Welding allows parts to coalesce along their contacting surfaces by applying
heat, pressure, or both, and often adding a filler material. The most common
industrial welding methods are fusion processes in which workpieces are melted
at their common surfaces.
Solid-state welding joins parts by applying heat and pressure. The
temperature is usually below the melting point of the materials joined.
Hot-press, ultrasonic, diffusion, and explosive welding are examples of
solid-state welding.
Fusion welding methods, chiefly gas, arc, and resistance, are the most widely
used and are less restrictive as to the materials that can be joined. Diffusion
welding is employed primarily to join high-strength materials.
Arc welding: In arc welding, an arc between an
electrode and the workpiece generates heat. Shielding the molten weld metal from
the atmosphere with gases fed in or generated by the weld reaction is often
critical. Unwanted gases react with the molten metal to cause strength-reducing
oxides and inclusions within the weld. Arc-welding processes vary mainly in the
way the weld is shielded and the methods of applying filler material.
Shielded-metal arc welding (SMAW): This form of
welding, also called stick welding, is usually done manually, with the welder
feeding a consumable, coated electrode in the work area. The flux coating
provides arc stabilizers; gases to displace air; metal; and slag to protect,
support, and insulate the weld metal. Many electrode and flux coatings are
available and are matched to type, size, and position of the material welded.
SMAW is suitable for portable applications. Labor and material costs are
high, but its simplicity and versatility make it the most commonly used welding
process.
Gas-metal arc welding (GMAW): This process
generates an arc between a consumable electrode wire and the workpiece.
Shielding is provided by gas that flows over the weld area from the welding gun.
Combinations of shielding gas, power source, and electrode significantly affect
metal transfer across the arc. A variety of gases are used, depending on the
metals reactivity and the joint design. For example, argon is used most often,
though carbon dioxide is added when welding ferrous metals. Carbon dioxide is
sometimes used alone to shield steels. GMAW processes are fast, can be
automated, and work in all positions.
Flux-core arc welding (FCAW): This process is
very similar to gas-metal arc welding. Weld heat is produced from an arc between
the work and a continuously fed filler-metal electrode. The electrode is hollow
with flux in the core. The core may provide shielding gases, deoxidizers, and
slag-forming materials. In some cases, materials may be added to promote arc
stability, enhance weld-metal properties, and improve weld contour. FCAW is used
only on ferrous metals, chiefly mild and low-alloy steels. Some FCAW electrodes
are self-shielding, but others require an external shielding gas, usually carbon
dioxide, supplied through a nozzle. This process works in all positions,
producing a fast, clean weld. Both GMAW and FCAW are sometimes referred to as
MIG welding. They require less skill than SMAW.
Gas-tungsten arc welding (GTAW): In GTAW, an arc
is generated between a nonconsumable tungsten electrode and the work areas. Wire
filler metal is fed in separately. Work is shielded by helium or argon gas. The
process, also called TIG (tungsten inert-gas welding), is more expensive than
GMAW but is used on thinner metals such as aluminum, magnesium, titanium, and
high-alloy steels.
GTAW may use dc or ac. When ac is used with argon shielding, arc cleaning
action is produced at aluminum and magnesium joint surfaces. This removes oxides
and is useful in removing porosity in aluminum. Helium used with dc provides
deeper penetration but requires stringent cleaning of aluminum and magnesium. Ac
is preferred for aluminum and magnesium.
GTAW costs more than SMAW and is much slower than GMAW, but it provides a
high-quality weld in a very wide range of thicknesses, positions, and
geometries. The process can be fully automated.
Submerged arc welding (SAW): Heat is produced
from an arc between the work and a continuously fed filler-metal electrode. A
blanket of granular fluxing material preplaced on the work protects the molten
weld puddle from the surrounding atmosphere. The process is limited to flat, or
nearly flat, workpieces. Both flux and filler wire can be fed automatically, and
the process is most commonly used in mechanized operations.
Plasma arc welding (PAW):Hot ionized gases shield
the work area in this process. The plasma is sometimes supplemented with a
separate shielding gas. The welding gun, like that used with GTAW, has a
nonconsumable tungsten electrode. Filler material, if used, is fed in
separately. The weld produced is also similar to GTAW, but the process is much
faster and arc control is superior. Plasma arc welding produces a deep, narrow,
uniform beam and is suitable for refractory metals, low-alloy steels, stainless
steels, aluminum, and titanium. It is most frequently used for high-quality
welds on high-strength, thin-section material. The process tolerates great
variations in joint alignment and does not generate a high-frequency arc.
Resistance welding: In resistance welding,
coalescence is produced by heat generated by resistance to the flow of electric
current through the parts being joined. The assembly heats up, and pressure is
applied by the welding machine through the electrodes. No fluxes or filler
metals are needed.
The process is commonly used as a mass-production technique requiring special
fixtures and automatic handling equipment. Resistance welding can be applied to
almost all steels and aluminum alloys and some dissimilar metal bonds. Heating
is localized. Common stock thickness range is 0.004 to 0.75 in.
The process may be continuous, as when welding pipe seams, or finite, as in
spot welding. There are no limitations on welding position. This process works
well with robotics.
Percussive arc welding: This technique can be
considered a special case of dc resistance welding. In percussive arc welding,
power is usually supplied by a capacitor bank that is directly short-circuited
across the parts to be welded. The charge is concentrated at the point where the
electrode nib contacts with the part. The nib vaporizes, establishing an ionized
area. This provides a localized arc for the remaining capacitor charge that
puddles an area on the surface of both parts.
The weld head moves forward quickly before the materials solidify. This
forces the interface to alloy, producing an excellent low-penetration, low
heat-affected-zone weld, usually having excellent character and grain structure.
Percussive arc welding is most commonly used for stud-welding precision parts.
However, the process is sensitive to humidity. Also, large welds tend to be
very noisy, with a loud percussive repeat which gives the process its name. The
process also is relatively slow because of the time required to recharge the
capacitor bank.
Oxyfuel gas welding (OFW): This group of welding
processes uses the heat produced by a gas flame to melt the base metal (and the
filler metal, when used). Various fuel-gas combinations are burned in oxygen to
produce the necessary heat. Oxygen and acetylene is the most commonly used
combination. The mixture produces a temperature of around 5,600°F. Oxy-acetylene
torches can be used with or without a filler metal.
Propane, butane, natural gas, and hydrogen, in combination with air or
oxygen, are used to weld nonferrous, low-melting-temperature materials in
special applications. However, such hydrocarbon fuel gases are not generally
suited to welding ferrous materials, because the flame atmosphere is oxidizing
or the heat output of the flame is too low.
Because OFW requires minimal equipment, it is inexpensive and suitable for
manual methods. However, it is slow.
Specialty systems include:
Electron-beam welding (EBW): EBW uses energy from
a fast-moving beam of electrons focused on the base material. The electrons
strike the metal surface, giving up their kinetic energy almost completely in
the form of heat. Welds are made in a vacuum, which eliminates contamination of
the weld material by gases. The high vacuum produces a stable beam.
With high beam energy, a hole can be melted through the material. This hole
is moved along the joint by moving either the electron gun or the workpiece and
is maintained as the metal at the front melts and flows around to the rear,
where it solidifies. Welds can be made without a hole, where melting takes place
by conduction of heat from the surface, but such welds are slower.
EBW can produce deep, narrow, almost parallel-sided welds with low total heat
input and relatively narrow heat-affected zones. The depth-to-width ratio can be
as high as 30:1 in some cases. The low energy input allows welding close to
heat-sensitive components. Also, projecting the electron beam makes it possible
to weld in otherwise inaccessible locations.
Though equipment costs are high, indirect savings can result from reduced
joint-preparation costs, ability to weld in a single pass, high welding speed,
and low distortion. However, the process can only be used to produce a tight
butt or lap joint.
Laser-beam welding (LBW): Fusion is obtained by
directing a concentrated beam of coherent light to a very small spot. Laser
beams combine low heat input with an intensity greater than the electron beam.
Since the heat is provided by a beam of light, there is no physical contact
between the workpiece and welding equipment. Welds can be made through
transparent materials.
LBW is flexible because the laser beam can be moved under digital control to
seam weld any shape. Laser welding plays an important role in microelectronics
and light-gauge metal-welding applications that require precise welding control.
LBW can be used with a variety of metals, including low-alloy and stainless
steels, aluminum alloys, lead, titanium, refractory metals, and high-temperature
alloys. It vaporizes the metal at the laser's point of focus, producing a deep
penetration column of vapor. The vapor column is surrounded by a liquid pool,
which is moved along the joint to produce welds with depth-to-width ratios
greater than 8:1. However, speeds may be low in seam welding because the welds
actually consist of a series of overlapping spot welds.
Certain characteristics of lasers can affect the costs of applying a system.
Joint preparation and fixturing costs may be higher because lasers require close
joint fit. However, filler material and edge machining are not used, so joint
finishing costs are saved. The time it takes to load and unload parts is reduced
because laser welding is done in an open atmosphere. Also, initial equipment
costs normally are higher than those for conventional welders, but operating
costs are comparable or lower.
Electroslag welding (ESW): Heat for ESW is
provided by an electrically conductive molten slag that is resistance heated by
the welding current. Electrodes are fed continuously into this molten pool of
slag at temperatures over 3,200°F.
Electroslag welding is suitable for welding joints in thick metal plates
because the weld pool is confined and molded by copper dams. However,
electroslag welds must be completed in one continuous process. Restarting a weld
after a stop can result in a large discontinuity. ESW tends to produce a large
grain size because of the large mass of metal that is molten at a given time and
the slow cooling of the metal.
Solid-state welding techniques
include:
Inertia welding:This is one of the few solid-state
welding processes that have been generally accepted by industry. Kinetic energy
of a flywheel is converted to heat by friction between the workpieces. One part
to be joined is fixed; the other is clamped in a spindle chuck. The flywheel (to
which the movable part is attached) is accelerated, and at a predetermined speed
driving power is cut, and the parts are forced together.
The major limitation of the process is that one of the two parts to be joined
must be axially symmetric. But inertia welding has few of the material and part
cleanliness limitations of other solid-state welding techniques.
Inertia-welding advantages include fast, uniform production welds, clean
operation, low energy costs, and minimum labor skill required. The amount of
upset of parts can be controlled to close tolerances, and a complete-interface
weld can be obtained.
Ultrasonic welding (USW): This process joins
metals by inducing high-frequency vibrations in overlapping metals in the area
to be joined. Fluxes and filler metals are not required. Electrical current does
not pass through the weld metal, and only localized heating is generated. The
temperature produced is below the melting point of the materials, so no melting
occurs during the welding.
Workpieces are clamped together between two jaws or sonotrode tips, and
vibrations are transmitted through one or both of the tips oscillating in a
plane parallel to the weld interface. The oscillating shear stress results in
elastic hysteresis, localized slip, and plastic deformation at the contacting
surfaces. This disrupts surface films and leads to metal-to-metal contact.
Ultrasonic welding has many applications in the assembly of electrical
products. It is typically used to attach oxide-resistant contact buttons to
switches; leads to coils of aluminum foil, sheet, or wire; and fine wire leads
and elements to other components. For plastics, USW is used for both spot and
continuous-seam fabrication and for closures on foil or plastic envelopes and
pouches.