By Emily Newton 
How hot is hot? There’s no standard definition for a low melting point metal, but considering most metals melt at over 1,000°F (537°C), those metals that disintegrate on the stovetop or even in a person’s hand are certainly unique. These metals and their alloys are great for casting, especially when they form a eutectic mixture.
The metal with the lowest melting point is mercury at -38°F (-39°C), which is liquid at room temperature. In contrast, tungsten has the highest melting point, only liquefying at 6,191°F (3,422°C). The 15 lowest melting point metals are:
Many of these low melting point metals commonly make up fusible alloys. However, not all are suitable for casting.
Some metals, like polonium and francium, are extremely radioactive. Mercury and lead are toxic and too dangerous to use for many casting applications. Therefore, just because a metal has a low melting point doesn’t mean it’s useful for casting.
Some useful properties of low melting point metals for casting — depending on the application — can include a high enthalpy of fusion, low volume changes, high thermal conductivity and low wettability. Other properties people typically value in metals for casting include high diffusivity, high corrosion resistance and low flammability. It’s rare for someone to want their metal sculptures or car parts to catch fire.
Another useful chemical property of many alloys is that they’re eutectic. This quality means they have a lower melting point when combined than that of any of their individual constituents. For example, anesthetic drugs lidocaine and prilocaine are both solids at room temperature, but when combined, they form a liquid with a melting point of 61°F (16°C).
Eutectic metal alloys are useful for casting because they require less energy input to melt than their individual components. That translates to less time, money and fuel needed to prepare them for pouring into a mold.
Low-melting alloys typically fall into the following categories:
Additionally, gallium can be used for casting on its own because of its low toxicity and melting point. Some chemists play practical jokes on unsuspecting guests by serving them tea with a gallium spoon — it instantly melts in contact with the hot liquid. Gallium also destroys many other metals, such as aluminum and copper, on contact.
Many fusible alloys — those which melt at low temperatures — have specific names or patents.
This non-toxic alloy melts at 144°F (62°C). Field’s metal contains 32.5% bismuth, 51% indium and 16.5% tin. Itl is useful for rapid prototyping and small-run die casting, although it is typically more expensive than Wood’s metal or Rose’s metal. Contact with liquid Field’s metal can cause third-degree burns despite its lower-than-average melting point.
Useful for casting, this fusible alloy is made from 50% bismuth, 25% to 28% lead and 22% to 25% tin. Ingesting Rose’s metal is toxic due to the lead content. Its melting point hovers around 212°F (100°C). Many people use it as a pipe filler material and as solder for cast iron railings.
This eutectic alloy is toxic to touch or breathe near, but it’s useful for casting custom metal parts. Some people use it to make metal inlays or casts of hard-to-duplicate keys. The alloy is comprised of 50% bismuth, 26.7% lead, 13.3% tin and 10% cadmium by mass. It melts at 158°F (70°C).
Cerrosafe is a mixture of 42.5% bismuth, 37.7% lead, 11.3% tin and 8.5% cadmium. Many people use it for making reference castings due to its thermal expansion properties during cooling. Cerrosafe contracts for the first half hour, which makes it easy to remove from a mold. It then expands over the next several days. It melts between 158°F (70°C) and 190°F (88°C).
This eutectic alloy initially expands, then shrinks. It’s useful for crafting turbine blades, jet blades, sprinkler heads, sealing adjustment screws and many other mechanical parts. It has a melting point of 136°F (57.8°C) and contains bismuth, lead, indium and tin.
This bismuth-base eutectic alloy is useful for proof casting in die and tool shops, low-temperature soldering and making dental models. It consists of bismuth, lead, indium, tin and cadmium, and it melts at 117°F (47°C), the temperature for which it is named.
Hobbyists who want to make metal sculptures or other objects at home usually aren’t equipped to melt materials like gold, silver or iron. An oven simply isn’t hot enough to liquefy most metals.
Additionally, low melting point metals are useful for casting in plastic molds that would otherwise melt at high temperatures. Since most 3D printers use plastic as a medium, fusible alloys pair well with using a 3D printer to create molds.
Using low melting point metals for casting is also safer overall. Many can still cause severe burns, but gallium, for example, becomes liquid at just 86°F (30°C), making it safe to handle.
Anyone who has recently become more familiar with metals for casting has almost certainly encountered instances of people using them for 3D printing. This fabrication method is much more efficient than many traditional options and gives users more customization options. Let’s look at one recent example of how 3D printing improved metal casting and another illustrating how people can apply these concepts to applications beyond that fabrication method.
In one example, a university team investigated the lost-PLA method as an alternative to conventional casting. Traditional approaches typically use the lost-foam process, which involves creating Styrofoam models that people place into sand molds and pouring molten metal into those shapes. The Styrofoam dissolves due to the heat of the molten metal, leaving negative space. Although this is an effective casting method, it is also costly and time-intensive due to the Styrofoam required.
The lost-PLA method uses a roll of PLA filament and a 3D printer to create precise molds more affordably. The filament has a low melting point and an equally low cost, making it ideal for short-run metal casting projects.
This example shows that innovation does not necessarily mean abandoning all past methods. Instead, people can focus on the aspects with notable downsides, tackling those with new technologies, including 3D printers.
Gallium and indium are two low-melting point metals, and they both played an important role in an innovation that opened opportunities for using 3D printers to create metal objects at room temperature. This effort began when researchers used a solution of micron-scale copper particles suspended in water. They then added an indium-gallium alloy that was liquid at room temperature and stirred the two components to mix them.
That action causes the copper particles and liquid metal to stick together, forming a metallic gel inside the aqueous solution. The scientists also clarified that an even distribution of the copper particles in the gel prevented issues that might otherwise clog a 3D printer.
The researchers confirmed that this method allows printing products that are up to 97.5% metal and highly conductive. Although metal casting will remain relevant for the foreseeable future, people also need to apply what they know about metals with low melting points to find other methods that may meet new needs.
Most metals are known for their high melting points, but there are exceptions. People who want to use metal for casting can take advantage of low melting point materials for added safety, convenience and compatibility with 3D-printed or handcrafted plastic molds.
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