Fusible Alloys: A Versatile Design Component and Manufacturing Tool

Gary Kardys | December 02, 2014

Fusible alloys or low-melting alloys have some interesting properties, but often are overlooked. Their solidification characteristics make them useful for tooling applications as well as components in product design.

Fusible alloys have low melting points, usually below 300 degrees Fahrenheit or 150 degrees Celsius. Many fusible alloys have eutectic compositions, which provide an alloy with a distinct melting point similar to a pure metal. Non-eutectic fusible alloys would melt over a range of temperature and act slushy between their liquidus and solidus temperatures. Many fusible alloys are based on bismuth alone or bismuth in combination with lead, tin, antimony, gallium, cadmium, zinc and indium. Some fusible alloys are based on gallium or indium.

Bismuth-based fusible alloys are desirable because of the pure bismuth's characteristic of expanding 3.3% upon solidification. The bismuth content in a fusible alloy is adjusted to produce a fusible alloy with desirable shrinkage or expansion characteristics. Bismuth alloys containing more than 55% bismuth expand while those with less than 48% contract during solidification. Alloys with bismuth levels between 48-55% exhibit little change in volume when they solidify.

Indium- and gallium-based fusible alloys have the ability to wet and adhere to ceramic and glass surfaces, which can be a useful characteristic in sealing and bonding applications. The fusible alloy compositions also are engineered to produce the desired melting temperature range or melting point for specific technology applications.

Fusible Alloys as a Design Component

Materials with high strength or heat resistance are not necessarily required or even desired when designing some engineered products. In certain design situations, the engineer wants to have a tab that breaks away or separates, a column that buckles or crushes to absorb impact, a plug that liquefies to prevent explosions or a component that transforms to indicate a specific temperature.

Because of these characteristics, fusible alloy components are critical elements in fire sprinklers, steam boiler relief plugs, fuse parts and thermostats. The ability to melt at a specific low temperature is used in making steam boiler safety plugs, automatic safety sprinklers, fusible valves, thermal protection devices, thermal fuses, fusible links, refrigeration safety plugs and even fusible chain loops.

Patents on alloys for fusible plug as well fusible plug designs continue to be granted. Without fusible alloy relief plugs, many boilers and pressure vessels would have exploded over the years when valve or thermostat failures caused overheating conditions. Several international standards are available to control fusible plug quality and safety (for example see BS 1123). Fusible alloy with eutectic compositions have also been used as temperature standards.

The turkey that many in the U.S. enjoy during the holidays uses a fusible alloy in the pop-up thermometer that comes with the food package. The fusible alloy melted when the proper internal temperature is reached, which allows a spring to push out the indicator.

Fusible alloy compositions can be engineered to produce an environmentally benign mercury replacements or a “liquid metal alloy”, which is molten at or below room temperature. Thermometers, tilt switches and MHD inertial sensors all make use of liquid metal fusible alloys. While gallium-indium-tin alloys are much less toxic than mercury, surface wetting and reactivity issues must be addressed in some sensor applications. Accurate, high-bandwidth and rugged magnetohydrodymic inertial or angular rate sensors utilize a toroid of liquid metal.

Silicone, petroleum or other polymer-based heat transfer materials can be liquid at or below room temperature. However, these organic liquids do not possess the high electrical and thermal conductivity characteristic of a liquid metal fusible alloys heat transfer medium. High thermal conductivity fusible alloys are useful in thermal interfaces, heat transfer systems for cooling or heating, constant-temperature heat treating baths and other thermal management devices.

Fusible alloys are also used as radiation shielding blocks for nuclear medicine or radiation therapy applications. The lower melting point bismuth fusible alloys have been replacing lead-based alloys because their lower melting point makes them easier and safer to work with. The shrinkage characteristic bismuth can provide more accurate shielding blocks.

Novel applications continue to be developed where fusible alloys are a key design component. For instance, a patent was recently granted using fusible alloys in a reconfigurable fluidic shutter for selectively shielding an antenna array. Researchers have found that a fusible metal, Galinstan, works well as an electrical interconnection in flexible sensor-skin (microfluidic channels) and lab on chip devices. The high fluidity of fusible alloys enables the formation and alignment of very fine conductive pathways. Batteries for utility electricity storage using liquid metal electrodes promise much greater efficiency.

Temperature sensitive microelectronics, OLEDS and organic photovoltaic cells are usng fusible alloys as electrode element or interconnect because the low melting point avoid thermal damage to the components and molten fusible alloys can be printed (for example, cathode for an organic electronics component patent).

Fusible Alloys as a Manufacturing Tool

Fusible alloys are also useful in making molds, dies or cores for electroforming, potting, encapsulation and wax pattern casting. These molding or net shaping applications do not require a high temperature die, mandrel or mold. In potting or encapsulation applications, a fusible alloy mold is made, the electronic components are placed in the mold, the electrically insulating resin or potting compound is poured into the mold to encase and protect the electronic device. After the encapsulant or potting resin is cured, the metal is cracked or broken away.

Fusible alloys are also used to replicate or proof geometries or surfaces of components by casting a layer of the fusible alloy onto a part. Bismuth-based fusible alloys characteristic of expansion during solidification enables the capturing of fine surface textures down to fingerprints on components.

Tube bending can be improved with fusible alloys. In the first step, a fusible tube bending alloy is poured into the tube and solidified. During the tube bending process, the solid fusible alloy in the tube acts like an internal mandrel, which allows smaller radii tubing to be formed without collapse or pinching of the tube. In electroforming applications, metal is electrodeposited around a fusible alloy core and then melted away to produce the electroformed part. Fusible alloy mandrels are used as cores during filament winding or composite laminate formation where the core is melted out after winding is completed or the composite is cured.

Internal thread metal alloy components can be difficult to make using additive manufacturing, also known as 3D. Tapping holes after the additive process might be more efficient. The difficulty comes in holding a complex prototype part for thread tapping after completing the 3D printing process. Another matching clamping part could be printed, but that might prevent an expensive additive manufacturing unit from forming the next design iteration. In this instance, fusible alloys are regularly used to hold irregular shapes components such as jet engine blades during a machining, grinding or finishing process.

In fixturing or work holding applications, the part is placed in the pocket or hole of a steel box, then the fusible alloy is melted and poured around the part. The fusible metal expands slightly when it freezes around the part, which securely clamps the component. After machining or grinding, the fusible alloy is melted out of the fixture and saved for resuse. Fusible alloys are used in fixturing, chucks, jigs, drill guides and other work holding applications.

Eyeglass manufacturers attach optical lens blanks to surfacing blocks for grinding and polishing to specific prescriptions. Fusible alloys can be excellent materials for use in these applications, as lens blocking alloys because molten fusible alloys that conform to the lens blank, are easily recycled by re-melting and do no thermal damage to plastic.

A newer application of fusible alloys in manufacturing is the continuous casting of glassy metal alloy sheet. The process the inventors describe is similar to the float glass or Pilkington process where molten glass is poured onto molten tin and then solidified into glass sheet. A steady flow of new design component and manufacturing tool applications will be developed for fusible alloys in the future.

More Resources:

IHS Supplier Directory

Fusible Alloy Datasheets