A low melting point is one of the unique properties of bismuth which give it so many different uses in today’s society. One of the important ways that bismuth is used industrially is in low-melt alloys. Low melt alloys are usually made from combinations of bismuth, lead, tin, zinc, and indium. They are commonly used in parts that need to be joined or bent, while maintaining strength and durability. They are also used in machining to support or or anchor different parts, in casting for art or industrial purposes, and in soldering.
Low melt alloys, also known as fusible alloys, allow metals to easily form a liquid at lower temperatures, and then resolidify into the wanted shape. Commonly, these temperatures are below 183 degrees celsius. Low melt alloys can be eutectic, which means that they form a liquid quickly at a certain temperature, or non-eutectic, which means that they form a liquid gradually as they heat up, with a slushy transition period.
From left to right: bismuth, lead, tin, zinc, and indium
There are several different types of bismuth-based low melt alloys that are used for a variety of reasons. One of the important qualities of bismuth based alloys is that when they melt and then resolidify, they can expand up to 3.3%. Usually, bismuth low melt allies contain about 40-55% bismuth, and the more bismuth there is, the more it expands. This makes bismuth- based alloys ideal for accurate casting, as the material pushes farther into the crevices of the mold as it expands, and is therefore less likely to leave voids in the finished piece. Its low melting point also means that the molds can be made of a wider range of materials. Bismuth is also less toxic than some other low-melt alloy metals, especially cadmium and lead. The following are some commonly used types of Bismuth-based alloys, and what they can be used for.
Wood’s metal: contains bismuth, tin, lead and cadmium. Wood’s metal is non-eutectic, and melts at a range of 70-76 °C. This bismuth-based alloy is commonly used for soldering and in casting. Its incredibly low melting point means that it can be cast in wood to make metal inlays, and is used in restoring antiques to repair pieces without causing further damage. It is also used as a filler when bending metal tubes. The tube is filled with wood’s metal which prevents the tube from collapsing as it is bent, and then the wood’s metal is melted out. Wood’s metal is also commonly used in fire sprinkler systems. Since it has a low melting point, the piece of Wood’s metal in the valve will melt in a fire, setting off the sprinkler.
From left to right: Wood metal alloy, Rose metal alloy, Field metal alloy
Rose metal contains bismuth, lead, and tin. Like wood’s metal, it is about 50% bismuth and is non-eutectic, but has a slightly higher melting point at about 95-98 °C. Rose metal has a lot of the same uses as Wood’s metal, but is safer because it contains no no cadmium. It is also slightly more cost-effective. Rose metal is mainly used as a solder, but is also used in art casting, since it does not contract as it cools.
Field’s metal contains only bismuth, indium and tin. This alloy is eutectic and has a melting point of about 62°C. Field’s metal contains less bismuth than the others and the indium it contains makes it much more expensive, but it is sometimes preferred as it doesn’t contain the toxic metals lead and cadmium. It is often used in die casting and rapid prototyping, and is sometimes used in casting small parts for jewelry.
There are several more bismuth based low melt alloys that have incredibly important industrial uses. Many companies come up with their own mixtures and ratios for their own specific purposes, and often, these alloys are kept a well-guarded secret. The versatility of low melt alloys means that we are constantly finding new applications for them and they have become a vital step in many industrial processes. With these processes constantly developing, bismuth is becoming more important and more well-known every day.