Turbine airfoil cooling system with leading edge impingement cooling system turbine blade investment casting using film hole protrusions for integral wall thickness control

A method of forming an airfoil (12), including: abutting end faces (72) of cantilevered film hole protrusions (64) extending from a ceramic core (50) against an inner surface (80) of a wax die (68) to hold the ceramic core in a fixed positional relationship with the wax die; casting an airfoil including a superalloy around the ceramic core; and machining film cooling holes (34) in the airfoil after the casting step to form an pattern of film cooling holes comprising the film cooling holes formed by the machining step and the cast film cooling holes (102) formed by the film hole protrusions during the casting step.

FIELD OF THE INVENTION

The invention relates to wall thickness control during investment casting of hollow parts having film cooling passages.

BACKGROUND OF THE INVENTION

Investment casting may be used to produce hollow parts having internal cooling passages. During the investment casting process, wax is injected into a wax cavity to form a wax pattern between a core and a wax die. The wax die is removed, and the core and wax pattern are dipped into the ceramic slurry to form a ceramic shell around the wax pattern. The wax pattern is thermally removed, leaving a mold cavity. Molten metal is cast between the ceramic core and the ceramic shell, which are then removed to reveal the finished part.

Any movement between the ceramic core and the wax die may result in a distorted wax pattern. Since the ceramic shell forms around the wax pattern, and the ceramic shell forms the mold cavity for the final part, this relative movement may result in an unacceptable part. Likewise, any movement between the ceramic core and the ceramic shell when casting the airfoil itself may result in an unacceptable part. Specifically, cooling channels formed into a wall of the finished part require that the wall, which is formed by the mold cavity, meet tight manufacturing tolerances. As gas turbine engine technology progresses, so does the need for more complex cooling schemes. These complex cooling schemes may produce passages that range in size from relatively small to relatively large, and hence manufacturing tolerances are becoming more prominent in the design of components.

The nature of the investment casting process, where two discrete parts must be held in a single positional relationship during handling and multiple casting operations, makes holding the tolerances difficult. In addition, the ceramic core itself is relatively long and thin when compared to the wax die and ceramic shell. As a result, when heated, the ceramic core may distort from its originally intended shape. Likewise, the ceramic core may not expand in all dimensions in exactly the same manner as the wax die and/or the ceramic shell. This relative movement may also change the mold cavity and render the final part unacceptable.

In order to overcome this relative shifting, U.S. Pat. No. 5,296,308 to Caccavale et al. describes a ceramic core having bumpers on the ceramic core that touch, or almost touch, the wax die during the wax pattern pour. This controls a gap between the ceramic core and the wax die, and likewise controls a gap between the ceramic core and the ceramic shell. Controlling the gap minimizes shifting between the ceramic core and the ceramic shell, and this improves control of the wall thickness of the airfoil. The bumpers are positioned at key stress regions to counteract distortions. The final part may have a hole where the bumpers were located, between an internal cooling passage and a surface of the airfoil, which allows cooling fluid to leak from the internal cooling passage.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have devised an innovative ceramic core that will enable wall thickness control without the unwanted cooling air leakage associated with the prior art. Specifically, the core disclosed herein forms the typical serpentine cooling passages in the conventional manner, but further includes film hole protrusions that extend from the conventional core. The film hole protrusions are configured to abut an inner surface of a wax die, and then an inner surface of a ceramic shell, in a manner that holds the ceramic core in a fixed positional relationship with the wax die and the ceramic shell. Each film hole protrusion will generate a respective hole in a subsequently cast airfoil. However, unlike the prior art, where the associated holes are minimized, or avoided altogether, to minimize cooling air leakage, the holes associated with the film hole protrusions disclosed here are instead sized and shaped to become film cooling holes, and positioned to be part, if not all, of a pattern of film cooling holes within a film cooling arrangement. By sizing, shaping, and positioning the film hole protrusions in this way there is no unwanted loss of cooling fluid. Instead, the resulting hole and associated cooling fluid flowing there through are innovatively used as part of a film cooling arrangement.

FIG. 1shows a blade10for a gas turbine engine (not shown) having an airfoil12with a base14, a tip16, a leading edge18, a trailing edge20, a pressure side22, and a suction side24. A film cooling arrangement30may have multiple groups32of film cooling holes34. Each group32may form its own pattern, such as a row36as is visible in this exemplary embodiment. Other patterns are envisioned, however, and are considered within the scope of this disclosure. Each of these film cooling holes34is configured to eject an individual stream of cooling fluid, such as air. The individual streams unite with each other and flow along a surface38of the airfoil, between hot gases and the airfoil surface38, thereby protecting the airfoil surface38from the hot gases. An outlet40of the film cooling hole34may be shaped to enhance the surface coverage. The shape may include that of a diffuser, which slows down the air escaping from the film cooling hole34. In one exemplary embodiment the shape may take the 10-10-10 configuration known to those in the art.FIG. 2shows the suction side24of the airfoil12.

FIG. 3shows an exemplary embodiment of a core50, which may be made of ceramic. The core50includes a core base52, a core tip54, a core leading edge56, a core trailing edge58, and core passageway structures60, from a pressure side62of the core50. In the blade10the core passageway structures60form internal passageways (not shown) that carry cooling fluid through the component. Extending from the core passageway structures60are a plurality of film hole protrusions64. It can be seen that the plurality of film hole protrusions64are positioned to coincide with the film cooling holes34ofFIGS. 1 and 2. Specifically, the film hole protrusions64located at the core tip54are positioned so that they form film cooling holes34that become part of the pattern/row36disposed parallel to the tip16of the airfoil12ofFIG. 1. There are fewer film hole protrusions64located at the core tip54than there are film cooling holes34located at the tip16in this exemplary embodiment. In this exemplary embodiment, the remaining needed film cooling holes34at the tip16not formed by the film hole protrusions64would need to be formed through a secondary machining operation. In an alternate exemplary embodiment, there could be as many film hole protrusions64as needed to form all of the film cooling holes34in the row36at the tip16. Likewise, there could be fewer film hole protrusions64than there are film cooling holes34on the entire airfoil12, which would necessitate subsequent machining to create the remaining needed film cooling holes34, or there could be as many film hole protrusions64as there are film cooling holes34on the entire airfoil12. In the exemplary embodiment ofFIG. 3, locations for the film hole protrusions64are selected to coincide with both a desired location of a film cooling hole and a location that will help maintain a shape of the core50within the wax die.

FIG. 4shows a suction side66the core50ofFIG. 3, and more film hole protrusions64extending from the core passageway structures60. The film hole protrusions64can extend from any or all of the pressure side62, the suction side66, the core base52, and the core tip54; wherever a film cooling hole is needed. Likewise, the film hole protrusion64need not form a film cooling hole, but can instead form, for example, a shank impingement cooling hole. The film hole protrusions64can be located anywhere there exists an arrangement for cooling a surface of the blade10.

FIG. 5shows a close-up of a film hole protrusion64extending from the core passageway structures60and contacting a wax die68. Each film hole protrusion64is formed by a body70having an end face72that may be enlarged with respect to the body70. The body70and end face72may be shaped to form the film cooling hole34with the shaped outlet40. An exemplary shaped outlet40may include a 10-10-10 configuration as is known to those in the art.FIG. 6shows a close-up of a film hole protrusion64extending from one of the core passageway structures60near the base14of the airfoil, and a film hole protrusion64extending from approximately half way in between the base14and the tip16. However, any location may be selected if a film cooling hole34is to be formed there.

As can be seen inFIG. 8, the film hole protrusion64may extend from a surface74of the core50such that an axis76of elongation of the body70outside the core50forms an acute angle78with the core surface74. The result is that the body70extending from the core surface74of the core50is cantilevered with respect to the core surface74. Stated another way, the end face72is laterally offset along the core surface74with respect to where the body70meets the core50.

As can be seen inFIG. 8, the end face72rests on and flush with (i.e. conforms to) an inner surface80of the wax die68. Collectively, then, the end faces72define a profile that conforms to a profile defined by the inner surface80of the wax die68to effect a conforming fit between the two. By resting flush with the inner surface, no (or little) wax can get between the end face72and the inner surface80. This results in a clean cooling hole outlet40, devoid of a need to eliminate flashing from the casting process through subsequent machining.

During handling and casting operations the wax die imparts frictional and normal forces to the end face72. Due to the cantilevered nature of the arrangement, this creates a bending moment around where the body70and the core50meet. This cantilevered arrangement renders the body70less able to resist forces imparted to it by an inner surface80of the wax die. For this reason, care must be taken to prevent damage to the film hole protrusion64. This tradeoff is, however, considered acceptable in order to create film cooling holes34that are oriented to direct cooling fluid so they travel with the hot gases, or alternately, counter current with the hot gases.

In order to resist this bending moment, while still maintaining a positional relationship between the core50and the wax die68, (and subsequently between the core50and the ceramic shell), the body70and the core50must not only be strong enough resist breaking, but must also be configured to permit a desired amount of flex, and yet mitigate any unwanted flex. In an exemplary embodiment where some flex is permitted, the positional relationship maintained by the film hole projections64is essentially a single, fixed positional relationship with a permissible tolerance. In an exemplary embodiment, it may be preferable to reduce and/or eliminate any flex. In an exemplary embodiment where no flex is permitted, the positional relationship maintained by the film hole projections64is essentially a single, fixed positional relationship without a permissible tolerance.

It can also be seen that the body70may include a first geometry82(defining the axis76of elongation) and a second geometry84of a larger and/or increasing cross sectional area. The second geometry84may define a diffuser portion of the subsequently formed film cooling hole34. Thus, the film hole protrusion64, which is defined by the first geometry82and the second geometry84(i.e. the portions of the body70exterior to the core surface74), may actually increase in cross sectional area the further it gets from the core surface74. In addition,FIG. 8shows an alternate exemplary embodiment where the body70includes a third geometry86that extends into the core10. This third geometry86may be present when the body70is a discrete component and is inserted into the core10, such as when the core50is a green body. In such an exemplary embodiment the body70may be quartz, or a sintered or unsintered (green body) powder metallurgy structure. The core50may be sintered with the body70installed in the desired position to form a sintered core10with film hole protrusions64extending there from.

Alternately, the body70with the third geometry86may be joined to a completed core by, for example, inserting the third geometry86into recesses and bonding the body70to the core50. This bonding may be accomplished by means known to those in the art, such as by using adhesives, or soldering, brazing, or welding etc. For example, a quartz body70may be inserted to a recess in the pressure side62and/or the suction side66. If discrete bodies70are assembled into the core, the discrete bodies70may optionally be configured to form a cooling hole34that is different than other cooling holes machined into the casting. For example, the discrete bodies70may be larger to ease handling/assembly. The relatively larger film cooling hole resulting from the enlarged discrete bodies70may simply be larger than the other machined cooling holes, or alternately, they may serve an additional function, such as being sized to permit dust to be ejected from the internal cooling passage of the component.

WhileFIG. 8shows a cross section of the film hole protrusion64extending from the core surface74on the pressure side62of the core50, another or plural other film hole protrusions64may extend from the suction side66of the core50. In such an arrangement the core50would then be held in a fixed positional relationship with the wax die68. This would define a gap90between the core50and the wax die68, and the gap90ultimately defines the wall thickness of the airfoil12. The film hole protrusions64are of sufficient strength that they can withstand forces generated by the core50when the core50attempts to change its shape due to thermal stress. Thus, the shape of the core50is maintained and held in its proper position relative to the wax die68. This means that the respective dimensions of the gap90are maintained all around the core50, and this maintains dimensional control of a wax pattern cavity92. Since the gap90defines the wall thickness of the airfoil12, better dimensional control of the wall thickness is maintained using this configuration.

FIGS. 9-14continue to depict the investment casting process using the structure disclosed herein. InFIG. 9, wax has been introduced into the wax pattern cavity92and a wax pattern94has been formed between the core50and the wax die68. The film hole protrusion64holds the single, positional relationship between the core50and the wax die68during the casting of the wax pattern94. InFIG. 10the wax die68has been removed, leaving the core50and the surrounding wax pattern94. Any wax that may have found its way on the end face72may be removed in this step, to ensure good contact between the end face72and the ceramic shell. InFIG. 11the core50and wax pattern94have been dipped in a ceramic slurry to form the ceramic shell96. The end face72is exposed to the ceramic slurry and thus interfaces with the ceramic shell96, thereby forming a structure that bridges the core50and the ceramic shell96. In an exemplary embodiment the ceramic shell96bonds to the end face72, thereby forming a monolithic core50and ceramic shell96arrangement. In this configuration where the two are bonded to each other, not only is the gap90maintained, but lateral movement of the end face72along the inner surface80of the ceramic shell96is also prevented. This prevents the core50from moving relative to the inner surface80, such as up or down inFIG. 11, and thereby maintains an even tighter positional relationship there between.

InFIG. 12the wax pattern94has been removed from between the core50and the ceramic shell96. This can be done thermally, or via any means known to those in the art. This leaves the core50, the ceramic shell96, and a mold cavity98defined there between, where the mold cavity98is bridged by the film hole protrusions64. By bridging this mold cavity98, the film hole protrusions64continue to hold the core50in the single, positional relationship with the ceramic shell96. InFIG. 13molten metal has been cast into the mold cavity98and around the film hole protrusion64. Once solidified, this forms the wall100of the airfoil12. The film hole protrusions64again hold the core50and the ceramic shell96in the fixed positional relationship, despite thermal and mechanical stresses that may occur when the relatively hot molten metal is poured, (or injected forcibly), into the mold cavity98.

InFIG. 14the core50and the ceramic shell96have been removed through chemical leaching or any other technique known to those in the art. What remains is the cast blade10having the cast airfoil12with the wall100having a cast film cooling hole102with a shaped outlet40where the film hole protrusion64was previously located. The cast film cooling hole102shown in this exemplary embodiment includes a diffuser104where the second geometry84of the body70was disposed. The cast film cooling hole102or holes formed by this casting process may constitute only a portion of the film cooling holes34needed to form the pattern (i.e. a row) of film cooling holes34that may be part of a greater film cooling hole arrangement30. A remainder of film cooling holes34needed to complete the desired pattern may be machined after the casting operation. Stated another way, the pattern of film cooling holes34in the airfoil12may include one or more cast film cooling holes102as well as film cooling holes that are machined into the airfoil12subsequent to the casting operation. For this to happen, the locations selected for the film hole protrusions64must be such that at least two goals are achieved. First, the fixed positional relationship must be maintained. Second, the cast film cooling holes102resulting from the presence of the film hole protrusions64are to be positioned such that they naturally become part of a pre-planned pattern of film cooling holes.

One advantage of forming the pattern using a combination of cast cooling holes and subsequently machined cooling holes is that more than one pattern and associated film cooling arrangement30can be fabricated from a single casting configuration. For example, should it be determined that the subsequently machined cooling holes should have a decreased or increased diameter, that change can be accommodated using the same core50. Increased cooling may be desired when, for example, a given gas turbine engine is upgraded to operate at a higher temperature to increase efficiency. In this instance, the blade remains the same, but more cooling is necessary. The greater cooling needed with the finished upgraded blades can be accomplished by machining different, or more, film cooling holes in the same casting that can be used to make finished blades for the engine before it was upgraded. Further, should it be determined that fewer machined film cooling holes are necessary, the unwanted holes would simply not be drilled. Consequently, the arrangement and method disclosed herein provide increased flexibility.

From the foregoing it can be seen that the inventors have devised a unique and innovative positioning arrangement that improves dimensional control of the mold cavity while not creating a structure that leaks air from the cooling passage of the resulting airfoil. The result is improved dimensional control of the wall thickness of the airfoil, and less subsequent machining needed to form film cooling holes. Consequently, this represents an improvement in the art.