Abstract:
A seal is provided between first and second relatively movable members. The seal comprises a liner attached to the first relatively movable member which liner is abraded by the second relatively movable member. The abradable liner comprises a metallic, open porous structure formed by fusing a metallic feedstock with an energy beam to build up sequential deposits of the fused feedstock.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a seal comprising an abradable liner. 
       BACKGROUND OF THE INVENTION 
       [0002]    With reference to  FIG. 1 , a ducted fan gas turbine engine generally indicated at  10  has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake  11 , a propulsive fan  12 , an intermediate pressure compressor  13 , a high-pressure compressor  14 , combustion equipment  15 , a high-pressure turbine  16 , and intermediate pressure turbine  17 , a low-pressure turbine  18  and a core engine exhaust nozzle  19 . A nacelle  21  generally surrounds the engine  10  and defines the intake  11 , a bypass duct  22  and a bypass exhaust nozzle  23 . 
         [0003]    The gas turbine engine  10  works in a conventional manner so that air entering the intake  11  is accelerated by the fan  12  to produce two air flows: a first air flow A into the intermediate pressure compressor  14  and a second air flow B which passes through the bypass duct  22  to provide propulsive thrust. The intermediate pressure compressor  13  compresses the air flow A directed into it before delivering that air to the high pressure compressor  14  where further compression takes place. 
         [0004]    The compressed air exhausted from the high-pressure compressor  14  is directed into the combustion equipment  15  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  16 ,  17 ,  18  before being exhausted through the nozzle  19  to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors  14 ,  13  and the fan  12  by suitable interconnecting shafts. 
         [0005]    The compressor rotor blades are surrounded by a compressor casing. A small gap, or clearance, is provided radially between the tips of the compressor rotor blades and the compressor rotor casing. The compressor casing is provided with an abradable liner on its radially inner surface immediately around the tips of the compressor rotor blades. These abradable liners wear preferentially relative to the material of the tips of the compressor rotor blades during engine service. The abradable liners reduce over tip leakage between the tips of the compressor rotor blades and the compressor casing and hence reduce the associated loss in engine efficiency and engine performance. 
         [0006]    Abradable liners are also used on the turbine casing, which is arranged radially around stages of turbine rotor blades. Abradable liners are also used on other components of gas turbine engines where there is a requirement to form a seal between a rotatable or movable component and a static component. 
         [0007]    Conventional abradable liners are typically thermally sprayed (eg plasma sprayed) metal coatings for compressors, and thermally sprayed ceramic coatings for turbines. These coatings have to provide a balance in mechanical properties between a requirement to be soft enough to be abraded and hard enough to resist erosion. Thus thermally sprayed abradables generally have a metallic matrix (eg based on Ni, Cr or Al) and a dislocator phase to impart improved cutting characteristics. 
         [0008]    However, there are disadvantages associated with thermally sprayed abradables. in particular, the coatings are generally limited to thicknesses of about 2 to 3 mm, as increased thicknesses can increase susceptibility to cracking and spalling due to thermal expansion coefficient mismatches and residual stresses. Other disadvantages include: difficulties with process control which can lead to coating variability and defects, resulting in blade damage or erosive abradable failures; and significant powder feedstock wastage (typically about 50%) during spraying. 
         [0009]    An alternative to thermally sprayed abradables consists of a liner formed from a mesh of sintered fine iron-based wires brazed directly to a casing. Such liners are commonly used at operational temperatures of greater than 350° C. However, the precise structure of the mesh can be difficult to control during manufacture of the liner. Further, in use, heat can be generated through friction between the rotating member and the mesh. 
       SUMMARY OF THE INVENTION 
       [0010]    Accordingly, an aim of the present invention is to provide an abradable liner with improved characteristics. 
         [0011]    Thus, a first aspect of the present invention provides a seal between first and second relatively movable members, the seal comprising a liner attached to the first relatively movable member which liner is abraded by the second relatively movable member, the abradable liner comprising a metallic, open porous structure formed by fusing a metallic feedstock with an energy beam to build up sequential deposits of the fused feedstock. 
         [0012]    Using an energy beam to build up sequential deposits of the fused feedstock allows precise control of the structure of the liner to be achieved. In this way product variability can be reduced. Also the structure of the liner can be functionally tailored throughout its thickness to meet the attributes required for the intended operational environment. 
         [0013]    The seal may have any one or, to the extent that they are compatible, any combination of the following optional features. 
         [0014]    The energy beam may be e.g. a laser beam, electron beam or electric arc. The feedstock may be a metal powder or a metal wire. 
         [0015]    The liner thickness may be greater than 2 mm and preferably greater than 5 or 10 mm. The liner thickness may be up to 50 mm. Relatively thick liners are possible due to the possibility to tailor and control the properties of the liner, and to the ability to form a strong bond (eg by local melting) at the interface between the liner and the first member. 
         [0016]    The first and second members may be relatively rotatable members. For example, one of the first and second members may be a rotor and the other of first and second members may be a stator. In particular, the rotor may comprise a plurality of rotor blades, the stator may comprise a casing, the abradable liner can be attached to the casing, and the liner can be abraded by the rotor blades. However, the seal may be used in other applications, such as oil or air seals on shafts, discs and drums e.g. with knife type fins cutting into the abradable liner. 
         [0017]    The metallic feedstock may typically be nickel, iron, chromium or cobalt, or may be an alloy of nickel, iron, chromium or cobalt, the choice of metal depending on the application. 
         [0018]    The abradable liner can have a plurality of sub-layers of varying pore size. For example, the abradable liner may have a top sub-layer having a relatively small pore size, and an underlying sub-layer having a relatively large pore size. The top sub-layer can thus provide good cutting and sealing properties, while the underlying sub-layer can have an enhanced compliance which reduces strain energy in the liner, and allows the liner to collapse to an extent during rubbing, to reduce the energy required to cut the abradable 
         [0019]    He seal may further comprise one or more baffle plates which traverse the thickness of the abradable liner to reduce fluid leakage over the surface of the first movable member through the liner. 
         [0020]    The pores of the abradable liner can be filled with a further material, such as a polymer, further metal or ceramic, eg to provide enhanced cutting or sealing characteristics. 
         [0021]    Preferably, the first and second relatively movable members are components of a gas turbine engine. 
         [0022]    A second aspect of the present invention provides a method of forming a seal between first and second relatively movable members, the method comprising:
       fusing a metallic feedstock with an energy beam to build up sequential deposits of the fused feedstock into a metallic, open porous structure,   wherein the metallic, open porous structure is attached to the first member to form an abradable liner on the first member so that, in use, the abradable liner can be abraded by the second movable member.       
 
         [0025]    Thus the method can be used to produce a seal of the first aspect. Accordingly, optional features of the first aspect apply also to the method of the second aspect. 
         [0026]    The metallic, open porous structure may be attached to the first member by fusing an initial deposit of feedstock onto the first member, and building up the sequential deposits of fused feedstock thereupon. Alternatively, the metallic, open porous structure may attached to the first member (eg by brazing, welding, mechanical fixture or a combination thereof) after the structure has been formed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: 
           [0028]      FIG. 1  shows a schematic longitudinal cross-section through a ducted fan gas turbine engine; 
           [0029]      FIG. 2  shows a schematic cross-section through a seal comprising an abradable liner attached to a gas turbine engine casing; 
           [0030]      FIG. 3  shows a scanning electron microscope micrograph of a typical liner open porous structure; and 
           [0031]      FIG. 4  shows a scanning electron microscope micrographs of the typical liner open porous structure at a different magnification. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]      FIG. 2  shows a schematic cross-section through a seal comprising an abradable liner  30  attached to a gas turbine engine casing  31 . The tip of a compressor or turbine blade  32 , which is one of a row of such blades attached to a rotor disc, passes over the casing with a clearance gap  33  determined by abrasion of the liner by the blade tips. 
         [0033]    The liner has a metallic, open porous structure. At the surface of the liner, an outer layer  30   a  of the liner  30  has a fine structure with a relatively small pore size which provides good cutting and sealing properties. Beneath the outer layer  30   a,  a mid-layer  30   b  of the liner has a coarser structure with a relatively large pore size, and possibly thinner pore walls, which provides the mid-layer with increased compliance and deformability, helping to mechanically isolate the outer layer from the casing  31  and to reduce build ups of strain energy in the liner which might otherwise lead to cracking or spallation of the liner. The mid-layer is thus significant in allowing relatively thick liners to be produced, eg up to 50 mm in thickness. The mid-layer can also collapse to an extent during rubbing, to reduce the blade energy required to cut the abradable. Joining the mid-layer to the casing, an inner layer  30   c  of the liner has a structure with a pore size between that of the outer layer and the mid-layer. Having a smaller pore size than the mid-layer helps to increase the number of attachment points between the liner and the casing and hence to increase the strength of the attachment, while having a larger pore size than the outer lay maintains a degree of compliance and deformability that can help to reduce stress build up at the liner/casing interface. 
         [0034]    A plurality of spaced baffle plates  34  traverse the thickness of the liner  30  in order to prevent gas leakage over the surface of the casing through the open porosity of the liner (ie by-passing the clearance gap  33 ). 
         [0035]    The metallic, open porous structure of the liner  30  can conveniently be produced by melting a metal powder feedstock with a laser beam to build up sequential fused deposits of the metal. However, other feedstocks may be used, such as wire feedstocks. Also other energy beams may be used, such as electron beams or electric arcs. Suitable deposition procedures are known to the skilled person. For example U.S. Pat. No. 6,811,744 proposes forming three-dimensional structures defined by CAD solid models using layer-by-layer deposition. U.S. Pat. No. 4,323,756 proposes a method of fabricating bulk articles by sequential layer deposition of powder of wire feedstock fused using an energy beam. U.S. Pat. No. 5,398,193 propose a method of 3 dimensional rapid prototyping through controlled layer-wise deposition, and U.S. Pat. No. 4,863,538 proposes a method and apparatus for producing parts by selective sintering a layer of powder. 
         [0036]      FIGS. 3 and 4  show scanning electron microscope micrographs at two different magnifications of a typical liner open porous structure. The structure resembles a foam, but with regularly positioned pores whose size is typically constant at a given height within the liner but can vary through the thickness of the liner. The structure of the liner is defined by a computer generated model, and this information is then sliced into a large number of deposition layers (eg hundreds or thousands of layers). A laser is then guided by each layer of information, over a fine metal powder layer to melt together the metal particles. A layer of fresh powder is then swept over the previous layer and melted in a second laser pass to deposit the next layer. The process is repeated for the subsequent layers. Conveniently, the metal powder may be fed as a stream of powder directly into the laser beam at the point of deposition, and then rastered with the laser. 
         [0037]    The layer-by-layer deposition procedure provides great control over the structure of the layer. It also allows liners to be formed with enhanced uniformity and repeatability, and with low incidence of manufacturing defects. Feedstock wastage may be 10% or less, compared with about 50% typical for thermal spraying. 
         [0038]    The liner  30  can be grown directly on the casing  31 . Alternatively, the liner can be produced separately and then attached directly, eg through brazing, welding or mechanical fixture, to the casing or to a carrier tile which is then fixed to the casing. 
         [0039]    The metal feedstock for liner can vary depending on the application. However, typically, the metal can be nickel, iron, chromium or cobalt, or an alloy thereof. In some applications, it may be advantageous to fill the pores of the structure with eg polymer, metallic or ceramic material. This can be with the aim of providing an enhanced seal or improved cutting characteristics. 
         [0040]    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. For example, although described above in relation to an abradable liner attached to a casing for sealing against the tips of rotor blades, such a liner may be used to form seals in other area of a gas turbine engine, such as oil or air seals on shafts, discs and drums with eg knife type fins cutting into the liner. Also the liner can be on the moving component rather than the static component. 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. 
         [0041]    All references referred to above are hereby incorporated by reference.