Provision of a substrate for use in additive layer manufacture

Abstract

A method of additive layer manufacture (ALM) of an article in an ALM machine by producing a substrate 24 in-situ in the machine by cohering powder 27 together and then forming the article on the subst

Description

The present invention relates to a method of additive layer manufacture of an article in an additive layer manufacturing machine and has particular reference to provision of a substrate for use in such manufacture. Additive layer manufacture is a well-established procedure for manufacture of three-dimensional articles. In this procedure, an article is produced in an additive layer manufacturing machine by selectively heating and melting fusible powder material, for example plastics or metallic material, by irradiation with a scanning energy beam such as an electron or laser beam. Irradiation and the resulting melting and subsequent fusion are carried out in relation to each of a succession of layers of the powder material deposited on a support so that material in each layer is melted in accordance with a predefined pattern and fused not only within the layer, but also to any previously fused material of an underlying layer, whereby the article shape is created on a layer-by-layer or additive basis. Manufacture is typically carried out on a support in the form of a raisable and lowerable table in order to be able to progressively lower the support in equal steps in the course of article manufacture so that each layer of powder material can be deposited with its top surface lying in a defined reference plane. The machine also includes a powder material feed and distribution system, which typically comprises a reservoir of powder material and a spreader for spreading and levelling successive layers of powder material extracted from the reservoir. The spreader is conventionally a reciprocatingly movable blade controlled to execute a movement in one direction to spread and level powder material to form a layer and a movement in the opposite direction to spread and level powder material to form a next layer. The blade edge during movement describes a plane representing the reference plane, in relation to which a machine co-ordinate system governing energy beam scanning is calibrated. Rather than deposit the layers of powder material directly on the top of the table, use is conventionally made of a separately constructed and supplied intermediate substrate, termed a start plate, which defines a support surface for article support and which is generally treated as a consumable in relation to each cycle of article manufacture. The start plate is usually metallic and both thermally and electrically conductive so as to dissipate heat and, where it arises, electrical charge. Such a plate functions during article manufacture as a heat and charge sink. At the start of a cycle of article manufacture using such a start plate it is necessary to level the plate so that the support surface it defines, more particularly the plane in which that surface lies, is exactly parallel with the reference plane described by the movement of the spreader blade and at a predetermined height, more particularly at predetermined spacing below the reference plane. The spacing is equal to the depth of the table downward step and hence a given powder layer thickness. This ensures that in operation of the machine the top of each deposited layer lies in the reference plane and at the correct height for the required layer thickness. Positioning of the start plate so that its support surface is precisely parallel with the plane described by the blade and precisely at the correct height is undertaken by a machine operator and as a consequence can be the source of error, for which compensation has to be provided by calibration of software of the machine with reference to a known centre of the plate. In addition, in the case of an electron beam as scanning energy beam and metallic powder as powder material the start plate also has to be heated at the start of each manufacturing cycle to the sintering temperature of the respective powder material so that the deposited first layer, which is laid directly onto the top surface of the plate, will sinter and facilitate subsequent fusion to the plate, whereby the progressively formed article remains fixed to the plate and thus securely located during manufacture. Heating the plate and maintaining the temperature can, in relative terms, consume a considerable amount of energy and time. There is also a risk of warping of the plate due to intrinsic internal stresses, which is some cases can render a plate unusable. At the end of a manufacturing cycle the manufactured article can be separated from the start plate by a light separating force or, if that is not sufficient, by wire cutting. The plate, although in principle reusable, is usually discarded, which can represent an appreciable cost burden in the case of, for example, a run of successive builds of identical articles, particularly if the plate is an accurately machined part. However, because of cost implications and the preference for single-use plates the plates are not usually precision-machined, but simply ground to have parallel planar major surfaces. In some cases the degree of planarity and parallelism is not sufficiently accurate for use in an additive layer manufacturing process with its usual extremely close tolerances. Moreover, the plates are commonly of stainless steel, which is compatible with many of the metal powders used in such a process, but some such powders will satisfactorily adhere only to plates of a material with the same or similar composition and in that case need plates of more exotic materials, which can be difficult to produce and use. It would therefore be desirable to provide a method of additive layer manufacture of an article in an additive layer manufacturing machine without the disadvantages outlined above with respect to the support substrates, such as start plates, used in prior art methods, especially methods involving use of an electron beam for powder melting and fusion. In particular, it would be desirable to eliminate the effort connected with substrate positioning prior to the start of a manufacturing process and the associated susceptibility to error and also to provide a substrate which is free or substantially free of issues with respect to planarity and parallelism and which offers advantages in terms of cost. Other objects and advantages of the invention will be apparent from the following description. According to the present invention there is provided a method of additive layer manufacture of an article in an additive layer manufacturing machine, comprising the steps of producing a substrate in situ in the machine from powder by causing the powder to cohere to form a stable support body and thereafter manufacturing an article in the machine by depositing successive layers of fusible powder on the substrate and selectively fusing areas of the layers in accordance with a predetermined shape of the article. Through production of the required substrate in situ in the machine where additive layer manufacture of the article is to be carried out as a next step, the need to import a separately produced substrate into the machine and the accompanying time and effort involved in correctly positioning the substrate in relation to, especially, a reference plane for article manufacture carried out on the substrate are eliminated. Instead, the substrate can be produced from powder material utilising the equipment of the machine itself, especially an energy beam used for the step of additive layer manufacture of the article. In that case, machine metrology and co-ordinate systems used in the layer-by-layer manufacture of the article are on hand for production of the substrate, so that the latter can be appropriately positioned as part of the production process, especially with respect to a reference plane relating to powder distribution, within the machine without the aligning procedures essential for an imported substrate as in the prior art. Further, the potential difficulties associated with substrate accuracy in the case of substrates separately produced outside the machine, such as surface planarity and internal stresses, can be avoided by substrate production from powder within the machine using the machine systems geared up for production of the article from powder. Provision of the substrate thus becomes a fully integrated and self-contained aspect of machine operation for article manufacture. For preference the step of producing the substrate comprises sintering the powder for the substrate, thus production from the powder of a cohesive body consisting of a solid mass of the powder in which the powder particles adhere to one another. Such a sintered body has the necessary rigidity and structural integrity to function as a firm support for the article to be subsequently produced thereon by additive layer manufacture. The powder for the substrate is preferably sintered by heating to a temperature sufficient to cause sintering, but not melting of the powder. The extent of heating can be selected to provide a desired degree of sinter, but with the heating remaining below the known melting point of the respective powder. Since the substrate is produced in situ in an additive layer manufacturing machine use can advantageously made of the same machine systems, which are provided for additive layer manufacture, to create the substrate. Thus, in the case of a machine using an electron beam in the manufacturing process the same beam, but operating with a reduced beam current and / or dwell time during beam scanning or faster beam scanning speed, can be utilised to heat the substrate powder to a lower level leading to sintering rather than melting. The step of producing the substrate preferably comprises depositing the powder for the substrate in successive layers up to a predetermined depth of the support body. This is a procedure analogous to article additive layer manufacture and accordingly performable by the same machine systems provided for powder deposit during that manufacture. As in the case of powder heating by machine equipment, i.e. a beam generating, focusing and scanning system, powder deposition by systems integrated in the machine provides a substantial economic advantage by comparison with use of separate systems and procedures for outside creation of substrates for later importation into the machine. In one preferred procedure for producing the substrate the powder deposited for that purpose is caused to cohere on a layer-by-layer basis, which has the advantage of permitting change of heating parameters between different layers, for example to heat initial layers at a gentler rate and subsequent layers at a faster rate to balance speed of creation of the substrate with risk of layer surface degradation due to charge-build-ups and consequent repulsion of particles. In an alternative procedure, the powder for the substrate is caused to cohere after all layers have been deposited, such as by heating only the top surface of the body formed by the powder layers and allowing heat to dissipate into and heat the powder below. Although not obligatory, a significant advantage arises if the powder for the substrate and the powder for the article are the same. This means that the potential issue in the prior art of an article to be produced from a particular powder being unable to satisfactorily adhere to a substrate of unlike material, such as stainless steel, is eliminated. If the powder of the article is the same as the powder of the substrate the two components are of the same material and will adhere to one another during the phase of article manufacture. Further, the powder for the substrate and the powder for the article can then be drawn from a common powder feedstock of the machine, which represents a further economy in substrate production. Apart from the mentioned common use of machine systems for powder deposition and powder heating the two steps of producing the substrate and manufacturing the article can advantageously comprise levelling the powder used in those steps by a common levelling device of the machine. This results in the major advantage that the reference plane for the top of the substrate, thus the article support surface, is established by the powder levelling device used for article manufacture, as a result of which the previously mentioned difficulties associated with planarity and parallelism of the surfaces of an imported substrate and precise location of the top surface of such a substrate in the reference plane no longer arise: the levelling device of the machine determines the plane of the top surface of the substrate so that this plane and the reference plane are one and the same. The levelling device itself is preferably a linearly movable blade, which represents a simple means suitable for levelling powder layers of both the substrate and the article. The substrate can be of various forms, but preferably is plate-shaped and thus suitable for a wide range of differently shaped and sized articles to be manufactured. An advantage of in situ production of the substrate is that it can be individually created in each manufacturing cycle with specific dimensions appropriate to the requirements of an article to be manufactured in that cycle. In the case of an article of small area, the substrate can be of correspondingly small area and thus provide a saving in quantity and cost of material. In that connection, appropriate measures can be undertaken in a case where the predetermined shape of the article to be manufactured has an overhang which has to be supported to avoid fracture or collapse. The method may then include an additional step of producing a support structure for the overhang during manufacture of the article. As the article is manufactured, so an ultimately sacrificial support structure standing on the substrate separately from the article is conjunctively produced layer-by-layer until start of the overhang, which is then formed on top of the structure. The support structure can be in the form of, for example, a leg or legs of suitable size and, like the article itself, can be created from fused rather than sintered powder. As indicated in the foregoing, the method is particularly suitable for performance in an additive layer manufacturing machine in which at least one of causing the powder for the substrate to cohere and fusing of the powder of the article can be carried out by electron beam irradiation. A preferred example of a method according to the present invention will now be more particularly described with reference to the accompanying drawing, in which: Fig. 1 is a diagrammatic elevation of part of an additive layer manufacturing machine in which in situ production of a substrate is shown taking place by a method exemplifying the present invention; and Fig. 2 is a diagrammatic elevation of the entire machine, to reduced scale, in which additive layer manufacture of an article on the substrate is shown being carried out by that method. The method of the present invention relates to the field of additive layer manufacture for generation of three-dimensional articles of predefinable shapes by selective melting and fusion of powder material, especially a metallic material such as, for example, Ti-6AI-4V titanium-aluminium-vanadium alloy, in successively deposited layers of the powder material. In the described example of the method, melting is achieved by the action of an electron beam, although thermal energy could also be provided by another suitable energy source. The powder material could, however, also be a plastics or other meltable material. In the case of the preferred use of an electron beam, manufacture is carried out in an additive layer manufacturing machine 10 shown in highly diagrammatic form in Fig. 2. The environment for operation with an electron beam is provided by a housing 11 bounding a vacuum chamber 12 in which a vacuum, as a precondition for propagation of an electron beam 13, can be established. The beam 13 is generated by an electron gun 14 located at the top of the housing 11 and oriented to direct the generated beam downwardly along a vertical neutral axis. The beam 13 can be focused and deflected relative to the axis by, for example, an electromagnetic focusing and deflecting lens assembly 15, deflection taking place in X and Y directions as exemplified by the arcuate double arrow for opposite directions in the X sense. Appropriately controlled beam deflection allows movement of the point of incidence of the beam on a target to cover an area of predetermined shape corresponding with an individual cross-sectional layer of an article undergoing manufacture. Provided in the vacuum chamber 12 at a spacing below the electron gun 14 and in the target region of the generated beam is a raisable and lowerable support, thus a support movable in Z direction as indicated in Fig. 1 by an associated double arrow, for a bed formed from the successively deposited layers of the powder material. The support has the form of a stage consisting of a support table 16 which is mounted on a post or posts and guided for vertical movement in a shaft, the boundary wall or walls of which confines or confine the material bed, when present, to the table. Above the shaft, the table is enclosed by a surround 17 with a planar surface with which the top of a support substrate on the table 16 - or subsequently the top of a powder material layer on that substrate - is alignable to lie in a common plane, which defines a reference plane for, in particular, determination of the beam focus and deflection within a co-ordinate system. The vertical movement of the table 16 is provided by a drive (not shown) which can be a pistoncylinder unit, spindle drive, rack-and-pinion drive, linear motor or any other suitable means of imparting periodic reciprocating linear movement. The cycle of movement of the table 16 in the context of additive layer manufacture of an article in a conventional procedure commences with positioning the top of the support substrate on the table below the reference plane to the extent of a defined depth or thickness of a first layer of the powder material, spreading powder material on the substrate to form the first layer with its top surface in the reference plane and selectively melting and thereby fusing powder material in the layer by heat generated by the focused electron beam 13 under progressive scanning of the area of the layer corresponding with an associated cross-section of the article. Thereafter, the procedure continues by lowering the table to the extent of the defined layer depth in readiness for formation of a second such layer, spreading powder material on the fused first layer to form the second layer also with a top surface thereof in the reference plane and selectively melting powder material in that layer to fuse the material not only together in that layer, but also to the already-fused material of the first layer thereunder. This procedure is repeated so that the selective melting of successive layers of powder material by scanning of the material bed by the electron beam 13 forms, in conjunction with incremental descent of the table 16, successive cross-sectional layers of an article of desired shape in X, Y and Z directions. Spreading of the powder material to form each layer is by way of a spreader which has a spreader blade 18 and which is movable over a build zone - represented by the region of the table 16 and substrate thereon - to smoothly distribute powder material into a layer having a level top surface lying in the above-mentioned reference plane for that surface. Movement of the spreader blade 18 for reciprocating travel in X direction, as indicated in Fig. 1 by a wheel running on a dashed-line track and an associated double arrow, over the build zone is provided by a drive (not shown) which can be a cogged belt and pinion drive, spindle drive, rack-and-pinion drive, linear motor or any other suitable means of imparting periodic reciprocating linear movement. Spreaders of such a kind are used in various forms of construction in prior art machines. The powder material for the layers forming the powder material bed is supplied by a dispenser 19 suitably positioned in relation to the table 16, for example above the surround 17 on the lefthand side of the table. Dispensers are known in a variety of forms in the prior art and can comprise, for example, a hopper containing a charge of powder material sufficient for manufacture of an individual article or given number of articles. Typically, the spreader itself is equipped with a dispenser and fed from such a hopper or periodically from a powder reserve outside the housing 11. The movements of the table 16 and spreader blade 18 in the course of a manufacturing cycle, that is to say creation of an individual cross-sectional layer of the article, are appropriately correlated. Specifically, the drive of the support table and the drive of the spreader are controlled to vertically move the table and horizontally move the spreader blade at specific times and in specific directions during each cycle, such control being provided by a suitable control system (not shown) with programmable software. The general construction and operation of additive layer manufacturing machines using an electron beam to selectively melt and fuse powder on a layer-by-layer basis are known in the art and do not require more detailed description. The method exemplifying the present invention is intended to be performed by a machine of the kind described, but can also be carried out on a machine of different design, but with features appropriate to performance of the method. The method exemplifying the invention is distinguished from methods of the prior art by production of the substrate in situ in the machine 10 followed preferably directly by manufacture of the desired article on the substrate. A first phase of the method, namely production of the substrate in the machine, is illustrated by way of example in Fig. 1 and a second phase, namely manufacture of the article in the same machine, is illustrated by way of example in Fig. 2. Both the substrate and the article are generated on a layer-by-layer basis from powder material drawn from the same feedstock, namely the powder reserve held in the dispenser 19, and the layers of both the substrate and the article are spread and levelled by the same component, namely the spreader blade 18, so that the top of the substrate will necessarily lie in the reference plane correlated with the beam coordinate system. A distinction between the substrate and the article as explained further below is that, in the preferred example, the powder material of the former is merely sintered to cause particle adhesion within a cohesive body and the powder material of the latter is melted to cause particle fusion within an amorphous solid mass. Although the method is a two-stage procedure, in the case of a special form of article with an overhang, i.e. a cantilever section, a support structure for the overhang can be produced in the second stage as an extension of the substrate; however, the structure is produced from fused powder and as such equates with a sacrificial part of the article. Referring now to Fig. 1 showing in situ substrate production in the first stage of the method, the table 16 is positioned by its drive so that its top surface is below the reference plane here conveniently coincident with the plane of the shaft surround 17, by a distance equal to the depth of each layer of the substrate. The machine 10 is, beforehand, placed in operational condition for electron beam generation by pumping down or evacuating the housing 11 in a hermetically sealed state to create a high-level vacuum in the chamber 12. A first layer 20 of powder material, in particular a material - such as a metallic powder -from which the article is to be subsequently manufactured, from the dispenser 19 is spread over the top of the support table 16 by the spreader blade 18 to the predetermined layer depth, for example 0.1 mm, with the layer top surface lying in the reference plane. The electron gun 14 is then energised to generate the electron beam 13 which is focused and deflected by the lens assembly 15 to progressively scan a selected area of the layer 20 with a beam current and at a predetermined scanning speed or with a dwell time at each point in the beam scan path sufficient to heat the powder particles to an extent causing them to adhere to one another, but not to melt and fuse. The heated layer thus produces in effect a skin of sintered powder on part or all of the top surface of the table. Although the entire layer could be heated, preferably heating is confined to a selected smaller area, for example of circular shape, corresponding with a substrate shape and area size appropriate to support of the article to be subsequently produced. Since heating of the layer 20 by the electron beam 13 imparts a charge to the powder material and particles at the layer top surface may accumulate a static negative charge liable to cause mutual repulsion and explosive departure from the layer the beam current is kept low and scanning speed high to assist with counteracting the effects of negative charging. After sintering of the first powder material layer 20 of the substrate is complete the table 16 is lowered by the layer depth and a second layer 21 of powder material is deposited on the sintered layer 20 and levelled by the spreader bar 18. The sintered layer 20 acts as a charge ‘sink’ and decreases the risk of particle repulsion and escape from the second layer 21, which is preferably heated and sintered in similar manner to the first layer, thus with controlled management of the parameters of heating by the beam 13. Sintering of the layer 21 also produces adhesion by sintering to the layer thereunder, i.e. the layer 20. This procedure is repeated with a number of succeeding layers until it is assessed that that there is a sufficient depth of sintered powder material on the table 16 to allow an increase in beam energy and therefore charge without causing particle repulsion and escape at the top surface of the last-laid layer. Further layers of powder material can then be laid and individually sintered by the action of the beam 13 with use of a higher beam current and slower beam scanning speed so as to accelerate the speed of creation of the sintered layers in relation to the preceding layers. When the entire sintered mass has attained a depth sufficient to dissipate charge to an extent allowing a further increase in energy input subsequent layers of powder material can be laid and sintered with even more aggressive parameters of beam heating action so that the final layers can produced relatively quickly to complete in situ production of the substrate. A substrate with dimensions corresponding with those of a typically used start plate produced separately and outside the machine in a conventional process may be able to be completed in, for example, approximately 20 minutes. Fig. 1 shows initial production of the substrate advanced to the point of sintering four sintered layers, which are designated 20, 21 and 22 and are, of course, depicted with greatly exaggerated depth, and a fourth layer in the course of formation by initial laying and levelling of its constituent powder material 23 by the spreader blade 18. As indicated in the foregoing, a substantial number of other layers is formed on those four layers in order to complete the substrate. Additive layer manufacture of the desired article can directly follow production of the substrate, which in completed form is denoted in Fig. 2 by 24 and has a thickness of, for example, 15 mm. Since the substrate is produced in the same machine intended for manufacture of the article, creation of the substrate can be regarded as an initial phase and creation of the article itself as a subsequent phase of a continuous or substantially continuous manufacturing process. Referring now more specifically to Fig. 2, manufacture of the article can be carried out, for example, in an essentially conventional additive layer manufacturing procedure in which following completion of the substrate 24 the table 16 is lowered to the extent of a predetermined depth or thickness of a layer of the article, which conveniently can be the same as the individual layer depth of the substrate. A first layer 25 of the same powder material as that used for the substrate is then deposited on and spread by the spreader blade 18 over the top of the substrate, which may require heating beforehand if due to a delay between the end of the initial phase and start of the subsequent phase of the method the residual heat from the sintering process drops to a level below that needed for required adhesion of the layer 25 to the substrate 24. If reheating of the substrate is needed, this can be performed with the same final parameters of heating applicable to the substrate production. After the powder material for the first layer 25 has been laid and levelled to form the layer the layer is scanned by the beam 13 to heat the powder material to such an extent as to melt and thereby fuse the powder particles into a solid mass, which, due to the melting, also attaches to the top of the sintered substrate. The attachment not only locates the subsequently manufactured article in three dimensions, but also provides a thermally and electrically conductive connection with the substrate for heat and charge dissipation into the latter. Scanning of the layer 25 in its powder state is carried out selectively in accordance with the shape of an individual cross-section, which is assigned to the layer, of the article. Following the action of the beam the layer 25 will consist of an area or areas of fused powder material corresponding with that cross-section and an area or areas of unfused, thus still non-adherent powder in particulate state. Manufacture of the article can then continue by deposit of successive layers 26 of the powder material one on top of the other in conjunction with incremental lowering of the table 16 prior to each deposit, followed by beam heating of each deposited layer to selectively fuse the material of the layer, again in accordance with an individual crosssection of the article assigned to that layer, not only within the layer, but also to the already-fused material of the respective layer below it. In the early stages of manufacture of the article less beam energy is deployed to heat the material of the layers since the sintered powder material of the substrate 24 has lower thermal conductivity than, for example, a metal substrate, as a consequence of which the substrate in relative terms functions less effectively as a heat sink and accordingly may be less able to counteract a tendency of initial layers of the article to overheat if the energy input is not managed in a controlled manner. Deposit and selective fusion of succeeding powder material layers is continued in the same way, but with greater beam energy when there is greater distance from the substrate, until the article is formed as a solid body with the desired shape and dimensions provided by the totality of fused areas of the layers. The finished article can usually be separated from the substrate 24 by manually breaking off under a light separating load, as can the substrate from the table 16. If necessary, however, either can be separated by wire-cutting. Fig. 2 shows the article manufacture advanced to the stage of completion of the first layer 25 and six subsequent layers 26 and also deposit and levelling of powder material 27 by the spreader bar 18 for formation of a further subsequent layer. The layers are again shown with greatly exaggerated thicknesses. The illustrated layers are succeeded by a large number of further layers depending on the intended height of the article concerned. Also shown in Fig. 2, by way of arbitrary example, are measures taken in a case where the article being manufactured has an overhang 28, here represented by a projecting cantilever portion of the top two formed layers 26 and the further layer undergoing formation thereabove. In order to support the overhang a support structure 29, in this case in the form of a plurality of very thin, spaced-apart legs, is formed from discrete sections of each deposited powder material layer of the article, these sections being heated and fused in the same manner and at the same time as that area or those areas of the layer forming a cross-section of the actual article. The legs adhere to the substrate 24 in the same manner as the article proper and extend to and connect with the underside of the overhang. To that extent the support structure 29 may be regarded as a sacrificial or extraneous part of the article and is later removed by breaking the legs off the substrate as well as the article. Any residual material of the legs can be removed from the article by grinding or another suitable process. After completion of manufacture of the article, the shape of which is indicated in Fig. 2 merely as part of a block, further articles each with a respective in situ sintered substrate can be produced in the same way in successive manufacturing cycles. It may be possible to produce a new substrate for a next article directly on top of an article just completed; the first layer for the new substrate is then laid on the top or final layer of that article, namely the fused powder material of the layer as well as the still-surrounding unfused material. Equally, it may be possible reuse a substrate for manufacture of a further article, particularly of the same kind. It will be self-evident that rather than just a single article, multiple articles can be simultaneously produced on a single substrate 24. This is achieved simply by appropriate programming of a control for the beam focusing and deflection lens assembly 15 to determine the beam scanning path. In the described example, production of the substrate is carried out on the basis of layer-by-layer deposit of powder material and individual heating and sintering of each layer before deposit of the next layer thereon. As already mentioned, it is alternatively possible, while still employing layer-by-layer deposit so as to take advantage of the available equipment of the machine, to undertake deposit of all layers and then heat the entirety from the top surface to allow heat to dissipate into and heat the powder material below. It is also possible to combine the two approaches and heat and sinter not each layer individually, but a number of layers as a proportion of the whole. A method exemplifying the invention as described in the foregoing offers the significant advantage that an article support or start plate for an additive layer manufacturing process can be created within the machine used for the process and directly in advance of the process. The method not only potentially saves cost, but also overcomes the difficulties connected with the quality and accuracy of an outside-produced start plate and eliminates the complication involved in correctly positioning such a plate in the machine.

Claims

1. A method of additive layer manufacture of an article in an additive layer manufacturing machine, comprising the steps ofproducing a substrate in situ in the machine from powder by causing the powder to cohere to form a stable support bodyand thereafter manufacturing an article in the machine by depositing successive layers of fusible powder on the substrate and selectively fusing areas of the layers in accordance with a predetermined shape of the article.

2. A method according to claim 1, wherein the step of producing the substrate comprising sintering the powder for the substrate.

3. A method according to claim 2, wherein the powder for the substrate is sintered by heating to a temperature causing sintering, but not melting of the powder.

4. A method according to any one of the preceding claims, wherein the step of producing the substrate comprising depositing the powder for the substrate in successive layers up to a predetermined depth of the support body.

5. A method according to claim 4, wherein the powder for the substrate is caused to cohere layer-by-layer.

6. A method according to claim 4, wherein the powder for the substrate is caused to cohere after all layers have been deposited.

7. A method according to any one of the preceding claims, wherein the powder for the substrate and the powder for the article are the same.

8. A method according to claim 7, wherein the powder for the substrate and the powder for the article are drawn from a common powder feedstock of the machine.

9. A method according to any one of the preceding claims, wherein the step of producing the substrate and the step of manufacturing the article comprise levelling the powder used in those steps by a common levelling device of the machine.

10. A method according to claim 9, wherein the levelling device is a linearly movable blade.

11. A method according to any one of the preceding claims, wherein the substrate is plate-shaped.

12. A method according to any one of the preceding claims, wherein the predetermined shape of the article to be manufactured has an overhang and the method includes producing a support for the overhang during manufacture of the article.

13. A method according to any one of the preceding claims, wherein at least one of causing the powder for the substrate to cohere and fusing of the powder of the article is carried out by electron beam irradiation.

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