US20130025746A1

US20130025746A1 - Twin roll sheet casting of bulk metallic glasses and composites in an inert environment

Info

Publication number: US20130025746A1
Application number: US13/452,816
Authority: US
Inventors: Douglas C. Hofmann; Scott N. Roberts; William L. Johnson
Original assignee: Apple Inc
Current assignee: Apple Inc
Priority date: 2011-04-20
Filing date: 2012-04-20
Publication date: 2013-01-31

Abstract

Sheet casting of metallic glasses and twin roll sheet casting of bulk metallic glasses and composite in an inert environment. Samples may be heated by RF to a temperature in the semi-solid region. After semi-solid processing, the partial liquid then may be poured or injected to achieve the desired shape. Plates of metallic glasses and/or metallic glass matrix composites may be formed (for example, through diecasting) and serve as a pre-form for rolling. In this configuration, the plates may be lowered through a radio frequency coil into compressing wheels, directly next to or below the coil. As the plates pass through the coil they may heat to above the glass transition temperature. Next, they may be fed into the rolling wheel to thermoplasically form the plates into thinner sheets.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to, and the benefit of, U.S. Provisional Patent Application No. 61/477,522, filed Apr. 20, 2011 and titled “Twin Roll Sheet Casting of Bulk Metallic Glasses and Composites in an Inert Environment,” the disclosure of which is hereby incorporated herein in its entirety.

TECHNICAL FIELD

Embodiments described herein relate generally to sheet casting of metallic glasses, and more particularly to twin roll sheet casting of bulk metallic glasses and composites in an inert environment.

BACKGROUND DESCRIPTION

Bulk metallic glasses (BMGs) and bulk metallic glass matrix composites (BMGMCs), in general, possess mechanical properties which potentially make them highly desirable engineering materials. Despite this, as of yet, these alloys are not being utilized on a wide scale outside of a few niche applications. This is widely due to the inability to make uniform sheet material free of oxidation, cracking or other contamination.
While twin roll casting is a common method for producing thin sheets of malleable materials, producing BMGs or BMGMCs by the method is extremely challenging, due to the brittle nature of metallic glass. If a BMG were rolled through a rolling mill at room temperature, the sheet would crack due to the low fracture toughness and high hardness of the glass. In fact, some rolling can be achieved at room temperature, but only in very small rolling increments. BMGMCs can be rolled at room temperature, but the glass matrix phase still cracks, as with the single phase BMG. The crystalline microstructure of the BMGMC prevents the sheet from cracking into multiple pieces. Although sheets can be made, the sample is highly damaged after rolling.
Rolling at elevated temperatures, specifically in the supercooled liquid region, has been widely done with metallic glasses. This type of processing is referred to a thermoplastic forming. However, when done in air, the BMG or BMGMC oxidizes badly, creating a brittle part. Typical rolling occurs by heating a BMG up in an oven and then placing the sample through a rolling mill in open air. Since rolling mills and ovens are typically too large to fit into vacuum chambers, oxidation cannot be avoided.
Fully melting a BMG or BMGMC in open air and then pouring it into a rolling mill cannot be done in open air. Above the liquidus temperature, when the alloy is fully molten, it will oxidize too quickly to be formed.

BRIEF SUMMARY OF THE FIGURES

FIG. 1 depicts an example of a water-cooled crucible and induction coil used to process BMGs and BMGMCs prior to pouring through rolling wheels, as seen from the inside of a vacuum chamber.

FIG. 2 depicts graphite crucibles after (on the left) and before (on the right) processing, in accordance with embodiments described herein.

FIG. 3 depicts a vacuum chamber from the outside. Visible are the hose used to pull vacuum (bottom), the handle used to inject liquid into the rolling mill or to pour a crucible (right) and the motor powering the rolling mill (left).

FIG. 4 depicts A custom-built rolling mill inside the vacuum chamber. The wheels in this case are made from brass to promote cooling and vitrification of the BMG or BMGMC. The feed-through for the motor is shown as in the induction coil.

FIG. 5 depicts a graphite crucible moving through the coil. After processing the BMG or BMGMC in the coil, the crucible is moved above the rolling mill and the liquid is poured into the wheels.

FIG. 6 depicts a feed-through for the motor to power the rolling mill. The motor can be kept out of the vacuum chamber.

FIG. 7 depicts a result of the manufacturing processes described herein. A BMGMC processed semi-solidly was poured into the wheel. The sheet, which was still hot after exiting the rolling mill, folded into this shape. Imm thick sections are visible, indicating the uniform thickness of the plate.

FIG. 8 depicts an example of a thin sheet of a BMGMC after semi-solid processing.

FIG. 9 is a drawing showing one example of twin roll casting in a vacuum chamber. The alloy is processed in the induction coil until a desired temperature is reached, then the coil is dumped into the rolling mill, forming a plate or thin sheet.

FIG. 10 shows another embodiment of twin roll casting, in which a solid rod or plate is passed through an induction coil, heating it, before the sample is injected through the rolling mill. This is an example of thermoplastic processing of plates.

FIG. 11 shows an example of how certain processes can be made semi-continuous. A hopper feeds the crucible which, in turn, pours the liquid into the wheels. The plates are collected in a separate chamber. Once the hopper is empty, the collection container is closed off from the rest of the chamber and opened. After removing the plates, the container is opened up to the vacuum chamber and vacuum is pulled. This “airlock” design speeds up production.

FIG. 12 depicts one example of how multiple rolling wheels can be used to achieve a desired effect, whether it is a thinner plate, forming a shape, cooling the plate, and so forth.

FIG. 13 is one example of how wires can be formed using the technique. The liquid can be either injected or poured into wheels that have a wire groove in them. The exiting wire can then be wound around a drum.

FIG. 14 shows one example of how net-shapes can be formed using the wheels. In this case, a corrugated plate is formed. The mold can be altered to form any repeating shape.

DETAILED DESCRIPTION

Sample Definitions
For the purposes of this disclosure, “twin roll casting” may be defined as a manufacturing technique by which a bulk metallic glass or a bulk metallic glass matrix composite is fed through a series of rolling wheels to produce a thin plate in a vacuum chamber under vacuum or an inert environment.
Bulk metallic glass (BMG): For the purposes of this disclosure, a bulk metallic glass is defined as an alloy which can be quenched into a vitreous state at a relatively large casting thickness (generally over 1 mm).
In-situ composite or bulk metallic glass matrix composite (BMGMC): For the purposes of this disclosure, an in-situ composite is defined as an alloy which, upon rapid cooling (1-1000 Kelvin/sec), chemically partitions into two or more phases, one being an amorphous matrix and the other(s) being crystalline inclusions. In contrast to an ex-situ composite, in which the glass-forming alloy is mechanically combined with the second phase through an infiltration process.
General Discussion
Embodiments disclosed herein may produce thin, semi-continuous or continuous sheets of BMGs or BMGMCs with thicknesses between approximately 0.1 mm and approximately 10 mm that are largely, or fully, free of oxygen or other contamination caused by environmental casting conditions. One possible advantage of such processes over conventional rolling techniques is that the rolling occurs completely within a vacuum chamber under the negative pressure of an inert gas, which may prevent the BMG or BMGMC from oxidizing and/or crystallizing. As such, the heating of the sample prior to rolling may achieved through radio frequency (RF) heating, which generally creates a magnetic field that couples to the BMG samples, or by a vacuum-safe oven. The RF heating can be generated by water cooled RF coils. Such coils can be incorporated into a vacuum chamber through a vacuum feed-through. When electricity is supplied to the coil, RF heating occurs without any vapor or other contaminants that will compromise the environment of the chamber. Additionally, the RF coil may allow a sample to be heated to a prescribed temperature prior to processing. For this technique, several important temperature ranges prior to rolling may be used. BMG and BMGMC may be fed through the rolling mill at a temperature above the glass transition temperature but below the crystallization temperature. This is referred to as the “supercooled liquid region.” BMG samples may be fed into the rollers above their liquidus temperatures. This is referred to as the “molten state.” In the case of the BMGMCs, a temperature between the solidus and liquidus temperature may be reached prior to rolling. This is called the “semi-solid region.” Certain embodiments may produce semi-solidly processed thin plates of BMGMCs with a highly coarsened microstructure. The semi-solid processing technique is described in the U.S. Patent, “Semi-Solid Processing of Bulk Metallic Glass Matrix Composites, PCT/US2008/058896.” which is incorporated by reference herein as if set forth in its entirety.
Sample Technical Description
Embodiments discussed herein may involve placing a rolling mill with two or more wheels inside of a vacuum chamber and forcing a heated BMG or BMGMC sample through the wheels to create a thin plate. The heating method used to achieve the desired pre-rolled temperature may be done using an RF coil with electricity provided by a power supply. Since rolling mills are typically extremely large and contain a motor built-in, it may be useful to separate the rolling mill from the motor. Unless the motor holds a vacuum, the motor may be placed outside of the chamber and the rolling mill may be placed inside the chamber. The motor supplies power to the wheels through one or more drive shafts that enter the chamber with a vacuum feed-through.
The samples to be rolled may be heated in various geometries depending on the desired sheet, all using inductive heating. To form a sheet of a monolithic BMG from the liquid, the BMG may be placed inside of a crucible, melted and then either poured or injected into the rolling wheels.
To create a semi-solidly processed plate of BMGMC, samples may be placed in a crucible and heated by RF to a temperature in the semi-solid region. After semi-solid processing, the partial liquid then may be poured or injected into the wheels to achieve the desired shape.
In another method, plates of BMGs and BMGMCs may be heated and injected into the rolling wheels without using a crucible. Plates of BMGs may be formed (for example, through die-casting) and semi-solidly processed plates of BMGMCs may be formed (as one example, through semi-solid forging) and these plates may serve as a pre-form for rolling. In this configuration, the plates may be lowered through an RF coil into the wheels, directly next to or below the coil. As the plates pass through the coil they may heat to above the glass transition temperature. Next, they may be fed into the rolling wheel to thermoplasically form the plates into thinner sheets. As an example, 3 mm thick cast plates may be rolled to 0.5 mm thick sheets by the process.
Generally, there may be certain advantages of the current process over existing methods to form BMG or BMGMC sheets. For example, BMGs and BMGMCs oxidize badly in open air and thus heating and rolling them outside of an inert environment produces poor quality parts. By enclosing the heating and rolling events within a vacuum chamber, these problems may be eliminated. Further, semi-solidly processed composites have certain very good mechanical properties. However, they can be difficult to produce. Heating the BMGMC into its semi-solid region often is challenging because the alloy reacts with the crucible during processing. In the present embodiment, a BMGMC can be heated semi-solidly in a crucible before pouring into the rolling wheels. In addition, semi-solidly processed parts can be heated above the glass transition of the matrix prior to entering the rolling wheels.
The Figures
FIG. 1 depicts an example of a water-cooled crucible and induction coil used to process BMGs and BMGMCs prior to pouring through rolling wheels, as seen from the inside of a vacuum chamber.
FIG. 2 depicts graphite crucibles after (on the left) and before (on the right) processing, in accordance with embodiments described herein.
FIG. 3 depicts a vacuum chamber from the outside. Visible are the hose used to pull vacuum (bottom), the handle used to inject liquid into the rolling mill or to pour a crucible (right) and the motor powering the rolling mill (left).
FIG. 4 depicts A custom-built rolling mill inside the vacuum chamber. The wheels in this case are made from brass to promote cooling and vitrification of the BMG or BMGMC. The feed-through for the motor is shown as in the induction coil.
FIG. 5 depicts a graphite crucible moving through the coil. After processing the BMG or BMGMC in the coil, the crucible is moved above the rolling mill and the liquid is poured into the
FIG. 6 depicts a feed-through for the motor to power the rolling mill. The motor can be kept out of the vacuum chamber.
FIG. 7 depicts a result of the manufacturing processes described herein. A BMGMC processed semi-solidly was poured into the wheel. The sheet, which was still hot after exiting the rolling mill, folded into this shape. Imm thick sections are visible, indicating the uniform thickness of the plate.
FIG. 8 depicts an example of a thin sheet of a BMGMC after semi-solid processing.
FIG. 9 is a drawing showing one example of twin roll casting in a vacuum chamber. The alloy is processed in the induction coil until a desired temperature is reached, then the coil is dumped into the rolling mill, forming a plate or thin sheet.
FIG. 10 shows another embodiment of twin roll casting, in which a solid rod or plate is passed through an induction coil, heating it, before the sample is injected through the rolling mill. This is an example of thermoplastic processing of plates.
FIG. 11 shows an example of how certain processes can be made semi-continuous. A hopper feeds the crucible which, in turn, pours the liquid into the wheels. The plates are collected in a separate chamber. Once the hopper is empty, the collection container is closed off from the rest of the chamber and opened. After removing the plates, the container is opened up to the vacuum chamber and vacuum is pulled. This “airlock” design speeds up production.
FIG. 12 depicts one example of how multiple rolling wheels can be used to achieve a desired effect, whether it is a thinner plate, forming a shape, cooling the plate, and so forth.
FIG. 13 is one example of how wires can be formed using the technique. The liquid can be either injected or poured into wheels that have a wire groove in them. The exiting wire can then be wound around a drum.
FIG. 14 shows one example of how net-shapes can be formed using the wheels. In this case, a corrugated plate is formed. The mold can be altered to form any repeating shape.
Sample Variations and Modifications
There are several variations to the current design that may be useful in certain conditions. For example, both rods and plates can be heated using a variety of coil designs before entering the rolling wheels. Rods may have more uniform heating then plates so they can be heated relatively uniformly in an induction coil prior to injecting into the wheels. By using a rod or plate as the pre-form for the rolling, a uniform plate can be produced.
There are many possible modifications for the design of the wheels. Multiple wheels can be used to achieve a straight plate shape or to cool the alloy. In the current design, the rolling wheels are made from brass or copper to supply cooling to the liquid before rolling. Multiple wheels could be used to either increase the cooling rate or to shape the alloy during rolling.
In one modification, a mold pattern could be applied to the wheels to form corrugated plates or other shapes. As the liquid passes through the rolling mill, it is imparted a repeatable shape from the wheels. This could also include producing wires from the processing, by machining the shape of wires into the wheels.
Additionally, complex designs can be applied to the wheels to create more uniform sheets. Grooves can be machined into the wheels to create a reservoir for the liquid so that a uniform sheet is pulled. The rolling wheels can also be heated or cooled to help control the temperature of the sheet or wire.
This technique might also be applied to other materials that oxidize in open air, e.g. crystalline Ti or Zr alloys.

Claims

1. A method for fabricating a sheet of metallic glass matrix composite, the method comprising:

placing a rolling mill inside a vacuum chamber;

heating a bulk metallic glass matrix composite having a metallic glass matrix above a glass transition temperature of the metallic glass matrix into a semi-solid region;

rolling the bulk metallic glass matrix composite with the rolling mill inside the vacuum chamber; and

forming the sheet of the bulk metallic glass matrix composite.

2. The method of claim 1, the method further comprising providing a motor outside the vacuum chamber.

3. The method of claim 1, further comprising providing an inert gas into the vacuum chamber.