Patent Application
20160318095
The present invention relates generally to a method of molding and de-molding of bulk metallic glasses (BMGs) into complex shapes.
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 1012 Pa·s at the glass transition temperature to 107 Pa·s and in some cases to 105 Pa·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:
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,
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).
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:
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:
For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying figures, in which:
Also, while not all elements may be labeled in each figure, all elements with the same reference number indicate similar or identical parts.
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:
In one embodiment, step b) can be achieved by molding over the bulk metallic glass mold insert with another bulk metallic glass BMGsupport) 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:
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<2×10−6 K−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>5×10−6 K−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:
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
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.
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.
As seen in
The backing mold 6 and master mold 2 are aligned and a BMG feedstock material 8 is positioned in between as shown in
Finally, as seen in
As seen in
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) BMGpart, BMGsupport and BMGinsert are 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,
Finally, as seen in
Blow molding at temperatures of Tg<Tblowmold<Tx×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
In the alternative, as depicted in
The wetting angle reflects the driving force that causes the BMG130 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 BMG130 is repelled by the master mold 32 and reduces contact, and if the wetting angle is 0 degrees, the BMG130 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 BMG130 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
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:
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
As depicted in
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.
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.
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.
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.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US14/62850 | 10/29/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61896986 | Oct 2013 | US |
