On January 31, 2013, the US Patent & Trademark Office published a patent application from Apple that reveals new method for fabricating a sheet of metallic glass matrix composite. Metallic glass is a form of liquidmetal. In order to make cool products with this exotic material one must first invent equipment that could properly processes and control it. Today's patent relates to sheet casting of metallic glasses, and more particularly to twin roll sheet casting of bulk metallic glasses and composites in an inert environment. If you don't think that this is about liquidmetal, the conclusion of our report will make it crystal clear.
Apple's Patent Background
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.
About Apple's Patent Figures
Patent 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.
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.
Apple credits Douglas Hofmann, Scott Roberts and William Johnston as the inventors of this patent application which was filed in Q2 2012. Douglas Hofmann is the surprise here as he's not an Apple engineer. No, Hofmann is Materials Scientist and Metallurgist at NASA Jet Propulsion Laboratory/California Institute of Technology. Yet before working for NASA, Hoffmann worked as a Research and Development Scientist at Liquidmetal Technologies. Does that clarify what Apple's invention is about? Evidently Apple acquired the patent at some point in time. Other Liquidmetal Reports from Patently Apple: One (with liquidmetal video), Two, Three.
With that good news, what is Apple going to create with this exotic material now that they've laid the groundwork with this new equipment.
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