This process was apparently developed first in the 1880’s, but it was not perfected until many years later. The parts to be welded are connected to one terminal of an electric circuit, and a rod called an electrode is connected to the other terminal.
The electrode is usually made of metal that has about the same composition as the workpiece and is called filler metal because it melts and fills the joint. An arc is created when the electrode is touched to the workpiece and slightly withdrawn. An arc is like a miniature lightning bolt. It occurs because the two ends of the electrical circuit are close enough for the current to jump the gap.
Watch a welder at work, and you will see a bright flash pass from the welding tool to the piece of metal that is being joined. (CAUTION! You should never watch a welder without proper protection for your eyes. Otherwise, the bright light from the arc may permanently damage your vision.)
The temperature of the arc has been estimated at about 10,000 degrees Fahrenheit. This is more than enough to melt most metals. As the arc is drawn along the joint, the metal melts together with the tip of the electrode and then quickly solidifies. When arc welding was new, the welds were not as smooth and strong as engineers had hoped. They did not know that impurities, gases from the atmosphere, and oxidation affected the welded joint.
The joints were made better by coating the electrode with certain chemicals that partly turned into gas and partly melted in the arc. The melted chemicals are called a flux. The flux forms a protective blanket over the newly formed weld, and the gas acts as a shield to keep out the atmosphere. This process became known as manual shielded metal arc welding, and accounts for most of the welding done today.
By World War II, submerged arc welding had been introduced. In this process the electrode is a coil of wire mechanically fed to the arc. Flux is dropped from a moving hopper just ahead of the arc. This process is very fast.
Another development called gas metal arc welding did away with the flux by substituting protective gases such as argon, helium, and carbon dioxide around the arc and weld.
A process called gas tungsten arc welding is often employed to weld thin parts and metals, such as aluminum, that are not easily welded by other processes. Many of the arc welding processes can be mechanized or made completely automatic.