Fusible alloys, or low-melting alloys, have some interesting properties, but are often 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° F. 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 temperatures 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 solely on gallium or indium.

Fusible alloys as design components

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.

Figure 1: Bismuth crystal. Bismuth is commonly used as a fusible alloy. Source: Imunarriz / CC BY-SA 3.0Figure 1: Bismuth crystal. Bismuth is commonly used as a fusible alloy. Source: Imunarriz / CC BY-SA 3.0

Patents on alloys for fusible plugs and fusible plug designs are common. 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 alloys with eutectic compositions have also been used as temperature standards.

Fusible alloy compositions can be engineered to produce an environmentally benign mercury replacement, or a “liquid metal alloy,” which is molten at or below room temperature. Thermometers, tilt switches and magnetohydrodynamic (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 MHD 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 characteristics of a liquid metal fusible alloy 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 offers can provide more accurate shielding blocks.

Future applications

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 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 now using fusible alloys as electrode elements or interconnects because the low melting point avoids thermal damage to the components.