Sven Johanson, age 13, of Chattanooga, Tenn., for his question:
HOW DO YOU WELD METAL?
Welding is a method of permanently joining two pieces of metal, usually by means of heat. There are about 40 welding processes but four methods are most commonly used: arc welding, resistance welding, gas welding and brazing.
Arc welding joins metals by using heat produced by an electric arc. The welder uses an electrode holder, an electrode or welding rod and a generator that produces an electric current. There are many variations of arc welding.
Shielded metal arc welding forms an electric arc between the metal and a flux covered electrode. The heat of the electric arc melts the metal, the electrode and the flux.
In the submerged arc welding method, the arc is covered by powdered flux dropped from a container attached to the welding tool. The arc forms between the metal and a consumable wire electrode that adds filler metal to the weld. Wire electrode is fed continuously from a coil.
Resistance welding joins metals by means of heat produced by resistance to the flow of an electric current. This process does not use filler metal or flux. The metals are clamped together and electrodes apply pressure on opposite sides.
There are two main methods of resistance welding. In resistance spot welding, rod shaped electrodes form spot welds along the surface of the metals. In resistance seam welding, electrodes in the form of rollers create a continuous seam or bead across the metals.
Gas welding uses the heat of a gas torch to join two metal parts. The most common welding gas is acetylene mixed with oxygen.
Brazing joins two exceptionally close fitting parts by means of a metal filler metal, such as brass, bronze or a silver alloy. The powdered filler metal and a special flux are placed on the two parts and heated in an oven or with a gas torch. The filler metal melts and flows into the joint where it spreads into, and alloys with, the surface of the parts. The heat applied in brazing is not high enough to melt the parts.
Often the operation of gas welding requires a filler metal. The welder holds a welding rod in one hand and directs the torch flame at it over the joint. He may apply a flux to the metal beforehand.
Other welding processes use electron beams. Lasers and ultrasonic waves to produce the necessary energy to join metals. These processes require complex, special equipment.
Manufacturers use welding to make a wide variety of products, including automobiles, home appliances and furniture. Construction companies use it in bridges and buildings.
In most welding processes, the heated metals must be shielded to prevent absorption of nitrogen and oxygen from the air. If the metals absorb these gases, the weld becomes weak and brittle. Shielding is provided by spraying an inactive gas such as argon, carbon dioxide or helium on the metals during welding, or applying a nonmetallic flux to the metal before welding.