Patent Description:
When manufacturing complex parts using powder injection molding, the volume reduction of the part occurring during sintering is often anisotropic, i.e. the deformations caused by the volume reduction are often more important in one direction when compared to the other. Accordingly, the shape of the part may change during sintering, which may prevent parts from being manufactured within tight dimensional tolerances.

Moreover, some gas turbine engine panel components, such as for example some combustor heat shield panels, may have features which are relatively hard to mold. Such features may include, for example, a curved shape, angled retention elements and/or angled cooling holes. Accordingly, manufacturing these components using a molding process while being able to easily remove the component from the mold cavity may require the use of a mold with a complex configuration, which may render the use of a molding process such as powder injection molding to manufacture these components undesirable and/or impractical.

<CIT> and <CIT> disclose methods of metal injection moulding a part including producing individual components of the part as separately moulding green sections which are then debindered to form brown sections. <CIT> discloses a method comprising preparing a first component part by metal injection moulding and injecting a second component part to the first part to form a multicomponent part. <CIT> discloses a double-layer metal sheet and method of fabricating the same.

The present invention provides a method of forming a component from a part in a green state through a powder injection molding process as set forth in claim <NUM>.

<FIG> illustrates a gas turbine engine <NUM> of a type preferably provided for use in subsonic flight, generally comprising in serial flow communication a fan <NUM> through which ambient air is propelled, a compressor section <NUM> for pressurizing the air, a combustor <NUM> in which the compressed air is mixed with fuel and ignited for generating an annular stream of hot combustion gases, and a turbine section <NUM> for extracting energy from the combustion gases.

The combustor <NUM> is housed in a plenum <NUM> supplied with compressed air from compressor section <NUM>. The combustor <NUM> typically comprises a combustor shell <NUM> defining a combustion chamber <NUM> and a plurality of fuel nozzles <NUM> for atomizing fuel, which are typically equally circumferentially distributed on the dome end panel of the combustor shell <NUM> in order to permit a substantially uniform temperature distribution in the combustion chamber <NUM> to be maintained. The combustor shell <NUM> is typically made out from sheet metal.

Annular rows of circumferentially segmented heat shield panels <NUM> are mounted to the inner surface of the combustor shell <NUM> to thermally shield the same. Each row of heat shield panels <NUM> may cover the full circumference of the combustor shell <NUM>. As shown in <FIG>, some of the heat shield panels <NUM> may be mounted to the dome panel of the combustor shell <NUM> and others to the axially projecting portions of the combustor shell <NUM>. Depending on the intended application, the heat shield panels <NUM> may fully cover the inner surface of the combustor shell <NUM> from the dome end to the opposed discharged end of the combustor. Alternatively, the heat shield panels may be only provided on specific portions, such as the dome end wall, of the combustor shell <NUM>.

The heat shield panels <NUM> have cold side surfaces or back surfaces which are spaced from the inner surface of the combustor shell <NUM> to define a back cooling space <NUM> such that cooling air may circulate therethrough to cool the heat shield panels <NUM>. Holes are typically defined in the combustor shell <NUM> to allow cooling air to flow from the plenum <NUM> to the back cooling space <NUM> between the heat shield panels <NUM> and the combustor shell <NUM>.

The turbine section <NUM> generally comprises one or more stages of rotor blades <NUM> extending radially outwardly from respective rotor disks, with the blade tips being disposed closely adjacent to a stationary annular turbine shroud <NUM> supported from the engine casing. In a particular embodiment, the turbine shroud <NUM> is segmented in the circumferential direction and accordingly includes a plurality of similar or identical circumferentially adjoining shroud segments <NUM> together defining the annular turbine shroud <NUM>. The turbine shroud <NUM> defines a portion of the radially outer boundary of the engine gas path. In a particular embodiment, each shroud segment <NUM> is individually supported and located within the engine <NUM> by an outer housing support structure (not shown).

There is described herein a method of forming a component from a green part, where different areas of the green part have different solid loadings (different proportions of binder/powder material). In a particular embodiment, such variations in solid loading results in a desired distribution of local deformations during the sintering process, for example to correct an anisotropic shrink otherwise obtained with a uniform solid loading, or to deform the part from an initial shape of the green part to a desired final shape of the component. Although exemplary components are provided herein as components of the gas turbine engine <NUM>, it is understood that the methods described also apply to other types of components.

The green part is made from one or more appropriate type(s) of feedstock, and obtained by powder or metal injection molding. The feedstock is a mixture of a material powder and of a binder which may include one or more binding material(s). Examples of possible powder materials include high temperature resistant powder metal alloys, such as a cobalt alloy or nickel-based superalloy, or ceramic, glass, carbide or composite powders or mixtures thereof, mixed with an appropriate binder. Other high temperature resistant material powders which may include one material or a mix of materials could be used as well. In a particular embodiment, the binder includes an organic material which is molten above room temperature (<NUM>) but solid or substantially solid at room temperature. The binder may include various components such as surfactants which are known to assist the injection of the feedstock into mold for production of the green part. In a particular embodiment, the binder includes a mixture of binding materials, for example including a lower melting temperature polymer, such as a polymer having a melting temperature below <NUM> (e.g. paraffin wax, polyethylene glycol, microcrystalline wax) and a higher melting temperature polymer or polymers, such as a polymer or polymers having a melting temperature above <NUM> (e.g. polypropylene, polyethylene, polystyrene, polyvinyl chloride). "Green state" or "green part" as discussed herein refers to a molded part produced by the solidified binder that holds the powder material together.

In a particular embodiment, the powder material is mixed with the molten binder and the suspension of injection powder and binder is injected into a mold cavity and cooled to a temperature below that of the melting point of the binder. Alternately, the feedstock is in particulate form and is injected into the mold cavity of the heated mold where the binder melts, and the mold is then cooled until the binder solidifies. Use of other processes to create the green part is also possible.

In a particular embodiment, the component(s) manufactured from green part(s) as discussed above include the heat shield panels <NUM>, an exemplary construction of which is shown in <FIG>. Each heat shield panel <NUM> has a platform section <NUM> having opposed cold and hot facing sides <NUM> and <NUM>, and cold side details extending from the cold facing side <NUM> of the platform section <NUM>. According to the illustrated embodiment, the cold side details include retaining elements in the form of threaded studs <NUM>. Alternate possible cold side details include, but are not limited to, various types of elongated features such as heat exchange promoting structures, rails, bosses, divider walls, ribs, pin fins, etc..

The threaded studs <NUM> are used to retain the heat shield panel <NUM> in place, and in a particular embodiment protrude through holes defined in the combustor shell <NUM> and are threadingly engaged to fasteners, such as for example self-locking nuts, from outside of the combustor shell <NUM>. Other types of retaining elements may alternately be used.

A thermal barrier coating, such as a ceramic coating (TBC), may be applied to the hot facing side <NUM> of the platform section <NUM>. Holes, such as effusion holes and dilution holes (not shown), may be defined through the platform section <NUM>. The effusion holes allow the cooling air to flow from the back cooling space <NUM> to the front or hot facing side <NUM> of the heat shield panels <NUM>.

In the embodiment shown, both the cold and hot facing sides <NUM> and <NUM> have a curved shape, more particularly an arcuate shape corresponding to an arcuate portion of a cylinder. Other types of curved shapes can also be used depending on the configuration of the surface to be protected by the heat shield panel <NUM>, including, but not limited to, a shape corresponding to part of a cone, a sphere, or a toroid.

In a particular embodiment, the heat shield panel <NUM> is obtained from a green part having an initial shape which is different from the final desired shape of the heat shield panel <NUM>. The initial shape is selected such as to be able to reach the final shape through deformation, and such as to be easier to mold than the final shape. For example, the initial shape corresponds to a mold cavity and mold configuration having a reduced degree of complexity with respect to that which would be required to mold the heat shield panel <NUM> directly in its final shape. In a particular embodiment and as shown in <FIG>, the green part <NUM>' defining the heat shield panel <NUM> is formed with its platform section <NUM> having a planar configuration and with the studs <NUM> extending perpendicularly with respect to the planar platform section <NUM>. The studs <NUM> of the green part <NUM>' thus each extend along a respective axis, with the axes being parallel to one another.

At least one portion of the green part <NUM>' is selected to undergo a different local volume reduction than another portion of the green part <NUM>' during the sintering process, in order to obtain the final shape of the component. In the embodiment shown, the curved final shape is achieved by having the platform section <NUM> of the green part <NUM>' composed of two superposed layers <NUM>, <NUM> each defining one of the cold and hot facing sides <NUM>, <NUM>, with the layer <NUM> defining the hot facing side <NUM> undergoing a greater volume reduction than the layer <NUM> defining the cold facing side <NUM>; the difference in volume reduction between the two connected layers <NUM>, <NUM> causes the platform section <NUM> to curve during the sintering process to reach the desired final shape, as will be further described below.

The green part <NUM>' is formed with the layer <NUM> on the cold facing side <NUM> having a first volume of powder material VP1 and a first volume of binder VB1, defining a solid loading of VP1/(VP1 + VB1), and a corresponding volumetric proportion of binder of VB1/(VP1 + VB1), and with the layer <NUM> of the hot facing side <NUM> having a second volume of powder material VP2 and a second volume of binder VB2, defining a solid loading defined as VP2/(VP2 + VB2), and a corresponding volumetric proportion of binder of VB2/(VP2 + VB2). In order to obtain the greater volume reduction in the layer <NUM> defining the hot facing side <NUM>, the solid loading of that layer <NUM> is selected to be smaller than the solid loading in the layer <NUM> defining the cold facing side <NUM>. In other words, the volumetric proportion of binder in the layer <NUM> defining the hot facing side <NUM> is larger than the volumetric proportion of binder in the layer <NUM> defining the cold facing side <NUM>.

In a particular embodiment (which is outside the scope of the claims), and with reference to <FIG>, the green part <NUM>' is entirely formed (e.g. molded) from a feedstock having the desired solid loading of the layer <NUM> defining the hot facing side <NUM>. The greater solid loading of the layer <NUM> defining the cold facing side <NUM> is obtained by locally heating that layer <NUM> to vaporize part of the binder therein, until the desired solid loading is reached. The layer <NUM> defining the cold facing side <NUM> is locally heated at a temperature equal to or higher than the vaporization temperature of the binder, but lower than the sintering temperature of the powder material to avoid sintering. In a particular embodiment the layer <NUM> is heated to a temperature equal to or above the vaporization temperature of one or more polymer(s) present in the binder but below the vaporization temperature of one or more other polymer(s) in the binder, such as to vaporize only some of the components of the binder. Accordingly, the higher vaporizing temperature polymer(s) in the binder remain present.

In the embodiment shown, the layer <NUM> is locally heated using a laser beam <NUM>, which may be defocussed and/or moved (e.g. wobbled) to limit the temperature increase in the layer <NUM>. Alternate local heating tools may be used, including, but not limited to, a heat gun, white light, any other appropriate type of radiation and/or method of heat transfer. The layer <NUM> defining the hot facing side <NUM> is supported on a setter <NUM> during the heating process. The setter <NUM> may be made of a material acting as a heat sink, and may include cooling passages for circulation of a coolant (e.g. water) therethrough.

In another embodiment, and with reference to <FIG>, the green part <NUM>' is formed (e.g. molded) by forming the two layers <NUM>, <NUM> from different feedstocks having the desired solid loadings. The layers <NUM>, <NUM> may be co-molded, or formed separately and assembled in the green state.

The green part forming the heat shield panel <NUM>' is then submitted to a debinding operation to remove most or all of the binder. The green part <NUM>' can be debound using various debinding solutions and/or heat treatments known in the art, to obtain a brown part <NUM>". Debinding may be done in shape-retaining conditions (e.g. with the part being supported in alumina powder) and accordingly in a particular embodiment, the part does not substantially deform during the debinding process.

Referring to <FIG>, the brown part <NUM>" forming the heat shield panel <NUM> is placed against a setter <NUM> and sintered. The surface of the hot facing side <NUM> is put into contact with a shaping surface <NUM> of the setter <NUM>; the surface of the hot facing side <NUM> has an initial shape which does not conform to the shaping surface <NUM>. The shaping surface <NUM> corresponds to the final shape which is desired for the surface of the hot facing side <NUM>. The smaller proportion of powder material in the layer <NUM> defining the hot facing side <NUM> creates a greater volumetric reduction is that layer <NUM> during the sintering process, when compared to the layer <NUM> defining the cold facing side <NUM>. The platform section <NUM> thus curves toward the setter <NUM> during the sintering process until the surface of the hot facing side <NUM> conforms to the shaping surface <NUM>, as shown in <FIG>.

The sintering operation can be done in air, an inert gas environment, a reducing atmosphere (H<NUM> for example), or a vacuum environment depending on the composition of material to be obtained. In a particular embodiment, sintering is followed by a heat treatment also defined by the requirements of the material of the finished part. In some cases, it may be followed with hot isostatic pressing (HIP). Coining may also be performed to further refine the profile of the part. It is understood that the parameters of the sintering operation can vary depending on the composition of the feedstock, on the method of debinding and on the configuration of the part.

From <FIG>, it can be seen that the deformation of the platform section <NUM> changes the relative orientation of the studs <NUM>; the angle between the respective stud axes changes as the platform section <NUM> deforms to reach the final desired angle α (i.e. the angle desired in the final shape of the part) once the surface of the hot facing side <NUM> conforms to the shaping surface <NUM>. Accordingly, the studs <NUM> may be molded as extending parallel to each other and have an angled orientation with respect to one another once the heat shield panel <NUM> reaches its final shape. In a particular embodiment, this allows for the use of mold cavities and mold structures having a simpler shape, which may be easier and/or less expensive to configure. In a particular embodiment, the platform section <NUM> is deformed to create an undercut or cavity not defined during the molding of the platform section <NUM>, thus avoiding the use of a sacrificial element or other insert in the mold.

Referring to <FIG>, in another particular embodiment, the component(s) manufactured from green part(s) as discussed above include the turbine shroud segments <NUM> forming part of the turbine shroud <NUM>.

The shroud segment <NUM> includes an arcuate platform <NUM> having an inner gas path surface <NUM> which is adapted to be exposed to the hot combustion gases during engine operation and an opposed outer cold surface <NUM>. The platform <NUM> extends between circumferentially opposed ends <NUM> which mate with the circumferential end of the abutting shroud segments to form the shroud. Axially spaced-apart front and rear legs <NUM> extend radially outwardly from the outer surface <NUM> of the platform <NUM>. The legs <NUM> are each provided with a respective axially projecting hook or rail portion <NUM> for engagement with corresponding mounting flange projections of the surrounding support structure in the engine. When assembled in the engine, a shroud plenum <NUM> is defined between the legs <NUM> and between the outer surface <NUM> of the platform <NUM> and the support structure, for receiving cooling air from a cooling air source, for example bleed air from the compressor section <NUM>. It is understood that the retention elements formed by the legs <NUM> and hook portions <NUM> are shown as an example only and can be replaced by any other appropriate type of retention elements.

Although not shown, various recesses or slots may be defined in the shroud segment <NUM>, for example for receiving sealing members therein, including, but not limited to, radially extending slots in the legs <NUM>, in the side of the legs facing the plenum <NUM>, and/or in the circumferential ends <NUM> of the platform. Other features may be provided in the shroud segment <NUM>, including, but not limited to, cooling holes and passages, angular timing features, pockets, and platforms.

The shroud segment <NUM> is manufactured from a green part, obtained by powder injection molding, using similar materials and processes as those described above for the heat shield panel <NUM> and which accordingly will not be repeated herein. A variety of removable and/or sacrificial inserts can be used in the molding process to define the internal features such as cooling passages.

At least one portion of the green part is selected to undergo a different local volume reduction than another portion of the green part during the sintering process, in order to obtain a desired final shape. In a particular embodiment and with reference to <FIG>, the desired final shape (in solid lines) is compared with the shape obtained after sintering when the green part is produced with a uniform solid loading (in dotted lines). It can be seen that for this exemplary shroud segment <NUM>, the shrink or volume reduction occurring during the sintering process is anisotropic: the volume reduction is greater in the legs <NUM> than in the platform <NUM>, causing the part to deform away from the desired shape. The portion(s) of the shroud segment <NUM> which deviate(s) from the desired shape by more than a predetermined threshold are thus selected to undergo a different local volume reduction through a change in solid loading, in order to change the local deformation produced during sintering.

In the embodiment shown, the part of the retention elements formed by the distal part 60d of each leg <NUM> and the associated hook portion <NUM> is thus selected to undergo a smaller volume reduction than the remainder of the shroud segment, to reduce or minimize the shape distortion created during the sintering process. The green part is thus formed with the distal part 60d of each leg <NUM> and the hook portions <NUM> having a solid loading greater than the remainder of the shroud segment (platform <NUM> and proximal part 60p of each leg <NUM>). In other words, the volumetric proportion of binder in the distal part 60d of each leg <NUM> and in the hook portions <NUM> is smaller than the volumetric proportion of binder in the platform <NUM> and in the proximal part 60p of each leg <NUM>.

In a particular embodiment, the green part is formed (e.g. molded) by forming the distal part 60d of each leg <NUM> and the hook portions <NUM> with a different feedstock from that used to form the platform <NUM> and proximal part 60p of each leg <NUM>, the two feedstocks having the desired different solid loadings. The portions made from different feedstocks may be co-molded, or formed separately and assembled in the green state. Although the transition between the two portions is shown here as a plane, it is understood that the transition may have any other appropriate shape, including, but not limited to, dovetail, tongue-and-groove, mortise and tenon, dowel and pin, zig-zag, or any other shape where the two portions define one or more complementary engagement member(s) engaged with one another.

The green shroud segment is then submitted to debinding and sintering, as discussed above.

It is understood that the heat shield <NUM> and shroud segment <NUM> shown herein are exemplary embodiments and that the component may alternately be any other component manufactured from a powder injection molding process for creating an intermediary green part, i.e. a part including a solidified binder that holds a material powder together with the binder being removed before the part is in its final form. Examples of such components include, but are not limited to, vane segments, vane rings, heat shields and other combustor components, fuel nozzle portions, arcuate shroud plates, folded brackets having a configuration which may be otherwise obtained through sheet metal folding, and components other than gas turbine engine components. Examples of component features which may be produced as extending at an initial angle in the green state and which may reach an orientation defined by a different angle after sintering, as discussed above, include, but are not limited to, attachment members, pins, fins, rails, ribs, walls, bosses, bumps, dents, etc. extending from the component and/or cooling holes or other apertures defined therein or therethrough.

In any embodiment, the possible range of variations in solid loading between portions of the component is dictated by the minimum and maximum possible solid loading for the particular feedstock used: the solid loading in all portions must be sufficiently high for the green part to be able to maintain its shape, and sufficiently low for the feedstock to be shapable to create the green part, for example to be injectable in the case of powder injection molding. The determination of minimum and maximum possible solid loading values for a particular feedstock is within the knowledge of the person of ordinary skill and will not be detailed herein. The difference between the values of the solid loading in the portions of the component is at least <NUM>. In a particular embodiment, the difference is at most <NUM> or <NUM>. Other values may also be possible.

In any embodiment where the green part is formed (e.g. molded) from different feedstocks having different solid loadings, the two feedstocks can be made from the same materials, i.e. same powder material and same binder, or alternately, may include different materials, particularly different materials having similar requirements (temperature, time) for sintering.

Claim 1:
A method of forming a component (<NUM>; <NUM>) from a part (<NUM>') in a green state through a powder injection molding process, the method comprising:
forming the green part (<NUM>') having an initial planar shape including:
injection-molding at least one first portion of the green part (<NUM>') in the initial shape of the flat panel with a first feedstock; and
injection-molding at least one second portion of the green part (<NUM>') with a second feedstock to obtain the component, wherein:
the at least one first portion has a first volume of powder material VP1 and a first volume of binder VB1, the at least one first portion having a first solid loading defined as VP1/(VP1 + VB1); and
the at least one second portion has a second volume of powder material VP2 and a second volume of binder VB2, the at least one second portion having a second solid loading defined as VP2/(VP2 + VB2), the first solid loading greater than the second solid loading, wherein a difference between the values of the first and second solid loadings is at least <NUM>; and
curving the green part (<NUM>') having the initial planar shape by debinding and sintering the part (<NUM>') to obtain the component (<NUM>; <NUM>), wherein the curving includes obtaining a final desired shape of a curved panel after the sintering by having the first and second solid loadings producing a different local volume reduction in the at least one first portion with respect to the at least one second portion (<NUM>, 60d, <NUM>), wherein sintering the green part (<NUM>') is performed with a surface of the green part placed against a shaping surface (<NUM>) of a setter (<NUM>), the surface of the green part (<NUM>') not conforming to the shaping surface (<NUM>), the surface of the green part (<NUM>') defined by the at least one second portion, and the first solid loading being greater than the second solid loading causing the at least one first portion to undergo the smaller local volume reduction than the at least one second portion during sintering to deform the part to conform the surface to the shaping surface (<NUM>) of the setter (<NUM>).