COMPRESSIVE SINTERING APPARATUS COMPRISING PROTECTED OPPOSING

RAMS

BACKGROUND OF THE INVENTION

Priority Data

This application claims priority to U.S. Provisional Patent Application serial no. 62/158,326, filed May 7, 2015, the entire contents of which are incorporated herein by reference.

1. Field of the Invention.

[0001] The present invention relates to methods and systems for compressive sintering having protected opposing rams.

2. Description of the Related Art.

[0002] Spark plasma sintering ("SPS"), also referred to as direct current sintering ("DCS") and field assisted sintering ("FAS"), is a pressure assisted high-speed powder consolidation/sintering technology related to hot-pressing ("HP") capable of processing conductive and nonconductive materials. The mechanisms of DCS that provide rapid densification and material property enhancement are still under investigation. However the most commonly accepted mechanisms are rapid heating rates, joule heating of conductive powders and an electric field influence on densification.

[0003] During a typical DCS process, either an ON-OFF DC pulse (typically referred to as SPS) or a constant direct current is applied to a sample contained within a tooling material composed of graphite, metal, ceramic, or a composite to generate joule heat. The heat is transferred to the sample by conduction, and, if the sample is conductive, electrical current can flow through it directly generating joule heat within the sample itself.

[0004] DCS's operational or "monitored" temperatures (200°C-2400°C.) are commonly 200°C to 500°C lower than with conventional sintering, classifying DCS as a low temperature sintering technology. Material processing (pressure and temperature rise and hold time) is typically completed in short periods of approximately five to twenty -five minutes. The relatively low temperatures combined with fast processing times ensure tight control over grain growth and microstructure, enhancing material properties directly related to microstructure, such as strength, toughness, and electrical, thermal, and optical properties.

[0005] Typically, DCS systems involve the use opposing rams to contact and compress a sinterable material contained within a die cavity. However, current DCS, SPS and HP furnaces show significant wear on the pressing faces of the rams due to constant carbide reaction between graphite tooling and metal ram bodies in addition to overheating due to low pressure conditions, which increases contact resistance. Removal of the worn rams for refinishing or replacement is a labor intensive process, especially for larger pressing systems. In addition, the damage layer (carbide reaction zone) on the surface typically has a depth of from about 0.03-0.04", which limits the number of times the rams can be turned down to create a fresh surface without impacting performance.

[0006] Thus, while DCS, SPS and HP methods and systems are known, there is a need to provide a compressive sintering apparatus comprising components that are resistant to wear and damage without significant cost or complexity. This is a significant challenge particularly for DCS and SPS systems since high electric currents must pass through the parts with minimal impact on the component and operation of the system as a whole.

SUMMARY OF THE INVENTION

[0007] The present invention relates to an apparatus for compressively sintering a material and forming a sintered product. The compressive sintering apparatus comprises a die set and a vacuum chamber into which the die set is placed. The die set comprises a die casing and opposing rams forming a die cavity loaded with material to be sintered and is configured to compress the material during sintering. At least one of the opposing rams comprises a surface protection layer in contact with the material to be sintered. The present invention further relates to the die set and to a method of compressively sintering. [0008] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIG 1-3 show various embodiments of the method and apparatus of the present invention while FIG 4 shows a slug design used for measuring component performance.

DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention relates to a method and system/apparatus for sintering materials under compression.

[0011] The compressive sintering apparatus of the present invention, which can be, for example, a hot press, a spark plasma sintering (SPS) system, or a direct current sintering (DCS) system, involves the use of pressure and high temperature to convert a material to be sintered, especially in particulate or powder (fine particulate) form, to a higher density product. SPS and DCS processes are similar, with SPS using a pulsed direct current to generate heat and DCS using a non-pulsed direct current. Hot pressing and DCS differ in how and where the heat is generated. For example, DCS is a pressure assisted direct current heated sintering process that utilizes uniaxial force and direct current to consolidate powder material. Specifically, the application of DC voltage and current between powder material particles creates localized heating within a conductive powder compact and within the conductive die assembly (die set). Due to heat being generated within the die set and potentially within the powder, high heating rates are achievable, in contrast to conventional hot pressing where heat must be transferred into the die set from the exterior by radiant heating elements. During direct current sintering, heat is generated in and around the sample, rapidly heating it and limiting particle/grain growth due to the speed of the process. The entire process - from powder to finished bulk sample - is completed quickly, with high uniformity and without changing the particles' characteristics, specifically grain size and microstructure. In addition to sintering the same apparatus can be applied to diffusion bonding, and a heat treating type process where no sintering takes place.

[0012] In a DCS process, it is believed that the electrical current flowing between particles can assist in removing fine impurities and gases on and between the surfaces of the particles due to dielectric breakdown of surface oxides and local heating. In addition, the higher heating rates achievable allow the fine powders to be heated to high temperatures before grain coarsening can occur, allowing the powder to retain a high surface area to contribute to the sintering process, which progresses quickly.

[0013] Force (pressure) also plays an important and predictable role in curbing particulate growth and influencing overall densities in SPS and DCS systems. For example, force multiplies diffusion throughout the sample as the material moves under pressure, especially during early sintering stages. Both too much and too little pressure can negatively influence the process. In large samples where high density is required, force is commonly increased in stages to enhance out-gassing at low temperatures and sintering diffusion at higher temperatures. Accordingly, accurate manipulation of force can enhance the process.

[0014] In order to provide the proper pressure during sintering, the compressive sintering systems of the present invention use opposing rams, particularly a pair of rams, that compress material contained within a die during sintering. Any ram design or type known in the art can be used, including, for example, a liquid cooled metallic pressing ram, and the rams can be made of any material capable of withstanding conditions of compressive sintering, including, for example, steel, stainless-steel, a copper-based, a super alloy or a composite. The opposing rams have at least one surface in contact with the material in the die during sintering.

[0015] In the present invention, in order to prevent thermal and/or chemical degradation and damage of the rams over time, a protective layer is provided on the contacting surface of at least one of the rams, and preferably, on both opposing rams. This layer can be, for example, metallic, carbon, ceramic or a composite thereof and creates a barrier from the sintering material at high temperature, thereby preventing damage or wear to the metallic ram. . [0016] The surface protection layer can vary in thickness and geometry depending, for example, on the size of the ram, the material to be sintered, and the sintering conditions. For example, the layer may have a thickness of from about 0.1 inches to about 2 inches, including about 0.2 inches to about 1 inch and 0.25 inches to about 0.75 inches. In addition, the surface protection layer may cover the entire ram surface in contact with the material in the die or can cover a portion of the surface, particularly the center portion which typically experiences higher temperatures. The protective layer may also be of a segmented design allowing large faces to be covered using multiple fitted pieces. Furthermore, the surface protection layer can be a coating applied to the ram surface or can be a separate layer of material attached or bonded to the ram surface, such as a faceplate or end cap. Preferably the surface protection layer is replaceable and can be removed as necessary with another one put in its place with minimal labor and machine downtime. Additional optional layers may also be included. For example, an optional intermediate layer can be used at the interface of the surface protection layer and the ram to provide a diffusion barrier to prevent, for example, carbon diffusion into the ram material. This diffusion barrier can be a thin metallic or ceramic layer resistant to carbon diffusion formation, such as, for example, Ni, Cu, Nb, Mo, Ti, TiN, TiB ₂ , or Ta. The optional intermediate layer is preferably thin compared to the surface protection layer and can be applied as a separate sheet or as coating.

[0017] Thus, the compression sintering apparatus of the present invention comprises opposing rams, wherein at least one of the rams comprises a surface protection layer, such as an attached faceplate, in contact with the material to be sintered. Specific embodiments are shown in FIG 1, FIG 2, and FIG 3. However, it should be apparent to those skilled in the art that this is merely illustrative in nature and not limiting, being presented by way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the present invention. In addition, those skilled in the art should appreciate that the specific conditions and configurations are exemplary and that actual conditions and configurations will depend on the specific system. Those skilled in the art will also be able to recognize and identify equivalents to the specific elements shown, using no more than routine experimentation. [0018] As shown in FIG 1, sintering apparatus 100 includes vacuum chamber 102 located within load frame 104 and further includes observation window 106 and temperature measurement device 108, both of which are incorporated into vacuum chamber 102. The material to be sintered (typically a powder material), is loaded into die set 111 and is placed within vacuum chamber 102 of sintering apparatus 100 wherein the SPS process is performed. More specifically, as shown, die set 111 includes die casing 112 and two opposing rams, lower ram 120 and upper ram 122, forming die cavity 110 in which the material to be sintered is placed. Sintering apparatus 100 further includes hydraulic power unit 116 and hydraulic press cylinder 118. The hydraulic power unit provides power to the hydraulic press cylinder, which in turn is used to move the lower ram and the upper ram up and down to manipulate the mechanical force (or pressure) applied, thereby compressing the sinterable material during the process. The force may be measured and monitored, such as with a load cell. In addition, a DC power supply 114 provides the necessary electric current within vacuum chamber 102 during the compression. As shown, sintering apparatus 100 also includes vacuum pump 124, which allows the apparatus to operate under negative atmospheric pressure. Gas 126 can also be injected into vacuum chamber 102 if desired during the process. Central control system 128 can be used to control the different aspects of the sintering apparatus during use. For example, the control system can be used to control the DC power supply, the hydraulic power unit, the vacuum pump, as well as to control the amount of any inert gas introduced to the vacuum chamber during use.

[0019] As shown in FIG 1, both of the opposing rams comprise surface protection layers. In particular, lower ram 120 comprises lower faceplate 140 and upper ram 122 comprises upper faceplate 142. A more detailed view of upper ram 122 is specifically shown in FIG 2. Thus, upper faceplate 142 is attached to upper ram 122 using a plurality of fasteners, bolts or screws. FIG 3 shows a detailed view of upper faceplate 142.

[0020] As a specific example, a faceplate was prepared from a carbon-carbon composite material, which was found to have a significantly higher electrical resistivity compared to metallic based systems but improved high temperature strength. To test the suitability of such a faceplate material, an 80mm slug with no thermal insulation was used to achieve high peak current levels (~7,800A). The slug assembly, shown in FIG 4, was heated to 1450°C at 50°C/min and held for 5 min at temperature.

[0021] The operational temperature for the faceplate was measured. The carbon-carbon composite faceplate was found to reach a peak temperature of 720°C and generated a peak ram temperature of 415°C, due to its higher electrical resistivity resulting in more joule heating. However this carbon-carbon composite does not have a yield point in a classical sense, and it can be used up to these temperatures and higher without consequence.

[0022] Experiments were also run using 347 stainless steel and Iconel 600 as the faceplate material. Both of these metal plates performed similarly to the carbon-carbon composite, with plate temperatures of ~575°C. The Inconel 600 plate was found to have a slightly higher temperature due to a lower thermal conductivity and higher electrical resistivity.

[0023] Current usage was also measured and found to be identical for the three materials studied, taking -7800A to heat the slug to 1450°C. However, the voltage differed significantly for the metal based materials compared to the carbon-carbon composite plate, with the metal systems having lower voltage than the carbon-carbon composite faceplate system. Power off free cooling of the sample was found to be nearly identical for all of the faceplate systems studied.

[0024] The performance of the carbon-carbon composite faceplates was tested under true operational conditions using a standard 40mm die assembly. The test conditions included a heating rate of 100°C/min to 2000°C for a 5 min hold under 20MPa of pressure. In addition, 4 layers of radial felt and 1 layer of felt on top and bottom was used for insulation. The results showed that a carbon-carbon composite faceplate used as a surface protection layer for a ram assembly had excellent operational characteristics under standard operational conditions. The peak ram temperature reached was ~300°C, well below the 450°C operational limit for the material, and the carbon-carbon composite faceplate temperatures did not exceed 600°C.

[0025] As a comparative example, testing was also conducted with a 316 SS faceplate, using the same set up and conditions shown in FIG 4. However, it was found that, after the faceplate temperatures reached >300°C, the stress in the faceplate quickly exceeded the yield strength of the material. This resulted in warping of the faceplate, with the edges of the plate bowing towards the sintering material and forming a dish in the center with a depth on the order of 0.020-0.030". Similar results were found with a 321H SS and 347 SS faceplate, showing similar warping occurring with the central dish depth in the 0.080" range, as well as with a higher grade material (Inconel 600), also resulting in similar warping at the same temperature ranges as the stainless steels. Thus, while these materials may be useful for different sintering conditions, a carbon-carbon composite surface protection layer is preferred. Degradation of the ram pressing face would be minimized, especially during long term use, which would be expected to allow for easy end user replacement in the event of catastrophic damage to the ram faceplate.

[0026] The foregoing description of preferred embodiments of the present invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings, or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.