Patent Publication Number: US-2017356308-A1

Title: Semi-annular component and method of manufacture

Description:
TECHNICAL FIELD 
     The present disclosure concerns a method of manufacturing a semi-annular component having a lattice structure, and a semi-annular component having such a lattice structure, in particular a seal carrier for a gas turbine engine. 
     BACKGROUND TO THE INVENTION 
     A seal carrier is an example of a semi-annular component for a gas turbine which is typically cast and finished by machining. Casting and machining may be a relatively inexpensive method for producing high strength and accurate parts. Whilst casting typically requires a minimum component thickness, machining can be used to remove excess material for weight reduction. One consideration in the design of machined components is therefore the access for machining tools. 
     In this context, “semi-annular” is to be understood to refer to any section of an annulus, for example (but without limitation) a half annulus, quarter annulus, a tenth annulus or a three-fifth annulus. Furthermore the curvature need not be strictly in line with the circumference of a circle but is a close approximation thereto. 
     Seal carriers typically have both structural and thermal functionality. Structurally, seal carriers may be configured to transfer tangential and axial loads from aerodynamic components, such as a high pressure guide vane. Thermally, seal carriers may be configured to insulate radially outer casing components from a hot annular gas flow of the gas turbine, and configured to suspend a seal segment from such radially outer casing components so as to accommodate thermal expansion and aid in transferring axial, radial and tangential loads from the seal segment to the outer casing. 
     STATEMENT OF THE INVENTION 
     According to a first aspect of the disclosure there is provided a method of manufacturing a semi-annular component for a casing arrangement of a gas turbine by additive manufacture, the component having a lattice structure and, in use, extending circumferentially about a central axis of the gas turbine so that a radial direction extends towards the central axis, the method comprising: fusing material at successive build locations along a build direction to progressively form the component so that members of the lattice structure have an elongate direction having a circumferential component; wherein the build direction lies in a plane substantially normal to the central axis; and wherein the build direction is inclined with respect to the radial direction at each build location so that members of the lattice structure are progressively formed along their elongate direction. 
     A member of the lattice structure may be progressively formed along its elongate direction when the formation of the respective member begins at a first node of the member and progresses along the elongate direction towards the other node. 
     The build direction may be inclined with respect to the radial direction by an angle of between 35° and 110°, or between 45° and 90°. The elongate direction of each of the members of the lattice structure corresponds with the direction of the maximum dimension of each member (i.e. the length, where the length is greater than the width or height). The build direction as projected onto a plane normal the central axis may be inclined with respect to the radial direction by an angle of between 35° and 110°. 
     The semi-annular component may be formed by selective deposition additive manufacture such as direct laser deposition (DLD), direct metal deposition (DMD) and laser metal deposition (LMD). The process of fusing material may comprise selectively melting a surface of a metal substrate using a radiation source to form a melt pool on the surface and selectively depositing metal powder into the melt pool to progressively form the component. 
     The build direction may be substantially vertical and configured or oriented with respect to the component or the coordinate system for the component so that the members of the lattice structure are inclined with respect to the vertical axis by no more than 30°. Accordingly, the lattice structure may be self-supporting as it is being manufactured. Self-supporting in this case means that the lattice does not require ancillary support structures to be built into the semi-annular component which are to be machined off after additive manufacture. Therefore, the part can be manufactured to its final shape without the need for machining. 
     The members of the lattice structure may be arranged in a quadrilateral configuration to define quadrilateral interstices. A diagonal of each quadrilaterial interstice may be aligned with the circumferential direction. 
     The semi-annular component may be a seal carrier in accordance with the second aspect described below. 
     According to second aspect of the disclosure there is provided a seal carrier for a casing arrangement of a gas turbine engine comprising a casing structure and a shroud seal segment, the seal carrier comprising: a casing attachment point for coupling the seal carrier with a casing structure; a carrier arm configured to extend radially inwardly from the casing structure to support a shroud seal segment; wherein the casing attachment point is axially spaced apart from the carrier arm; and a semi-annular lattice structure extending between the casing attachment point and the carrier arm, the lattice structure comprising a plurality of elongate members each extending along a respective elongate direction having a circumferential component. The shroud seal segment may depend from the seal carrier and be configured to minimise the clearance between the casing structure and the tips of turbine blades of the intermediate or high pressure turbines in order to increase the efficiency of the engine. 
     The seal carrier may further comprise a radially outer barrier plate for insulating the casing from an axial gas flow through the seal carrier. The axial gas flow may affect the clearance between the shroud seal segment and the tips of the high pressure turbine blades due to the casing structure expanding or contracting with temperature changes and therefore pushing the shroud seal segments towards the tips of the blades or pulling the shroud seal segment away from the tips of the blades. Therefore, careful control of the temperature at the casing structure can optimise the clearance and is achieved through the use of the barrier plate. 
     The barrier plate may comprise a plurality of openings to allow a gas flow through the seal carrier to the casing. At least one opening may be elliptical (or at least partly elliptical) with a minimum aspect ratio of 2:1, wherein the major axis of each elliptical opening is in the circumferential direction of the seal carrier. Alternatively or additionally, at least one opening may be quadrilateral, wherein a diagonal of the quadrilateral is substantially aligned with the circumferential direction. The edges of the quadrilateral may be inclined with respect to the circumferential direction by no more than 30°. The openings may also be a mixture of elliptical and quadrilateral openings as described. The openings may be provided for selectively allowing a gas flow through the seal carrier to the casing, for example, under the action of a pump to draw gas flow through the openings to selectively heat the casing. 
     Each opening may be at least partly defined by an overhanging boundary which overhangs the opening with respect to the build direction (e.g. an upper boundary). There may be an opposing supported boundary. When the build direction is vertical, an overhanging boundary may comprise any portion of the boundary defining a respective opening that faces at least partly downwardly (i.e. the normal direction with respect to the boundary has a downward component). The overhanging boundary may form part of an ellipse, for example an ellipse having a minimum aspect ratio of 2:1 wherein the major axis of the ellipse is in the circumferential direction. An opposing supported boundary may be non-elliptical, for example circular or simply linear (e.g. horizontal). Accordingly, the opening may be teardrop shaped, with portions of an overhanging boundary of the opening being more inclined towards the vertical than opposing portions of the supported opening. In other examples, an opening may have an overhanging boundary in the form of two sides inclined by no more than 60° (for example no more than 55° or no more than 45°) from the vertical. For example, the opening may be triangular with a horizontal supported boundary and two sides forming the overhanging boundary. 
     The casing attachment point may be an intermediate casing attachment point disposed between a forward and an aft portion of the barrier plate, or alternatively, the seal carrier may additionally comprise such an intermediate casing attachment point. 
     The barrier plate may comprise a continuous portion circumferentially adjacent to the intermediate casing attachment point, the continuous portion may extend between the forward and aft portions and may have a substantially constant radius. 
     The members of the lattice structure may be arranged in a quadrilateral configuration to define quadrilateral interstices, for example square or rhombic. 
     The seal carrier may be manufactured by a method in accordance with the first aspect of the invention. 
     According to a third aspect of the disclosure there is provided a casing arrangement for a gas turbine engine comprising: a casing structure, a shroud seal segment and a seal carrier according to the second aspect; the casing structure having a carrier attachment point cooperating with the intermediate casing attachment point of the seal carrier so that the seal carrier depends from the casing structure; and a recess in the barrier plate of the seal carrier for accommodating the carrier attachment point of the casing. The axial extent of the recess may be no more than 130% of the axial extent of the carrier attachment point of the casing. The casing arrangement may provide a gas path seal between the shroud seal segment and the tips of turbine blades, for example in the intermediate or high pressure turbine, in order to maximise the energy extracted from an annular gas flow through the gas turbine. 
     The carrier attachment point of the casing and intermediate casing attachment point of the seal carrier may comprise projections configured to interlock with one another. 
     The recess may be circumferentially adjacent the continuous portion and the casing arrangement may comprise a plurality of seal carriers; wherein the barrier plates of the plurality of seal carriers are arranged so that there is an alternating arrangement of continuous portions and recesses when assembled. 
     The intermediate casing attachment point may extend radially outwardly from the barrier plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described by way of example only, with reference to the Figures, in which: 
         FIG. 1  is a schematic sectional side view of a gas turbine engine; 
         FIG. 2  is a schematic cross-sectional view of a casing arrangement at the high pressure turbine of a gas turbine engine; 
         FIG. 3  is a schematic cross-sectional view of a previously considered seal carrier with a barrier plate; 
         FIG. 4  is a schematic cross-sectional view of a further seal carrier; 
         FIG. 5  is a schematic radially-outer view of the outside of the seal carrier of  FIG. 4 ; 
         FIG. 6  is a schematic perspective view of the seal carrier of  FIGS. 4 and 5 ; 
         FIGS. 7A and 7B  schematically illustrate a partially-formed seal carrier according to  FIGS. 4-6  during additive manufacturing; 
         FIG. 8  schematically illustrates a partially-formed seal carrier according to  FIGS. 4-6  in a further configuration during additive manufacture; 
         FIG. 9  is a flow chart showing the steps for producing the seal carrier of  FIGS. 4-6  by additive manufacturing. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS AND SOME EMBODIMENTS 
     With reference to  FIG. 1 , an example gas turbine engine is generally indicated at  10 , having a central (also referred to as a principal) and rotational axis  11 . The engine  10  comprises, in axial flow series, an air intake  12 , a propulsive fan  13 , an intermediate pressure compressor  14 , a high-pressure compressor  15 , combustion equipment  16 , a high-pressure turbine  17 , an intermediate pressure turbine  18 , a low-pressure turbine  19  and an exhaust nozzle  20 . A nacelle  21  generally surrounds the engine  10  and defines both the intake  12  and the exhaust nozzle  20 . 
     The gas turbine engine  10  works in the conventional manner so that air entering the intake  12  is accelerated by the fan  13  to produce two air flows: a first air flow into the intermediate pressure compressor  14  and a second air flow which passes through a bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  14  compresses the air flow directed into it before delivering that air to the high pressure compressor  15  where further compression takes place. 
     The compressed air exhausted from the high-pressure compressor  15  is directed into the combustion equipment  16  where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines  17 ,  18 ,  19  before being exhausted through the nozzle  20  to provide additional propulsive thrust. The high  17 , intermediate  18  and low  19  pressure turbines drive respectively the high pressure compressor  15 , intermediate pressure compressor  14  and fan  13 , each by suitable interconnecting shaft. 
     Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. By way of example such engines may have an alternative number of interconnecting shafts (e.g. two) and/or an alternative number of compressors and/or turbines. Further the engine may comprise a gearbox provided in the drive train from a turbine to a compressor and/or fan. 
       FIG. 2  schematically shows a portion of an example high pressure turbine  17  of the gas turbine engine  10  in cross section. The high pressure turbine  17  comprises a casing arrangement  24  and a high pressure nozzle guide vane  26  and high pressure turbine blades  28  radially within the casing arrangement  24 . The hot gas flow from the combustion equipment stage  16  enters the high pressure turbine  17  in the direction of the arrows A. The gas flow is expanded past the high pressure nozzle guide vanes  26  to increase the circumferential velocity of the gas flow and thereby drive the high pressure turbine blades  28  in rotation. 
     The casing arrangement  24  of the high pressure turbine  17  comprises an annular casing structure  30 , a plurality of seal carriers  32  and a plurality of shroud seal segments  34  which partly define the radially-outer gas-washed surface for flow through the high pressure turbine  17 . Each seal carrier  32  is semi-annular and the plurality of seal carriers  32  are assembled to form an annular seal carrier  32  which is located radially inward of the casing structure  30  and attached to the casing structure  30 . In this particular example, there are twenty seal carriers each having an angular extent of 18 degrees. Each shroud seal segment  34  is semi-annular and the plurality of shroud seal segments  34  are assembled to form an annular shroud seal segment  34  which is located radially inward of the casing structure  30  and the seal carrier  32 , and which depends from the seal carrier  32 . In this particular example, a shroud seal segment  34  is provided for each seal carrier  32 , and has the same angular extent as the seal carrier  32 . The seal carriers  32  and shroud seal segments  34  extend circumferentially around a central axis which in this example is common with (coaxial with) the central axis  11  of the gas turbine  10 . 
     The casing arrangement  24  of  FIG. 2  is shown with a previously considered example of a seal carrier  32  which will be described with reference to  FIG. 3 .  FIG. 3  shows a cross section of the seal carrier  32  in a plane extending through the central axis  11 . The seal carrier  32  comprises an intermediate casing attachment point  36 , an aft casing attachment point  38 , an intermediate carrier arm  40 , an aft carrier arm  42  and a barrier plate  44  comprising a forward portion  46  and an aft portion  48 . 
     The barrier plate  44  is semi-annular and comprises a forward portion  46  having a first radius and an aft portion  48  having a second smaller radius, with a step-change or shoulder  58  therebetween. The step-change corresponds to the location of an intermediate casing attachment point  36 . The aft portion  48  of the barrier plate  44  forms part of an aft support structure  49  having a generally triangular configuration, as will be described in detail below. 
     The intermediate casing attachment point  36  and the aft casing attachment point  38  are axially spaced apart and are in the form of hooks which protrude radially outwardly from the barrier plate  44  and aft support structure  49  respectively. The hooks  36 ,  38  extend axially rearwardly to couple with complimentary hooks  50  on the casing structure  30  (which may be referred to as carrier attachment points of the casing), to thereby attach the seal carrier  32  to the casing structure  30 . The seal carrier  32  also comprises a bolt hole  54  in the forward portion of the barrier plate  46  for coupling to the casing structure  30  with a bolt  52  (see  FIG. 2 ). 
     The intermediate carrier arm  40  and aft carrier arm  42  extend radially inwardly from the intermediate casing attachment point  36  and the aft casing attachment point  38  respectively. At the radially inner end of each carrier arm  40 ,  42  there is provided a respective seal attachment point  41 ,  43 , which in this example are in the form of axially extending hooks to which the shroud seal segment  34  can be attached. 
     A stiffening member  56  extends radially and axially (i.e. at an angle inclined with respect to the axial direction) from a location on the intermediate carrier arm  40  towards (i.e. near to) the intermediate seal attachment point  41  to a location on the aft carrier arm  42  towards (i.e. near to) the aft casing attachment point  38 , and thereby is substantially conical. In this example, the intermediate carrier arm  40 , the aft portion of the barrier plate  44  and the stiffening member  56  form the substantially triangular aft support structure  49 . 
     As best shown in  FIG. 2 , in the assembled casing arrangement the high pressure nozzle guide vanes  26  abut a circumferentially and radially extending forward surface  60  of the intermediate carrier arm  40  to exert tangential and axial forces on the intermediate carrier arm  40 . 
     The example seal carrier  32  is cast and machined. The design of the example seal carrier  32  is partly informed by particular limitations of the casting process. For example, the minimum wall thickness of the stiffening member  56  when casting is relatively high and may therefore add unnecessary weight to the seal carrier  32 . 
     The barrier plate  44  is configured to insulate the casing structure  30  from the hot gas flowing through the seal carrier. Openings (not shown) in the barrier plate  44  provide a path for the hot gas to be drawn through to the casing structure  30  by a pump (not shown) to control the temperature at the casing structure  30 , for example as described in US Patent Publication No. US 2001/0229306. The closer the barrier plate  44  is to the casing structure  30 , the more accurately and efficiently the temperature at the casing structure  30  can be controlled. 
     However, as best shown in  FIG. 2 , the forward portion  46  of the barrier plate  44  is radially closer to the casing structure  30  than the aft portion  48  of the barrier plate  44  due to the step change or shoulder  58  between the forward portion  46  and the aft portion  48  where the radius of the barrier plate  44  is reduced. The reduced radius of the barrier plate  44  at this location allows for insertion of the carrier attachment point  50  of the casing under the intermediate casing attachment point  36  of the seal carrier  32 . Whilst the carrier attachment point  50  of the casing is of limited axial extent, the axial extent of the aft portion  48  at the reduced radius is significantly greater in order to allow for access of machining tools below the intermediate casing attachment point after casting. Machining is done to ensure the accurate geometry of the part and proper interfacing between the intermediate casing attachment point  36  and the corresponding carrier attachment point  50  of the casing structure. 
     A further example of a seal carrier will now be described with reference to  FIGS. 4-6 . In this particular example, the seal carrier  60  is interchangeable with the seal carrier  32  described above, and may be incorporated in the gas turbine engine  10  described above with respect to  FIG. 1 , and in the casing arrangement described above with respect to  FIG. 2 . 
     The example seal carrier  60  is manufactured by additive manufacture, and more particularly by selective deposition additive manufacture such as direct laser deposition (DLD), also known as direct metal deposition (DMD) or laser metal deposition (LMD), as will be described in detail below. 
       FIG. 4  shows a cross section of the seal carrier  60  in a plane extending through the central axis  11 . The seal carrier  60  comprises an intermediate casing attachment point  62 , an aft casing attachment point  64 , an intermediate carrier arm  66 , an aft carrier arm  68  and a barrier plate  70  comprising a forward portion  72  and an aft portion  74 . 
     The overall arrangement of the seal carrier  60  is similar to that described above with respect to the seal carrier  32  of the previously considered example except as follows. In particular, the intermediate casing attachment point  62  of the seal carrier  60  is disposed between the forward portion  72  and the aft portion  74  of the barrier plate  70 , whereas the aft casing attachment point  64  is disposed at the aft end of the aft portion  74  of the barrier plate. 
     The intermediate casing attachment point  62  and the aft casing attachment point  64  are therefore axially spaced apart and are in the form of hooks which protrude radially outwardly from the barrier plate  70  and extend axially rearwardly to couple with corresponding hooks  50  (or carrier attachment points) of the casing structure  30 , to thereby attach the seal carrier  60  to the casing structure  30 . 
     As best shown in  FIG. 5 , in this example the seal carrier  60  has two intermediate casing attachment points  62  angularly spaced apart, each one being at a respective angular end of the seal carrier. The barrier plate  70  comprises recesses  76  at locations corresponding to the intermediate casing attachment points  62 , and an angularly-adjacent continuous portion  78  therebetween. The continuous portion  78  axially extends from the forward portion  72  to the aft portion  74  at a substantially constant radius. 
     As shown in  FIG. 5 , the recesses  76  have an angular extent which substantially corresponds to the angular extent of the corresponding intermediate casing attachment point  62 . Further, over at least an angular portion of the barrier plate  70 , the recess  76  is of limited axial extent so that the aft portion  74  of the barrier plate returns to the same radius as the forward portion of the barrier plate downstream of the recess  76 . In particular, the axial extent of at least a portion of the recess  76  is no more than 130% of the axial extent of the carrier attachment point  50  of the casing structure. In the particular example shown, the axial extent of the carrier attachment point  50  is approximately 7.5 mm, whereas portions of the recess  76  have an axial extent of between 8 mm and 10 mm. In contrast, in the previously considered example seal carrier  32  described above, the axial extent of the aft portion  48  of the barrier plate  44  rearwardly of the step change  58  that is provided to accommodate a carrier attachment point  50  of the same dimension (7.5 mm) is approximately 22 mm. 
     Since the continuous portions  78  are angularly (or circumferentially) adjacent to the recesses  76 , when the plurality of seal carriers  60  are assembled into an annular seal carrier, the barrier plates of the seal carriers  60  are arranged so that there is an alternating arrangement of continuous portions  78  and recesses  76  in the barrier plates  70 . 
     The intermediate carrier arm  66  and aft carrier arm  68  are in the form of radially inwardly protruding arms as described above, and are each provided with a respective seal attachment point  67 ,  69  at their radially inner ends to which a shroud seal segment can be attached. 
     A forward surface  82  of the intermediate carrier arm  66  is configured to abut with the high pressure nozzle guide vane  26 , and to receive axial and tangential loading from the high pressure nozzle guide vane  26 , as described above. 
     A stiffening member in the form of an annular lattice structure  84  extends between a location on the intermediate carrier arm  66  towards the intermediate seal attachment point  67  and a portion on the intermediate carrier arm towards the aft casing attachment point  64 , thereby provide reinforcement for the intermediate carrier arm  66  against the axial load from the high pressure nozzle guide vanes  26 . The lattice structure  84  therefore extends between the intermediate carrier arm  66  and the aft casing attachment point  64 , via the aft carrier arm  68 . In other examples, the lattice structure  84  may extend substantially directly to the aft casing attachment point  64 . 
     The lattice structure  84  comprises an arrangement of members  86  defining quadrilateral interstices. In this particular example, the angle between the lattices members is 90°, but in other examples the lattice members may define other shapes, such as rhombi having a greater circumferential extent than axial extent. Further, in this particular example one of the diagonals of each of the interstices extends in the circumferential direction of the seal carrier  60 . 
     The applicant has found that the lattice structure  84  results in significant weight savings as compared with the stiffening member  56  of the seal carrier  32 , since additive manufacturing can produce the lattice according to the structural requirements and without the limitations of a minimum wall thickness as required for casting. Further, whilst it may be possible to remove some material from a cast part using machining, access requirements may hinder any such machining step. 
     Further, the recesses  76  under the intermediate casing attachment point  62  are smaller than in the previously considered example seal carrier  32 , in particular since there is no requirement to provide machining access under the casing attachment point  62 . As such, a region of the aft portion  74  of the barrier plate  70  downstream of the recess  76  is at the same radius as the forward portion  72  of the barrier plate  70 . Accordingly, in use, the temperature at the casing structure  30  may be controlled more accurately and evenly over the length of the seal carrier  60 . 
     In order to accommodate the control of the temperature at the casing structure  30 , the barrier plate  70  comprises openings  88 . In one example of a seal carrier as shown in  FIGS. 5 and 6 , these openings  88  are elliptical (with the major axis of each opening  88  extending in the circumferential direction). In an alternative example, the openings may be quadrilateral with a diagonal axis of each quadrilateral opening being in the circumferential direction. Otherwise, the holes may be a mixture of elliptical or quadrilateral openings. In other examples the openings may be of other shapes, each opening having an upper boundary (or overhanging boundary, defined with respect to the build direction) partly defining the opening and having an overhang angle of no more than 60 degrees from the vertical. 
     The seal carrier  60  described above with reference to  FIGS. 4-6  is manufactured using selective deposition additive manufacture. 
     A method of manufacturing a semi-annular component with a lattice structure will now be described with reference to  FIGS. 7-9 . It will be appreciated that the method of manufacturing may be applicable to a wide range of different semi-annular components, and is not limited to the manufacture of a seal carrier as described above. Nevertheless, the method will be described, by way of example only, with respect to manufacture of the example seal carrier  60  described with respect to  FIGS. 4-6 . 
     Some additive manufacturing methods make use of a supporting bed, for example a bed of powder which is applied layer by layer and selectively fused or sintered to form a component. In such example methods, the bed of powder supports the product as it is formed, and so it is possible to manufacture components having features which are not initially directly supported from below by another feature of the component. 
     In contrast, in selective deposition additive manufacturing, material is deposited on previously-formed or deposited portions of the component, to progressively build a component along a build direction (which may be substantially vertical) in a self-supporting manner. In certain examples, selective deposition additive manufacturing may be conducted on a tool configured to support portions of a component which would not otherwise be self-supporting, or support arms may be incorporated into the component which can later be removed. In selective deposition additive manufacture, material can be deposited so that newly-deposited material overhangs the supporting material by a limited extent. Such newly-deposited material is said to overhang by an overhang angle, typically measured from the vertical. 
     The applicant has found that, in order to reliably and accurately manufacture a self-supporting structure in selective deposition additive manufacturing, an overhang angle of an overhanging part should be no more than 60° from the vertical axis (i.e. at least 30° from the horizontal plane). The applicant has found that the surface finish of the component may be affected by the overhang angle of the component, such that a smaller overhang angle from the vertical axis generally results in a better surface finish. The surface finish will affect the life of the component, therefore it is important to select a suitable surface finish for the life of the component. For a component which will endure high stresses, a smaller angle from the vertical axis may be required in order to have an acceptable surface finish and therefore an acceptable life. For a component which will endure lower stresses, a larger angle from the vertical axis may be allowable. However, the angle from the vertical axis should not exceed 60°. 
     In the particular example of manufacture of the seal carrier  60 , described below, the maximum overhang angle is approximately 45°-55° from the build direction, which itself is substantially vertical. 
     Before the deposition of material commences to form the component, the orientation of the component to be manufactured is defined with respect to the build direction. The orientation of a component which is yet to be manufactured can be defined with respect to its coordinate system. In the case of a semi-annular component, the coordinate system can be defined with respect to the axial direction and the radial direction. Since the component to be manufactured is semi-annular, it has first and second angular ends. In the example manufacturing method to be described below, the build direction for additive manufacture is not aligned with a radial direction or axial direction, but instead the build direction is oriented such that the component is manufactured substantially from a first angular end towards a second angular end, as will be described below. 
       FIGS. 7A and 7B  show an example orientation of the component  60  as partially-formed with respect to a build direction  98  for additive manufacture.  FIG. 7A  shows a cross-sectional view of the component through a plane normal to the axial direction, whereas  FIG. 7B  shows a view of the component along the radial direction at a first angular end (the lower end in  FIGS. 7A and 7B ). In both drawings, the component  60  is partially-formed on a workpiece  94  received on a bed  90 , as will be described in detail below. 
     In this particular example, the build direction for additive manufacture is substantially vertical, and the coordinate system for the component  60  to be manufactured is oriented so that the build direction  98  lies in a plane normal to the central axis  11 , and is orthogonal (i.e. at 90°) with respect to the radial direction at a first end of the component. Accordingly, the coordinate system of the component is oriented so that a first build location of the component to be formed (i.e. where material is initially fused) corresponds to a radial slice of the first angular end of the component. Therefore, the build direction is substantially tangential with the circumferential direction at the first build location. As shown in  FIG. 7A , owing to the semi-annular profile of the component the circumferential direction diverges from the build direction at subsequent build locations. 
     In this particular example, the lattice members  86  of the seal carrier  60  are at 90° from each other such that they define substantially square interstices. A diagonal of the interstices is aligned with the circumferential direction, and so the lattice members  86  are inclined at substantially 45° from the vertical axis at the start of the build process. 
     Similarly, it may be desirable to limit the extent to which the boundary of the openings  88  of the of the example seal carrier  60  are inclined by more than 60° from the vertical axis. The use of elliptical openings  88  in which the major axis is in the circumferential direction, as compared with circular openings, generally has the effect of inclining the tangential direction around the boundary of the opening towards the vertical direction, and thereby limits the proportion of the boundary of the openings  88  having a shallow overhang angle. Similarly, using quadrilateral openings  88  where one of the diagonals is in the circumferential direction enables shallow overhang angles (i.e. angles of more than 60° from the vertical) to be prevented. 
       FIG. 8  shows an alternative example orientation of the component to be manufactured relative the build direction  98 . In this example the build direction  98  is substantially vertical and the coordinate system for the component is oriented so that the circumferential direction at the first angular end of the component is inclined with respect to the build direction. In particular, a bed  90  carrying the workpiece  94  on which the component is to be formed is tilted at an angle θ relative the horizontal, which in this example is 18°, and the component is oriented so that the circumferential direction curves towards the vertical as the component is manufactured. Accordingly, in this example the build direction lies in a plane normal the central axis of the component and is inclined with respect to the radial direction at the first angular end of the component by approximately 72°. In this particular example, the component  60  has an angular extent of 18°, and so the build direction will be substantially tangential to the circumferential direction (orthogonal with respect to the radial direction) at the second angular end of the component. 
     It will be appreciated that, in the case of the seal carrier  60 , the elongate direction of the lattice members are inclined by 45° with respect to the vertical when the circumferential direction is aligned with (i.e. tangential with) the build direction (i.e. when the build direction is at 90° with respect to the radial direction), and may be inclined by more than 45° with respect to the vertical when the build direction is inclined with respect to the circumferential direction (i.e. when the build direction is greater or less than 90° with respect to the radial direction). 
     As shown in  FIGS. 7A, 7B and 8 , a starting workpiece  94  is provided for selective deposition additive manufacture in order that a melt pool may be formed in which to fuse metal powder. In this particular example the starting workpiece is a rectangular metal workpiece received on a bed  90 . 
     To begin material deposition, a laser head  92  directs a laser at a first build location on the workpiece  94 , so that the laser heats the first build location on the workpiece  94  and forms a melt pool. It then injects a small amount of metal powder  96  into the melt pool which melts and solidifies when the laser moves to the next build location. As such, the seal carrier  60  is built up by adding metal  96  to form successive layers at successive build locations. 
       FIG. 9  shows a flow chart for the process of manufacturing a semi-annular component with a self-supporting lattice structure. In block  902 , the orientation of the component to be manufactured is defined relative the build direction, as described above. In block  904 , material is fused at successive build locations to progressively form the component along the build direction. 
     It will be appreciated that the build locations may be defined automatically based on a geometry of the component to be manufactured. Further, it will be appreciated that the component to be manufactured, and the orientation of the component with respect to the build direction, may be based on an assessment of overhang angles during manufacture of the component, particular overhang angles of members of the lattice. 
     In particular, it will be appreciated that the minimum overhang angle of a lattice member may be achieved when the build direction is aligned with the circumferential direction (i.e. orthogonal to the radial direction) at the respective build location. For example, where a lattice structure has lattice members defining substantially square interstices in a diamond pattern, the minimum overhang angle may be 45°. Accordingly, the degree to which an overhang angle of a lattice member deviates (increase) from the minimum overhang angle may be a function of the angular extent of a component, and may increase as the angular extent of a component increases. 
     Further, it will be appreciated that a lattice structure may have a configuration of quadrilaterals other than square, such as a rhombic arrangement. In particular, a rhombic arrangement in which the axial extent of each rhombus is less than the circumferential extent may result in a minimum overhang angle of less than 45°. Such arrangements may be selected to mitigate excessive deviation from the minimum overhang angle, such as in components having a large angular extent. 
     Whilst a particular example of the above method has been described with respect to a seal carrier for a gas turbine engine, it will be appreciated that the method is applicable to the manufacture of other semi-annular components. In particular, the method can be used to manufacture other semi-annular components for a gas turbine, such as seal carriers for other parts of a gas turbine, intermediate and high pressure shroud seal segments, or any semi-annular component with a lattice structure. 
     It will be understood that the invention is not limited to the embodiments described above and various modifications and improvements can be made without departing from the concepts described herein. Except where mutually exclusive, any of the features may be employed separately or in combination with any other features and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.