Welding is a method of permanently joining two pieces of metal, usually by means of heat. Manufacturers use welding in making many products, including automobiles, home appliances, and furniture. Construction firms use it in erecting bridges, buildings, and other structures. The production of electronic equipment involves sophisticated microwelding processes.
There are more than 60 welding processes. Each process falls into one of three groups: (1) fusion welding, (2) solid-state bonding, and (3) brazing and soldering. However, some metallurgists do not consider brazing and soldering technically to be forms of welding.
Fusion Welding uses heat to partially melt the metal surfaces that are to be joined. When the metal cools and hardens, the two pieces are connected by a welded joint that may be as strong as any other part of the metal.
Most fusion welding processes also use a filler metal, which is added to the weld in the form of a welding rod or a consumable electrode. The heat of the welding process melts the rod or electrode, which mixes with the melted base metal. The filler metal thus fills in the joint and strengthens it. The seam of hardened filler and base metal is called the fusion zone.
In most fusion welding processes, the heated metals must be shielded from hydrogen, nitrogen, and oxygen in the atmosphere. If the metals absorbed these gases, the weld could be weak or brittle. Shielding may be provided by spraying the metals during welding with an inactive gas, such as argon, carbon dioxide, or helium. Another method involves applying a nonmetallic flux to the metals before welding. The heat of welding melts the flux, which covers and protects the metals. A third way of shielding the metals is to weld in a vacuum.
There are a number of methods of fusion welding. They include the following.
Arc Welding joins metals by using heat from an electric arc. The welder uses an electrode holder, an electrode or welding rod, and a generator that produces an electric current. One type of arc welding, shielded metal arc welding, forms an electric arc between the metals and a flux-covered electrode. Heat from the electric arc melts the metal, the electrode, and the flux. In submerged arc welding, the electric arc is covered by powdered flux from a container attached to the welding tool. The arc forms between the metals and a consumable wire electrode that adds filler metal to the weld. The consumable wire electrode is fed continuously through the welding tool from a coil.
Another type of arc welding is gas tungsten arc welding. In this process, an electric arc is directed between the metals and a bare wire electrode made from tungsten, which does not melt in the arc's heat. Argon or helium gas shields the metals. The welder must use a separate welding rod if filler metal is required. In plasma arc welding, argon or a similar gas is electrically heated until it forms an ionized gas called a plasma. An arc of plasma is directed on the metals to weld them, and a filler metal is supplied separately. The plasma arc also shields the metals. A plasma arc produces extremely high temperatures and can be used to weld metals that are difficult to join by other methods.
Resistance Welding joins metals by means of the 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. An electric current passing through the electrodes meets resistance when it flows from one metal to the other. The resulting heat melts the metals and welds them together. In resistance spot welding, rod-shaped electrodes form spot welds along the metals. In resistance-seam welding, electrodes in the form of rollers create a continuous seam.
Gas Welding uses heat from a gas torch to join two metals. The most common welding gas is acetylene mixed with oxygen. If the job requires a filler metal, the welder dips a welding rod into the liquid metal between the pieces being joined. A flux may be applied to the metal before welding.
Other Fusion Welding Processes include those that use electron beams and lasers to produce the energy necessary to join metals. These methods require complex, specialized equipment.
Solid-State Bonding uses pressure as well as heat to bond metal and other materials. Common methods of solid-state bonding include the following.
Deformation Bonding joins metals partly by means of pressure great enough to deform the pieces. The pressure forces the surfaces so close together that a strong joint forms by atomic attraction. Methods of exerting pressure include roll bonding, friction or inertia welding, explosive bonding, and ultrasonic bonding. In most cases, deformation bonding also involves heating metals to temperatures just below their melting points, so that atoms will move to the joint and fill in any gaps.
Diffusion Bonding resembles deformation bonding but uses pressure insufficient to noticeably deform pieces. Diffusion bonding is especially suitable for joining unusual or reactive materials.
Brazing and Soldering use a melted filler metal to join unusually close-fitting parts. The filler metal and a special flux are applied to the parts, which are then heated in an oven or with a gas torch. The temperature is too low to melt the parts. But the filler metal melts into the joint and welds the parts together. Brazing uses such fillers as brass, bronze, or a silver alloy. Soldering uses lead-tin alloys or other metals with comparatively lower melting points.