Patent Publication Number: US-2023140657-A1

Title: Method for making turbine engine components using metal injection molding

Description:
BACKGROUND 
     Metal casting involves pouring molten metal or alloy into a mold, and allowing the poured molten material to cool and solidify into a part shaped by the mold. The part may be retrieved from the mold, for example, by breaking or disassembling the mold. Turbine engine parts such as, for example, turbine shroud segments, have complex shapes and can be manufactured using traditional manufacturing methods such as sand casting, forging, and the like. 
     In a metal injection molding (MIM) process, a metal powder feedstock is injected into a mold to form a part, and, as in traditional casting methods, the mold is disassembled to retrieve the molded part. MIM has been used to create some turbine engine parts having relatively simple geometries with near-net shape at high volumes. By reducing the investment in forging and casting processing steps such as investment mold building, post processing, and the like, and requiring only minimal machining, and in some cases MIM can significantly reduce costs compared to traditional manufacturing methods. 
     Mold design for MIM requires that the mold be openable after injection without causing damage to the molded part. Making turbine engine parts with undercut regions, which include overhangs and complex curvatures, with MIM can result in die lock, where the mold used in the MIM process is difficult or impossible to open after feedstock injection without causing damage to the molded part released from the mold. As a result, current MIM design practice limits geometries to simple components, and makes more complex parts with undercut regions difficult to mold without damaging the part or requiring undesirable additional machining steps. In some cases, to make parts with undercut regions, mold designs require creating multiple slides that come together to create the complex shapes of the undercuts and overhangs adjacent to the undercuts, but such design requirements add costs to the tooling and injection process, and reduce the desirability of MIM. 
     SUMMARY 
     To address problems with die lock and allow more complex turbine engine part geometries to be made using metal injection molding (MIM), molding techniques are needed to allow the mold to be opened after injection without causing an undesirable amount of damage to complex features of the molded part. In general, the present disclosure is directed to an insert placed in a mold for use in a metal injection molding (MIM) process to form a turbine engine part with a complex feature such as, for example, an undercut region. During the MIM process, the insert is placed in the mold such that the feedstock injected into the mold forms the undercut region about at least a portion of an exterior surface of the insert. The insert shapes and supports the undercut and any overhangs adjacent to the undercut during the injection process, and maintains the shape of the undercut region during mold removal and in post-molding processing steps. Positioning the insert in the mold can solve complex issues with die lock and allow the mold to be opened after injection without causing undue damage to the turbine engine part or requiring additional machining steps. 
     In various examples, suitable inserts include removable cores, telescoping or collapsible tools in selected cavities of the mold, and the like. 
     In one aspect, the present disclosure is directed to a method for manufacturing a turbine shroud segment with at least one undercut region. The method includes forming a removable insert including an external surface corresponding to at least a portion of a wall of the undercut region in the turbine shroud segment; placing the removable insert in a mold including a mold cavity corresponding to a shape of the turbine shroud segment; injecting a metal injection molding (MIM) feedstock into the mold cavity and around the removable insert to form a shroud green body with the at least one undercut region; and sintering the shroud green body to form the shroud body. 
     In another aspect, the present disclosure is directed to a method for manufacturing a turbine shroud segment with an undercut region including an overhang and at least one arcuate wall portion. The method includes: providing a mold with a mold cavity corresponding to a shape of the turbine shroud segment, wherein the mold cavity includes an adjustable tool with an external surface corresponding to a shape of the at least one arcuate wall portion in the undercut region; adjusting the telescoping tool to provide a desired configuration of the arcuate wall portion; injecting with a metal injection molding (MIM) process a metal powder mixture into the mold cavity and around the adjustable tool to form a shroud green body having the undercut region; and sintering the shroud green body to form the shroud body. 
     In another aspect, the present disclosure is directed to a method for manufacturing a turbine shroud body with at least one undercut region with an overhang and an arcuate wall beneath the overhang. The method includes forming a sacrificial insert including a body of a soluble material chosen from polymers, waxes, and metal alloys, wherein the body of the sacrificial insert has an external surface corresponding to at least a portion of a wall of the undercut region; placing the sacrificial insert in a mold including a mold cavity corresponding to a shape of the turbine shroud segment; injecting with a metal injection molding (MIM) process a base metal powder mixture into the mold cavity and around the sacrificial insert to form a shroud green body with the undercut region; and sintering the shroud green body to form the shroud body and at least partially dissolve the sacrificial insert. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1 A  is a plan view of an example of a metal injection molded (MIM) turbine shroud segment. 
         FIG.  1 B  is schematic, cross-sectional view of the turbine shroud segment of  FIG.  1 A . 
         FIG.  2    is a schematic cross-sectional view of a mold suitable for making the turbine shroud segment of  FIGS.  1 A- 1 B  using a metal injection molding (MIM) process, and including removable mold inserts. 
         FIG.  3    is a schematic cross-sectional view of an embodiment of a mechanical mold insert suitable for use with the mold of  FIG.  2    in a MIM process. 
         FIG.  4    is a flow chart of an embodiment of the method of the present disclosure for making a MIM part using a removable mold insert. 
     
    
    
     Like symbols in the drawings indicate like elements. 
     DETAILED DESCRIPTION 
     Referring to  FIGS.  1 A- 1 B , an example of a turbine engine component that can be made by the MIM processes described herein, in this case a turbine shroud segment  10 , includes axially spaced-apart forward and aft hooks  12  and  14  extending radially outwardly from an arcuate platform  16 . The platform  16  has an opposite radially inner hot gas flow surface  20  adapted to be disposed adjacent to the tip of the turbine blades. Internal cooling passages  22  are defined in the platform  16 . The internal cooling scheme shown in  FIG.  1    is for illustration purposes only, and the shape and dimensions of the shroud segment  10  can vary widely depending on the intended turbine engine application. 
     The turbine shroud segment  10  further includes a first undercut region  24  and a second undercut region  26 . The undercut regions  24 ,  26  each include a respective wall  28 ,  30 , each which may include at least a portion  32 ,  34  with an arcuate shape. The undercut regions  24 ,  26  further include respective adjacent overhangs  36 ,  38 , which protrude in a direction generally normal to the respective undercut walls  28 ,  30 . 
     In other examples, suitable turbine engine components that can be made using MIM processes of the present disclosure can include turbine blades, compressor vanes, low pressure (LP) turbine blade tracks, compressor blade tracks, and the like. 
     As noted above, the turbine shroud segment  10  may be formed in a mold by a metal injection molding (MIM) process. Referring now to the schematic depiction in  FIG.  2   , a MIM mold  100  includes a mold cavity  102  corresponding to the shape of the turbine shroud segment  10  of  FIGS.  1 A- 1 B  (cooling passages not shown in  FIG.  2    for clarity), and is shaped to form the undercut regions  24 ,  26  with walls  28 ,  30  and overhangs  36 ,  38 . The mold cavity  102  typically is slightly larger than that of the desired finished part to account for the shrinkage that occurs during subsequent processing steps such as, for example, de-binding and sintering. 
     However, in some cases the walls  28 ,  30  and the overhangs  36 ,  38  in the undercut regions  24 ,  26  may be difficult to reproducibly form with MIM, and may be damaged as the green part is removed from the mold following the molding process and prior to sintering the part. The fragility of the walls  28 ,  30  and the overhangs  36 ,  38  make the mold  100  difficult to separate at a separation line  101  following the completion of the MIM process, and the resultant die lock can make MIM molding of complex parts difficult, or even impossible. 
     The present disclosure is directed to a method for molding a part with MIM in which at least one removable insert  150 ,  152  is positioned in the mold cavity  102  within the undercut regions  24 ,  26  to support the overhangs  36 ,  38  and form the walls  28 ,  30  during molding and sintering of a green part, as well as during subsequent separation of the mold  100  from the MIM molded part. In some examples, optional fasteners  154  such as pins, screws, adhesives and the like, may be used to support or removably attach the inserts  150 ,  152  in the mold cavity  102 . 
     In some examples, the removable inserts  150 ,  152  include a solid body or a hollow body having an exterior surface  156 ,  158 . The exterior surfaces  156 ,  158  are shaped such that the MIM feedstock entering the mold cavity  102  collects about the removable inserts  150 ,  152  to form the walls  28 ,  30  of the finished part  10  ( FIGS.  1 A- 1 B ) and to prevent the sagging or unwanted collapse of the overhangs  36 ,  38 . In some examples, the inserts  150 ,  152  may include an optional performance coating layer (not shown in  FIG.  2   ) such as, for example, a layer of a lubricant or a mold release composition, to ease insertion or removal of the inserts from the mold cavity  102 . In some examples, the body of the removable inserts  150 ,  152  may be formed from a first material, and a coating layer on the exterior of the body may include a second material different from the first material to provide, for example, increased resistance to abrasion, increased lubricity to assist flow of the MIM feedstock about the insert, and the like. 
     In some examples, the removable inserts  150 ,  152  are formed from a material having a melting temperature sufficient to remain chemically and physically stable at temperatures corresponding to the injection temperatures of the MIM feedstock material. In addition, the removably inserts  150 ,  152  should be readily removable from the mold cavity  102  prior to, during, or after the consolidation heat treatment cycle of the MIM part, which is referred to herein as sintering. 
     For example, in some embodiments the removable inserts  150 ,  152  can be made of a polymeric material that can be dissolved using an acid or base solution, an aqueous solution, water, an organic solvent, or combinations thereof, following molding but prior to the sintering process. In some examples, the inserts  150 ,  152  can be made of a material that dissolves and vaporizes when heat is applied to the mold  100  prior to or during the sintering process such as, for example, a polymeric material, a wax, or a low melting point metal such as a tin/bismuth based alloy. 
     In another embodiment, the removable inserts  150 ,  152  include a ceramic material. In some cases, the inserts  150 ,  152  are pre-sintered ceramic bodies placed in the mold cavity  102 , and may be removed following molding and prior to or after subsequent sintering steps. 
     After the inserts  150 ,  152  are properly positioned in the mold cavity  102  to form the undercut regions  24 ,  26 , the walls  28 ,  30  and the overhangs  36 ,  38  of the desired part to formed using MIM ( FIGS.  1 A- 1 B ), the assembly of the mold  100  is completed and the mold cavity  102  is filled be injecting a base metal powder mixture, otherwise known as a MIM feedstock through the port  104  in the direction of the arrow A. 
     The MIM feedstock generally includes a binder and a metal powder. A variety of binders may be used in the MIM feedstock including, but not limited to, waxes, polyolefins such as polyethylenes and polypropylenes, polystyrenes, polyvinyl chloride, polyacrylics, cyanoacrylates, polytetrafluoroethylene (PTFE) and other fluoropolymers, and mixtures and combinations thereof. The metal powder used in the MIM feedstock can be selected among a wide variety of metal powders, including, but not limited to, Ni, Ti, Cu, Al, steel, alloys thereof, and combinations thereof. A suitable mixture will provide sufficient fluidity to carry the feedstock from an injection port  104  through passages  106  to flow around the removable inserts  150 ,  152  and fill substantially all of the mold cavity  102 . 
     Once the MIM feedstock is injected into the mold  100 , it is allowed to solidify in the passages  106  of the mold cavity  102  to form a green compact part around the inserts  150 ,  152 . After the green compact part has cooled and solidified, the mold  100  is disassembled in the direction of the arrow B about the separation line  101 , and in some examples the green shroud segment with its embedded inserts  150 ,  152  can be removed from the mold  100 . In other examples, the green shroud segment and the removable inserts  150 ,  152  may remain in the mold cavity  102  following molding and during the sintering process. The term “green” is used herein to refer to the state of a formed body made of sinterable powder or particulate material that has not yet been heat treated to the sintered state. 
     Conditioning operations, including de-binding and sintering, are then performed on this green shroud segment to remove the binder material and to consolidate the molded metal shroud segment into a dense metal part having mechanical properties similar to the material in casted or wrought form. In some examples, at least some of the conditioning operation (e.g. sintering) are performed at high temperatures which are well beyond the melting point of the inserts  150 ,  152 , which can concurrently dissolve or vaporize the inserts  150 ,  152  during the heat treatment cycle of the MIM shroud segment without requiring any extra manufacturing operations. The use of a low melting point material insert  150 ,  152  such as a polymer, wax, or low melting point metal alloy in combination with a MIM process can in some cases eliminate the need for a separate insert removal operation. The melting temperature of most polymeric materials are well below the sintering temperatures of metal powders, and plastic inserts and the like may be completely dissolved/vaporized without performing any dedicated insert removal operations. The sintering temperature of various metal powders is well-known in the art and can be easily determined by an artisan familiar with powder metallurgy. 
     Next, the resulting sintered shroud segment body may be subjected to any appropriate metal conditioning or finishing treatments, such as grinding and/or coating to obtain the final product shown in  FIGS.  1 A- 1 B . 
     In another embodiment shown schematically in  FIG.  3   , a removable insert  250  is formed from a mechanical assembly including a first arm  260  for attachment of the insert  250  to the mold, and a second arm  262  including an external surface  263  shaped to form the undercut regions of a part and support an overhang in the undercut regions. In some embodiments, the first arm  260  and the second arm  262  may be joined with fasteners  264 , or may be joined by a threaded rod  270  such that the insert  250  is adjustable for use in forming undercut regions of various shapes and sizes. In some examples (not shown in  FIG.  3   ), the second arm  262  may be formed from a plurality of telescoping segments to provide additional adjustability. 
     In another example, the removable insert  250  can be a pneumatic or hydraulic device that can expand a cylinder to engage a segment or a bladder formed in a suitable shape. In another example, the mechanical assembly could be active through a gear, sprocket or lever such as, a toggle a clamp or a press. 
       FIG.  4    is directed to a method  300  for manufacturing a turbine shroud body with at least one undercut region. The method includes at step  302  forming a removable insert comprising an external surface corresponding to at least a portion of a wall of the undercut region in the turbine shroud segment. In step  304 , the removable insert is placed in a mold including a mold cavity corresponding to a shape of the turbine shroud segment. In step  306 , a metal injection molding (MIM) feedstock is injected into the mold cavity and around the removable insert to form a shroud green body with the at least one undercut region. In step  308 , the shroud green body is sintered to form the shroud body. 
     The above described shroud manufacturing method has several advantages including design flexibility, simplified production process, manufacturing lead-time reduction, production cost savings, no need for hazardous materials to dissolve casting ceramic cores, and the like. Polymeric materials, waxes and low melting point metals can be readily shaped and can be less fragile than ceramic materials, and have fewer design limitations in term of shape and size when compared to ceramics. More complex shroud shapes can thus be realized using MIM processes. 
     Various embodiments of the invention have been described. These and other embodiments are within the scope of the following claims.