Patent Publication Number: US-2011052441-A1

Title: Method and device for hot isostatic pressing of alloyed materials

Description:
FIELD OF THE INVENTION 
     The subject matter disclosed herein relates to a method and container for forming billets using hot isostatic pressing and, more particularly, to a method and container for preventing diffusion of metals between a high value powder alloy and the container used for hot isostatic pressing. 
     BACKGROUND OF THE INVENTION 
     Metallurgical techniques have been developed for the manufacture of a metal billet or other object from metal powders created in a predetermined particle size by e.g., microcasting or atomization. Usually highly alloyed with Ni (nickel), Cr (chromium), Co (cobalt), and Fe (iron), these powders are consolidated into a dense mass approaching 100 percent theoretical density. The resulting billets have a uniform composition and dense microstructure providing for the manufacture of components having improved toughness, strength, fracture resistance, and thermal expansion coefficients. Such improved properties can be particularly valuable in the fabrication of e.g., rotary components for a turbine where high temperatures and/or high stress conditions exist. 
     The consolidation of these metal powders into a dense mass typically occurs under high pressures and temperatures in a process referred to as hot isostatic pressing (HIP). Typically, the powders are placed into a container (sometimes referred to as a “can”) that has been sealed and its contents placed under a vacuum. The container is also subjected to an elevated temperature and pressurized on the outside using an inert gas such as e.g., argon to avoid chemical reaction. For example, temperatures as high as 480° C. to 1315° C. and pressures from 51 MPa to 310 MPa or even higher may be applied to process the metal powder. By pressurizing the container that is enclosing the powder, the selected fluid medium (e.g., an inert gas) applies pressure to the powder at all sides and in all directions. Under the extreme temperatures and pressures of the HIP process, the container is substantially deformed or crushed as the volume of the powder decreases during the HIP process and the container becomes joined to the surface of the billet created by the compacted powder. 
       FIGS. 1 and 2  provide an exemplary illustration of conventional containers in the HIP process.  FIG. 1  provides a schematic illustration of a portion of a container  101  before being subjected to the extreme temperature and pressure of the HIP process. Container  101  encloses the powder mixture  105  intended for compaction and provides a seal to prevent the ingress of the fluid used for pressurization e.g., argon during the HIP process. Before pressurization, the walls  110  between top  100  and bottom  135  are basically straight and/or without deformation. Top  100  and bottom  135  are also undeformed before the HIP process. Powder  105  rests within container  101  and is not joined thereto. 
       FIG. 2  illustrates the same portion of container  101  after being subject to the HIP process. The conditions of the HIP process have now converted the powder into a metal billet  106 . The change in density from powder to a solid metal has also resulted in a rather dramatic change in volume. As the volume decreased, container  101  also deformed with the change from powder  105  to billet  106 .  FIG. 2  illustrates that wall  110  has now taken on an e.g., an arcuate shape, and top  100  and bottom  135  may undergo deformations as well. 
     In addition to the visible changes that have occurred, certain microscopic events also occur during the HIP process. More specifically, during the several hours over which the HIP process occurs, unwanted diffusion effects are created. Elements will migrate from the container to the powder and from the powder to the container during the HIP process. For example, container  101  is conventionally manufactured from low carbon steel or authentic stainless steel such as 304SS. Fe and C (carbon) can diffuse from the container into the metal powder. Conversely, Cr and other elements in the powder can diffuse into the container. Additionally, an unwanted diffusion layer containing e.g., Cr, Ni, and Fe will develop between the container and the billet. Therefore, the cross-diffusion of components creates a region of undesired compositions near the surface of the billet and also represents loss of the substantially expensive, highly alloyed powder used to create the billet. 
     Unfortunately, depending upon the shape desired for billet  106  (or the shape of the ultimate component to be constructed from billet  106 ), the above-described diffusion effects for container  101  may require the removal of valuable material from its surface. Again, because of the substantial costs of the original powder, this loss is undesirable. Therefore, an improved device that provides for the reduction or elimination of such diffusion effects and the loss of high value powder materials during HIP treatment would be useful. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention provides an improved method and container for forming billets using hot isostatic pressing. The method and container prevent or control the diffusion of metals between a high value powder alloy and the container used for hot isostatic pressing. Objects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     In one exemplary embodiment, the present invention provides a container for compaction processing of a powder. The container includes a container top, a container bottom, and an outer wall located between and connecting the container top and the container bottom to define an interior for the receipt of the powder. A diffusion barrier is positioned along the container top, container bottom, and outer wall so as to separate the container from the powder during the compaction processing. 
     In another exemplary embodiment, a container for compaction processing of a powder is provided. The container includes a container top, a container bottom and an outer wall located between and connecting the container top and the container bottom to define an interior for the receipt of the powder. One or more of the container top, the container bottom, and the outer wall are constructed from the same alloy composition as the powder in order to prevent diffusion between the powder and the container or parts thereof. Alternatively, one or more of the container top, the container bottom, and the outer wall are constructed from an alloy similar to the powder that does not allow for a detrimental alloy phase to form in the can/billet interface during the HIP cycle. 
     In still another exemplary aspect of the present invention, a method for improving the use of material during hot isostatic pressing is provided. The method includes the step of providing a container for the receipt of a powder intended for hot isostatic pressing. The container includes a top, a bottom, and an outer wall connecting the top and the bottom to define an interior of the container. The method also includes positioning a diffusion barrier along the container so as to separate the powder from the container during the hot isostatic pressing. A powder is inserted into the interior of the container. The container is then submitted to hot isostatic pressing while preventing or minimizing the diffusion of elements between the container and the powder. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  is a schematic cross-section along one side of a container before subjection to a HIP process. 
         FIG. 2  is a schematic cross-section along one side of the container of  FIG. 1  after undergoing the pressure and temperature of the HIP process. 
         FIG. 3  is a schematic cross-section view along one side of an exemplary embodiment of a container according to the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     To provide advantageous improvements as described herein, the present invention provides an improved method and container for forming billets using hot isostatic pressing and, more particularly, to a method and container for preventing diffusion of metals between a high value powder alloy and the container used for hot isostatic pressing. For purposes of describing the invention, reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment, can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     An exemplary embodiment of a container  201  according to the present invention is shown in  FIG. 3 . For purposes of illustration, one side of the container  201  is shown in cross-section. Container  201  is illustrated in  FIG. 3  with a powder  205  in the interior and in a condition before undergoing the deformations of the HIP process. 
     Container  201  includes a container top  200 , container bottom  235 , and outer wall  210 . For this exemplary embodiment, container  201  may be constructed of conventional materials as previously mentioned e.g., an authentic stainless steel such as 304SS. As shown in  FIG. 3 , top  200 , bottom  235 , and outer wall  210  are constructed as a single piece. However, container  201  may include other constructions as well including constructions where top  200 , bottom  235 , and outer wall  210  are created as one or more separate components. 
     Container  201  also includes a diffusion barrier  220  separating the high value powder material  205  from the container top  200 , bottom  235 , and outer wall  210 . Diffusion barrier  220  operates to prevent diffusion and is positioned as a layer or inner liner on container  201  located between powder  205  and container  201 . Diffusion barrier  220  prevents or minimizes the migration of elements from powder  205  into container  201  or from container  201  into powder  205 . 
     Diffusion barrier  220  is constructed from one or more materials specifically selected to prevent the diffusion process. A variety of materials may be used depending upon the composition of powder  205 , container  201 , and the conditions of the HIP process. For example, the diffusion barrier  220  could be constructed from various metal nitrides, sulphides, carbides, carbon nitrides or metal oxides. Ceramic material may also be used. In certain applications, diffusion barrier  220  may be constructed from a metal alone such as e.g., tantalum, gold, silver, or copper. Other materials may be applied as well. Again, the objective of material selection for diffusion barrier  220  is to prevent or impede the diffusion of materials between container  201  and powder  205 . 
     A variety of techniques may be used to position diffusion barrier  220  along the inside of container  201 . Diffusion barrier  220  may, for example, be constructed of a metal foil that is placed along the inside of the container. The foil could be specifically constructed according to the geometry of container  201  or could be applied as overlapping sheets before placement of powder  205  into container  201 . Various plating techniques could also be used to deposit diffusion barrier  220  upon the interior of container  201 . For example, electroplating or electroless plating could be used to deposit the desired thickness of barrier material as a layer  220  upon container  201 . Chemical vapor deposition can also used to deposit materials of the desired thickness on container  201  to create diffusion barrier  220 . Ceramic coating could also be applied through a variety of techniques including e.g., plasma spraying. Using the teachings disclosed herein, one of skill in the art will understand that various other methods may also be used in order to apply diffusion barrier  220 . 
     In the conventional container of  FIG. 1  and the exemplary embodiment of the present invention shown in  FIG. 3 , a difference in composition between the material used in constructing the container and the alloy used for creating the powder mixture will provide a driving force for diffusion during the HIP process. In still another exemplary embodiment of the present invention, in order to prevent the unwanted cross-diffusion of components between the container and a high value powder material, a container for HIP processing can be constructed of the same alloy or a similar alloy as the high value powder material used to create the billet in the HIP process. By using a container and powder having the same or similar overall composition of alloy, the driving force causing diffusion during the HIP process is minimized or eliminated, and a diffusion barrier such as barrier  220  may be omitted. In addition, such a construction for the container could be used to eliminate a manufacturing step of removing the container from the surface of the billet. 
     While the present subject matter has been described in detail with respect to specific exemplary embodiments and methods thereof, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing may readily produce alterations to, variations of, and equivalents to such embodiments. Accordingly, the scope of the present disclosure is by way of example rather than by way of limitation, and the subject disclosure does not preclude inclusion of such modifications, variations and/or additions to the present subject matter as would be readily apparent to one of ordinary skill in the art.