Patent Publication Number: US-11649737-B2

Title: Forged cast forged outer case for a gas turbine engine

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The instant application is a continuation application of U.S. patent application Ser. No. 14/948,768, filed Nov. 23, 2015, which claims the benefit of provisional application Ser. No. 62/083,968, filed Nov. 25, 2014. 
    
    
     BACKGROUND 
     The present disclosure relates to a gas turbine engine and, more particularly, to a case therefore. 
     A Mid Turbine Frame (MTF) of a gas turbine engine typically includes a plurality of hollow vanes arranged in a ring-vane-ring structure. The rings define inner and outer boundaries of a core gas path while the vanes are disposed across the gas path. Tie rods extend through the hollow vanes to interconnect an engine mount ring and a bearing compartment. The MTF is subject to thermal stresses from combustion gases along the core gas path, which may reduce operational life thereof. 
     The MTF, sometimes referred to as an inter-turbine frame, is located generally between a high pressure turbine stage and a low pressure turbine stage of a gas turbine engine to support one or more bearings and to transfer bearing loads through to an outer engine case. The MTF system is thus a load bearing structure that provides rotor containment in the unlikely event a turbine shaft shear event should occur. The MTF is typically a forged structure that requires high strength for containment, and relatively significant machining to minimize weight and provide effective interfaces for various attachments. 
     SUMMARY 
     A case assembly for a gas turbine engine according to one disclosed non-limiting embodiment of the present disclosure includes a cast case section cast case section configured to be welded between a forward case section and an aft case section. 
     A further embodiment of the present disclosure includes, wherein the cast case section includes a machined interface. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the cast case section includes a raised boss. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the cast case section includes a machined surface. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the forward case section and the aft case section are forged. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the forged forward case section is configured for containment of a high pressure turbine rotor stage. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the forged forward case section includes a forward flange. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the forged aft case section is configured for containment of a low pressure turbine rotor stage. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the forged aft case section includes an aft flange. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the forged forward case section and the forged aft case section define respective forward and aft containment zones. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the forged forward case section and the forged aft case section define respective forward and aft containment zones. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, further comprising a forward weld between the forged forward case section and the cast case section outside of the forward containment zone and an aft weld between the cast case section and the forged aft case section outside of the aft containment zone. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the case is a mid-turbine frame. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the cast case section contains Inconel 718. 
     A case assembly for a gas turbine engine according to a another disclosed non-limiting embodiment of the present disclosure includes a forged forward case section that defines a forward containment zone around an axis; a forged aft case section that defines an aft containment zone around the axis; and a cast case section around the axis, the cast case section welded to the forged forward case section and the forged aft case section. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the cast case section including a multiple of bosses. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the cast case section includes an interface. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the cast case section includes a raised boss. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the cast case section includes a machined surface. 
     A further embodiment of any of the foregoing embodiments of the present disclosure includes, wherein the cast case section is manufactured of a lower strength but equivalent alloy of the forged forward case section and the forged aft case section. 
     The foregoing features and elements may be combined in various combinations without exclusivity, unless expressly indicated otherwise. These features and elements as well as the operation thereof will become more apparent in light of the following description and the accompanying drawings. It should be understood, however, the following description and drawings are intended to be exemplary in nature and non-limiting. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various features will become apparent to those skilled in the art from the following detailed description of the disclosed non-limiting embodiment. The drawings that accompany the detailed description can be briefly described as follows: 
         FIG.  1    is a schematic cross-sectional view of a geared architecture gas turbine engine; 
         FIG.  2    is a perspective view of the engine modules of the engine; 
         FIG.  3    is an exploded view of a Mid-Turbine Frame module; 
         FIG.  4    is a cross-sectional view of the Mid-Turbine Frame module through a tie-rod; 
         FIG.  5    is a side view of an outer MTF case of the Mid-Turbine Frame; 
         FIG.  6    is a cross-sectional view of an outer MTF case of the Mid-Turbine Frame; 
         FIG.  7    is a perspective view of the outer MTF case of the Mid-Turbine Frame module; 
         FIG.  8    is an expanded view of machined surfaces of the cast case section of the outer MTF case exterior; and 
         FIG.  9    is an expanded view of machined surfaces of the cast case section of the outer MTF case interior. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    schematically illustrates a gas turbine engine  20 . The gas turbine engine  20  is disclosed herein as a two-spool turbofan that generally incorporates a fan section  22 , a compressor section  24 , a combustor section  26  and a turbine section  28 . Alternative engines architectures such as a low-bypass turbofan may include an augmentor section (not shown) among other systems or features. Although schematically illustrated as a turbofan in the disclosed non-limiting embodiment, it should be understood that the concepts described herein are not limited to use with turbofans as the teachings may be applied to other types of turbine engines to include but not limited to a three-spool (plus fan) engine wherein an intermediate spool includes an intermediate pressure compressor (IPC) between a low pressure compressor and a high pressure compressor with an intermediate pressure turbine (IPT) between a high pressure turbine and a low pressure turbine as well as other engine architectures such as turbojets, turboshafts, open rotors and industrial gas turbines. 
     The fan section  22  drives air along a bypass flowpath and a core flowpath while the compressor section  24  drives air along the core flowpath for compression and communication into the combustor section  26  then expansion through the turbine section  28 . The engine  20  generally includes a low spool  30  and a high spool  32  mounted for rotation about an engine central longitudinal axis A relative to an engine case assembly  36  via several bearing compartments  38 - 1 ,  38 - 2 ,  38 - 3 ,  38 - 4 . The bearing compartments  38 - 1 ,  38 - 2 ,  38 - 3 ,  38 - 4  in the disclosed non-limiting embodiment are defined herein as a forward bearing compartment  38 - 1 , a mid-bearing compartment  38 - 2  axially aft of the forward bearing compartment  38 - 1 , a mid-turbine bearing compartment  38 - 3  axially aft of the mid-bearing compartment  38 - 2  and a rear bearing compartment  38 - 4  axially aft of the mid-turbine bearing compartment  38 - 3  It should be appreciated that additional or alternative bearing compartments may be provided. 
     The low spool  30  generally includes an inner shaft  40  that interconnects a fan  42 , a low-pressure compressor (“LPC”)  44  and a low-pressure turbine (“LPT”)  46 . The inner shaft  40  drives the fan  42  through a geared architecture  48  to drive the fan  42  at a lower speed than the low spool  30 . The high spool  32  includes an outer shaft  50  that interconnects a high-pressure compressor (“HPC”)  52  and high-pressure turbine (“HPT”)  54 . A combustor  56  is arranged between the HPC  52  and the HPT  54 . The inner shaft  40  and the outer shaft  50  are concentric and rotate about the engine central longitudinal axis A that is collinear with their longitudinal axes. 
     Core airflow is compressed by the LPC  44  then the HPC  52 , mixed with the fuel and burned in the combustor  56 , then expanded over the HPT  54  and the LPT  46 . The HPT  54  and the LPT  46  drive the respective high spool  32  and low spool  30  in response to the expansion. 
     In one example, the gas turbine engine  20  is a high-bypass geared architecture engine in which the bypass ratio is greater than about six (6:1). The geared architecture  48  can include an epicyclic gear system  58 , such as a planetary gear system, star gear system or other system. The example epicyclic gear train has a gear reduction ratio of greater than about 2.3, and in another example is greater than about 2.5 with a gear system efficiency greater than approximately 98%. The geared turbofan enables operation of the low spool  30  at higher speeds which can increase the operational efficiency of the LPC  44  and LPT  46  and render increased pressure in a fewer number of stages. 
     A pressure ratio associated with the LPT  46  is pressure measured prior to the inlet of the LPT  46  as related to the pressure at the outlet of the LPT  46  prior to an exhaust nozzle of the gas turbine engine  20 . In one non-limiting embodiment, the bypass ratio of the gas turbine engine  20  is greater than about ten (10:1), the fan diameter is significantly larger than that of the LPC  44 , and the LPT  46  has a pressure ratio that is greater than about five (5:1). It should be understood, however, that the above parameters are only exemplary of one embodiment of a geared architecture engine and that the present disclosure is applicable to other gas turbine engines including direct drive turbofans. 
     In one non-limiting embodiment, a significant amount of thrust is provided by the bypass flow due to the high bypass ratio. The fan section  22  of the gas turbine engine  20  is designed for a particular flight condition—typically cruise at about 0.8 Mach and about 35,000 feet. This flight condition, with the gas turbine engine  20  at its best fuel consumption, is also known as bucket cruise Thrust Specific Fuel Consumption (TSFC). TSFC is an industry standard parameter of fuel consumption per unit of thrust. 
     Fan Pressure Ratio is the pressure ratio across a blade of the fan section  22  without a Fan Exit Guide Vane system. The low Fan Pressure Ratio according to one non-limiting embodiment of the example gas turbine engine  20  is less than 1.45. Low Corrected Fan Tip Speed is the actual fan tip speed divided by an industry standard temperature correction of (“T”/518.7) 0.5  in which “T” represents the ambient temperature in degrees Rankine. The Low Corrected Fan Tip Speed according to one non-limiting embodiment of the example gas turbine engine  20  is less than about 1150 fps (351 m/s). 
     With reference to  FIG.  2   , the engine case assembly  36  generally includes a plurality of modules, including a fan case module  60 , an intermediate case module  62 , a Low Pressure Compressor (LPC) module  64 , a High Pressure Compressor (HPC) module  66 , a diffuser module  68 , a High Pressure Turbine (HPT) module  70 , a mid-turbine frame (MTF) module  72 , a Low Pressure Turbine (LPT) module  74 , and a Turbine Exhaust Case (TEC) module  76 . It should be understood that additional or alternative modules might be utilized. 
     With reference to  FIG.  3   , the MTF module  72 , in this example, generally includes an outer MTF case  80 , a mid-turbine frame (MTF)  82  with a multiple of hollow vanes  84 , a multiple of tie rods  86 , a multiple of tie rod nuts  88 , an inner case  90 , a HPT seal  92 , a heat shield  94 , a LPT seal  96 , a multiple of centering pins  98 , and a borescope plug assembly  100 . The MTF module  72  supports the mid-bearing compartment  38 - 3  through which the inner and outer shafts  40 ,  50  are rotationally supported ( FIG.  4   ). It should be appreciated that various other components may additionally or alternatively be provided within the MTF  82 , for example only, the LPT seal  96  may alternatively be referred to as an intermediate seal in other engine architectures. 
     Each of the tie rods  86  are mounted to the inner case  90  and extend through a respective vane  84  to be fastened to the outer MTF case  80  with the multiple of tie rod nuts  88  that are at least partially received into a respective feature  89  formed in the outer MTF case  80 . That is, each tie rod  86  is typically sheathed by a vane  84  through which the tie rod  86  passes ( FIG.  4   ). The other vanes  84  may alternatively or additionally provide other service paths. The multiple of centering pins  98  are circumferentially distributed between the vanes  84  to engage bosses  102  on the MTF  82  to locate the MTF  82  with respect to the inner case  90  and the outer MTF case  80 . It should be understood that various attachment arrangements may alternatively or additionally be utilized. 
     With reference to  FIG.  5   , the outer MTF case  80  is manufactured in a multiple of sections, here identified as a forged forward case section  120 , an forged aft case section  122 , and a cast case section  124  that is welded therebetween. The forged forward case section  120 , the forged aft case section  122 , and the cast case section  124  are defined around the engine axis A. In this example, the material of the sections  120 ,  122 ,  124  may include an age-hardened Inconel such as 718. 
     It should be appreciated that casting is the process where metal is heated until molten, then, while in the molten or liquid state, it is poured into a mold or vessel to create a desired shape. Casting facilitates manufacture of components that are relatively large, complicated, intricate or otherwise unsuitable for the forging process. In contrast, forging is the application of thermal and mechanical energy to steel billets or ingots to cause the material to change shape while in a solid state. Forging offers uniformity of composition and structure. Forging results in metallurgical recrystalisation and grain refinement as a result of the thermal cycle and deformation process. This strengthens the resulting alloy particularly in terms of impact and shear strength. 
     The forged forward case section  120  and the forged aft case section  122  each include a respective interface flange  126 ,  128  that permits the outer MTF case  80  to be fastened to respective forward and aft engine cases e.g. the diffusion module  68  and the TEC module  76  ( FIG.  2   ). In this example, the outer MTF case  80  may at least partially form the High Pressure Turbine (HPT) module  70  and the Low Pressure Turbine (LPT) module  74 . 
     The forged forward case section  120  and the forged aft case section  122  at least partially form containment zones  130 ,  132  for at least one rotor of the respective HPT  54  and the LPT  46 . That is, the forged forward case section  120  and the forged aft case section  122  are located radially outboard of at least the last rotor  54 -A ( FIG.  4   ) of the HPT  54  and the first rotor  46 -A ( FIG.  4   ) of the LPT  46  to contain a blade-out incident. Blade-out requirements are readily provided for by the higher impact properties typical of a forged structure. 
     With reference to  FIG.  6   , forward weld  140  is located between the forged forward case section  120  and the cast case section  124  outside of the containment zone  130  while an aft weld  142  between the cast case section  124  and the forged aft case section  122  outside of the containment zone  132 . That is, the welds  140 ,  142  are located outside of the containment zones. The welds  140 ,  142  may also be non-machined welds to avoid exposing indications. 
     The cast case section  124  forms the multiple of features  89  such as raised bosses  150  ( FIG.  7   ) and other features that are formed thereby. At least some of the features  89  may be “dummy” features  152  to provide equivalent circumferential feature distribution to maintain equivalent thermal expansion about the entire periphery. That is, the “dummy” feature  152  does not provide an interface but merely balances other interface features located, for example, one hundred eighty degrees around the cast case section  124 . 
     The relative complexity of the cast case section  124  due to the multiple of features  89  defined thereby is readily applicable to casting. That is, the cast case section  124  is cast to an essentially final shape that requires but minimal machining ( FIGS.  8  and  9   ). The relative minimal machining of the features  89  may, for example, only require that a machined surface  160  ( FIGS.  8  and  9   ) formed for attachment of various connections, sensors, and other devices such as the tie rod nut  88 . 
     The cast case section  124 , being casted rather than forged, facilitates relatively large, compound, fillets and/or blended fillets. The relatively large, compound, fillets are also readily easily cast which otherwise required cutter access between features  89  such as adjacent bosses  150 . Conversely, relatively small fillets are readily cast to decrease weight. Casting thus results in a relatively lighter weight and easier to manufacture structure rather than a forged area that may require relatively more significant all around machining to reduce weight. 
     The material of the cast case section  124  may include an age-hardened Inconel such as 718 that is of a lower strength than that of the forged forward case section  120  and the forged aft case section  122  which are also manufactured of 718. Since no rotational hardware is located inboard of the cast case section  124 , the cast case section  124  may provide the relatively lower impact properties typical of a cast structure. In this example, the material of the cast case section  124  may include an age-hardened Inconel such as 718 that is of a lower strength than that of the forged forward case section  120  and the forged aft case section  122 . 
     The use of the terms “a,” “an,” “the,” and similar references in the context of description (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or specifically contradicted by context. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., it includes the degree of error associated with measurement of the particular quantity). All ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. It should be appreciated that relative positional terms such as “forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like are with reference to normal operational attitude and should not be considered otherwise limiting. 
     Although the different non-limiting embodiments have specific illustrated components, the embodiments of this invention are not limited to those particular combinations. It is possible to use some of the components or features from any of the non-limiting embodiments in combination with features or components from any of the other non-limiting embodiments. 
     It should be appreciated that like reference numerals identify corresponding or similar elements throughout the several drawings. It should also be appreciated that although a particular component arrangement is disclosed in the illustrated embodiment, other arrangements will benefit herefrom. 
     Although particular step sequences are shown, described, and claimed, it should be understood that steps may be performed in any order, separated or combined unless otherwise indicated and will still benefit from the present disclosure. 
     The foregoing description is exemplary rather than defined by the limitations within. Various non-limiting embodiments are disclosed herein, however, one of ordinary skill in the art would recognize that various modifications and variations in light of the above teachings will fall within the scope of the appended claims. It is therefore to be understood that within the scope of the appended claims, the disclosure may be practiced other than as specifically described. For that reason the appended claims should be studied to determine true scope and content.