Electric resistance welding facts for kids
Electric resistance welding (ERW) is a way to join metal parts together. It uses an electric current to heat the metal where the parts touch. This heat melts the metal, creating a strong, permanent bond.
ERW is used a lot in making things like steel pipes and car bodies. The electric current can come from special tools called electrodes that also press the metal together. Sometimes, a magnetic field helps heat the metal.
There are different types of electric resistance welding. These include spot welding, seam welding, flash welding, and projection welding. How hot the metal gets depends on things like the size of the metal pieces, if they have a coating, what the electrodes are made of, and how much electric current is used.
Small pools of melted metal form where the parts touch. This happens because that spot has the most electrical resistance. A strong electric current (from 100 to 100,000 Amperes) passes through the metal to create this heat. Generally, ERW methods are good because they use energy well and don't cause much pollution. However, they work best for thinner materials.
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Spot Welding: Joining Metal with Dots
Spot welding is a type of resistance welding. It joins two or more metal sheets that overlap each other. Imagine you want to attach two pieces of paper with a single dot of glue. Spot welding does something similar with metal.
Usually, two copper tools called electrodes are used. They clamp the metal sheets together and send an electric current through them. When the current passes, the spot where the metal sheets touch gets very hot. This is because there's more electrical resistance there.
The rising heat causes the metal to melt, forming a small pool. This usually happens in less than a second. When the current stops, the copper electrodes cool the spot weld quickly. This makes the melted metal harden under pressure, creating a strong bond. Copper is great at cooling because it conducts heat very well.
Sometimes, if too much heat is used, or not enough pressure, the melted metal might burst out. This is called expulsion. When this happens, the weld might be weaker. To check a weld's quality, people often do a "peel test." This involves trying to peel the welded pieces apart.
Spot welding has many good points. It uses energy efficiently and doesn't deform the metal much. It's also very fast and can be done by machines. You don't need extra material to fill the gap. Spot welding is often used in the automotive industry. A single car can have thousands of spot welds!
Seam Welding: Making Continuous Lines
Resistance seam welding is like spot welding, but it creates a continuous line, or "seam," instead of just dots. It joins two similar metal pieces. This process is often done automatically by machines.
Like spot welding, seam welding uses two electrodes. But in seam welding, these electrodes are usually shaped like discs or wheels. They rotate as the metal passes between them. This lets the electrodes stay in contact with the metal, making long, continuous welds. The wheels can also help move the metal along.
A special device called a transformer sends low-voltage, high-current electricity to the metal. The part of the metal being welded has high electrical resistance. This heats it up to its melting point. The hot, soft metal is then pressed together by the welding wheels. This creates a strong, uniform bond. Most seam welders use water to cool the electrodes and other parts because of all the heat generated.
Seam welding creates very strong welds. A properly made seam weld can even be stronger than the original metal itself. A common use for seam welding is in making round or rectangular steel pipes. It was once used to make steel beverage cans, but now most cans are seamless aluminum.
Seam welding can be done in two ways:
- Intermittent seam welding: The wheels move to a spot, stop to make a weld, then move again. This continues until the weld is finished.
- Continuous seam welding: The wheels keep rolling as each weld is made, creating a smooth, unbroken line.
Older Methods: Low-Frequency ERW
There was an older way of doing electric resistance welding called Low-Frequency Electric Resistance Welding (LF-ERW). This method was used to weld seams in pipes for oil and gas. It was common from the 1920s until about 1970.
In the LF-ERW process, a flat sheet of steel was shaped into a cylinder. Then, a low-frequency electric current was passed between the edges of the steel. This heated the edges, and they were pressed together to form a bond without needing any extra welding material.
However, over time, welds made with low-frequency ERW pipes were found to have problems. They could corrode easily, crack, or not be bonded well. Because of these issues, the low-frequency method is no longer used to make pipes. A newer, high-frequency ERW process replaced it in 1970. This newer method makes much higher quality welds and is still used today for new pipelines.
Other Ways to Weld with Electricity
Besides spot and seam welding, there are other types of electric resistance welding:
- Flash welding: This method doesn't use any extra filler metal. The two pieces of metal are placed a small distance apart. When electricity is applied, the gap creates an electric arc, which is like a tiny lightning bolt. This arc melts the metal. Once the metal is hot enough, the pieces are pressed together, joining them.
- Projection welding: This is a special kind of spot welding. Instead of flat surfaces, one or both metal pieces have small raised bumps, or "projections." The heat from the electricity focuses on these bumps. This allows for stronger welds or for welding thicker pieces of metal. Projection welding is often used to attach things like nuts or bolts to metal plates. It's also good for joining crossed wires or bars.
See also
In Spanish: Soldadura por resistencia para niños
- List of welding processes
