Melting metal together is most commonly called welding, though the specific term depends on what you’re trying to accomplish. If you’re joining two metal pieces into one, the process is called fusion welding. If you’re combining different metals to create a new material with different properties, that’s called alloying. And if you’re pouring molten metal into a shape, that’s casting. Each process involves melting, but they serve very different purposes.
Fusion Welding: Joining Two Pieces Into One
Fusion welding is the most direct answer to “melting metal together.” It’s a manufacturing process where two metal parts are heated until they melt at the joint, creating a shared pool of liquid metal that solidifies into a single continuous piece. The atoms from both parts intermix in the molten pool, and as the metal cools, new crystal grains grow across what used to be the boundary between the two pieces. The result is a bond where the two parts are literally fused at an atomic level.
Most fusion welding adds a filler material, essentially a metal rod or wire that melts into the joint along with the base pieces. This fills gaps and reinforces the connection. Arc welding, the most common type, uses electricity as a heat source and reaches temperatures around 10,000°F. That’s far more heat than other metal-joining methods, which is why welded joints can be as strong as the original metal itself.
The area surrounding a weld, called the heat-affected zone, doesn’t melt but still undergoes significant changes. The intense heat causes the metal’s internal crystal structure to rearrange, which can make the surrounding material harder, softer, or more brittle depending on the specific metal and how hot it got. This is why welding is as much about controlling heat as it is about melting things together.
Brazing and Soldering: Melting Metal Between Pieces
Not all metal-joining processes melt the pieces themselves. Brazing and soldering both work by melting a separate filler metal that flows between two solid parts and bonds them when it cools. Think of it like a metallic glue that happens to be liquid at high temperatures. The base pieces stay solid the entire time.
The only technical difference between brazing and soldering is temperature. Soldering happens below 840°F (450°C) and is the method used for electronics, plumbing, and jewelry. Brazing happens above 840°F and creates stronger joints suited for industrial applications, HVAC systems, and bicycle frames. Both rely on capillary action: the liquid filler metal gets drawn into the tight gap between parts the same way water wicks into a paper towel.
Interestingly, brazed joints can sometimes outperform welded ones in raw pulling strength. In one study comparing cobalt-chromium alloy joints, brazed connections withstood an average of 792 megapascals of pulling force, nearly double the 404 megapascals that laser-welded joints could handle. The tradeoff is that brazed joints are more vulnerable to corrosion over time. Which method is “better” depends entirely on what the joint needs to survive.
Alloying: Creating a New Metal Entirely
When you melt two or more different metals together not to join pieces but to create a completely new material, that’s called alloying. Steel, bronze, and brass are all alloys. Steel is iron mixed with carbon. Bronze is copper and tin. Brass is copper and zinc. Each combination produces a material with properties none of the individual metals have on their own.
Alloying is far more complex than just stirring two liquids together. When different metals melt and mix, the process involves heat transfer, mass transfer, chemical reactions, and phase transformations all happening simultaneously. The atoms of different metals arrange themselves into new crystal structures as the mixture cools, and the specific arrangement determines whether the result is harder, more flexible, more corrosion-resistant, or better at conducting heat than the original metals.
One particularly useful phenomenon in alloying is the eutectic point, where a specific ratio of two metals produces a mixture with a lower melting point than either metal alone. This is why solder (a tin-lead or tin-silver alloy) melts at temperatures low enough to use on delicate electronics, even though the individual metals in it melt at much higher temperatures.
Casting: Melting Metal Into a Shape
Casting is the process of melting metal and pouring it into a mold to create a specific shape. It’s one of the oldest metalworking techniques and is still the go-to method for making complex shapes that would be difficult or too expensive to machine from a solid block. Engine blocks, pipe fittings, sculptures, and jewelry are all commonly cast.
The process starts with a mold that contains a negative impression of the desired shape. Molten metal is poured into the mold (usually from a crucible), fills every cavity, and then cools and solidifies. The mold is then broken away or opened to reveal the finished piece.
Solid-State Joining: No Melting Required
It’s worth knowing that some processes join metals without melting them at all. Friction stir welding, for example, uses a spinning tool that generates intense heat through friction and physically stirs the softened (but still solid) metal from two pieces together. The metal gets hot enough to become pliable, but no liquid metal is ever created. The U.S. Department of Energy describes it as a “quintessential solid-state-bonding technology” that forms joints in much less time than traditional diffusion bonding, where two pieces are simply pressed together at high temperature until atoms migrate across the boundary.
These solid-state methods are especially valuable for metals like aluminum that behave poorly when melted and resolidified. They also avoid the heat-affected zone problems that come with fusion welding, since the temperatures involved are significantly lower.
Quick Reference for Metal-Melting Terms
- Welding (fusion welding): Melting two metal pieces together at the joint so they become one piece. Temperatures around 10,000°F for arc welding.
- Brazing: Melting a filler metal between two solid pieces at temperatures above 840°F.
- Soldering: Same as brazing but below 840°F. Used for electronics and plumbing.
- Alloying: Melting different metals together to create a new material with different properties.
- Casting: Melting metal and pouring it into a mold to form a shape.
- Solid-state welding: Joining metals through heat and pressure without any melting.