Abstract:
An additive layer manufacturing method includes the steps of: a) laying down powder layer on powder bed, and b) focussing energy on an area of powder layer to fuse area of powder layer and thereby form a cross-section of the product; wherein steps a) and b) are repeated to form successive cross-sections of product, and wherein at least one of said steps b) involves focussing energy on an area of respective powder layer which is unsupported by a previously formed cross-section of product to thereby form a downwardly facing surface of product. Method is at least some of said successive steps b) involve focussing energy on a support area of respective powder layer, to fuse support area and thereby form successive cross-sections of a support pin within powder bed, support pin extending outwardly from downwardly facing surface of product when it is formed, so as to support downwardly facing surface.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from British Patent Application Number 1310762.8 filed 17 Jun. 2013, the entire contents of which are incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Disclosure 
         [0003]    The present invention relates to an additive layer manufacturing (ALM) method, and more particularly relates to an ALM for the production of a three-dimensional product via successive fusion of parts of a powder bed, said parts corresponding to successive cross-sections of the product. 
         [0004]    2. Description of the Related Art 
         [0005]    Additive layer manufacturing has become more widely used over recent years in order to produce three-dimensional products. Electron Beam Melting (EBM) is a particular type of ALM technique which is used to form fully dense metal products (such as component parts for gas turbine engines in the aerospace industry). The technique involves using an electron beam in a high vacuum to melt metal powder in successive layers within a powder bed. Metal products manufactured by EBM are fully dense, void-free, and extremely strong. 
         [0006]      FIG. 1  illustrates a known configuration of apparatus  1  which is used in an EBM method to produce a three-dimensional metal product  2  from metal powder  3 . The apparatus comprises an adjustable height work platform  4  upon which the product  2  is to be built, a powder dispenser  5  such as a hopper, a rake  6  or other arrangement operable to lay down a thin layer of the powder  3  on the work platform  4  to form the powder bed  7 , and an electron beam column  8  for directing and focussing an electron beam  9  downwardly on the powder bed  7  in order to melt parts of uppermost layer of the powder bed  7 . The entire apparatus is housed within a vacuum housing and the operative parts are computer controlled. 
         [0007]    During operation, the electron beam column  8  is energised under the control of the computer to focus the electron beam onto the powder bed  7  and to scan the beam across the powder bed to melt a predetermined area of the top layer of the powder bed  7  and thereby form a cross-section of the three-dimensional product  2 . 
         [0008]    The three-dimensional product  2  is built up by the successive laying down of powder layers on the powder bed  7  and melting of the powder in predetermined areas of the layers to form successive cross-sections of the product  2 . During a work cycle the work platform  4  is lowered successively relative to the electron beam column  8  after each added layer of powder has been melted, ready for the next layer to be laid down on top. This means that the work platform  4  starts in an initial position which is higher than the position illustrated in  FIG. 1 , and in which position a first layer of powder of necessary thickness is laid down on the work platform  4  by the rake  6 . In order to prevent damage to the work platform  4  by the electron beam  9 , the first layer of powder is typically made thicker than the other applied layers, thereby preventing melt-through by the electron beam  9 . This is why the product  2  appears spaced above the work platform  4  within the powder bed  7  in  FIG. 1 . The work platform  4  is then successively lowered for the laying down of a new powder layer for the formation of a new cross-section of the product  2 . 
         [0009]    When the electron beam  9  impinges on the top layer of powder within the powder bed  7 , the kinetic energy of the electrons is transformed into heat which melts the powder to form the respective cross-section of the product  2 . The layer previously scanned usually serves as a rigid support for the next layer above. However, when the product has a shape which defines an overhanging or downwardly facing surface  10  such as is illustrated in  FIG. 1 , then the top layer of powder being scanned by the beam  9  will not have a rigid support beneath it. In this context, a downwardly facing surface  10  is defined as one whose orientation enables its projection onto a horizontal two-dimensional plane below it, as illustrated schematically in  FIG. 2 . 
         [0010]    If no support is provided beneath downwardly facing surfaces  10  of the product  2  as it is formed, then localised overheating can occur during melting of the powder by the beam  9  which can result in poor surface finish to the product. Also, distortion of the product can occur and so it has been proposed previously to provide some means of mechanically fixing the product in place relative to a metallic substrate or base plate  11  on which the product is formed. 
         [0011]    Previously proposed support structures  12  designed to avoid the problems mentioned above in relation to unsupported downwardly facing surfaces generally consist of an array of thin walls that are manufactured at the same time as the product and from the powder via the same EBM technique. These thin walls are created so as to extend between the downwardly facing surface  10  and another solid surface. The other solid surface can either be a base plate  11  on which the product is formed, or a previously formed upwardly facing surface of the product in the case where the downwardly facing surface  10  is formed above such a surface. These structures  12  are typically referred to as ‘wafers’, and are illustrated schematically in  FIGS. 3 and 4 . The wafers are designed to be removable from the downwardly facing surface  10  by hand or machine tools during a subsequent finishing procedure. 
         [0012]    As illustrated most clearly in  FIG. 4 , the thin-walled structures or wafers  12  may be provided in a lattice configuration as viewed from below the downwardly facing surface  10 , which thus defines an array of small spaces  13  between the wafers  12 . It is common for powder feedstock to become trapped in these spaces  13  as the wafers are built up during the EBM process, and the trapped powder can then be difficult to remove and separate from the fused powder forming the wafers  12  themselves during the subsequent finishing process. The trapped powder is thus typically discarded rather than being recycled for subsequent use. Powder feedstock used to manufacture component parts of gas turbine engines is typically very expensive, so this wastage increases the overall manufacturing cost. 
         [0013]    It has often been found that the wafer supports  12  produced by prior art methods can be difficult to remove from some intricately shaped products during subsequent finishing processes. 
         [0014]    The deposition of wafer supports  12  must start on a solid substrate, which as mentioned above can either be an area of the component being manufactured, or an underlying base plate. The surface finish and geometrical tolerance of the component in contact with the wafers is also reduced and the total foot print of the supported component is increased. Both reduce the manufacturing efficiency for the component 
         [0015]    Distortion of the downwardly facing surface  10  represents another problem that can arise when utilising wafer supports  12 . This distortion typically arises from the formation of concave regions  14  on the supported surface in the area between each wafer support, as illustrated schematically in  FIG. 5 . 
       OBJECTS AND SUMMARY 
       [0016]    It is a preferred object of the present invention to provide an improved ALM method for the production of a three-dimensional product. 
         [0017]    According to the present invention, there is provided an additive layer manufacturing (ALM) method for the production of a three-dimensional product via successive fusion of parts of a powder bed, said parts corresponding to successive cross-sections of the product, the method comprising the steps of: a) laying down a powder layer on said powder bed, and b) focussing energy on a predetermined area of said powder layer to fuse said area of the powder layer and thereby form a cross-section of the product; wherein steps a) and b) are repeated to form successive cross-sections of the product, and wherein at least one of said steps b) involves focussing said energy on an area of the respective powder layer which is at least partially unsupported by a previously formed cross-section of the product to thereby form a downwardly facing surface of the product, the method being characterised in that at least some of said successive steps b) involve focussing energy on a support area of the respective powder layer which is spaced from the predetermined area of the powder layer, to fuse the support area and thereby form successive cross-sections of a support pin within the powder bed, the support pin extending outwardly from the downwardly facing surface of the product when it is formed, so as to support the downwardly facing surface. 
         [0018]    Preferably, at least some of said successive steps in which energy is focussed on a support area of a respective powder layer also involve focussing energy on a said predetermined area of the powder layer to fuse said area of the powder layer and thereby form a cross-section of the product, the support area and the predetermined area being spaced apart. 
         [0019]    Said support pin preferably extends generally downwardly from said downwardly facing surface of the product. 
         [0020]    Said successive steps in which energy is focussed on a support area of the respective powder layer may involve focussing energy on a plurality of said support areas in spaced relation to one another, to thereby form successive cross-sections of a plurality of said support pins, the support pins being formed in a spaced array within the powder bed. 
         [0021]    Preferably, said support pins are parallel to one another. Alternatively, however, the pins may be non-parallel to one another. 
         [0022]    In preferred embodiments the or each support pin is approximately cylindrical, and may optionally have a diameter in the range 0.2 mm to 2 mm. It should be noted, however, that the pins can have alternative cross-sectional profiles such as, for example, square or hexagonal. 
         [0023]    In some embodiments of the invention the or each said support area is circular, and energy is focussed on successive said support areas of respective powder layers which are in alignment to one another to form successive circular cross-sections of the or each support pin which is thus cylindrical. In such an embodiment, the or each support pin may thus be formed so as to extend vertically within the powder bed. 
         [0024]    Alternatively, the or each said support area is approximately elliptical, and energy is focussed on successive said support areas of respective powder layers which are imbricated to form successive elliptical cross-sections of the or each support pin which is thus cylindrical. In such an embodiment, the or each support pin may thus be formed so as to extend non-vertically within the powder bed. 
         [0025]    In preferred embodiments of the method, the or each support pin has a free end which is formed within the powder bed. 
         [0026]    Preferably, the free end of the or each said support pin is spaced from any other surface of the product, and is also spaced from any base plate used to support the powder bed. 
         [0027]    The free end of the or each said support pin may be formed by focussing energy on an initial support area which is supported only by underlying unfused powder in the powder bed, to fuse said initial support area and thereby form the free end. 
         [0028]    Preferably, the method involves Electron Beam Melting and is used to manufacture metal products. Accordingly, said powder is preferably metal powder, and said steps of focussing energy on said areas of the powder layers preferably involves the use of an electron beam to melt said areas of the powder layers. 
         [0029]    According to another aspect of the present invention, the above-defined method may be used to manufacture a component of a gas turbine engine, and involves the step of removing the or each said support pin from said product to form said component. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]    So that the invention may be more readily understood, and so that further features thereof may be appreciated, embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: 
           [0031]      FIG. 1  (discussed above) is a schematic vertical cross-sectional view showing a generally conventional apparatus suitable for use in an ALM method for the manufacture of a three-dimensional product from powder feedstock; 
           [0032]      FIG. 2  (discussed above) is a schematic cross-sectional view showing a product with a downwardly facing surface; 
           [0033]      FIG. 3  (discussed above) is a view similar to that of  FIG. 2 , but which shows the downwardly facing surface supported by prior art wafer supports; 
           [0034]      FIG. 4  is a schematic underneath plan view showing the wafer supports of  FIG. 3  in more detail; 
           [0035]      FIG. 5  is an enlarged cross-sectional view showing distortion of a downwardly facing surface of a product, between the prior art wafer supports; 
           [0036]      FIG. 6  is a schematic cross-sectional view showing part of a product having a horizontally extending, downwardly facing surface supported by a plurality of support pins formed via the method of the present invention; 
           [0037]      FIG. 7  is a schematic underneath plan view showing the arrangements of support pins in further detail; 
           [0038]      FIG. 8  is a schematic cross-sectional view showing a product having downwardly facing surfaces which can be manufactured via the method of the present invention; 
           [0039]      FIG. 9  is a schematic illustration showing an initial step of the method of the invention; 
           [0040]      FIG. 10  is a view similar to that of  FIG. 9 , but which shows a subsequent step of the method; 
           [0041]      FIG. 11  shows another subsequent step of the method involving the fusion of areas of a layer of powder; 
           [0042]      FIG. 12  is a plan view from above, showing the arrangement of the fused areas shown in  FIG. 11 ; 
           [0043]      FIG. 13  shows a subsequent step of the method; 
           [0044]      FIG. 14  shows yet another subsequent step of the method; 
           [0045]      FIG. 15  is a schematic cross-sectional view showing part of a product having an inclined and downwardly facing surface supported by a plurality of support pins which may formed via the method of the present invention. 
           [0046]      FIG. 16  is a horizontal cross-sectional view taken along line I-I of  FIG. 15 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0047]    Turning now to consider the drawings in more detail, the method of the present invention will now be described in detail, with particular reference to  FIGS. 6 to 16 . 
         [0048]    The technical effect of the present invention can most easily be understood with regard to  FIGS. 6 and 7  which show a product  20  which is manufactured by the method of the invention. The product  20  illustrated in  FIGS. 6 and 7  is a metal product and is formed via an ALM method from metal powder. 
         [0049]    As illustrated most clearly in  FIG. 6 , the product has a horizontally oriented lower surface  21  which is downwardly facing.  FIG. 6  also shows the downwardly facing surface  21  being supported by a plurality of narrow and elongate support pins  22  which extend downwardly from the downwardly facing surface  21 . 
         [0050]      FIG. 7  shows the arrangement of the support pins  22  as viewed from below the product  20  and shows the support pins  22  arranged in a generally regular array across the downwardly facing surface  20 . The pins  22  are formed via fusion of the same powder feedstock from which the product  20  is formed, and via a similar technique. The pins  22  effectively substitute the wafer support structures  12  of the prior art described above and thus serve to support the downwardly facing surface  21  as the product  20  is built up in a powder bed. The size and configuration of the pins can vary, but it has been found that pins having a circular cross-section and a diameter of approximately 0.8 mm provide particular advantages over the prior art wafer structures  12 . 
         [0051]    A supporting structure for the downwardly facing surface  21  which is formed from support pins  22  of the type illustrated has been found to be quicker and less expensive to produce than the prior art wafer structures  12 , which makes their use very significant in a commercial ALM context. 
         [0052]    It has been found that the supporting pins  22  do not trap un-melted powder feedstock between them to the same degree as prior art wafer structures, and they thus permit more efficient recycling of powder. It has also been found that the supporting pins  22  actually use less powder in their manufacture, which further reduces wastage of powder feedstock. The supporting pins  22  can also provide better control and reduction of distortion on the downwardly facing surface  22  and are also more easily removed during subsequent finishing of the product. As will become apparent from the following description of the method, the support pins  22  can be formed so as to have free ends formed within a powder bed, rather than needing to be built up from lower rigid surfaces such as might be defined by other parts of the product, or by a metal base plate inserted within the powder bed. 
         [0053]      FIG. 8  illustrates a vertical cross-section through an exemplary product  20  which is used herein to highlight key aspects of the present invention. As will be noted, the product  20  has two downwardly facing surfaces, namely a horizontally oriented surface  21  a similar to the one shown in  FIG. 6 , and a sloping surface  21  b which is inclined to the vertical in the orientation of the product shown. Additionally, beneath the sloping surface  21   b,  the product  20  also has a vertically oriented surface  23  which is not downwardly facing. 
         [0054]    The method of the present invention can be performed using apparatus generally similar to the apparatus shown in  FIG. 1 . Accordingly, particular reference is made herein to the use of Electron Beam Melting of metal powder feedstock. However, it is to be noted that the invention is not limited to EBM, and could be embodied in alternative ALM techniques in which a three-dimensional product is formed via successive fusion of parts of a powder bed, said parts corresponding to successive cross-sections of the product. 
         [0055]      FIG. 9  illustrates an initial step in the method of manufacturing the product, and shows the work platform  4  of an EBM apparatus in an initial raised position. An initial layer  24  of metal powder feedstock is laid on the work table  4  to start a powder bed  25 . The powder may be spread into the layer  24  via the rake  6  of the apparatus shown in  FIG. 1 . In a similar manner to prior art methods, the initial layer  24  of the powder bed  25  can be laid thicker than subsequent layers. 
         [0056]      FIG. 10  shows a subsequent step in which an electron beam  9  is focussed on and scanned across a predetermined area  26  of the initial powder layer  24 . The beam thus melts the powder in the predetermined area  26 , thereby fusing the area  26  and forming an initial cross-section of the product  20 . The shape of the cross-section is effectively defined by the shape of the predetermined area  26 . 
         [0057]    The table  4  is then lowered and another layer of powder is laid on top of the first layer  24 , thereby adding to the powder bed  25 , whereupon the electron beam  9  is again focussed on and scanned across an identically sized and positioned predetermined area of the top layer, thereby forming the next cross-section of the product, on top of the first cross-section. 
         [0058]    The steps of laying down a layer of powder and then focussing/scanning the electron beam over a predetermined area of the layer are repeated to form successive cross-sections of the product  20 , thereby gradually building the product from the bottom up. During the initial stages of the method, these steps are repeated to form identical and vertically aligned cross-sections of the product, thereby building up the lower part of the product having the vertical surface  23 . It is to be noted that during this stage of the method, the respective predetermined areas  26  of each successive layer of powder are thus all aligned with one another. 
         [0059]      FIG. 11  illustrates a stage during the formation of the product at which the lower part of the product  20  and its vertical surface  23  is complete. This drawing therefore shows the final cross-section of the lower part of the product having just been formed by melting a predetermined area  26  of the top layer of powder on the powder bed  25 . Before the table  4  is subsequently lowered ready for the next powder layer to be laid on the powder bed  25 , the electron beam is refocused, in turn, on a plurality of small spaced apart support areas  27 . The support areas are all spaced from the predetermined area of the same layer of powder which is fused to form the cross-section of the lower part of the product  20 . 
         [0060]      FIG. 12  shows the arrangement of the support areas  27  in plan view. As will be noted, the support areas  27  are substantially circular in shape and arranged in a series of rows which cooperate to define a generally regular array. The support areas  27  most preferably have a diameter of approximately 0.8 mm. 
         [0061]    As will be appreciated, focussing the electron beam  9  on each of the support areas  27  melts the powder in those areas, thereby fusing the powder. The fused support areas  27  of the top layer of powder thus form initial cross-sections of respective support pins  22  similar to those illustrated in  FIGS. 6 and 7 . The initial cross-sections of the support pins  22  which are formed in this way define free ends  28  of the respective support pins  22 . 
         [0062]    It is to be noted that the free ends  28  of the support pins  22  are thus formed in the top layer of the powder bed  25  (at the stage illustrated in  FIG. 11 ), and are spaced from all other rigid structures such as surfaces of the product  20  being formed and the work table  4 . The initial support areas  27  which are fused to define the ends  28  of the support pins are only supported by underlying powder in the powder bed  25 . 
         [0063]    A series of further successive layers of powder then continue to be laid on the powder bed  25 . When each layer has been laid, the electron beam  9  is focussed on correspondingly shaped and positioned support areas  27  to melt the powder material in the support areas and thereby steadily build up successive cross-sections of the support pins  22 , as shown schematically in  FIG. 13 . The successive support areas  27  of each powder layer which are melted to form each support pin  22  are thus aligned with one another, such that each support pin  22  is built up vertically. 
         [0064]    As will also be evident from  FIG. 13 , the electron beam  9  also continues to be focussed on respective predetermined areas  26  of the layers to melt the powder material in the predetermined areas and thereby define respective cross-sections of the central region of the product  20 . However, the predetermined areas  26  of each layer which are melted during this stage of the procedure differ from one another in the sense that each successive predetermined area  26  is slightly larger than the preceding one such that in each layer a region of the predetermined area  26  is partially unsupported by the previously formed cross-section of the central region of the product  20 . The inclined downwardly facing surface  21   b  is thus built up gradually in this way, layer by layer. 
         [0065]    As will also be noted from  FIG. 13 , each support pin  22  which was shown being started in  FIGS. 11 and 12 , is eventually completed by its final cross-section being defined by a support area  27  which becomes subsumed by the predetermined area  26  of the respective layer of powder. The support pins  22  thus extend outwardly from the inclined downwardly facing surface  21   b,  the pins extending vertically downwardly within the powder bed  25  and are parallel to one another. 
         [0066]      FIG. 13  also shows a second set of support pins  22  being built up in substantially the same manner as described above; with a series of further support areas  27  of each layer being melted to define successive cross-sections of the support pins  22 . The second set of support pins  22 , shown as incomplete in  FIG. 13 , will provide support for the subsequent formed horizontal downwardly facing surface  21  a of the product  20 . As will be noted, the second set of support pins  22  are formed by melting respective support areas  27  of powder layers in which the partially unsupported predetermined areas  26  are also melted. 
         [0067]      FIG. 14  shows a stage in the production process in which the predetermined area  26  of the top powder layer has a size and shape corresponding to the cross-section of the upper region of the product  20 .  FIG. 14  thus shows the creation of the first cross-section of the upper region of the product, and hence the horizontal downwardly facing surface  21   a  of the product. As will be noted, therefore, a very significant proportion of the predetermined area  26  of the upper powder layer is unsupported by the previously formed cross-section of the product  20 . However, the downwardly facing surface defined by the top predetermined area  26  is supported by the previously built up support pins  22  beneath the surface  21   a,  the support pins thus extending downwardly from the surface  21   a.    
         [0068]    The subsequent cross-sections of the relatively wide upper region of the product  20  are then formed by melting substantially identical predetermined regions  26  of successive powder layers in a generally conventional manner. 
         [0069]    As will be appreciated, when the product  20  has been fully formed via the method described above, it may be removed from the EBM apparatus and from the powder bed  25 , whereupon the support pins  22  can be removed during a subsequent finishing process. As indicated above, the support pins  22  have been found to be significantly easier, and less wasteful, to remove than prior art wafer structures. 
         [0070]    As will be appreciated, the invention has been described above with specific reference to an embodiment in which the support pins  22  are parallel to one another and are formed such that they extend substantially vertically within the powder bed. This is achieved by melting support areas  27  of successive layers which are substantially circular and which are arranged in alignment with one another, such that respective cross-sections of the pins  22  are built up vertically. However,  FIGS. 15 and 16  illustrate an alternative method in which the pins  22  are formed so to extend non-vertically within the powder bed  25 , such that the pins are non-parallel to the vertical working axis of the machine 
         [0071]      FIG. 15  shows an inclined downwardly facing surface  21  b of a product, from which depend a plurality of parallel support pins  22 . However, as can be seen immediately, the pins  22  make an acute angle to the vertical axis z rather than being oriented vertically as shown in  FIG. 6 .  FIG. 16 , which illustrates a similar view to  FIG. 12  described above, shows how this achieved. 
         [0072]    As will be noted from  FIG. 16 , in this embodiment, the support areas  27  of each powder layer are elliptical in shape, rather than circular as was the case in the embodiment described above and as shown in  FIG. 12 . Furthermore, as will be appreciated having regard to  FIG. 15 , the elliptical support areas  27  pertaining to each support pin are melted in successive powder layers in an imbricated manner, such that the successive horizontal cross-sections of each support pin are partially horizontally offset from one another. In this manner, the support pins  22  are built up so as to still be cylindrical in form, but so that they are non-vertical within the powder bed  25 . This type of support structure can be very useful and offers increased flexibility over prior art wafer support structures. 
         [0073]    When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or integers. 
         [0074]    The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. 
         [0075]    While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention.