Arc
Welding
These
processes use a welding power supply to
create and maintain an electric arc between an electrode and the base material
to melt metals at the welding point. They can use either direct current (DC)
or alternating current (AC),
and consumable or non-consumable electrodes. The welding region is
sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and
filler material is sometimes used as well.
Power
Supplies
To supply
the electrical power necessary for arc welding processes, a variety of
different power supplies can be used. The most common welding power supplies
are constant current power
supplies and constant voltage power
supplies. In arc welding, the length of the arc is directly related to the
voltage, and the amount of heat input is related to the current. Constant
current power supplies are most often used for manual welding processes such as
gas tungsten arc welding and shielded metal arc welding, because they maintain
a relatively constant current even as the voltage varies. This is important
because in manual welding, it can be difficult to hold the electrode perfectly
steady, and as a result, the arc length and thus voltage tend to fluctuate.
Constant voltage power supplies hold the voltage constant and vary the current,
and as a result, are most often used for automated welding processes such as
gas metal arc welding, flux cored arc welding, and submerged arc welding. In
these processes, arc length is kept constant, since any fluctuation in the
distance between the wire and the base material is quickly rectified by a large
change in current. For example, if the wire and the base material get too
close, the current will rapidly increase, which in turn causes the heat to
increase and the tip of the wire to melt, returning it to its original
separation distance.
The type
of current used plays an important role in arc welding. Consumable electrode
processes such as shielded metal arc welding and gas metal arc welding
generally use direct current, but the electrode can be charged either
positively or negatively. In welding, the positively charged anode will
have a greater heat concentration, and as a result, changing the polarity of
the electrode affects weld properties. If the electrode is positively charged,
the base metal will be hotter, increasing weld penetration and welding speed.
Alternatively, a negatively charged electrode results in more shallow
welds. Nonconsumable electrode processes, such as gas tungsten arc
welding, can use either type of direct current, as well as alternating current.
However, with direct current, because the electrode only creates the arc and
does not provide filler material, a positively charged electrode causes shallow
welds, while a negatively charged electrode makes deeper welds. Alternating
current rapidly moves between these two, resulting in medium-penetration welds.
One disadvantage of AC, the fact that the arc must be re-ignited after every
zero crossing, has been addressed with the invention of special power units
that produce a square wave pattern
instead of the normal sine wave,
making rapid zero crossings possible and minimizing the effects of the problem.
Processes
One of
the most common types of arc welding is shielded metal arc welding (SMAW), it
is also known as manual metal arc welding (MMAW) or stick welding. Electric
current is used to strike an arc between the base material and consumable
electrode rod, which is made of filler material (typically steel) and is
covered with a flux that protects the weld area from oxidation and contamination by
producing carbon dioxide (CO2)
gas during the welding process. The electrode core itself acts as filler
material, making a separate filler unnecessary.
The
process is versatile and can be performed with relatively inexpensive
equipment, making it well suited to shop jobs and field work. An operator
can become reasonably proficient with a modest amount of training and can
achieve mastery with experience. Weld times are rather slow, since the
consumable electrodes must be frequently replaced and because slag, the residue
from the flux, must be chipped away after welding. Furthermore, the
process is generally limited to welding ferrous materials, though special
electrodes have made possible the welding of cast iron, nickel, aluminum,
copper, and other metals.
Gas metal arc welding (GMAW),
also known as metal inert gas or MIG welding, is a semi-automatic or automatic
process that uses a continuous wire feed as an electrode and an inert or
semi-inert gas mixture to protect the weld from contamination. Since the
electrode is continuous, welding speeds are greater for GMAW than for SMAW.
A related
process, flux-cored arc welding (FCAW),
uses similar equipment but uses wire consisting of a steel electrode
surrounding a powder fill material. This cored wire is more expensive than the
standard solid wire and can generate fumes and/or slag, but it permits even
higher welding speed and greater metal penetration.
Gas tungsten arc welding (GTAW),
or tungsten inert gas (TIG) welding, is a manual welding process that uses a
nonconsumable tungsten electrode, an inert or
semi-inert gas mixture, and a separate filler material. Especially useful
for welding thin materials, this method is characterized by a stable arc and
high quality welds, but it requires significant operator skill and can only be
accomplished at relatively low speeds.
GTAW can
be used on nearly all weldable metals, though it is most often applied to stainless steel and
light metals. It is often used when quality welds are extremely important, such
as in bicycle, aircraft and naval applications. A
related process, plasma arc welding, also uses a tungsten electrode but uses
plasma gas to make the arc. The arc is more concentrated than the GTAW arc,
making transverse control more critical and thus generally restricting the
technique to a mechanized process. Because of its stable current, the method
can be used on a wider range of material thicknesses than can the GTAW process
and it is much faster. It can be applied to all of the same materials as GTAW
except magnesium, and automated welding of stainless steel is one important
application of the process. A variation of the process is plasma cutting, an
efficient steel cutting process.
Submerged arc welding (SAW)
is a high-productivity welding method in which the arc is struck beneath a
covering layer of flux. This increases arc quality, since contaminants in the
atmosphere are blocked by the flux. The slag that forms on the weld generally
comes off by itself, and combined with the use of a continuous wire feed, the
weld deposition rate is high. Working conditions are much improved over other
arc welding processes, since the flux hides the arc and almost no smoke is
produced. The process is commonly used in industry, especially for large
products and in the manufacture of welded pressure vessels. Other arc welding
processes include atomic hydrogen welding, electroslag welding (ESW), electrogas welding, and stud arc welding. ESW
is a highly productive, single pass welding process for thicker materials
between 1 inch (25 mm) and 12 inches (300 mm) in a vertical or close
to vertical position.
Gas
Welding
The most
common gas welding process is oxyfuel welding, also known as oxyacetylene
welding. It is one of the oldest and most versatile welding processes, but in
recent years it has become less popular in industrial applications. It is still
widely used for welding pipes and tubes, as well as repair work.
The
equipment is relatively inexpensive and simple, generally employing the
combustion of acetylene in oxygen to produce a welding flame
temperature of about 3100 °C (5600 °F). The flame, since it is
less concentrated than an electric arc, causes slower weld cooling, which can
lead to greater residual stresses and weld distortion, though it eases the welding
of high alloy steels. A similar process, generally called oxyfuel cutting, is
used to cut metals.
Resistance
Resistance
welding involves the generation of heat by passing current through the
resistance caused by the contact between two or more metal surfaces. Small
pools of molten metal are formed at the weld area as high current
(1000–100,000 A) is passed through the metal. In
general, resistance welding methods are efficient and cause little pollution,
but their applications are somewhat limited and the equipment cost can be high.
Spot welding is a popular resistance
welding method used to join overlapping metal sheets of up to 3 mm
thick. Two electrodes are simultaneously used to clamp the metal sheets
together and to pass current through the sheets. The advantages of the method
include efficient energy use,
limited workpiece deformation, high production rates, easy automation, and no
required filler materials. Weld strength is significantly lower than with other
welding methods, making the process suitable for only certain applications. It
is used extensively in the automotive industry—ordinary cars can have several
thousand spot welds made by industrial robots. A
specialized process, called shot welding, can be used to spot weld
stainless steel.
Like spot
welding, seam welding relies on two electrodes
to apply pressure and current to join metal sheets. However, instead of pointed
electrodes, wheel-shaped electrodes roll along and often feed the workpiece,
making it possible to make long continuous welds. In the past, this process was
used in the manufacture of beverage cans, but now its uses are more
limited. Other resistance welding methods include butt welding, flash welding, projection welding,
and upset welding.
Energy
Beam
Energy
beam welding methods, namely laser beam welding and electron beam welding, are
relatively new processes that have become quite popular in high production
applications. The two processes are quite similar, differing most notably in
their source of power. Laser beam welding employs a highly focused laser beam,
while electron beam welding is done in a vacuum and uses an electron beam. Both
have a very high energy density, making deep weld penetration possible and
minimizing the size of the weld area. Both processes are extremely fast, and
are easily automated, making them highly productive. The primary disadvantages
are their very high equipment costs (though these are decreasing) and a susceptibility
to thermal cracking. Developments in this area include laser-hybrid welding,
which uses principles from both laser beam welding and arc welding for even
better weld properties, laser cladding, and x-ray welding.
Solid
State
Like the
first welding process, forge welding, some modern welding methods do not
involve the melting of the materials being joined. One of the most popular, ultrasonic welding, is
used to connect thin sheets or wires made of metal or thermoplastic by
vibrating them at high frequency and under high pressure. The equipment
and methods involved are similar to that of resistance welding, but instead of
electric current, vibration provides energy input. Welding metals with this
process does not involve melting the materials; instead, the weld is formed by
introducing mechanical vibrations horizontally under pressure. When welding
plastics, the materials should have similar melting temperatures, and the
vibrations are introduced vertically. Ultrasonic welding is commonly used for
making electrical connections out of aluminum or copper, and it is also a very
common polymer welding process.
Another
common process, explosion welding,
involves the joining of materials by pushing them together under extremely high
pressure. The energy from the impact plasticizes the materials, forming a weld,
even though only a limited amount of heat is generated. The process is commonly
used for welding dissimilar materials, including bonding aluminum to carbon
steel in ship hulls and stainless steel or titanium to carbon steel in
petrochemical pressure vessels.
Other
solid-state welding processes include friction welding (including friction stir welding), magnetic pulse welding, co-extrusion
welding, cold welding, diffusion bonding, exothermic welding, high frequency welding,
hot pressure welding, induction welding,
and roll welding.
Solid
state welding processes classification chart :
Welding processes flow chart :
