Welding, in engineering, any process in which
two or more pieces of metal are joined together by the application
of heat, pressure, or a combination of both. Most of the processes
may be grouped into two main categories: pressure welding, in
which the weld is achieved by pressure; and heat welding, in which
the weld is achieved by heat. Heat welding is the most common
welding process used today. Brazing and soldering are other means
of joining metals.
With the development of new techniques during
the first half of the 20th century, welding replaced bolting and
riveting in the construction of many types of structures, including
bridges, buildings, and ships. It is also a basic process in the
automotive and aircraft industries and in the manufacture of machinery.
Along with soldering and brazing, it is essential in the production
of virtually every manufactured product involving metals.
The welding process best suited to joining
two pieces of metal depends on the physical properties of the
metals, the specific use to which they are applied, and the production
facilities available. Welding processes are generally classified
according to the sources of heat and pressure used.
The original pressure process was forge welding.
Forge welding was practiced for centuries by blacksmiths and other
artisans. The metals are brought to a suitable temperature in
a furnace, and the weld is achieved by hammering or other mechanical
pressure. Forge welding is used rarely in modern manufacturing.
The welding processes most commonly employed today include gas
welding, arc welding, and resistance welding. Other joining processes
include Thermit welding, laser welding, and electron-beam welding.
Gas welding is a nonpressure process using heat from a gas flame.
The flame is applied directly to the metal edges to be joined
and simultaneously to a filler metal in wire or rod form, called
the welding rod, which is melted to the joint. Gas welding has
the advantage of involving equipment that is portable and does
not require an electric power source. The surfaces to be welded
and the welding rod are coated with flux, a fusible material that
shields the material from air, which would result in a defective
weld.
Arc-welding processes, which have become the most important welding
processes, particularly for joining steels, require a continuous
supply of either direct or alternating electrical current. This
current is used to create an electric arc, which generates enough
heat to melt metal and create a weld .
Arc welding has several advantages over other
welding methods. Arc welding is faster because of its high heat
concentration, which also tends to reduce distortion in the weld.
Also, in certain methods of arc welding, fluxes are not necessary.
The most widely used arc-welding processes are shielded metal
arc, gas-tungsten arc, gas-metal arc, and submerged arc.
In shielded metal-arc welding, a metallic electrode, which conducts
electricity, is coated with flux and connected to a source of
electric current. The metal to be welded is connected to the other
end of the same source of current. By touching the tip of the
electrode to the metal and then drawing it away, an electric arc
is formed. The intense heat of the arc melts both parts to be
welded and the point of the metal electrode, which supplies filler
metal for the weld. This process, developed in the early 20th
century, is used primarily for welding steels.
In gas-tungsten arc welding, a tungsten electrode is used in place
of the metal electrode used in shielded metal-arc welding. A chemically
inert gas, such as argon, helium, or hydrogen, is used to shield
the metal from oxidation. The heat from the arc formed between
the electrode and the metal melts the edges of the metal. Metal
for the weld may be added by placing a bare wire in the arc or
the point of the weld. This process can be used with nearly all
metals and produces a high-quality weld. However, the rate of
welding is considerably slower than in other processes.
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In gas-metal welding, a bare electrode is shielded from the air
by surrounding it with argon or carbon dioxide gas or by coating
the electrode with flux. The electrode is fed into the electric
arc, and melts off in droplets to enter the liquid metal that
forms the weld. Most common metals can be joined by this process.
Submerged-arc welding is similar to gas-metal arc welding, but
in this process no gas is used to shield the weld. Instead, the
arc and tip of the wire are submerged beneath a layer of granular,
fusible material formulated to produce a proper weld. This process
is very efficient but is generally only used with steels.
In resistance welding, heat is obtained from the resistance of
metal to the flow of an electric current. Electrodes are clamped
on each side of the parts to be welded, the parts are subjected
to great pressure, and a heavy current is applied briefly. The
point where the two metals meet creates resistance to the flow
of current. This resistance causes heat, which melts the metals
and creates the weld. Resistance welding is extensively employed
in many fields of sheet metal or wire manufacturing and is particularly
adaptable to repetitive welds made by automatic or semiautomatic
machines.
In Thermit welding, heat is generated by the
chemical reaction that results when a mixture of aluminum powder
and iron oxide, known as Thermit, is ignited. The aluminum unites
with the oxygen and generates heat, releasing liquid steel from
the iron. The liquid steel serves as filler metal for the weld.
Thermit welding is employed chiefly in welding breaks or seams
in heavy iron and steel sections. It is also used in the welding
of rail for railroad tracks.
The use of electron beams and lasers for welding has grown during
the second half of the 20th century. These methods produce high-quality
welded products at a rapid rate. Laser welding and electron-beam
welding have valuable applications in the automotive and aerospace
industries.
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