Welding is a materials joining process in which two (or more) parts are
coalesced at their contacting surfaces by the suitable application of heat and/or pressure. The assemblage of parts that are
joined by welding is called a weldment.
Many welding processes are accomplished by heat alone, with no pressure
applied, others by a combination of heat and pressure, and still others by pressure alone with no external heat supplied.
In some welding processes a filler material is added to facilitate coalescence. Welding is most commonly associated
with metal parts, but the process is also used for joining plastics.
Welding is a relatively new process, its commercial and technological
importance derives from the following:
- Welding provides a permanent joint. The welded
parts become a single entity.
- The welded joint can be stronger than the parent
materials, if a filler material is used that has strength properties superior to those of the parents, and proper welding
techniques are used.
- Welding is usually the most economical way to
join components in terms of material usage and fabrication costs. Alternative mechanical methods of assembly require more
complex shapes alterations (for example, drilling of holes) and addition of fasteners (for example, rivets or bolts). The
resulting mechanical assembly is usually heavier than a corresponding weldment.
- Welding is not restricted to the factory environment.
It can be accomplished in the field.
Although Welding has the advantages indicated, it also has certain limitations
and drawbacks (or potential drawbacks):
- Most welding operations are performed manually
and are expensive in terms of labour cost. Many welding operations are considered skilled trades, and the labour to perform
these operations may be scarce.
- Most welding processes, involving the use of
high energy, are inherently dangerous.
- Since welding accomplishes a permanent bond
between the components, it does not allow for convenient disassembly. If there is a need for occasional disassembly of the
product (for repair or maintenance), then welding should not be used as the assembly method.
- The welded joint can suffer from certain quality
defects that are difficult to detect. The defects can reduce the strength of the joint.
Although Welding has the advantages indicated, it also has certain limitations and drawbacks (or potential drawbacks):
Welding involves localised coalescence or joining together of two metallic
parts at their faying surfaces. The faying surfaces are the part surfaces in contact or close proximity that are to be joined.
Welding is usually performed on parts made of the same metal, but some welding operations can be used to join dissimilar metals.
Types of Welding Processes
Over 50 different types of welding operations have been catalogued by
the American Welding Society. They use various types or combinations of energy to provide the required power. Welding processes
can be divided into two major groups: (1) fusion welding and (2) solid-state welding.
Fusion welding processes use heat to melt the base
metals. In many fusion welding operations, a filler metal is added to the molten pool to facilitate the process and provide
bulk and strength to the welded joint. A fusion welding operation in which no filler metal is added is referred to as an autogenous
weld. The fusion category comprises the most widely used welding processes and includes the following general groups (initials
in parentheses are designations of the American Welding Society):
Arc welding (AW). Arc welding refers to a group of
welding processes in which heating of the metals is accomplished by an electric arc. Some arc welding operations also apply
pressure during the process, and most utilise a filler metal.
Resistance welding (RW). Resistance welding achieves
coalescence using heat from electrical resistance to the flow of a current passing between the faying surfaces of two parts
held together under pressure.
Oxyfuel gas welding (OFW). These joining processes
use an oxyfuel gas, such as a mixture of oxygen and acetylene, to produce a hot flame for melting the base metal and filler
metal, if one is used.
Other fusion welding processes. In addition to the preceding types, there
are other welding processes that produce fusion of the metals joined. Examples include electron beam welding and laser
Certain arc and oxyfuel processes are also used for cutting metals.
Solid-state welding refers to joining processes in which coalescence
results from application of pressure alone or a combination of heat and pressure. If heat is used, the temperature in the
process is below the melting point of the metals being welded. No filler metal is utilised in solid-state processes. Some
representative welding processes in this group include the following:
Diffusion welding (DFW). In diffusion welding, two
surfaces are held together under pressure at an elevated temperature and the parts coalesce by solid-state fusion.
Friction welding (FRW). In this process, coalescence
is achieved by the heat between two surfaces.
Ultrasonic welding (USW). Ultrasonic welding is performed
by applying a moderate pressure between the two parts and using an oscillating motion at ultrasonic frequencies in a direction
parallel to the contacting surfaces. The combination of normal and vibratory forces results in shear stresses that remove
surface films and achieve atomic bonding of the surfaces.
(GMAW or MIG)
- Welds all
commercial metals and alloys.
can be done in all positions.
speeds are higher than those with shielded metal arc welding.
- Long welds
can be deposited without stops and starts.
- Spray transfer
allows deeper penetration than with shielded metal arc welding.
post weld cleaning is required.
Gas Tungsten Arc Welding
superior quality welds, generally free of defects.
- Free of
the spatter that occurs with other arc welding processes.
- Used with
or without filler metal as required for the specific application.
- Allows excellent
control of root pass penetration.
- It allows
precise control of the welding variables.
Cored Arc Welding
- Weld metal
deposit is high quality.
- Weld appearance
is excellent - smooth, uniform welds.
contour of horizontal fillet welds.
- Allows many
sheets to be welded over a wide thickness range.
- High operating
factor allows the process to be easily mechanized.
- High deposition
rate allows for high current density.
- Higher tolerance
for contaminants that may cause weld cracking.
is relatively simple, inexpensive, and portable.
- The covered
electrode protects the filler metal and the weld metal from harmful oxidation.
gas shielding or granular flux is not required.
- The process
is less sensitive to wind and draft than gas shielded arc welding processes.
- It can be
used in areas where access is limited.
- The process
is suitable for most of the commonly used metals and alloys.