Abstract:
A floating collar is metal injected molded with an excess portion intended to be separated, such as by shearing, from the reminder of the molded floating collar to leave a chamfer thereon and/or remove injection marks.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of application Ser. No. 11/782,234, filed Jul. 24, 2007, now U.S. Pat. No. 7,543,383 issued on Jun. 9, 2009. 
    
    
     TECHNICAL FIELD 
     The application relates generally to gas turbine engine combustors and, more particularly, to a method of manufacturing a fuel nozzle floating collar therefor. 
     BACKGROUND OF THE ART 
     Gas turbine combustors are typically provided with floating collar assemblies or seals to permit relative radial or lateral motion between the combustor and the fuel nozzle while minimizing leakage therebetween. Machined floating collars are expensive to manufacture at least partly due to the need for an anti-rotating tang or the like to prevent rotation of the collar about the fuel nozzle tip. This anti-rotation feature usually prevents the part from being simply turned requiring relatively expensive milling operations and results in relatively large amount of scrap material during machining. 
     There is thus a need for further improvements in the manufacture of fuel nozzle floating collars. 
     SUMMARY 
     In one aspect, there is provided a method of manufacturing a floating collar adapted to be slidably engaged on a fuel nozzle for providing a sealing interface between the fuel nozzle and a combustor wall, the method comprising: metal injection moulding a generally cylindrical part having an axis, a collar portion and a sacrificial portion, the sacrificial portion including at least a shoulder projecting radially inwardly from one end of said collar portion along an inner circumferential wall of the collar portion, the shoulder and the circumferential wall defining a corner, and while the cylindrical part is still in a substantially dry green condition, forming a chamfer at said one end of said collar portion on an inside diameter of the collar portion by applying axially opposed shear forces on opposed sides of the corner to shear off the sacrificial portion from said collar portion along a shearing line extending angularly outwardly from said corner. 
     In a second aspect, there is provided a method for manufacturing a floating collar adapted to provide a sealing interface between a fuel nozzle and a gas turbine engine combustor, comprising: a) metal injection moulding a green part including a floating collar portion and a feed inlet portion, the feed inlet portion bearing injection marks corresponding to the points of injection, b) separating the feed inlet portion from the floating collar portion to obtain a floating collar free of any injection marks, and c) debinding and sintering the floating collar portion 
     Further details of these and other aspects of the present invention will be apparent from the detailed description and figures included below. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       Reference is now made to the accompanying figures depicting aspects of the present invention, in which: 
         FIG. 1  is a schematic cross-sectional view of a gas turbine engine having an annular combustor; 
         FIG. 2  is an enlarged cross-sectional view of a dome portion of the combustor illustrating a floating collar slidably mounted about a fuel nozzle tip and axially trapped between a heat shield and a combustor dome panel; 
         FIG. 3  is an isometric view of the floating collar shown in  FIG. 2 ; 
         FIG. 4  is a cross-sectional view of a mould used to form the floating collar; 
         FIG. 5  is a cross-sectional view of the moulded green part obtained from the metal injection moulding operation, the feed inlet material to be discarded being shown in dotted lines; 
         FIG. 6  is a cross-sectional schematic view illustrating how the moulded green part is sheared to separate the collar from the material to be discarded; and 
         FIG. 7  is a cross-section view of the collar after the shearing operation, the sheared surface forming a chamfer on the inside diameter of the collar. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a gas turbine engine  10  of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan  12  through which ambient air is propelled, a multistage compressor  14  for pressurizing the air, a combustor  16  in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section  18  for extracting energy from the combustion gases. 
     The combustor  16  is housed in a plenum  17  supplied with compressed air from compressor  14 . The combustor  16  has a reverse flow annular combustor shell  20  including a radially inner liner  20   a  and a radially outer liner  20   b  defining a combustion chamber  21 . As shown in  FIG. 2 , the combustor shell  20  has a bulkhead or inlet dome portion  22  including an annular end wall or dome panel  22   a . A plurality of circumferentially distributed dome heat shields (only one being shown at  24 ) are mounted inside the combustor  16  to protect the dome panel  22   a  from the high temperatures in the combustion chamber  21 . The heat shields  24  can be provided in the form of high temperature resistant casting-made arcuate segments assembled end-to-end to form a continuous 360° annular band on the inner surface of the dome panel  22   a . Each heat shield  24  has a plurality of threaded studs  25  extending from a back face thereof and through corresponding mounting holes defined in the dome panel  22   a . Fasteners, such as self-locking nuts  27 , are threadably engaged on the studs from outside of the combustor  16  for securely mounting the dome heat shields  24  to the dome panel  22   a . As shown in  FIG. 2 , the heat shields  24  are spaced from the dome panel  22   a  by a distance of about 0.1 inch so as to define an air gap  29 . In use, cooling air is admitted in the air gap  29  via impingement holes (not shown) defined though the dome panel  22   a  in order to cool down the heat shields  24 . 
     A plurality of circumferentially distributed nozzle openings (only one being shown at  26 ) are defined in the dome panel  22   a  for receiving a corresponding plurality of air swirler fuel nozzles (only one being shown at  28 ) adapted to deliver a fuel-air mixture to the combustion chamber  21 . A corresponding central circular hole  30  is defined in each of the heat shields  24  and is aligned with a corresponding fuel nozzle opening  26  for accommodating an associated fuel nozzle  28  therein. The fuel nozzles  28  can be of the type generally described in U.S. Pat. Nos. 6,289,676 or 6,082,113, for example, and which are incorporated herein by reference. 
     As shown in  FIGS. 2 and 3 , each fuel nozzle  28  is associated with a floating collar  32  to facilitate fuel nozzle engagement with minimum air leakage while maintaining relative movement of the combustor  16  and the fuel nozzle  28 . Each floating collar  32  comprises an axially extending cylindrical portion  36  and a radially extending flange portion  34  integrally provided at a front end of the axially extending cylindrical portion  36 . The axially extending cylindrical portion  36  defines a central passage  35  for allowing the collar  32  to be axially slidably engaged on the tip portion of the fuel nozzle  28 . First and second inner diameter chamfers  37  and  39  are provided at opposed ends of the collar  32  to eliminate any sharp edges that could interfere with the sliding movement of the collar  32  on the fuel nozzle  28 . The chamfers  37  and  39  extend all around the inner circumference of the collar  32 . The radially extending flange portion  34  is axially sandwiched in the air gap  29  between the heat shield  24  and the dome panel  22   a . An anti-rotation tang  38  extends radially from flange portion  34  for engagement in a corresponding slot (not shown) defined in a rearwardly projecting surface of the heat shield  24 . 
     As can be appreciated from  FIG. 4 , the floating collar  32  can be produced by metal injection moulding (MIM). The MIM process is preferred as being a cost-effective method of forming precise net-shape metal components. The MIM process eliminates costly secondary machining operations. The manufacturing costs can thus be reduced. The floating collar  32  is made from a high temperature resistant powder injection moulding composition. Such a composition can include powder metal alloys, such as IN625 Nickel alloy, or ceramic powders or mixtures thereof mixed with an appropriate binding agent. Other high temperature resistant compositions could be used as well. Other additives may be present in the composition to enhance the mechanical properties of the floating collar (e.g. coupling and strength enhancing agents). 
     As shown in  FIG. 4 , the molten metal slurry used to form the floating collar  32  is injected in a mould assembly  40  comprising a one-piece male part  42  axially insertable into a two-piece female part  44 . The metal slurry is injected in a mould cavity  46  defined between the male part  42  and the female part  44 . The gap between the male and female parts  42  and  44  corresponds to the desired thickness of the walls of the floating collar  32 . The female part  44  is preferably provided in the form of two separable semi-cylindrical halves  44   a  and  44   b  to permit easy unmoulding of the moulded green part. 
     The male part  42  has a disc-shaped portion  48 , an intermediate cylindrical portion  50  projecting axially centrally from the disc-shaped portion  48  and a terminal frusto-conical portion  52  projecting axially centrally from the intermediate cylindrical portion  50  and tapering in a direction away from the intermediate cylindrical portion  50 . An annular chamfer  54  is defined in the male part  42  between the disc-shaped portion  48  and the intermediate cylindrical portion  50 . The annular chamfer  54  is provided to form the inner diameter chamfer  39  of the collar  32 . An annular shoulder  56  is defined between the intermediate cylindrical portion  50  and the bottom frusto-conical portion  52 . 
     The female part  44  defines a central stepped cavity including a rear shallow disc-like shaped cavity  58 , a cylindrical intermediate cavity  60  and a front or feed inlet cylindrical cavity  62 . The disc-like shaped cavity  58 , the intermediate cavity  60  and the feed cavity  62  are aligned along a central common axis A. The disc-like shaped cavity  58  has a diameter d 1  greater than the diameter d 2  of the intermediate cavity  60 . Diameter d 2  is, in turn, greater than the diameter d 3  of the feed cavity  62 . The disc-like shaped cavity  58 , the intermediate cavity  60  and the feed cavity  62  are respectively circumscribed by concentric cylindrical sidewalls  64 ,  66  and  68 . First and second axially spaced-apart annular shoulders  70  and  72  are respectively provided between the disc-like cavity  58  and the intermediate cavity  60 , and the intermediate cavity  60  and the front cavity  62 . 
     After the male part  42  and the female part  44  have been inserted into one another with a peripheral portion of the disc-like shaped portion  48  of the male part  42  sealingly abutting against a corresponding annular surface  74  of the female part  44 , the mould cavity  46  is filled with the feedstock (i.e. the metal slurry) by injecting the feedstock axially endwise though the feed cavity  62  about the frusto-conical portion  52 , as depicted by arrows  74 . 
     After a predetermined setting period, the mould assembly  40  is opened to reveal the moulded green part shown in  FIG. 5 . The moulded green part comprises a floating collar portion  32 ′ and a sacrificial or “discardeable” feed inlet portion  76  (shown in dotted lines) to be separated from the collar portion  32 ′ and discarded. As can be appreciated from  FIG. 5 , the collar portion  32 ′ has a built-in flange  34 ′ and an inner diameter chamfer  39 ′ respectively corresponding to flange  34  and chamfer  39  on the finished collar product shown in  FIG. 3 , but still missed the inner diameter chamfer  37  at the opposed end of the floating collar. As will be seen hereinafter, the chamfer  37  is subsequently formed by separating the sacrificial portion  76  from the collar portion  32 ′. 
     In the illustrated example, the sacrificial feed inlet portion  76  comprises a shoulder  78  extending radially inwardly from one end of the collar portion  32 ′ opposite to flange  34 ′ and an axially projecting hollow cylindrical part  80 . The shoulder  78  extends all around the entire inner circumference of the collar portion  32 ′. The shoulder  78  and the cylindrical wall  81  of the collar portion  32 ′ define a sharp inner corner  82 . The sharp inner corner  82  is a high stress concentration region where the moulded green part will first start to crack if a sufficient load is applied on shoulder  78 . Also can be appreciated from  FIG. 5 , the thickness T 1  of the shoulder  78  is less than the wall thickness T 2  of the collar portion  32 ′. The shoulder  78  is thus weaker than the cylindrical wall  81  of the collar  32 ′, thereby providing a suitable “frangible” or “breakable” area for separating the sacrificial feed inlet portion  76  from the collar portion  32 ′. 
     As schematically shown in  FIG. 6 , the sacrificial feed inlet portion  76  can be separated from the collar portion  32 ′ by shearing. The shearing operation is preferably conducted while the part is still in a dry green state. In this state, the part is brittle and can therefore be broken into pieces using relatively small forces. As schematically depicted by arrows  84  and  86 , the moulded green part is uniformly circumferentially supported underneath flange  34 ′ and shoulder  78 . An axially downward load  88  is applied at right angles on the inner shoulder  78  uniformly all along the circumference thereof. A conventional flat headed punch (not shown) can be used to apply load  88 . The load  88  or shearing force is applied next to inner corner  82  and is calibrated to shear off the sacrificial portion  80  from the collar portion  32 ′. As shown in dotted lines in  FIG. 6 , the crack initiates from the corner  88  due to high stress concentration and extends angularly outwardly towards the outer support  86  at an angle θ comprised between 40-50 degrees, thereby leaving a sheared chamfer  37 ′ (see  FIG. 7 ) on the inner diameter of the separated collar portion  32 ′. The shear angle θ can be adjusted by changing the diameter of the outer support  86 . For instance, if the diameter of the outer support  86  is reduced so as to be closer to the inner corner  82 , the shear angle θ will increase. Accordingly, the location of the intended shear line can be predetermined to consistently and repeatedly obtain the desired inner chamfer at the end of the MIM floating collars. This avoids expensive secondary machining operations to form chamfer  37 . The sheared chamfer  37  has a surface finish which is a rougher than a machined or moulded surface, but is designed to remain within the prescribed tolerances. There is thus no need to smooth out the surface finish of the sheared chamfer  37 . Also, since the sacrificial portion  76  bears the injection marks left in the moulded part at the points of injection, there is no need for secondary machining of the remaining collar portion  32 ′ in order to remove the injection marks. 
     Once separated from the collar portion  32 ′, the sacrificial feed inlet portion  76  can be recycled by mixing with the next batch of metal slurry. The remaining collar portion  32 ′ obtained from the shearing operation is shown in  FIG. 7  and is then subject to conventional debinding and sintering operations in order to obtain the final net shape part shown in  FIG. 3 . 
     The above description is meant to be exemplary only, and one skilled in the art will recognize that changes may be made to the embodiments described without departing from the scope of the invention disclosed. For example, a line of weakening could be integrally moulded into the part or cut into the surface of the moulded part to provide a stress concentration region or frangible interconnection between the portion to be discarded and the floating collar portion. Also, it is understood that the part to be discarded could have various configurations and is thus limited to the configuration exemplified in  FIGS. 5 and 6 . Still other modifications which fall within the scope of the present invention will be apparent to those skilled in the art, in light of a review of this disclosure, and such modifications are intended to fall within the appended claims.