WO2014012187A1 - Die compaction powder metallurgy - Google Patents

Abstract

There is provided a method of producing a green compact that completely encases an internal structure. The method includes: providing a deformable shell which defines a cavity; providing a metal powder suitable for powder metallurgy; in a rigid die, surrounding the deformable shell with the metal powder; and compacting the metal powder and the deformable shell to generate the green compact that completely encases the internal structure, where the internal structure is defined by the deformable shell.

Description

DIE COMPACTION POWDER METALLURGY

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority of U.S. Provisional Patent

Application No. 61/673,988 filed July 20, 2012, which is incorporated herein by reference in its entirety for all purposes.

FIELD

[0002] The present disclosure relates generally to manufacturing of metal composite components using die compaction powder metallurgy methods.

BACKGROUND

[0003] In many engineering applications components are subjected to various types of loads. Often the material choice is governed by which material exhibits the best overall performance. For example, concrete has tremendous compressive strength but poor tensile properties. In applications where beams experience bending, a portion of the concrete beam will exhibit very high resistance to failure (compressive areas) whereas it will fail rather quickly in tensile areas. To remedy this disparity in strength characteristics one of two options is available where (1) a material which has a more even compressive-to-tensile strength ratio could be chosen or (2) an internal reinforcement could be used to upgrade the tensile strength.

[0004] In the first option a metal could be used, which has much improved tensile strength but would render the beam overly heavy and costly. The second option is preferred and is in fact the driving force behind the widespread use of rebar in concrete structures. The rebar, whether it be metallic, polymeric, or composite-based, has very high tensile strengths and effectively carries the tensile load while the concrete carries the majority of the compressive load.

[0005] The above example is effectively a composite component with two distinct structures: the external concrete matrix and the internal rebar. Key aspects that make this type of composite structure both economically attractive and mechanically efficient are:

• The matrix completely surrounds, or encases, the internal rebar structure.

• The composite component is manufactured in one or two steps. • The internal structure has a measurable effect on the functional performance of the composite component.

[0006] There are many examples of composite components, for example: steel- belted tires, foam-core artificial bones, and fluid-filled foams. There are a few examples where a metal external structure (matrix) is used. Some examples of composites that have a metal external structure include: cast-in inserts and flux-cored welding/brazing rods.

[0007] In the case of cast-in inserts, internal structures are held in a casting mold while the external molten metal is poured around them, much the same as concrete rebar. For example, stainless steel bolts may be cast directly into a copper anchor. The bolts provide a high strength connection point while the copper anchor provides corrosion resistance. Another example of a cast-in insert is a hollow rear control arm for automobiles. The control arm is manufactured by Alcoa using state-of-the-art vacuum riserless

casting/pressure riserless casting (VRC-PRC) methods. The VRC-PRC method casts molten aluminum around a hollow preform, producing a composite component where the internal structure is a hollow preform and the exterior structure is the solidified molten aluminum.

[0008] The manufacture of metal composite components using one of the above- described techniques suffer at least one of the following economic challenges:

• Typically high-energy and multi-step processes resulting in high production costs;

• Low production cycle times; and

• Poor material utilization.

[0009] Another manufacturing technique that may be used to produce metal composite components is powder metallurgy (PM). One example of a composite component is a functionally graded material or compact. A functionally graded material or compact may be characterized by the variation in composition and structure gradually over volume, resulting in corresponding changes in the properties of the material.

[0010] Seyferth & Czubarow (1995) disclose a method for producing a functionally graded material in which ceramic or metal in powdered form were mixed with a binder and applied to a substrate in layers. The amount of binder in the applied mixture allowed for controlling of shrinkage during the sintering process. However, as no compaction is applied to the powder the degree of densification of the composite would be limited.

[0011] Belhadjamida et al. (2011) disclose a functionally graded compact formed by applying a layer of powder/binder mixture to a green compact. A "green compact" would be understood to refer to a metal powder part that has been pressed (compacted) into a desired shape. It has some porosity and has not yet been heat-treated (sintered). The powder/binder mixture is composed of a different material than the green compact thereby imparting different physical and mechanical properties to the exterior of the compact. This green compact with the applied layer is then sintered to bond the exterior layer and sinter the compact.

[0012] Similar to Belhadjamida et al., Watanabe et al. (1981) disclose a method of coating green PM compacts with an exterior shell. The exterior shell was applied to a mold using electrolysis, after which the whole composite a green compact is inserted into the formed shell. The shell and the compact are then sintered together.

[0013] McGee & Mikoda (1975) disclose a bi-material green compact formed by placing a divider longitudinally in a die and inserting different powders on either side of the divider. After the material was inserted, the divider was removed, and the powder was compacted using die compaction. The method is a simple and economical way to produce a graded material using die compaction; however, it doesn't allow for engineering densification distributions within the compact.

[0014] Although these processes produce functionally graded materials or compacts, they use multi-step processes and/or do not use die compaction.

[0015] Powder metallurgy has also been disclosed in the production of functionally graded materials or compacts that incorporate hollow metal spheres. Cellular materials can be classified into three types: closed cell metal foams, open cell metal foams and hollow sphere structures. Hollow sphere structures, can be classified into two categories: (1) partial hollow sphere structures, and (2) syntactic hollow sphere structures. Conventional metal foams (i.e. closed cell metal foams and open cell metal foams) have been excluded from this discussion as these materials are not produced using powder metallurgy manufacturing techniques.

[0016] Partial hollow sphere structures consist of hollow spheres bonded together with void space between the spheres. Syntactic hollow sphere structures are similar to partial structures, though a metal matrix surrounds the hollow spheres. Hollow sphere structures may be fabricated by placing hollow metal spheres in a die and sintering the form to bond the spheres. With partial structures, the void space is unfilled. In syntactic structures, a metal matrix is used to fill the void space. [0017] The benefit of hollow sphere structures over conventional metal foams is that hollow sphere structures exhibit better compressive strength due to a greater bulk material content while still retaining a relatively low density (Sanders & Gibson, 2003). Furthermore, hollow sphere structures are capable of absorbing large amounts of energy at low to moderate stress levels compared to wrought metals. Greater bulk material allows hollow sphere structures to carry more load then conventional metal foams (Neville & Rabiei, 2008). Currently, hollow sphere structures have a wide base of applications which include: light weight construction, energy absorption applications, explosion protection, sound absorption and various medical applications (Andersen et al., 2007).

[0018] Partial hollow sphere metal foams yield very low densities in the range of 20% of the solid density, however, they show poor compressive strengths due to the lack of bulk material to sustain load (Andersen et al., 2000; Rabiei & Vendra, 2009). Syntactic hollow sphere metal foams are similar partial hollow sphere metal foams in that the spaces between the hollow spheres are filled with a metal alloy. After sintering, the composite structure shows a higher compressive strength compared to hollow sphere metal foams, at the expense of higher relative densities approaching 40% (Neville & Rabiei, 2008).

[0019] Andersen et al. (2000) disclosed the production of partial hollow sphere metal foams from PM using 316L stainless steel spheres with a diameter between 0.5 mm and 3 mm. The 316L stainless steel (SS-316L) spheres were created by coating Styrofoam spheres with a mixture of water, binder and 316L powder. Spheres were then formed into cylinders with nominal diameters and heights each equaling about 24 mm. Debinding and sintering was performed in place.

[0020] The geometrical characteristics of the test samples were determined. Samples

SS-A-2.5 (2.0-3.0 mm diameter, mean wall thickness 250 μηι, 1.43 mg/cm³) and SS-A-0.75

(0.5-1.0 mm diameter, mean wall thickness 40 μηι, 1.05 mg/cm³) both yielded relative densities of 20.7% and 15.2%, respectively; however, the nominal compressive yield strength was only 9.0 MPa and 7.4 MPa, respectively; roughly 4% of sintered 316L powder

(Andersen et al., 2000). The naming convention used for describing the various samples is:

XX-YY-ZZ, where XX is the hollow sphere material, YY is the matrix material and ZZ is the average sphere diameter. For partial hollow sphere structures where no matrix material is used, A is used to represent air in the case of regular hollow sphere structures.

[0021] Neville & Rabiei (2008) disclose a syntactic hollow sphere structures made with spheres created using by Augustin & Hungerbach (2009) using a PM processes. Two materials were evaluated; a low carbon steel (LC) and 316L stainless steel; in addition to three sphere diameters: 3.7 mm, 2.0 mm and 1.4 mm. Three rectangular samples were created, each with a PM matrix that matched the sphere material. The samples were created by filling a die (51 x 51 x 89 mm) with spheres and after which vibration was applied. Next, the PM matrix was added and the die was vibrated further to achieve a high apparent density (Neville & Rabiei, 2008). The samples were finally sintered in place. The relative densities for samples LC-PM-3.7, LC-PM-1.4 and SS-PM-2.0 can be calculated as 38.9%, 32.4% and 37.5%. These relative densities are higher than those presented by Andersen et al. (2000) for partial hollow sphere structures.

[0022] Rabiei and O'Neill (2005) disclosed a method of producing syntactic hollow sphere structures using low carbon and stainless steel hollow spheres (both produced by PM methods) surrounded by aluminum 356 casting alloy (CAI). The production process involved placing the hollow spheres in a casting mold and encasing aluminum around the spheres. The relative densities for samples LC-CAI-3.7 and SS-CAI-3.7 were calculated to be 42 and 43%, respectively. This is slightly (5-10% relative density) higher than the relative densities presented by Neville et al. (2008) using a PM alloy rather than a casting as the matrix.

[0023] It is, therefore, desirable to provide a method for manufacturing a metal composite component that obviates or mitigates at least one disadvantage of previous methods.

SUMMARY

[0024] It is an object of the present disclosure to obviate or mitigate at least one disadvantage of previous methods for manufacturing metal composite components.

[0025] According to one aspect of the present disclosure, there is provided a method of producing a green compact that completely encases an internal structure. The method includes: providing a deformable shell which defines a cavity, the deformable shell being sufficiently strong to withstand compaction pressures; providing a metal powder suitable for powder metallurgy; in a rigid die, surrounding the deformable shell with the metal powder; and compacting the metal powder and the deformable shell to generate the green compact that completely encases the internal structure, the internal structure defined by the deformable shell. [0026] The deformable shell may have a shell strength of at least 680 MPa at 400

MPa of compaction pressure. The deformable shell may be able to be crushed between 25- 50% of its diameter at 400 MPa of compaction pressure, without failure or cracking.

[0027] The cavity may be partially or fully filled with a filler material.

[0028] The deformable shell may define a hole and the method may further include: heating the produced green compact to a temperature sufficient to liquefy the filler material but below a sintering temperature, and allowing at least a portion of the liquefied filler material to escape the cavity through the hole defined by the shell.

[0029] The filler material may include a fluid, for example a shear thickening fluid, such as a wax, for example stearic acid, or a metal powder, such as Alumix 321 powder.

[0030] The method may further include sintering the produced green compact.

[0031] Compacting the metal powder may deform the deformable shell.

[0032] The deformable shell may be a shell that includes AA3003 aluminum.

[0033] According to another aspect of the present disclosure, there is provided a green compact that includes: compacted metal powder; and a deformable shell completely encased by the compacted metal powder, the deformable shell being sufficiently strong to withstand compaction pressures.

[0034] The deformable shell may have a shell strength of at least 680 MPa at 400

MPa of compaction pressure. The deformable shell may be able to be crushed between 25- 50% of its diameter at 400 MPa of compaction pressure, without failure or cracking.

[0035] The deformable shell may be partially or fully filled with a filler material.

[0036] The filler material may include a fluid, for example a shear thickening fluid, such as a wax, for example stearic acid, or a metal powder, such as Alumix 321 powder.

[0037] The deformable shell may be a shell that includes AA3003 aluminum.

[0038] According to still another aspect of the present disclosure, there is provided a deformable shell for use in powder metallurgy, the deformable shell being sufficiently strong to withstand compaction pressures and defining a cavity, wherein the cavity is partially or fully filled with a filler material.

[0039] The deformable shell may have a shell strength of at least 680 MPa at 400

MPa of compaction pressure.

[0040] The deformable shell may be able to be crushed between 25-50% of its diameter at 400 MPa of compaction pressure, without failure or cracking. [0041] The filler material may include a fluid, for example a shear thickening fluid, such as a wax, for example stearic acid, or a metal powder, such as Alumix 321 powder.

[0042] The deformable shell may be a shell that includes AA3003 aluminum.

[0043] Other aspects and features of the present disclosure will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments in conjunction with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the present disclosure will now be described, by way of example only, with reference to the attached Figures.

[0045] Fig. 1 is a schematic illustrating a method according to the present disclosure.

[0046] Fig. 2 is an illustration of an exemplary method according to the present disclosure.

[0047] Fig. 3 is an illustration of heating a green compact to liquefy filler material.

[0048] Fig. 4 is a graph illustrating the bulk density vs. compaction pressure for a conventional compact and a compact made according to the present disclosure.

[0049] Fig. 5 is a graph illustrating relative density of the powder metallurgy matrix alone vs. compaction pressure for: (i) a green compact without an internal structure and (ii) a green compact having an unfilled internal structure.

[0050] Fig. 6 is an illustration of a density map of cross sections of a conventional compact (left) and a compact made according to the present disclosure (right), both compacts made at 100 MPa.

[0051] Fig. 7 is an illustration of a density map of cross sections of a conventional compact (left) and a compact made according to the present disclosure (right), both compacts made at 400 MPa.

[0052] Fig. 8 is a graph illustrating the compressive green strength vs. compaction pressure for a compact made according to the present disclosure.

[0053] Fig. 9 is an illustration of a density map of cross sections of a conventional compact (left) and a compact made according to the present disclosure (right), both compacts made at 100 MPa.

[0054] Fig. 10 is an illustration of a density map of a cross section of a compact made at 100 MPa according to the present disclosure. [0055] Fig. 1 1 is an illustration of a density map of cross sections of a conventional compact (left) and a compact made according to the present disclosure (right), both compacts made at 400 MPa.

[0056] Fig. 12 is an illustration of a density map of a cross section of a compact made at 400 MPa according to the present disclosure.

[0057] Fig. 13 is a photograph of a sintered metal product formed from a compact made at 400 MPa according to the present disclosure.

DETAILED DESCRIPTION

[0058] Powder metallurgy (PM) manufacturing process is a manufacturing technology which forms parts from powdered metal products. One of the most common PM processing techniques is the press-and sinter method. In a "press-and-sinter" method, powder is first pressed or compacted in a rigid die under high pressure to form a green compact. Green compacts are very near the final desired shape ("near-net-shape") but are characterized by incomplete densification of the powder. That is, the bulk density of the part is lower than the theoretical full density of the solid metal. To set the internal microstructure, thereby setting the mechanical strength, green compacts are heat treated at elevated temperatures

(sintered).

[0059] Powder metallurgy techniques may have advantages over conventional manufacturing processes, such as:

• Reduced raw material wastage;

• Reduced secondary machining required; and/or

• Increased volume, reduced cost production capabilities.

[0060] Compared to other processes, such as casting, chip generating machining

(e.g. grinding, milling), forging and extruding, PM techniques may offer advantages in terms of material utilization and energy demands. Dale (201 1) discloses a study which quantified the amount of raw material remaining in a part as well as how much energy is consumed per kilogram of produced part. The results of the study show that PM has reduced material wastage and lower overall energy demands compared to competing processes. It should be noted that while the deep drawing processing technique is comparable in performance, it is limited to sheet metal raw materials. While reducing raw material wastage has direct cost benefits, other cost savings may be realized through PM processes. In titanium alloy aerospace applications, the raw alloy in ingot form costs four times more than high strength steels. Using PM titanium alloys has been shown to result in a 40% cost savings on raw materials (Fang, 2010). The automotive industry, PM's largest consumer, enjoys similar raw material costs savings but its primary benefit is the significant costs savings derived from the lack of secondary machining required. By leveraging the benefits of PM techniques, metal composite components may be manufactured more economically, with improved

performance, or both.

[0061] Broadly, the present disclosure provides a powder metallurgy manufacturing process which can produce a composite component having a metal matrix that completely encases an internal structure. The method may use a "press-and-sinter" powder metallurgy die compaction technique to create the composite component. Alternative powder metallurgy manufacturing techniques which may be used include cold isotatic pressing (CIP) or hot isotatic pressing (HIP).

[0062] A cold isotatic process subjects a component to isostatic fluid pressure in a high pressure containment vessel. The powder is contained in a rubber part from in the shape of the desired part. The benefit of the CIP is that the part has a highly uniform density after compaction in conjunction with a high degree of part geometry. However, CIP is a slow process which results in poor dimensional tolerances that may require additional machining processes after sintering.

[0063] A hot isotatic process is similar to cold isostatic compaction, though compaction is performed at elevated temperatures in an effort to combine compaction and sintering into a single step. A HIP process may result in near full density components. An inert gas may be used, for example argon, to reduce chemical reactions with the material. In hot isotatic processes, the chamber may be heated, causing the pressure inside the vessel to increase. Gas pumping may be used to achieve the necessary pressure level. Pressure is applied to the material from all directions (hence the term "isostatic"). With CIP and HIP, hollow internal structures are not formed since fill material would be unable to escape the shell.

[0064] In one aspect of the present disclosure, there is provided a method of producing a green compact that completely encases an internal structure. The exemplary method is illustrated in Fig. 1. The method (10) includes: providing a deformable shell (12) which defines a cavity; providing a metal powder (14) suitable for powder metallurgy; in a rigid die, surrounding the deformable shell with the metal powder (16); and compacting the metal powder and the deformable shell (18) to generate the green compact (20) that completely encases the internal structure, where the internal structure is defined by the deformable shell.

[0065] A deformable shell is advantageous since it reduces residual stresses in the compact during compaction. In particular powder metallurgy processes, when using a rigid shell, the powder may be overly stressed and could crack upon ejection of the green compact. Using deformable shells may aid in raising the local density of the powder near the shell, thus improving compaction characteristics. To achieve similar improvement in compaction characteristics, conventional compaction methods would have to increase the compaction pressure. A shell would be understood to be deformable if it is able to be crushed at least 25% of its diameter when subjected to 400 MPa, without failure or cracking. In particular examples, a deformable Alumix 321 shell may be crushed to 50% of its diameter at 400 MPa.

[0066] The cavity shell may be partially or fully filled with a filler material. The filler material may be a metal powder or a fluid, for example a shear thickening fluid, such as a wax, for example stearic acid, or other similar material.

[0067] Filling the cavity shell with a material that has similar densification properties as the surrounding powder metallurgy matrix used in the formation of the green compact may provide an increase in local densification and improved sintering characteristics. This may be accomplished, for example, by filling the cavity shell with a material that is the same as or similar to the powder metallurgy matrix. In a specific example, this may be accomplished by filling a deformable aluminum 3003 shell with Alumix 321 powder.

[0068] Using cavity shells filled with a material that has similar densification properties as the surrounding powder metallurgy matrix may be beneficial when forming specific components that would otherwise have areas of low density. Placing the filled cavity shell in such areas may increase the densification without reducing the sintering

performance, thereby resulting in sintered metal products with improved strength.

[0069] The deformable shell may define a hole and the method may further include: heating the produced green compact to a temperature sufficient to liquefy the filler material but below a sintering temperature (22), and allowing at least a portion of the liquefied filler material to escape the cavity through the hole defined by the shell.

[0070] The method may further include sintering (24) the produced green compact.

[0071] Compacting the metal powder may deform the deformable shell. [0072] The deformable shell may be a shell comprising AA3003 aluminum.

[0073] In another aspect of the present disclosure, there is provided a green compact that includes: compacted metal powder; and a deformable shell completely encased by the compacted metal powder. The deformable shell may be partially or fully filled with a filler material. The filler material may be a metal powder, a fluid, for example a shear thickening fluid, such as a wax, for example stearic acid, or other similar material. The deformable shell may be a shell comprising AA3003 aluminum.

[0074] In yet another aspect of the present disclosure, there is provided a deformable shell for use in powder metallurgy, the deformable shell defining a cavity, where the cavity is partially or fully filled with a filler material. The filler material may be a fluid, for example a shear thickening fluid, such as a wax, for example stearic acid. The deformable shell may be a shell comprising AA3003 aluminum.

Powder Production Methods

[0075] The method according to the present disclosure may use any metal powder, irrespective of the source or method used to make the powder. The method according to the present disclosure may use, for example: an elemental powder, a pre-alloyed powder, or a combination thereof. Additives (such as lubricants) may be added to the metal powder. One example of a metal powder which may be used in the method according to the present disclosure include Alumix 321 powder by ECKA Granules. Other examples include:

aluminum powder, titanium powder, and magnesium/lithium powder.

[0076] An elemental powder is a metal powder having at least one metal element in powder form. An elemental powder may be a mixture of different elemental powders that are combined so that they produce the desired alloy when the compacted powder mixture is sintered. For example, a mix of tin and copper powders will form bronze when compacted and sintered. A pre-alloyed metal powder is a powder having particles of the metal alloy in question. Elemental metals are often softer than alloys. Accordingly, an elemental powder may have better compaction characteristics than the corresponding pre-alloyed equivalents and an elemental powder may, therefore, yield a resulting green compact with a higher density than the corresponding pre-alloyed powder (German, 2005).

[0077] Three broad categories of exemplary methods employed by the powder metal industry to produce metal powder include: physical, mechanical and chemical. Exemplary sub-methods are listed below within each category. • Physical: gas, water, and centrifugal atomization;

• Mechanical: machining, milling, and mechanical alloying; and

• Chemical: decomposition of a solid by a gas, electrolytic production, and thermal decomposition.

[0078] The selection of the production technique by a metal powder production company depends on factors such as, for example: material chemical characteristics, physical characteristics (shape, size, chemical), powder quality and the economic climate (Meluch, 2009).

[0079] Physical production techniques may include, for example: gas and water atomization, and centrifugal atomization. Gas and water atomization involve obliterating a stream of molten metal using gas (N₂, Ar, He, Air) or water into fine particles that solidify into powder particles. This process can be applied to any metal or alloy that can be melted; however, reactive metals such as Al, Mg, Ti, Zr are only compatible with gas atomization. Water atomization generally yields more irregular powder morphology with a relatively large powder particle size. Similarly, employing air also produces an irregular powder morphology shape with a similar powder particle size. Inert gas yields more spherical powder morphology with a relatively small powder particle size. Atomization is the most dominate powder production method currently employed by the PM industry because favorable economics and high production rates. Furthermore, atomization is the only effective method of producing alloy powders on large scales.

[0080] Mechanical production techniques may include, for example: machining, milling and mechanical alloying. Machining consists of turning or grinding solid stock into a powder with a curled morphology. The drawback of this production method is machining fluid contamination and work hardened particles that yield poor compaction characteristics. Milling produces powder by applying impact (e.g. ball mill) to a coarse material feed. The major requirement of this method is the feed material must be brittle; therefore, the feed is often manipulated into a brittle form (hydration) pre-milling, after milling, this process is reversed to restore material ductility. Mechanical alloying is a combination of grinding, milling and alloying into a single step. This process employs a raw powder mixture as the feed in a ball mill. The benefit of this process is that compressive load induces alloying, work hardening, and particle fracture in a cycle. This can result in a very fine grain size which gives excellent mechanical properties in the final PM product. Furthermore, it is possible to synthesize alloys that would be otherwise impossible to produce.

[0081] Decomposition of a solid by a gas is a chemical production technique that is often employed to produce, for example, Fe, Mo, W or Cu powders. Additionally, it is virtually the sole method used to produce refractory powders (e.g. W, Mo) since their respective melting temperatures are too high to produce powders with more atomization techniques. The chemistry behind the process involves the reduction of an oxide into a metallic form by way of CO or H₂ as the reducing agent. The final powder product has a spongy morphology that is characterized by internal porosity. Electrolytic production is used to produce elemental powders from an initially solid purified metallic plate. The plate is used as the anode of an electrochemical cell. A current is passed through the anode to a cathode by way of an electrolyte. This process causes the initial plate to be dissolved and then precipitated on the cathode. The precipitated metal is finally ground into a powder which has usually has dendritic morphology (e.g. Cu, Ag) or an angular morphology (e.g. Fe, Mn, Co).

Compaction methods

[0082] The method according to the present disclosure may use any powder compaction method used in the powder metallurgy manufacturing industry. The method according to the present disclosure may use, for example: (1) cold isostatic compaction, (2) hot isostatic compaction, or (3) uniaxial die compaction.

[0083] Cold isostatic compaction (CIP) is performed by applying compaction pressure to the powder via pressurized fluid at approximately room temperature. The powder is contained in a rubber part formed in the shape of the desired part. The benefit of CIP is that the part has a highly uniform density after compaction, and a high geometrical tolerance. However, CIP is a slow process and may result in poor dimensional tolerances. Parts formed using CIP may require additional machining processes after sintering.

[0084] Hot isostactic compaction (HIP) is similar to cold isostatic compaction, although compaction is performed at elevated temperatures in an effort to combine compaction and sintering into a single step. This process results in near full density components.

[0085] Uniaxial die compaction may be used, for example, to produce high part volumes with little material waste. With uniaxial die compaction, powder is placed in a die (in the shape of the desired part) and pressure is applied. The powder densifies and produces a green compact. Pressure can either be applied in one direction (single action compaction), or pressure can be applied from the top and the bottom simultaneously (double action compaction). In single action compaction, force can be applied from the top, bottom, or both. For example, the force can be applied by four separate punches at the top and bottom, for a total of eight punches. In uniaxial die compaction, the force applied can be varied from punch to punch, as well as the timing of the punches can be varied.

[0086] A floating die set-up is designed to simulate double action compaction without the equipment needed to apply force via top and bottom punches. The floating die simulates double action compaction by allowing the die by to move relative to the top and bottom punches during the compaction process. As compaction begins, the bottom punch and the die move at the same rate while the top punch is fixed. As the powder is compacted, die wall friction becomes sufficiently high enough to change the relative movement of the die and the bottom punch. Once this happens, the force exerted by the top punch is increased, thus creating a double action effect. The compaction process has four stages. In the first stage (termed the "rearrangement stage"), as pressure is applied by the punches, the powder particles rearrange themselves to reduce the volume of empty space without any

deformation occurring to the powder; in this stage there is the most rapid increase in density. In the second stage (termed the "localized deformation" stage), the pressure application continues and particles begin to deform where they interact; for example, two spheres with a point contact will begin to have a flattened contact. In the third stage (termed the

"homogeneous deformation" stage), there is homogeneous deformation where voids between the particles begin to collapse and the initial partial shape is no longer recognizable and most particles will have a polygonal shape. In the fourth stage (term the "bulk

compression" stage), there is bulk deformation and the voids are no longer collapsing and the whole compact is behaving as a solid; there is a very little increase in density in the fourth stage.

[0087] In some instances, uniaxial die compaction may exhibit die wall friction. As a result of die wall friction, when force is applied to the top (in single action compaction) the application force is degraded along the length of the compact. In double action compaction this same phenomenon occurs, however, the application force is degraded in both directions with the minimum applied force occurring in the center. This location is called the density split. Naturally, as a result of the difference in force application there exists density gradients that are the result of the degradation of force application. Density gradients are of particular importance because they can cause part failure or poor mechanical properties in areas of low density.

[0088] In an effort to combat density gradients, lubrication can be employed. Two exemplary routes of lubrication application include: solid wax mixed into the powder, and a liquid sprayed on to the die wall. In some situations solid wax is preferable because it has been found to be more economically viable and environmentally friendly. In exemplary methods, between 0.7 and 1.5 wt% lubricant can added to the powder before compaction depending on powder material. In addition to reducing density gradients, lubrication may additionally reduce wear on tooling and thus extend tool life. Lubrication may be expelled prior to sintering by raising the temperature to a sufficiently high temperature.

[0089] Other factors that play a role in the compaction process include, for example: powder morphology and powder size. Powder size is important because it is more difficult to compress smaller particles. This is because smaller particles are often harder and strain harden quickly. The powders that press to the highest green densities are large, soft particles. Furthermore, powder shape effects the compaction process because there tends to be an increased particle friction in smaller and irregularly shaped powders, for example non- spherical powders. In particular examples, green density of powders reaches a maximum at a blend of approximately 70% coarse, 30% fine particles; this is a result of a very high apparent density caused by smaller particles filling voids created by larger particles.

[0090] Powder shape has distinct compaction characteristics. Particles with a sponge shape, or morphology, are often difficult to compress because it takes a great deal of energy to collapse the internal porosity. Furthermore, irregular particles, while harder to compress often exhibit increased green strength properties in comparison to spherical particles because of the interlocking characteristics of the irregular shapes. Spherical particles have excellent flow characteristics and low inter-particle friction; however, they exhibit relatively low green strength characteristics. Most often, irregular shaped particles will yield lower green densities but higher green strengths than their rounded and spherical counter parts.

[0091] One example of a die compact method is illustrated in Fig. 2. As illustrated, in panel (1), the die (100) is partially filled with the powder (102) and the bottom punch (104) is installed. The deformable shell (106) is filled with stearic acid (108) and the opening (110) is directed downwards. In panel (2), the remaining powder is filled around the deformable shell and the top punch (112) is inserted. In panel (3), force is applied to the top punch in order to compact the powder until a desired compaction pressure is attained. The deformable shell deforms according to this compaction pressure resulting in a green compact having a fully encased internal structure which is filled with stearic acid.

[0092] As illustrated in Fig. 3, the die walls may be heated and the top punch load may be reduced, though not completely removed, in order to liquefy the filler material - in this case, stearic acid. The liquefied stearic acid escapes through the opening in the shell and through the porous compacted powder, resulting in a green compact having a fully encased internal structure which is empty of the filler material.

[0093] It would be understood that the opening does not need to be directed downward in order for the filler material to escape so long as the die and green compact can be moved into a position where the liquid form of the filler material can flow out of the opening. For example, the opening may be in an upward direction on compaction and the die and green compact may be inverted to allow the liquefied filler material to flow out of the opening. In other examples, the opening may be in a sideward direction on compaction and the die and green compact may be spun in order to generate centrifugal force that directs liquefied filler material from the opening.

Sintering

[0094] The method according to the present disclosure may additionally employ sintering to bond particles together after the powder is compacted, thereby enhancing mechanical properties of the object. Sintering involves heating the compact to temperatures approaching the melting point of the material in question, thereby bonding adjacent particles together.

[0095] Sintering can occur in two forms: (1) solid state, or (2) liquid phase. Solid state sintering is performed below the melting point of the material and functions by reducing the compact surface energy by way of mass transport mechanisms. Liquid phase sintering is performed at or very near the melting point of the material. During the sintering process a liquid phase is formed which drastically increases the mass transport mechanisms.

Aluminum alloys are exclusively sintered using liquid phase sintering as they are often difficult to sinter as a result of an oxide layer easily form on the powder surface. In some examples, for example green compacts made with Alumix 321 powder, sintering may be effected at 630 °C held for 25 minutes. Internal Structures

[0096] The green compacts produced according to the present disclosure include at least one completely encased internal structure. The internal structure is generated using a deformable shell that defines a cavity. The deformable shell may be referred to as a "cavity shell".

[0097] The cavity shell may be unfilled, partially filled with a material, or fully filled with a material. The shell defines the shape of the internal structure encased in the green compact, taking into account that the shell is deformed when the metal powder is compacted. The shell may be, for example, metal which has good ductility, is easy to machine, and has a melting point below but within 10% of the sintering temperature of the metal powder. In some examples, the metal may be AA3003 aluminum or a steel alloy.

[0098] The material used to partially or fully fill the shell may be referred to as a

"filler", "fill material", "filler material", "cavity fill", or similar variations. The cavity fill may be, for example, any material that provides sufficient stiffness to the shell to preferentially compact the metal powder and avoid collapse of the cavity shell, but not so stiff as to cause cracking of the compact. An appropriate stiffness is determined by the density, bulk modulus, elastic modulus, and yield strength of the filler material. Cavity fill material is selected to match the compliance needs of the surrounding powder metallurgy matrix during compaction. In particular examples, the filler material combined with the shell would provide a stiffness that closely matches the evolving stiffness of the powder during compaction. Foam cavity fills, such as polystyrene fills used in the spheres developed by Andersen (2000), Neville & Rabiei (2008) and Rabiei & O'Neill (2005), do not provide sufficient stiffness to the shell. The fill material should additionally have a melting point below the temperature used to sinter the compact, and should be compatible with the metal powder being used.

[0099] The cavity fill may be, for example, stearic acid which is a lubricant and does not, therefore, contaminate, alter, or interfere with powder densification or sintering. Other cavity fills which may be used include, for example: fluids; powder metallurgy powders such as Alumix 321 powder; and shear thickening (dilatant) fluids, which are fluids in which viscosity increases with the rate of shear strain.

[00100] The method according to the present disclosure may use a cavity shell formed using any known method that provides a shell with sufficient strength to withstand

compaction pressures. When the shell is made of Alumix 321 , the shell may have a strength of at least 680 MPa at 400 MPa of compaction pressure. Spheres developed by Andersen (2000), Neville & Rabiei (2008) and Rabiei & O'Neill (2005) lack the strength to withstand compaction pressures used in the method according to the present disclosure. Cavity shells, for example spheres, may be formed using, for example: cupping with molds, powder metallurgy, chip cutting, or assembly of separate parts. The cavity shell may be fully filled or partially filled with a filler.

[00101] In exemplary methods, the internal structure can be fabricated by forming a hole in a spherical cavity shell and adding the selected cavity fill, such as a fluid, for example a shear thickening fluid, such as a wax, for example stearic acid. The hole may or may not be closed after the material is added.

[00102] In order to reduce the possibility of premature, complete collapse of deformable shells having unfilled cavities during compaction, the surface of the deformable shells having unfilled cavities should be free from holes or openings therein. When using deformable shells having unfilled cavities, the pressure used to produce the compact may be reduced, for example by 50-100 MPa, in order to reduce the possibility of premature, complete collapse of deformable shells. Since the filler material reduces the possibility of premature, complete collapse of deformable shells having filled cavities, the surface of the shells may or may not include holes or openings in the surface therein.

[00103] The PM production method for forming a cavity shell first starts by mixing a suspension of metal powder, water and binding agent to create a uniform mixture.

Polystyrene shapes, in this example spheres, are then fluidized in a chamber where the powder suspension is sprayed onto the spheres; after which the spheres are spayed with a steady air flow to alloy the mixture to dry on the foam spheres. The spheres can be sintered if improved mechanical properties are desired. The final sintering process allows for the pyrolization of the internal polystyrene core, which leaves a hollow sphere.

[00104] Geiger (1946) discloses a precursor to the PM method of producing hollow spheres. In the method disclosed by Geiger, refractory hollow spheres were produced by creating a mixture of binder and ceramic material. Combustible pellets several millimeters in diameter were coated in the mixture which was then sintered to remove the internal core and allow the mixture to set. Other methods of producing hollow spheres which may be used in the method according to the present disclosure include, for example: Andersen et al., 2000; Augustin & Hungerbach, 2009; Ochsner & Augustin, 2007; Andersen et al., 2007; and Hollomet, n.d.. [00105] Cavity shells produced using other methods may also be used in the method according to the present disclosure. For example, a cavity shell may be formed by forging a structure, drilling an internal cavity, and plugging the holes used to create the internal cavity.

[00106] In one example of a cavity shell that can be used in the present method, Roush & Clark (1952) disclose a method that employed both powder metallurgy and casting techniques to produce hollow poppet valve heads. First a powder compact is formed in the shape of the internal void desired. This compact is then placed in a casting mold and part is cast around the PM compact. In this technique, the PM compact is on the order of 50-60% relative density and suitable compact materials include: copper aluminum, titanium or iron powder.

[00107] The selection of the casting material and the compact material are

codependent since the casting material melting temperature must be sufficiently low as not to affect the powder compact during the casting process. After the casting process, the cast and internal compact are immersed in acid which dissolves the internal compact and leaves a hollow cavity. The hole left by the PM compact can be fitted with a plug which is welded into place.

[00108] In another example of a cavity shell that can be used in the present method, Levinstein & Butts (1969) disclose a method for the production of a hollow metal articles using an internal core (consisting of one of the following metal alloys: aluminum, magnesium, zinc, tin, or cadmium) coated by way of electron or vapour disposition process with powders such as (70-95% of one of the following: iron nickel cobalt copper silver). Once the core is coated with the metal powder the whole assembly is sintered to a temperature above the core melting point but below the exterior melting point. This allows for the diffusion of the core into the outer shell, thus leaving a hollow interior.

[00109] In yet another example of a cavity shell that can be used in the present method, Voice & Junfa (2008) disclose a method of engineering hollow fan blades for gas turbines. This method employed filling a hollow container with a metal powder (Ti6AL4V60) and a metal insert of the same material, coated in a pattern of stop off material, such as yttrium. The container is sealed and forged to consolidate the powder, after which the container is removed from the consolidated powder. A hole is drilled in the tip of the preform and a pipe is connected to the end of the metal insert. The preform and the attached pipe are forged to define the shape of the turbine blade during the forging process; an inert gas is supplied to the turbine blade to inflate the regions where the stop off material was applied to the metal insert to create the internal voids. Lastly, the turbine blade may be finished with necessary grinding or machining operations to attain the proper dimensional tolerances.

[00110] Still other techniques have been employed to develop hollow parts using PM methods, including, for example: the production of heat exchanger header by Greune (1989) and the production of a hollow ballistic charge by Nguyen (1992).

[00111] In a still further example of a cavity shell that can be used in the present method, a wrought aluminum hollow structure may be used. Such a wrought aluminum hollow structure may provide cost savings in comparison to PM produced hollow structures since the production of PM produced hollow structures requires complex, high grade powder and, as a result, up to 50% of the cost for PM produced hollow structure production is attributed to powder (Ochsner & Augustin, 2007).

Example 1

[00112] Preparation of a filled cavity shell. A commercially available spherical cavity shell was used to make a filled cavity shell. A through-hole was drilled in the cavity shell and the resulting shell was submerged in liquefied 100% stearic acid, held at a temperature of approximately 80°C. Once the stearic acid filled the cavity shell, the filled cavity shell was allowed to resolidify at room temperature. Stearic acid surrounding the shell was removed and the filled shell was cleaned. The properties of the shell and the stearic acid are listed in Table 1 , below.

Table 1

Example 2

[00113] Formation of a green compact. Alumix 321 powder (manufactured by Ecka Granules ®) is a powder metallurgy version of AA6061 aluminum alloy. The powder was poured into a 15 mm diameter cylindrical die with the bottom punch installed. The die was filled halfway with the metal powder. A filled shell cavity shell produced as described in Example 1 was placed in the center of the die. Additional metal powder was added to surround the cavity shell and fill the die to the desired height.

[00114] The metal powder was settled with tapping and the top punch inserted. The top punch was actuated and the metal powder was compacted to the desired compaction pressure. The resulting green compact was removed from the die.

Example 3

[00115] Generation of an unfilled internal structure in a green compact. A green compact produced as described in Example 2 was reloaded in the die at a pressure about ¼ of the compaction pressure. The die was warmed to 80 °C to melt the stearic acid. The melted stearic acid flowed through the openings in the cavity shell and through the matrix of the green compact since the matrix is porous.

Example 4

[00116] Testing Bulk Densification. Bulk density of compact vs. compaction pressure was determined for conventional compacts and for compacts made according to Example 2. The resulting compaction curves are shown in Fig. 4 where the curve for Alumix 321 powder compacts (that is, a conventional compact), is included as a reference. Note that the conventional compacts were of identical size to those of the compacts made according to Example 2. The initial tap density refers to the bulk density of the compacts after vibrating the die to settle the powder.

[00117] As expected, the density of the compact made according to Example 2 is lower given that the internal structure is not filled with metal and, therefore, has a lower density than the surrounding PM matrix. While the final heights of the internal structure and conventional compacts were the same, their initial heights were not. The tap densities of the two compacts are very similar due to the fact that, while the compact made according to Example 2 weighed less, it was initially taller.

[00118] Fig. 5 is a graph illustrating the relative density of the powder metallurgy matrix alone vs. compaction pressure for: (i) a green compact without an internal structure and (ii) the green compact of Example 3, which has an unfilled internal structure. The relative density (percentage of wrought aluminum density) shows very little difference between the two compacts. It can be concluded that the presence of an internal structure in a compact does not affect the bulk or overall densification of the powder.

Example 5

[00119] Testing Density Distribution. The density distribution of compacts produced according to Example 2 was examined to determine what effects the internal structure has on the powder densification. Visual inspection of sectioned samples and density maps produced using optical densitometry methods (Fig. 6 and Fig. 7) were performed as described by Beck, G., Selig, S., Doman, D. A., & Plucknett, K. ("Densitometry Analysis to Determine Density Distribution in Green Compacts" (2011) International Conference on Powder Metallurgy & Particulate Materials, San Francisco.)

[00120] In the conventional sample pressed at 100 MPa, the density distribution was somewhat uniform, with low density regions in the middle of the compact and very low regions at the bottom. In contrast, in the compact pressed at 100 MPa having an internal structure, a distinct localized high density band exists. This high density band may be visually seen from the highly localized band of high densification of PM matrix around the internal structure, as evidenced by shiny areas of PM matrix. More detailed density maps for conventional compact pressed at 100 MPa and compacts made according to the present disclosure pressed at 100 MPa are illustrated in Fig. 6. The density maps show that, in stark contrast to the conventional compact, high densities are observed in the middle (core) of compacts made according to the present disclosure. Furthermore, not only is the location of the high density different, compacts made according to the present disclosure see much higher local densities at the same compaction pressure.

[00121] Fig. 7 shows density maps for compacts made at 400 MPa. The density distribution is more uniform, as would be expected at such elevated compaction pressures. However, it is again evident that compacts made according to the present disclosure have higher densities in the core of the compact.

Example 6

[00122] Testing compressive green strength. Compressive green strength (CGS) testing was performed to assess the strength of compacts made according to Example 2.

Compressive green strength (CGS) testing was completed in a Carver® Model C hydraulic press. The testing was completed by placing the compact between two flat platens lubricated with a solid stearate. A load cell connected to a National Instruments portable data acquisition system was placed above the top platen to measure the failure load. For the CGS testing, the engineering stress was considered; the testing regimen involved loading the compact with the hydraulic press until an observed failure occurred. The CGS of the tested compacts was defined as the ultimate compressive strength recorded before the compact failed by fracture. The CGS of the compacts made according to Example 2 was found to increase with pressure, as illustrated in Fig. 8, and has similar strengths as conventional samples (not shown).

Example 7

[00123] Preparation of an Alumix 321 filled cavity shell. A commercially available spherical cavity shell was used to make a filled cavity shell. A through-hole was drilled on side of the cavity shell and an Alumix 321 powder was inserted (using a funnel) into the resulting shell. A green compact was formed in a manner similar to that described in

Example 2.

Example 8

[00124] Reclosing the openings in the shell structure. A through-hole was drilled in a commercially-available spherical cavity shell and the resulting shell was submerged in liquefied 100% stearic acid, held at a temperature of approximately 80 °C. Once the stearic acid filled the cavity shell, the filled cavity shell was allowed to resolidify at room temperature. Stearic acid surrounding the shell was removed and the filled shell was cleaned.

[00125] A commercially available two part epoxy was then inserted into the through- holes created by the drilling process and allowed to cure. The epoxy used was: West System 105 Epoxy Resin, 206 Hardener, Density: 1180 kg/m³. Since the epoxy was a thermosetting polymer, it did not melt and the filled cavity shell remained filled after compaction and sintering.

Example 9

[00126] Formation of a green compact using an unfilled cavity shell. Alumix 321 powder was poured into a 15 mm diameter cylindrical die with the bottom punch installed.

The die was filled halfway with the metal powder. A commercially available spherical cavity shell, traditionally used for food processing and chemical mixing applications as an agitator, was placed in the center of the die. Additional metal powder was added to surround the cavity shell and fill the die to the desired height.

[00127] The metal powder was settled with tapping and the top punch inserted. The top punch was actuated and the metal powder was compacted to the desired compaction pressure. The resulting green compact was removed from the die.

Example 10

[00128] Sintering. The green compacts of Examples 3, 7 and 9 were sintered at 630°C for 25 minutes. The sintered compacts resulting from the green compacts of Examples 3 and 9 have a hollow interior cavity. In the sintered compact resulting from the green compact of Example 7, the Alumix 321 powder in the spherical cavity shell is left in the shell and sintered.

Example 11

[00129] Comparison of density maps for different green compacts. Green compacts were made: (a) without a deformable shell, (b) with a wax-filled aluminum 3003 deformable shell, and (c) with an Alumix 321 powder-filled aluminum 3003 deformable shell, by pressing a cylindrical compact 15mm in diameter at a pressure of 100, 200, 300 or 400 MPa as discussed in Example 2.

[00130] Optical densitometry was performed on the resulting green compacts to generate density maps of the samples. The density maps for the green compacts generated at 100 MPa are shown in Fig. 9 (left: no deformable shell; right: wax-filled deformable shell) and Fig. 10 (Alumix-321 powder-filled deformable shell). The density maps for the green compacts generated at 400 MPa are shown in Fig. 1 1 (left: no deformable shell; right: wax- filled deformable shell) and Fig. 12 (Alumix-321 powder-filled deformable shell).

[00131] The density maps for the green compacts with the wax-filled and powder-filled deformable shells generated at both 100 MPa and 400 MPa show similar increases in local densification around the shell. This is expected since both filled cavity shells exhibit compressible deformation responses, that is, both the wax and the powder densify as the surrounding powder metallurgy matrix densifies. Both sets of density maps show

approximately 2-3% density difference between the compacts formed using wax- and powder-filled deformable shells, though the density of the powder-filled deformable shell more closely matches the density of the powder metallurgy matrix immediately surrounding the shell.

Example 12

[00132] Sintering of green compacts made with an Alumix 321 powder-filled aluminum 3003 deformable shell. Green compacts were made with an Alumix 321 powder-filled aluminum 3003 deformable shell, by pressing a cylindrical compact 15mm in diameter at a pressure of 100, 200, 300 or 400 MPa as discussed in Example 2.

[00133] These green compacts were sintered as discussed in Example 10. Since the powder-filled cavity shells densify along with the powder metallurgy matrix, the green compacts exhibited little geometric distortion and no observable cracking or failures. On sintering, the deformable shells appear to have melted and the powder filling the shell has sintered in the same fashion as the powder metallurgy matrix, resulting in a substantially homogenous and cohesive sintered metal product. The sintered product made from the green compact formed at 400 MPa is shown in Fig. 13.

[00134] In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that these specific details are not required. The above-described embodiments are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art without departing from the scope, which is defined solely by the claims appended hereto.

References

Aluminum Association, T. (2008). "Alcoa Earns 'Casting of the Year' Award for BMW Rear Control Arm.

Andersen, O., W. Hungerbach, et al. (2007). "inno.zellmet - a concerted approach towards the application of non-foam type cellular metals." Thermec 2006, Pts 1-5 539-543: 1892-1897.

Andersen, O., U. Waag, et al. (2000). "Novel metallic hollow sphere structures." Advanced

Engineering Materials 2(4): 192-195.

ASTM (2003). ASTM B328-96 Standard Test Method for Density, Oil Content, and

Interconnected Porosity of Sintered Metal Structural Parts and Oil-Impregnated

Bearings, ASTM.

Augustin, C. and W. Hungerbach (2009). "Production of hollow spheres (HS) and hollow sphere structures (HSS)." Materials Letters 63(13-14): 1 109-1 112.

Belhadjhamida, A., D. Williams and J. Davies (2011). "Manufacture of Composite

Components by Powder Metallurgy." US Patent Application No. US 201 1/0070119 A1

Capus, J. (2011). "Powder metallurgy, progress and the eco-friendly car." Metal Powder Report: 16-18.

Dale, J. R. (2011). "Sustainability Manufacturing within the PM Industry." International

Journal of Powder Metallurgy 47(1): 23-25.

Donaldson, I. W., T.E. Geiman, and R.K. Wlliams (2011). A novel approach for

manufacturing powder-forged connecting rods. 2011 Conference on Powder

Metallurgy & Particulate Materials. San Francisco.

Ecka Granules (2012). "ECKA Alumix Typical Chemical Composition, Physical

Characteristics, and Sintered Properties." Internet publication from http://www.ecka- granules.com/index.php?id=153&typ=15&anwendung=3&L=2

Fang, Z. Z. (2010). "Powder metallurgy titanium— Challenges and opportunities."

International Journal of Powder Metallurgy 46(5): 9-10.

Geiger, C. F. (1946). "Method of Making Refractory Bodies and Product Thereof." U.S.

Patent No. 2,553,759.

German, R. M. (2005). Powder metallurgy and particulate materials processing: the

processes, materials, products, properties and applications. Princeton, New Jersey,

Metal Powder Industries Federation. Gibbs, S. (2012). "Using Cast Inserts." Internet publication from

http://www.metalcastingdesign.com/content/view/799/315/

Greune, C. (1989). "Apparatus for the Production by Powder Metallurgy of a section of a

Header Pipe of a Heat Exchanger". U.S. Patent No. 4,867,412.

Gruver, J. H. (1917). "Sheet Metal Hollow Ball and Method of Making the Same." U.S. Patent

No. 1 ,278,914.

Hall, I. W., M. Guden, et al. (2000). "Crushing of aluminum closed cell foams: Density and strain rate effects." Scripta Materialia 43(6): 515-521.

Hollomet. (n.d.). "globomet." Internet publication retrieved January, 28, 2012, from

http://www.hollomet.com/cms/produkte/globomet. html?L=1

Isometall. (n.d.). "Ball Manufacturing Process." Internet publication retrieved January 31 ,

2012, from http://www.isometall.de/kugelherstellung.htm.

Levinstein, M. A. and W. R. Butts (1969). "Method for making a hollow metal article." U.S.

Patent No. 3,466, 166.

Lutheran, M. E. (201 1). "State of the PM industry in North America— 2011." Advances in

Powder Metallurgy & Particulate Materials, in press.

Martin, A., R. Knight, et al. (1961). "The Forming of Hollow Shapes in Beryllium By Loose

Sintering and Hot Pressing." Powder Metallurgy (7): 268-282.

MatWeb. (2012). "Alclad Aluminum 3003-O." Internet publication retrieved April 18, 2012, from:http://www.matweb.com/search/DataSheet.aspx?MatGUID=dbe8df6a132547b4

982311 b9698e63be&ckck=1.

McGee, S. W. and J. H. Mikoda (1975). "Powder Metallurgy Composite." U.S. Patent No.

3,859,016.

Meluch, L. (2009). Warm Compaction of Aluminium Alloy Alumix 123. PhD Thesis, University of Birmingham.

MPIF (2010). "Standard Test Methods for Metal Powders and Powder Metallurgy Products."

Determination of Density of Compacted or Sintered Powder Metallurgy (PM)

Products. Princeton, Metal Powder Industries Federation: 59-62.

Neville, B. P. and A. Rabiei (2008). "Composite metal foams processed through powder metallurgy." Materials & Design 29(2): 388-396.

Nguyen, C. (1992). "Process for Producing a Hollow Charge with a Metallic Lining." U.S.

Patent No. 5, 119,729. Novak, P. J. (1972). "Hollow Sphere and Structural Elements for Constructing the Same."

U.S. Patent No. 3,691 ,704.

Ochsner, A. and C. Augustin (2007). Multifunctional Metallic Hollow Sphere Structures,

Springer.

Oliveira, B. F., L. A. Da Cunda, et al. (2009). "Hollow sphere structure: a study of mechanical behaviour using numerical simulation." Materialwissenschaft und Werkstofftechnik 40(3): 144-153.

O'Neil, M. J. (2006). The Merck index : an encyclopedia of chemicals, drugs, and biologicals.

Whitehouse Station, N.J., Merck.

Rabiei, A. and A. T. O'Neill (2005). "A study on processing of a composite metal foam via casting." Materials Science and Engineering a-Structural Materials Properties

Microstructure and Processing 404(1-2): 159-164.

Rabiei, A. and L. J. Vendra (2009). "A comparison of composite metal foam's properties and other comparable metal foams." Materials Letters 63(5): 533-536.

Rachor, L. and H. Lotz (1966). "Fuel Material for Nuclear Reactors and Process and

Apparatus for its Manufacture." U.S. Patent No. 3,284,314.

Roush, M. S. and C. C. Clark (1952). "Method of Making Hollow Shapes." U.S. Patent No.

2,609,576.

Sanders, W. S. and L. J. Gibson (2003). "Mechanics of hollow sphere foams." Materials

Science and Engineering a-Structural Materials Properties Microstructure and

Processing 347(1-2): 70-85.

Schatz, J. W. (1910). "Process of Making Hollow Metallic Balls." U.S. Patent No. 955,698. Seyferth, D. and P. Czubarow (1995). "Method for preparation of a functionally gradient material." U.S. Patent No. 5,455,000.

Vesenjak, M., T. Fiedler, et al. (2008). "Behaviour of Syntactic and Partial Hollow Sphere

Structures under Dynamic Loading." Advanced Engineering Materials 10(3): 185-191. Voice, W. E. and M. Junfa (2008). "Method of manufacturing a metal article by powder

metallurgy." U.S. Patent No. 7,407,622.

Wang, T. G. and D. D. Elleman (1982). "Method and Apparatus for Producing Gas-Filled

Hollow Spheres." U.S. Patent No 4,344,787.

Watanabe, T., O. Mitsuo, A. Ohta and Y. Kondo (1981). "Method for preparing a composite metal sintered article." U.S. Patent No 4,261 ,745. Zhan-you, L, Z. Xian-rong, et al. (2007). "Expansion of spherical cavity of strain-softening materials with different elastic moduli of tension and compression." Journal of Zheiianq University - Science A 8(9): 1380-1387.

Claims

WHAT IS CLAIMED IS:

1. A method of producing a green compact that completely encases an internal structure, the method comprising:

providing a deformable shell which defines a cavity, the deformable shell being sufficiently strong to withstand compaction pressures;

providing a metal powder suitable for powder metallurgy;

in a rigid die, surrounding the deformable shell with the metal powder;

compacting the metal powder and the deformable shell to generate the green compact that completely encases the internal structure, the internal structure defined by the deformable shell.

2. The method according to claim 1 wherein the deformable shell has a strength of at least 680 MPa.

3. The method according to claim 1 or 2 wherein the deformable shell is able to be crushed between 25-50% of its diameter at 400 MPa, without failure or cracking.

4. The method according to any one of claims 1 to 3 wherein the cavity is partially or fully filled with a filler material.

5. The method according to claim 4, wherein the filler material is a metal powder or a fluid.

6. The method according to claim 5, wherein the fluid is a shear thickening fluid.

7. The method according to claim 6, wherein the shear thickening fluid is stearic acid.

8. The method according to claim 5, wherein the metal powder is Alumix 321 powder.

9. The method according to claim 6 or 7 wherein the deformable shell defines a hole and the method further comprises: heating the produced green compact to a temperature sufficient to liquefy the filler material but below a sintering temperature, and

allowing at least a portion of the liquefied filler material to escape the cavity through the hole defined by the shell.

10. The method according to any one of claims 1 to 9 wherein the method further comprises sintering the produced green compact.

11. The method according to any one of claims 1 to 10 wherein compacting the metal powder deforms the deformable shell.

12. The method according to any one of claims 1 to 1 1 wherein the deformable shell is a shell comprising AA3003 aluminum.

13. A green compact comprising:

compacted metal powder; and

a deformable shell completely encased by the compacted metal powder, the deformable shell being sufficiently strong to withstand compaction pressures.

14. The green compact according to claim 13 wherein the deformable shell has a strength of at least 680 MPa.

15. The green compact according to claim 13 or 14 wherein the deformable shell is able to be crushed between 25-50% of its diameter at 400 MPa, without failure or cracking.

16. The green compact according to any one of claims 13 to 15 wherein the deformable shell is partially or fully filled with a filler material.

17. The green compact according to claim 16, wherein the filler material is a metal powder or a fluid.

18. The green compact according to claim 17, wherein the fluid is a shear thickening

19. The green compact according to claim 18, wherein the shear thickening fluid is stearic acid.

20. The green compact according to claim 17, wherein the metal powder is Alumix 321 powder.

21. The green compact according to any one of claims 13 to 20 wherein the deformable shell is a shell comprising AA3003 aluminum.

22. A deformable shell for use in powder metallurgy, the deformable shell being sufficiently strong to withstand compaction pressures and defining a cavity, wherein the cavity is partially or fully filled with a filler material.

23. The deformable shell according to claim 22 wherein the deformable shell has a strength of at least 680 MPa.

24. The deformable shell according to claim 22 or 23 wherein the deformable shell is able to be crushed between 25-50% of its diameter at 400 MPa, without failure or cracking.

25. The deformable shell according to any one of claims 22 to 24, wherein the filler material is a metal powder or a fluid.

26. The deformable shell according to claim 25, wherein the fluid is a shear thickening fluid.

27. The deformable shell according to claim 26, wherein the shear thickening fluid is stearic acid.

28. The deformable shell according to claim 25, wherein the metal powder is Alumix 321 powder. The deformable shell according to any one of claims 22 to 28 wherein the deformable a shell comprising AA3003 aluminum.