Patent Application
20230286045
Embodiments of the disclosure relate generally to a material joining process for the fabrication of compact heat exchangers. More particularly, embodiments of the disclosure relate to methods of joining (e.g., diffusion welding, diffusion bonding) materials to form an article and to articles including the article.
Many of today's high-performance technologies (e.g., nuclear reactors, spacecraft, concentrated solar plants and hydrogen cells) require advanced materials. The advanced materials are made of metals and/or ceramics that can withstand extreme (e.g., temperature, environmental) conditions or meet exacting specifications. Compact heat exchangers for nuclear applications use advanced materials to withstand the extreme environment in which they operate.
Diffusion welding is a technique to join advanced materials for the fabrication of compact heat exchangers. Diffusion welding is a solid-state joining technique where atomic diffusion across contacting surfaces forms a bond between components. This is induced by high temperatures and pressures. Diffusion welding is conventionally achieved by hot pressing sheets of material where heat and pressure are simultaneously applied for long periods of time to join mating surfaces of the materials. INCONEL® 617 is a metal alloy that has been investigated for use as an advanced material because of its material properties. The desirable material properties of INCONEL® 617 result in it being a preeminent candidate for the fabrication of compact heat exchangers for nuclear applications. There have been multiple endeavors to diffusion weld INCONEL® 617. Although fully bonded interfaces in conventional diffusion welded INCONEL® 617 have been achieved, grain boundary migration across the bonding interface is hindered by extensive precipitation at the bonding interface. Bonds of this nature have been observed to have degraded elevated-temperature mechanical properties compared to the wrought product form of INCONEL® 617.
As is known in the art, diffusion welding is not limited to joining materials that are nickel based. Diffusion welding is capable of joining similar and dissimilar metals, such as titanium alloy/stainless steel, nickel alloy 800H, nickel alloy GH4099, Ti/Al, and stainless steel/copper. For fabricating compact heat exchangers, a number of sheets with defined cooling recesses are bonded via diffusion welding. Optimized diffusion welding parameters can produce a microstructure that has metallurgical continuity and achieve similar properties as the material in the wrought-product form.
Many endeavors in diffusion welding of INCONEL® 617 have used hot pressing; these are well known in the art. The microstructure and high-temperature mechanical properties of INCONEL® 617, which was hot-pressed at 1150° C. for 2 hours under a uniaxial pressure of 14.7 MPa, have been studied. The results showed very limited grain boundary migration across the interface. This poor GB migration was often reported in diffusion welding of INCONEL® 617 using hot pressing with different temperature, hold time, and pressure. Such unsatisfactory GB migration was often attributed to the extensive chromium-rich carbides and aluminum-rich oxides along the interface, which inhibited atomic diffusion over the interface. Microstructural discontinuities at the interface of the two materials are detrimental to the mechanical properties of the joined material. This is evidenced by the noticeable reduction of rupture strength of the diffusion weld compared to that of the base metal. Due to the extensive precipitates and limited GB migration, hot-pressed INCONEL® 617 was found to have significantly reduced creep performance with rupture at the interface. A 2.5 μm thick pure nickel interlayer has been used in the prior art to bond INCONEL® 617 plates. Nevertheless, microstructural discontinuities were still seen at the interface with no noticeable grain boundary migration. In the prior art, the nickel interlayer also caused secondary phase particles and pores at the interface, which weakened the bonding strength of the weld. Post-weld heat treatment at a certain temperature may achieve some grain boundary migration, but this process has limitations such as inefficiency, geometric distortion, and oxidation for large diffusion-welded stacks. The embodiments of the disclosure seek to remedy the above deficiencies of diffusion welding.
A method of forming an article is disclosed and comprises placing a first material and a second material in a die of a direct current sintering apparatus. The second material directly contacts the first material. An electric current and pressure are applied to the first material and the second material to form an article.
An additional method of forming an article is disclosed and comprises placing a nickel-based material in direct contact with one or more other nickel-based materials to form a stack of nickel-based materials. An electric current and pressure are applied to the stack of nickel-based materials to join the nickel-based material and the one or more other nickel-based materials.
An article is also disclosed and comprises a first material comprising a first nickel alloy. A second material comprises a second nickel alloy and is diffusion bonded to the first material. An interface between the first material and the second material is substantially free of voids and cracks.
Illustrations presented herein are not meant to be actual views of any particular material, component, or system, but are merely idealized representations that are employed to describe embodiments of the disclosure.
The following description provides specific details, such as material types, dimensions, and processing conditions in order to provide a thorough description of embodiments of the disclosure. However, a person of ordinary skill in the art will understand that the embodiments of the disclosure may be practiced without employing these specific details. Indeed, the embodiments of the disclosure may be practiced in conjunction with conventional fabrication techniques employed in the industry. In addition, the description provided below does not form a complete process flow, system, or method for forming an article for the fabrication of a compact heat exchanger for nuclear applications. Only those process acts and structures necessary to understand the embodiments of the disclosure are described in detail below. Additional acts to form a compact heat exchanger may be performed by conventional techniques. Further, any drawings accompanying the present application are for illustrative purposes only and, thus, are not drawn to scale. Additionally, elements common between figures may retain the same numerical designation.
According to embodiments described herein, a material joining process for forming an article may be utilized to fabricate the article configured to withstand extreme conditions or meet exacting specifications, such as compact heat exchangers, or similar devices. The material joining process includes a diffusion welding (e.g., diffusion bonding) process. Diffusion welding may be accomplished by a technique, such as, spark plasma sintering (SPS) or electric-field-assisted sintering (EFAS). Specifically, the material joining process includes passing a high current density (e.g., electric current) between adjacent materials and applying pressure to form the article. The article may exhibit equivalent or improved material properties, such as creep, elevated-temperature fatigue, and creep-fatigue, to material properties of the base material (e.g., base-metal, wrought product form).
Each sheet of material may exhibit a length of from about 14.14 mm to about 283 mm, and a width of from about 14.14 mm to about 283 mm. For example, the length of the material may be from about 20 mm to about 220 mm in length, such as from about 50 mm to about 200 mm in length, from about 80 mm to about 150 mm in length, or from about 100 mm to about 200 mm in length. The width of the material may be from about 20 mm to about 220 mm, such as from about 50 mm to about 200 mm in width, from about 80 mm to about 150 mm in width, or from about 100 mm to about 200 mm in width. The sheet of material may exhibit a thickness of from about 1.25 mm to about 1.70 mm. However, a greater material thickness may be used if the electric current is increased. Furthermore, if the sheet of material exhibits a round shape, each sheet may comprise a diameter of from about 20 mm to about 220 mm, such as a diameter of 283 mm and a thickness of 1.35 mm.
With combined reference to
The material compositions of the first 202 and second 206 materials may be selected based on a difference in melting points between the two materials. For example, the melting point of the first material may be substantially the same as the second material, with the material compositions of the first 202 and second 206 materials being different than one another. Alternatively, the first material and the second material may have a difference in melting point of less than about 50° C., such as less than about 40° C., less than about 30° C., or less than about 20° C. By way of example only, the melting point of the first material 202 may be in the range of about 1300° C. to about 1400° C., such as from about 1320° C. to about 1390° C., or from about 1330° C. to about 1380° C. and the melting point of the second material 206 may be in the range of from about 1300° C. to about 1400° C., such as from about 1320° C. to about 1390° C., or from about 1330° C. to about 1380° C. To modify the thermoelectric properties of the materials 202 and 206, an additional barrier 205 may be placed between the first 202 material and the die 204. An additional barrier 205 may be placed between the second material 206 and the die 204. The barrier 205 may be a carbon-carbon composite plate or a tantalum foil.
The process 100 shown in
The electric current applied to the punch (e.g., upper punch 210, lower punch 212) may range from about 1240 amps (A) to about 50,000 A, such as from about 1240 A to about 48,000 A, from about 1300 A to about 46,000 A, from about 1325 A to about 42,000 A, from about 10,000 A to about 50,000 A, from about 20,000 A to about 50,000 A, from about 30,000 A to about 50,000 A, from about 40,000 A to about 50,000 A, or from about 45,000 A to about 50,000 A. The electrical current may be selected depending on the dimensions and other properties of the first and second materials 202, 206. The magnitude of electric current applied corresponds to a fabrication temperature during the material joining process. The fabrication temperature may include one or more of a temperature of a punch material, a temperature of a die material, a temperature of the first material 202, a temperature of the second material 206, or a temperature of the barrier 205. The fabrication temperature generated by the applied electric current may depend on the resistivity and the thickness of the material of the punch, the geometry of the punch (e.g., upper punch 210, lower punch 212), the geometry of the die 204, the material of the die 204, and the material compositions of the first 202 and second 206 materials. For example, a stack of 3 sheets of material, all having the same composition and through which an electric current of 1240 A, 1322 A and 1377 A are applied may generate a fabrication temperature of 1050° C., 1100° C., and 1150° C., respectively. Specifically, an electric current 1377A may generate a fabrication temperature of 1150° C. using a punch defined by a cylindrical shape.
The electric current is directly dependent on the desired fabrication temperature, in addition to the material properties listed above, such as the geometry of the punch, the barrier, and the chemical composition of the materials being joined. In some embodiments, the fabrication temperature may be from about 1050° C. to about 1200° C., such as from about 1100° C. to about 1200° C., from about 1100° C. to about 1190° C., or from about 1100° C. to about 1180° C. In further embodiments, the article is fabricated at a temperature of about 1150° C. It is noted that a temperature of the first material 202 and the second material 206 may differ from (e.g., may be less than, may be greater than) the fabrication temperature during the material joining process. By way of non-limiting example, the fabrication temperature may be about 1150° C., while the temperature of the first material 202 and the second material 206 may be about 1200° C., as shown in
While
Additional processing parameters, such as bonding atmosphere (e.g., such as a vacuum, an argon atmosphere, a helium atmosphere), heating rate, and surface finish may be selected to achieve the desired material properties of the article 215. For example, the heating rate may be in a range of about 1° C. per minute (° C./min) to about 300° C. ° C./min, such as from about 10° C. to about 250° C., from about 50° C. to about 200° C., or from about 75° C. to about 175° C. The heating may be monitored by a pyrometer 226 external to the die 204. The surfaces of each of the first material 202 and the second material 206 may be processed to exhibit substantially smooth and planar surfaces. The process of preparing 101 the surface of the materials may include polishing (e.g., abrasive polishing) the surface of the materials. By way of example only, silicon carbide (SiC) abrasive papers, such as SiC abrasive papers ranging from about 240 grit to about 1200 grit, may be used. Alternatively, the preparation 101 of the surfaces may include wire brushing, sand blasting, emery cloth polishing, or using degreasers, nitric acid, sodium hydroxide, or hydrofluoric acid to clean and decontaminate the first material 202 and the second material 206. The preparation 101 may further include cleaning the surface to be bonded with a solvent, such as with ethanol, distilled water, or acetone. A lap may be used to smooth the surfaces of the first material 202 and the second material 206. The preparation 101 may further include using an ultrasonic agitator bath to prepare the substantially smooth and planar surfaces. The duration of conducting the ultrasonic agitator bath may be from about 5 minutes to about 10 minutes.
The application of electric current and pressure to the apparatus 200 containing the first material 202 and the second material 206, for a desired hold time, forms the article 215. The application of both electric current and pressure may be controlled by the system controller 220. In preparation for the application of the electric current, and after the first 202 and second 206 materials are placed in the die 204, the apparatus 200 may be evacuated and back filled with argon, helium, or another inert gas. The evacuated pressure may be in the rage of from about 1×10−2 to about 3×10−2 Torr. The current and pressure are applied to the die 204. After applying the electric current 106 and applying the pressure 108, the article 215 is removed 110 from the die 204 of the apparatus 200. The article 215 may be cooled by the cooling system 228 before removal or may be allowed to cool slowly. An optional post-forming treatment (e.g., a heat treatment followed by a quenching or air cooling) 112 may be conducted on the article 215 to form a treated article. The post-forming treatment 112 may include heating and cooling of the article 215 to achieve the desired properties of the treated article. The article 215 may be heated (e.g., solution-annealed) to a temperature of from about 1050° C. to about 1200° C., such as from about 1100° C. to about 1200° C. In some embodiments, the article 215 is heated to a temperature of about 1150° C. The post-forming treatment 112 may be conducted for a duration of from about 6 hours to about 12 hours at the chosen temperature. After the post-forming treatment 112, the article is cooled, such as furnace cooled or quenched (e.g., water quenched or oil quenched).
The article 215 formed by the process 100 described above may exhibit substantially the same or improved material properties, such as creep, elevated-temperature fatigue, and creep-fatigue, as the material properties of the wrought-product form. Additional acts to form a compact heat exchanger for nuclear applications from the article 215 may be performed by conventional techniques. It is believed that the material joining process 100 may overcome the challenges of a conventionally diffusion welded INCONEL® 617, while also significantly decreasing energy usage. By using the material joining process (e.g., SPS, EFAS) of the disclosure, the article 215 may be formed quickly and efficiently due to the Joule heating of the first and second materials 202, 206 and a shorter processing time. The resulting article 215 is also more capable of maintaining the original material properties in the wrought product form. Maintaining or improving the properties of the wrought-product form is desirable because of the often-stringent performance requirements of the compact heat exchangers, such as gas-cooled intermediate heat exchangers for nuclear applications.
During the material joining process, according to embodiments of the disclosure, a material boundary (e.g., the contact surface 208, an interface) between the first material 202 and the second material 206 may be improved.
Without being bound by any theory, it is believed that the application of the high current density may increase vacancy defect migration and, thus, improve the diffusion coefficient of the joined materials. The vacancy defect migration may occur at a higher rate when assisted by high current density. The higher amount of vacancy defect migration may lead to a greater amount of grain boundary migration, as shown schematically in
The following examples serve to explain embodiments of the present invention in more detail. These examples are not to be construed as being exhaustive or exclusive as to the scope of this invention.
Diffusion Welding (DW) of Alloy 617 using an EFAS process was conducted using a Direct Current Sintering Furnace DCS-5 from Thermal Technology, LLC. (USA). The chemical composition of the Alloy 617 sheets is shown below in Table 1. The Alloy 617 sheets in solution-annealed condition were utilized as the starting material in this study. The Alloy 617 sheets (heat XX6083UK) with a thickness of 1.6 mm were procured from High Temp Metals, Inc. (USA). The sheets were sectioned to 14.14 mm×14.14 mm (diagonal length of 20 mm) coupons. Both surfaces of the square coupons were mechanically ground using SiC abrasive papers up to 1200 grit to achieve flat and parallel surfaces as well as to remove surface oxides, defects, and contaminations. The prepared sheets were raised with distilled water, cleaned with acetone in an ultrasonic bath for 10 minutes, raised with ethanol, and finally air-dried. The surface roughness of three random sheets was measured using a Veeco profilometer. Measurements showed a peak Ra of 0.029 μm and Rz of 0.97 μm. Three coupons were stacked together and loaded into 20-mm inner diameter graphite die. The square sample was smaller than the die cavity and therefore, allowed unconstrained compression during the DW (material flow in the direction perpendicular to the applied force due to creep and/or yielding at the bonding temperature). Graphite foils with a thickness of 0.127 mm were placed in the contact surfaces of the die assembly for easy removal of the sample after the experiment.
The EFAS process was conducted at the temperatures of 1050° C., 1100° C. and 1150° C., at a pressure of 10 MPa, 20 MPa and 30 MPa, and hold time of 10 minutes, 30 minutes, and 90 minutes. The total electric current flowing through the die assembly was 1240 A, 1322 A, and 1377 A for the temperature of 1050° C., 1100° C., and 1150° C., respectively. The current passing through the sample, however, depended on several factors including the material properties, punch and die geometries, and sample size. The process parameters are shown in Table 2.
The EFAS process was compared to a conventional hot pressing process, which parameters are also in Table 2. The comparative experiments were conducted using an Oxy-Gon hot press furnace in an argon environment. During hot pressing, the sample in the graphite die was subjected to uniaxial pressure and elevated temperature at the same time. The sample was heated by conventional resistive heating elements in the hot pressing furnace while compressed by upper and lower hydraulic rams. No electric current passed through the sample during hot pressing (HP). The hot pressing was performed under pressure of 20 MPa and 30 MPa, hold time of 30 min and 90 min, and temperatures of 1150° C. and 1200° C., as listed in Table 2. Due to a temperature gradient in the die assembly during the EFAS process (explained below), the hot pressing experiments were conducted at 1200° C. with a zero-current analog of EFAS performed at 1150° C. (e.g., EFAS-#7 vs. HP-#2, EFAS-#8 vs. HP-#4). The heating rate during hot pressing was set at 100° C./min. Subsequent cooling of the samples was conducted in the hot press chamber with an uncontrolled cooling rate of ˜50° C./min.
During the EFAS process, the electric current initiated from the upper electrode passed through the die as well as a stack of three sheets of Alloy 617, due to the good electrical conductivity of Alloy 617. The primary heating mechanism of the three sheets was Joule heating.
As shown in
During the EFAS method, the temperature was monitored using a pyrometer 726 aimed at a hole on the graphite die. Sample temperature was measured by two type C thermocouples 704 and 702 running through the upper and lower punches.
To characterize the samples produced through the EFAS method, they were sectioned to reveal the diffusion-weld interfaces. The sectioned samples were mounted in Bakelite and mechanically ground and polished following conventional metallography procedures. Vibratory polishing was carried out in 0.02 μm colloidal silica suspension for about 4 hours. The weld interfaces were inspected using a light optical microscope (Keyence VHX6000) and a scanning electron microscope (FEI Quanta FEG 650) equipped with energy-dispersive X-ray spectroscopy and electron-backscattered diffraction. Electron-backscattered diffraction maps (EBSD) were captured with a step size of 2 μm. The EBSD data was post-processed in orientation imagery microscopy (OIM) analysis software. Data with less than a 0.1 confidence index was removed from the data set. The grain size of the samples was measured. Grains were defined as a minimum of 5 pixels. A grain boundary (GB) was defined as a disorientation angle of 5° between neighboring pixels. Grain boundary migration across the interface was quantified as the percent of the interface with grain boundary migration compared to the total length of the interface. Optical micrographs were taken over the entire length of both interfaces and the summation of both interfaces was used to quantify the grain boundary migration. Grain boundaries were revealed with a 10% oxalic electrochemical etch at 2.2 V for 30 seconds. To characterize the fine precipitates of the diffusion-welded Alloy 617, characterization using transmission electron microscopy (TEM) was performed. Lift-out samples for TEM analysis were prepared using the FEI Contra 3D FEG dual-beam focused ion beam (FIB) microscope. TEM imaging and Electron Dispersion Spectroscopy analysis were performed using a FEI Tecnai TF30-FEG STwin TEM.
As shown in
In contrast to article 800a, the article 800b exhibited a substantially homogeneous crystallographic structure in a contacting region (e.g., bonding interface) 810b where the contact surface 806b of the first material 802b was adjacent to (e.g., in direct contact with) the contact surface 808b of the second material 804b. The article 800b was joined at a temperature of 1150° C., heated with Joule heating, and an applied pressure of 20 MPa with a hold time of 90 min. The contacting region 810b of the article 800b was substantially free (e.g., lacks) of voids and was substantially free (e.g., lacks) from cracks. The contacting region 810b was also substantially free from pores (e.g., lacks porosity). The article 800b exhibited a variety of different precipitates at the contacting region 810b but these precipitates were slowed so that grain boundary migration was increased. The composition of these precipitates varied depending on the process parameters used and the location across a width of the specimen. The formation of precipitates, however, depended on the fabrication temperature generated by the electric field. Therefore, the application of electric current and temperature retarded precipitation and achieved significant grain boundary migration across the interface.
A further example in
A summary of the properties of the samples prepared by EFAS and HP are shown below in Table 3.
As seen in Table 3, precipitates were observed at the interface of the two materials in the hot pressing experiments. Because of these precipitates, very limited grain boundary migration was achieved by hot pressing. These precipitates were caused by passivation oxides of Alloy 617. Although the passivation oxides protect Alloy 617 from oxidation and corrosion, these stable surface oxides largely restrict diffusion across the interface during diffusion welding. In contrast to hot pressing, precipitates were not observed in the EFAS sample fabricated at 1150° C. Considering that both the EFAS and hot pressing experiments were conducted at a temperature equivalent to 1150° C., the difference in precipitation between the EFAS and hot pressing samples is most likely a consequence of the applied electric current. Not only is the EFAS environment highly reductive, it has been shown that one of the thermal effects of EFAS is surface cleaning of metals and dielectric breakdown of metal oxides.
For Alloy 617, the above results show the application of electric current impeded precipitation. Since precipitates were not formed along the diffusion-weld interface for the sample fabricated at 1150° C., significant grain boundary migration across the diffusion-weld interface was achieved by the EFAS process. The high current density also enhanced vacancy defect migration and thus improved atom diffusion, which may increase the mechanical strength of the diffusion-welded interfaces. Samples heated at the same temperatures (1150° C.) by the hot pressing process and by the EFAS process did not result in the same diffusion-welded interface. Extensive precipitates were distributed along the interface of the hot-pressed samples, which limited grain boundary migration across the interface. In contrast, the samples exposed to the EFAS process under different temperatures demonstrated the importance of welding at the correct temperature.
Therefore, the impact of an applied electric current during the diffusion welding of Alloy 617 was determined. For the Alloy 617 sheets diffusion-welded using the EFAS process, the applied current had a significant influence on precipitation and grain boundary migration. Coupled with diffusion welding at the correct temperature, the EFAS process achieved superior interface quality over hot pressing based on the following observations: 1) at 1150° C. interfacial precipitates were eliminated by the EFAS process and 2) substantial grain boundary migration (88.5%) was achieved by the EFAS process across the interface compared with 3.6% for the HP process at 1150° C.
While embodiments of the disclosure may be susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and have been described in detail herein. However, it should be understood that the disclosure is not limited to the particular forms disclosed. Rather, the disclosure encompasses all modifications, variations, combinations, and alternatives falling within the scope of the disclosure as defined by the following appended claims and their legal equivalents.
This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 63/269,302, filed Mar. 14, 2022, the disclosure of which is hereby incorporated herein in its entirety by this reference.
This invention was made with government support under Contract Number DE-AC07-05-ID14517 awarded by the United States Department of Energy. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63269302 | Mar 2022 | US |
