Patent Application
20210362228
The present invention relates generally to methods and systems for sintering titanium metal particles together to form a solid part.
The Reduction Expansion Synthesis (RES) concept in broad terms is as follows: chemical radicals, which are released by thermal decomposition in an inert atmosphere, of solid compounds (e.g., urea) can remove oxygen complexes from nearby structures. Traditional RES processes require intimate mixing of the metal precursors with urea or alternative reduction agents. Previously developed processes based on the RES concept include the batch generation of sub-micron metal and metal alloy particles, metal thin film formation, metal part formation from mixtures of metal and metal oxide particles, graphene from graphite oxide, and even stable tin/carbon anodes for batteries.
In all “metal” variants of RES, the key to the secondary step is the reaction between reducing radicals and metal oxide species. It is postulated that the radicals interact with oxygen atoms in metal oxides to create products such as CO2 and H2O that subsequently leave the process reactor as a gas. The heavy metal atoms/metallic clusters produced via the removal of oxygen are not volatile and do not leave the reactor. Instead, based on proper arrangement of materials within the bed, these species migrate, as per the Ostwald Ripening (OR) mechanism, leading to metal particle growth and sintering. The process has been demonstrated to lead to lead to growth and sintering of metal, specifically Ni, Fe and Cr, objects from beds consisting originally of particles of metal and oxide. This leads to the creation of designed solid metal objects, thus this type of RES, RES-Sintered Metals (RES-SM) is a form of metal additive manufacturing.
Prior variants of RES do not work for metal oxides, including titania, which are too stable for reduction by radicals even at 1000° C. The primary method for titanium part creation is the metal powder injection molding (MIM) process, as RES cannot be used to create titanium brown bodies. Many variations on the MIM process have been studied, including the use of titanium hydride as a ‘sintering aid’; however, in all cases both high temperature (>1000° C.) and high pressure, more than 1000 atmospheres are required to create a brown body. Also, the MIM process is multi-step: i) A green body containing a binder (wax) is formed by high pressure injection into a metal mold followed by moderate heating; ii) The binder is removed in a slow heating step (debinding); iii) The metal is sintered at high temperature to create a titanium brown body.
Embodiments in accordance with the invention relate a method for sintering metal particles together to form a solid part. Initially, a mold holding titanium metal particles of a titanium metallic powder is positioned in a chamber and, after flowing a gas mixture comprising hydrogen over the titanium metal particles in the chamber, the chamber is positioned in a furnace that is preheated at a target temperature, where the target temperature is at least a decomposition temperature of titanium hydride. While maintaining the flow of the gas mixture, the titanium metal particles are heated to create a metallic product.
Embodiments in accordance with the invention are best understood by reference to the following detailed description when read in conjunction with the accompanying drawings.
Embodiments in accordance with the invention are further described herein with reference to the drawings.
The following description is provided to enable any person skilled in the art to use the invention and sets forth the best mode contemplated by the inventor for carrying out the invention. Various modifications, however, will remain readily apparent to those skilled in the art, since the principles of the present invention are defined herein specifically to provide a method for hydrogen enhanced atomic transport (HEAT) for sintering metal particles together to form a solid part.
In theory, HEAT sinters metal because it takes place under conditions that favor metal transport via volatile metal hydride. The hydride forms at one location in the bed of metal particles and then decomposes at another spot. The net result is a form of atomic metal transport. Historically known as “Ostwald Ripening,” atomic transport of any type in a bed of particles, or droplets, is known to lead to particle/droplet growth and sintering. Thus, HEAT, which creates a volatile atomic species leads to metal particle growth and sintering via an Ostwald Ripening type process.
Embodiments herein describe a method for additive manufacture of brown titanium parts (titanium open pore metallic foam) from titanium particles. The method is mechanistically related to Reduction Expansion Synthesis-Sintered Metal (RES-SM). At elevated temperatures and ambient pressure a gas mixture containing hydrogen and inert gas is flowed over a titanium particle bed contained in an inert (e.g. graphitic) mold. At relatively low temperatures (>650° C.) and with sufficient time (ca. 4 hours) brown bodies that mimic the shape of the original mold form. These brown bodies are composed of two phases: titanium and titanium hydride.
The general procedure for HEAT is to heat a bed of metal particles in a mold of the desired part shape in a gas including hydrogen, or hydrogen and an inert gas such as argon/nitrogen/helium, etc. The temperature for the process should be near or above the known decomposition temperature of the metal hydride. For example, titanium particles can be sintered together to form a solid at 650° C. or above. This is consistent with the fact that the decomposition temperature of TiH2 decomposes above about 550° C.
Observations support a model that the successful production of brown titanium/titanium hydride metal bodies using HEAT occurs via the following mechanism: Titanium hydride, TiH2 readily forms in a hydrogen containing environment as predicted by thermodynamic phase diagrams. It is anticipated that, as per kinetic theory, the titanium hydride concentration reaches steady state due to a balance between formation and the decomposition process. It is postulated that this species has a relatively high vapor pressure at elevated temperatures, leading to “transport” of this species in molecular form within the bed. Subsequent decomposition leads to titanium deposition/transport.
The precise nature of the titanium hydride transport is not known but could be via a combination of surface and gas diffusion. Transport of material within a particle bed, either surface or gas phase, at the atomic/molecular scale is well-known to lead to Ostwald Ripening (OR). Specifically, OR leads to large particles grown at the expense of small particles. A similar “metal radical” model is used to explain the growth of large metal particles in the gas phase during catalytic etching as well as the enhanced rates of particle growth in supported catalysts under reaction conditions. In the case of the beds to titanium particles treated in this description, the process is postulated to lead to growth in the average particle size via sintering and neck formations between particles. The ultimate product is a titanium brown body.
The development of the HEAT process represents a further evolution of Reduction Expansion Synthesis (RES), a set of technologies based on “reduction” chemistry. RES chemistry in all cases starts with a primary step: thermal decomposition of solids, such as urea, under inert gas. This primary step creates volatile “reducing” radicals that react with metal oxide particles (always the starting material) to reduce them to the metal state. Once the particles are in the metal state, the process can be harnessed to create a wide range of products based on designed secondary reactions.
The inherent chemical limitations of the RES-SM method to only a few metals led to the development of HEAT. Indeed, RES cannot work to create metal objects from oxides, including titania, which are too stable for reduction by radicals even at 1000° C. yet, providing the motivation for finding an RES-like process for metal additive manufacturing of titanium parts. Titanium and titanium alloys are widely used in aerospace, automotive, chemical and biomedical industry because of their great strength, low weight and excellent corrosion resistance. Also, titanium is very expensive, thus “subtractive” manufacture involves a significant cost because cut metal must be reprocessed. Thus, HEAT was developed to create a new “primary step” for titanium sintering. The generation of mobile but short-lived titanium hydride species is postulated to be the primary step in HEAT. The subsequent HEAT secondary steps lead to OR, very similar to that in RES.
In HEAT and RES-SM, the dominant means of metal transport is hypothesized to be via atomic metal species, which leads, via OR, to sintering. HEAT, like RES-SM, creates brown bodies at ambient pressure and at a temperature far below the melting temperature.
Embodiments herein create brown, designed, titanium and titanium alloy metal parts more quickly and with less investment than any commercial process for creating metal parts of designed shape from particles including laser particle bed sintering and related metal additive manufacturing technologies, and metal powder injection molding (MIM). In practice, MIM and RES are potential competitors in the low cost, high throughput “solid metal parts from particles using a mold” market. Additive manufacturing is a far more expensive, slower, but more precise and flexible technology.
An advantage to the HEAT process, similar to the advantages of RES, is that neither high temperature nor pressure is required to create a titanium brown body. Only one step is required: Heat (>650 C) titanium particles in a mold at ambient pressure in a gas containing hydrogen for a few hours.
In one or more embodiments, the precursor material is titanium powder (Sigma Aldrich—Ti powder, 325 mesh, 99.9% metal basis), average particle size (˜40 microns). The weight of the input titanium powder can be 0.5+/−0.1 gins. Examples of key treatment parameters, and apparent bulk bonding for samples is shown in TABLE I.
In block 202, a mold and stand can placed in the center of quartz tube (e.g., a 50 cm×2.5 cm diameter). In block 204, the quartz tube can be flushed with a gas mixture such as UHP argon (Praxair, Salinas, Calif., USA) or a pre-mix gas of argon and 2% hydrogen (Praxair, Salinas, Calif., USA) at ˜50 sccm for a fixed duration (e.g., 30 minutes).
In block 206, the gas flow can be reduced to, for example, ˜10 sccm, and immediately after the gas flow is reduced, the tube can be placed inside a furnace (e.g., Lindburgh-Blue M 24″ single zone) that is pre-heated to a target temperature such that the mold/sample are at the furnace center. In block 208, the furnace is held at a target temperature for a predetermined time (e.g., four hours) while the gas flow is maintained. In block 210 at the completion of the predetermined time, gas flow can be increased to, for example, 50 sccm, and then, the tube can be quickly removed from the furnace.
In block 212, the quartz tube/sample cooled under gas flow at ambient temperature, generally for 30 minutes. In block 214, the tube can be opened, and the sample removed.
Five tools were employed to characterize the solid objects created as described herein. (i) A Rigaku Mini-flex 600 X-ray diffractometer (Rigaku Corporation, Tokyo, Japan) operated at 40 kV and 15 mA with a Cu metal target (1.54 Å Kα line) was used for x-ray diffraction of crystal structure. Data was collected in the 20 range of 100 to 90° at 3-5°/minute, step width 0.02°. Structural and refinement data analysis and were performed using Jade 9. (ii) Micron and sub-micron scale morphology of Ti specimens was studied with a Zeiss Neon 40 scanning electron microscope (ZEISS International, Oberkochen, Germany), where key parameter settings were a 30 μm aperture and an accelerating voltage of 20 kV. (iii) An Archimedes principle device, ‘Ohaus density kit’, was unsuccessfully employed to determine density. Multiple tests suggested that diffusion of water into the pores of the brown bodies was invalidating the results. (iv) Finally, a micrometer with an error of 0.02 mm was used to measure dimensions of the “cylinders.” The values reported are based on average five measurements of thickness and diameter. (v) A model 5892 Instron compressive instrument was used in ambient temperature compression mode (max capacity 100 kN) to determine stress/strain behavior. In these embodiments, the brown body “cylinders” were not confined by any device during compression.
The major, and unique, finding of these embodiments is that the HEAT process, that is low temperature sintering of Ti particles in hydrogen gas, enables brown bodies of Ti/TiH to form at ambient pressure and temperatures as low as 650° C. The key results for sintering conducted in hydrogen forming gas (2 vol % H2, 98% Ar) at ambient pressure: i) the titanium particles did not form solid bodies at temperatures below 650° C., ii) brown bodies that mimicked the shape of the graphite molds formed after heating at 650° C. or higher, iii) The brown bodies were primarily titanium metal, but contained some TiH as well, and iv) although solid density was low (<40%) all brown bodies could tolerate at least 5000 atm of pressure with no visible cracking and little strain. Brown bodies able to tolerate high pressure are henceforth labelled as strong brown bodies (SBB). v) High density metal was found to form during cold compression at lower pressures than normally required for titanium. vi) Density increased with increases in firing temperature.
Samples sintered in argon only (control) were significantly different than those produced in forming gas. In particular: i) ‘Brown bodies’ only appeared to form after heat treatments of 850° C. or higher, ii) The apparent brown bodies were unstable, for example samples produced at 850° C. completely decomposed to powder even after compression at relatively low (ca. 2000 atm) pressure. Samples produced at 950° C. in Ar did not pulverize at pressure, but large, easily visible cracks appeared even at 1000 atm. For this reason, these are labelled weak brown bodies (WBB) iii) The WBB were 100% a single phase of metallic titanium. Thus, control study results indicated that hydrogen gas was a key to the observed low temperature sintering/strong brown body formation observed in HEAT.
Solid cylinders with the same radii as the mold formed above 650° C. for HEAT, and above 850° C. for control processes. At temperatures lower than 650 C, for both HEAT produced (forming gas used) and control process produced (Ar only gas), solid structures appeared to be present, but after even modest shaking only powder was found.
The phases of Ti found after treatment were found to be a function of the gas present during firing. After treatment in argon at all temperatures, only titanium metal is present, and the X-ray diffraction levels are identical to those of the precursor particles. In contrast after HEAT treatment in forming gas, although the spectra is dominated by metal peaks, there is always evidence of TiH phases. For all samples treated in forming gas there are diffraction lines unique to TiH species, as well as distortions in peak shapes, and distortion in the baseline. These features are not observed in the samples fired in Ar.
Employing integrated X-ray powder diffraction software to determine phase fractions from integrated line areas yields an estimate after treatment in forming gas at 650° C. of 5% TiH and 95% Ti metal. After treatment in forming gas at 850° C., embodiments herein yield an approximate composition of 30% TiH and 70% Ti metal. Although the absolute values are only roughly quantitative, the embodiments do provide a valid qualitative indication that the amount of hydride increases with firing temperature. Test sample results are shown below in TABLE II.
Scanning electron microscope (SEM) images support the embodiments of HEAT; however, the evidence is subtle. Unlike the very evident inter-particle necking observed in earlier RES-SM studies of solid, designed shapes formed from metal/metal oxide particle mixes using the RES-SM method, there are only subtle differences observed between samples prepared in Ar/H2 mixes (HEAT process) relative to those created in Argon only (control) under otherwise identical conditions as shown in
Ar/H2 samples 302, 306 using HEAT at 850° C. Ar only samples 304, 308 at 850° C. Ar/H2 HEAT samples 302, 306 show metal flow; “softer” particle edges, evidence of sintering between particles, and examples of stacked layers of 306, characteristic of titanium alloys with multiple phases.
All of the above listed differences between Ar/H2 and Ar only treated samples can be observed in
None of the features of the SEM results are inconsistent with the other findings. First, there is a great deal of void space, in agreement with density studies. Second, XRD results indicate that there are multiple phases present in the HEAT samples. Third, shaking and compression studies indicate that only HEAT samples are held together by metallic bonds throughout. The HEAT samples are stable to shaking and only metallic bonds can explain the plastic deformation observed under high pressure. In contrast, the thermally sintered control samples, produced over a range of temperatures (550-850 C), shatter in a brittle fashion under shaking or compression.
In the absence of a mold, compressed HEAT generated SBB demonstrate the existence of metallic bonding. Brown bodies sintered at 750° C. are modified in shape by high pressure compression (approximately 5000 atm), but only modestly. For example, the diameter of near-cylinders increases by roughly 2.5%. These modest dimensional changes are indicative of metallic bonding holding particles together throughout the sample. In contrast, WBB samples made by sintering in pure argon at 750° C. were found to be brittle. Even after relatively low loading (ca. 1000 atm) the samples completely crumbled. This indicates true metallic bonding did not exist in the control samples.
It is interesting to compare the density versus the compressive force data for the HEAT samples with earlier reports regarding the use of cold compression to create brown bodies from titanium and titanium hydride powders. It should be noted in making the comparison, many earlier reports regard titanium hydride, not pure titanium, and that all prior studies utilized confining hard tool steel solid molds during compression. Hence, it is anticipated that the earlier outcomes shown in
As shown in
All empirical evidence shows the HEAT process leads to the formation of metallic bonded titanium brown bodies of any desired shape (matching the mold) from titanium particles treated at ambient pressure and temperature as low as 650° C. The pressure and temperatures and pressures required to create these brown bodies are significantly lower than those employed in prior work; ca >1000° C., and >2000 atmospheres of pressure. In contrast, brown bodies generated in the control studies, that is samples prepared identically to the HEAT process below 950° C., except for the absence of hydrogen gas, are very brittle. Samples treated in the control fashion at 950 C are less brittle, but still crumble at far lower pressure than that tolerated by HEAT samples produced at only 650 C.
The data suggests a simple model of the mechanism of metallic bonded brown body formation: Hydrogen gas creates a short-lived volatile titanium compound that mobilizes titanium atoms or clusters. The hydrides decompose, releasing hydrogen gas, and metal atom or clusters. The Ti species released in this process re-bond with existing particles, and this eventually leads to particle sintering and the creation of metallic “connections” between existing titanium particles in the bed. The process of sintering described is anticipated by one of the oldest models of particle growth: Ostwald Ripening.
All observations are consistent with this model. First, visual inspections and simple “shaking” tests reveal that only brown bodies produced in the pressure of hydrogen at temperatures greater than 850° C. are mechanically stable. Indeed, the control samples prepared at 850° C. or less are clearly very brittle, even under low loads. Second, XRD studies reveal that some titanium hydride is present in all HEAT prepared samples. This is clear evidence that TiH2 exists in the samples, a finding anticipated by/consistent with the model. Note: Another fact consistent with the model: the Ti/TiH2 phase diagram indicates both metal and hydride phases should coexist at the temperatures employed. Third, SEM reveals the existence of features such as softened edges and evidence of direct particle-particle sintering, anticipated by OR process. The existence of step structures observed in SEM, similar to structures observed in ‘stabilized multi-phase titanium alloys’, is consistent with the XRD results showing the coexistence of metal and hydride phases in the HEAT samples. Fourth, the finding that HEAT prepared samples modestly plastically deform under pressure is clearly an indication that these samples have metallic bonds formed between particles. In contrast, the control samples created at 850° C. or lower completely shatter under pressure, revealing a lack of metallic bonding between particles. This supports the supposition that hydrogen is needed to create metallic bonding.
In addition to the data reported being consistent with the provided model, a number of reports in literature are consistent with the postulate that short lived volatile metal containing species can lead to gross modification of metals. The earliest reports of atoms/radicals restructuring metals were published more than 90 years ago. All evidence suggests the restructuring occurred via metal transport in the form of unstable tin hydride. There are also several reports of the restructuring of platinum due to the action of “radical” species, formed homogenously during the combustion process of both ethylene and hydrogen oxidation. Reportedly, these radicals react with platinum foils, films, catalysts etc. to create very short-lived volatile species. Upon decomposition, new platinum structures form. The net result is gross scale reconstruction over time, at temperature hundreds of degrees below melting, often clearly observed without any magnification.
One alternative model of hydrogen induced titanium sintering is that surface oxide is a barrier to sintering, and hydrogen removes surface oxygen; enabling sintering. In previous studies the surface reduction by hydrogen appears to initiate above 1000° C., a temperature never achieved in this study. In all those studies with pure titanium, sintering is conducted at 1200° C. or even higher, that is 550° C. higher than that required to observe sintering in the present embodiments. The inclusion of TiHx particles is shown to enhance sintering, presumably acting as a hydrogen source for oxygen removal. In sum, it is not clear those results pertain to the work described herein for which there are no TiHx, particles, and sintering is significant even at 650° C.
All data indicates that the HEAT process; specifically treating a mold filled with titanium particles to above 650° C. in a flowing ambient pressure gas containing hydrogen, creates a metallic bonded brown body on the order of 40% metal density. The brown body will also mimic the mold shape. Based on prior work with titanium brown bodies formed using Ti-MIM, it is anticipated that second step of hot isostatic pressing will lead to complete densification.
The HEAT process may have advantages relative to current commercial processes. For example, HEAT creates a titanium brown body in a single step in a soft mold. In contrast, Ti-MIM requires a sequence of steps to create a similar brown body: i) mixing metal and binder, ii) high pressure injection and high pressure molds, iii) moderate temperature green body formation and iv) very slow debinding at 900° C. Laser sintering from particles requires extremely expensive equipment, is slow, and requires significant expertise. Arguably, it is also not true additive manufacturing as those particles not sintered are removed from the bed after each multi-micron scale “level” is complete. The unused particulate material is often more than half of all material employed.
Finally, all data collected is consistent with a simple model: In the presence of hydrogen at temperatures exceeding approximately 650° C., some form of titanium hydride is produced. The vapor pressure is sufficient to carry titanium short distances within the bed before decomposing. Titanium atom/clusters generated by the decomposition are released and these bond with existing particle surfaces. The net result, as anticipated by the Ostwald Ripening model, is sintering and metal bonding between particles.
Embodiments herein are designed for comparison with existing metal “additive manufacturing” technologies, particularly mold-based MIM. In the MIM process, as in true metal additive manufacture by laser sintering, the final part is required to be a fully dense metal object. Alternatively, the “brown” Ti parts created here can be considered “designed” open cell Ti/TiH metal foams of high compressive strength. Cellular metal foams, as discussed elsewhere may have properties for some applications superior to fully dense metal parts. For example in
It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention and it is not intended to be exhaustive or limit the invention to the precise form disclosed. Numerous modifications and alternative arrangements may be devised by those skilled in the art in light of the above teachings without departing from the spirit and scope of the present invention. It is intended that the scope of the invention be defined by the claims appended hereto.
In addition, the previously described versions of the present invention have many advantages, including but not limited to those described above. However, the invention does not require that all advantages and aspects be incorporated into every embodiment of the present invention.
All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted.
This application claims the benefit of U.S. Provisional Application No. 63/027,783, filed May 20, 2020, which is hereby incorporated in its entirety by reference.
| Number | Date | Country | |
|---|---|---|---|
| 63027783 | May 2020 | US |
