The disclosure relates generally to molds and processes of using molds to create amorphous metal parts. In particular, the disclosure provides a hybrid mold and molding process that combines the benefits of investment casting with the functional parameters of diecast molding.
Investment casting is a technique for making complex near-to-net or near net shaped parts where the mold is formed around a pattern of wax. Investment casting is often utilized to produce metallic parts having complicated shapes or geometries, often where the shape or geometry is problematic using a diecasting technique.
Amorphous metals are a family of high strength and hardness alloys that provide a number of useful properties. In particular, for example, amorphous alloys display excellent strength-to-weight ratios, resistance to corrosion, environmental durability, as well as high elastic strain limits that approach 2.0% (much higher than other non-amorphous metallic alloys).
Amorphous alloys maintain optimal utility when the alloy exhibits substantially non-crystalline structure. However, manipulation of these alloys in the absence of crystal formation is difficult and requires melted materials to be cooled at a high cooling rate. These cooling rates are not practical for fabrication using investment casting. As such, amorphous alloy composed parts are typically prepared via diecast molding techniques.
Diecasting is a process characterized by injecting molten metal under pressure and vacuum into a mold cavity. Typical mold cavities are formed from steel or other like material and require significant capital costs. Diecasting, however, is limited to less complex part designs and, where amorphous metals are concerned, less mold durability (a significant cost in fabricating amorphous metal parts). Further, where parts made of amorphous metal are concerned, excessive post molding modification is often required, for example, stock may be added to the amorphous metal part which is then removed by machining and polishing. In the end it is very difficult to fabricate near-to-net shaped parts using diecasting where amorphous alloys are concerned.
The present disclosure is provided to overcome the limitations of investment casting, and of die-casting, parts composed of amorphous metals.
A hybrid investment-diecasting mold for fabrication of amorphous alloy parts is described herein. Hybrid investment diecasting molds in accordance with embodiments herein include a thin inner layer useful in investment casting and a thicker outer layer useful in diecasting, the two layers functioning together to form the hybrid mold.
Inner investment casting materials are typically composed of thermal-conductive ceramics such as alumina, silicon, nitride, silicon nitride, and the like, or lower conductivity ceramics like silica or zirconia. This layer provides a mold for investment casting amorphous alloys.
Outer diecasting mold materials typically have high heat capacity and thermal conductivity so as to act as a heat sink during use, and include materials like steel, stainless steel, aluminum, copper and brass. The outer diecasting mold supports the inner investment casting material and acts as a heat sink for the molten amorphous alloy to properly quench.
The hybrid investment-diecasting mold as described herein is integrated with the use of a diecasting machine where the molten amorphous alloy may be injected, under vacuum, to the cavity formed in the hybrid mold. Dissolution of the inner investment layer from the properly quenched amorphous alloy composed part provides a net-to-shape part that requires little or no post fabrication processing.
Methods of part fabrication in accordance with the investment-diecasting mold are also provided herein. A wax print is prepared from a desired part and positioned in an outer diecasting mold leaving a gap between the wax print and outer diecasting mold assembly. Using the appropriate filling conduit, the gap is filled with the appropriate investment ceramic material to prepare a thin, typically about 1 mm to 4 mm thick, investment casting layer on the wax print. After removal of the wax print from the investment diecasting mold, the mold is placed in a diecasting machine and molten amorphous alloy injected. The amorphous alloy part is quenched and the investment diecasting mold removed from its surface. Vibration or pressure wash can be used to dislodge the near-to-net or near net shape amorphous alloy part from the investment diecasting mold. Where appropriate the part can undergo further processing.
In one embodiment, an investment-diecasting mold is described. The investment-diecasting mold has an inner investment casting mold for defining a cavity. The cavity is shaped as a negative imprint for at least a portion of a part. The investment-diecasting mold also has an outer diecasting mold which encases and operatively contacts the inner investment mold.
In some aspects, the inner investment casting mold is composed of a thermally conductive ceramic material, and in some cases, is composed of alumina, silicon, nitride or silicon nitride. In other aspects, the investment casting mold has a thickness of from about 1 mm to about 4 mm.
In other aspects, the outer diecasting mold is composed of a material like steel, aluminum, copper or brass. The outer diecasting mold can have an appropriate mass to act as a heat sink for quenching parts, particularly parts made from amorphous alloy. It is also envisioned that the outer diecasting mold can define one or more cooling conduits for inclusion of water, brine, NaOH or oil.
Another embodiment herein is a method comprising filing a gap formed between a wax print and an outer diecasting mold with an investment casting material; dissolving the wax print such that it leaves a cavity formed by the investment casting material; and injecting molten amorphous alloy into the cavity formed in the investment casting material, thereby forming a desired part out of amorphous alloy in the shape of the wax print. The investment casting material can be dissolved from the part, which is then removed from the diecasting mold.
In some aspects, the method further comprises processing the amorphous alloy part after removal from the outer diecasting mold. The outer diecasting mold should have a sufficient mass and thermal conductivity to quench the desired part, and in some cases, can include cooling conduits for passing a fluid through to quench the desired part.
In other aspects, the method is directed at forming a housing for an electronic device, and in particular, a housing for a mobile phone.
Embodiments also include electronic device, particularly handheld electronic device. Electronic device comprise a housing composed of amorphous alloy, a display positioned within the housing and a cover positioned over the display. The amorphous alloy housing is at least 0.1 mm in thickness and formed to a near-to-net or near net shape by investment-diecasting.
In some aspects, the investment-diecasting is performed with an investment-diecasting mold, and the mold has an inner investment casting mold for defining a cavity, where the cavity is shaped as a negative imprint for the housing; and an outer diecasting mold, where the outer diecasting mold encases and operatively contacts the inner investment casting mold.
In other aspects, the housing is formed of a BMG and the housing is at least 0.5 mm thick. The electronic device can be a wearable electronic device or it can be a mobile phone.
The disclosure will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:
Reference will now be made in detail to representative embodiments illustrated in the accompanying drawings. It should be understood that the following descriptions are not intended to limit the embodiments to one preferred embodiment. To the contrary, it is intended to cover alternatives, modifications, and equivalents as can be included within the spirit and scope of the described embodiments as defined by the appended claims.
The following disclosure relates generally to hybrid, investment-diecasting molds, and to the use of investment-diecasting molds in the creation of amorphous metal composed parts. Embodiments in accordance with the present disclosure combine the benefits of investment casting with the utility of a diecasting machine for fabrication of near-to-net shape amorphous alloy parts.
Embodiments herein utilize the benefit of investment casting, allowing for complex part geometries, including undercuts, with the durability, injection parameters, and thermal conductivity of molds in a diecasting machine. This unique combination of investment casting and diecasting allows for near-to-net or near net shaped amorphous alloy part production, a process not possible using either technique alone. In some aspects, the ability to form a near net shaped part includes parts that have geometric shapes and geometric overhangs. Conventional part fabrication often requires molding a part and post process machining geometric overhangs or openings. These overhangs can require adhering the overhang onto the part in some cases. In other aspects, the post processing not required in embodiments herein can introduce defects into the part due to removal of material or modifications of material not required by the investment-diecasted part.
Embodiments include investment-diecasting molds fabricated of an inner investment casting material and an outer thermally conductive material appropriate for diecasting. In some aspects the inner investment casting material is a layer made of a ceramic shell, while an outer diecasting layer is a mold made of materials having sufficient heat capacity and thermal conductivity to quench the inner ceramic shell during use with amorphous alloys.
The term “investment-diecasting mold” refers to the combination of an inner and outer layer of materials, where the combination allows for high pressure injection, under vacuum, of molten amorphous alloy using a metal piston. In some embodiments the investment-diecasting mold is used at room temperature, or more typically used at below room temperatures, and the resultant amorphous alloy part is prepared at a near-to-net shape.
Compositions of the inner investment casting ceramic shell may include castable variants, or powder with binder, of a high thermally conductive ceramic, for example, alumina, silicon nitride, sialon, silicon carbide, aluminum nitride, tungsten carbide, boron nitride, graphite or combinations thereof. Embodiments herein may also be composed of a lower conductivity ceramic, whether castable, or powder mixed with binder, for example, silica or zirconia (depending on thickness).
Embodiments may also include a high thermal conductivity metal/ceramic powder mixed with a suitable binder to form a high thermal conductivity paint or paste. Example metals in such paints or pastes include copper, aluminum, brass, zinc, steel, stainless steel, nickel, chromium, tungsten, silver, or combinations thereof. Example binders for use in paints or paste embodiments include: sodium silicate, potassium silicate, aluminum phosphate, silica aluminum phosphate, silica, alumina and combinations thereof.
In typical embodiments the inner investment layer is between about 0.5 mm in thickness and about 5 mm in thickness. In other embodiments the inner investment layer is between about 1 mm in thickness and about 4 mm in thickness.
In one typical embodiment, the inner investment casting ceramic shell is composed of a castable alumina cement, in another typical embodiment the inner ceramic shell is composed of aluminum powder mixed with sodium silicate.
Compositions of the outer mold, necessary for diecasting processes (for example, necessary such that the inner shell does not fail (explode) when filled with molten amorphous metal at high pressure) require sufficient heat capacity and thermal conductivity to act as a heat-sink and thereby quench the inner investment casting shell when in use with amorphous metal. Possible outer mold materials include: steel, stainless steel, aluminum, copper, or brass.
Embodiments herein also include fabrication processes or methods for producing parts composed of amorphous metal. Fabrication processes allow for the production of near-to-net shape parts which require limited post-fabrication machining.
An “amorphous alloy” is an alloy having an amorphous content of more than 50% by volume, typically more than 90% by volume and most typically more than 95% by volume. In some aspects an amorphous alloy can have an amorphous content of about 99% or more and up to about 100% by volume. Note that, amorphous by volume means to exhibit a disorderly atomic scale or arrangement as compared to most metals, which are highly ordered in atomic structure. Materials in which such a disordered structure is produced directly from the liquid state during cooling are often referred to as “glasses,” hence the name bulk metallic glasses (BMG), see below. There are additional ways besides rapid cooling to produce amorphous metals, including physical vapor deposition and melt spinning. Regardless, amorphous alloys are considered to be a class of materials and will be treated as such throughout this disclosure.
In one embodiment, amorphous alloys can be described as (Nr, Ti)a(Ni, Cu, Fe)b(Be, Al, Si, B)c, where “a” is in the range of from about 30 to about 75 atomic percent, “b” is in the range of from about 5 to 60 atomic percent, and “c” is from about 0 to about 50 atomic percent. In addition, these amorphous alloys can accommodate substantial amounts of other transition metals, including but not limited to Nb, Cr, V, and Co.
In addition, amorphous alloys can also be described as ferrous metal based materials, for example including Fe, Ni, or Co. Exemplary compositions of such compositions include Fe72Al5Ga2P11C6B4. Another illustrative composition is Fe72Al7Zr10Mo5W2B15.
Other embodiments include amorphous alloys composed of zinc and titanium. These amorphous alloy compositions tend to exhibit high strength and hardness. For example, Zr and Ti-based amorphous alloys typically have yield strengths of 250 ksi or higher and hardness values of 450 Vickers or higher. Typical amorphous alloy compositions herein also have high elastic strain limits that approach up to 2.0%.
As noted above, one class of amorphous alloys are BMG. BMG is a class of metallic materials that may be solidified and cooled at relatively slow rates, and retain their amorphous, non-crystalline state at room temperature. If the cooling rate of an amorphous alloy is not sufficient, termed the critical cooling rate, crystals may form inside the alloy, so that the benefits of the amorphous state can be lost. As such, one challenge to fabrication of BMG parts is partial crystallization in the BMG during the cooling process.
Crystal formation in an amorphous alloy provides a level of uncertainty to the quality of parts formed therefrom, uncertainty that can translate to increased costs and failure rates for parts fabricated from BMG alone. In order to obtain a cooling rate equal to or above the critical cooling rate, heat is extracted from the BMG itself. As such, the thickness of a BMG material is often a limiting factor on whether the critical cooling rate may be ascertained. The thickness of BMG, for a particular fabricating technique, that aligns with the critical cooling rate is termed the critical thickness.
BMGs do not experience a liquid/solid crystallization transformation upon cooling, as with conventional metals. Rather, the highly fluid, non-crystalline form of the metal found at high temperatures (near a melting temperature Tm) becomes more viscous as the temperature is reduced (near a Tg), eventually taking on the outward physical properties of conventional solids.
Although there is no liquid/crystallization transformation for a BMG, a “melting temperature” Tm may be defined as the thermodynamic liquidus temperature of the corresponding crystalline phase. The viscosity of the BMG at the melting temperature could lie in the range of about 0.1 poise (or lower) to about 10,000 poise. The cooling rate of the molten metal to form a BMG part, for example, is typically such that the time-temperature profile during cooling does not traverse through a crystallized region in a TTT diagram. The crystallization temperature Tx is where crystallization is most rapid and occurs in the shortest time scale.
The supercooled liquid region, which is the temperature region between Tg and Tx, is a manifestation of the extraordinary stability against crystallization of BMGs. In this temperature region the BMG can exist as a high viscosity liquid. The viscosity of the BMG in the supercooled liquid region can vary between 1012 Pas at the glass transition temperature down to 105 Pas at the crystallization temperature, the high temperature limit of the supercooled liquid region. Liquids with such viscosities can undergo substantial plastic strain under an applied pressure.
Other amorphous alloy metals are known as described in U.S. Pat. No. 9,057,120, 9,103,009, 9,056,353, and 9,044,805, each of which is incorporated by reference for all purposes.
As noted above, a significant problem for the production and use of most amorphous alloys is crystal formation. Crystallization of amorphous alloys can have detrimental effects on the materials properties, particularly where toughness and strength are necessitated. Crystal formation is typically related to the amorphous alloy cooling rate, where an insufficient cooling rate often results in some amount of crystal formation.
Embodiments herein are described in greater detail with reference to
In one embodiment herein, the desired part can be used in the fabrication of electronic devices and/or articles integrated in electronic devices. Embodiments herein provide the amorphous alloy or BMG near-to-net shaped parts integral to electronic devices.
An electronic device herein can refer to any electronic device known in the art, for example, mobile telephone, smart phone, computer, electronic e-mail sending or receiving device, health-monitoring device, wearable electronic device, DVD player, Blue-Ray disc player, video game console, and the like. Electronic devices or articles integrated into an electronic device can also refer to a display, TV monitor, book-reader, web-browser, computer monitor, and the like or to accessories such as casings, laptop housings, smart phone housings, laptop track pads, keyboard, mouse, speakers, etc.
In one embodiment, a portable electronic device can include a cover sheet and an enclosure or housing made of the BMG formed parts described herein. The cover sheet can be composed of a polished glass, sapphire or other hardened transparent material. The housing and cover sheet come together to form an interior volume configured to enclose the various electronic components of the device. For example, the housing may define an opening in which a display is positioned. The cover sheet is positioned over the display and forms a portion of the exterior of the device. The display may include a liquid crystal device (LCD), or organic light-emitting diode (OLED) display, or other suitable display elements or components.
In accordance with embodiments herein, the housing may be formed from BMG as described herein. The housing embodiments may be of a thickness above 0.1 mm, and more typically above 0.5 mm.
As shown in
The outer diecasting molds, in accordance with the disclosure, often include significant mass (arrow 216) to act as a heat sink for maintaining an appropriate cooling rate for amorphous metal during operation of the investment-diecasting mold. The mass of the outer mold is determined by the size of the amorphous metal part to be formed, the composition of the amorphous alloy, and the required cooling rate of the amount and thickness of the amorphous alloy. Generally, the mold is of sufficient uniform mass around the part to allow for a uniform cooling rate.
Compositions of the outer mold, require sufficient mass, but also require a material that has sufficient heat capacity and thermal conductivity to act as a heat-sink and thereby quench the inner investment casting shell when in use with embodiments herein, for example, parts composed of amorphous metal. Possible outer mold materials include: steel, stainless steel, aluminum, copper, or brass.
As noted above, investment-diecasting molds of the present disclosure have a thin, typically between about 0.5 mm and about 5 mm, and in some cases between about 0.5 mm and about 4 mm, and most typically between about 1 mm and 4 mm, gap 300 between an outer surface 107 of the target wax print 106 (when in the mold) and the inner surface 302 of the outer mold assembly 200. Although not shown, this gap can be of uniform thickness or variable thickness as is discussed more thoroughly below. In addition, although some portion of the investment-diecasting mold always includes the gap, the gap does not have to be created between the entirety of the wax part and inner surface of the outer mold. As such, embodiments herein include any number of aspects where the gap exists between the entire wax part and the inner wall of the outer mold to very discrete areas between the wax print outer surface and inner surface of the outer mold. The gap will act as the formation site for the inner or investment layer that is created between the outer mold and the wax part.
Once the investment-diecasting mold is established, the wax print 106 is removed using steam heat or solvent (see
An amorphous alloy part 100 may now be fabricated using the precision and detail of the inner investment casting mold having the heat sink and cooling capacity of the outer diecasting mold.
Still referring to
As illustrated in
In some embodiments, the inner investment mold is dissolved off. Part removal, with or without the investment mold attached, can be accomplished from the outer mold via vibration or use of a pressure wash, for example. In typical cases the fabricated amorphous metal part is ready for its intended use, typically in a near-to-net shape. In some instances, a fabricated part may require additional processing, although the amount of processing will be smaller than a comparable piece prepared by standard diecasting techniques.
As shown in
In some embodiments, the outer diecasting mold assembly may also include cooling conduits throughout to facilitate quenching of the amorphous material. Fluids for use in the cooling conduits include water, brine, oil, NaOH and the like. Cooling conduits can be defined throughout the outer mold and are designed to maximize the cooling rate of desired amorphous metal parts.
In operation 1208, once the outer mold is prepared and in place, the wax print is positioned within the outer mold cavity. In operation 1210, a gap is defined between the wax print and the inner wall or surface of the outer mold where the gap corresponds to the thickness and position of the inner investment casting mold.
In operation 1212, the appropriately composed inner investment casting material is filled into the gap to form the two layer hybrid investment-diecasting mold of the disclosure. Note that the type of investment casting material and thickness of material are important for thermoconductivity. Investment casting material, for example ceramics, can be filled from the backside or through a hole in the wax print. Presence, thickness, an uniformity of the investment casting layer in the investment-diecasting mold is designed to maximize both utility of the investment layer and cost of the layer or the underlying outer diecasting mold. In some cases where little or no value is occasioned by the investment layer, the gap between the outer mold and desired part would not be present, and would only be present at portions where the underlying part or outer diecasting mold necessitate the layer.
In operation 1214, once the inner investment cast is set, the wax print can be dissolved out of the investment-diecasting mold (via steam or solvent, for example), leaving a precisely defined negative space for the desired part.
In operation 1216, the hybrid investment-diecasting mold may then be loaded onto a diecasting machine for injection molding of the appropriate amorphous alloy into the investment-diecasting mold. The molten amorphous alloy is injected into the hybrid investment-diecasting mold.
In operation 1218, the investment-diecasting mold quenches the amorphous alloy by the heat sink aspects of the outer mold and the thermoconductivity of the inner investment cast layer. In some embodiments, this includes use of flowing fluids through cooling conduits in the outer diecasting mold. In some embodiments, quenched amorphous alloy parts maintain the investment casting layer until removed via solvent.
In operation 1220, the investment-diecast amorphous alloy part can then be removed from the outer mold via vibration or under a pressure wash. In some aspects, the formed part is further processed using known techniques, but is typically in a near-to-net shape. The outer diecasting mold is then inspected for damage, and a determination made as to the durability of the mold. Typical hybrid molds herein provide a significant advantage, in that the outer mold can be re-used numerous times due to the protection afforded by the inner investment mold.
The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the described embodiments. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the described embodiments. Thus, the foregoing descriptions of the specific embodiments described herein are presented for purposes of illustration and description. They are not targeted to be exhaustive or to limit the embodiments to the precise forms disclosed. It will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings.
This application is a non-provisional patent application of U.S. Provisional Patent Application No. 62/235,115, filed Sep. 30, 2015 and titled “INVESTMENT DIECASTING MOLD,” the disclosure of which is herein by reference in its entirety.
Number | Date | Country | |
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62235115 | Sep 2015 | US |