Patent Description:
Components in the turbine section are typically formed of a superalloy and may include thermal barrier coatings to extend temperature capability and lifetime. Ceramic matrix composite ("CMC") materials are also being considered for turbine components. Among other attractive properties, CMCs have high temperature resistance. Despite this attribute, however, there are unique challenges to implementing CMCs.

<CIT> discloses a prior art mandrel as set forth in the preamble of claim <NUM>.

From one aspect, there is provided a method for forming multiple ceramic matrix composite (CMC) articles as recited in claim <NUM>.

There is also provided a mandrel for forming multiple ceramic matrix composite articles as recited in claim <NUM>.

In this disclosure, like reference numerals designate like elements where appropriate and reference numerals with the addition of one-hundred or multiples thereof designate modified elements that are understood to incorporate the same features and benefits of the corresponding elements.

Fabrication of CMC articles may begin with forming a ceramic fiber fabric into the geometry of the article to be produced. For hollow articles, ceramic fibers are braided or woven around a mandrel to form a fiber preform. The geometry of the mandrel is analogous to the geometry of the article such that the fiber preform takes the shape of the article. Subsequently, an interface coating is applied to the fiber preform, followed by densification with a ceramic matrix material to form the final or near final article. This fabrication process, however, can be time-consuming and wasteful of the ceramic fibers. For instance, there may be substantial set-up time of the machinery for each mandrel to apply the ceramic fibers. Moreover, there is a lead-in region to each mandrel where the braiding or weaving is started before the geometry of the article. The lead-in region is not part of the final article and is thus removed later on, thereby resulting ceramic fiber waste.

<FIG> illustrates a mandrel <NUM> for forming multiple ceramic matrix composite (CMC) articles. As will be appreciated from this disclosure, the mandrel <NUM> facilitates addressing one or more of the aforementioned challenges in CMC articles. The articles produced using the mandrel <NUM> are not particularly limited and may be turbine vanes, turbine blades, turbine blade outer air seals, or combustor pieces.

The mandrel <NUM> includes a continuous mandrel body <NUM> disposed along an axis A. The body <NUM> is continuous in that it is a one-piece structure that contains no mechanical joints. The continuity of the body <NUM> reduces the number of parts used in the process and may also facilitate enhancing the strength and rigidity of the mandrel <NUM> to resist loads on the mandrel <NUM> during processing. In this regard, the body <NUM> may be formed by <NUM>-D printing, machining, or other process from graphite material, polymer material, metallic material, or composites of these materials.

The body <NUM> defines a first end 22a and an opposed second end 22b. The body <NUM> is comprised of several functional sections. In order along the axis A from the first end 22a, the body <NUM> includes a pilot lead-in section <NUM>, a first article section <NUM>, a tapered transition section <NUM>, and a second article section <NUM>. The geometries of the articles will be formed on the article sections <NUM>/<NUM>, while the pilot lead-in section <NUM> and the transition section <NUM> serve to aid processing. In this example, two articles are to be produced via the two article sections <NUM>/<NUM>. However, it is to be appreciated that the mandrel <NUM> may include additional transition sections <NUM> and article sections <NUM>, as shown in <FIG>, for producing three or more articles.

As indicated above, the pilot lead-in section <NUM> serves to facilitate processing. For example, the process of braiding or weaving ceramic fibers around the mandrel <NUM> begins at the pilot lead-in section <NUM>. In this regard, the pilot lead-in section <NUM> is relatively narrow at the first end 22a and continuously flares from the first end 22a to the start of the first article section <NUM>. This gradual tapering facilitates initiation of the braiding or weaving at a relatively small cross-section of the mandrel <NUM> and then gradually transitions to the larger cross-section at the beginning of the first article section <NUM>.

The article sections <NUM>/<NUM> have geometries that are analogous to the articles that are to be produced, or portions thereof. In this example, the article sections <NUM>/<NUM> are airfoil-shaped and are identical to each other in geometry in order to produce identical articles, although the article sections <NUM>/<NUM> could alternatively have different geometries in order to produce different articles. The airfoil shape continuously flares from one end to the other. If the airfoil article being produced is a single cavity design, the airfoil-shape of the sections <NUM>/<NUM> will be of the entire or substantially entire airfoil geometry. If the airfoil article being produced is multi-cavity, the airfoil-shape will be a section of the entire airfoil geometry. For instance, the airfoil shape is analogous to the geometry of a leading cavity of the airfoil and thus has a geometry that is analogous to the leading edge and portions of the pressure and suction sides of the airfoil. In this case, the articles produced will be airfoil leading end cavity tubes. Alternatively, the airfoil shape may be analogous to the geometry of an intermediate cavity tube or a trailing cavity tube of the airfoil. Such tubes would then subsequently be assembled together with a skin wrap to produce a full airfoil. Alternatively, articles other than airfoils may be produced, in which case the article sections <NUM>/<NUM> will have a geometry that is analogous to the geometries of those articles.

Since the airfoil shapes flare, the opposed ends thereof are unequal in cross-sectional geometry. As a result, if the second article section <NUM> followed immediately after the first article section <NUM> (without section <NUM>), there would be a size mismatch such that the mandrel <NUM> would have a steep step, which may be difficult to braid or weave over and may be a location of weakness where the mandrel could incur damage during processing. In this regard, the mandrel <NUM> includes the tapered transition section <NUM> between the two article sections <NUM>/<NUM>. The taper of the transition section <NUM> provides a more gradual rate of change between the differently sized ends of the sections <NUM>/<NUM>, or different geometries of the sections <NUM>/<NUM> for articles of different geometry, thereby enabling continuous, smooth braiding/weaving and facilitating the elimination of weak points. For instance, in the example shown, the transition section <NUM> continuously tapers from the first article section <NUM> to the second article section <NUM>. In most cases, the transition section <NUM> will be relatively short, as the difference in the cross-sectional geometries between the opposed ends of the airfoil shape is relatively small. As an example, the axial length of the transition section <NUM> is from <NUM>% to <NUM>% of the length of each of the article sections <NUM>/<NUM>. In further examples, the sides of the transition section may have an angle of approximately <NUM> degrees to <NUM> degrees relative to the axis A.

For the braiding or weaving process, the mandrel <NUM> is placed into a machine, such as a braider, for applying the ceramic fibers over the mandrel <NUM> to produce a seamless fabric sleeve <NUM>. The fabric sleeve <NUM> is braided or woven onto the mandrel <NUM> beginning at the pilot lead-in section <NUM> and then progressively proceeds along the mandrel <NUM> to the first article section <NUM>, to the tapered transition section <NUM>, and then to the second article section <NUM>. This results in a continuous, seamless fabric sleeve <NUM> over the mandrel <NUM>, i.e. a fabric-covered mandrel <NUM>. The portions of the fabric sleeve <NUM> that are on the first article section <NUM> and the second article section <NUM> form, respectively, first and second article preforms 34a/34b.

The fabric-covered mandrel <NUM> is subsequently divided, such as by cutting, to separate the first article preform 34a from the second article preform 34b, shown in <FIG>. The portions of the fabric sleeve <NUM> that are over the pilot lead-in section <NUM> and the transition section <NUM> are not part of the final articles. Thus, the fabric-covered mandrel <NUM> is divided at the locations between the first article section <NUM> and the tapered transition section <NUM>, between the second article section <NUM> and the tapered transition section <NUM>, and between the pilot lead-in section <NUM> and the first article section <NUM>.

An interface coating is applied to the article preforms 34a/34b, and then the article preforms 34a/34b are densified with a ceramic matrix material to thereby form, respectively, first and second CMC articles. At some point after the dividing, such as after application of the interface coating and an initial stage of densification, or after densification, the mandrel <NUM> is removed. The densification process is not particularly limited, but may include chemical vapor infiltration, polymer impregnation and pyrolysis, melt infiltration, or combinations thereof. Example ceramic matrices are silicon-containing ceramic, such as but not limited to, a silicon carbide (SiC) matrix or a silicon nitride (Si3N4) matrix. Ceramic fibers are formed of bundles of filaments and may include, but are not limited to, silicon carbide (SiC) fibers or silicon nitride (Si3N4) fibers. The CMC may be, but is not limited to, a SiC/SiC ceramic matrix composite in which SiC fiber fabric is disposed within a SiC matrix. The architecture pattern of the fibers in the fabric may be, but is not limited to, triaxial braid or a harness satin weave.

<FIG> illustrates a modified example of the mandrel <NUM>. In this example, rather than tapering continuously as in transition section <NUM>, the tapered transition section <NUM> first flares from the first article section <NUM> to an intermediate axial location A1 along the tapered transition section <NUM>. The tapered transition section <NUM> then tapers from the intermediate axial location A1 to the second article section <NUM>. In this case, at least a portion of the fabric sleeve <NUM> over the transition region <NUM> forms a part of the final article. For instance, the fabric-covered mandrel <NUM> is divided at the location A1 to separate the preforms 134a/134b. The fabric in the transition section <NUM> may then be folded outwardly as shown in <FIG> to form what will be a platform on the final airfoil article.

<FIG> shows a pictorial representation of an example of the method for forming the CMC articles. After forming the preforms using the mandrel <NUM>, the interface coating is applied. The coating may be, but is not limited to, boron nitride or carbon, and may be applied by vapor infiltration, for example. The mandrel <NUM> is subsequently removed from the preforms. For instance, the mandrel (cut pieces) are mechanically withdrawn from the preforms. The preforms are then densified with the ceramic matrix material. The timing of these actions with regard to one another may be varied. For example, the dividing may be conducted either before or after the deposition of the interface coating or at an intermediate stage of the densification. The mandrel removal will most typically be after the dividing, as wider portions of the mandrel <NUM> cannot be withdrawn through narrower portion of the fabric sleeve <NUM>. However, if techniques other than mechanical withdrawal are used it may be possible to remove the mandrel <NUM> prior to the dividing.

Claim 1:
A method for forming multiple ceramic matrix composite (CMC) articles, the method comprising:
providing a continuous mandrel body (<NUM>) being a one-piece structure that contains no mechanical joints and disposed along an axis (A), the continuous mandrel body (<NUM>) defines, in order along the axis from a first end (22a) thereof, a pilot lead-in section (<NUM>), a first article section (<NUM>), a tapered transition section (<NUM>; <NUM>), and a second article section (<NUM>);
beginning at the pilot lead-in section (<NUM>), progressively forming a continuous fabric sleeve (<NUM>) on the continuous mandrel body (<NUM>) from the pilot lead-in section (<NUM>) to the first article section (<NUM>), to the tapered transition section (<NUM>; <NUM>), and then to the second article section (<NUM>) to thereby form a fabric-covered mandrel (<NUM>), wherein the portions of the continuous fabric sleeve (<NUM>) that are on the first article section (<NUM>) and the second article section (<NUM>) form, respectively, a first article preform (34a; 134a) and a second article preform (34b; 134b);
dividing the fabric-covered mandrel (<NUM>) to separate the first article preform (34a; 134a) from the second article preform (34b; 134b); and
densifying the first article preform (34a; 134a) and the second article preform (34b; 134b) with a ceramic matrix material to thereby form, respectively, a first CMC article and a second CMC article.