Patent Application
20150075744
The present invention generally relates to fabricating structures including metallic glass-based materials using low pressure casting techniques.
Metallic glasses, also known as amorphous alloys, embody a relatively new class of materials that is receiving much interest from the engineering and design communities. Metallic glasses are characterized by their disordered atomic-scale structure in spite of their metallic constituent elements—i.e. whereas conventional metallic materials typically possess a highly ordered atomic structure, metallic glass materials are characterized by their disordered atomic structure. Notably, metallic glasses typically possess a number of useful material properties that can allow them to be implemented as highly effective engineering materials. For example, metallic glasses are generally much harder than conventional metals, and are generally tougher than ceramic materials. They can also be relatively corrosion resistant, and, unlike conventional glass, they can have good electrical conductivity. Importantly, the manufacture of metallic glass materials lends itself to relatively easy processing in certain respects. For example, the manufacture of a metallic glass can be compatible with an injection molding process.
Nonetheless, the manufacture of metallic glasses presents challenges that limit their viability as engineering materials. For example, metallic glasses are typically formed by raising a metallic alloy above its melting temperature, and rapidly cooling the melt to solidify it in a way such that its crystallization is avoided, thereby forming the metallic glass. The first metallic glasses required extraordinary cooling rates, e.g. on the order of 106 K/s, and were thereby limited in the thickness with which they could be formed. Indeed, because of this limitation in thickness, metallic glasses were initially limited to applications that involved coatings. Since then, however, particular alloy compositions that are more resistant to crystallization have been developed, which can thereby form metallic glasses at much lower cooling rates, and can therefore be made to be much thicker (e.g. greater than 1 mm). These metallic glasses that have compositions that can allow them to be made to be thicker are known as ‘bulk metallic glasses’ (“BMGs”).
In addition to the development of BMGs, ‘bulk metallic glass matrix composites’ (BMGMCs) have also been developed. BMGMCs are characterized in that they possess the amorphous structure of BMGs, but they also include crystalline phases of material within the matrix of amorphous structure. For example, the crystalline phases can exist in the form of dendrites. The crystalline phase inclusions can impart a host of favorable materials properties on the bulk material. For example, the crystalline phases can allow the material to have enhanced ductility, compared to where the material is entirely constituted of the amorphous structure. BMGs and BMGMCs can be referred to collectively as BMG-based materials. Similarly, metallic glasses, metallic glasses that include crystalline phase inclusions, BMGs, and BMGMCs can be referred to collectively as metallic glass-based materials or MG-based materials.
Systems and methods in accordance with embodiments of the invention fabricate objects including metallic glass-based materials using low-pressure casting techniques. In one embodiment, a method of fabricating an object that includes a metallic glass-based material includes: introducing molten alloy into a mold cavity defined by a mold using a low enough pressure such that the molten alloy does not conform to features of the mold cavity that are smaller than 100 μm; and cooling the molten alloy such that it solidifies, the solid including a metallic glass-based material.
In another embodiment, the mold cavity is characterized by extrusion symmetry.
In yet another embodiment, the entirety of the solid includes a metallic glass-based material.
In still another embodiment, only some portion less than the entirety of the solid includes a metallic glass-based material.
In still yet another embodiment, cooling jets are used to cool the molten alloy such that it solidifies.
In a further embodiment, introducing the molten alloy into the mold cavity includes using gas to force the molten alloy into the mold cavity.
In a still further embodiment, introducing the molten alloy into the mold cavity includes using at least a partial vacuum to cause a pressure differential that causes the molten alloy to be drawn into the mold cavity.
In a yet further embodiment, introducing the molten alloy into the mold cavity includes using a hydraulic ram to apply pressure to the molten alloy and thereby introduce it into the mold cavity.
In a still yet further embodiment, introducing the molten alloy into the mold cavity includes pouring molten alloy into the mold cavity.
In another embodiment, introducing the molten alloy into the mold cavity further includes using at least a partial vacuum to cause a pressure differential that causes the molten alloy to be drawn into the mold cavity.
In still another embodiment, introducing the molten alloy into the mold cavity further includes using a piston to apply a force to the molten alloy causing the molten alloy to be compelled into the mold cavity.
In yet another embodiment, the mold cavity defines the shape of a gear.
In still yet another embodiment, a method of fabricating an object that includes a metallic glass-based material includes: introducing molten alloy into a mold cavity defined by a mold using a pressure of less than approximately 100 psi; and cooling the molten alloy such that it solidifies, the solid including a metallic glass-based material.
In a further embodiment, the molten alloy is introduced into the mold cavity at a pressure of less than approximately 15 psi.
In a yet further embodiment, the molten alloy is introduced into the mold cavity at a pressure of less than approximately 5 psi.
In a still further embodiment, the mold cavity is characterized by extrusion symmetry.
In a still yet further embodiment, the entirety of the solid includes metallic glass-based material.
In another embodiment, only some portion less than the entirety of the solid includes a metallic glass-based material.
In yet another embodiment, cooling jets are used to cool the molten alloy.
In still another embodiment, introducing the molten alloy into the mold cavity includes using gas to force the molten alloy into the mold cavity.
In still yet another embodiment, introducing the molten alloy into the mold cavity includes using at least a partial vacuum to cause a pressure differential that causes the molten alloy to be drawn into the mold cavity.
In a further embodiment, introducing the molten alloy into the mold cavity includes using a hydraulic ram to apply pressure to the molten alloy and thereby introduce it into the mold cavity.
In a yet further embodiment, introducing the molten alloy into the mold cavity includes pouring molten alloy into the mold cavity.
Turning now to the drawings, systems and methods for fabricating objects including a metallic glass-based material using low pressure casting techniques are illustrated.
While metallic glass-based materials are characterized by a host of desirable materials properties, it has proved to be challenging to economically fabricate objects that include metallic glass-based materials so as to harness their desirable materials properties. For example, although molten metallic glass compositions can be cast into molds to form them into desired shapes, using conventional casting techniques can result in a number of imperfections in the cast part. Thus, for example, in accordance with many conventional casting techniques, molten alloy is forced into a mold cavity at relatively high pressure (e.g. greater than approximately 10,000 psi); as a result, after the material cools, the solidified material may replicate the microscale features that can be unintentionally present in the mold—e.g. the roughness embodied in the surface finish of the mold. For example, if the mold has a rough surface finish, that rough surface can be unintentionally replicated on the solidified material because of the high pressure under which the material is cast. This can be undesirable in a number of respects. For example, a rough surface can be detrimental to the cast part's operation. For instance, where gears are fabricated, a rough surface finish can exacerbate the detrimental effects of ‘wear and tear’ as compared to what the gear would experience if it had a smoother surface finish.
Furthermore, casting molten alloys at high pressures, as is conventionally done, can cause other undesired outcomes. For example, in many instances the solidified material will be so tightly adhered to the mold that it will be difficult to remove. The mechanics of this outcome are generally understood to be as follows: when molten alloy is introduced at high pressure, the molten alloy can be compelled to conform to, and/or intertwine with, the rough features of the surface of the mold such that when the melt solidifies, it interlocks with the mold surface to an extent that makes the removal of the cast part from the mold difficult. In many instances, removing a part cast under high pressure from a mold results in damage to the part, the mold, or both. For example,
To circumvent this outcome, split molds are often used; split molds can facilitate the removal of the cast part from the mold. However, where split molds are used, the pressurized molten alloy often conforms to the parting line, and consequently, the cast part includes the parting line. For example,
Thus, in many embodiments of the invention, molten alloy is cast into molds at low pressures to avoid the aforementioned undesired outcomes, and is thereafter cooled so as to form a casting that includes a metallic glass-based material. For example, in many embodiments, a molten alloy is cast into a mold at a low enough pressure such that the alloy adopts the macroscale geometry of the mold cavity, but does not replicate the microscale features of the mold cavity. This casting method can result in a host of advantages. For instance, the surface finish of the cast part can be largely a function of the surface tension of the molten alloy rather than the surface roughness of the mold. Moreover, because the molten alloy is not being so forcefully compressed against the surface of the mold, as the molten alloy cools—and correspondingly shrinks in volume—it can more easily retract from the surface of the mold. As a result, the solidified part can be more easily removed from the mold. Consequently, split molds do not necessarily have to be used. Further, as can be appreciated, the extent of any post-casting processing to finish the desired part can generally be reduced.
Low-pressure casting techniques are now discussed below in greater detail.
In many embodiments of the invention, low pressure casting techniques are implemented to fabricate structures. As alluded to previously, although metallic glass based materials can be made to possess a host of desirable materials properties, their practicable implementation as a viable engineering material has yet to be fully realized. This is partly due to an incomplete understanding of the materials properties that metallic glass-based materials can be made to possess. For example, although metallic glass-based materials have been used in the construction of microscale gears, progress has been slow in manufacturing such gears at a macroscale. U.S. patent application Ser. No. 13/928,109 entitled “Systems and Methods for Implementing Bulk Metallic Glass-based Macroscale Gears” to Hofmann et al. discuss this problem and discloses a strategy for the viable manufacture of macroscale metallic-glass based gears. The disclosure of U.S. patent application Ser. No. 13/928,109 is hereby incorporated by reference in its entirety, especially as it pertains to the manufacture of macroscale gears. By way of example,
Hofmann et al. further disclose that metallic glass-based materials can be made to be particularly well-suited in the manufacture of compliant mechanisms in U.S. Pat. Pub. No. US 2014/0020794. The disclosure of U.S. Pat. Pub. No. US 2014/0020794 is hereby incorporated by reference in its entirety, especially as it pertains to compliant mechanisms that include metallic glass-based materials. Moreover, Hofmann et al. further disclose that metallic glass-based materials can be made to be particularly well suited in the manufacture of strain wave gears in U.S. Pat. Pub. No. US 2014/0224050. The disclosure of U.S. Pat. Pub. No. US 2014/0020794 is hereby incorporated by reference in its entirety, especially as it pertains to strain wave gears and strain wave gear components that include metallic glass-based materials.
In addition to disclosing metallic glass-based material compositions that more easily lend themselves as viable engineering materials, and particular components that can demonstrate improved performance when fabricated from metallic-glass based materials, Hofmann et al. have further disclosed particular fabrication techniques that can more easily enable any of a variety of geometries to be fabricated from metallic glass-based materials. For example, Hofmann et al. disclose depositing metallic glass-based compositions in a layer-by-layer manner (e.g., akin to additive manufacturing techniques) to build up a desired geometry in U.S. Pat. Pub. No. US 2014/0202595. The disclosure of U.S. Pat. Pub. No. US 2014/0202595 is hereby incorporated by reference in its entirety, especially as it pertains to depositing metallic glass-based material compositions in a layer-by-layer manner. Similarly, Hofmann et al. disclose using ultrasonic consolidation to adjoin ribbons of metallic glass-based material compositions and to thereby build up a desired geometry in U.S. patent application Ser. No. 14/259,608. The disclosure of U.S. patent application Ser. No. 14/259,608 is hereby incorporated by reference in its entirety, especially as it pertains to using ultrasonic consolidation to adjoin ribbons of metallic glass-based material compositions to thereby build up a desired geometry. Hofmann et al. further disclose techniques for coating objects with metallic glass-based materials in U.S. Pat. Pub. No. US 2014/0141164. The disclosure of U.S. Pat. Pub. No. US 2014/0141164 is hereby incorporated by reference in its entirety, especially as it pertains to coating objects with metallic glass-based materials.
Notably, as discussed above, metallic glass-based materials can be implemented using heritage conventional casting techniques. However, as also mentioned above, the heritage casting techniques can result in a number of deficiencies. Accordingly, in many embodiments of the invention, low pressure-casting techniques are implemented that help circumvent the above-identified issues. For example,
As can be appreciated, introducing the molten alloy at low pressures can avoid undesirably replicating the rough surface finish of a mold onto a cast part. Additionally, introducing the molten alloy at low pressure instead of high pressure allows the solidified cast part to be more easily removed from the mold. For example, as discussed above, when molten alloy is introduced at high pressure, it can undesirably intertwine with the surface of the mold, thereby making it difficult to remove from the mold. By contrast, using the low pressure casting techniques described herein, the cast part can be cast so as not to interlock with the mold surface to such an extent that it becomes difficult to remove. Moreover, as the cast part cools, the molten alloy composition can be such that its cooling causes it to shrink in volume, which can allow it to be more easily removed 406 from the mold.
The molten alloy can be cooled 404 using any suitable technique. As can be appreciated, the extent to which the molten alloy develops an amorphous structure is largely a function of the rate that the molten alloy cools. Thus for instance, in many embodiments, cooling jets are used to rapidly cool the molten alloy such that metallic glass forms. Of course, it should be clear that any of a variety of techniques may be used to cool the molten alloy so as to cause the formation of metallic glass. In many embodiments, the molten alloy is cooled so rapidly that the entire casting is characterized by an amorphous structure. In several embodiments, the molten alloy is cooled to an extent such that it only partially forms an amorphous structure.
In many instances the mold in which the molten alloy is cast is characterized by extrusion symmetry. In other words, the mold geometry has a similar cross-section throughout its length. For example, in many embodiments, the mold geometry is cylindrical. In some embodiments, the geometry of the mold cavity defines a rectangular prism. Having extrusion symmetry can allow the cast part to be easily ejected from the mold—e.g. the cast part can be ejected along its longitudinal axis. Note that split molds need not necessarily be used where the mold is characterized by extrusion symmetry.
As can be appreciated, the above described process is compatible with any of a variety of casting techniques. For example,
Similarly,
While several particular casting methods are discussed, it should be clear that the described low pressure casting techniques can be implemented using any of a variety of arrangements. Generally, low pressure casting techniques in accordance with many embodiments of the invention can be implemented using any arrangement that is capable of introducing molten alloy into a mold at a low pressure (e.g. such that the surface tension of the melt largely governs the surface finish of the cast part; such that the melt does not conform to any features within the mold that are longer than 100 μm in length; and/or such that the melt is introduced at a pressure less than approximately 100 psi), and cooling the melt so as to form a metallic glass-based material. The techniques are not limited to implementation by the above-described arrangements.
As can be appreciated, these techniques are versatile and can be used to fabricate any of a variety of geometries in accordance with embodiments of the invention. The casting of gears is particularly well-suited to harness the advantages achieved by low pressure casting techniques, and the casting of gears is now described in greater detail.
The above described techniques are suitable to advantageously fabricate any of a variety of geometries. In many embodiments, the described low pressure casting techniques are used to fabricate gears. For example,
Note that the pressure under which the molten alloy is introduced into the mold can be tuned to obtain the desired geometry in accordance with embodiments of the invention. For example, where more conformity with the mold geometry is desired, a relatively higher pressure can be applied. Contrariwise, where less conformity with the geometry of the mold cavity is desired, the molten alloy can be introduced into the mold at a relatively lower pressure.
It should be clear that the molten alloy can still be made to substantially conform to the geometry of a mold cavity to a desired extent even when cast under low pressure. For example.
Importantly, as mentioned previously, low pressure casting techniques in accordance with many embodiments of the invention can allow the surface finish of the cast part to be made relatively smooth. In general, the surface tension within the melt can facilitate the creation of a smooth finish. By way of example,
The above-described techniques can be implemented using any of a variety of arrangements. For example,
The above-described fabricated gears can be utilized in any of a variety of ways. For example,
More generally, as can be inferred from the above discussion, the above-mentioned concepts can be implemented in a variety of arrangements in accordance with embodiments of the invention. Accordingly, although the present invention has been described in certain specific aspects, many additional modifications and variations would be apparent to those skilled in the art. It is therefore to be understood that the present invention may be practiced otherwise than specifically described. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive.
The current application claims priority to U.S. Provisional Application No. 61/879,820, filed Sep. 19, 2013, the disclosure of which is incorporated herein by reference.
The invention described herein was made in the performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the Contractor has elected to retain title.
| Number | Date | Country | |
|---|---|---|---|
| 61879820 | Sep 2013 | US |
