Abstract:
A ducted fan gas turbine engine aerofoil is made by electron beam welding together at least two metal sheets ( 10 ) and ( 12 ) and electron beam welding that sub assembly via an end to a root that has been manufactured in a separate operation, and then heating the whole to a temperature that will convert the electron beam welds to diffusion bonds.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. application Ser. No. 11/140,908, filed 1 Jun. 2005 and currently pending, which claims priority to GB 0412915.1, filed 10 Jun. 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the manufacture of aerofoil blades of the kind used in ducted fan gas turbine engines, wherein the aerofoils are located via respective roots, in and about the rim of a rotary disk within a ducted fan gas turbine engine. 
     More specifically, the present invention has best efficacy where used in the manufacture of gas turbine engine fan blades, the aerofoils of which are hollow. 
     It is known to manufacture a hollow fan blade by forming two half aerofoils, one of which provides a concave exterior surface, and the other of which provides a convex exterior surface, and both include a half root portion. The formed halves are then placed in a die and heated so as to enable diffusion bonding of the halves and super-plastic expansion and separation in known manner of the interior surfaces of the joined aerofoils to cause movement of the aerofoils into their respective curved forms. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to provide an improved method of making and joining a hollow aerofoil and root. 
     According to the present invention there is provided a method of making an at least substantially hollow aerofoil having a separately manufactured root comprises the steps of welding at least two metal sheets together about their edges, manufacturing a root having a surface shaped to receive an end of said joined sheets, welding said end to said surface, and then holding the resulting assembly in holding means via said sheets and heating the assembly to convert the weld joints to diffusion bonds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described by way of example and with reference to the accompanying drawings in which: 
         FIG. 1  is a longitudinal cross section through an aerofoil on line  1 - 1  of  FIG. 2 . 
         FIG. 2  is a diagrammatic sketch of a ducted fan gas turbine engine including a stage of fan aerofoils in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 . two sheets of metal,  10  and  12 , which may be titanium or aluminium, are welded together around their edges  14 . A third, much thinner metal sheet  16  of the same material as sheets  10  and  12 , is trapped between sheets  10  and  12 , and is further fixed by the weld referred to hereinbefore. A root member  18  that has been manufactured separate from the sheets  10 ,  12  and  16 , is provided with a surface  20  to which, via an end, the assembly of sheets  10 ,  12  and  16  is fixed by e.g. electron beam welding, or linear friction welding. Root member  18  is so shaped as to be a sliding fit in a respective groove in the rim of a fan disk  22  of engine  24  in  FIG. 2 . 
     All of the parts making up the assembly are of a common material e.g. titanium or aluminium, and in the present example of the invention, it is intended that they be diffusion bonded after the welding operation. However, where thin plate  16  is used, lengthwise strip portions thereof are later required to stretch in opposing directions laterally of the sheet length, so as to provide a stiffening member for the aerofoil. 
     Therefore, a number of strips of a diffusion bond preventative such as Yttria are glued on to each side of sheet  16  prior to its insertion between sheets  10  and  12 . 
     When the assembly is completed as described so far, it is placed in a suitable die which will enable forming sheets  10  and  12  into an aerofoil shape, and subjected to heat and temperature, the magnitudes of both of which are well known in the diffusion bonding and super-plastic forming field. Piping is connected to the interior of the sheets and an inert gas pumped in so as to cause sheets  10  and  12  to move away from each other to form the aerofoil shape dictated by the die, and simultaneously pull spaced portions of sheet  16  in opposing directions, to form the stiffening member. Also effected is the conversion of all of the welded joints peripherally of the sheets and between the ends of the sheets and root  18  to diffusion bonds, wherein material from each part migrates across the joint interface and eliminates it. 
     Attaching the root  18  to sheets  10 ,  12  and  16  at the stage in the process described provides the advantage that the following diffusion bonding process relieves stresses that are generated in the joint area during welding, thus obviating the need to perform a separate operation to achieve that effect. Further, it has been found that the resulting strength of the finished article is such that thinner sheets may be used without detriment. 
     Exclusion of sheet  16  will enable the manufacture of a completely hollow aerofoil having a root attached in the manner as described with reference to  FIG. 1 . In this example, that surface on one of the sheets that will be an interior surface when the two sheets are assembled, will have yttria applied to that area not required to diffusion bond. 
     An alternative method of manufacturing an aerofoil blade and root, is to weld sheets  10  and  12 , or sheets  10 ,  12  and  16  together as described hereinbefore, and then super-plastically form them into the desired aerofoil shape, prior to welding them to root  18 . The finished aerofoil can then be welded to root  18 . The whole will then be heated to achieve conversion of the root weld to a diffusion bond, again as described hereinbefore. 
     A further alternative method of manufacturing an aerofoil blade and root, is to weld sheets  10  and  12 , or weld sheets  10  and  12  and  16 , together as described hereinbefore, and then to diffusion bond them together, prior to welding them to the root  18 . The sheets  10  and  12 , or the sheets  10 ,  12  and  16  are then super-plastically formed into the desired aerofoil shape. The heating used by the super-plastic forming process relieves stresses in the joint area during welding and to form a diffusion bond.