Abstract:
In various embodiments, powders with engineered particle-size distributions are slip or pressure casted to produce homogeneous parts without the need for additives such as flocculating or deflocculating agents.

Description:
RELATED APPLICATION 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/830,892, filed Jun. 4, 2013, the entire disclosure of which is hereby incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    In various embodiments, the present invention relates to slip casting with or without applied pressure, in particular to such slip or pressure casting of metal bodies. 
       BACKGROUND 
       [0003]    Powder metallurgy techniques have been utilized in the fabrication of various types of finished metallic parts. For example, a flexible bag may be filled with metal powder and placed in a mold that approximates the final pressed shape and dimensions of the part. The bag and mold are sealed, and cold isostatic pressing is utilized to form a pressed “green body” of the metal powder. The green body may then be sintered in order to increase its density and subsequently machined to its final desired dimensions. While such powder metallurgy techniques are useful for particular applications, their utility is limited when the final parts have more complicated shapes. The more complicated the shape, the more excess powder must be placed in the bag and mold in order to prevent cracking during pressing. Thus, the ratio of the weight of the sintered part to the new weight of the part in its final machined form may exceed 4:1, resulting in high machining, materials, and labor costs. Moreover, parts fabricated with such powder metallurgy techniques rarely achieve final sintered densities that exceed 94% of their theoretical densities. Such parts may also have disadvantageously large grain sizes, e.g., greater than 40 microns. 
         [0004]    Another powder metallurgy technique utilized for the production of complex shapes is slip casting, in which a “slip,” i.e., a suspension of fine metal powder in water, dispersing agents (for the stabilization of the powder against colloidal forces), one or more solvents (for control of slip viscosity and facilitating casting), and a binder for strengthening the cast shape are placed in a mold, the liquids are drawn away, and the resulting green body is sintered for densification and strength. Unfortunately, slip casting typically requires the use of a deflocculant or suspension aid to prevent the settling and agglomeration of the powder and to maintain a desirable slip viscosity. Such deflocculants include, e.g., alcohols, other organic liquids, or alginates (i.e., ammonium and sodium salts of alginic acids). The use of such additives increases the cost, complexity, environmental impact and materials-handling requirements for a slip-casting process. 
         [0005]    In view of the foregoing, there is a need for simplified powder metallurgy-based techniques for the fabrication of complex metal parts that do not utilize exotic additives and that enable high part densities and small final grain sizes. 
       SUMMARY 
       [0006]    In accordance with various embodiments of the present invention, metal parts (or “bodies”) are produced utilizing slip or pressure casting of metal powders having specific powder-size distributions that obviate the need for the use of added organic or inorganic suspension aids during casting. The metal powders are suspended in a liquid that includes or consists essentially of water, e.g., deionized (DI) water, and poured into a mold for casting. As mentioned above, the liquid does not include any flocculating or deflocculating additives. In addition, preferred embodiments utilize powders that consist essentially only of one or more metals and do not contain solid or powder agents such as binders or plasticizers. The mold is porous (and may include or consist essentially of, e.g., gypsum), and the liquid in which the powder is suspended is drawn into the mold via capillary action. The resulting green body is sintered to increase its density and final machining (if desired and/or necessary) is performed to shape the final part. The metal powder may include or consist essentially of, e.g., one or more refractory metals. For example, the metal powder may include or consist essentially of tungsten (W), tantalum (Ta), niobium (Nb), zirconium (Zr), molybdenum (Mo), and/or titanium (Ti). 
         [0007]    In general, the slip does not exhibit dilatant or thixotropic flow, for example, when the slip is being poured into the mold; rather, the viscosity of the slip is preferably approximately constant as a function of the shear rate (i.e., with changes in shear rate) applied to the slip. Moreover, in preferred embodiments of the invention the slip and/or mold is not agitated (e.g., stirred, shaken, rotated, and/or vibrated) to promote mixing or settling of the metal powder within the slip; rather, the engineered particle size distribution of the powder particles within the slip provides resistance to powder settling within the slip before and during casting (i.e., the slip is colloidally stable rather than, e.g., colloidally unstable or even metastable, in preferred embodiments) while also facilitating low-resistance pourability of the slip itself in the absence of agitation. Similarly, in preferred embodiments of the present invention no electrical voltage or current is applied to the mold or slip during the casting process to influence settling behavior. In addition, the green bodies and molded parts (before and after sintering) are generally not functionally or mechanically graded, i.e., the parts exhibit few if any gradients in composition, microstructure, mechanical properties, porosity, residual stress, grain size, particle size, etc. Instead, the mechanical and functional properties of bodies and parts fabricated in accordance with embodiments of the present invention (e.g., grain size of unsintered or sintered parts, and/or powder particle size of slips and/or green bodies) are substantially homogeneous in various (or even all) directions. Furthermore, embodiments of the invention feature slips that are substantially free of acidic or basic pH-modifying agents; such agents may have deleterious effects (e.g., corrosion and/or chemical reaction) on the metal powder particles utilized in preferred embodiments and are generally not necessary for engineering colloidal stability of slips described herein. 
         [0008]    As utilized herein, the terms “body,” “part,” and “article” refer to bulk three-dimensional objects (as opposed to mere grains of powder) with shapes as simple as a slab but also more complex shapes such as crucibles and other volumes having convexity and/or concavity. For complex shapes, the final shape (not considering any process-related shrinkage) is typically formed by casting (as opposed to bulk compression) and sintering, which may be followed by machining. 
         [0009]    In an aspect, embodiments of the invention feature a method of producing a shaped part. A powder is suspended in a liquid comprising or consisting essentially of water, thereby forming a slip. The powder has a particle-size distribution d10 between 0.15 micron and 0.5 micron, d50 between 0.6 micron and 1 micron, and d90 between 2.4 microns and 3 microns, where a particle-size distribution dX of Y denotes that X % of particles have a size less than Y. The slip is introduced into a mold having a shape approximately equal to a desired shape of the shaped part. The particle-size distribution of the powder (i) substantially prevents separation of the powder from the liquid by agglomeration and/or sedimentation and (ii) maintains a substantially homogeneous distribution of powder particles within the liquid. Thereafter, at least a portion of the liquid drains out of the slip to produce a green body comprising or consisting essentially of the powder, and the green body is sintered to produce the shaped part. 
         [0010]    Embodiments of the invention may include one or more of the following in any of a variety of different combinations. The particle-size distribution of the powder may be d10 of approximately 0.3 micron, d50 of approximately 0.8 micron, and d90 of approximately 2.7 microns. After sintering, the shaped part may have a density between approximately 95% and approximately 99% of theoretical density, or even between 97% and 99% of theoretical density. After sintering, the shaped part may have a grain size (e.g., average grain size) between approximately 10 microns and approximately 20 microns. The powder may include or consist essentially of one or more metals, e.g., one or more refractory metals. The powder may include or consist essentially of tungsten, tantalum, niobium, zirconium, molybdenum, and/or titanium. The green body may be sintered in hydrogen. The green body may be sintered at a temperature between approximately 3000° F. and approximately 5000° F. 
         [0011]    The powder may be produced by providing an initial powder having a particle-size distribution d10 of approximately 0.42 micron, d50 of approximately 1.8 micron, and d90 of approximately 3.8 microns, deagglomerating a portion of the initial powder, and blending the deagglomerated portion of the initial powder with a second portion of the initial powder. The portion of the initial powder may be deagglomerated by ball milling. 
         [0012]    The density of the green body may be between approximately 30% and approximately 40% of theoretical density. The liquid may consist essentially of deionized water. The mold may be porous. Substantially all of the liquid from the slip may drain into the mold to form the green body. Neither the slip nor the mold may be agitated (e.g., shaken, stirred, vibrated, and/or rotated) after the slip is introduced into the mold and before the green body is produced. A super-atmospheric pressure (which may be hydrostatic) may be applied to the mold and/or to the slip during and/or after introducing the slip into the mold. The powder may be substantially homogeneously distributed within the green body (i.e., there may be no noticeable gradients or bands in particle or grain size formed within the green body). The mold may contain substantially only the slip. The shaped part may be machined to a desired size and/or shape. 
         [0013]    In another aspect, embodiments of the invention feature a method of producing a shaped part. A powder is suspended in water without flocculating or deflocculating additives, thereby forming a slip. The slip is introduced into a mold having a shape approximately complementary to (i.e., enclosing a space for introduction of slip therewithin approximately equal to) a desired shape of the shaped part substantially without separation of the powder from the water (and/or settling or sedimentation of the powder within the water) thereduring. Thereafter, at least a portion of the liquid is allowed to drain out of the slip to produce a green body including or consisting essentially of the powder, and the green body is sintered to produce the shaped part. 
         [0014]    Embodiments of the invention may include one or more of the following in any of a variety of different combinations. The powder may have a particle-size distribution (i) substantially preventing separation of the powder from the water by at least one of agglomeration or sedimentation and (ii) maintaining a substantially homogeneous distribution of powder particles within the liquid. The particle-size distribution may be d10 between 0.15 micron and 0.5 micron, d50 between 0.6 micron and 1 micron, and d90 between 2.4 microns and 3 microns, a particle-size distribution dX of Y denoting that X % of particles have a size less than Y. The powder may include or consist essentially of one or more metals (e.g., refractory metals such as tungsten). A super-atmospheric pressure may be applied to the mold and/or the slip during and/or after introducing the slip into the mold. The mold may contain substantially only the slip. 
         [0015]    These and other objects, along with advantages and features of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and may exist in various combinations and permutations. As used herein, the terms “approximately” and “substantially” mean±10%, and in some embodiments, ±5%. The term “consists essentially of” means excluding other materials that contribute to function, unless otherwise defined herein. Nonetheless, such other materials may be present, collectively or individually, in trace amounts. As used herein, “consisting essentially of at least one metal” refers to a metal or a mixture of two or more metals but not compounds between a metal and a non-metallic element or chemical species such as oxygen or nitrogen (e.g., metal nitrides or metal oxides); such non-metallic elements or chemical species may be present, collectively or individually, in trace amounts, e.g., as impurities. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0017]      FIG. 1  is a schematic cross-section of a slip being introduced into a mold in accordance with various embodiments of the invention; 
           [0018]      FIG. 2  is a schematic cross-section of a green body within a mold in accordance with various embodiments of the invention; 
           [0019]      FIG. 3  is a schematic cross-section of an article after casting in accordance with various embodiments of the invention; 
           [0020]      FIG. 4  is a micrograph depicting the microstructure of a cast article in accordance with various embodiments of the invention; and 
           [0021]      FIGS. 5A-5C  are schematic cross-sections of fabrication of an article via pressure casting in accordance with various embodiments of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    In order to enable slip and pressure casting without the need for added suspension aids, the metal powder utilized in accordance with embodiments of the present invention has a particle-size distribution (PSD) that maintains an advantageous ratio between the powder sedimentation rate and the viscosity of the slip. In various embodiments, the metal powder has a PSD d10 between 0.15 micron and 0.5 micron, e.g., 0.3 micron, a PSD d50 between 0.6 micron and 1 micron, e.g., 0.8 micron, and a PSD d90 between 2.4 microns and 3 microns, e.g., 2.7 microns. (As known to the skilled practitioner, a PSD d10 value of X indicates that 10% of the powder particles have a size less than X.) Powders having desired PSDs may be prepared via, for example, deagglomeration and blending of commercially available powders. In various embodiments, the slip has a viscosity 0.7 and 1.3 Pa-s, and preferably between 0.9 and 1.1 Pa-s. In contrast, use of powders having conventional PSDs without suspension aids typically results in high sedimentation rates and low viscosities (e.g., below 0.5 Pa-s) unsuitable for slip casting. 
         [0023]    For example, in an embodiment, a tungsten powder having a small Fisher sub-sieve sizer (FSSS) particle size is identified. (As known to the skilled practitioner, FSSS particle size represents the average particle size as determined by air permeability, assuming perfectly spherical powder particles.) In an embodiment, the FSSS particle size of the starting powder is between 0.5 micron and 1 micron, e.g., 0.7 micron. For example, an initial powder may be HC70S tungsten powder having an FSSS particle size of 0.7 micron, available from H.C. Starck GmbH of Goslar, Germany. Such powder may have a PSD d10 of 0.42 micron, d50 of 1.8 micron, and d90 of 3.8 microns. All or a portion of the initial powder may be deagglomerated by, e.g., ball milling with tungsten carbide milling balls in order to reduce agglomeration of the powder or ultrasonification (i.e., application of ultrasound energy) for a time sufficient to reduce or substantially eliminate agglomeration of the powder. 
         [0024]    In an embodiment, a powder blend may be subsequently produced by blending an unmilled portion of the initial powder with a portion that has been deagglomerated in order to form a powder having the desired particle-size distribution. An exemplary powder blend may include, e.g., between 60% and 80% (e.g., 70%) by weight unmilled powder and between 20% and 40% (e.g., 30%) by weight deagglomerated powder. In an embodiment utilizing HC70S tungsten powder as the initial powder, the resulting powder blend may have a PSD d10 of 0.3 micron, d50 of 0.8 micron, and d90 of 2.7 microns. 
         [0025]    After the powder having the engineered particle-size distribution is produced, the powder is suspended in a liquid including or consisting essentially of water, e.g., DI water. The resulting slip preferably contains between 30% and 40% (e.g., approximately 35%) of the solid particles by volume. As shown in  FIG. 1 , the slip  100  is poured into a porous mold  110 , e.g., a mold including or consisting essentially of gypsum, resin, one or more polymeric materials (e.g., polystyrene), and/or plaster of paris, having the desired shape and dimensions for a pre-sintered part (i.e., the shape and dimensions that, after sintering, provide the part with substantially the final desired shape and dimensions). In various embodiments of the invention, external pressure is applied while filling the mold  110  with the slip  100 , as described in more detail below. For example, the slip  100  may be pumped into the mold  110  at a pressure exceeding atmospheric pressure. The density of the cast slip  100  may be, for example, between 30% and 40%, e.g., approximately 34%, as determined by gravimetric methods. The liquid suspending the powder is then absorbed into mold  110 , as shown in  FIG. 2 , resulting in a green body  200  shaped by the mold  110 . As shown in  FIG. 3 , the green body  200  is removed from the mold  110  and subsequently sintered for densification, producing a sintered part  300 . High-pressure air and/or vacuum may be applied via lines  530 ,  540  (either together or in sequence) in order to facilitate removal of the green body  200  from the mold  110 . In an exemplary embodiment, the green body  200  is sintered in a hydrogen ambient. The sintering may be performed at temperatures between approximately 3000° F. and approximately 5000° F., e.g., approximately 4000° F., for a time period between approximately 2 hours and approximately 7 hours, e.g., approximately 5 hours. 
         [0026]    After sintering, the part  300  may have a grain size smaller than approximately 30 microns, e.g., between approximately 10 microns and approximately 20 microns. The density of part  300  may be between approximately 95% and approximately 99% of its theoretical density, e.g., approximately 97%. The part may be utilized in its as-cast and as-sintered form, or may be machined into a desired shape, as, e.g., a crucible, a heat shield, a seamless tube, or other hollow or conical shape.  FIG. 4  is an optical micrograph of the microstructure of a part  300  fabricated from W powder in accordance with embodiments of the present invention. As shown in  FIG. 4 , the grain size of the part  300  ranges between approximately 10 microns and approximately 20 microns. In  FIG. 4 , the grains of part  300  have been revealed via etching with Murakami&#39;s etchant, known to those of skill in the art to be a mixture of potassium ferricyanide (K 3 Fe(CN) 6 ), potassium hydroxide (KOH), and water. 
         [0027]    Embodiments of the invention utilize pressure casting of powders having engineered PSDs to form metal parts.  FIG. 5A  depicts a pressure casting apparatus  500  that may be utilized in embodiments of the present invention. As shown, apparatus  500  features a mold  110  partially or substantially encased within a pressure jacket  510  that may include or consist essentially of one or more mechanically strong and rigid materials capable of resisting the pressures imparted upon the slip while preventing deformation or fracture of the mold  110 . As shown, the pressure jacket  510  (and the mold  110 ) may be composed of multiple different parts that may be separated (see  FIG. 5C ) to facilitate removal of the cast part from the mold  110 . Pressure jacket  510  of  FIGS. 5A-5C  is depicted as being composed of pressure-jacket portions  510 - 1 ,  510 - 2 . The slip  100  is introduced into the mold  110  via a slip feed line  520  through which the slip  100  may be pumped under applied pressure. The apparatus  500  also includes pressure lines  530 ,  540  for the introduction of, e.g., high-pressure air (or other gas, e.g., inert gas), to apply pressure to the slip  100  during casting. For example air of a first super-atmospheric pressure (i.e., having a pressure greater than atmospheric pressure) may be introduced into pressure line  530  to apply pressure to the mold  110  and the slip  100 , and either air of a second pressure less than the first super-atmospheric pressure or vacuum may be applied via the pressure line  540 . The applied pressure may be substantially hydrostatic pressure, and it may advantageously decrease the amount of time required for the casting process (due to, e.g., increased outflow of water from the slip  100  during casting) and/or increase the density (and/or improve other mechanical properties) of the resulting green body. Pressures of greater than approximately 10 bars, greater than approximately 20 bars, or even greater than approximately 40 bars may be applied during casting in accordance with various embodiments of the present invention. 
         [0028]      FIG. 5B  depicts apparatus  500  after the slip  100  has been introduced into the mold  110  via the slip feed line  520 . After introduction of the slip  100 , pressure is applied to the slip  100  within the mold  110  as detailed above in reference to  FIG. 5A , resulting in a green body  200  shaped by the mold  110 . As shown in  FIG. 5C , the pressure jacket  510  and/or mold  110  may be separated into multiple portions to facilitate removal of the green body  200  from the mold  110 . After pressure casting, the green body  200  may be sintered for densification, producing a sintered part  300 . In an exemplary embodiment, the green body  200  is sintered in a hydrogen ambient. The sintering may be performed at temperatures between approximately 3000° F. and approximately 5000° F., e.g., approximately 4000° F., for a time period between approximately 2 hours and approximately 7 hours, e.g., approximately 5 hours. 
         [0029]    The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.