GB2569649A

GB2569649A - Powder supply in additive layer manufacturing apparatus

Info

Publication number: GB2569649A
Application number: GB1721772.0A
Authority: GB
Inventors: Thomas Richardson William
Original assignee: Reliance Precision Ltd
Current assignee: Reliance Precision Ltd
Priority date: 2017-12-22
Filing date: 2017-12-22
Publication date: 2019-06-26
Anticipated expiration: 2037-12-22
Also published as: GB2569649B; GB201721772D0

Abstract

Additive layer manufacturing apparatus 10 for fusing powder with an electron beam 15 comprising layer forming means 14 and a housing 11 bounding a vacuum chamber 12. Powder supply 17, which is located outside vacuum chamber 12, may be detachable and can be placed in an evacuated state. Vacuum chamber 12 is communicable with the evacuated supply means 17, perhaps by a door, for the transfer of stored powder to the layer forming means 17. Between the powder supply 17 and the vacuum chamber 12, there may be chutes 27 and 28 and/or a transfer chamber 22 that acts as an air lock. The chutes may be positioned automatically when the doors 25 and 29 are opened. The supply means may have a valve comprising a rotary drum 26 and the amount powder in the power supply may be measured, monitored and communicated. The layer forming device 14 may comprise levelling blades 31 and a powder reservoir 30 with capacity for enough powder to form many layers. Included is a method for replenishing powder in a vacuum chamber without disrupting the vacuum environment.

Description

POWDER SUPPLY IN ADDITIVE LAYER MANUFACTURING APPARATUS

The present invention relates to additive layer manufacturing apparatus, especially to powder supply in such apparatus, and to a method of supplying powder for use in additive layer manufacturing.

Additive layer manufacturing is a manufacturing process in which material is deposited on a substrate in layers and processed layer-by-layer in order to build up a threedimensional article from the processed layers. One of the most prominent technologies employed in this area is powder bed fusion in which a thin layer of powder - typically metal or plastic - is selectively melted by an energy source such as a laser or an electron beam. The melted area of the powder layer forms a cross-sectional part of the article, whereas unmelted powder in the layer is discarded and usually recycled at the end of the process. After each layer has been selectively melted, a new layer of powder is deposited and then also selectively melted so that the complete article is constructed on a layer-by-layer basis.

An electron beam consists of a focused stream of high-velocity negatively charged electrons which deliver energy when striking the powder bed. For example, a focused beam of approximately 200 microns diameter may deliver in excess of 1 kilowatt of power to the bed. The beam is produced by a source or gun, for example using thermionic emission, and is accelerated by an acceleration voltage of, for example, 10 kilovolts or more. The beam can be readily steered, focused and shaped using a series of controllable coils producing electromagnetic fields which influence the electrons making up the beam. The source and coils together with other components form an electron beam column.

When an electron beam is used as the energy source, the process must take place in a vacuum created in a vacuum chamber. If the process were to take place in atmospheric air, the beam electrons would frequently collide with gas molecules in the air, which would dissipate much of the power in the beam and diminish the beam focus, with the consequences of poor resolution and slow processing time or no processing at all. Accordingly, the process is usually carried out in a sub-atmospheric pressure of 10’ ³ millibars or lower, which is six orders of magnitude lower than atmospheric pressure.

In current commercial examples of machines for that purpose all of the powder to be used in the process has to be loaded into a hopper or hoppers in the vacuum chamber prior to starting the process of melting the powder using the electron beam. The vacuum chamber is then pumped down to its operating vacuum pressure. This has several drawbacks:

The chamber has to be relatively large to accommodate a hopper or hoppers of sufficient size to contain all of the powder that may be needed to complete a large build, for example, up to 200 kilograms of titanium powder.

In the case of a cubic vacuum chamber, which is the simplest and most robust form of construction, the electron beam column must be further away from the powder bed in order to accommodate large powder hoppers seated above the powder melting surface so as to dispense powder under gravity. This results in longer electron beam working distances and hence a larger electron beam spot size. As a result, only lower resolution articles can be built.

The vacuum chamber must be vented at the end of each build in order to refill the hoppers with powder, which results in a significant loss of processing time. Working capital required is higher. All of the powder needed must be available at the start of the build, because more powder cannot be introduced into the vacuum chamber part way through the build. If the powder could be replenished part way through a build, then only part of the total amount of powder, which is expensive, would have to be available at the start of the build.

When a vacuum chamber is vented, any residual powder is exposed to air and atmospheric humidity. It is well known in the vacuum equipment industry that large surface areas outgas considerably, resulting in long pump-down times. A large body of powder has a substantial surface area, with the consequence that, if it is exposed to air and atmospheric moisture, the next time the powder is used it will take a lengthy period of time to outgas and pump down times will be longer to the detriment of production time.

There is therefore scope for improvement in the powder supply arrangement in additive layer manufacturing apparatus employing an electron beam in a vacuum environment, particularly so that reliance on storage within a vacuum chamber of a large quantity of the powder needed for a manufacturing process can be avoided.

It would also be desirable to provide a supply arrangement capable of being replenished with powder during a manufacturing process without disrupting the vacuum environment.

Other objects and advantages will be apparent from the following description.

According to a first aspect of the present invention there is provided additive layer manufacturing apparatus comprising a housing bounding a vacuum chamber in which additive layer manufacturing can be carried out in a vacuum by the action of an electron beam on successive layers of fusible powder, layer forming means in the vacuum chamber for forming each powder layer and powder supply means outside the chamber for storing powder for supply to the layer forming means, the powder supply means being capable of being placed in an evacuated state substantially corresponding with a vacuum in the vacuum chamber and the vacuum chamber being communicable with the supply means in the evacuated state thereof for transfer of stored powder from outside the vacuum chamber to the layer forming means in the vacuum chamber.

Apparatus embodying the present invention has the significant advantage of location of powder supply means in rather than outside of a vacuum chamber of the apparatus so that transfer of powder to the layer forming means in the chamber can be made. This allows use of a smaller-size vacuum chamber freed of the need to accommodate large hoppers and thus without the associated disadvantage of increased spacing between the zone of emission and zone of action of the electron beam. With such an arrangement of external powder supply means it becomes possible for the layer forming means to operate with just the quantity of powder needed for a specific task or specific operating time, replenishment being provided by transfer of a further powder batch from a powder reserve which is held by the powder supply means and which itself may be capable of being replenished. In that case, the disadvantages of the prior art apparatus associated with venting the chamber, namely loss of operating time and exposure of residual powder to the atmosphere, are eliminated. In addition, acquisition of the expensive powder can be more easily correlated with current needs rather than locked into maintenance of a substantial stock within the apparatus.

Preferably, the apparatus is provided with a powder transfer port for the communication of the vacuum chamber with the supply means, such a port providing a dedicated means of access for powder transfer independently of any other loading or unloading facility. Such a transfer port is preferably openable and closable to control the communication, which allows access to the layer forming means for the purpose of powder transfer only at times selected by the user. When communication is terminated by closure of the port, actions can be undertaken at the externally located supply means without concern for the operating state of the apparatus, in particular the pressure in the vacuum chamber. For preference, the apparatus comprises a movable door for opening and for sealably closing the transfer port, such a door being of any suitable kind such as a swing door or sliding door, but constructed or equipped to ensure sealed closure of the vacuum chamber, especially to provide hermetic separation between the vacuum chamber in an evacuated state and the supply means if not in an evacuated state.

For preference, the supply means after transfer of the stored powder is transferrable to a non-evacuated state and in that state is refillable with powder in situ or removable at least in part for refilling or for exchange for a filled replacement so that further powder can be supplied to the layer forming means without loss of a vacuum in the vacuum chamber. Such an operating capability and a configuration of the supply means make it possible to replenish the supply means with powder at the apparatus or to remove part or all of the supply means for replenishing at a separate location or for replacement by another, already-filled supply means or part of the supply means. Whichever procedure is adopted, it can be carried out without influence on the vacuum chamber, which can be maintained at sub-atmospheric pressure. Operation of the electron beam for powder fusion can thus be continued during the replenishing or replacement procedure so that manufacturing does not need to be interrupted. When the supply means has been suitably restocked or replaced the original or replacement supply means can be placed in the evacuated state so that transfer of a further batch or batches of powder to the layer forming means can be carried out when needed. For these purposes, the supply means is preferably mounted on the housing, which bounds the vacuum chamber, to be detachable therefrom entirely or in part. If the supply means is of simple construction it may be straightforward to remove it entirely and refit in a filled state or alternatively the supply means can be designed so that just a part intended to actually hold powder can be removed and refitted.

In a preferred construction the supply means comprises a powder reservoir for storing the powder and a transfer chamber intermediate the reservoir and the vacuum chamber, such a transfer chamber serving to provide an intermediary stage between stored powder and the vacuum chamber. This may facilitate design of the supply means with respect to the creation of sub-atmospheric pressure in the region adjacent the vacuum chamber. In that case, the supply means may also comprise closure means for closing such a transfer chamber relative to the reservoir, so that the reservoir can be conveniently isolated when replenishing is to be carried out. In a preferred embodiment the reservoir can be in a constantly evacuated state substantially corresponding with a vacuum in the vacuum chamber, the transfer chamber being transferrable between a non-evacuated state and an evacuated state so as to function as a lock between the vacuum chamber and the reservoir. In such an embodiment, powder is stored in a vacuum from the outset and thus prevented from contact with ambient air and absorption of humidity, which can impair the characteristics of the powder and its use in the apparatus.

The supply means preferably also comprises a dispensing valve for dispensing powder from the reservoir to the lock chamber, such a valve serving to control discharge of powder from the reservoir and being able to meter the powder to be transferred to the vacuum chamber. In one convenient embodiment, the valve comprises a rotary drum with a powder receiving compartment fillable and emptiable under gravity as a function of the rotational setting of the drum. A valve with such a configuration can be simple to operate, either manually or under automatic control by a control system of the apparatus, and selective design of the compartment volume provides a simple means of determining the quantity of powder for discharge on a metered basis.

For preference, the supply means is provided with connecting means for connection with a source of sub-atmospheric pressure for placing the supply means in the evacuated state thereof. Such connecting means can be connected with, for example, a pump for pumping down the supply means, whether a dedicated pump or a pump otherwise serving for pumping down the vacuum chamber itself.

The apparatus preferably includes guide means for guiding powder through the supply means and into the vacuum chamber, particularly guide means arranged to provide a directed delivery of powder to the region of the layer forming means so that the powder is not dispersed. In a simple embodiment, the guide means is arranged to guide the powder under gravitational force, so that there is no need for the complication of positive displacement means, and in that case the guide means can comprise a transfer chute extending between and into both the vacuum chamber and the supply means when the vacuum chamber and supply means are in communication. The transfer chute can extend, for example, at an inclination from the supply means into the chamber so that transfer from the outside to the inside of the vacuum chamber housing can take place without any form of disruptive transition. Such a chute can be arranged to be automatically movable into an operative position when the vacuum chamber and supply means are placed in communication and into an inoperative position when communication of the vacuum chamber and supply means is terminated. Thus, for example, if access to the vacuum chamber is controlled by way of a port and a door as described above, movement of the transfer chute to extend between the vacuum chamber and the supply means can be linked with opening the door, such as through a mechanical coupling.

The guide means can comprise, in addition to such a transfer chute, a discharge chute for guiding powder discharged from a stock thereof stored in the supply means. Such a discharge chute can serve to guide powder from, for example, the mentioned reservoir to the transfer chute. Various forms of guide means, whether using multiple chutes or a different guidance system entirely, are conceivable.

For preference, the apparatus comprises measuring means for deriving from the supply means a measurement indicative of the quantity of powder therein, for example a weight measurement. This provides a convenient means of assessing the amount of residual powder in the supply means so that, for example, action to refill or replace can be taken in good time. Alternatively, the apparatus can comprise monitoring means for monitoring the quantity of powder in the supply means and signalling depletion or imminent depletion of that quantity. Other arrangements for detecting when replenishment is needed are equally possible.

A particular advantage of the invention is that the quantity of powder actually held within the vacuum chamber can be reduced to the amount appropriate to immediate needs. Accordingly, in a preferred construction of the layer forming means this comprises a reservoir for storing a quantity of powder received from the supply means, the layer forming means being movable to provide metered discharge of powder to form each layer. The reservoir could take the form of, for example, a bar which is provided with a slot serving as the reservoir and which is horizontally movable on a spreading plane at a small spacing above a preceding powder layer so that powder can fall out of the slot and, for example, be levelled by levelling means incorporated in the layer forming means, such as levelling blades depending from the bar. With advantage, the storage capacity of the reservoir of the layer forming means is sufficient for a quantity of powder usable to form a predetermined plurality of layers of given area, for example 100 layers of 50 micron thickness over a 300 millimetre diameter; the storage capacity is thus computed by reference to a particular predefined need, rather than merely of an arbitrary size. This allows operation of the apparatus for a suitable period of time, for example up to about an hour, before exhausting the powder in that reservoir. Prior to that time, fresh powder can be transferred from the supply means.

According to a second aspect of the present invention there is provided a method of supplying powder for use in additive layer manufacturing carried out in a vacuum in a vacuum chamber by the action of an electron beam on successive layers of fusible powder individually formed by layer forming means in the vacuum chamber, comprising the steps of storing a quantity of powder outside the vacuum chamber in powder supply means, establishing a vacuum in the vacuum chamber, placing the powder supply means in an evacuated state substantially corresponding with the vacuum in the vacuum chamber, placing the vacuum chamber in communication with the powder supply means in that state and during such communication transferring powder from the supply means to the layer forming means.

The advantages associated with such a method are those indicated with respect to the apparatus of the first aspect of the invention.

For preference, the method comprises the further steps of terminating the communication of the vacuum chamber with the powder supply means prior to exhausting the stored powder through transfer to the layer forming means, transferring the powder supply means to a non-evacuated state and refilling the powder supply means with powder.

In a preferred example, the powder is stored in the powder supply means constantly in a vacuum substantially corresponding with that of the vacuum in the vacuum chamber and the steps of placing the powder supply means in an evacuated state and placing the vacuum chamber in communication with the powder supply means comprise evacuating a lock chamber between the vacuum chamber and the stored powder and placing the evacuated lock chamber in communication with both the vacuum chamber and the stored powder.

The step of transferring is preferably carried out in metered batches and preferably comprises transferring the powder solely by way of gravitational force.

The method can also include the step of providing an indication of the amount of powder remaining in the powder supplying means.

With advantage, a quantity of powder sufficient for a predetermined number of layers less than the number needed to complete manufacture of an article of a given volume is stored in the layer forming means and the step of transferring comprising periodically replenishing the powder stored in the layer forming means to the extent required for complete manufacture of the article.

A preferred embodiment of the additive layer manufacturing apparatus and preferred example of the method according to the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which:

Fig. 1 is a schematic elevation of additive layer manufacturing apparatus embodying the invention, showing powder supply means of the apparatus in an inoperative state; and

Fig. 2 is a schematic elevation similar to Fig. 1, but showing the powder supply means in an operative state.

Referring now to the drawings there is shown additive layer manufacturing apparatus 10 comprising a housing 11 bounding a vacuum chamber 12, the housing being in the form of a pressure vessel able to be evacuated to a sub-atmospheric pressure of 10^-3millibars or less depending on the specific requirements for a particular manufacturing process. Pumping down to achieve the targeted pressure is by way of a suction pump (not shown). Manufacturing is carried out in a lower region of the chamber 12, in which an incrementally lowerable table 13 for supporting an article to be manufactured by an additive layer process, also called 3D printing, is located. In this process, a fusible powder, especially a metal powder, is spread by a layer forming device 14 within the chamber to form a thin and even layer over the table and is then selectively melted and fused by the action of heat in a predefined area. The powder in that area forms, after solidification, a cross-sectional layer of the article. Residual powder surrounding the area and ultimately the article as a whole plays no further part in the manufacturing process as such, other than helping to support subsequent powder layers deposited thereabove, and is eventually recovered and recycled. After formation of such a crosssectional layer of the article the table 13 is lowered by a layer thickness and a further such layer of powder is distributed by the layer forming device 14 over the solidified part of the preceding layer to form, by selective melting and fusing in the same or a different predefined area, a next cross-sectional layer of the article. The process is repeated until construction of the article on a layer-by-layer basis is completed.

The heat for melting the powder material to induce fusion is supplied by an electron beam 15 generated and transmitted by an electron beam column 16 mounted on the housing 11. The column 16 comprises an electron source (not shown), generally a cathode of electron emissive material capable of emitting electrons under applied voltage, a series of lens (not shown) for focussing the electrons for propagation as a beam of defined cross-sectional size and shape along a column axis and a deflector (not shown) for deflecting the beam relative to the axis. The deflector is operable under programmed computer control in such a way as to cause the beam to scan each successive powder layer on the table 13 at high speed to melt powder in an area corresponding with a cross-sectional shape of the article as described in the preceding paragraph.

Additive layer manufacture by this process and the basic construction of equipment to perform manufacture are generally known and have been described only to the extent necessary for a background understanding of the embodiment of the present invention.

As indicated in the introduction, in conventional additive layer manufacturing equipment, particularly industrial machines, operating on the basis of powder fusion by the action of an electron beam in a vacuum chamber the powder is stored in a hopper or hoppers within the vacuum chamber, which imposes constraints on, inter alia, optimum chamber size, quantity of stored powder, the replenishing of powder and associated loss of production time. In the case of apparatus embodying the present invention these issues are addressed by relocation of the powder source to outside the vacuum chamber, thus by provision of a powder supply unit 17 externally of the housing 11 to supply powder to the layer forming device 14 internally of the housing.

For this purpose the housing 11 is formed in one side wall with a transfer port 18 openable and closable by a movable door 19 which, when closed, hermetically seals the vacuum chamber 12 relative to the ambient atmosphere. The door 19 can be a swing door, a sliding door or a door embodying some other form of motion for transfer between a position of sealably closing and a position of opening the port 18. The door is shown in a closed state in Fig. 1 and in an open state in Fig. 2, in the latter case in an arbitrarily depicted displaced position.

Also provided at the mentioned chamber side wall is a flanged projection 20 which shrouds the port 18 and provides an access to the vacuum chamber 12. The flange of the projection 20 forms a coupling point for the powder supply unit 17.

The supply unit 17 comprises a casing 21 which contains a powder transfer chamber 22 and a powder reservoir 23 in the form of a hopper with a discharge duct, the casing being detachably mounted on the flanged projection 20 by way of a mating flange and an intermediate sealing gasket (not shown). Mounting in this way is merely exemplifying; mounting by any other suitable means is equally possible. Similarly, the transfer chamber 22 can be of any suitable shape consistent with its function as a powder conduit between the reservoir and the access provided by the projection 20.

Communication between the vacuum chamber 12 and the transfer chamber 22 by way of the port 18 with opened door 19 is possible, when a vacuum prevails in the chamber 12, if the transfer chamber is evacuated to substantially the same vacuum level, for example 10^-3 millibars. The casing 21 is constructed to withstand internal subatmospheric pressure and the desired level of vacuum can be established in the transfer chamber 22 by way of a pumping-down and venting connection 24.

Communication between the transfer chamber 22 and the reservoir 23 is controlled by a further openable and closable door 25 providing, when closed, a hermetic seal relative to the discharge duct of the reservoir and by a rotary drum 26 of a valve at the base of the reservoir hopper. The door 25, which like the door 19 can be a swing door, sliding door or some other form of door, is shown in a closed state in Fig. 1 and an open state in Fig. 2, in the latter case also in an arbitrarily depicted displaced position. The valve drum 26 has a powder-receiving pocket 26a which in a first rotational setting of the drum 26 as shown in Fig. 1 faces upwardly to accept a charge of powder from the reservoir 23 and in a second rotational setting representing half a revolution from the first setting and as shown in Fig. 2 faces downwardly to discharge the accepted powder into the discharge duct and, if the door 25 is open, thence into the transfer chamber 22. Although only one pocket 26a is shown, the drum preferably has two diametrically opposite pockets so that one is always filling or filled while the other is emptying or emptied.

The powder supply arrangement based on the external supply unit 17 is structured on dispensing powder into the vacuum chamber 12, specifically into the layer forming device 14, under gravity. Accordingly, the supply unit 17 is elevated in relation to the layer forming device 14 and the powder hopper of the reservoir 23 of the supply unit 17 represents the highest part of the supply unit as fitted to apparatus housing 11. Flow under gravity of powder released from the hopper is initially guided - when both doors 19 and 25 are open - by a discharge chute 27 arranged at an angle in the discharge duct to direct powder descending from the valve drum 26 downwardly into the transfer chamber 22 and also forwardly towards the transfer port 18. Further guidance of the powder flow is provided by a transfer chute 28 pivotable between an inoperative setting in which it is wholly located in the vacuum chamber as shown in Fig. 1 and an operative setting in which, with the door 19 displaced to open the transfer port 18, it extends between and into both the vacuum chamber 12 and transfer chamber 22 at such an angle as to receive powder from the discharge chute 27 and to direct the powder to a point where it can drop onto the layer forming device 14. Acceptance of powder by the layer forming device 14 from the transfer chute 28 takes place when the device 14, which is reciprocatingly movable above and across the table 13 as indicated by the double arrow in Fig. 1, is positioned at one side of the table to be below the lower end of the transfer chute 28 as shown in Fig. 2. The pivot movement of the transfer chute 28 is preferably linked with the movement of the door 19 so that when the latter is moved to open the port 18 the chute is automatically pivoted from its inoperative setting to its operative setting and conversely when the door is moved to close the port.

The layer forming device 14 has the form of a horizontally and reciprocatingly movable bar 29 with a powder reservoir 30 having a discharge slot at the side of the bar facing the table 13 and two levelling blades 31 depending from the bar on either side thereof to respectively lead and trail in the direction of movement of the device 14. Powder continuously discharged from the slot at the base of the reservoir 30 during movement of the bar 29 falls onto the table 13 and is spread to form thereon a thin layer, which is levelled and smoothed by the blades 31. The blades 31, which can be fixed or vertically adjustable, define the thickness of the spread powder layer, for example 50 microns, and the length of the slot in the base of the reservoir and the travel or stroke of the bar together determine the area of the layer, for example 300 x 300 millimetres. The mentioned dimensions merely serve to provide an example of scale and actual dimensions will be governed by such factors as the type and size of apparatus, type of size of article to be manufactured, power and deflection range of the electron beam, type of powder and other parameters of construction and use.

Since the reservoir 30 is intended to be replenished, during apparatus operation, by powder from the much larger reservoir 23 of the external supply unit 17 the reservoir 30 of the layer forming device 14 is designed to be of relatively small capacity, for example sufficient to hold 300 to 500 cubic centimetres of powder in the case of the exemplifying dimensions mentioned in the preceding paragraph. This volume of powder is sufficient for enough layers, for example 100, to keep the apparatus in continuous operation for a suitable length of time, for example up to about 50 minutes in the case of a layer forming and powder fusion time of about 30 seconds per layer. During operation, movement of the layer forming device 14 can be paused to enable replenishing of the reservoir 30 of the device 14 when the latter is positioned on the right in the vacuum chamber 12 as shown in Fig. 2. Replenishing is initiated by rotation of the valve drum 26 to discharge, from the pocket 26a, a received charge or batch of powder from the reservoir 23 of the supply unit 17 onto the discharge chute 27. From there, the powder is guided via the transfer chute 28 to fall directly into the reservoir 30 of the layer forming device 14 positioned under the lower end of the transfer chute. This replenishing procedure can be performed rapidly without the need to interrupt the manufacturing process. However, since the reservoir 23 of the supply unit 17 can be of substantial capacity and, as described further below, the external powder source represented by the supply unit can itself be replenished, additive layer manufacturing by the apparatus 10 can be carried out for an extended period of time regardless of the small powder capacity of the internal reservoir 30.

As previously mentioned, the flanged casing 21 of the supply unit 17 is detachably mounted on the flanged projection 20 of the vacuum chamber housing 11, thus allowing the unit as a whole to be removed from the housing and separated from the rest of the apparatus 10. Removal can - and is intended - to be carried out with the apparatus in operation, in which case the door 19 is closed to seal the transfer port 18. Removal is needed when the powder reserve in the reservoir 23 of the supply unit 17 is exhausted or nearly exhausted, which can be determined in various ways, for example by provision of a strain gauge or other weight or load measuring sensor detecting reduction in weight or load as the powder stock reduces, or an optical, electrical or other form of level sensor detecting level change in the reservoir 23, for example descent to a threshold. Once removed, the supply unit 17 can be replaced by an identical substitute unit, but with filled reservoir; the substitute unit is fitted to the flanged projection 20 of the vacuum chamber housing 11 in the same way as the unit it has replaced.

Use of the apparatus 10, in particular the powder supply arrangement in the context of performance of additive layer manufacturing by the apparatus, especially for carrying out the method exemplifying the invention, is self-evident from the preceding description. When operation of the apparatus is commenced - with a vacuum established in the vacuum chamber 12, the electron beam column 16 placed in operating condition in readiness for generation of the electron beam 15 and the supply unit 17 in place with a filled reservoir 23 - the transfer chamber 22 can be pumped down to the same sub-atmospheric pressure as the vacuum chamber 12 by way of the connection 24. When this pressure is established in the transfer chamber 22 the door 19 can be moved to open the port 18 and place the vacuum chamber 12 and transfer chamber 22 in direct communication. At the same time, the transfer chute 28 is pivoted into the operative setting shown in Fig. 2.

In the case of the described embodiment, the supply unit 17 is a self-contained module in which the reservoir 23, that is to say the powder hopper and discharge duct, is kept at a preselected sub-atmospheric pressure, specifically the same pressure as that intended to prevail in the vacuum chamber 12. The reservoir 23 is separated from the transfer chamber 22 by the hermetically sealing door 25, in which case the transfer chamber acts as a lock chamber between the reservoir 23 and the vacuum chamber 12. The reservoir 23 can be filled with powder and evacuated, at the outset, by way of a suitable port (not shown) at, for example, the top of the hopper.

Accordingly, after sub-atmospheric pressure at the appropriate level has been established in the transfer chamber 22 the door 25 can be opened as shown in Fig. 2 to expose the discharge chute 27 and place the vacuum chamber 12 in communication with the reservoir 23 via the transfer chamber 22. With the pocket 26a of the rotary drum 26 of the valve between the hopper and the discharge duct filled with a charge of powder the drum can now be rotated through half a revolution to discharge the charge of powder under gravity onto the discharge chute 27 and then by way of the transfer chute 28 through the port 18 and into the vacuum chamber 12 to drop into the reservoir 30 of the appropriately positioned bar 29 of the layer forming device 14. With the reservoir 30 thus filled, the bar 29 can be moved across the table 13 to dispense powder to form a layer, which is levelled by the blades 31. Thereafter the column 16 is operated to generate the electron beam 15 and produce controlled deflection of the beam to melt powder of the layer in a predetermined area corresponding with a crosssectional shape of an article to be manufactured. The manufacturing process by melting and fusion of powder through electron beam energy is well-known and is not an aspect of the present invention as such.

After a number of such powder layers has been formed by the layer forming device 14 and the content of the reservoir 30 in the bar 29 exhausted or nearly exhausted the reservoir 30 can be replenished by the simple expedient of repeat rotation of the valve drum 26 to discharge a fresh batch of powder from the reservoir 23 of the supply unit 17 into the vacuum chamber 12 to then drop into the reservoir 30 of the bar 29 when located below the lower end of the transfer chute 28. This replenishing process on a batch basis can be repeated as often as necessary, at least up to the point of exhausting the powder stock in the main reservoir 23. There is no need to specially switch off the electron beam, which is switched off in any case in the intervals between action on layers.

When the hopper of the reservoir 23 of the supply unit 17 is exhausted or substantially exhausted the unit 17 is replaced, as a module, by closing the door 19 to re-seal the transfer port 18, venting the transfer chamber 22 by way of the connection 24 to reestablish ambient pressure and then detaching the supply unit 17 from the coupling formed by the flanged projection 20. A replacement supply unit is fitted to the coupling as already described and the sequence of steps repeated to place the vacuum chamber 22 in communication with the transfer chamber 22 and reservoir 23 of the new supply unit, from which powder can then be discharged to resume batch filling of the reservoir 30 in the layer forming device 14.

The powder present in the reservoir 23 of the supply unit 17 is, in this embodiment, stored under vacuum as explained in the foregoing. This ensures that the powder remains dry, in particular does not absorb humidity from the ambient atmosphere, and is thus in an optimum condition for flowability both into and within the apparatus 10. In addition, since a large volume of powder has a large surface area any moisture or air trapped in the centre of the powder volume has a long and convoluted escape path, which lengthens outgassing time and consequently pump-down time to create a vacuum in the transfer chamber 22. Storage of the powder under vacuum is one method for reducing or eliminating these disadvantages.

However, to the extent that it is necessary or desirable the powder could be kept dry by other means including heating or use of a desiccant and in that case powder storage under vacuum in the external supply unit is superfluous. The door 25 providing a closure between the transfer chamber 22 and the reservoir 23 can be eliminated, together with the discharge chute 27 if, for example, the reservoir - in particular the hopper of the reservoir - is arranged to discharge more or less directly onto the transfer chute 28. Pumping down of the chamber 22 to sub-atmospheric pressure will in these circumstances lead to establishing the same pressure throughout the interior of the supply unit. In that case it may be feasible to provide for refilling of the hopper, for example by way of an openable cover, in situ, as long as refilling does not occupy such a length of time that it leads to an undesirably or unacceptably lengthy pause in the apparatus operation; a pause longer than, for example, 5 minutes may lead to cooling down of the last-laid powder layer and possible discontinuities in the article being manufactured. A further possibility would be configuration of the supply unit so that just the powder hopper of the reservoir is removable, either for refilling or for replacement by a refilled reservoir, preferably hermetically sealed to exclude ingress of humidity. Such a removable hopper could also include, for example, the rotary valve or other such element for metered discharge of powder, thus a self-contained cartridge. In such a construction, the entire supply unit as such need not be detachable from the vacuum chamber housing 11; merely the powder-containing part of the supply unit is removable. Various possibilities of powder storage in and powder replenishing of the external supply unit are conceivable, subject to the requirements of locating the bulk powder source outside the vacuum chamber and providing transfer of powder from that source to the layer forming means within the vacuum chamber by way of a transfer chamber that can be evacuated for compatibility with the vacuum environment of the vacuum chamber. This permits, as already indicated, not only a more compact size and more optimised design of the vacuum chamber by removal of the principal powder source to outside the chamber, but also scope to operate the apparatus for a more extended period of time without the disruption liable to be caused by venting the vacuum chamber to carry out powder refilling of a hopper or hoppers within the chamber.

Claims

1. Additive layer manufacturing apparatus comprising a housing bounding a vacuum chamber in which additive layer manufacturing can be carried out in a vacuum by the action of an electron beam on successive layers of fusible powder, layer forming means in the vacuum chamber for forming each powder layer and powder supply means outside the vacuum chamber for storing powder for supply to the layer forming means, the powder supply means being capable of being placed in an evacuated state substantially corresponding with a vacuum in the vacuum chamber and the vacuum chamber being communicable with the supply means in the evacuated state thereof for transfer of stored powder from outside the vacuum chamber to the layer forming means in the vacuum chamber.

2. Apparatus according to claim 1, wherein the housing is provided with a powder transfer port for the communication of the vacuum chamber with the supply means.

3. Apparatus according to claim 2, wherein the transfer port is openable and closable to control the communication.

4. Apparatus according to claim 3, comprising a movable door for opening and for sealably closing the transfer port.

5. Apparatus according to any one of the preceding claims, wherein the supply means after transfer of the stored powder is transferrable to a non-evacuated state and in the evacuated state thereof is refillable with powder in situ or is removable at least in part for refilling or for exchange for a filled replacement so that powder can be supplied to the layer forming means without loss of a vacuum in the vacuum chamber.

6. Apparatus according to claim 5, wherein the supply means is mounted on the housing to be detachable therefrom entirely or in part.

7. Apparatus according to any one of the preceding claims, wherein the supply means comprises a powder reservoir for storing the powder and a transfer chamber intermediate the reservoir and the vacuum chamber.

8. Apparatus according to claim 7, wherein the supply means comprises closure means for closing the transfer chamber relative to the reservoir.

9. Apparatus according to claim 8, wherein the reservoir is in a constantly evacuated state substantially corresponding with a vacuum in the vacuum chamber, the transfer chamber being transferrable between a non-evacuated state and an evacuated state so as to function as a lock between the vacuum chamber and the reservoir.

10. Apparatus according to any one of claims 7 to 9, wherein the supply means comprises a dispensing valve for dispensing powder from the reservoir to the transfer chamber.

11. Apparatus according to claim 10, wherein the valve comprises a rotary drum with a powder receiving compartment fillable and emptiable under gravity as a function of the rotational setting of the drum.

12. Apparatus according to any one of the preceding claims, wherein the supply means is provided with connecting means for connection with a source of subatmospheric pressure for placing the supply means in the evacuated state thereof.

13. Apparatus according to any one of the preceding claims, comprising guide means for guiding powder through the supply means and into the vacuum chamber.

14. Apparatus according to claim 13, wherein the guide means is arranged to guide the powder under gravitational force.

15. Apparatus according to claim 14, wherein the guide means comprises a transfer chute extending between and into both the vacuum chamber and the supply means when the vacuum chamber and supply means are in communication.

16. Apparatus according to claim 15, wherein the transfer chute is automatically movable into an operative position when the vacuum chamber and supply means are placed in communication and into an inoperative position when communication of the vacuum chamber and supply means is terminated.

17. Apparatus according to any one of claims 14 to 16, wherein the guide means comprises a discharge chute for guiding powder discharged from a stock thereof stored in the supply means.

18. Apparatus according to any one of the preceding claims, comprising measuring means for deriving from the supply means a measurement indicative of the quantity of powder therein.

19. Apparatus according to any one of the preceding claims, comprising monitoring means for monitoring the quantity of powder in the supply means and signalling depletion or imminent depletion of that quantity.

20. Apparatus according to any one of the preceding claims, wherein the layer forming means comprises a reservoir for storing a quantity of powder received from the supply means and is movable to provide metered discharge of powder from that reservoir to form each layer.

21. Apparatus according to claim 19, wherein the storage capacity of the reservoir of the layer forming means is sufficient for a quantity of powder usable to form a predetermined plurality of layers of given area.

22. Apparatus according to claim 19 or claim 20, wherein the layer forming means comprises levelling means for levelling each layer of discharged powder.

23. A method of supplying powder for use in additive layer manufacturing carried out in a vacuum in a vacuum chamber by the action of an electron beam or successive layers of fusible powder individually formed by layer forming means in the vacuum chamber, comprising the steps of storing a quantity of powder outside the vacuum chamber in powder supply means, establishing a vacuum in the vacuum chamber, placing the powder supply means in an evacuated state substantially corresponding with the vacuum in the vacuum chamber, placing the vacuum chamber in communication with the powder supply means in that state and during such communication transferring powder from the supply means to the layer forming means.

24. A method according to claim 23, comprising the further steps of terminating the communication of the vacuum chamber with the powder supply means prior to exhausting the stored powder through transfer to the layer forming means, transferring the powder supply means to a non-evacuated state and refilling the powder supply means with powder.

25. A method according to claim 23 or claim 24, wherein the powder is stored in the powder supply means constantly in a vacuum substantially corresponding with that of the vacuum in the vacuum chamber and the steps of placing the powder supply means in an evacuated state and placing the vacuum chamber in communication with the powder supply means comprise evacuating a lock chamber between the vacuum chamber and the stored powder and placing the evacuated lock chamber in communication with both the vacuum chamber and the stored powder.

26. A method according to any one of claims 23 to 25, wherein the step of transferring is carried out in metered batches.

27. A method according to any one of claims 23 to 25, wherein the step of transferring comprises transferring the powder solely by way of gravitational force.

28. A method according to any one of claim 23 to 27, comprising the step of providing an indication of the amount of powder remaining in the powder supplying means.

29. A method according to any one of claims 23 to 28, where a quantity of powder sufficient for a predetermined number of layers less than the number needed to complete manufacture of an article of a given volume is stored in the layer forming means and the step of transferring comprising periodically replenishing the powder stored in the layer forming means to the extent required for complete manufacture of the article.