus further comprises a laser 8 arranged to generate a laser beam 9 which is incident upon the substrate at the same location. A powder hopper (not shown) is provided at an upper end of the feed conduit 60. The feed conduit 60 may be considered to be a form of powder feeding head.

The feed conduit 60 is connected via a connector 18 to a piezoelectric actuator 16. A frequency generator 24, via an amplifier 22, causes the piezoelectric actuator 16 to vibrate. The frequency waveform applied to the piezoelectric actuator 16 may be displayed on an oscilloscope or other display device 28. Optics 32 are arranged to condition the laser beam before it is incident upon the substrate 6. The optics are protected by a screen 34.

An important difference between the apparatus shown in Figure 4 and the apparatus shown in Figure 3 is that the nozzle 62 is directly above the location on the substrate 6 onto which powder is to be delivered. Powder thus falls from the nozzle 62 directly downwards onto the substrate 6. It is a combination of the vibration of the nozzle 62 and gravity which causes the powder to fall from the nozzle. Since the powder is delivered vertically onto the substrate, rather than at an angle, the delivery of powder onto the substrate is unaffected by the direction of movement of the substrate. It is therefore not necessary to deliver powder onto the substrate from more than one nozzle 62.

The laser beam 9 is incident upon the substrate 6 at an angle. In the absence of suitable optics, this would cause the laser beam 9 to form a beam spot which is elliptical in shape on the substrate 6. Such a beam spot would mean that omnidirectional operation of the apparatus was not possible. To counter this, the conditioning optics 32 are arranged to adjust the laser beam 9 to have a non-circular cross-section, such that the beam spot formed by the laser on the substrate 6 is circular. The optics 32 shown are a schematic illustration only. The optics may for example comprise a pair cylindrical lenses, or may comprise diffractive optics. The precise form of optics needed in order to provide the laser beam 9 with a circular beam spot on the substrate 6 will be apparent to those skilled in the art.

In a modification of the apparatus shown in Figure 4, the laser beam 9 is supplemented with a plurality of other laser beams (not shown in Figure 4) generated by other lasers. The laser beams may be incident upon the substrate at the same position and with substantially the same incidence angles, but from different locations. For example, when viewing the apparatus from above, three laser beams may be provided, the beams being angularly spaced apart by 120°. Alternatively, four laser beams may be provided, the beams being angularly spaced apart by 90°. Other numbers of laser beams may be used. Where a plurality of laser beams are used, optics to correct the beam spot to a circular form may not be needed. Superposition of the beam spots formed by the different laser beams may give a combined beam spot which has a shape that allows omni-directional operation of the apparatus (for example the beam spot may be rotationally symmetric when rotated by 90° or 180°).

Figure 5 shows in perspective and cross-section views a modified version of the feed conduit shown in Figure 4. A single pipe nozzle 162 tapers outwards at an upper end to form a powder receiving portion 164. A powder delivery conduit 166 passes into the powder receiving portion 164. The powder receiving portion 164 and the powder delivery conduit 166 may together be considered to comprise a feed conduit 160. The powder delivery conduit 166 is suspended such that there is a gap between a lowermost end of the powder delivery conduit and a bottom surface of the powder receiving portion. This gap defines a chamber within which powder will sit before entering the single pipe nozzle 162.

The powder receiving portion 164 is connected via a bracket 168 to a piezoelectric actuator 170. The powder delivery conduit 166 is connected by a second bracket 172 to the same piezoelectric actuator 170 (the two brackets 168, 172, are connected to opposite ends of the piezoelectric actuator). The entire arrangement is connected to a support bracket 174 which may be used to mount the apparatus on a suitable mounting.

The powder delivery conduit 166 has a taper 176 at its lowermost end, the taper ending at an aperture 178. The powder receiving portion 164 of the feed conduit 160 includes a taper 180 at its lowermost end, which is connected to the single pipe nozzle 162.

In use, powder (not shown) is delivered from a powder hopper (not shown) to the powder delivery conduit 166. The powder then flows from the aperture 178 in the powder delivery conduit 166 into the chamber defined in the powder receiving portion 164. The powder is then directed by the taper 180 into the single pipe nozzle 162, and passes down through the single pipe nozzle and onto a substrate (not shown). In other words, powder passes from a powder hopper (not shown) via the feed conduit 160, into the single pipe nozzle 162. Powder then passes from the single pipe nozzle 162 onto a substrate (not shown). A laser, for example of the type shown schematically in Figure 4, may be used to direct radiation onto the powder delivered to the substrate, thereby performing direct laser deposition. An advantage of the arrangement shown in Figure 5 is that the use of the powder delivery conduit 166 ensures that large variations of the amount of powder waiting in the powder receiving portion 164 to be fed into the single pipe nozzle 162 do not occur. The chamber defined by the gap between the powder delivery conduit 166 and the single pipe nozzle 162 contains a self-regulating amount of powder (the upper level of the powder coinciding with the bottom of the powder delivery conduit 166). The chamber and the powder delivery conduit may be considered to be an isolation apparatus, since they provide some isolation of the nozzle from pressure arising from powder held in the powder delivery conduit.

The aperture 178 may also be considered to be an isolation apparatus, since it provides some isolation of the nozzle from pressure arising from powder held in the powder delivery conduit.

In the absence of the powder delivery conduit 166, the powder receiving portion 164 would at some times be almost full of powder and at other times would be almost empty. The weight of powder in the powder receiving portion 164 would press downwards on the powder which is passing through the taper 180 into the single pipe nozzle 162. In some instances this weight may cause the powder to jam, such that the flow of powder into the single pipe nozzle stops. This interruption of the powder flow is undesirable, since it will interrupt direct laser deposition. The interruptions may, to some extent be suppressed by applying high amplitude vibrations via a piezoelectric actuator. However, this may cause secondary issues such as damage to the feed conduit, excessive noise, etc. Alternatively, stopping and starting the vibration may be used to restart the flow of powder into the single pipe nozzle. However, a steady continuous flow of powder is often preferable when performing direct laser deposition.

These issues are avoided when using the feed conduit 160 shown in Figure 5. The majority of powder being that is to be delivered is held in the powder delivery conduit 166. The aperture 178 in the powder delivery conduit is sufficiently large that it does not suffer from jamming of the powder within the aperture, however, it is sufficiently small that the majority of the pressure arising from the powder held in the delivery conduit is not transmitted downwards to the taper 180 of the powder receiving portion. The diameter of the aperture is such that powder to passes freely through the aperture when the vibration is applied to the nozzle and, and does not pass through the aperture when the vibration is switched off.

The aperture may have a diameter which is in the range 1 to 6 mm, preferably in the range 3 to 5 mm, and most preferably around 4mm, for powder with particle sizes in the range 40-150 micrometers. The aperture may have a diameter which is in the range 1 to 2 mm for powder with particle sizes in the range 40-150 micrometers.

Generally, a small amount of powder is present in the powder receiving portion 164 of the feed conduit 160. The amount of powder is sufficiently low that the pressure applied by the powder is not sufficient to cause jamming at the upper end of the single pipe nozzle 162. In effect, the aperture 178 in the powder delivery conduit 166 isolates the powder receiving portion 164 from the majority of the pressure arising from the powder held in the powder delivery conduit. It is this which prevents clogging of the single pipe nozzle 162. The flow of powder through the aperture 178 is self-regulating, i.e. the flow of powder will be such that powder present in the feed conduit extends up to, but not beyond, the bottom surface of the aperture.

Experimental results have shown that the delivery of powder using an apparatus of the type shown in Figure 5 is steady. Not only does the flow of powder not suffer from interruptions, but the rate of flow itself is very constant and does not suffer from fluctuations (fluctuations may be seen if an apparatus as shown in Figure 4 were to be used).

As previously mentioned, the aperture 178 has the effect of isolating the powder receiving portion 164 from the majority of the pressure arising from powder in the powder delivery conduit 166. However, some pressure will be transmitted through the aperture 178 and onto the powder passing into the single pipe nozzle 162. In order to avoid this happening, the apparatus may be modified such that the aperture 178 is no longer directly above the single pipe nozzle 162, but instead is laterally offset (e.g. offset to one side). This offset enhances the isolation, and ensures that the weight of powder in the powder delivery conduit 166 does not press directly onto powder passing into the single pipe nozzle 162. The offset may be considered to be an isolation apparatus, since it provides some isolation of the nozzle from pressure arising from powder held in the powder delivery conduit. Figures 6 and 7 show feed conduits of the type shown in Figure 3, but with more detail. The apparatus shown in figures 6 and 7 is referred to as a powder feeding head. It is viewed in perspective from above in Figure 6a, in perspective from below in Figure 6b, and in partial cross-section in Figure 7.

The powder feeding head 200 comprises a powder repository 202 which surrounds an upper portion of a protective ring 204. The protective ring 204 comprises annular metal components fitted together such that they provide a central channel through which a radiation beam (not shown) may pass. A piezoelectric actuator 206, which is also annular in form, is provided around part of the protective ring 204.

The powder repository 202 is connected to feed conduits 210 which feed the powder to single pipe nozzles 222. Each single pipe nozzle 222 is arranged to direct powder onto a substrate (not shown) to allow direct laser deposition to be performed in the manner described above in relation to Figure 3.

Each feed conduit 210 comprises a diagonal powder delivery channel 208 (i.e. at an angle to the vertical) and a vertical powder delivery channel 224. The diagonal powder delivery channel 208 and the vertical powder delivery channel 224 have the same diameter. However, an upper end of the vertical powder delivery channel 224 includes an outward taper 220, such that the diameter of the upper end of the vertical powder delivery channel is greater than the diameter of the lower end of the diagonal powder delivery channel 208.

The diagonal orientation of the powder delivery channel 208, acts as a pressure isolation apparatus. That is to say, it provides some isolation of the single pipe nozzle 222 from pressure arising from powder held in the powder repository 202.

The diameter of the diagonal powder delivery channel 208 may be chosen to be narrow, such that restricted powder flow is provided by the diagonal powder delivery channel. Where this is done, the narrow diameter of the diagonal powder delivery channel acts as a pressure isolation apparatus. In other words, it provides some isolation of the single pipe nozzle 222 from pressure arising from powder held in the powder repository 202. The vertical powder delivery 224 channel may also have a narrow diameter, and for example may have the same diameter as the diagonal powder delivery channel (as shown in figure 7).

The diagonal powder delivery channel 208 may have a diameter which is narrower than the vertical powder delivery channel 224. Where this is done, the vertical powder delivery channel 224 may be thought of as a chamber, which holds the powder prior to it passing through the nozzle defined at the bottom of the funnel. This chamber helps to provide a steady and continuous flow of powder to the single pipe nozzle 222. In conventional use, the chamber defined by the vertical powder delivery channel will always be full of powder, and the level of powder in the chamber is thus self-regulating.

The diagonal powder delivery channel 208 may have a diameter which is in the range 1 to 6 mm, preferably in the range 3 to 5 mm, and most preferably around 4mm, for powder with particle sizes in the range 40-150 micrometers. The diagonal powder delivery channel 208 may have a diameter which is in the range 1 to 2 mm for powder with particle sizes in the range 40-150 micrometers.

The vertical powder delivery channel 224 may have a diameter which falls in any of the above ranges. It may have a wider diameter than the diagonal powder delivery channel, which may for example be wider than 6 mm.

The powder feeding head 200 is configured to ensure that the pressure exerted on powder entering the single pipe nozzle 222 is not sufficient to cause jamming of the powder. Referring to figure 8, the entrance of the single pipe nozzle 222 is offset with respect to a central axis 240 of the vertical powder delivery channel 224. The offset may be considered to be an isolation apparatus, since pressure generated by powder in the feed conduit 210 is not exerted directly on the entrance to the single pipe nozzle 222.

In addition, a taper 214 is provided at the entrance of the single pipe nozzle 222. The taper provides a slope 215 which is angled with respect to the horizontal. Pressure generated by powder in the feed conduit 210 is exerted on the slope 215, but the angle of the slope is such that only a fraction of this pressure is transmitted into the entrance of the single pipe nozzle. The angle may for example be between 10 and 20 degrees from the horizontal. In some instances the angle may be less than 10 degrees, and may even be zero degrees (i.e. horizontal). Where this is the case, the pressure from powder in the feed conduit 210 which is transmitted into the entrance of the single pipe nozzle is decreased, and may even be zero. The vibration of the powder feeding head is sufficient to ensure that delivery of the powder takes place.

A rotatable connector 242 connects the feed conduit 210 to the single pipe nozzle 222. The rotatable connector has a spherical outer surface, and is received in a correspondingly shaped opening in the feed conduit 210. The connector is therefore rotatable relative to the feed conduit. This allows adjustment of the direction in which the single pipe nozzle 222 points, and correspondingly allows adjustment of the angle of the taper 214 at the entrance of the single pipe nozzle.

Within the rotatable connector 242 a taper 244 is provided to guide powder into a delivery channel 246 which is dimensioned such that it couples correctly to the single pipe nozzle 222. The taper 242 and delivery channel 246 may be considered to form an extension of the feed conduit 210.

In use, powder is delivered to the powder feeding head 200 from a powder hopper (not shown). The powder passes through the powder repository 202 and into the feed conduits 210. The powder passes via the tapers 214 into the single piper nozzles 222, and from the single pipe nozzles onto a substrate (not shown).

Optics (not shown) may be provided within the protective screen 204.

Embodiments of the invention may be provided with different nozzles, and a mechanism for selecting an appropriate nozzle, for example to deliver a line of powder with a particular width to the substrate. For example, referring to Figure 4, the feed conduit 60 and nozzle 62 may be one of several which are each connected to a different piezo-electric actuator. A desired feed conduit may be positioned over the substrate. Once the feed conduit has been positioned, vibration of the piezo-electric actuator may be begun so that powder is delivered onto the substrate.

In an alternative arrangement, a single piezo-electric actuator may be provided together with a connection mechanism to connect it to the feed conduit which is located over the substrate. The piezo-electric actuator may therefore be used to induce vibration of different feed conduits at different times.

The feed conduits may be separately mounted, so that vibration of one feed conduit does not cause the other feed conduits to vibrate. Alternatively, the feed conduits may be linked together by a connection that does not transmit vibration (e.g. formed from leather or a suitable rubber), hi some instances, some transmission of vibration to a feed conduit which is not delivering powder may take place without inducing powder to fall from that feed conduit (for example if the amplitude of the vibration is sufficiently low). The feed conduits may be arranged in a manner which corresponds to the barrel of a revolver gun, the desired feed conduit being selected by rotating the barrel. If only one feed conduit is connected to a source of powder at any given time, and the other feed conduits are empty, then it does not matter whether vibration is transmitted to the other feed conduits since no powder will be delivered by those feed conduits.

The feed conduits may be connected to different sources of powder, for example powder hoppers containing different powders.

Different powders may be delivered to the substrate by the same nozzle, by using a selection mechanism which selects between different powder sources. For example, referring to Figure 4, the feed conduit 60 may be arranged to receive powder from a particular powder source. A selection mechanism may comprise an actuator arranged to replace the powder source with a different powder source, to deliver a different powder to the substrate. Where this is done, the feed conduit may be short, so that it retains only a small amount of powder, thereby allowing relatively fast switching between different powders. Suitable powder hoppers (not shown) may be used to deliver the powder to the feed conduit.

An advantage of the invention is that it allows powder to be delivered to the substrate 6 at a relatively low velocity, compared with prior art direct laser deposition apparatus. Prior art direct laser deposition apparatus use carrier gas to deliver the powder to the substrate. A high gas pressure is required in order to carry the powder to the substrate (typically the powder travels a significant distance), and this results in the powder having a high velocity when it is incident upon the substrate. The high velocity of the powder causes a significant proportion of the powder to bounce away from the laser beam spot. This powder is not melted by the laser beam and so is wasted. In some instances the powder may bounce onto optics which are used to condition the laser beam, thereby contaminating the optics. In some cases a proportion of the un-melted powder may be collected and re-used. However, in many cases this is not possible and only a small proportion of the powder delivered to the substrate is melted by the laser. Since the powder is expensive this has a significant impact on the cost of forming components using direct laser deposition. The invention solves these problems by allowing powder to be delivered onto the substrate with a low velocity. Provided that the area over which powder is delivered is less than the beam spot size, close to 100% of the powder is melted by the melt pool, thereby providing very efficient use of the powder.

When a carrier gas is used to deliver powder to a substrate, the powder will be focussed by the gas at a distance which is typically 5 to 15 mm from the end of the delivery nozzle. Beyond the focal point, the powder diverges and cannot be usefully used for direct laser deposition. The invention provides a stream of powder which has negligible divergence over a distance of 10s of cm. Therefore, the invention allows direct laser deposition to be performed using a substrate which is located 10s of cm away from the powder feeding head. This is advantageous because it allows room for other pieces of equipment to be provided. In addition, it allows optical lenses (used to focus the laser) to be more kept further away from the substrate, thereby reducing the risk that the lenses may be contaminated.

Due to the manner in which the invention operates, i.e. delivering powder by using a combination of gravity and vibration, the nozzles 14, 52, 62, 162, 222 may be narrower than is generally possible using prior art gas delivery systems. For example, a typical prior art gas delivery system has a nozzle with a diameter of between 3 and 5mm. Gas delivery systems having nozzles narrower than this may suffer from problems, since this may not allow gas to flow sufficiently quickly through the delivery system to carry the powder. If the gas flow rates were to be reduced, then the flow of powder would become non-uniform, so that delivery of powder might oscillate, be intermittent, or even stop. The problems associated with gas delivery are avoided by the described embodiments of the invention. This means that the nozzles 14, 52, 62, 162, 222 may be of any desired size. This allows nozzles 14, 52, 62 to be used having diameters less than 3mm, for example 2mm, lmm, 0.5mm or less.

An additional advantage of the invention over conventional gas-based powder delivery systems is that it allows smaller powder particles to be used. In prior art gas- based delivery systems, smaller particles tend to become airborne when they are expelled using gas through a nozzle. Where this- occurs only a small proportion of the powder is incident upon a substrate, a significant proportion of the powder being carried away by the air (or other gases). This causes substantial amounts of contamination of for example optics used to condition the laser beam, and other surfaces of the apparatus. Since the embodiments of the invention do not use gas to deliver powder, this disadvantage is avoided. This allows powders having smaller particle sizes to be used. The powder may comprise particles having diameters of less than 40 microns, less than 20 microns, less than 10 microns, or less than 1 micron.

The invention may use particles having diameters greater than 40 microns, for example particles having diameters up to 150 microns may be used. In some instances particles having diameters of up to 200 microns may be used.

An additional advantage of not using gas to deliver the powder to the substrate is that the possibility of gas contamination is avoided. Gas contamination may in some instances cause porosity contamination of a component formed using direct laser deposition. This contamination is avoided when using the invention, since gas is not used to deliver the powder to the substrate.

A further advantage of not using gas to deliver the powder to the substrate is that powder is not blown onto cooling areas of material. Where this occurs in gas based systems the powder adheres to the cooling material and causes unwanted surface roughness.

A further advantage of not using gas to deliver the powder to the substrate is that the powder may be delivered in a vacuum. This is useful because it avoids the possibility that a draft may cause the stream of powder to be deflected out of position. Such a draft could be caused for example by movement of the substrate during direct laser deposition. An additional advantage of working in a vacuum is that an electron- beam source may be used instead of a laser. The nozzle and substrate may be provided in a vacuum chamber (not shown in the figures).

Although the embodiments of the invention have referred to using a laser, other radiation sources could be used. For example, as mentioned above an electron- beam source may be used (or some other source of charged particles). The term radiation source is therefore not intended to be limited to a source of electromagnetic radiation. It is appropriate therefore to refer to direct radiation deposition than direct laser deposition.

As mentioned above, the invention allows the flow of powder to a substrate to be easily interrupted, by interrupting the waveform applied to the piezoelectric actuator. This convenient manner of interrupting the flow of powder onto the substrate contrasts with prior art gas-flow based powder delivery systems, in which the supply of powder cannot easily be interrupted (several seconds is needed to allow gas pressure to dissipate, during which powder continues to be supplied).

The embodiments of the invention all use vibration and gravity to deliver powder onto a substrate. They do not use gas to carry the powder onto the substrate. Although it is not shown in the illustrated embodiments, in some instances it may be desired to use a gas-based system to carry powder, for example from a storage container to the powder hopper 2. In general, although the delivery of powder onto the substrate is via a combination of gravity and vibration, carrier gas may be used to move powder in other parts of the apparatus.

The embodiments of the invention may be arranged, through the choice of nozzle diameter, the number of feed conduits, and the amplitude and frequency of vibration, to direct a wide range of powder volumes onto the substrate. For example, less than 12 grams per hour of powder may be directed onto the substrate (or substrates), or more than 3kg per hour.

The frequency applied to the powder feeding head may be a resonant frequency of the of the powder feeding head, in order to maximise vibration of the head and thereby maximise the rate of powder flow (where this is desired). The resonant frequency of the powder feeding head will be influenced by the weight of powder present in the powder feeding head, and so will change over time (as powder is used up). The applied frequency may for example be arranged to track the resonant frequency. This may be achieved for example by monitoring the magnitude of vibration of the powder feeding head.

The frequency applied to the powder feeding head may be sonic or ultrasonic (the term ultrasonic frequency is generally understood to mean frequencies of 20 kHz or higher). Using an ultrasonic frequency has the advantage that it minimises audible noise. The frequency of the vibration may be low, for example 2 kHz or lower. Although the embodiments of the invention describe the use of a piezo-electric actuator to apply the vibration to one or more feed conduits, it will be appreciated that other types of actuator such as a magnetic actuator or a motor may be used.

Although the embodiments of the invention refer to moving the substrate beneath the laser beam spot, it will be appreciated that it is possible to move the laser spot (and powder delivery apparatus) relative to the substrate. Movement of the substrate (or laser beam spot and powder delivery apparatus) may be controlled by a computer, for example a computer numerical control (CNC) system.

The term 'nozzle' is not intended to imply a specific size or dimension, but is instead intended to include any opening suitable for delivering powder.

More than one powder hopper may be used, for example to provide more than one powder to the feed conduits.

The use of an aperture in order to reduce the pressure applied to powder entering a nozzle has been described in relation to some of the above embodiments. The aperture may, with any necessary modifications, be applied to other embodiments. The aperture may be offset with respect to a nozzle, to isolate the nozzle from pressure applied by powder through the aperture.

Although the aperture is described as being provided in a powder delivery conduit, and is shown as being at a lowermost end of the powder delivery conduit, the aperture may be provided in other suitable locations, hi general, the aperture should be located above the nozzle, in a feed conduit which is arranged to feed powder to the nozzle. The term 'aperture' is intended to mean a narrowing followed by an opening out of the path along which the powder flows.

Where an aperture is provided, for example in the manner described above, interrupting the flow of powder from a nozzle is quickly and easily achieved by stopping vibration applied to the nozzle. The flow is re-started by re-starting the vibration.

Where an aperture is provided, for example in the manner described above, the flow of powder from the nozzle (or nozzles) is self-limiting. This is because the amount of powder held beneath the aperture remains substantially constant.

As an alternative to providing an aperture, other pressure isolation apparatus may be used to reduce the pressure applied to powder entering a nozzle, as explained in relation to the above embodiments. These include providing a configuration that delivers a constant supply of powder to a chamber above the nozzle entrance, the supply of powder being small compared to the volume of the hopper. They also include offsetting an axis of a feed conduit with respect to the nozzle entrance, so that pressure is not delivered downwards through the feed conduit onto the nozzle entrance. They also include delivering powder to the nozzle entrance via a slope which has an angle relative to the horizontal selected such that pressure transmitted to the nozzle entrance is not sufficient to cause jamming at the nozzle entrance. Any suitable combination of these pressure isolation apparatus may be used.

Where a chamber is provided above the nozzle entrance to give a constant supply of powder, some pressure isolation may be provided between the chamber and powder in other upstream parts of the apparatus. For example, one or more additional chambers may be provided, the chambers being separated using pressure isolation apparatus (e.g. apertures provided between the chamber). For example a cascade of chambers may be provided, successive chambers being separated by apertures.

Although embodiments of the invention have been described in relation to direct laser deposition, the invention may be applied in other fields, and is not limited to direct laser deposition except where explicitly stated.

Claims

1. A direct radiation deposition apparatus comprising a feed conduit and a nozzle arranged to deliver powder to a substrate, and a radiation source arranged to simultaneously direct a radiation beam onto the substrate and thereby melt the powder, wherein the apparatus further comprises an actuator arranged to apply a vibration to the nozzle which causes powder to fall from the nozzle onto the substrate.

2. The apparatus of claim 1, wherein the apparatus further comprises a pressure isolation apparatus, arranged such that in use it provides some isolation of the nozzle from pressure arising from the weight of powder to be delivered to the substrate.

3. The apparatus of claim 2, wherein the degree of pressure isolation provided by the pressure isolation apparatus is sufficient that jamming of powder in the nozzle does not occur when vibration is being applied to the nozzle.

4. The apparatus of claim 2 or claim 3, wherein the pressure isolation apparatus comprises a chamber which is located upstream of the nozzle, and a powder delivery conduit which is arranged to deliver powder to the chamber, the powder delivery conduit and the chamber being configured such that in use the volume of powder present in the chamber remains substantially constant.

5. The apparatus of claim 4, wherein the powder delivery conduit extends into the chamber, such that a powder delivering opening at a lower end of the powder delivery conduit defines a self-regulating level for the powder.

6. The apparatus of claim 4 or claim 5, wherein the powder delivery conduit comprises an aperture.

7. The apparatus of any of claims 4 to 6, wherein the apparatus further comprises a member which is moveable into and out of contact with an opening at the bottom of the chamber, thereby defining the nozzle from which the powder falls onto the substrate.

8. The apparatus of claim 2 or claim 3, wherein the pressure isolation apparatus comprises an aperture provided in the feed conduit.

9. The apparatus of claim 8, wherein the diameter of the aperture is such that powder passes freely through the aperture when the vibration is applied to the nozzle and, and does not pass through the aperture when the vibration is switched off.

10. The apparatus of claim 8 or claim 9, wherein the diameter of the aperture is between 1 and 6 mm.

11. The apparatus of claim 2 or claim 3, wherein the pressure isolation apparatus comprises providing at least part of the feed conduit at an angle relative to the vertical.

12. The apparatus of claim 2 or claim 3, wherein the pressure isolation apparatus comprises an offset of the axis of the feed conduit relative to an entrance of the nozzle, such that pressure exerted by powder in the feed conduit is not directed vertically downwards onto the entrance of the nozzle.

13. The apparatus of claim 12, wherein an entrance of the nozzle is sloped, the slope having an angle relative to the horizontal which is such that pressure transmitted to the nozzle entrance by powder in the feed conduit is not sufficient to cause jamming of the powder at the nozzle entrance.

14. The apparatus of any preceding claim, wherein an entrance of the nozzle tapers outwardly.

15. The apparatus of any preceding claim, wherein the actuator is arranged to apply an ultrasonic vibration to the nozzle.

16. The apparatus of any preceding claim, wherein the apparatus comprises two or more nozzles.

17. The apparatus of any preceding claim, wherein the radiation source is an electron beam source and the radiation beam is an electron beam.

18. The apparatus of any of claims 1 to 16, wherein the radiation source is a laser and the radiation beam is a laser beam.

19. The apparatus of claim 18, wherein the apparatus includes optics arranged to direct the laser beam perpendicularly onto the substrate, and the one or more nozzles are arranged to direct powder onto the substrate in a non- vertical direction.

20. The apparatus of claim 18, wherein the apparatus includes optics arranged to direct the laser beam onto the substrate in a non-perpendicular direction, and the nozzle is arranged to direct powder onto the substrate in a vertical direction, the optics being arranged to provide the laser beam with a non-circular cross-section, such that the beam spot formed by the laser on the substrate is circular.

21. The apparatus of claim 18, wherein the laser is one of a plurality of lasers, arranged to generate laser beams which are directed onto the substrate from different directions but with substantially the same angle of incidence.

22. The apparatus of any preceding claim, wherein the nozzle is located in a vacuum chamber.

23. A powder delivery apparatus comprising a nozzle arranged to deliver powder and a feed conduit arranged to deliver powder to the nozzle, wherein the apparatus further comprises a pressure isolation apparatus which in use provides some isolation of the nozzle from pressure arising from the weight of powder to be delivered by the apparatus.

24. The powder delivery apparatus of claim 23, wherein the degree of pressure isolation provided by the pressure isolation apparatus is sufficient that jamming of powder in the nozzle does not occur when vibration is being applied to the nozzle.

25. The apparatus of claim 23 or claim 24, wherein the pressure isolation apparatus comprises a chamber which is located upstream of the nozzle, and a powder delivery conduit which is arranged to deliver powder to the chamber, the powder delivery conduit and the chamber being configured such that in use the volume of powder present in the chamber remains substantially constant.

26. The apparatus of claim 25, wherein the powder delivery conduit extends into the chamber, such that a powder delivering opening at a lower end of the powder delivery conduit defines a self-regulating level for the powder.

27. The apparatus of claim 26, wherein the apparatus further comprises a member which is moveable into and out of contact with an opening at the bottom of the chamber, thereby defining the nozzle from which the powder is delivered.

28. The apparatus of claim 23 or claim 24, wherein the pressure isolation apparatus comprises an aperture provided in the feed conduit.

29. The apparatus of claim 28, wherein the diameter of the aperture is such that powder passes freely through the aperture when the vibration is applied to the nozzle and, and does not pass through the aperture when the vibration is switched off.

30. The apparatus of claim 23 or claim 24, wherein the pressure isolation apparatus comprises an offset of the axis of the feed conduit relative to an entrance of the nozzle, such that pressure exerted by powder in the feed conduit is not directed vertically downwards onto the entrance of the nozzle.

31. The apparatus of claim 30, wherein an entrance of the nozzle is sloped, the slope having an angle relative to the horizontal which is such that pressure transmitted to the nozzle entrance by powder in the feed conduit is not sufficient to cause jamming of the powder at the nozzle entrance.

32. A method of direct radiation deposition comprising directing a radiation beam onto a substrate, and simultaneously delivering powder to the location at which the radiation beam is incident upon the substrate such that the powder is melted by the radiation beam, wherein the powder is delivered to the substrate from a nozzle by applying a vibration to the nozzle such that the powder falls from the nozzle onto the substrate.

33. The method of claim 32, wherein the apparatus further comprises a pressure isolation apparatus, arranged such that in use it provides some isolation of the nozzle from pressure arising from the weight of powder to be delivered to the substrate.

34. The method of claim 33, wherein the degree of pressure isolation provided by the pressure isolation apparatus is sufficient that jamming of powder in the nozzle does not occur when vibration is being applied to the nozzle.

35. The method of claim 33 or claim 34, wherein the pressure isolation apparatus comprises a chamber which is located upstream of the nozzle, and a powder delivery conduit which is arranged to deliver powder to the chamber, the volume of powder present in the chamber remaining substantially constant.

36. The method of any of claims 32 to 35, wherein the nozzle and the substrate are in a vacuum.

37. The method of any of claims 32 to 36, wherein the radiation source is an electron beam source and the radiation beam is an electron beam.

38. The method of any of claims 32 to 37, wherein the flow of powder onto the substrate is interrupted by interrupting the vibration of the nozzle.