Welding and is a fabrication process that joins materials, usually metals or thermoplastics, by causing coalescence. This is often done by melting the workpieces and adding a filler material to form a pool of molten material that cools to become a strong joint, but sometimes pressure is used in conjunction with heat, or by itself, to produce the weld. This is in contrast with soldering brazing, which involves melting a lower-melting-point material between the workpieces to form a bond between them.
Many different energy sources can be used for welding, including a gas flame, an electric arc, a laser, an electron beam, friction, and ultrasound. While often an industrial process, welding can be done in many different environments, including open air, underwater and in space. Regardless of location, however, welding remains dangerous, and precautions must be taken to avoid burns, electric shock, poisonous fumes, and overexposure to ultraviolet light.
Until the end of the 19th century, the only other welding process was forge welding, which blacksmiths had used for centuries to join metals by heating and pounding them. Arc welding and oxyfuel welding were among the first processes to develop during the 1800s, and resistance welding followed soon after. Welding technology advanced quickly during the early 20th century as World War I and World War II drove the demand for reliable and inexpensive joining methods. Following the wars, several modern welding techniques were developed, including manual methods like shielded metal arc welding, now one of the most popular welding methods, as well as semi-automatic and automatic processes such as gas metal arc welding, submerged arc welding and flux-cored arc welding. Developments continued with the invention of laser beam welding and electron beam welding in the latter half of the century. Today, the science continues to advance. Robot welding is becoming more commonplace in industrial settings, and researchers continue to develop new welding methods and gain greater understanding of weld quality and properties.
Welding processes
Arc welding
Arc welding processes use
a welding
power supply to create an electric arc between an electrode and the base
material to melt metals at the welding point. They can use either direct (DC) or alternating
(AC) current, 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 energy necessary for arc welding processes, a number
of different power supplies can be used. The most common classification is
constant current power supplies and constant voltage power supplies. In arc welding,
the voltage is directly related to the length of the arc, and the current is
related to the amount of heat input. 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 in arc welding also plays an important role in
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 has an impact on weld
properties. If the electrode is positively charged, it will melt more quickly,
increasing weld penetration and welding speed. Alternatively, a negatively
charged electrode results in more shallow welds. Non-consumable 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.
Methods
Shielded
metal arc welding a pipe
One of the most common types of arc welding is shielded metal arc welding (SMAW),
which is also known as manual metal arc welding (MMA) or stick welding. Electric
current is used to strike an arc between the base material and consumable
electrode rod, which is made of steel and is covered with a flux that
protects the weld area from oxidation and contamination by producing CO2
gas during the welding process. The electrode core itself acts as
filler material, making a separate filler unnecessary. The process is
very versatile, requiring little operator training and inexpensive
equipment. However, 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
speciality electrodes have made possible the welding of cast iron,
nickel, aluminium, copper, and other metals. The versatility of the
method makes it popular in a number of applications, including repair
work and construction.
Gas metal
arc welding (GMAW), also known as metal inert gas (MIG) welding, is a
semi-automatic or automatic welding process that uses an 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. However, because of the additional equipment, the process is
less portable and versatile, but still useful for industrial applications. The
process can be applied to a wide variety of metals, both ferrous and
non-ferrous. 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 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 non-consumable electrode made of tungsten, 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. It 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 furthermore, 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
no smoke is produced. The process is commonly used in industry, especially for
large products. Other arc welding processes include atomic
hydrogen welding, carbon arc
welding, electroslag
welding, electrogas
welding, and stud
arc welding.
Gas welding a
steel armature
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 more than 3000°C. 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. Other gas welding methods, such as air acetylene
welding, oxygen
hydrogen welding, and pressure gas
welding are quite similar, generally differing only in the type of gases
used. A water torch is
sometimes used for precision welding of items such as jewelry. Gas welding is
also used in plastic
welding, though the heated substance is air, and the temperatures are much
lower.
Resistance welding
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 amounts of 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. 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 flash welding, projection
welding, and upset welding.
Energy beam welding
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.
Solid-state welding
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, such as the welding of aluminum with steel in ship hulls
or compound plates. Other solid-state welding processes include co-extrusion
welding, cold welding,
diffusion
welding, friction
welding, high frequency
welding, hot pressure
welding, induction welding, and roll
welding.
Geometry
Common
welding joint types – (1) Square butt joint, (2) Single-V preparation joint, (3)
Lap joint, (4) T-joint.
Welds can be geometrically prepared in many different ways. The four most
basic types of weld joints are the
square butt joint, the
single-V preparation joint, the lap
joint, and the T-joint. Other variations
exist as well—for example, double-V preparation joints are characterized by the
two pieces of material each tapering to a single center point at one-half their
height. Single-U and double-U preparation joints are also fairly common—instead
of having straight edges like the single-V and double-V preparation joints, they
are curved, forming the shape of a U. Lap joints are also commonly more than two
pieces thick—depending on the process used and the thickness of the material,
many pieces can be welded together in a lap joint geometry.
Often, particular joint designs are used exclusively or almost exclusively by
certain welding processes. For example, resistance spot welding, laser beam
welding, and electron beam welding are most frequently performed on lap joints.
However, some welding methods, like shielded metal arc welding, are extremely
versatile and can weld virtually any type of joint. Additionally, some processes
can be used to make multipass welds, in which one weld is allowed to cool, and
then another weld is performed on top of it. This allows for the welding of
thick sections arranged in a single-V preparation joint, for example.
The
cross-section of a welded butt joint, with the darkest gray representing the
weld or fusion zone, the medium gray the heat-affected zone, and the lightest
gray the base material.
After welding, a number of distinct regions can be identified in the weld
area. The weld itself is called the fusion zone—more specifically, it is where
the filler metal was laid during the welding process. The properties of the
fusion zone depend primarily on the filler metal used, and its compatibility
with the base materials. It is surrounded by the heat-affected zone, the area that had its
microstructure and properties altered by the weld. These properties depend on
the base material's behavior when subjected to heat. The metal in this area is
often weaker than both the base material and the fusion zone, and is also where
residual stresses are found.
Quality
Most often, the major metric used for judging the quality of a weld is its
strength and the strength of the material around it. Many distinct factors
influence this, including the welding method, the amount and concentration of
heat input, the base material, the filler material, the flux material, the
design of the joint, and the interactions between all these factors. To test the
quality of a weld, either destructive or nondestructive testing methods are
commonly used to verify that welds are defect-free, have acceptable levels of
residual stresses and distortion, and have acceptable heat-affected zone (HAZ)
properties. Welding codes and specifications exist to guide welders in proper
welding technique and in how to judge the quality of welds.
Heat-affected zone
The effects of welding on the material surrounding the weld can be
detrimental – depending on the materials used and the heat input of the welding
process used, the HAZ can be of varying size and strength. The thermal
diffusivity of the base material plays a large role—if the diffusivity is
high, the material cooling rate is high and the HAZ is relatively small.
Alternatively, a low diffusivity leads to slower cooling and a larger HAZ. The
amount of heat inputted by the welding process plays an important role as well,
as processes like oxyacetylene welding have an unconcentrated heat input and
increase the size of the HAZ. Processes like laser beam welding give a highly
concentrated, limited amount of heat, resulting in a small HAZ. Arc welding
falls between these two extremes, with the individual processes varying somewhat
in heat input.
Distortion and cracking
Welding methods that involve the melting of metal at the site of the joint
necessarily are prone to shrinkage
as the heated metal cools. Shrinkage, in turn, can introduce residual stresses
and both longitudinal and rotational distortion. Distortion can pose a major
problem, since the final product is not the desired shape. To alleviate
rotational distortion, the workpieces can be offset, so that the welding results
in a correctly shaped piece. Other methods of limiting distortion, such as
clamping the workpieces in place, cause the buildup of residual stress in the
heat-affected zone of the base material. These stresses can reduce the strength
of the base material, and can lead to catastrophic failure through cold cracking, as in the
case of several of the Liberty
ships. Cold cracking is limited to steels, and is associated with the
formation of martensite as the
weld cools. The cracking occurs in the heat-affected zone of the base material.
To reduce the amount of distortion and residual stresses, the amount of heat
input should be limited, and the welding sequence used should not be from one
end directly to the other, but rather in segments. The other type of cracking,
hot cracking or
solidification cracking, can occur in all metals, and happens in the fusion zone
of a weld. To diminish the probability of this type of cracking, excess material
restraint should be avoided, and a proper filler material should be
utilized.
Weldability
The quality of a weld is also dependent on the combination of materials used
for the base material and the filler material. Not all metals are suitable for
welding, and not all filler metals work well with acceptable base materials.
Steels
The weldability of steels is inversely proportional to a
property known as the hardenability of the steel, which measures the
ease of forming martensite
during heat treatment. The hardenability of steel depends on its chemical
composition, with greater quantities of carbon and other alloying elements resulting in a higher hardenability and
thus a lower weldability. In order to be able to judge alloys made up of many
distinct materials, a measure known as the equivalent carbon content is used to
compare the relative weldabilities of different alloys by comparing their
properties to a plain carbon
steel. The effect on weldability of elements like chromium and vanadium, while not as great as carbon, is more significant than that of copper and nickel, for example. As the equivalent carbon content
rises, the weldability of the alloy decreases. The disadvantage to using plain
carbon and low-alloy steels is their lower strength—there is a trade-off between
material strength and weldability. High strength, low-alloy steels were developed
especially for welding applications during the 1970s, and these generally easy to weld materials have
good strength, making them ideal for many welding applications.
Stainless steels,
because of their high chromium content, tend to behave differently with respect
to weldability than other steels. Austenitic grades of stainless steels tend to
be the most weldable, but they are especially susceptible to distortion due to
their high coefficient of thermal expansion. Some alloys of this type are prone
to cracking and reduced corrosion resistance as well. Hot cracking is possible
if the amount of ferrite in the weld is not controlled—to
alleviate the problem, an electrode is used that deposits a weld metal
containing a small amount of ferrite. Other types of stainless steels, such as
ferritic and martensitic stainless steels, are not as easily welded, and must
often be preheated and welded with special electrodes.
Aluminum
The weldability of aluminum
alloys varies significantly, depending on the chemical composition of the alloy
used. Aluminum alloys are susceptible to hot cracking, and to combat the
problem, welders increase the welding speed to lower the heat input. Preheating
reduces the temperature gradient across the weld zone and thus helps reduce hot
cracking, but it can reduce the mechanical properties of the base material and
should not be used when the base material is restrained. The design of the joint
can be changed as well, and a more compatible filler alloy can be selected to
decrease the likelihood of hot cracking. Aluminum alloys should also be cleaned
prior to welding, with the goal of removing all oxides, oils, and loose
particles from the surface to be welded. This is especially important because of
an aluminum weld's susceptibility to porosity due to hydrogen and dross due to oxygen.
Unusual conditions
While many welding applications are done in controlled environments such as
factories and repair shops, some welding processes are commonly used in a wide
variety of conditions, such as open air, underwater, and vacuums (such as space). In open-air applications, such
as construction and outdoors repair, shielded metal arc welding is the most
common process. Processes that employ inert gases to protect the weld cannot be
readily used in such situations, because unpredictable atmospheric movements can
result in a faulty weld. Shielded metal arc welding is also often used in underwater
welding in the construction and repair of ships, offshore platforms, and
pipelines, but others, such as flux cored arc welding and gas tungsten arc
welding, are also common. Welding in space is also possible—it was first
attempted in 1969 by Russian cosmonauts, when they performed experiments to
test shielded metal arc welding, plasma arc welding, and electron beam welding
in a depressurized environment. Further testing of these methods were done in
the following decades, and today, researchers are continuing to develop methods
for using other welding processes in space such as laser beam welding,
resistance welding, and friction welding. Advances in these areas could prove
indispensable for projects like the construction of the new International Space Station, which
will likely rely heavily on welding for joining in space the parts that were
manufactured on Earth.
Safety issues
Welding, without the proper precautions, can be a dangerous and unhealthy
practice. However, with the use of new technology and proper protection, the
risks of injury and death associated with welding can be greatly reduced.
Because many common welding procedures involve an open electric arc or flame,
the risk of burns is significant. To prevent them, welders wear protective clothing in the form of heavy leather gloves and protective long sleeve jackets to avoid
exposure to extreme heat and flames. Additionally, the brightness of the weld
area leads to a condition called arc
eye in which ultraviolet light causes the inflammation of
the cornea and can burn the retinas of the eyes. Goggles and helmets with dark face plates are worn to prevent this
exposure, and in recent years, new helmet models have been produced that feature
a face plate that self-darkens upon exposure to high amounts of UV light. To
protect bystanders, transparent welding curtains often surround the welding
area. These curtains, made of a polyvinyl chloride plastic film, shield
nearby workers from exposure to the UV light from the electric arc, but should
not be used to replace the filter glass used in helmets.
Welders are also often exposed to dangerous gases and particulate matter. Processes like flux-cored arc
welding and shielded metal arc welding produce smoke containing particles of various types of oxides. The size of the particles in question
tends to influence the toxicity of the
fumes, with smaller particles presenting a greater danger. Additionally, many
processes produce various gases, most commonly carbon dioxide and ozone, and fumes that can prove dangerous if ventilation
is inadequate. Furthermore, because the use of compressed gases and flames in
many welding processes pose an explosion and fire risk, some common precautions
include limiting the amount of oxygen in the air and keeping combustible
materials away from the workplace.
Costs and trends
As an industrial process, the cost of welding plays a crucial role in
manufacturing decisions. Many different variables affect the total cost,
including equipment cost, labor cost, material cost, and energy cost. Depending on
the process, equipment cost can vary, from inexpensive for methods like shielded
metal arc welding and oxyfuel welding, to extremely expensive for methods like
laser beam welding and electron beam welding. Because of their high cost, they
are only used in high production operations. Similarly, because automation and
robots increase equipment costs, they are only implemented when high production
is necessary. Labor cost depends on the deposition rate (the rate of welding),
the hourly wage, and the total operation time, including both time welding and
handling the part. The cost of materials includes the cost of the base and
filler material, and the cost of shielding gases. Finally, energy cost depends
on arc time and welding power demand.
For manual welding methods, labor costs generally make up the vast majority
of the total cost. As a result, many cost-savings measures are focused on
minimizing the operation time. To do this, welding procedures with high
deposition rates can be selected, and weld parameters can be fine-tuned to
increase welding speed. Mechanization and automatization are often implemented
to reduce labor costs, but this frequently increases the cost of equipment and
creates additional setup time. Material costs tend to increase when special
properties are necessary, and energy costs normally do not amount to more than
several percent of the total welding cost.
In recent years, in order to minimize labor costs in high production
manufacturing, industrial welding has become increasingly more automated, most
notably with the use of robots in resistance spot welding (especially in the
automotive industry) and in arc welding. In robot welding, mechanized devices both hold the
material and perform the weld, and at first, spot welding was its most common
application. But robotic arc welding has been increasing in popularity as
technology has advanced. Other key areas of research and development include the
welding of dissimilar materials (such as steel and aluminum, for example) and
new welding processes, such as friction stir, magnetic
pulse, conductive
heat seam, and laser-hybrid welding. Furthermore,
progress is desired in making more specialized methods like laser beam welding
practical for more applications, such as in the aerospace and automotive
industries. Researchers also hope to better understand the often unpredictable
properties of welds, especially microstructure, residual stresses, and a weld's tendency to
crack or deform.
Notes
- ^ Weman, page 5.
References
- Most inline citations are only visible in edit mode. They refer to the
following works:
Here is the link to Getting Killer Welds I recommend:
Here's all you have to do...
Just
click the link below and You can soon be experiencing How to Weld Like a Pro starting right now... today!
|