US20160318095A1

US20160318095A1 - Molding and De-Molding of Metallic Glass Using Non-Disposable Molds

Info

Publication number: US20160318095A1
Application number: US15/033,173
Authority: US
Inventors: Jan Schroers; Rodrigo Miguel Ojeda Mota; Jittisa Ketkaew; Punnathat Bordeenithikasem
Original assignee: Yale University
Current assignee: Yale University
Priority date: 2013-10-29
Filing date: 2014-10-29
Publication date: 2016-11-03
Also published as: EP3063307A1; EP3063307A4; WO2015066145A1

Abstract

A reusable mold comprising a flexible bulk metallic glass mold insert comprising one or more mold cavities removably coupled to a support mold and a method of making the reusable mold. The support mold is removable from the flexible bulk metallic glass mold insert without macroscopic elastic flexing or deforming of the support mold. The reusable mold may be used for molding one or more bulk metallic glass parts and the one or more molded bulk metallic glass parts may be released from the flexible bulk metallic glass mold insert by elastically flexing the flexible mold insert.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Application Ser. No. 61/896,986, the subject matter of which is herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to a method of molding and de-molding of bulk metallic glasses (BMGs) into complex shapes.

BACKGROUND OF THE INVENTION

Bulk metallic glasses (BMGs), also known as bulk solidifying amorphous alloy compositions, are a class of amorphous metallic alloy materials that are regarded as prospective materials for a vast range of applications because of their superior properties, including high yield strength, large elastic strain limit, and high corrosion resistance.
A unique property of BMGs is that they have a super-cooled liquid region (SCLR), ΔTsc, which is a relative measure of the stability of the viscous liquid regime. The SCLR is defined by the temperature difference between the onset of crystallization, Tx, and the glass transition temperature, Tg of the particular BMG alloy. These values can be conveniently determined by using standard calorimetric techniques such as DSC (Differential Scanning calorimetry) measurements at 20° C./min.
Generally, a larger ΔTsc is associated with a lower critical cooling rate, though a significant amount of scatter exists at ΔTsc values of more than 40° C. Bulk-solidifying amorphous alloys with a ΔTsc of more than 40° C., and preferably more than 60° C., and still more preferably a ΔTsc of 70° C. or more, are very desirable because of the relative ease of forming. In the SCLR, the bulk solidifying alloy behaves like a high viscous fluid. The viscosity for bulk solidifying alloys with a wide SCLR decreases from 10¹²Pa·s at the glass transition temperature to 10⁷Pa·s and in some cases to 10⁵Pa·s. Heating the bulk solidifying alloy beyond the crystallization temperature leads to crystallization and immediate loss of the superior properties of the alloy and it can no longer be formed.
Superplastic forming (SPF) of an amorphous metal alloy involves heating it into the SCLR and forming it under an applied pressure. The method is similar to the processing of thermoplastics, where the formability, which is inversely proportional to the viscosity, increases with increasing temperature. In contrast to thermoplastics, the highly viscous amorphous metal alloy is metastable and eventually crystallizes.
Crystallization of the BMG (or amorphous metal alloy) must be avoided for several reasons. First, it degrades the mechanical properties of the BMG. From a processing standpoint, crystallization limits the processing time for hot-forming operation because the flow in crystalline materials is at least an order of magnitude higher than in the liquid BMG. Crystallization kinetics for various BMGs allows processing times between minutes and hours in the described viscosity range. This makes the superplastic forming method a finely tunable process that can be performed at convenient time scales, enabling the net-shaping of complicated geometries. Since similar processing pressures and temperatures are used in the processing of thermoplastics, techniques used for thermoplastics, including compression molding, extrusion, blow molding, and injection molding have also been suggested for processing BMGs as described for example in U.S. Pat. No. 8,641,839 to Schroers et al. and U.S. Pat. Pub. No. US2013/0306262 to Schroers et al., the subject matter of each of which is herein incorporated by reference in its entirety.
BMGs are an ideal material for small geometries because they are homogeneous and isotropic. This is due to the fact that no “intrinsic” limitation such as the grain size in crystalline materials is present. Also, since thermoplastic forming is done isothermally and the subsequent cooling step can be carried out slowly, thermal stresses can be reduced to a negligible level.
Molding of BMGs on the nano, micro, and macro length scale is known in the art as described for example in U.S. Pat. No. 8,641,839 to Schroers et al. and U.S. Pat. Pub. No. US2013/0306262 to Schroers et al., the subject matter of each of which is herein incorporated by reference in its entirety.
However, objects on the millimeter length scale in all three dimensions, and also having combined micron length scale features, or that require micron size precision, are very challenging to mold from BMGs because methods for fabricating the molds are limited. Many mechanical parts including, for example, watch movement parts, biomedical implants and devices, and resonators, among many others, are on this miniature length scale (which is typically about one millimeter but covers the length scale from about 100 nm to about 1 cm).
One method of micro molding has focused on the use of silicon molds. However, parts fabricated using silicon molds are limited in depth to typically less than about 300 microns. In addition, this method is not suitable for the fabrication of miniature parts and even micro parts of BMGs through thermoplastic molding for the following reasons:

- 1) The linear thermal expansion coefficient of polycrystalline silicon is 2.6×10⁻⁶C⁻¹, while the thermal expansion coefficient of BMGs varies between ˜10×10⁻⁶C⁻¹−25×10⁻⁶C⁻¹(summary in²). Thus, a typical mismatch, Δα, is about 10×10⁻⁶C⁻¹. Such a Δα creates stresses when cooling after the molding operation back to ambient temperature. These stresses can cause severe problems in the de-molding of the BMG from the silicon mold. In addition, these stresses can lead to mechanical locking of the BMG part in the mold, particularly when a plurality of cavities is molded, leading to bending and breaking of the mold and limit the mold to a one-time use;
- 2) Silicon lithography is typically limited to parts having a depth of less than about 300 microns;
- 3) Silicon molds are expensive, particularly when used for millimeter to centimeter size parts; and
- 4) Silicon molds cannot be reused because of their brittle nature, issues regarding mechanical locking, and the precision with which the BMG replicates mold features. In very limited cases for smallest aspect ratios (i.e., <1) and simple features, several usages of the silicon mold may be possible. However, in this instance, the usages are generally limited to less than 5 usages.

As a consequence, the use of silicon molds as a working mold for BMGs that can be used multiple times and in a parallel molding process is simply not possible.
The process of filling mold cavities based on thermoplastic molding of BMGs has been widely demonstrated. However all of these processes (except when very simple structures are used with a small aspect ratio, e.g., <0.5 and very large draft angle) lack the ability to reuse the mold. As a consequence, miniature molding of BMG parts has been prohibitively expensive and limited to high-value added specialty applications.
De-molding forces have two origins. The first demolding force is a chemical bond between mold and part. Chemical bonding can be readily avoided between many mold-BMG combinations when a mold material is chosen with a significantly higher (i.e., at least 10 times) flow stress than the part material as described for example in U.S. Pat. Pub. No. 2010/0098967 to Schroers et al., the subject matter of which is herein incorporated by reference in its entirety. In this instance, the mold does not plastically deform, which is a requirement for avoiding a chemical bond.
The second demolding force is mechanical locking, in which the part is mechanically locked into the mold, meaning that the part cannot be removed from the mold without destroying the mold. The origin for a mechanical locking is in the geometry of the mold cavity or roughness, where undercuts cause mechanical locking, and in the difference in thermal expansion coefficient between mold and part material, Δα. A typical mold material for micron size molding is silicon. As discussed above, the mismatch in linear thermal expansion coefficient, Δα, can cause severe problems in the de-molding of the BMG out of the silicon mold. The stresses can also lead to mechanical locking of the BMG part in the mold (during parallel processing), leading to bending of the mold, and breaking of the mold.
For example, FIG. 1 shows demolding forces that are proportional to Δα. This situation is typical in parallel miniature molding wherefore for most cases Δα should be minimized. FIG. 1 depicts the origin of the de-molding challenge in parallel miniature molding. Since the thermal expansion coefficient for the mold is always smaller than the one of the part, after molding, when the part is cooled to be released, it shrinks faster than the mold. As a consequence the parts are forced against the surface of the mold cavity towards the center. This increases required de-molding forces, cause stresses in the mold which may bend the mold, and higher stresses can cause the mold even to fracture. Reducing Δα is effective in reducing required de-molding forces.
Mold roughness is relatively insensitive to mold size, meaning that a similar absolute roughness is present in small size molds as in large molds. Thus, with decreasing mold size, the roughness to mold size ratio increases, and the relative roughness increases. Therefore de-molding is generally more challenging for small size parts.
Thus, it would be desirable to provide a molding process and hardware for molding BMGs that also allows for de-molding, thereby allowing for re-usage of the molds and which can be carried out massively parallel (i.e., with a plurality of mold cavities).

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved mold for molding bulk metallic glass (BMG) parts.
It is another object of the present invention to provide a method of making a mold for molding BMG parts.
It is still another object of the present invention to provide a method of making a mold for molding BMG parts in which the mold can be used multiple times.
It is another object of the present invention to provide a method of molding BMG parts comprising miniature and/or micro-scale features.
It is another object of the present invention to provide a method of molding BMG parts comprising combinations of length scales with macro scale dimensions including miniature and/or micro-scale features.
It is another object of the present invention to provide a method of molding BMG parts comprising miniature and/or micro-scale features exhibiting complex geometries.
It is another object of the present invention to provide a method of molding a plurality of miniature BMG parts massively in parallel.
To that end, in one embodiment, the present invention relates generally to a reusable mold comprising:
a flexible bulk metallic glass mold insert comprising one or more mold cavities removably coupled to a support mold;
wherein the support mold is removable from the flexible bulk metallic glass mold insert without macroscopic elastic flexing or deforming of the support mold.
In another embodiment, the present invention relates generally to a method of making a reusable mold comprising one or more mold cavities, the method comprising the steps of:

- a) deforming a bulk metallic glass (BMG_flexible) feedstock against a surface of a master mold to replicate the surface of the master mold and create a flexible bulk metallic glass mold insert comprising one or more mold cavities;
- b) removably coupling the flexible bulk metallic glass insert to a support mold,
  - wherein the support mold is coupled to a surface of the flexible bulk metallic glass insert opposite the replicated surface; and
- c) removing the master mold from the flexible bulk metallic glass insert after the support mold has been removed from the insert mold.
  - wherein the support mold is removable from the flexible bulk metallic glass mold insert without macroscopic deforming of the support mold.

In still another embodiment, the present invention relates generally to a method of molding a bulk metallic glass part using a reusable mold comprising a flexible bulk metallic glass mold insert removably coupled to a support mold, the flexible bulk metallic glass mold insert comprising one or more mold cavities, the method comprising the steps of:

- a. heating a bulk metallic glass to be molded (BMG_part) into a supercooled liquid region for the bulk metallic glass;
- b. disposing the bulk metallic glass (BMG_mold) into the one or more mold cavities in the flexible bulk metallic glass mold insert (BMG_insert);
- c. cooling the bulk metallic glass to solidify the bulk metallic glass within the one or more mold cavities and create the molded bulk metallic glass part;
- d. removing the support mold from the flexible bulk metallic glass insert without macroscopic deforming of the support mold (BMG_support); and
- e. releasing the one or more molded bulk metallic glass parts from the one or more mold cavities in the BMG_insertby elastically flexing the BMG_insert.

BRIEF DESCRIPTION OF THE FIGURES

For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying figures, in which:

FIG. 1 depicts de-molding forces present in parallel miniature molding.

FIG. 2 depicts the fabrication of a mold with flexible BMG mold inserts in accordance with the present invention.

FIG. 3 depicts the fabrication of a mold with flexible BMG mold inserts where BMGinsert and BMGsupport are of the same BMG and separated by a separation layer in accordance with the present invention.

FIG. 4A depicts a master mold replicated through blow molding of a first BMG while FIG. 4B depicts a flexible BMG mold insert fabricated as in FIG. 4A but not separated from the master mold.

FIG. 5 depicts fabrication of a mold using wetting phenomena to generate thin BMG inserts.

FIG. 6 depicts replication of a mold with another BMG.

FIG. 7 depicts examples of miniature part geometries molded in accordance with the present invention.

Also, while not all elements may be labeled in each figure, all elements with the same reference number indicate similar or identical parts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention describes a method of molding and de-molding BMGs based on thermoplastic molding which can be carried out massively parallel, and in which a plurality of mold cavities are used. A key aspect of the invention described herein is that the molds may be reused multiple times. This is achieved by using a mold that includes a thin BMG mold insert that can be elastically bent upon demolding, reducing de-mold forces through optimizing thermal expansion mismatch. The present invention also describes various fabrication methods for making these reusable molds.
In one embodiment, the present invention relates generally to a reusable mold comprising:
a flexible bulk metallic glass mold insert comprising one or more mold cavities removably coupled to a support mold;
wherein the support mold is removable from the flexible bulk metallic glass mold insert without macroscopic elastic flexing or deforming of the support mold.
In one embodiment, the mold insert is highly elastic. Thus, in one embodiment, the mold insert may comprise a percent elasticity of at least 1.3%.
In one embodiment, the flexible bulk metallic glass mold insert may comprise features on a miniature-length scale. In another embodiment, the flexible bulk metallic glass mold insert comprises features having multiple length scales with at least one feature having a length scale on the miniature scale.
In another embodiment, the present invention relates generally to a method of making a reusable mold comprising one or more mold cavities, the method comprising the steps of:

- a) deforming a bulk metallic glass (BMG_flexible) feedstock against a surface of a master mold to replicate the surface of the master mold and create a flexible bulk metallic glass mold insert comprising one or more mold cavities;
- b) removably coupling the flexible bulk metallic glass insert to a support mold, and
- c) removing the master mold from the flexible bulk metallic glass mold insert after the support mold has been removed from the flexible bulk metallic glass mold insert.
  - wherein the support mold is coupled to a surface of the flexible bulk metallic glass insert opposite the replicated surface; and
  - wherein the support mold is removable from the flexible bulk metallic glass mold insert without macroscopic deforming of the support mold.

In one embodiment, step b) can be achieved by molding over the bulk metallic glass mold insert with another bulk metallic glass BMG_support) to create a support. In another embodiment, the bulk metallic glass mold insert and support are made of the same BMG and are separated by a separation layer.
Thus, in another embodiment, the present invention also relates generally to a method of making a reusable mold comprising one or more mold cavities, the method comprising the steps of:
deforming a sandwich comprising of a thin bulk metallic glass (for forming the flexible bulk metallic glass mold insert) which is separated by a separation layer and a thick bulk metallic glass of the same type for the support mold against the surface of the master mold, wherein the thin bulk metallic glass mold insert layer faces the surface of the master mold.
The requirements for a suitable mold material for molding BMGs based on thermoplastic forming include:

- (1) high elasticity of at least 1.3%
- (2) a small Δα between the BMG being molded and the BMG used for the flexible mold insert,
- (3) high hardness,
- (4) sufficient toughness, i.e., >5 MPa m−½,
- (5) high wear resistance,
- (6) precise machinability or formability, and
- (7) sufficient strength at forming temperature.

Some crystalline metals may fulfill some of these requirements, but are challenged in terms of precision, especially when economically viable top-down approaches are used. For various BMGs, these requirements are fulfilled and these BMGs would thereby qualify as a mold material for molding other BMGs based on a thermoplastic molding process.
The strength of a BMG (at a given temperature) scales with the glass transition temperature, Tg. For the same reason, a measure of the cohesive energy, a, also scales with Tg. As a consequence, in order to achieve Δα=0, BMGs for mold and part with identical Tg are required. However, identical Tg would not allow a molding operation since the mold must be significantly stronger (higher Tg) than the part (low Tg) at the molding temperature. For crystalline materials, this condition requires the use of very different materials for mold and part, e.g., high strength steels and soft aluminum alloys.
Surprisingly, the inventors have found that for BMGs one can design combinations where ΔTg differs by less than 10%, thereby exhibiting very small, Δα<2×10⁻⁶K⁻¹, but that still exhibit sufficient difference in strength (i.e., the strength of the mold material is at least 10 times the strength of the part material). Such combinations drastically reduce de-molding forces by preventing chemical bonding due to the order of magnitude difference in strength between mold and part BMG and the reduced stresses due to the small Δα.
The use of BMGs as a mold to mold another BMG has been demonstrated for combinations with a 1 arge Δα(i.e. Δα>5×10⁻⁶K⁻¹), for example in U.S. Pat. Pub. No. 2010/0098967 to Schroers et al., the subject matter of which is herein incorporated by reference in its entirety. This prior art represents an extreme simple molding and de-molding operation with aspect ratio<0.5 and pyramid shaped features exhibiting very large draft angles, >30 degrees. However, the prior art has been incapable of molding and de-molding parts (without irreversible destroying the mold or on the macro scale where split molds can be used) of aspect ratio>1 (the aspect ratio is not only for the entire structure but also for parts of the structure) and negligible draft angle<1 degree. The present invention enables molding and de-molding of a plurality of parts of high complexity and aspect ratio with even negligible draft angle.
This is achieved by:

- using molds that consist of a thin and flexible BMG insert that is designed such that it can be removed without macroscopic flexing (elastic deformation) from the backing mold and subsequently flexed (macroscopically elastically deformed) to enable de-molding of the BMG parts.
- selecting BMG mold/part combinations with
  - minimized Δα (general case)
  - maximized αpart−αmold (some specific cases)

While various BMG materials are usable for making the flexible BMG insert and the support mold, what is most important is the relative properties of the support mold, insert and molded BMG part. Their strength at the forming temperature must be at least one order of magnitude different. In addition, the BMG that is used to replicate another BMG (as seen for example in FIG. 2, support mold to replicated insert, to replicate part) must have the higher strength at the processing temperature. In general, it is also beneficial that the support mold and mold insert BMGs are higher Tg alloys such as Zr-based, Ni-based, and Ze-based. However, in some instances, where low Tg parts are made, the BMGs can be alloys that are Pd-based or even Pt-based BMGs.
The BMG insert will in some instances bend and flex significantly. Therefore, it is beneficial to have a BMG that is not inherently brittle, and the ideal insert BMGs will have a Poisson's ratio of at least 0.32, more preferably a Poisson's ratio of at least 0.34, and most preferably a Poisson's ratio of at least 0.36. Poisson's ratio is the ratio of the relative contraction strain, or transverse strain normal to the applied load, to the relative extension strain, or axial strain in the direction of the applied load. When a sample of material is stretched in one direction it tends to get thinner in the other two directions perpendicular or parallel to the direction of flow. This phenomenon is called the Poisson effect and Poisson's ratio is a measure of this effect.
Examples of some suitable combinations for the fabrication method described in Example 1 include, for example, PdNiCuP alloys for the insert and ZrTiNiCuBe alloys for the molded part or PdNiCuP alloys for the insert and PtNiCuP alloys for the molded part. Examples of some suitable combinations for fabrication of the mold insert by the method described below in Example 2 include, for example, ZrAlNiCu alloys for the insert and support mold and PdNiCuP alloys for the part. Examples of some suitable combinations for fabrication of the mold insert by the method described below in Example 3 include, for example, ZrAlNiCu alloys for the insert, ZrTiNiCuBe alloys for the support mold and PdNiCuP alloys for the part. For the wetting example described in Example 4, ZrNbCuNiAl alloys may be used as the insert when using Si molds with W layer for wetting. In one embodiment, the wetting layer is separate from the BMG mold insert and may comprise, for example, tungsten. Here however, the alloy seems to react with the W layer and dissolve some. Thus, while the process still works, the surface is a bit rough.
Examples of other materials usable in the present invention for the support mold, BMG mold insert and molded BMG part include those listed in Table 1. However, as discussed above, what is important is the relative properties of the BMG used for the support mold, flexible BMG mold insert and the molded BMG part.

TABLE 1

Properties of Various BMGs

Alloy	T_g(° C.)	T_x(° C.)	ΔT_sc(° C.)

Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5	349	426	77
Zr₄₄Ti₁₁Cu₁₀Ni₁₀Be₂₅	350	471	121
Zr₅₇Nb₅Cu_15.4Ni_12.6Al₁₀	405	470	65
Zr_58.5Nb_2.8Cu_15.6Ni_12.8Al_10.3	400	480	80
Pd₄₃Ni₁₀Cu₂₇P₂₀	305	406	101
Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3	130	184	54
Pt_57.5Cu_14.7Ni_5.3P_22.5	236	325	89
Fe₄₈Cr₁₅Mo₁₄Er₂C₁₅B₆	573	620	47
Mg₆₅Cu₂₅Y₁₀	155	219	64
Zr₆₅Al₁₀Ni₁₀Cu₁₅	368	473	105

The step of deforming the BMG feed stock against the surface of the master mold to form the flexible MBG mold insert can be performed in various ways.
For example, in one embodiment, the BMG feed stock is deformed by increasing the temperature of the BMG feed stock to a processing temperature, between the glass transition temperature and the crystallization temperature of the BMG feed stock and applying pressure to plastically deform the BMG feed stock between a backing mold and the master mold and create the flexible BMG mold insert. Thereafter, the backing mold can be removed from the flexible BMG mold insert without any macroscopic flexing of the backing mold.
In another embodiment, the BMG feed stock is deformed against the surface of the master mold by increasing the temperature of the BMG feed stock to a blow molding temperature between the glass transition temperature and the crystallization temperature of the BMG feed stock, and blow molding the BMG feed stock at the blow molding temperature and at low pressure to replicate the surface of the master mold and create the flexible BMG mold insert.
In another embodiment, the BMG feed stock is deformed against the surface of the master mold by heating the BMG feed stock into a super cooled liquid region of the BMG feed stock and creating a favorable wetting behavior between the master mold and the BMG feed stock to provide a reduction in surface energy and cause the BMG feed stock to cover the surfaces of the one or more mold cavities. For example the wetting angle may be between about 5 and about 90 degrees and in one embodiment, the wetting angle is about 30 degrees.
The following examples describe methods of fabricating flexible BMG inserts in accordance and molds incorporating the flexible BMG inserts in accordance with the present invention.

Example 1

FIG. 2 depicts a fabrication method of a mold comprising a flexible BMG mold insert with a precise surface and an easily removable surface in accordance with the present invention.
As seen in FIG. 2, step (a), a master mold 2 comprising a material that can withstand the forming pressure (1-100 MPa) and forming temperature (150-800° C., depending on BMG) of the BMG is attached to a backing plate 4. In one embodiment, the master mold 2 comprises a plurality of master molds 2 that can be used for parallel processing. While FIG. 2, step (a) depicts three master molds 2, by plurality of master molds 2 what is meant is that there are at least two master molds 2, preferably at least five master molds. However, there may be any number of master molds 2 disposed on the backing plate 4, depending on the complexity of the part being replicated, among other factors.
FIG. 2, step (b) depicts a backing mold 6 that is machined or, in a separate step thermoplastically formed, from a first BMG. If the backing mold 6 is machined, a range of materials can be used for forming the backing mold 6 including, but not limited to, Ni-based alloys, brasses, aluminum, and BMGs with a softening temperature higher than the softening temperature of the BMG used for the mold insert and higher than the softening temperature of the BMG for the final part. Machining of the backing mold 6 must leave a cavity between the master mold 2 and the backing mold 4 ranging from 50 microns to 2 mm to allow for the fabrication of the flexible BMG mold insert. Regions that can be readily removed, such as the larger horizontal regions shown in FIG. 2, step (b), have no thickness constraints. The cavity of the backing mold 6 is machined such that corners are rounded and ideally a draft angle is realized.
The backing mold 6 and master mold 2 are aligned and a BMG feedstock material 8 is positioned in between as shown in FIG. 2, step (b). Before applying pressure to plastically deform the BMG feedstock material 8, the temperature is increased to Tg<Tprocess<1.3×Tx (Tg: glass transition temperature, Tx: crystallization temperature measured during heating with 20K/min). A pressure is applied typically between 1-100 MPa to deform the BMG feedstock material 8 such that it replicate both surfaces and forms the BMG mold insert 10 shown in FIG. 2, step (c). The processing conditions are chosen such that the surface to the master mold 2 is precisely replicated. The surface to the backing mold 6 is less critical.
FIG. 2, step (c) depicts the final molding condition of the BMG mold insert 10.
FIG. 2, step (d) shows that in order to release the master part 2 from the mold, one must first remove the backing mold 6 from the BMG mold insert 10. This can easily be accomplished because the backing mold 6 is designed to allow for such removal. Due to the designed round edges and draft angle, the backing mold 6 is capable of being removed from the BMG mold insert 10 without flexing (macroscopically elastically deforming).
FIG. 2, step (e) depicts that the BMG flexible mold insert 10 after the master mold 2 has been released by elastically bending the BMG mold insert 10. Here the large elastic strain limit of BMGs of ˜2% (for typical crystalline materials <0.5%) and the geometry effect of strain in beam bending, ε=t/d are utilized (t: thickness of insert, d: radius over which it is bend), meaning that thin sections can be bent (small d) proportional to cit.
Finally, as seen in FIG. 2, step (f), the final working mold 12 is formed, which can be used to thermoplastically mold BMG parts (and other thermoplastic materials). The limitation of the BMG material for molding in the molding process is that its softening behavior must be lower than that of the BMG mold. That is, the strength at processing temperature of the BMG mold 12 must be at least one order of magnitude higher than that of the BMG part being molded.

Example 2

FIG. 3 depicts a fabrication method of a mold comprising a flexible BMG mold insert with a precise surface which is separated by a separation layer from the support which is of the same BMG. Flexible BMG mold insert (BMG_insert) combined with BMG support (BMG_support) where the same BMG is used for BMG_insertand BMG_support. Both layers are separated by a separation layer that prevents chemical bond of BMG_insertwith BMG_support. Upon de-molding the support (BMGsupport) is first removed from BMG_insert, and subsequently BMG_insertfrom BMG_part.
As seen in FIG. 3, a master mold 2 comprising a material that can withstand the forming pressure (1-100 MPa) and forming temperature (150-800° C., depending on BMG) of the BMG is attached to a backing plate 4. In one embodiment, the master mold 2 comprises a plurality of master molds 2 that can be used for parallel processing. While FIG. 3 depicts three master molds 2, by plurality of master molds 2 what is meant is that there are at least two master molds 2, preferably at least five master molds. However, there may be any number of master molds 2 disposed on the backing plate, depending on the complexity of the part being replicated, among other factors.
Flexible insert and support are fabricated by a sandwich of the same BMG 8 where the BMG layer of the flexible mold is thin, approximately the thickness of the small features. This layer is separated by a separation layer 5. This separation layer 5 must deform continuously with the insert and support during fabrication to prevent chemical bonding of the two. One can use any liquid or readily deformable solid for the separation layer 5 that fulfills the conformity requirement during forming. One example includes salts, including molten salt fluids such as Dynalene MS-1, available from Dynalene, Inc. Other similar salts and molten salt fluids would also be known to those skilled in the art and are usable in the present invention.
After forming the sandwich over the master mold (and in a real forming operation over the BMG material used to fabricate a part) BMG_part, BMG_supportand BMG_insertare cooled to a temperature where all of the bulk metallic glasses are sufficiently hardened that the following demolding sequence does not cause plastic deformation.
Demolding is achieved by first removing the support from the flexible insert and subsequently the insert from the formed bulk metallic glass part or plurality of parts.
For example, as discussed above in Example 1, FIG. 2, step (d) shows that in order to release the master part 2 from the mold, one must first remove the backing mold 6 from the BMG mold insert 10. This can easily be accomplished because the backing mold 6 is designed to allow for such removal. Due to the designed round edges and draft angle, the backing mold 6 is capable of being removed from the BMG mold insert 10 without flexing (macroscopically elastically deforming).
FIG. 2, step (e) depicts that the BMG flexible mold insert 10 after the master mold 2 has been released by elastically bending the BMG mold insert 10. Here the large elastic strain limit of BMGs of ˜2% (for typical crystalline materials <0.5%) and the geometry effect of strain in beam bending, ε=t/d are utilized (t: thickness of insert, d: radius over which it is bend), meaning that thin sections can be bent (small d) proportional to ε/t.
Finally, as seen in FIG. 2, step (f), the final working mold 12 is formed, which can be used to thermoplastically mold BMG parts (and other thermoplastic materials). The limitation of the BMG material for molding in the molding process is that its softening behavior must be lower than that of the BMG mold. That is, the strength at processing temperature of the BMG mold 12 must be at least one order of magnitude higher than that of the BMG part being molded.

Example 3

FIGS. 4A and 4B depict another method of forming a BMG mold insert, which utilizes blow molding for fabrication of the flexible BMG mold insert.
Blow molding at temperatures of T_g<T_blowmold<T_x×1.3 and at low pressure <1 MPa has been demonstrated for BMGs as described, for example, in U.S. Pat. Pub. No. 2011/0079940 to Schroers et al., the subject matter of which is herein incorporated by reference in its entirety. The highest precision of this process is demonstrated with respect to the surface facing the mold.
As depicted in FIG. 4A, a master mold 20, which may comprise silicon or a range of other materials, is replicated by blow-molding of a BMG 22 at a first temperature to create a flexible mold insert 24. Blow-molding precisely replicates the master mold surface but leaves rounded corners on the opposite side. At room temperature or at a temperature significantly below Tg, the flexible mold insert 24 is elastically flexed to release the flexible mold insert 24 from the master mold 20. In the alternative, the master mold 20 may be etched to facilitate removal of the flexible mold insert 24. As depicted in FIG. 4A, a disposable ceramic compact 28 disposed in a mold frame 30 may be used to back the flexible mold insert 24. In this instance, the flexible mold insert 24, disposable ceramic compact 28 and the mold frame 30 together represent the working mold.
In the alternative, as depicted in FIG. 4B, a flexible mold insert 24 is fabricated as described above for FIG. 4A, but is not separated from the master mold 20. After the master mold 20 is replicated by blow molding to create the flexible mold insert 24 from a first bulk metallic glass at first temperature T1, the backside of the bulk metallic glass insert 24 is replicated with a second bulk metallic glass 26 at a second temperature T2 that is less than the blow molding temperature T1. The master mold 20 may then be subsequently removed either through etching or through a sequential release of the second bulk metallic glass 26 and then elastic flexing of the bulk metallic glass insert 24.

Example 4

FIG. 5 depicts another mold fabrication method in which a favorable wetting behavior is created between the master mold 32 and a first bulk metallic glass 30 (BMG1). When bringing the BMG1 30 into contact with the master mold 32, the master mold 32 exhibits a surface that creates a wetting angle of 90 degrees >θ>5 degrees, and the reduction of surface energy acts as driving force to cover surface of the master mold 32 with a thin layer of the BMG1 30. Surfaces of the master mold 32 that exhibit the required θ can be fine-tuned (release from master mold, thickness of BMG insert layer) through controlled oxidation to allow for release of the BMG1 from the master mold and to control the thickness of the BMG1 layer and create the flexible mold insert 34. For example, in one embodiment, the master mold may comprise a wetting layer to facilitate favorable wetting behavior. In one embodiment, this wetting layer comprises tungsten. However, molybdenum or other refractory or high temperature metals can also be used as the wetting layer in the practice of the invention. Other wetting layers would also be known to those skilled in the art.
The wetting angle reflects the driving force that causes the BMG1 30 to cover the surface of the master mold 32. If the wetting angle is 90 degrees, it behaves neutral, if the wetting angle is larger than 90 degrees (up to a maximum of 180 degrees), the liquid BMG1 30 is repelled by the master mold 32 and reduces contact, and if the wetting angle is 0 degrees, the BMG1 30 is highly attracted to the master mold 32 and forms a very thin layer. Such small wetting angles are typically formed through chemical bonding, and thus the BMG1 can no longer be separated from the mold or a wetting layer applied thereon.
In a preferred embodiment, the wetting angle is about 30 degrees. However, the best wetting angle results in a thin layer (approximately 50 microns) of BMG1 30 being formed on the surface of the master mold 32. The better the wetting (i.e., smaller angle), the thinner the layer. However, at the same time a larger enough angle is needed so that the BMG1 does not react with the wetting layer (e.g., tungsten) or with the surface of the master mold 32. It has been observed that metals on metal wets well, such as BMG1 on tungsten, with angles close to 0 degrees. However, in reality, there are always oxides on the surface which increase the wetting angle.
Finally, just silicon is not sufficient because the wetting angle is approximately 90 degrees and silicon is also covered by a natural oxide layer. Thus, the wetting angle of silicon with BMGs is about 130 degrees.
Thus, as seen in FIG. 5, a bulk metallic glass BMG1 30 is used to form a flexible BMG insert 34 by means of a favorable wetting angle so that as the BMG1 is heated into the SCLR of the particular BMG1, the wetting angle provides a reduction of surface energy to allow the BMG1 30 to cover the surface of the master mold 30 and forth the bulk metallic glass insert 34. Thereafter, as described above with respect to FIG. 4B, the backside of the bulk metallic glass insert 34 is replicated with a second bulk metallic glass 36. The master mold 30 may then be subsequently removed either through etching or through a sequential release of the second bulk metallic glass 36 and then elastic flexing of the bulk metallic glass insert 34.
As described herein in another embodiment, the present invention relates generally to a method of molding a bulk metallic glass part using a reusable mold comprising a flexible bulk metallic glass mold insert removably coupled to a support mold, the flexible bulk metallic glass mold insert comprising one or more mold cavities, the method comprising the steps of:

- a. heating a bulk metallic glass to be molded into a supercooled liquid region for the bulk metallic glass;
- b. disposing the bulk metallic glass into the one or more mold cavities in the flexible bulk metallic glass mold insert;
- c. cooling the bulk metallic glass to solidify the bulk metallic glass within the one or more mold cavities and create the molded bulk metallic glass part;
- d. removing the support mold from the flexible bulk metallic glass insert without macroscopic flexing of the support mold; and
- e. releasing the one or more molded bulk metallic glass parts from the one or more mold cavities in the flexible bulk metallic glass mold insert by elastically flexing the flexible mold insert.

Thus, once the bulk metallic glass insert is formed using one of the methods described above in Examples 1 to 4, the bulk metallic glass insert may be used in molding and demolding operations to mold and thus create miniature BMG parts. In addition, as described herein, the use of the molds containing such flexible BMG mold inserts can be used to create miniature BMG parts in a highly parallel manner as shown in FIG. 6.
As depicted in FIG. 6, a mold is created having a bulk metallic glass backing mold 50 with a flexible BMG mold insert 52 disposed therein. The backing mold 50 is designed such that it can be readily, without macroscopic flexing, removed from the flexible mold insert 52. Once the mold is created, a BMG part 54 may be replicated by heating the BMG usable for the BMG part 54 into the SCLR for that particular BMG (temperature T3), where T3<T2<T1 and disposing the BMG into the mold. The glass transition temperatures of the BMG part 54, the BMG backing mold 50 and the flexible BMG mold insert 52 are such that the strength of flexible mold insert 52 is at least 10 times that of the BMG backing mold 50 at T2, and the strength of the BMG backing mold 50 is at least 10 times that of the BMG part 54 at T3.
Thereafter, the backing mold 50 is removed from the flexible mold insert 32 without macroscopic flexing. Subsequently, the BMG part 54 may be released from the mold b y elastically flexing the flexible mold insert 52.
The inventors have found that the use of the flexible mold insert 52 allows one to fabricate BMG parts having undercuts. It is noted that the limitation of the size of the undercut is given by the specific geometry but is also imposed by the amount or degree to which the flexible mold insert 52 can elastically bend.
Molding can be either carried out in air, in an inert gas environment or in vacuum. Molding conditions of the BMG part 54 must be such that crystallization during replication of the flexible bulk metallic glass mold insert 52 does not occur. Cooling rates do not have to be fast, only fast enough to avoid crystallization. However, in one embodiment fast cooling may be undertaken as a separate processing step to achieve a more ductile state of the bulk metallic glass.
For de-molding, the first step involves removing the backing mold 50 at a temperature that is significantly below the Tg of the BMG part 54 to prevent plastic deformation of the BMG part 54. In one embodiment, this temperature may be room temperature. This can be achieved without large elastic flexing of the backing mold 50 because the interface between the backing mold 50 and the flexible mold insert 52 is designed to allow for easy removal of the backing mold 50 from the flexible mold insert 52, including attributes such as round edges, small Δα, and specific draft angle, by way of example and not limitation.
Once the backing mold 50 is removed, the flexible mold insert 52 can be released from the BMG part 54. This is achieved through flexing (elastic deforming) of the flexible mold insert 52 due to the inherent elasticity and the thin dimensions of the BMG used for the flexible mold insert 52. Once released, the working mold comprising the backing mold 50 and the flexible mold insert 52, is reassembled by inserting flexible mold insert 52 into the backing mold 50 for the next molding cycle. This cycle can be repeated many times using the same flexible mold insert 52 and backing mold 50.
Depending on the mold cavity geometry and the number of cavities being filled and their complexity, different requirements for Δα exist. These requirements can be grouped into two classes.
In the first class, αpart−αmold is maximized. In this instance, the mold geometry is such that the part separates everywhere from the nearest mold surface. In addition, for geometries where at least a fraction of the part is pushing again the nearest mold surface upon cooling, Δα=1 is the requirement to minimize de-molding forces. FIG. 7B depicts examples of miniature part geometries that require αpart−αmold to be maximized. In this instance, upon cooling, the BMG retracts from the closest mold surface at various points.
In the second class, such as for parallel molding when parts are connected through an overflow and for most geometries, Δα=0 is the condition required for minimizing de-molding forces. FIG. 7A depicts examples of miniature part geometries that require Δα=0. In this instance, upon cooling, at least some fraction of the BMG part pushes against the nearest mold surface.
Thus, it can be seen that the BMGs can be used to prepare a reusable mold for molding other BMGs, especially for molding BMG parts having miniature or micron-sized features and/or that exhibit a complex geometry.
It should also be understood that the following claims are intended to cover all of the generic and specific features of the invention described herein and all statements of the scope of the invention that as a matter of language might fall there between.

Claims

1. A reusable mold comprising:

a flexible bulk metallic glass mold insert comprising one or more mold cavities removably coupled to a support mold;

wherein the support mold is removable from the flexible hulk metallic glass mold insert without macroscopic deforming of the support mold.

2. The reusable mold according to claim 1, wherein the mold insert is highly precise on the mold cavity side.

3. The reusable mold according to claim 1, wherein the mold insert comprises a percent elasticity of at least 1.3%.

4. The reusable mold according to claim 1, wherein the flexible bulk metallic glass mold insert comprises features on a miniature-length scale.

5. The reusable mold according to claim 1, wherein the flexible bulk metallic glass mold insert comprises features having multiple length scales with at least one feature having a length scale on the miniature scale.

6. The reusable mold according to claim 1, wherein the flexible bulk metallic glass mold insert is reusable multiple times.

7. The reusable mold according to claim 1, wherein the flexible bulk metallic glass mold insert and the support mold comprise the same bulk metallic glass and are separated by a separation layer,

wherein the separation layer prevents chemical bonding of the support layer and the bulk metallic glass mold insert.

8. The reusable mold according to claim 7, wherein the separation layer deforms continuously with the bulk metallic glass mold insert and the support layer.

9. The reusable mold according to claim 7, wherein the separation layer comprises a salt that is capable of providing a degree of wetting of the bulk metallic glass that is greater than 90 degrees.

10. A method of making a reusable mold comprising one or more mold cavities, the method comprising the steps of:

a) deforming a bulk metallic glass feedstock against a surface of a master mold to replicate a surface of the master mold and create a flexible bulk metallic glass mold insert comprising one or more mold cavities;

b) removably coupling the flexible bulk metallic glass insert to a support mold, and

c) removing the master mold from the flexible bulk metallic glass insert after the support mold has been removed from the insert mold;

wherein the support mold is coupled to a surface of the flexible bulk metallic glass mold insert opposite the replicated surface; and

wherein the support mold is removable from the flexible bulk metallic glass mold insert without macroscopic elastic flexing or deforming of the support mold.

11. A method of making a reusable mold according to claim 10, wherein the step of deforming the bulk metallic glass feed stock comprises deforming a sandwich comprising of a thin bulk metallic glass layer separated by a separation layer and a thick bulk metallic glass layer of the same type against the surface of the master mold to replicate the surface of the master mold;

wherein the thin bulk metallic glass layer creates the flexible bulk metallic glass mold insert and the thick bulk metallic glass layer creates the support mold; and

wherein the thin bulk metallic glass layer faces the surface of the master mold.

12. The method of making a reusable mold according to claim 10, wherein the flexible bulk metallic glass mold insert and the support mold comprise different bulk metallic glasses.

13. The method of making a reusable mold according to claim 10, wherein a strength of the bulk metallic glass mold insert at the processing temperature of the bulk metallic glass of the support mold is at least one order of magnitude higher.

14. The method according to claim 10, wherein the step of deforming the bulk metallic glass feed stock against the surface of the master mold comprises:

a. increasing the temperature of the bulk metallic glass feed stock to a processing temperature, wherein the processing temperature is between the glass transition temperature and the crystallization temperature of the bulk metallic glass feed stock;

b. applying pressure to plastically deform the bulk metallic glass feed stock between a backing mold and the master mold and create the flexible bulk metallic glass mold insert; and

c. removing the backing mold from the flexible bulk metallic glass mold insert, wherein the backing mold is removed without any macroscopic flexing.

15. The method according to claim 10, wherein the step of deforming the bulk metallic glass feed stock against the surface of the master mold comprises:

a. increasing the temperature of the bulk metallic glass feed stock to a blow molding temperature, wherein the blow molding temperature is between the glass transition temperature and the crystallization temperature of the bulk metallic glass feed stock; and

b. blow molding the bulk metallic glass feed stock at the blow molding temperature and at low pressure to replicate the surface of the master mold and create the flexible bulk metallic glass mold insert;

c. deforming another BMG over the flexible bulk metallic glass insert, wherein the flexible bulk metallic glass insert covers the master mold.

16. The method according to claim 15, wherein the blow molding pressure is less than 1 MPa.

17. The method according to claim 10, wherein the step of deforming the bulk metallic glass feed stock against the surface of the master mold comprises:

a. heating the bulk metallic glass feed stock into a super cooled liquid region of the bulk metallic glass feed stock; and

b. creating a favorable wetting behavior between the master mold and the bulk metallic glass feed stock to provide a reduction in surface energy and cause the bulk metallic glass feed stock to cover the surfaces of the one or more mold cavities.

18. The method according to claim 17, wherein the wetting angle is between about 5 and about 90 degrees.

19. The method according to claim 18, wherein the wetting angle is about 30 degrees.

20. The method according to claim 10, wherein a Poisson's ratio of the flexible bulk metallic glass mold insert is at least 0.32.

21. A method of molding a bulk metallic glass part using a reusable mold comprising a flexible bulk metallic glass mold insert removably coupled to a support mold, the flexible bulk metallic glass mold insert comprising one or more mold cavities, the method comprising the steps of:

a. heating a bulk metallic glass to be molded into its supercooled liquid region;

b. disposing the bulk metallic glass into the one or more mold cavities in the flexible bulk metallic glass mold insert;

c. cooling the bulk metallic glass to solidify the bulk metallic glass within the one or more mold cavities and create the molded bulk metallic glass part;

d. removing the support mold from the flexible bulk metallic glass insert without macroscopic flexing of the support mold; and

e. releasing the one or more molded bulk metallic glass parts from the one or more mold cavities in the flexible bulk metallic glass mold insert by elastically flexing the flexible mold insert.

22. The method according to claim 21, wherein the disposing step is carried out through an applied pressure.

23. The method according to claim 21, wherein a plurality of cavities are filled in one molding operation and a change in thermal expansion difference, Au, between the flexible bulk metallic glass insert and the bulk metallic glass being molded is less than 10×10⁻⁶K⁻¹.

24. The method according to claim 21, wherein a plurality of cavities are filled in one molding operation and the change in thermal expansion difference, Δα, between the flexible bulk metallic glass insert and the bulk metallic glass being molded is less than 5×10⁻⁶K⁻¹.

25. The method according to claim 21, wherein the molded bulk metallic glass part comprises a complex geometry.

26. The method according to claim 25, wherein the complex geometry comprises one or more undercuts.

27. The method according to claim 21, wherein the support mold is removed from the flexible bulk metallic glass insert at a temperature that is at least 5% below the processing temperature of the molded bulk metallic glass part.

28. The method according to claim 21, wherein the support mold is removed from the flexible bulk metallic glass insert at a temperature that is below the glass transition temperature of the molded bulk metallic glass part.

29. The method according to claim 28, wherein the support mold is removed from the flexible bulk metallic glass insert at about room temperature.

30. The method according to claim 21, wherein the strength of the flexible bulk metallic glass insert is at least 10 times that of the bulk metallic glass being molded at the processing temperature of the bulk metallic glass being molded.

31. The method according to claim 21, wherein when the same bulk metallic glass is used for the bulk metallic glass mold insert and the support mold, the strength at forming temperature of the bulk metallic glass is at least one order of magnitude higher than that of the molded bulk metallic glass part.

32. The method according to claim 21, wherein when different bulk metallic glasses are used for the bulk metallic glass mold insert and the support mold, the strength of the bulk metallic glass insert is at least one order of magnitude higher than that of the support mold during forming of the support mold.

33. The method according to claim 32, Wherein the strength of the support mold is at least one order of magnitude higher than that of the molded bulk metallic glass part during forming of the molded bulk metallic glass part.