Patent Publication Number: US-2019195135-A1

Title: Titanium alloy compressor case

Description:
FIELD OF THE INVENTION 
     The present invention provides fire protection for titanium and titanium alloys, and specifically, for titanium and titanium alloys used as compressor cases in gas turbine engines. 
     BACKGROUND OF THE INVENTION 
     Titanium and certain titanium alloys are subject to ignition resulting in fires in the presence of oxygen under certain conditions. This can occur in machinery when titanium structures are exposed to friction, such as results from rubbing or when debris is lodged between a rotating part and a stationary part, at least one of the rotating part and stationary part being comprised of titanium. One type of machinery that employs titanium and its alloys are gas turbine engines, and in particular, gas turbine engines used in aircraft and aerospace applications, which take advantage of the high strength and low density of titanium and its alloys. 
     Gas turbine engines operate by burning fuel and extracting energy from the combusted fuel to generate power. Atmospheric air is drawn into the engine from the environment, where, in the compressor section it is compressed in multiple stages to significantly higher pressure and higher temperature. A portion of the compressed air is then mixed with fuel and ignited in the combustor section to produce high energy combustion gases. The high energy combustion gases then flow through the turbine section of the engine, which includes a plurality of turbine stages, each stage comprising turbine vanes and turbine blades mounted on a rotor. The high energy combustion gases create a harsh environment, causing oxidation, erosion and corrosion of downstream hardware. The turbine blades extract energy from the high energy combustion gases and turn the turbine shaft on which the rotor is mounted. The shaft may produce mechanical power or may directly generate electricity. Hot gases exiting the turbine section pass into the exhaust section. Gases exiting the exhaust section are used to propel the aircraft. A portion of the compressed air is also used to cool components of the turbine engine downstream of the compressor, such as combustor components, turbine components and exhaust components. 
     Titanium and its alloy find particular use in the multiple stages of the compressor section in applications such as bladed disks, blisks, vanes and spools, and cases surrounding the rotating airfoils and stationary vanes. The limiting temperature in a compressor of titanium and its alloys is about 900°-1000° F., which is well below the ignition point of titanium. This limit is due to the creep strength of titanium and its alloys at temperatures above about 900°-1000° F. and higher, and its propensity to slowly absorb oxygen at these higher temperatures and gradually embrittle over time. However, titanium and its alloys may be ignited in these applications by mechanical rubbing, which may be caused by a number of factors including foreign or domestic objects lodged between the rotor and a casing, rotor imbalance wherein the rotating blade rubs against the titanium or titanium alloy case, rotor displacement or case bending. The high pressure case (HPC) is most susceptible to these problems as the clearances are very small, the rotational speeds of the compressor blades are the highest and the temperature is the highest, above about 700° F. 
     Current constructions for compressor section components such as compressor cases utilize superalloy materials, steel or nickel-based materials, adding weight to the component, which is undesirable in aerospace applications. Alternative constructions utilize thick-walled titanium or double-walled titanium construction, adding weight to the component and exotic thermal barrier coating (TBC) systems to minimize ignition in rub scenarios, adding cost to the component. What is needed is a construction that advantageously continues to utilize low density titanium or titanium alloys, yet reduces the likelihood of a titanium fire when rubbing conditions are encountered while avoiding the excess weight of superalloy constructions or excess titanium with expensive coating systems 
     BRIEF DESCRIPTION OF THE INVENTION 
     A titanium or titanium alloy is used as the base material for a component. A layer of material of high thermal conductivity material forming an integral liner is applied to the titanium or titanium alloy component at least along those areas in which a rub with an adjacent part is predicted or otherwise expected as a result of operations, or impingement of molten titanium from ignition of Ti blades and/or vanes. The high specific heat material should also have a coefficient of thermal expansion (CTE) that is compatible with the titanium or titanium alloy component base material. The application of high specific heat material to the titanium or titanium alloy base material also must be accomplished so that the high specific heat material does not react with the titanium or titanium alloy base material at temperatures of operation, forming detrimental phases or otherwise reacting only minimally. While the high thermal conductivity material may function as a wear surface, its primary purpose is to reduce the effect of heat due to friction from rub events or from impingement of molten titanium from ignition of Ti blades and/or vanes. 
     A component comprising a titanium or titanium alloy base material (Ti) further comprises a high specific heat material overlying at least a portion of the titanium or titanium alloy base material. The high specific heat material overlies the titanium or titanium alloy base material at least in areas in which a mechanical event such as a rub can occur which might otherwise overheat the base material to the point of ignition, or impingement of molten titanium from ignition of Ti blades or vanes. A diffusion barrier is intermediate the high specific heat material and the titanium or titanium alloy base material, the diffusion barrier being a thin layer overlying the titanium or titanium alloy base material and underlying the high specific heat base material. 
     The component of the present invention advantageously is light in weight due to the use of the low density titanium or titanium alloy, yet reduces the likelihood of a titanium fire and breach of the HPC case due to the use of a high specific heat liner material in areas in which mechanical events such as rubbing can occur or areas where molten titanium from ignition of Ti blades and/or vanes may occur. 
     The component of the present invention also is resistant to the formation of detrimental phases between the titanium or titanium alloy base material and the applied high specific heat surface material forming a liner when operated at elevated temperatures by the use of a diffusion barrier between the high specific heat material liner material which also has high thermal conductivity and the titanium or titanium alloy base material. Another advantageous feature of the diffusion barrier is to prevent localized melting which might occur in the event of a heavy rub onto the liner or impinging molten titanium from ignition of Ti blades and/or vanes. If either of these high heat input processes were to occur and no diffusion barrier existed between the titanium or titanium alloy case and the high specific heat surface material, interdiffusion between the two materials may occur, resulting in the formation of a molten layer, likely breach and combustion of the titanium or titanium alloy HPC case. The diffusion layer prevents the formation of low melting point phases. When the high specific heat material is sufficiently thick, it may be possible to fabricate the component without a diffusion layer as the high heat capacity layer can absorb sufficient heat during a rub event and/or impingement of molten Ti from a Ti blade/vane fire, such that no low melting point eutectic can form and the case will not deform, breach and ignite. 
     The component of the present invention will not distort as a result of differential thermal expansion between the applied high specific heat surface material and the titanium or titanium alloy base material, nor will the high specific heat layer spall when used at elevated temperatures since the high specific heat material forming the liner is selected based on having a CTE that is compatible with that of the titanium or titanium alloy base material. Further, the component may be tightly toleranced since the high specific heat material forming the thermal liner will expand or contract at or close to the same rate of the titanium or titanium alloy base material. Thus, no allowances that affect clearances have to be included to account for differential thermal expansion between the high specific heat liner material and the titanium or titanium alloy base material. 
     When the component of the present invention is used as a high pressure compressor forward case in an aircraft gas turbine, an improvement in fuel burn of 0.07% is attributed to the use of this liner/base material combination as compared to steel case alternative, while reducing the likelihood of a titanium fire in a titanium or titanium alloy case alternative. 
     Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    depicts an exemplary titanium case with the high specific heat material system of the present invention applied to the case opposite a rotating titanium blade. 
         FIG. 1 b    is an enlarged view of the exemplary titanium case of  FIG. 1   a.    
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Titanium and its alloys are used in those applications requiring strength, but in which weight must be kept to a minimum. Titanium and its alloys have a tensile strength that approaches that of steel, but has a density that is about one-half that of steel. As used herein, unless otherwise specified, the term titanium or the symbol Ti refers to both titanium and its alloys. 
     Titanium has tensile strength similar to that of low alloy steel, about 63 ksi. However, titanium has a density that is about 56% that of steel, steel having a density of about 7.88 gm/cm 3 , while Ti has a density of about 4.41 gm/cm 3 . Ti has a significant advantage over steel where strength-to-weight ratio is a concern. 
     Weight is a major consideration in virtually all aircraft applications, and strength-to-weight ratio is of particular concern for components of aircraft engines. Thus, for many applications, titanium is the material of choice. One titanium alloy used in aircraft and aircraft engine applications is Ti 6-2-4-2. Ti 6-2-4-2 has a nominal composition in weight percent, of 6% aluminum (Al), 2% tin (Sn), 4% zirconium (Zr), 2% molybdenum (Mo) and the balance titanium (Ti). Unless otherwise specified, all compositions are specified in weight percentages. Ti 6-4 is another titanium alloy used in aircraft applications and has a nominal composition of 6% Al, 4% vanadium (V) and the balance Ti. 
     While titanium provides a clear strength-to-weight advantage for most applications in which strength-to-weight is a major consideration, for applications involving contacting surfaces at high speeds, overheating and ignition of titanium as a titanium fire is a major concern. Of course, aircraft engine applications include such contacting components comprising titanium that operate at high speed, and uncontained titanium fires are a major concern. More specifically, the fan and compressor sections of the engine include titanium blades rotating at high speeds. When the mating cases comprise titanium, uncontained titanium fires are a possibility. One solution to preventing titanium fires, for example in the forward case of a high pressure compressor (HPC) of an aircraft engine is to fabricate the forward case of a steel, such as M152 steel. Of course, this imposes a weight penalty. Another solution, such as used in some high performance engine designs, is to provide a split case, in which a portion of the case is a liner comprising 17-4PH steel. This separate liner is attached to the case and held in place mechanically. This solution also imposes a weight penalty in that both the 17-4 liner and the mechanical attachments securing the liner to the remainder of the case add weight. Another solution is to provide thick Ti cases or double walled Ti cases along with exotic TBC systems, which undesirably add both weight and cost to the component. 
     The solution to the problem of titanium rubs and/or molten titanium containment is set forth in  FIGS. 1 a  and 1 b   . In  FIG. 1( a ) , a titanium case  50  includes a high specific heat material coating system applied as a liner  40 .  FIG. 1( b )  is an enlarged view of one of the liners  40  of  FIG. 1( a ) . In its broadest embodiment, a high specific heat material  10  system having a high coefficient of thermal expansion consistent with the CTE of the titanium substrate is applied over a titanium substrate  30 . This high specific heat material comprises a material, and more specifically an alloy such as a steel or superalloy, selected from the group consisting of IN909, IN783, M152, 15-5PH and 17-4PH. The composition of each of these alloys is provided in Table 1, below. In one embodiment, the substrate  30  is an HPC forward case comprising Ti 6-2-4-2. The high specific heat material  10  is applied to the liner region  40  of the forward case  50  to a thickness of from about 5 mils (0.005 inches) up to about 100 mils (0.100 inches) and preferably about 10-60 mils (0.010-0.060 inches). Since the thickness of HPC forward case  50  in the vicinity of the liner is about ¼″ (250 mils) in thickness, the applied high specific heat material is about ⅓ of the liner thickness. Thus as compared to the 17-4PH liner used in some high performance engine HPC forward cases, this solution allows for replacement of about 0.180 mils of 17-4PH steel with titanium, and specifically Ti 6-2-4-2. This solution thus provides the HPC forward case  50  with a significant weight reduction. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 IN909 
                 IN783 
                 M152 
                 15-5 PH 
                 17-4 PH 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Ni 
                 35-40 
                 26.0-30.0 
                 2.0-3.0 
                 3.5-5.5 
                 3.0-5.0 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Cr 
                 1.0 
                 max 
                 2.5-3.5 
                 11.0-12.5 
                 14.0-15.0 
                 15.0-17.5 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Co 
                 12-16 
                 Balance 
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Fe 
                 balance 
                 24.0-30.0 
                 Balance 
                 Balance 
                 Balance 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Nb 
                 4.3-5.2 
                 2.5-3.5 
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Ti 
                   
                   
                 0.1-0.4 
                   
                   
                   
                   
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Nb + Ti 
                   
                   
                   
                   
                   
                   
                 0.15-0.45 
                 0.15-0.45 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Al 
                   
                   
                 5.0-6.0 
                   
                   
                   
                   
                   
                   
               
               
                 B 
                 0.012 
                 max 
                 0.003-.012  
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 C 
                   
                   
                 0.003 
                 max 
                 0.08-0.13 
                 0.07 
                 max 
                 0.07 
                 max 
               
               
                 Mn 
                 1.0 
                 max 
                 0.5 
                 max 
                 0.5-0.9 
                 1.0 
                 max 
                 1.0 
                 max 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Si 
                   
                   
                 0.5 
                 max 
                 0.35 
                 max 
                 1.0 
                 max 
                 1.0 
                 max 
               
               
                 P 
                 0.015 
                 max 
                 0.015 
                 max 
                 0.025 
                 max 
                 0.04 
                 max 
                 0.04 
                 max 
               
               
                 S 
                 0.015 
                 max 
                 0.005 
                 max 
                 0.025 
                 max 
                 0.03 
                 max 
                 0.03 
                 max 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Cu 
                 0.5 
                 max 
                 0.5 
                 max 
                 0.5 
                 max 
                 2.5-4.5 
                 3.0-5.0 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 V 
                   
                   
                   
                   
                 0.25-0.4  
                   
                   
                   
                   
               
               
                 Mo 
                   
                   
                   
                   
                 1.5-2.0 
               
               
                   
               
            
           
         
       
     
     As noted, the high specific heat material must have a CTE that is compatible with titanium. Since high pressure compressors operate at elevated temperatures, significant differences in a CTE between the high specific heat material forming the liner and the substrate introduce significant stresses between the applied high specific heat material and the substrate. This differential thermal expansion and the resulting stresses can result in the spalling of the high specific heat material from the substrate at the operational temperatures of the compressor. Spalling may not occur immediately, but cyclic operation of the compressor eventually may result in spalling. Titanium has a CTE of about 5.6 in./in./° F. IN909 has a CTE of about 4.2 in./in./° F. (10 −6 ). IN783 has a CTE of about 5.6-7.15 in./in./° F. (10 −6 ). M152 has a CTE of about 7.2 in./in./° F. (10 −6 ). 15-5PH has a CTE of about 6.5-7.2 in./in./° F. (10 −6 ). 17-4PH has a CTE of about 6.5-7.2 in./in./° F. (10 −6 ). These CTE&#39;s are compatible with titanium up to the maximum operating temperature of a stage 4 HPC compressor case, which may exceed 800° F., so that significant stresses are not developed between the wear coating layer and the component substrate. 
     While high specific heat material  10  may be applied over the titanium substrate  30 , the wear coatings selected from the group consisting of IN909, IN783, M152, 15-5PH and 17-4PH react with titanium forming low melting point eutectic phases. To prevent the formation of these detrimental low melting point eutectic phases, a diffusion barrier layer, depicted as layer  20  in  FIG. 1 , is applied between over titanium substrate  30  and under high specific heat material  10 . Diffusion barrier layer  20  between the titanium substrate  30  and the high specific heat material liner  10  must be compatible with both the titanium base material and the liner material so as not to interact with either, while also preventing interaction between the titanium alloy base material and the material comprising the high thermal conductivity liner. As used herein, “interact’ is defined as forming a low melting point eutectic phase of sufficient thickness that adversely affects mechanical properties and/or CTE of the overall structure. A coating or layer of material that does not “interact” is further defined as “non-reactive” as used herein. Diffusion barriers comprising niobium (Nb), vanadium (V), chromium (Cr) and combinations thereof applied over titanium substrate  30  have been found to be effective, although any other material that does not interact with the titanium alloy base material and the applied high specific heat material while also preventing significant interdiffusion and formation of detrimental phases between them may be used. These diffusion barrier layers may be applied by any convenient method, such as by plating, cold spray, thermal spray or additive manufacturing processing. At least one layer of diffusion barrier material is applied to a thickness sufficient to prevent interdiffusion and interaction. In one embodiment, the thickness may comprise about 2-10 mils (0.002-0.010 inches), while in another embodiment the thickness may be about 5 mils (0.005 inches). However, the functional limit is based on the thickness required to prevent interdiffusion and not arbitrary numerical limits on thickness, which may be set forth for as manufacturing tolerances. 
     The wear coating and the diffusion barrier layer may be applied over the titanium alloy substrate by any convenient deposition process. However, it is preferred that these layers be applied by methods that minimize melting and/or interaction between the titanium base material and the material deposited as layers over the titanium base material. 
     While any method may be used to apply the diffusion barrier layer and the high specific heat material, thermal spray treatments undesirably increase the chemical interaction between underlying material and the material over it. This additional melting results in mixing of chemical compositions along the interface between the underlying material and the applied layers. 
     Two preferred methods for applying the diffusion barrier layer to the titanium substrate and the wear coating over the diffusion barrier layer include cold spray and additive manufacturing. 
     One or both of the diffusion barrier layer and the high specific heat material layer may be applied by cold spraying. In cold spraying, particles of material, typically powders of preselected size are accelerated at very high velocities with a non-reactive gas toward a substrate. The high speed of the particles as they strike the target surface deform the particles and may slightly deform target surface, and the rapid deceleration of the particles as they strike the target surface heat both the surface and the particles. While both the target surface and the powder particles are heated as the powder particles collide with the surface, the powder particles are small compared to the target surface. The energy resulting from the collision is sufficient to substantially deform the powder particles, at least most of the particles, but not sufficient to substantially affect the target surface, which is a much greater mass. Extremely localized melting at the atomic layer interface between the highly deformed powder particles and the substrate may occur. As one skilled in the art will understand, the particle distribution of the powders is within a tightly controlled size distribution so that there is little difference among the velocities of the particles, which affects both the momentum and kinetic energy of the powder particles accelerated toward the substrate by the gas. Non-reactive gases include nitrogen and argon, although other inert gases may also be used. This technique can provide an interface between the substrate and the overlying layer that is very thin and in which there is minimal mixing of the material forming the substrate and the applied material. 
     One or both the diffusion barrier layer  20  and the high specific heat material layer  10  may be applied by additive manufacturing. In additive manufacturing, a thin layer of powder material is applied over an underlying layer in a predetermined pattern. A laser or other suitable heat source provides thermal energy which follows the applied material, melting the applied material. The applied thermal energy is carefully controlled to melt the applied powder material. Of course, the applied thermal energy can be controlled to melt the applied powder material and a portion of the substrate, which provides controlled mixing of the substrate and the applied powder material. The applied thermal energy can be controlled to melt substantially all of the powder material while minimally affecting the substrate so that there is minimal mixing between the substrate and the applied material. In additive manufacturing, the substrate and the applied material may be the same material or may be different materials. In this circumstance, since the applied material and the substrate are different materials and since it is preferred that there be minimal mixing between the substrate and the applied material, it is preferred that the applied thermal energy be adjusted so that the substrate is minimally affected while substantially melting the applied material. In additive manufacturing, additional layers can be applied to achieve the desired thickness. Once the interface layer has been formed, the parameters for applying the additional layers of material can be altered to speed deposition without affecting the interface layer. 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.