The present invention is directed to a process of forming fully dense, near net-shaped parts from green laminae of powders of engineering materials, such as metals, ceramics, intermetallics, and composite materials. More specifically, the present invention is related to a powder metallurgy process for production of complex-shaped engineering parts in a simple, straight walled die with pressure supplied by a simple press.
There is need for a more universal, highly automated fabrication process capable of producing complex shaped, highly robust, engineered parts from metals, ceramics, hybrid materials, and metal, glass or ceramic matrix composites. Such a process should meet demanding engineering requirements including one or more of the following: complex shape, full density, graded property, internal cavities, and low cost, and allow a quick turn-around from drawing to finished product.
There is also need for a universally applicable process as an alternative to the conventional fabrication processing techniques of casting, forging, machining, cutting, joining, and certain near net-shape processing techniques, including thermal spraying, powder metal forging, weld deposition, sintering, and vapor deposition.
Many deficiencies exist in materials processing methods, such as casting, forging, and hot isostatic pressing. These may be discussed under two categories: cost and performance.
COST
Structures that are highly engineered are difficult to fabricate and are frequently very costly. Some examples for these include parts that are complex in shape, require graded property, are actively cooled (through internal channels or cavities), incorporate materials that act as sensors and actuators (piezoelectric ceramics, magnetostrictive and shape-memory materials, and electrorheological fluids). At the present time, these requirements can only be met by large increases in the labor content of the part's manufacturing cost.
Complex shaped structural ceramics (and other hard to machine materials) parts are expensive because of the high cost of machining them to shape.
Additionally, products made by conventional metal forming techniques require long production lead times due to design and fabrication of required shaped hard dies. Long lead times lead to increased cost to the user and the fabricator. Costs increase further if mid-stream design changes must be implemented.
PERFORMANCE
Alternatives to the conventional materials processing methods are needed to achieve improvements in performance. The process which is the subject of this invention provides needed solutions in the following specific problem areas:
(a) Utilization of complex-shaped, load-bearing advanced ceramics is limited by structural flaw sizes and high cost of machining to shape.
In order to reduce flaw size in ceramic structures, high pressures and temperatures must be employed. Existing (axial) hot pressing techniques are limited by low applied pressure requirements associated with the low strength of the hard graphite die. Increasing the temperature could compensate for the low pressure, but this could also yield materials with inferior mechanical properties and coarsened microstructures. Sintering aids may be used to increase density, but these stay in the final product as grain boundary phases, which in turn influence the performances of the material, particularly at elevated temperatures. For a discussion of hot pressed structural ceramics, see an article by Andre Ezis in Engineered Mat. Handbook, Vol. 4, Ceramics & Glasses, ASM, Metals Park, Ohio, pp. 186-193, 1991.
Hot isostatic pressing of ceramic preforms can be accomplished at pressures up to 320 Mpa or two to three times the axial hot pressing pressure (see an article by Hans T. Larker, entitled "Hot Isostatic Pressing" in Engineered Materials Handbook, Vol. 4, Ceramics & Glasses, ASM, Metals Park, Ohio, pp. 194-201, 1991). However, since the pressure medium is an inert gas, the powder preform must be encapsulated in a glass or a metal enclosure or can, which also traps volatile impurities such as fluorine (remaining from powder purification process) and humidity. These trapped impurities lower material strength. Encapsulation is a costly process and is usually used for simple shapes. Injection molded parts produce the best dimensional accuracy, due to their high green density and uniformity which result from the use of an organic binder. Injection molded parts, however, are confined to small sizes for efficient removal of the binder. Other problems associated with isostatic pressing include distortion due to gravity and high cost of HIP equipment designed for high pressures.
In the process of the present invention, high uniformity and density of starting green ceramic sheets (no binder removal problems), use of a simple shaped die, and pressures in the range 600-1,000 Mpa from a simple axial press can lead to improved material properties as well as to increased size and shape complexity capability at a lower cost.
(b) Some applications require a gradual or sudden change in properties such as thermal conductivity, thermal expansion, wear resistance, electrical conductivity as a function of distance from part surface. Variation of properties cannot be produced by casting, forging, or extrusion. Lengthy secondary processing steps or long powder metallurgical approaches conventionally must be utilized.
By way of contrast, the process of the present invention is ideally suited to produce gradient property and multi-layer structures.
(c) Conventionally, internal channels and cavities are formed in a metal or ceramic part by machining and brazing or diffusion bonding two halves of the part. These high-temperature joining processes can detrimentally affect the mechanical and corrosion properties of the part, and increase its cost of fabrication.
(d) At present, near net-shape consolidation of complex shaped parts from powder metals continues to involve costly processes. Hot isostatic pressing (HIP) is one such process. As is well known in the art, in the HIP process, a powder part or compacted powder is subjected, at elevated temperatures, to equal pressure from all directions, the pressure being transmitted by a pressurizing inert gas, usually argon. Typical conditions of the HIP process range from 20 to 300 MPa. pressure (approximately 100 Mpa. being the average), and 480.degree. C. to 1700.degree. C. temperature. The HIP temperature depends greatly on the nature of the metal alloy being consolidated. A review of the state-of-the-art of HIP processing, as applied to metal powders, is given by Peter E. Price and Steven P. Kohler in "Hot Isostatic Pressing of Metal Powders", Metals Handbook, 9.sup.th Edition, Vol. 7, ASM, Metals Park, Ohio, pp. 419-443. The high cost of pressure vessels and other equipment required for HIP canning of the compact before pressurization (to prevent oxidation and gaseous penetration of the consolidated product), the relatively long cycle time, and other factors, make HIP, overall, a costly process. Because of these and other disadvantages associated with HIP, several alternatives to the HIP process have emerged during recent years. Three of these alternatives, the CERACON process, Rapid Omnidirectional Compaction (ROC), and the STAMP process are described by Lynn Ferguson in an article titled "Emerging Alternatives to Hot Isostatic Pressing", International Journal of Powder Metallurgy and Powder Technology, Vol. 21(3), 1985.
The above noted alternatives to HIP attempt to approximate the isostatic pressure conditions of HIP while using conventional pressing equipment. In these alternatives, the pressurizing gas in the HIP vessel is replaced by a secondary pressing medium, which typically comprises granules of ceramic materials, glass or graphite. In these alternative processes, the advancing top punch of a conventional press pressurizes the secondary pressure medium, which transfers pressure to the workpiece. The result is consolidation of the workpiece under nearly isostatic conditions.
In these processes shape predictability is a major problem. A probable reason for this lies in the fact that, under pressure, plastic deformation of the compacting powder body of the manufactured object occurs at rates and directions which is defined by the elastic/plastic deformation of the surrounding medium. The compressibility ratios of the powder of the object and of the medium are not equal. Therefore, after pressing under a given set of pressure and temperature conditions, the achieved final densities (expressed as percentage of theoretical density) of the two materials are not equal. In light of this, it will be readily understood that if, for example, during pressing in the STAMP process full density is achieved in the powder mass of the manufactured part but not in the pressurizing medium, then the fully densified part being incompressible (its density can no longer be increased) continues to deform in the direction of the weaker and perhaps more openly packed pressurizing medium. This, of course, leads to distortion of the part. Frictional differences between the powder of the part and the surrounding medium also have a distorting effect in the STAMP and like processes, probably for reasons which are similar to the reasoning elucidated regarding compressibility differences.
A net result of the foregoing and related effects is that in the STAMP and like processes of the prior art (which substitute a non-gaseous secondary medium for the pressurizing gas of HIP) variations in the several processing parameters affect the final shape of the consolidated part, so that it is very difficult to hold close tolerances.
None of the powder metallurgy processes utilizing simple dies for consolidation of powders have considered the importance of relative compressibility of the pressurizing medium and of the powder of the manufactured part. An exception to this is a process disclosed in U.S. Pat. No. 4,673,549 by the inventor of the present invention. The disclosure in that patent offers a method comprising the steps of preparing a shaped, preferably ceramic, shell, placing it inside a metal or ceramic can, filling both the shell and space between the shell and the can with powder, out-gassing and sealing the can if necessary, heating the full can, and pressing it to consolidate the powder into a dense form; and separating the densified object within the shell from the densified shapes between the shell and the can.
The present invention provides a green powder laminae consolidation process for the production of near net-shape engineering parts. In the present invention laminae constitute both pressurizing medium and the part, and may be shaped to fit a simple shaped die (usually a cylindrical die). Therefore, laminate shape may be circular. Since there is no pressurizing medium, other than the green compacted powders of laminae, all laminae densify under applied load at the same rate. This eliminates or minimizes any distortion to the part shape during consolidation processing.
The invention differs in some significant ways from manufacturing method described by Feygin in U.S. Pat. No. 4,752,352. The method includes the steps of stacking individually contoured laminations to form a three-dimensional object, and bonding each lamination to the next lamination to form an integral object. While this approach could produce three-dimensional objects from compressed powder laminae, the method allows only processes like brazing and high energy beam (laser, electron-beam, plasma arc, etc.) melting to consolidate the part. Such forming operations taking place in free space exclude the application of high pressures to assist consolidation, since the part would have a tendency to spread in directions perpendicular to the applied load. Thus, the resultant part lacks the strength and chemical uniformity required of some of the demanding engineering applications. In the process provided by the present invention, consolidation takes place under high pressure and at elevated temperature to consolidate the part into a chemically uniform, substantially void free, high-strength part without melting or infiltrating with a molten material as in brazing. The present invention's novelty stems from its use of simple dies to produce complex shapes in fully dense state without significant grain growth, with elimination of need for a secondary material acting as the pressure transmitting medium. The process enables usage of separation compounds to create complex shapes.
The process of this invention is also highly suitable for fabrication of complex-shaped, near net-shape components from monolithic ceramics, Ceramic Matrix Composites (CMC's), Metal Matrix Composites (MMC's), intermetallics, and metals. Because the process is a high-pressure process, consolidation temperatures are used, which are lower than the temperatures required for sintering. This results in improved properties due to smaller grain size, and smaller structural flaws (voids and reaction layers).