With the correct selection of composition, some bulk metallic glasses (BMGs) have been demonstrated that have excellent combinations of hardness, fracture toughness, and wear resistance so that their use in gears and gearboxes is a potentially commercially viable application. For BMGs to be used as a low-cost alternative to steel gears, rapid fabrication strategies are needed to cast the BMGs into net-shaped gears that require little or no post-casting machining prior to use. Die casting, suction casting, and other cold-mold casting techniques have been widely demonstrated for BMGs in the past, but the unique nature of gears precludes traditional techniques from being used in an optimal way.

(a) A metallic glass that was cast into a mold using a forging apparatus. Large surface area contact with the mold prevents the mold from being removed and it must be peeled off. (b) A BMG gear cast into a steel mold using low-pressure suction. Although the part has taken the shape of the gear, a small gap exists between the mold and BMG gear. (c) With only the press of a finger, the BMG gear slides out of the mold. (d) Eight BMG gears cast using the same mold. (e) Microscope image of the teeth of the cast BMG gears from (d) with a smooth surface. (f) A BMG gear that has been fabricated with wire EDM showing a rough surface. (g) A machined steel gear showing the tooling marks.
The three most crucial elements for net-shape-casting a BMG gear (assuming that alloy selection has already been optimized) are:

  1. Making sure that the cast gear has the correct shape for use as a gear.
  2. The surface of the gear teeth needs to be as smooth as possible to prevent abrasive wear during the use of the gear.
  3. The gear needs to be able to be removed from the mold easily without any damage to the mold.

The solution to all three crucial elements for casting a gear is to use a lowpressure casting strategy. Typically, die casting or injection casting pressures of several tons to hundreds of tons are used to press the liquid BMG into the mold. However, almost all die casting uses multi-piece molds that split open (usually under hydraulics) so that cast parts can be removed. In the current research, BMGs are cast into molds that have extrusion symmetry, which means the part can be pressed out of the mold from the top or bottom without having to open the mold. This allows the teeth of the gear to be formed without parting lines. Utilizing low-pressure casting, not only can the BMG gears be cast and easily removed from the mold, but the surface of the gears can also be dramatically improved over what would be obtained if the gear were pressed against the rough mold surface. BMG gears that have been fabricated through casting have been demonstrated to have better wear performance than a gear with the same shape that has been ground or machined.

The current work demonstrates that BMGs can be repeatedly cast into nonsplit molds, can be removed from the molds without damage, and can have surface finishes that are superior to machined parts (due to the high surface tension of the liquid). By applying a low-pressure casting strategy, BMG gears can be fabricated into net-shaped gears that outperform machined gears in a process where they can be easily removed from the mold. This is a demonstration that low-pressure casting is actually advantageous for the fabrication of gears (not only for mold release using molds without any draft angle, but also for an improved surface finish).

This work was done by Douglas C. Hofmann and Andrew Kennett of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact the researchers at This email address is being protected from spambots. You need JavaScript enabled to view it..

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Refer to NPO-49318