Patent Application
20130112321
The present invention relates to rapid discharge processes for amorphous metal forming, and products made by these processes.
The Background is copied from and incorporated herein from U.S. application Ser. No. 12/409,253. Amorphous materials are a new class of engineering material, which have a unique combination of high strength, elasticity, corrosion resistance and processability from the molten state. Amorphous materials differ from conventional crystalline alloys in that their atomic structure lacks the typical long-range ordered patterns of the atomic structure of conventional crystalline alloys. Amorphous materials are generally processed and formed by cooling a molten alloy from above the melting temperature of the crystalline phase (or the thermodynamic melting temperature) to below the “glass transition temperature” of the amorphous phase at “sufficiently fast” cooling rates, such that the nucleation and growth of alloy crystals is avoided. As such, the processing methods for amorphous alloys have always been concerned with quantifying the “sufficiently fast cooling rate”, which is also referred to as “critical cooling rate”, to ensure formation of the amorphous phase.
The “critical cooling rates” for early amorphous materials were extremely high, on the order of 106° C./sec. As such, conventional casting processes were not suitable for such high cooling rates, and special casting processes such as melt spinning and planar flow casting were developed. Due to the crystallization kinetics of those early alloys being substantially fast, extremely short time (on the order of 10−3 seconds or less) for heat extraction from the molten alloy were required to bypass crystallization, and thus early amorphous alloys were also limited in size in at least one dimension. For example, only very thin foils and ribbons (order of 25 microns in thickness) were successfully produced using these conventional techniques. Because the critical cooling rate requirements for these amorphous alloys severely limited the size of parts made from amorphous alloys, the use of early amorphous alloys as bulk objects and articles was limited.
Over the years it was determined that the “critical cooling rate” depends strongly on the chemical composition of amorphous alloys. Accordingly, a great deal of research was focused on developing new alloy compositions with much lower critical cooling rates. Examples of these alloys are given in U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975, each of which is incorporated herein by reference. These amorphous alloy systems, also called bulk-metallic glasses or BMGs, are characterized by critical cooling rates as low as a few ° C./second, which allows the processing and forming of much larger bulk amorphous phase objects than were previously achievable.
With the availability of low “critical cooling rate” BMGs, it has become possible to apply conventional casting processes to form bulk articles having an amorphous phase. Over the past several years, a number of companies, including LiquidMetal Technologies, Inc. have undertaken an effort to develop commercial manufacturing technologies for the production of net shape metallic parts fabricated from BMGs. For example, manufacturing methods such as permanent mold metal die-casting and injection casting into heated molds are currently being used to fabricate commercial hardware and components such as electronic casings for standard consumer electronic devices (e.g., cell phones and handheld wireless devices), hinges, fasteners, medical instruments and other high value added products. However, even though bulk-solidifying amorphous alloys provide some remedy to the fundamental deficiencies of solidification casting, and particularly to the die-casting and permanent mold casting processes, as discussed above, there are still issues which need to be addressed. First and foremost, there is a need to make these bulk objects from a broader range of alloy compositions. For example, presently available BMGs with large critical casting dimensions capable of making large bulk amorphous objects are limited to a few groups of alloy compositions based on a very narrow selection of metals, including Zr-based alloys with additions of Ti, Ni, Cu, Al and Be and Pd-based alloys with additions of Ni, Cu, and P, which are not necessarily optimized from either an engineering or cost perspective.
In addition, the current processing technology requires a great deal of expensive machinery to ensure appropriate processing conditions are created. For example, most shaping processes require a high vacuum or controlled inert gas environment, induction melting of material in a crucible, pouring of metal to a shot sleeve, and pneumatic injection through a shot sleeve into gating and cavities of a rather elaborate mold assembly. These modified die-casting machines can cost several hundreds of thousands of dollars per machine. Moreover, because heating a BMG has to date been accomplished via these traditional, slow thermal processes, the prior art of processing and forming bulk-solidifying amorphous alloys has always been focused on cooling the molten alloy from above the thermodynamic melting temperature to below the glass transition temperature. This cooling has either been realized using a single-step monotonous cooling operation or a multi-step process. For example, metallic molds (made of copper, steel, tungsten, molybdenum, composites thereof, or other high conductivity materials) at ambient temperatures are utilized to facilitate and expedite heat extraction from the molten alloy. Because the “critical casting dimension” is correlated to the critical cooling rate, these conventional processes are not suitable for forming larger bulk objects and articles of a broader range of bulk-solidifying amorphous alloys. In addition, it is often necessary to inject the molten alloy into the dies at high-speed, and under high-pressure, to ensure sufficient alloy material is introduced into the die prior to the solidification of the alloy, particularly in the manufacture of complex and high-precision parts. Because the metal is fed into the die under high pressure and at high velocities, such as in high-pressure die-casting operation, the flow of the molten metal becomes prone to Rayleigh-Taylor instability. This flow instability is characterized by a high Weber number, and is associated with the break-up of the flow front causing the formation of protruded seams and cells, which appear as cosmetic and structural micro-defects in cast parts. Also, there is a tendency to form a shrinkage cavity or porosity along the centerline of the die-casting mold when unvitrified liquid is trapped inside a solid shell of vitrified metal.
Attempts to remedy the problems associated with rapidly cooling the material from above the equilibrium melting point to below the glass transition were mostly focused on utilizing the kinetic stability and viscous flow characteristics of the supercooled liquid. Methods have been proposed that involve heating glassy feedstock above the glass transition where the glass relaxes to a viscous supercooled liquid, applying pressure to form the supercooled liquid, and subsequently cooling to below glass transition prior to crystallizing. These attractive methods are essentially very similar to those used to process plastics. In contrast to plastics however, which remain stable against crystallization above the softening transition for extremely long periods of time, metallic supercooled liquids crystallize rather rapidly once relaxed at the glass transition. Consequently, the temperature range over which metallic glasses are stable against crystallization when heated at conventional heating rates (20° C./min) are rather small (50-100° C. above glass transition), and the liquid viscosity within that range is rather high (109-10′ Pa s). Owing to these high viscosities, the pressures required to form these liquids into desirable shapes are enormous, and for many metallic glass alloys can exceed the pressures attainable by conventional high strength tooling (<1 GPa). Metallic glass alloys have recently been developed that are stable against crystallization when heated at conventional heating rates up to considerably high temperatures (165° C. above glass transition). Examples of these alloys are given in U.S. Pat. Appl. 20080135138 and articles to G. Duan et al. (Advanced Materials, 19 (2007) 4272) and A. Wiest (Acta Materialia, 56 (2008) 2525-2630), each of which is incorporated herein by reference. Owing to their high stability against crystallization, process viscosities as low as 105 Pa-s become accessible, which suggests that these alloys are more suitable for processing in the supercooled liquid state than traditional metallic glasses. These viscosities however are still substantially higher than the processing viscosities of plastics, which typically range between 10 and 1000 Pa-s. In order to attain such low viscosities, the metallic glass alloy should either exhibit an even higher stability against crystallization when heated by conventional heating, or be heated at an unconventionally high heating rate which will extend the temperature range of stability and lower the process viscosity to values typical of those used in processing thermoplastics.
A few attempts have been made to create a method of instantaneously heating a BMG up to a temperature sufficient for shaping, thereby avoiding many of the problems discussed above and simultaneously expanding the types of amorphous materials that can be shaped. For example, U.S. Pat. Nos. 4,115,682 and 5,005,456 and articles to A. R. Yavari (Materials Research Society Symposium Proceedings, 644 (2001) L12-20-1, Materials Science & Engineering A, 375-377 (2004) 227-234; and Applied Physics Letters, 81(9) (2002) 1606-1608), the disclosures of each of which are incorporated herein by reference, all take advantage of the unique conductive properties of amorphous materials to instantaneously heat the materials to a shaping temperature using Joule heating. However, thus far these techniques have focused on localized heating of BMG samples to allow for only localized forming, such as the joining (i.e., spot welding) of such pieces, or the formation of surface features. None of these prior art methods teach how to uniformly heat the entire BMG specimen volume in order to be able to perform global forming. Instead, all those prior art methods anticipate temperature gradients during heating, and discuss how these gradients can affect local forming. For instance, Yavari et al (Materials Research Society Symposium Proceedings, 644 (2001) L12-20-1) write: “The external surfaces of the BMG specimen being shaped, whether in contact with the electrodes or with the ambient (inert) gas in the shaping chamber, will be slightly cooler than the inside as the heat generated by the current dissipates out of the sample by conduction, convection or radiation. On the other hand, the outer surfaces of samples heated by conduction, convection or radiation are slightly hotter than the inside. This is an important advantage for the present method as crystallization and or oxidation of metallic glasses often begin first on outer surfaces and interfaces and if they are slightly below the temperature of the bulk, such undesirable surface crystal formation may be more easily avoided.”
Another drawback of the limited stability of BMGs against crystallization above the glass transition is the inability to measure thermodynamic and transport properties, such as heat capacity and viscosity, over the entire range of temperatures of the metastable supercooled liquid. Typical measurement instruments such as Differential Scanning calorimeters, Thermo-Mechanical Analyzers, and Couette Viscometers rely on conventional heating instrumentation, such as electric and induction heaters, and are thus capable of attaining sample heating rates that are considered conventional (typically <100° C./min). As discuss above, metallic supercooled liquids can be stable against crystallization over a limited temperature range when heated at a conventional heating rate, and thus the measureable thermodynamic and transport properties are limited to within the accessible temperature range. Consequently, unlike polymer and organic liquids which are very stable against crystallization and their thermodynamic and transport properties are measureable throughout the entire range of metastability, the properties of metallic supercooled liquids are only measureable to within narrow temperature ranges just above the glass transition and just below the melting point.
Accordingly, a need exists to find a novel approach to instantaneously and uniformly heat the entire BMG specimen volume and thus enable global shaping of amorphous metals. In addition, from a scientific perspective, a need also exists to find a novel approach to access and measure these thermodynamic and transport properties of metallic supercooled liquids.
A proposed solution according to embodiments herein for nano- and micro-replication in metals is to use bulk-solidifying amorphous alloys. The embodiments herein include methods for forming nano- and/or micro-replication directly embossed in a bulk solidifying amorphous alloy comprising a metal alloy by superplastic forming of the bulk solidifying amorphous alloy at a temperature greater than a glass transition temperature (Tg) of the metal alloy.
The description will be more fully understood with reference to the following figures and data graphs, which are presented as exemplary embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:
a to 8d provide a series of photographic images of experimental results obtained using an exemplary rapid capacitor discharge forming method in accordance with the current invention.
a to 11e provide a set of schematics of an exemplary rapid capacitor discharge apparatus in accordance with the current invention.
a and 12b provide photographic images of a molded article made using the apparatus shown in
a to 15d provide a series of cartoons showing the process of joining two parts in one embodiment using a protrusion comprising an amorphous alloy in one embodiment.
a to 16c provide illustrations of a process of mating a protruding part (not to scale) of the first part being compressed by a tip in the shape of a plunger. The protruding part is shown as separate from the first part by exaggeration merely to show that the first part and the protruding part need not be the same. Also shown in the figures are the gradual changes in the shape of the protruding part.
a to 17c provide illustrations of a process for heat staking using rapid discharge.
a shows a shaped electrode in which section A-A is at the bottom right corner of the shaped electrode.
a and 19b show an embodiment for replicating text and logos with extremely high degrees of accuracy.
All publications, patents, and patent applications cited in this Specification are hereby incorporated by reference in their entirety.
The articles “a” and “an” are used herein to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. By way of example, “a polymer resin” means one polymer resin or more than one polymer resin. Any ranges cited herein are inclusive. The terms “substantially” and “about” used throughout this Specification are used to describe and account for small fluctuations. For example, they can refer to less than or equal to ±5%, such as less than or equal to ±2%, such as less than or equal to ±1%, such as less than or equal to ±0.5%®, such as less than or equal to ±0.2%, such as less than or equal to ±0.1%, such as less than or equal to ±0.05%.
Bulk-solidifying amorphous alloys, or bulk metallic glasses (“BMG”), are a recently developed class of metallic materials. These alloys may be solidified and cooled at relatively slow rates, and they retain the amorphous, non-crystalline (i.e., glassy) state at room temperature. Amorphous alloys have many superior properties than their crystalline counterparts. However, if the cooling rate is not sufficiently high, crystals may form inside the alloy during cooling, so that the benefits of the amorphous state can be lost. For example, one challenge with the fabrication of bulk amorphous alloy parts is partial crystallization of the parts due to either slow cooling or impurities in the raw alloy material. As a high degree of amorphicity (and, conversely, a low degree of crystallinity) is desirable in BMG parts, there is a need to develop methods for casting BMG parts having controlled amount of amorphicity.
The term “phase” herein can refer to one that can be found in a thermodynamic phase diagram. A phase is a region of space (e.g., a thermodynamic system) throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, chemical composition and lattice periodicity. A simple description of a phase is a region of material that is chemically uniform, physically distinct, and/or mechanically separable. For example, in a system consisting of ice and water in a glass jar, the ice cubes are one phase, the water is a second phase, and the humid air over the water is a third phase. The glass of the jar is another separate phase. A phase can refer to a solid solution, which can be a binary, tertiary, quaternary, or more, solution, or a compound, such as an intermetallic compound. As another example, an amorphous phase is distinct from a crystalline phase.
The term “metal” refers to an electropositive chemical element. The term “element” in this Specification refers generally to an element that can be found in a Periodic Table. Physically, a metal atom in the ground state contains a partially filled band with an empty state close to an occupied state. The term “transition metal” is any of the metallic elements within Groups 3 to 12 in the Periodic Table that have an incomplete inner electron shell and that serve as transitional links between the most and the least electropositive in a series of elements. Transition metals are characterized by multiple valences, colored compounds, and the ability to form stable complex ions. The term “nonmetal” refers to a chemical element that does not have the capacity to lose electrons and form a positive ion.
Depending on the application, any suitable nonmetal elements, or their combinations, can be used. The alloy (or “alloy composition”) can comprise multiple nonmetal elements, such as at least two, at least three, at least four, or more, nonmetal elements. A nonmetal element can be any element that is found in Groups 13-17 in the Periodic Table. For example, a nonmetal element can be any one of F, Cl, Br, I, At, O, S, Se, Te, Po, N, P, As, Sb, Bi, C, Si, Ge, Sn, Pb, and B. Occasionally, a nonmetal element can also refer to certain metalloids (e.g., B, Si, Ge, As, Sb, Te, and Po) in Groups 13-17. In one embodiment, the nonmetal elements can include B, Si, C, P, or combinations thereof. Accordingly, for example, the alloy can comprise a boride, a carbide, or both.
A transition metal element can be any of scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, mercury, rutherfordium, dubnium, seaborgium, bohrium, hassium, meitnerium, ununnilium, unununium, and ununbium. In one embodiment, a BMG containing a transition metal element can have at least one of Sc, Y, La, Ac, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Tc, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, and Hg. Depending on the application, any suitable transitional metal elements, or their combinations, can be used. The alloy composition can comprise multiple transitional metal elements, such as at least two, at least three, at least four, or more, transitional metal elements.
The presently described alloy or alloy “sample” or “specimen” alloy can have any shape or size. For example, the alloy can have a shape of a particulate, which can have a shape such as spherical, ellipsoid, wire-like, rod-like, sheet-like, flake-like, or an irregular shape. The particulate can have any size. For example, it can have an average diameter of between about 1 micron and about 100 microns, such as between about 5 microns and about 80 microns, such as between about 10 microns and about 60 microns, such as between about 15 microns and about 50 microns, such as between about 15 microns and about 45 microns, such as between about 20 microns and about 40 microns, such as between about 25 microns and about 35 microns. For example, in one embodiment, the average diameter of the particulate is between about 25 microns and about 44 microns. In some embodiments, smaller particulates, such as those in the nanometer range, or larger particulates, such as those bigger than 100 microns, can be used.
The alloy sample or specimen can also be of a much larger dimension. For example, it can be a bulk structural component, such as an ingot, housing/casing of an electronic device or even a portion of a structural component that has dimensions in the millimeter, centimeter, or meter range.
The term “solid solution” refers to a solid form of a solution. The term “solution” refers to a mixture of two or more substances, which may be solids, liquids, gases, or a combination of these. The mixture can be homogeneous or heterogeneous. The term “mixture” is a composition of two or more substances that are combined with each other and are generally capable of being separated. Generally, the two or more substances are not chemically combined with each other.
In some embodiments, the alloy composition described herein can be fully alloyed. In one embodiment, an “alloy” refers to a homogeneous mixture or solid solution of two or more metals, the atoms of one replacing or occupying interstitial positions between the atoms of the other; for example, brass is an alloy of zinc and copper. An alloy, in contrast to a composite, can refer to a partial or complete solid solution of one or more elements in a metal matrix, such as one or more compounds in a metallic matrix. The term alloy herein can refer to both a complete solid solution alloy that can give single solid phase microstructure and a partial solution that can give two or more phases. An alloy composition described herein can refer to one comprising an alloy or one comprising an alloy-containing composite.
Thus, a fully alloyed alloy can have a homogenous distribution of the constituents, be it a solid solution phase, a compound phase, or both. The term “fully alloyed” used herein can account for minor variations within the error tolerance. For example, it can refer to at least 90% alloyed, such as at least 95% alloyed, such as at least 99% alloyed, such as at least 99.5% alloyed, such as at least 99.9% alloyed. The percentage herein can refer to either volume percent or weight percentage, depending on the context. These percentages can be balanced by impurities, which can be in terms of composition or phases that are not a part of the alloy.
An “amorphous” or “non-crystalline solid” is a solid that lacks lattice periodicity, which is characteristic of a crystal. As used herein, an “amorphous solid” includes “glass” which is an amorphous solid that softens and transforms into a liquid-like state upon heating through the glass transition. Generally, amorphous materials lack the long-range order characteristic of a crystal, though they can possess some short-range order at the atomic length scale due to the nature of chemical bonding. The distinction between amorphous solids and crystalline solids can be made based on lattice periodicity as determined by structural characterization techniques such as x-ray diffraction and transmission electron microscopy.
The terms “order” and “disorder” designate the presence or absence of some symmetry or correlation in a many-particle system. The terms “long-range order” and “short-range order” distinguish order in materials based on length scales.
The strictest form of order in a solid is lattice periodicity: a certain pattern (the arrangement of atoms in a unit cell) is repeated again and again to form a translationally invariant tiling of space. This is the defining property of a crystal. Possible symmetries have been classified in 14 Bravais lattices and 230 space groups.
Lattice periodicity implies long-range order. If only one unit cell is known, then by virtue of the translational symmetry it is possible to accurately predict all atomic positions at arbitrary distances. The converse is generally true, except, for example, in quasi-crystals that have perfectly deterministic tilings but do not possess lattice periodicity.
Long-range order characterizes physical systems in which remote portions of the same sample exhibit correlated behavior. This can be expressed as a correlation function, namely the spin-spin correlation function: G(x,x′)=s(x),s(x′)
.
In the above function, s is the spin quantum number and x is the distance function within the particular system. This function is equal to unity when x=x′ and decreases as the distance |x−x′ | increases. Typically, it decays exponentially to zero at large distances, and the system is considered to be disordered. If, however, the correlation function decays to a constant value at large |x−x′|, then the system can be said to possess long-range order. If it decays to zero as a power of the distance, then it can be called quasi-long-range order. Note that what constitutes a large value of |x−x′| is relative.
A system can be said to present quenched disorder when some parameters defining its behavior are random variables that do not evolve with time (i.e., they are quenched or frozen)—e.g., spin glasses. It is opposite to annealed disorder, where the random variables are allowed to evolve themselves. Embodiments herein include systems comprising quenched disorder.
The alloy described herein can be crystalline, partially crystalline, amorphous, or substantially amorphous. For example, the alloy sample/specimen can include at least some crystallinity, with grains/crystals having sizes in the nanometer and/or micrometer ranges. Alternatively, the alloy can be substantially amorphous, such as fully amorphous. In one embodiment, the alloy composition is at least substantially not amorphous, such as being substantially crystalline, such as being entirely crystalline.
In one embodiment, the presence of a crystal or a plurality of crystals in an otherwise amorphous alloy can be construed as a “crystalline phase” therein. The degree of crystallinity (or “crystallinity” for short in some embodiments) of an alloy can refer to the amount of the crystalline phase present in the alloy. The degree can refer to, for example, a fraction of crystals present in the alloy. The fraction can refer to volume fraction or weight fraction, depending on the context. A measure of how “amorphous” an amorphous alloy is can be amorphicity. Amorphicity can be measured in terms of a degree of crystallinity. For example, in one embodiment, an alloy having a low degree of crystallinity can be said to have a high degree of amorphicity. In one embodiment, for example, an alloy having 60 vol % crystalline phase can have a 40 vol % amorphous phase.
An “amorphous alloy” is an alloy having an amorphous content of more than 50% by volume, preferably more than 90% by volume of amorphous content, more preferably more than 95% by volume of amorphous content, and most preferably more than 99% to almost 100% by volume of amorphous content. Note that, as described above, an alloy high in amorphicity is equivalently low in degree of crystallinity. An “amorphous metal” is an amorphous metal material with a disordered atomic-scale structure. In contrast to most metals, which are crystalline and therefore have a highly ordered arrangement of atoms, amorphous alloys are non-crystalline. Materials in which such a disordered structure is produced directly from the liquid state during cooling are sometimes referred to as “glasses.” Accordingly, amorphous metals are commonly referred to as “metallic glasses” or “glassy metals.” In one embodiment, a bulk metallic glass (“BMG”) can refer to an alloy, of which the microstructure is at least partially amorphous. However, there are several ways besides extremely rapid cooling to produce amorphous metals, including physical vapor deposition, solid-state reaction, ion irradiation, melt spinning, and mechanical alloying. Amorphous alloys can be a single class of materials, regardless of how they are prepared.
Amorphous metals can be produced through a variety of quick-cooling methods. For instance, amorphous metals can be produced by sputtering molten metal onto a spinning metal disk. The rapid cooling, on the order of millions of degrees a second, can be too fast for crystals to form, and the material is thus “locked in” a glassy state. Also, amorphous metals/alloys can be produced with critical cooling rates low enough to allow formation of amorphous structures in thick layers—e.g., bulk metallic glasses.
The terms “bulk metallic glass” (“BMG”), bulk amorphous alloy (“BAA”), and bulk solidifying amorphous alloy are used interchangeably herein. They refer to amorphous alloys having the smallest dimension at least in the millimeter range. For example, the dimension can be at least about 0.5 mm, such as at least about 1 mm, such as at least about 2 mm, such as at least about 4 mm, such as at least about 5 mm, such as at least about 6 mm, such as at least about 8 mm, such as at least about 10 mm, such as at least about 12 mm. Depending on the geometry, the dimension can refer to the diameter, radius, thickness, width, length, etc. A BMG can also be a metallic glass having at least one dimension in the centimeter range, such as at least about 1.0 cm, such as at least about 2.0 cm, such as at least about 5.0 cm, such as at least about 10.0 cm. In some embodiments, a BMG can have at least one dimension at least in the meter range. A BMG can take any of the shapes or forms described above, as related to a metallic glass. Accordingly, a BMG described herein in some embodiments can be different from a thin film made by a conventional deposition technique in one important aspect—the former can be of a much larger dimension than the latter.
Amorphous metals can be an alloy rather than a pure metal. The alloys may contain atoms of significantly different sizes, leading to low free volume (and therefore having viscosity up to orders of magnitude higher than other metals and alloys) in a molten state. The viscosity prevents the atoms from moving enough to form an ordered lattice. The material structure may result in low shrinkage during cooling and resistance to plastic deformation. The absence of grain boundaries, the weak spots of crystalline materials in some cases, may, for example, lead to better resistance to wear and corrosion. In one embodiment, amorphous metals, while technically glasses, may also be much tougher and less brittle than oxide glasses and ceramics.
Thermal conductivity of amorphous materials may be lower than that of their crystalline counterparts. To achieve formation of an amorphous structure even during slower cooling, the alloy may be made of three or more components, leading to complex crystal units with higher potential energy and lower probability of formation. The formation of amorphous alloy can depend on several factors: the composition of the components of the alloy; the atomic radius of the components (preferably with a significant difference of over 12% to achieve high packing density and low free volume); and the negative heat of mixing the combination of components, inhibiting crystal nucleation and prolonging the time the molten metal stays in a supercooled state. However, as the formation of an amorphous alloy is based on many different variables, it can be difficult to make a prior determination of whether an alloy composition will form an amorphous alloy.
Amorphous alloys, for example, of boron, silicon, phosphorus, and other glass formers with magnetic metals (iron, cobalt, nickel) may be magnetic, with low coercivity and high electrical resistance. The high resistance leads to low losses by eddy currents when subjected to alternating magnetic fields, a property useful, for example, as transformer magnetic cores.
Amorphous alloys may have a variety of potentially useful properties. In particular, they tend to be stronger than crystalline alloys of similar chemical composition, and they can sustain larger reversible (“elastic”) deformations than crystalline alloys. Amorphous metals derive their strength directly from their non-crystalline structure, which can have none of the defects (such as dislocations) that limit the strength of crystalline alloys. For example, one modern amorphous metal, known as Vitreloy™, has a tensile strength that is almost twice that of high-grade titanium. In some embodiments, metallic glasses at room temperature are not ductile and tend to fail suddenly when loaded in tension, which limits the material applicability in reliability-critical applications, as the impending failure is not evident. Therefore, to overcome this challenge, metal matrix composite materials having a metallic glass matrix containing dendritic particles or fibers of a ductile crystalline metal can be used. Alternatively, a BMG low in element(s) that tend to cause embitterment (e.g., Ni) can be used. For example, a Ni-free BMG can be used to improve the ductility of the BMG.
Another useful property of bulk amorphous alloys is that they can be true glasses; in other words, they can soften and flow upon heating. This can allow for easy processing, such as by injection molding, in much the same way as polymers. As a result, amorphous alloys can be used for making sports equipment, medical devices, electronic components and equipment, and thin films. Thin films of amorphous metals can be deposited as protective coatings via a high velocity oxygen fuel technique.
A material can have an amorphous phase, a crystalline phase, or both. The amorphous and crystalline phases can have the same chemical composition and differ only in the microstructure—i.e., one amorphous and the other crystalline. Microstructure in one embodiment refers to the structure of a material as revealed by a microscope at 25× magnification or higher. Alternatively, the two phases can have different chemical compositions and microstructures. For example, a composition can be partially amorphous, substantially amorphous, or completely amorphous.
As described above, the degree of amorphicity (and conversely the degree of crystallinity) can be measured by fraction of crystals present in the alloy. The degree can refer to volume fraction of weight fraction of the crystalline phase present in the alloy. A partially amorphous composition can refer to a composition of at least about 5 vol % of which is of an amorphous phase, such as at least about 10 vol %, such as at least about 20 vol %, such as at least about 40 vol %, such as at least about 60 vol %, such as at least about 80 vol %, such as at least about 90 vol %. The terms “substantially” and “about” have been defined elsewhere in this application. Accordingly, a composition that is at least substantially amorphous can refer to one of which at least about 90 vol % is amorphous, such as at least about 95 vol %, such as at least about 98 vol %, such as at least about 99 vol %, such as at least about 99.5 vol %, such as at least about 99.8 vol %, such as at least about 99.9 vol %. In one embodiment, a substantially amorphous composition can have some incidental, insignificant amount of crystalline phase present therein.
In one embodiment, an amorphous alloy composition can be homogeneous with respect to the amorphous phase. A substance that is uniform in composition is homogeneous. This is in contrast to a substance that is heterogeneous. The term “composition” refers to the chemical composition and/or microstructure in the substance. A substance is homogeneous when a volume of the substance is divided in half and both halves have substantially the same composition. For example, a particulate suspension is homogeneous when a volume of the particulate suspension is divided in half and both halves have substantially the same volume of particles. However, it might be possible to see the individual particles under a microscope. Another example of a homogeneous substance is air where different ingredients therein are equally suspended, though the particles, gases and liquids in air can be analyzed separately or separated from air.
A composition that is homogeneous with respect to an amorphous alloy can refer to one having an amorphous phase substantially uniformly distributed throughout its microstructure. In other words, the composition macroscopically comprises a substantially uniformly distributed amorphous alloy throughout the composition. In an alternative embodiment, the composition can be of a composite, having an amorphous phase having therein a non-amorphous phase. The non-amorphous phase can be a crystal or a plurality of crystals. The crystals can be in the form of particulates of any shape, such as spherical, ellipsoid, wire-like, rod-like, sheet-like, flake-like, or an irregular shape. In one embodiment, it can have a dendritic form. For example, an at least partially amorphous composite composition can have a crystalline phase in the shape of dendrites dispersed in an amorphous phase matrix; the dispersion can be uniform or non-uniform, and the amorphous phase and the crystalline phase can have the same or a different chemical composition. In one embodiment, they have substantially the same chemical composition. In another embodiment, the crystalline phase can be more ductile than the BMG phase.
The methods described herein can be applicable to any type of amorphous alloy. Similarly, the amorphous alloy described herein as a constituent of a composition or article can be of any type. The amorphous alloy can comprise the element Zr, Hf, Ti, Cu, Ni, Pt, Pd, Fe, Mg, Au, La, Ag, Al, Mo, Nb, Be, or combinations thereof. Namely, the alloy can include any combination of these elements in its chemical formula or chemical composition. The elements can be present at different weight or volume percentages. For example, an iron “based” alloy can refer to an alloy having a non-insignificant weight percentage of iron present therein, the weight percent can be, for example, at least about 20 wt %, such as at least about 40 wt %, such as at least about 50 wt %, such as at least about 60 wt %, such as at least about 80 wt %. Alternatively, in one embodiment, the above-described percentages can be volume percentages, instead of weight percentages. Accordingly, an amorphous alloy can be zirconium-based, titanium-based, platinum-based, palladium-based, gold-based, silver-based, copper-based, iron-based, nickel-based, aluminum-based, molybdenum-based, and the like. The alloy can also be free of any of the aforementioned elements to suit a particular purpose. For example, in some embodiments, the alloy, or the composition including the alloy, can be substantially free of nickel, aluminum, titanium, beryllium, or combinations thereof. In one embodiment, the alloy or the composite is completely free of nickel, aluminum, titanium, beryllium, or combinations thereof.
For example, the amorphous alloy can have the formula (Zr, Ti)a(Ni, Cu, Fe)b(Be, Al, Si, B)c, wherein a, b, and c each represents a weight or atomic percentage. In one embodiment, a is in the range of from 30 to 75, b is in the range of from 5 to 60, and c is in the range of from 0 to 50 in atomic percentages. Alternatively, the amorphous alloy can have the formula (Zr, Ti)a(Ni, Cu)b(Be)c, wherein a, b, and c each represents a weight or atomic percentage. In one embodiment, a is in the range of from 40 to 75, b is in the range of from 5 to 50, and c is in the range of from 5 to 50 in atomic percentages. The alloy can also have the formula (Zr, Ti)a(Ni, Cu)b(Be)c, wherein a, b, and c each represents a weight or atomic percentage. In one embodiment, a is in the range of from 45 to 65, b is in the range of from 7.5 to 35, and c is in the range of from 10 to 37.5 in atomic percentages. Alternatively, the alloy can have the formula (Zr)a(Nb, Ti)b(Ni, Cu)c(Al)d, wherein a, b, c, and d each represents a weight or atomic percentage. In one embodiment, a is in the range of from 45 to 65, b is in the range of from 0 to 10, c is in the range of from 20 to 40 and d is in the range of from 7.5 to 15 in atomic percentages. One exemplary embodiment of the aforedescribed alloy system is a Zr—Ti—Ni—Cu—Be based amorphous alloy under the trade name Vitreloy™, such as Vitreloy-1 and Vitreloy-101, as fabricated by Liquidmetal Technologies, CA, USA. Some examples of amorphous alloys of the different systems are provided in Table 1.
The amorphous alloys can also be ferrous alloys, such as (Fe, Ni, Co) based alloys. Examples of such compositions are disclosed in U.S. Pat. Nos. 6,325,868; 5,288,344; 5,368,659; 5,618,359; and 5,735,975, Inoue et al., Appl. Phys. Lett., Volume 71, p 464 (1997), Shen et al., Mater. Trans., JIM, Volume 42, p 2136 (2001), and Japanese Patent Application No. 200126277 (Pub. No. 2001303218 A). One exemplary composition is Fe72Al5Ga2P11C6B4. Another example is Fe72Al7Zr10Mo5W2B15. Another iron-based alloy system that can be used in the coating herein is disclosed in U.S. Patent Application Publication No. 2010/0084052, wherein the amorphous metal contains, for example, manganese (1 to 3 atomic %), yttrium (0.1 to 10 atomic %), and silicon (0.3 to 3.1 atomic %) in the range of composition given in parentheses; and that contains the following elements in the specified range of composition given in parentheses: chromium (15 to 20 atomic %), molybdenum (2 to 15 atomic %), tungsten (1 to 3 atomic %), boron (5 to 16 atomic %), carbon (3 to 16 atomic %), and the balance iron.
The aforedescribed amorphous alloy systems can further include additional elements, such as additional transition metal elements, including Nb, Cr, V, and Co. The additional elements can be present at less than or equal to about 30 wt %, such as less than or equal to about 20 wt %, such as less than or equal to about 10 wt %, such as less than or equal to about 5 wt %. In one embodiment, the additional, optional element is at least one of cobalt, manganese, zirconium, tantalum, niobium, tungsten, yttrium, titanium, vanadium and hafnium to form carbides and further improve wear and corrosion resistance. Further optional elements may include phosphorous, germanium and arsenic, totaling up to about 2%, and preferably less than 1%, to reduce melting point. Otherwise incidental impurities should be less than about 2% and preferably 0.5%.
In some embodiments, a composition having an amorphous alloy can include a small amount of impurities. The impurity elements can be intentionally added to modify the properties of the composition, such as improving the mechanical properties (e.g., hardness, strength, fracture mechanism, etc.) and/or improving the corrosion resistance. Alternatively, the impurities can be present as inevitable, incidental impurities, such as those obtained as a byproduct of processing and manufacturing. The impurities can be less than or equal to about 10 wt %, such as about 5 wt %, such as about 2 wt %, such as about 1 wt %, such as about 0.5 wt %, such as about 0.1 wt %. In some embodiments, these percentages can be volume percentages instead of weight percentages. In one embodiment, the alloy sample/composition consists essentially of the amorphous alloy (with only a small incidental amount of impurities). In another embodiment, the composition includes the amorphous alloy (with no observable trace of impurities).
In one embodiment, the final parts exceeded the critical casting thickness of the bulk solidifying amorphous alloys.
In embodiments herein, the existence of a supercooled liquid region in which the bulk-solidifying amorphous alloy can exist as a high viscous liquid allows for superplastic forming. Large plastic deformations can be obtained. The ability to undergo large plastic deformation in the supercooled liquid region can be used for the forming and/or cutting process. As oppose to solids, the liquid bulk solidifying alloy deforms locally which drastically lowers the required energy for cutting and forming. The ease of cutting and forming depends on the temperature of the alloy, the mold, and the cutting tool. As higher is the temperature, the lower is the viscosity, and consequently easier is the cutting and forming.
Embodiments herein can utilize a thermoplastic-forming process with amorphous alloys carried out between Tg and Tx, for example. Herein, Tx and Tg are determined from standard DSC measurements at typical heating rates (e.g. 20° C./min) as the onset of crystallization temperature and the onset of glass transition temperature.
The amorphous alloy components can have the critical casting thickness and the final part can have thickness that is thicker than the critical casting thickness. Moreover, the time and temperature of the heating and shaping operation is selected such that the elastic strain limit of the amorphous alloy can be substantially preserved to be not less than 1.0%, and preferably not being less than 1.5%. In the context of the embodiments herein, temperatures around glass transition means the forming temperatures can be below glass transition, at or around glass transition, and above glass transition temperature, but preferably at temperatures below the crystallization temperature TX. The cooling step is carried out at rates similar to the heating rates at the heating step, and preferably at rates greater than the heating rates at the heating step. The cooling step is also achieved preferably while the forming and shaping loads are still maintained.
The embodiments herein can be valuable in the fabrication of electronic devices using a BMG. An electronic device herein can refer to any electronic device known in the art. For example, it can be a telephone, such as a cell phone, and a land-line phone, or any communication device, such as a smart phone, including, for example an iPhone™, and an electronic email sending/receiving device. It can be a part of a display, such as a digital display, a TV monitor, an electronic-book reader, a portable web-browser (e.g., iPad™), and a computer monitor. It can also be an entertainment device, including a portable DVD player, conventional DVD player, Blue-Ray disk player, video game console, music player, such as a portable music player (e.g., iPod™), etc. It can also be a part of a device that provides control, such as controlling the streaming of images, videos, sounds (e.g., Apple TV™), or it can be a remote control for an electronic device. It can be a part of a computer or its accessories, such as the hard drive tower housing or casing, laptop housing, laptop keyboard, laptop track pad, desktop keyboard, mouse, and speaker. The article can also be applied to a device such as a watch or a clock.
The description herein of forming amorphous metal by rapid capacitor discharge is copied from and incorporated herein from U.S. application Ser. No. 12/409,253.
The current invention is directed to a method of uniformly heating, Theologically softening, and thermoplastically forming metallic glasses rapidly (typically with processing times of less than 1 second into a net shape article using an extrusion or mold tool by Joule heating. More specifically, the method utilizes the discharge of electrical energy (typically 100 Joules to 100 KJoules) stored in a capacitor to uniformly and rapidly heat a sample or charge of metallic glass alloy to a predetermined “process temperature” about half-way between the glass transition temperature of the amorphous material and the equilibrium melting point of the alloy in a time scale of several milliseconds or less, and is referred to hereinafter as rapid capacitor discharge forming (RCDF). The RCDF process of the current invention proceeds from the observation that metallic glass, by its virtue of being a frozen liquid, has a relatively low electrical resistivity, which can result in high dissipation and efficient, uniform heating of the material at rate such that the sample is adiabatically heated with the proper application of an electrical discharge.
By rapidly and uniformly heating a BMG, the RCDF method extends the stability of the supercooled liquid against crystallization to temperatures substantially higher than the glass transition temperature, thereby bringing the entire sample volume to a state associated with a processing viscosity that is optimal for forming. The RCDF process also provides access to the entire range of viscosities offered by the metastable supercooled liquid, as this range is no longer limited by the formation of the stable crystalline phase. In sum, this process allows for the enhancement of the quality of parts formed, an increase yield of usable parts, a reduction in material and processing costs, a widening of the range of usable BMG materials, improved energy efficiency, and lower capital cost of manufacturing machines. In addition, owing to the instantaneous and uniform heating that can be attained in the RCDF method, the thermodynamic and transport properties throughout the entire range of the liquid metastability become accessible for measurement. Therefore by incorporating additional standard instrumentation to a Rapid Capacitor Discharge set up such as temperature and strain measurement instrumentation, properties such as viscosity, heat capacity and enthalpy can be measured in the entire temperature range between glass transition and melting point.
A simple flow chart of the RCDF technique of the current invention is provided in
Once the sample is uniformly heated such that the entire sample block has a sufficiently low process viscosity, it may be shaped into a high quality amorphous bulk article via any number of techniques including, for example, injection molding, dynamic forging, stamp forging, blow molding, etc. However, the ability to shape a charge of metallic glass depends entirely on ensuring that the heating of the charge is both rapid and uniform across the entire sample block. If uniform heating is not achieved, then the sample will instead experience localized heating and, although such localized heating can be useful for some techniques, such as, for example, joining or spot-welding pieces together, or shaping specific regions of the sample, such localized heating has not and cannot be used to perform bulk shaping of samples. Likewise, if the sample heating is not sufficiently rapid (typically on the order of 500-105 K/s) then either the material being formed will lose its amorphous character, or the shaping technique will be limited to those amorphous materials having superior processability characteristics (i.e., high stability of the supercooled liquid against crystallization), again reducing the utility of the process.
The RCDF method of the current invention ensures the rapid uniform heating of a sample. However, to understand the necessary criteria for obtaining rapid, uniform heating of a metallic glass sample using RCDF it is necessary to first understand how Joule heating of metal materials occurs. The temperature dependence of the electrical resistivity of a metal can be quantified in terms of a relative change of resistivity per unit of temperature change coefficient, S, where S is defined as:
S=(1/ρ0)[dρ(T)/dT]To (Eq. 1)
where S is in units of (1/degrees-C.), ρ0 is the resistivity (in Ohm-cm) of the metal at room temperature To, and [dρ/dT]To is the temperature derivative of the resistivity at room temperature (in Ohm-cm/C) taken to be linear. A typical amorphous material has a large ρ0(80 μΩ-cm<ρ0<300 μΩ-cm), but a very small (and frequently negative) value of S (−1×10−4<s<+1×10−4).
For the small S values found in amorphous alloys, a sample of uniform cross-section subjected to a uniform current density will be ohmically heated uniformly in space, the sample will be rapidly heated from ambient temperature, T0, to a final temperature, TF, which depends on the total energy of the capacitor, given by the equation:
E=½CV2 (Eq. 2)
and the total heat capacity, CS (in Joules/C), of the sample charge. TF will be given by the equation:
T
F
=T
0
±E/C
S (Eq. 3).
In turn, the heating time will be determined by the time constant τRC=RC of the capacitive discharge. Here R is the total resistance of the sample (plus output resistance of the capacitive discharge circuit. Accordingly, in theory the typical heating rate for a metallic glass can be given by the equation:
dT/dt=(TF−T0)/τRC (Eq. 4)
By contrast, common crystalline metals have much lower ρ0 (1-30 μΩ-cm) and much greater values of S˜0.01-0.1. This leads to significant differences in behavior. For example, for common crystalline metals such as copper alloys, aluminum, or steel alloys, ρ0 is much smaller (1-20 μΩ-cm) while S is much larger, typically S˜0.01-0.1. The smaller ρ0 values in crystalline metals will lead to smaller dissipation in the sample (compared with the electrodes) and make the coupling of the energy of the capacitor to the sample less efficient. Furthermore, when a crystalline metal melts, ρ(T) generally increases by a factor of 2 or more on going from the solid metal to the molten metal. The large S values along with increase of resistivity on melting of common crystalline metals leads to extreme non-uniform Ohmic heating in a uniform current density. The crystalline sample will invariably melt locally, typically in the vicinity of the high voltage electrode or other interface within the sample. In turn, a capacitor discharge of energy through a crystalline rod leads to spatial localization of heating and localized melting wherever the initial resistance was greatest (typically at interfaces). In fact, this is the basis of capacitive discharge welding (spot welding, projection welding, “stud welding” etc.) of crystalline metals where a local melt pool is created near the electrode/sample interface or other internal interface within the parts to be welded.
As discussed in the Background, prior art systems have also recognized the inherent conductive properties of amorphous materials; however, what has not been recognized to date is that to ensure uniform heating of the entire sample it is also necessary to avoid the dynamic development of spatial inhomogeneity in the energy dissipation within the heating sample. The RCDF method of the current invention sets forth two criteria, which must be met to prevent the development of such inhomogeneity and to ensure uniform heating of the charge: uniformity of the current within the sample; and stability of the sample with respect to development of inhomogeneity in power dissipation during dynamic heating.
Although these criteria seem relatively straightforward, they place a number of physical and technical constraints on the electrical charge used during heating, the material used for the sample, the shape of the sample, and the interface between the electrode used to introduce the charge and the sample itself. For example, for a cylindrical charge of length L and area A=πR2 (R=sample radius), the following requirements will exist.
Uniformity of the current within the cylinder during capacity discharge requires that the electromagnetic skin depth, A, of the dynamic electric field is large compared to relevant dimensional characteristics of the sample (radius, length, width or thickness). In the example of a cylinder, the relevant characteristic dimensions will obviously be the radius and depth of the charge, R and L. This condition is satisfied when Λ=[ρ0τ/μ0]1/2>R, L. Here τ is the “RC” time constant of the capacitor and sample system, μ0=4π×10−7 (Henry/m) is the permittivity of free space. For R and L˜1 cm, this implies τ>10-100 μs. Using typical dimensions of interest and values of resistivity of amorphous alloys, this requires a suitably sized capacitor, typically capacitance of ˜10,000 μF or greater.
Stability of the sample with respect to development of inhomogeneity in power dissipation during dynamic heating can be understood by carrying out stability analysis which includes Ohmic “Joule” heating by the current and heat flow governed by the Fourier equation. For a sample with resistivity, which increases with temperature (i.e., positive S), a local temperature variation along the axis of the sample cylinder will increase local heating, which further increases the local resistance and heat dissipation. For sufficiently high power input, this leads to “localization” of heating along the cylinder. For crystalline materials, it results in localized melting. Whereas this behavior is useful in welding where one wishes to produce local melting along interfaces between components, this behavior is extremely undesirable if one wishes to uniformly heat an amorphous material. The present invention provides a critical criterion to ensure uniform heating. Using S as defined above, we find heating should be uniform when:
where D is the thermal diffusivity (m2/s) of the amorphous material, CS is the total heat capacity of the sample, and R0 is the total resistance of the sample. Using values of D and CS typical of metallic glass, and assuming a length (L˜1 cm), and an input power I2R0˜106 Watts, typically required for the present invention, it is possible to obtain a Scrit˜10−4-10−5. This criterion for uniform heating should be satisfied for many metallic glasses (see above S values). In particular, many metallic glasses have S<0. Such materials (i.e., with S<0) will always satisfy this requirement for heating uniformity. Exemplary materials that meet this criterion are set forth in U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975, the disclosures of which are incorporated herein by reference.
Beyond the fundamental physical criteria of the charge applied and the amorphous materials used there are also technical requirements to ensure that the charge is applied as evenly as possible to the sample. For example, it is important the sample be substantially free of defects and formed with a uniform cross-section. If these conditions are not met, the heat will not dissipate evenly across the sample and localized heating will occur. Specifically, if there is a discontinuity or defect in the sample block then the physical constants (i.e., D and Cs) discussed above will be different at those points leading to differential heating rates. In addition, because the thermal properties of the sample also are dependent on the dimensions of the item (i.e., L) if the cross-section of the item changes then there will be localized hot spots along the sample block. Moreover, if the sample contact surfaces are not adequately flat and parallel, an interfacial contact resistance will exist at the electrode/sample interface. Accordingly, in one embodiment the sample block is formed such that it is substantially free of defects and has a substantially uniform cross-section. It should be understood that though the cross-section of the sample block should be uniform, as long as this requirement is met there are no inherent constraints placed on the shape of the block. For example, the block may take any suitable geometrically uniform shape, such as a sheet, block, cylinder, etc. In another embodiment, the sample contact surfaces are cut parallel and polished flat in order to ensure good contact with the electrodes.
In addition, it is important that no interfacial contact resistance develops between the electrode and the sample. To accomplish this, the electrode/sample interface must be designed to ensure that the electrical charge is applied evenly, i.e., with uniform density, such that no “hot points” develop at the interface. For example, if different portions of the electrode provide differential conductive contact with the sample, spatial localization of heating and localized melting will occur wherever the initial resistance is greatest. This in turn will lead to discharge welding where a local melt pool is created near the electrode/sample interface or other internal interface within the sample. In light of this requirement of uniform current density, in one embodiment of the current invention the electrodes are polished flat and parallel to ensure good contact with the sample. In another embodiment of the current invention the electrodes are made of a soft metal, and uniform “seating” pressure is applied that exceeds the electrode material yield strength at the interface, but not the electrode buckling strength, so that the electrode is positively pressed against the entire interface yet unbuckled, and any non-contact regions at the interface are plastically deformed. In yet another embodiment of the current invention, a uniform low-energy “seating” pulse is applied that is barely sufficient to raise the temperature of any non-contact regions of the amorphous sample at the contact surface of the electrode to slightly above the glass transition temperature of the amorphous material, and thus allowing the amorphous sample to conform to the microscopic features of the contact surface of the electrode. In addition, in yet another embodiment the electrodes are positioned such that positive and negative electrodes provide a symmetric current path through the sample. Some suitable metals for electrode material are Cu, Ag and Ni, and alloys made substantially of Cu, Ag and Ni (Le., that contain at least 95 at % of these materials).
Lastly, provided that the electric energy is successfully discharged uniformly into the sample, the sample will heat up uniformly if heat transport towards the cooler surrounding and electrodes is effectively evaded, i.e., if adiabatic heating is achieved. To generate adiabatic heating conditions, dT/dt has to be high enough, or τRC small enough, to ensure that thermal gradients due to thermal transport do not develop in the sample. To quantify this criterion, the magnitude of τRC should be considerably smaller than the thermal relaxation time of the amorphous metal sample, τth, given by the following equation:
τth=csR2/ks (Eq. 6)
where ks and cs are the thermal conductivity and specific heat capacity of the amorphous metal, and R is the characteristic length scale of the amorphous metal sample (e.g. the radius of a cylindrical sample). Taking ks˜10 W/(m K) and cs˜5×106J/(m3 K) representing approximate values for Zr-based glasses, and R˜1×10−3 m, we obtain τth˜0.5 s. Therefore, capacitors with τRC considerably smaller than 0.5 s should be used to ensure uniform heating.
Turning to the shaping method itself, a schematic of an exemplary shaping tool in accordance with the RCDF method of the current invention is provided in
It should be understood that any source of electrical energy suitable for supplying sufficient energy of uniform density to rapidly and uniformly heat the sample block to the predetermined process temperature, such as, for example, a capacitor having a discharge time constant of from 10 μs to 10 milliseconds may be used. In addition, any electrodes suitable for providing uniform contact across the sample block may be used to transmit the electrical energy. As discussed, in one preferred embodiment the electrodes are formed of a soft metal, such as, for example, Ni, Ag, Cu, or alloys made using at least 95 at % of Ni, Ag and Cu, and are held against the sample block under a pressure sufficient to plastically deform the contact surface of the electrode at the electrode/sample interface to conform it to the microscopic features of the contact surface of the sample block.
Although the above discussion has focused on the RCDF method generally, the current invention is also directed to an apparatus for shaping a sample block of amorphous material. In one preferred embodiment, shown schematically in
Although an injection molding technique is discussed above, any suitable shaping technique may be used. Some alternative exemplary embodiments of other shaping methods that may be used in accordance with the RCDF technique are provided in
In another embodiment, shown schematically in
In yet another exemplary embodiment, shown schematically in
In yet another exemplary embodiment, shown schematically in
In still yet another embodiment, shown schematically in
Although the above discussion has focused on the essential features of a number of exemplary shaping techniques, it should be understood that other shaping techniques may be used with the RCDF method of the current invention, such as extrusion or die casting. Moreover, additional elements may be added to these techniques to improve the quality of the final article. For example, to improve the surface finish of the articles formed in accordance with any of the above shaping methods the mold or stamp may be heated to around or just below the glass transition temperature of the amorphous material, thereby smoothing surface defects. In addition, to achieve articles with better surface finish or net-shape parts, the compressive force, and in the case of an injection molding technique the compressive speed, of any of the above shaping techniques may be controlled to avoid melt front instability arising from high “Weber number” flows, i.e., to prevent atomization, spraying, flow lines, etc.
The RCDF shaping techniques and alternative embodiments discussed above may be applied to the production of small, complex, net shape, high performance metal components such as casings for electronics, brackets, housings, fasteners, hinges, hardware, watch components, medical components, camera and optical parts, jewelry etc. The RCDF method can also be used to produce small sheets, tubing, panels, etc. which can be dynamically extruded through various types of extrusion dyes used in concert with the RCDF heating and injection system.
In summary, the RCDF technique of the current invention provides a method of shaping amorphous alloys that allows for the rapid uniform heating of a wide range of amorphous materials and that is relatively cheap and energy efficient. The advantages of the RCDF system are described in greater detail below.
Rapid and Uniform Heating Enhances Thermoplastic Processability:
Thermoplastic molding and forming of BMGs is severely restricted by the tendency of BMGs to crystallize when heated above their glass transition temperature, Tg. The rate of crystal formation and growth in the undercooled liquid above Tg increases rapidly with temperature while the viscosity of the liquid falls. At conventional heating rates of ˜20 C/min, crystallization occurs when BMGs are heated to a temperature exceeding Tg by ΔT=30-150° C. This ΔT determines the maximum temperature and lowest viscosity for which the liquid can be thermoplastically processed. In practice, the viscosity is constrained to be larger thane ˜104 Pa-s, more typically 105-107 Pa-s, which severely limits net shape forming. Using RCDF, the amorphous material sample can be uniformly heated and simultaneously formed (with total required processing times of milliseconds) at heating rates ranging from 104-107 C/s. In turn, the sample can be thermoplastically formed to net shape with much larger ΔT and as a result with much lower process viscosities in the range of 1 to 104 Pa-s, which is the range of viscosities used in the processing of plastics. This requires much lower applied loads, shorter cycle times, and will result in much better tool life.
RCDF Enables Processing of a Much Broader Range of BMG Materials:
The dramatic expansion of ΔT and the dramatic reduction of processing time to milliseconds enable a far larger variety of glass forming alloys to be processed. Specifically, alloys with small ΔT, or alloys having much faster crystallization kinetics and in turn far poorer glass forming ability, can be processed using RCDF. For example, cheaper and otherwise more desirable alloys based on Zr, Pd, Pt, Au, Fe, Co, Ti, Al, Mg, Ni and Cu and other inexpensive metals are rather poor glass formers with small ΔT and strong tendency to crystallize. These “marginal glass forming” alloys cannot be thermoplastically processed using any of the currently practiced methods, but can easily be used with the RCDF method of the current invention.
RCDF is Extremely Material Efficient:
Conventional processes that are currently being used to form bulk amorphous articles such as die casting require the use of feedstock material volume that far exceeds the volume of the part being cast. This is because of the entire ejected content of a die in addition to castings includes gates, runners, sprue (or biscuit), and flash, all of which are necessary for the molten metal passage towards the die cavity. In contrast, the RCDF ejected content in most cases will only include the part, and in the case of the injection molding apparatus, a shorter runner and a much thinner biscuit as compared to die casting. The RCDF method will therefore be particularly attractive for applications involving processing of high-cost amorphous materials, such as the processing of amorphous metal jewelry.
RCDF is Extremely Energy Efficient:
Competing manufacturing technologies such as die-casting, permanent-mold casting, investment casting and metal powder injection molding (PIM), are inherently far less energy efficient. In RCDF, the energy consumed is only slightly greater than that required to heat the sample to the desired process temperature. Hot crucibles, RF induction melting systems, etc. are not required. Further, there is no need to pour molten alloy from one container to another thereby reducing the processing steps required and the potential for material contamination and material loss.
RCDF Provides a Relatively Small, Compact, and Readily Automated Technology:
Compared with other manufacturing technologies, RCDF manufacturing equipment will be small, compact, clean, and will lend itself readily to automation with a minimum of moving parts and an essentially all “electronic” process.
Environmental Atmosphere Control not Required:
The millisecond time scales required to process a sample by RCDF will result in minimal exposure of the heated sample to ambient air. As such, the process can be carried out in the ambient environment as opposed to current process methods where extended air exposure gives severe oxidation of the molten metal and final part.
Exemplary Embodiments from U.S. application Ser. No. 12/409,253
Examples 1 and 2 are copied from and incorporated herein from U.S. application Ser. No. 12/409,253.
The person skilled in the art will recognize that additional embodiments according to the invention are contemplated as being within the scope of the foregoing generic disclosure, and no disclaimer is in any way intended by the foregoing, non-limiting examples
To demonstrate the basic principle that for BMGs capacitive discharge with Ohmic heat dissipation in a cylindrical sample will give uniform and rapid sample heating a simple laboratory spot welding machine was used as a demonstration shaping tool. The machine, a Unitek 1048 B spot welder, will store up to 100 Joules of energy in a capacitor of ˜10 μf. The stored energy can be accurately controlled. The RC time constant is of order 100 μs. To confine a sample cylinder, two paddle shaped electrodes were provided with flat parallel surfaces. The spot welding machine has a spring loaded upper electrode which permits application of an axial load of up to ˜80 Newtons of force to the upper electrode. This, in turn permits a constant compressive stress ranging to ˜20 MPa to be applied to the sample cylinder.
Small right circular cylinders of several BMG materials were fabricated with diameters of 1-2 mm and heights of 2-3 mm. The sample mass ranged from ˜40 mg to about ˜170 mg and was selected to obtain TF well above the glass transition temperature of the particular BMG. The BMG materials were a Zr—Ti-based BMG (Vitreloy 1, a Zr—Ti—Ni—Cu—Be BMG), a Pd-based BMG (Pd—Ni—Cu—P alloy), and an Fe-based BMG (Fe—Cr—Mo—P—C) having glass transitions (Tg) at 340 C, 300 C, and ˜430 C respectively. All of these metallic glasses have S˜−1×10−4<<Scrit.
a to 8d show the results of a series of tests on Pd-alloy cylinders of radius 2 mm and height 2 mm (8a). The resistivity of the alloy is ρ0=190 μΩ-cm, while S˜−1×104 (C−1). Energies of E=50 (8b), 75 (8c), and 100 (8d) Joules were stored in the capacitor bank and discharged into the sample held under a under a compressive stress of ˜20 MPa. The degree of plastic flow in the BMG was quantified by measuring the initial and final heights of the processed samples. It is particularly important to note that the samples are not observed to bond to the copper electrode during processing. This can be attributed to the high electrical and thermal conductivity of copper compared to the BMG. In short, the copper never reaches sufficiently high temperature to allow wetting by the “molten” BMG during the time scale of processing (˜milliseconds). Further, it should be noted that there is little or no damage to the electrode surface. The final processed samples were freely removed from the copper electrode following processing and are shown in
The initial and final cylinder heights were used to determine the total compressive strain developed in the sample as it deformed under load. The engineering “strain” is given by H0/H where H0 and H are the initial (final) height of the sample cylinder respectively. The true strain is given by ln(H0/H). The results are plotted vs. discharge energy in
These tests results indicate that the plastic deformation of the BMG sample blank is a well-defined function of the energy discharged by the capacitor. Following dozens of tests of this type, it is possible to determine that plastic flow of the sample (for a given sample geometry) is a very well defined function of energy input, as is clearly shown in
In another example, a working prototype RCDF injection molding apparatus was constructed. Schematics of the device are provided in
The entire machine is comprised of several independent systems, including an electrical energy charge generation system, a controlled process pressure system, and a mold assembly. The electrical energy charge generation system comprises a capacitor bank, voltage control panel and voltage controller all interconnected to a mold assembly (60) via a set of electrical leads (62) and electrodes (64) such that an electrical discharge of may be applied to the sample blank through the electrodes. The controlled process pressure system (66) includes an air supply, piston regulator, and pneumatic piston all interconnected via a control circuit such that a controlled process pressure of up to ˜100 MPa may be applied to a sample during shaping. Finally, the shaping apparatus also includes the mold assembly (60), which will be described in further detail below, but which is shown in this figure with the electrode plunger (68) in a fully retracted position.
The total mold assembly is shown removed from the larger apparatus in
As shown in
Once the sample block is in position and in positive contact with the electrode the sample block is heated via the RCDF method. The heated sample becomes viscous and under the pressure of the plunger is controllably urged through the gate (84) into the mold (72). As shown in
This experiment provides evidence that complex net-shape parts may be formed using the RCDF technique of the current invention. Although the mold is formed into the shape of a ring in this embodiment, one of skill in the art will recognize that the technique is equally applicable to a wide variety of articles, including small, complex, net shape, high performance metal components such as casings for electronics, brackets, housings, fasteners, hinges, hardware, watch components, medical components, camera and optical parts, jewelry etc.
Even though there is no liquid/crystallization transformation for a bulk solidifying amorphous metal, a “melting temperature” Tm may be defined as the thermodynamic liquidus temperature of the corresponding crystalline phase. Under this regime, the viscosity of bulk-solidifying amorphous alloys at the melting temperature can lie in the range of about 0.1 poise to about 10,000 poise, and even sometimes under 0.01 poise. A lower viscosity at the “melting temperature” will provide faster and complete filling of intricate portions of the shell/mold with a bulk solidifying amorphous metal for forming the BMG parts. Furthermore, the cooling rate of the molten metal to form a BMG part has to such that the time-temperature profile during cooling does not traverse through the nose-shaped region bounding the crystallized region in the TTT diagram of
The supercooled liquid region, the temperature region between Tg and Tx is a manifestation of the extraordinary stability against crystallization of bulk solidification alloys. In this temperature region the bulk solidifying alloy can exist as a high viscous liquid. The viscosity of the bulk solidifying alloy in the supercooled liquid region can vary between 1012 Pa s at the glass transition temperature down to 105 Pa s 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. The embodiments herein make use of the large plastic formability in the supercooled liquid region as a forming and separating method.
One needs to clarify something about Tx. Technically, the nose-shaped curve shown in the TTT diagram describes Tx as a function of temperature and time. Thus, regardless of the trajectory that one takes while heating or cooling a metal alloy, when one hits the TTT curve, one has reached Tx. In
The schematic TTT diagram of
Typical differential scanning calorimeter (DSC) heating curves of bulk-solidifying amorphous alloys taken at a heating rate of 20 C/min describe, for the most part, a particular trajectory across the TTT data where one will likely see a Tg at a certain temperature, a Tx when the DSC heating ramp crosses the TTT crystallization onset, and eventually melting peaks when the same trajectory crosses the temperature range for melting. If one heats a bulk-solidifying amorphous alloy at a rapid heating rate as shown by the ramp up portion of trajectories (2), (3) and (4) in
Additional Exemplary Embodiments
The additional exemplary embodiments below are novel and based on the concepts of forming amorphous metal by rapid capacitor discharge as disclosed in U.S. application Ser. No. 12/409,253.
The embodiment relates to the use of discharge of current to heat up the area which one wishes to deform. In one embodiment, a composition comprising an amorphous alloy can be used to form a joining mechanism, such as a mechanical interlock, to join at least two separate parts. More than two parts can be joined using the presently described methods.
At least a portion of the alloy can be thermoplastically deformed, as shown in the schematic diagrams shown in
An embodiment herein relates to a method of heating a BMG pin or any other BMG piece. The current art involves conductively heating and thermoforming the BMG pin or BMG piece. Rapid discharge forming (RDF) can improve the cycle time, deformation of the BMG pin or piece and reduce crystallinity (from exposure to high temperatures for long durations). The possible methods that can be used are as follows: heat a whole piece using capacitive discharge; place electrodes in strategic locations so heating occurs locally in the area to be deformed during the capacitive discharge; or using a tip which can be an electrode itself to deform material in the same or subsequent step. These methods are shown in
One can apply a load to the pin either using a separate tool or an integrated tool and electrode. Then one can apply the rapid discharge of current through the workpiece by one of the methods shown in
The advantage of using rapid discharge of current to heat up the sample over using a thermally-conductive tip for conducting the heat into the part is that one can get a much higher heat penetration depth. So one can reach higher temperatures much quicker throughout the thickness of the whole sample, as supposed to just a small surface layer, which one probably get if one were using a conductive tip. This is partially due to the material of the sample having a low thermal-conductivity. Some metallic glasses have a thermal conductivity of 5 W/mK as compared to stainless steel which has a thermal conductivity of about 30 W/mK. In short, by rapid discharge, one is able to get more heat into the sample uniformly through the thickness by rapid heating. This way one heat the sample above the Tg of the metallic glass material in a short of time. This means one can work with a less viscous metallic glass and still relatively easily and quickly deform the metallic glass.
Rapid discharge can also be used as a cutting technique as shown in
There are advantages in the fabrication of complicated and intricate designs in structures and components using bulk solidifying amorphous alloys. The shrinkage of bulk solidifying amorphous alloys during casting or molding is very small; therefore, the as cast component can be used with minimal post-finishing. Furthermore, geometric factors such as ribs can be incorporated into the structure for better structural integrity. The bulk-solidifying amorphous alloy structures having nano- and/or micro-replications and components can be fabricated by either casting the amorphous alloys or molding the amorphous alloys.
Bulk amorphous alloys retain their fluidity from above the melting temperature down to the glass transition temperature due to the lack of a first order phase transition. This is in direct contrast to conventional metals and alloys. Since, bulk amorphous alloys retain their fluidity, they do not accumulate significant stress when cooled from their casting temperatures down to below the glass transition temperature, and as such dimensional distortions from thermal stress gradients can be minimized. Accordingly, intricate structures with large surface area and small thickness can be produced cost-effectively.
a and 19b show an embodiment for replicating text and logos with extremely high degrees of accuracy.
a) (labeled as “Pressing text+logos”) shows the process of forming a logo or text on a metallic glass workpiece using a positive mold of a tool or punch and performing a rapid discharge forming on the workpiece by locally heating up the surface in the vicinity of the electrode or heating the whole part and then pressing the shaped electrodes on the heated surface so as to leave an imprint on the surface of the metallic glass workpiece as shown in
One exemplary method for producing nano- and micro-replications in structures using a molding process comprises the following steps: providing a sheet feedstock of amorphous alloy being substantially amorphous; heating the feedstock to around the glass transition temperature by rapid discharge; forming nano- and/or micro-replication in the heated feedstock; and cooling the formed component to temperatures far below the glass transition temperature.
In the context of the embodiments herein, temperatures around glass transition means the forming temperatures can be below glass transition, at or around glass transition, and above glass transition temperature and below the melting temperature Tm, but preferably at temperatures below the crystallization temperature TX. The cooling step is carried out at rates similar to the heating rates at the heating step, and preferably at rates greater than the heating rates at the heating step. The cooling step is also achieved preferably while the forming and shaping loads are still maintained.
One potential embodiment of that is that one can have a zirconium based alloy and expose the surface to nitrogen to get a zirconium nitride layer forming that will have a gold colored surface appearance and it will also be very hard. In some situations one can expose the metallic glass under rapid discharge to not only nitrogen but some other vapor deposited material so as to cause formation of a ceramic-type layer, that can be a thin micro layer or even a nano layer of ceramic-type material that will further protect the metallic glass material from degradation, or even give the metallic glass material a really smooth surface like that of a diamond-like coating.
Ono will be able to form a micro structured coating as opposed to a macro structured feature like a logo or text which can be visible by the naked eye. For example, one can imprint very small features which can have functional benefits, for example, having a textured coating at a certain size scale that can, for example, provide a surface with hydrophobic properties.
Another embodiment relates to roll forming with or without texture formation on the surface of the metallic glass sheet that is passed through or extruded between rolls as shown in
Using rapid pulse discharge, one will pulse the discharge multiple times so generate rapid discharge in a semi-continuous manner. In order to cause rapid pulse discharge, one will select the frequency and power as appropriate to heat an area big enough for one to work on while the capacitor charges and allows one to provide the next pulse.
This application claims priority to U.S. Provisional Application No. 61/547,025, filed Oct. 13, 2011, which is incorporated herein in entirety by reference. This application is related to U.S. application Ser. No. 12/409,253, filed Mar. 23, 2009, PCT/US11/52170, filed Sep. 19, 2011, and PCT/US11/42852, filed Jul. 1, 2011, the disclosures of which are incorporated herein in entirety by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61547025 | Oct 2011 | US |
