PATENT ABSTRACT
A method for manufacturing a rotor shaft includes fabricating a first shaft portion that extends axially from a first end to a second end, fabricating a second shaft portion that extends axially from a first end to a second end, and coupling the second shaft portion to the first shaft portion with an explosive bonded joint such that the second shaft portion is aligned substantially concentrically with respect to the first shaft portion, and such that the bonded joint extends obliquely with respect to a centerline axis of symmetry of the rotor shaft.

PATENT DESCRIPTION
GOVERNMENT RIGHTS STATEMENT 
     The U.S. Government has certain rights in this invention pursuant to contract number F333615-94-2-4439. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to gas turbine engines, and more specifically to rotor shafts used with gas turbine engines. 
     At least some known gas turbine engines include a core engine having, in serial flow arrangement, a fan assembly, a high pressure compressor which compress airflow entering the engine, a combustor which burns a mixture of fuel and air, and low and high pressure turbines which each include a plurality of rotor blades that extract rotational energy from airflow exiting the combustor. The fan assembly and the low pressure turbine are coupled by a first shaft, and the high pressure compressor and the high pressure turbine are coupled by a second shaft. 
     During engine operation, the fan assembly and the low-pressure turbine are subjected to different operating temperatures, pressures, and stresses than those that the high pressure turbine and compressor are subjected. As a result, within at least some known gas turbine engines, the rotor shaft coupling the low pressure components is fabricated from a different material than the heavier, more durable material used in fabricating the rotor shaft that couples the high pressure components. However, because the low-pressure shaft extends the length of the gas turbine engine, a portion of the low-pressure shaft is exposed to the same temperatures and pressures as the high pressure turbine components. To facilitate optimizing engine weight considerations with operating stresses that may be induced to the shaft, at least some known low-pressure shafts include an upstream portion that is fabricated form a first material and a downstream portion that is fabricated from a second material. For example, a forward portion of the low-pressure shaft connected to the fan assembly and the aft portion of the low-pressure shaft connected to the low-pressure turbine may be fabricated from a nickel alloy, while an intermediate portion of the shaft extending through the compressor and high pressure turbine may be fabricated from a titanium alloy. Because such materials are dissimilar, explosive bonding is used to create a bonded joint that is then used to couple the two nickel shaft portions to the intermediate titanium alloy section of the shaft such that the bonded joint extends therebetween. 
     A low strength inner layer material is used to separate the plates used in forming the bonded joint. The inner layer material facilitates preventing the production of deleterious intermetallic compounds across the bonded joint. More specifically, the low strength inner layer material extends diametrically across the bonded joint, such that when the rotor shaft portions are coupled at the bonded joint, the inner layer of material extends substantially perpendicularly to a centerline axis of symmetry of the shaft. Within known bond joints, when the shaft is rotated, the low strength material resides completely in a plane of maximum shear stress. As a result, during engine operation, the inner layer material may significantly limit the performance of the bonded joint. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect of the invention, a method for manufacturing a rotor shaft is provided. The method comprises fabricating a first shaft portion that extends axially from a first end to a second end, fabricating a second shaft portion that extends axially from a first end to a second end, and coupling the second shaft portion to the first shaft portion with an explosive bonded joint such that the second shaft portion is aligned substantially concentrically with respect to the first shaft portion, and such that the bonded joint extends obliquely with respect to a centerline axis of symmetry of the rotor shaft. 
     In another aspect, a rotor shaft is provided. The rotor shaft includes a first shaft portion that extends axially from a first end to a second end, and a second portion that extends axially from a first end to a second end, wherein the first shaft portion is coupled to the second portion at a bonded joint such that the first shaft portion is substantially axially-aligned with respect to the second shaft portion. The bonded joint extends obliquely with respect to a centerline axis of symmetry of the rotor shaft. 
     In a further aspect of the invention, a gas turbine rotor shaft is provided. The gas turbine rotor shaft includes a first shaft portion, a second shaft portion, and a bonded joint extending therebetween. The bonded joint is substantially concentrically aligned with respect to said first and second portions, and is oblique with respect to a centerline axis of symmetry extending axially through the rotor shaft. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is schematic illustration of a gas turbine engine; 
     FIG. 2 is a partial perspective view of a known explosive bonded joint; 
     FIG. 3 is an enlarged side view of a known shaft bonded joint section created from the explosive bonded joint shown in FIG. 2; 
     FIG. 4 is a partial perspective end view of a bonded joint that may be used with a rotor shaft shown in FIG. 1; and 
     FIG. 5 is an enlarged side view of a shaft bonded joint section created from the explosive bonded joint shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a schematic illustration of a gas turbine engine  10  including a low pressure compressor  12 , a high pressure compressor  14 , and a combustor  16 . Engine  10  also includes a high pressure turbine  18  and a low pressure turbine  20 . Compressor  12  and turbine  20  are coupled by a first shaft  21 , and compressor  14  and turbine  18  are coupled by a second shaft  22 . A load (not shown) may also coupled to gas turbine engine  10  with first shaft  21 . In one embodiment, gas turbine engine  10  is an F110 available from General Electric Aircraft Engines, Cincinnati, Ohio. 
     In operation, air flows through low pressure compressor  12  and compressed air is supplied from low pressure compressor  12  to high pressure compressor  14 . The highly compressed air is delivered to combustor  16 . Airflow from combustor  16  drives turbines  18  and  20  and exits gas turbine engine  10  through a nozzle  24 . 
     FIG. 2 is a partial perspective view of a known explosive bonded joint  40 . FIG. 3 is an enlarged side view of a known shaft bonded joint section  41  created from explosive bonded joint  40 . Bonded joint  40  is formed by explosive bonding which enables the joining of dissimilar or metallurgically incompatible metals, such that a rotor shaft, such as shaft  21  may be fabricated from a plurality of different materials. 
     Specifically, bonded joint  40  is fabricated by creating an explosive bonded sandwich of plates  44  and  46  that are each fabricated from the same respective material as used in fabricating an upstream portion  48  of shaft  21  and a downstream portion  50  of shaft  21 . More specifically, plate  44  and shaft upstream portion  48  are each fabricated from a first material, and plate  46  and shaft downstream portion  50  are each fabricated from a second material. In the exemplary embodiment, the first material is a nickel alloy, and the second material is a titanium alloy. 
     Before plates  44  and  46  are explosively bonded together, a low strength inner layer  52  is positioned between plates  44  and  46  to separate plates  44  and  46 . In addition, because layer  52  is fabricated from a material that is not the same as either material used to fabricate shaft portions  48  and  50 , layer  52  facilitates preventing the production of deleterious intermetallic compounds. In the exemplary embodiment, layer  52  is fabricated from a niobium alloy. 
     After plates  44  and  46 , and layer  52  have been explosively bonded together in a known explosive bonding process, shaft section  41  is cut from plates  44  and  46  and used to couple shaft portions  48  and  50 . Specifically, when shaft portions  48  and  50  are coupled together, shaft section  41  extends therebetween, such that inner layer  52  extends diametrically across rotor shaft  21  and is substantially perpendicular to a centerline axis of symmetry  60  extending through shaft  21 . However, during operation, as shaft  21  rotates, shear stress is induced into shaft  21 . More specifically, because of an orientation of inner layer  52  with respect to shaft  21 , inner layer  52  resides completely in a plane of maximum shear stress as shaft  21  is rotated. As a result, during engine operation, inner layer material  52  may significantly limit the performance of the bonded joint. 
     FIG. 4 is a partial perspective end view of a bonded joint  100  that may be used with a rotor shaft, such as shaft  21 . Alternatively, bonded joint  100  may be used with shafts (not shown) not used in the aviation industry, such as, but not limited to, shafts used in automobile engines. FIG. 5 is an enlarged side view of shaft bonded joint section  102  created from explosive bonded joint  100 . Bonded joint  100  is formed by explosive bonding which enables the joining of dissimilar or metallurgically incompatible metals, such that a rotor shaft, such as shaft  21  may be fabricated from a plurality of different materials. 
     Specifically, bonded joint  100  is fabricated by creating an explosive bonded sandwich of plates  104  and  106  that are each fabricated from the same respective material as used in fabricating an upstream portion  108  of shaft  21  and a downstream portion  110  of shaft  21 . More specifically, plate  104  and shaft upstream portion  108  are each fabricated from a first material, and plate  106  and shaft downstream portion  110  are each fabricated from a second material. In the exemplary embodiment of FIGS. 4 and 5, the first material is a nickel alloy, and the second material is a titanium alloy. 
     Before plates  104  and  106  are explosively bonded together, a low strength inner layer  112  is positioned between plates  104  and  106  to separate plates  104  and  106 . In addition, because layer  112  is fabricated from a material that is not the same as either material used in fabricating shaft portions  108  and  110 , layer  112  facilitates preventing the production of deleterious intermetallic compounds. In one embodiment, layer  112  is fabricated from a niobium alloy. 
     After plates  104  and  106 , and layer  112  have been explosively bonded together in a known explosive bonding process, shaft section  102  is cut from plates  104  and  106  and used to couple shaft portions  108  and  110 . Specifically, when shaft portions  108  and  110  are coupled together, shaft section  102  extends therebetween, such that inner layer  112  extends diametrically across rotor shaft  21 . However, unlike inner layer  52  (shown in FIGS.  2  and  3 ), inner layer  112  extends obliquely with respect to a centerline axis of symmetry  120  extending through shaft  21 . More specifically, inner layer  52  is positioned at an oblique angle θ with respect to centerline axis of symmetry  120 . In one embodiment, angle θ is approximately 105° degrees. 
     During operation, as shaft  21  rotates, torsional shear stress is induced into shaft  21 . However, because inner layer  112  is oriented at an oblique angle θ, layer  112  and bonded joint  102  are moved from the plane of maximum shear stress, which facilitates improving load capacity of shaft  21 . Furthermore, because inner layer angle θ also facilitates improving torsion and bending stiffness of shaft  21 . In addition, angle θ also provides torque limiting for shaft  21 . Accordingly, shaft section  102  and bonded joint  100  facilitate improving a useful life of shaft  21 . 
     The above-described bonded joint is cost-effective and highly reliable. The shaft section including the bonded joint is formed at an oblique angle that facilitates shifting the bonded joint from the plane of maximum shear stress during shaft rotation. Furthermore, because the inner layer of the bonded joint is oriented obliquely with respect to the shaft, the bonded joint provides torque limiting for the associated shaft  21 . As a result, the bonded joint facilitates extending a useful life of the shaft in a cost-effective and reliable manner. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.