Welding Sheet metal
1 Welding
2 Welding processes
2.1 Arc welding
2.2 Gas welding
2.2.1. Equipment
2.2.2. Welding gun and wire feed unit
2.2.3. Power supply
2.2.4. Electrode
2.2.5. Shielding gas
2.2.6. Operation
2.2.7. Technique
2.2.8. Quality
2.2.9. Safety
2.3 Resistance welding
2.4 Energy beam welding
2.5 Solid-state welding
3. Geometry
4. Quality
4.1. Heat-affected zone
4.2. Distortion and cracking
4.3. Weldability
4.3.1. Steels
4.3.2. Aluminum
5. Unusual conditions
6. Safety issues
7. Costs and trends
8. Welding Glossary
9. Bending
2.1.1 Arc welding
2.1.2 Processes
2.1.3.Anode and cathode in electrochemical cells
2.1.4. Primary cell
2.1.5. Secondary cell
2.1.6 Other anodes and cathodes
2.1.7 Welding electrodes
2.1.8 Alternating current electrodes
2.1.9 Types of electrode
9.1.Simple or Symmetrical Bending
9.2.Complex or Unsymmetrical Bending
10. Sheet Metal
10.1. Introduction
10.2. Processes
10.3. Stretching
10.4. Drawing
10.5. Deep Drawing
10.6. Cutting
10.7. Bending and Flanging
10.8. Punching and Shearing
10.9. Spinning
1010. Press Forming
10.11. Roll Forming
10.11.1. Rollíng
11. Molding (process)
11.2.Types of Presses
1. Welding
Welding is a metal-joining process in which coalescence is obtained by heat and
pressure. It may also be defined as a metallurgical bond accomplished by the
attracting forces between atoms. Before these atoms can be bonded together,
absorbed vapors and oxides on contacting surfaces must be overcome. The number
one enemy to welding is oxidation, and, consequently, many welding processes are
performed in a controlled environment or shielded by an inert atmosphere. If force is
applied between two smooth metal surfaces to be joined, some crystals will crush
through the surfaces and be in contact. As more and more pressure is applied, these
areas spread out and other contacts are made. The brittle oxide layer is broken and
fragmented as the metal is deformed plastically. Coalescence is obtained when the
boundaries between the two surfaces are mainly crystalline planes. This process,
known as cold welding, will be discussed further in this chapter. The breaking
through or elimination of surface oxide layers happens when a weld is made
If temperature is added to pressure the welding of two surfaces will be facilitated, and
coalescence is obtained in the same manner as cold-pressure welding. As
temperature is increased the ductility of the base metal is increased and atomic
diffusion progresses more rapidly. Nonmetallic materials on interfacial surfaces are
softened, permitting them to be removed or broken up by plastic flow of the base
materials. Hot-pressure welds are more efficient but not necessarily stronger if the
atom-to-atom bond is the same(1).
Many welding processes have been developed. They differ widely in the manner that
heat is applied and in the equipment used. The principal processes are listed here(1).
1. Braze welding
A. Torch
B. Furnace
C. Induction
D. Resistance
E. Dip
F. Infrared
II. Forge welding
A. Manual
B. Machine
1. Rolling
2. Hammer
3. Die
III. Gas welding
A. Oxyacetylen
B. Oxyhydrogen
C. Air acetylene
D. Pressure
IV. Resistance welding
A. Spot
B. Projection
C. Seam
D. Butt
E. Flash
F. Percussion
G. High frequency
VI. Arc welding
A. Carbon electrode
1. Shielded
2. Unshielded
B. Metal electrode
1. Shielded
a. Shielded arc
b. Atomic hydrogen
c. Inert gas
d. Arc spot
e. Submerged arc
f. Stud
g. Electroslag
2. Unshieldde
a. Bare metal
b. Stud
VII. Special welding processes
A. Electron beam
B. Laser welding
C. Friction welding
D. Thermit welding
a. Pressure
b. Nonpressure
E. Flow welding
F. Cold welding
a. Pressure
b. Ultrasonic
G. explosive welding
H. Diffusion welding
2. Welding processes
2.1. 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 (DC) or alternating (AC) current, and consumable or nonconsumable 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.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.[2]
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.[7] 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 mediumpenetration 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.
2.1.2. Processes
Fig. Shielded metal arc welding
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 versatile and can be performed with relatively inexpensive equipment,
making it well suited to shop jobs and field work.[6] 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.[9] 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. Inexperienced operators may find it
difficult to make good out-of-position welds with this process.
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. As with SMAW, reasonable operator proficiency can be achieved with
modest training. Since the electrode is continuous, welding speeds are greater for
GMAW than for SMAW. Also, the smaller arc size compared to the shielded metal
arc welding process makes it easier to make out-of-position welds (e.g., overhead
joints, as would be welded underneath a structure).
The equipment required to perform the GMAW process is more complex and
expensive than that required for SMAW, and requires a more complex setup
procedure. Therefore, GMAW is less portable and versatile, and due to the use of a
separate shielding gas, is not particularly suitable for outdoor work. However, owing
to the higher average rate at which welds can be completed, GMAW is well suited to
production welding. 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 even higher welding speed and greater metal penetration.
Fig. Gas tungsten arc welding
Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding (also
sometimes erroneously referred to as heliarc 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.[4] 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.[6]
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.[5] Other arc welding processes
include atomic hydrogen welding, carbon arc welding, electroslag welding, electrogas
welding, and stud arc welding.
Fig. Gas welding a steel armature using the oxy-acetylene process.
An electrode is an electrical conductor used to make contact with a metallic part of a
circuit (e.g. a semiconductor, an electrolyte or a vacuum). The word was coined by
the scientist Michael Faraday from the Greek words elektron (meaning amber, from
which the word electricity is derived) and hodos, a way.[3]
2.1.3. Anode and cathode in electrochemical cells
Fig. Scheme of a discharging galvanic cell
An electrode in an electrochemical cell is referred to as either an anode or a cathode,
words that were also coined by Faraday. The anode is now defined as the electrode
at which electrons leave the cell and oxidation occurs, and the cathode as the
electrode at which electrons enter the cell and reduction occurs. Each electrode may
become either the anode or the cathode depending on the voltage applied to the cell.
A bipolar electrode is an electrode that functions as the anode of one cell and the
cathode of another cell.
2.1.4. Primary cell
A primary cell is a special type of electrochemical cell in which the reaction cannot be
reversed, and the identities of the anode and cathode are therefore fixed. The anode
is always the negative electrode. The cell can be discharged but not recharged.
2.1.5. Secondary cell
The case in an electrolytic cell. When the cell is being discharged, it behaves like a
primary or voltaic cell, with the anode as the negative electrode and the cathode as
the positive.
2.1.6. Other anodes and cathodes
In a vacuum tube or a semiconductor having polarity (diodes, electrolytic capacitors)
the anode is the positive (+) electrode and the cathode the negative (−). The
electrons enter the device through the cathode and exit the device through the
In a three-electrode cell, a counter electrode, also called an auxiliary electrode, is
used only to make a connection to the electrolyte so that a current can be applied to
the working electrode. The counter electrode is usually made of an inert material,
such as a noble metal or graphite, to keep it from dissolving.
2.1.7. Welding electrodes
In arc welding an electrode is used to conduct current through a workpiece to fuse
two pieces together. Depending upon the process, the electrode is either
consumable, in the case of gas metal arc welding or shielded metal arc welding, or
non-consumable, such as in gas tungsten arc welding. For a direct current system
the weld rod or stick may be a cathode for a filling type weld or an anode for other
welding processes. For an alternating current arc welder the welding electrode would
not be considered an anode or cathode.
2.1.8. Alternating current electrodes
For electrical systems which use alternating current the electrodes are the
connections from the circuitry to the object to be acted upon by the electrical current
but are not designated anode or cathode since the direction of flow of the electrons
changes periodically, usually many times per second.
2.1.9.Types of electrode
Electrodes for medical purposes, such as EEG, ECG, ECT, defibrillator
Electrodes for electrophysiology techniques in biomedical research
Electrodes for execution by the electric chair
Electrodes for electroplating
Electrodes for arc welding
Electrodes for cathodic protection
Inert electrodes for electrolysis (made of platinum)
2.2. Gas welding
Gas welding includes all he processes in which gases are used in combination to
obtain a hot flame. Those commonly used are acetylene, natural gas, and
hydrogen in combination with oxygen. Oxyhydrogen welding was the first gas
process to be commercially developed. The maximum temperature developed by
this process is 3600 F (1980 C). Hydrogen is produced by either the electrolysis
of water or passing steam over coke(1). The most used combination is the
oxyacetylene process, which has a flame temperature of 6300 F (3500 C).
Oxyacetylene Welding. An oxyacetylene weld is produced by heating with a flame
obtained from the combustion of oxygen and acetylene with or without the use of
a filler metal. Most often the joint is heated to a state of fusion and as a rule no
pressure is used. Oxygen is produced by both electrolysis and liquification of air.
Electrolysis separates water into hydrogen and oxygen by passing an electric
current through it. Most commercial oxygen is made by liquefying air and
separating the oxygen from the nitrogen. It is stored in steel cylinders at a
pressure of 2000 psi (14 MPa).
Acetylene gas (C2H2) is obtained by dropping lumps of calcium carbide in water.
The gas bubbles through the water, and any precipitate is slaked lime. The
reaction that takes place in an acetylene generator is
CaC2 + 2H2O = Ca(OH)2 + C2H2
The perfect gas laws describe the amount of gas available at a regulated
pressure from a pressurized cylinder(1). That is ;
P1V1/T1 = P2V2/T2
Where P1, V1, and T1 refer to the pressure, volume, and absolute temperature in
the cylinder; P2, V2, T2 refer to the regulated pressure, volume, and temperature.
The absolute temperature is T= 460 + t F. I the cylinder an regulated
temperatures are the same or nearly so, then
P1V1 = P2 V2
2.2.1. Equipment
To perform gas metal arc welding, the basic necessary equipment is a welding gun, a
wire feed unit, a welding power supply, an electrode wire, and a shielding gas supply.
2.2.2. Welding gun and wire feed unit
Fig. GMAW torch nozzle cutaway image. (1) Torch handle, (2) Molded phenolic
dielectric (shown in white) and threaded metal nut insert (yellow), (3) Shielding gas
nozzle, (4) Contact tip, (5) Nozzle output face
Fig. A GMAW wire feed unit
The typical GMAW welding gun has a number of key parts—a control switch, a
contact tip, a power cable, a gas nozzle, an electrode conduit and liner, and a gas
hose. The control switch, or trigger, when pressed by the operator, initiates the wire
feed, electric power, and the shielding gas flow, causing an electric arc to be struck.
The contact tip, normally made of copper and sometimes chemically treated to
reduce spatter, is connected to the welding power source through the power cable
and transmits the electrical energy to the electrode while directing it to the weld area.
It must be firmly secured and properly sized, since it must allow the passage of the
electrode while maintaining an electrical contact. Before arriving at the contact tip,
the wire is protected and guided by the electrode conduit and liner, which help
prevent buckling and maintain an uninterrupted wire feed. The gas nozzle is used to
evenly direct the shielding gas into the welding zone—if the flow is inconsistent, it
may not provide adequate protection of the weld area. Larger nozzles provide greater
shielding gas flow, which is useful for high current welding operations, in which the
size of the molten weld pool is increased. The gas is supplied to the nozzle through a
gas hose, which is connected to the tanks of shielding gas. Sometimes, a water hose
is also built into the welding gun, cooling the gun in high heat operations.[4]
The wire feed unit supplies the electrode to the work, driving it through the conduit
and on to the contact tip. Most models provide the wire at a constant feed rate, but
more advanced machines can vary the feed rate in response to the arc length and
voltage. Some wire feeders can reach feed rates as high as 30.5 m/min
(1200 in/min),[5] but feed rates for semiautomatic GMAW typically range from 2 to
10 m/min (75–400 in/min).[5]
2.2.3. Power supply
Most applications of gas metal arc welding use a constant voltage power supply. As
a result, any change in arc length (which is directly related to voltage) results in a
large change in heat input and current. A shorter arc length will cause a much greater
heat input, which will make the wire electrode melt more quickly and thereby restore
the original arc length. This helps operators keep the arc length consistent even
when manually welding with hand-held welding guns. To achieve a similar effect,
sometimes a constant current power source is used in combination with an arc
voltage-controlled wire feed unit. In this case, a change in arc length makes the wire
feed rate adjust in order to maintain a relatively constant arc length. In rare
circumstances, a constant current power source and a constant wire feed rate unit
might be coupled, especially for the welding of metals with high thermal
conductivities, such as aluminum. This grants the operator additional control over the
heat input into the weld, but requires significant skill to perform successfully.[7]
Alternating current is rarely used with GMAW; instead, direct current is employed and
the electrode is generally positively charged. Since the anode tends to have a greater
heat concentration, this results in faster melting of the feed wire, which increases
weld penetration and welding speed. The polarity can be reversed only when special
emissive-coated electrode wires are used, but since these are not popular, a
negatively charged electrode is rarely employed.[4]
2.2.4. Electrode
The selection of an electrode to be used in GMAW is a complicated decision, as it
depends on the process variation being used, the composition of the metal being
welded, the joint design, and the material surface conditions. The choice of an
electrode strongly influences the mechanical properties of the weld area, making it a
key factor in weld quality. In general, it is desirable that the welded metal have
mechanical properties similar to those of the base material, and that there be no
discontinuities, such as porosity, within the weld. To achieve these goals in different
materials using different GMAW variations, a wide variety of electrodes exist. All
contain deoxidizing metals such as silicon, manganese, titanium, and aluminum in
small percentages to help prevent oxygen porosity, and some contain denitriding
metals such as titanium and zirconium to avoid nitrogen porosity.[9] Depending on the
process variation and base material being used, the diameters of the electrodes used
in GMAW typically range from 0.7 to 2.4 mm (0.028–0.095 in), but can be as large as
4 mm (0.16 in). The smallest electrodes are associated with short-circuiting metal
transfer, while the pulsed spray mode generally uses electrodes of at least 1.6 mm
(0.06 in).[6]
Fig. GMAW Circuit diagram. (1) Welding torch, (2) Workpiece, (3) Power source, (4)
Wire feed unit, (5) Electrode source, (6) Shielding gas supply.
2.2.5. Shielding gas
Shielding gases are necessary for gas metal arc welding to protect the welding area
from atmospheric gases such as nitrogen and oxygen, which can cause fusion
defects, porosity, and weld metal embrittlement if they come in contact with the
electrode, the arc, or the welding metal. This problem is common to all arc welding
processes, but instead of a shielding gas, many arc welding methods utilize a flux
material which disintegrates into a protective gas when heated to welding
temperatures. In GMAW, however, the electrode wire does not have a flux coating,
and a separate shielding gas is employed to protect the weld. This eliminates slag,
the hard residue from the flux that builds up after welding and must be chipped off to
reveal the completed weld.
The choice of a shielding gas depends on several factors, most importantly the type
of material being welded and the process variation being used. Pure inert gases such
as argon and helium are only used for nonferrous welding; with steel they cause an
erratic arc and encourage spatter (with helium) or do not provide adequate weld
penetration (argon). Pure carbon dioxide, on the other hand, allows for deep
penetration welds but encourages oxide formation, which adversely affect the
mechanical properties of the weld. Its low cost makes it an attractive choice, but
because of the violence of the arc, spatter is unavoidable and welding thin materials
is difficult. As a result, argon and carbon dioxide are frequently mixed in a 75%/25%
or 80%/20% mixture, which reduces spatter and makes it possible to weld thin steel
Argon is also commonly mixed with other gases, such as oxygen, helium, hydrogen,
and nitrogen. The addition of up to 5% oxygen encourages spray transfer, which is
critical for spray-arc and pulsed spray-arc GMAW. However, more oxygen makes the
shielding gas oxidize the electrode, which can lead to porosity in the deposit if the
electrode does not contain sufficient deoxidizers. An argon-helium mixture is
completely inert, and is used on nonferrous materials. A helium concentration of
50%–75% raises the voltage and increases the heat in the arc, making it helpful for
welding thicker workpieces. Higher percentages of helium also improve the weld
quality and speed of using alternating current for the welding of aluminum. Hydrogen
is added to argon in small concentrations (up to about 5%) for welding nickel and
thick stainless steel workpieces. In higher concentrations (up to 25% hydrogen), it is
useful for welding conductive materials such as copper. However, it should not be
used on steel, aluminum or magnesium because of the risk of hydrogen porosity.
Additionally, nitrogen is sometimes added to argon to a concentration of 25%–50%
for welding copper, but the use of nitrogen, especially in North America, is limited.
Mixtures of carbon dioxide and oxygen are similarly rarely used in North America, but
are more common in Europe and Japan.
Recent advances in shielding gas mixtures use three or more gases to gain improved
weld quality. A mixture of 70% argon, 28% carbon dioxide and 2% oxygen is gaining
in popularity for welding steels, while other mixtures add a small amount of helium to
the argon-oxygen combination, resulting in higher arc voltage and welding speed.
Helium is also sometimes used as the base gas, to which smaller amounts of argon
and carbon dioxide are added. Additionally, other specialized and often proprietary
gas mixtures claim to offer even greater benefits for specific applications.[6]
The desirable rate of gas flow depends primarily on weld geometry, speed, current,
the type of gas, and the metal transfer mode being utilized. Welding flat surfaces
requires higher flow than welding grooved materials, since the gas is dispersed more
quickly. Faster welding speeds mean that more gas must be supplied to provide
adequate coverage. Additionally, higher current requires greater flow, and generally,
more helium is required to provide adequate coverage than argon. Perhaps most
importantly, the four primary variations of GMAW have differing shielding gas flow
requirements—for the small weld pools of the short circuiting and pulsed spray
modes, about 10 L/min (20 ft³/h) is generally suitable, while for globular transfer,
around 15 L/min (30 ft³/h) is preferred. The spray transfer variation normally requires
more because of its higher heat input and thus larger weld pool; along the lines of
20–25 L/min (40–50 ft³/h).[9]
2.2.6. Operation
Fig. GMAW weld area. (1) Direction of travel, (2) Contact tube, (3) Electrode, (4)
Shielding gas, (5) Molten weld metal, (6) Solidified weld metal, (7) Workpiece.
In most of its applications, gas metal arc welding is a fairly simple welding process to
learn, requiring no more than several days to master basic welding technique. Even
when welding is performed by well-trained operators, however, weld quality can
fluctuate, since it depends on a number of external factors. And all GMAW is
dangerous, though perhaps less so than some other welding methods, such as
shielded metal arc welding.[9]
2.2.7. Technique
The basic technique for GMAW is quite simple, since the electrode is fed
automatically through the torch. In gas tungsten arc welding, the welder must handle
a welding torch in one hand and a separate filler wire in the other, and in shielded
metal arc welding, the operator must frequently chip off slag and change welding
electrodes. GMAW, on the other hand, requires only that the operator guide the
welding gun with proper position and orientation along the area being welded.
Keeping a consistent contact tip-to-work distance (the stickout distance) is important,
because a long stickout distance can cause the electrode to overheat and will also
waste shielding gas. The orientation of the gun is also important—it should be held
so as to bisect the angle between the workpieces; that is, at 45 degrees for a fillet
weld and 90 degrees for welding a flat surface. The travel angle or lead angle is the
angle of the torch with respect to the direction of travel, and it should generally
remain approximately vertical. However, the desirable angle changes somewhat
depending on the type of shielding gas used—with pure inert gases, the bottom of
the torch is out often slightly in front of the upper section, while the opposite is true
when the welding atmosphere is carbon dioxide.[1]
2.2.8. Quality
Two of the most prevalent quality problems in GMAW are dross and porosity. If not
controlled, they can lead to weaker, less ductile welds. Dross is an especially
common problem in aluminum GMAW welds, normally coming from particles of
aluminum oxide or aluminum nitride present in the electrode or base materials.
Electrodes and workpieces must be brushed with a wire brush or chemically treated
to remove oxides on the surface. Any oxygen in contact with the weld pool, whether
from the atmosphere or the shielding gas, causes dross as well. As a result, sufficient
flow of inert shielding gases is necessary, and welding in volatile air should be
In GMAW the primary cause of porosity is gas entrapment in the weld pool, which
occurs when the metal solidifies before the gas escapes. The gas can come from
impurities in the shielding gas or on the workpiece, as well as from an excessively
long or violent arc. Generally, the amount of gas entrapped is directly related to the
cooling rate of the weld pool. Because of its higher thermal conductivity, aluminum
welds are especially susceptible to greater cooling rates and thus additional porosity.
To reduce it, the workpiece and electrode should be clean, the welding speed
diminished and the current set high enough to provide sufficient heat input and stable
metal transfer but low enough that the arc remains steady. Preheating can also help
reduce the cooling rate in some cases by reducing the temperature gradient between
the weld area and the base material.[3]
2.2.9. Safety
Gas metal arc welding can be dangerous if proper precautions are not taken. Since
GMAW employs an electric arc, welders wear protective clothing, including heavy
leather gloves and protective long sleeve jackets, to avoid exposure to extreme heat
and flames. In addition, the brightness of the electric arc can cause arc eye, in which
ultraviolet light causes the inflammation of the cornea and can burn the retinas of the
eyes. Helmets with dark face plates are worn to prevent this exposure, and in recent
years, new helmet models have been produced that feature a liquid crystal-type face
plate that self-darkens upon exposure to high amounts of UV light. Transparent
welding curtains, made of a polyvinyl chloride plastic film, are often used to shield
nearby workers and bystanders from exposure to the UV light from the electric
Welders are also often exposed to dangerous gases and particulate matter. GMAW
produces smoke containing particles of various types of oxides, and the size of the
particles in question tends to influence the toxicity of the fumes, with smaller particles
presenting a greater danger. Additionally, carbon dioxide and ozone gases can prove
dangerous if ventilation is inadequate. Furthermore, because the use of compressed
gases in GMAW 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.[10]
2.3. 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 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.
Fig. Spot welder
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
flash welding, projection welding, and upset welding.
2.4. 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
2.5. 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 (including friction stir welding), high frequency welding, hot pressure
welding, induction welding, and roll welding.
3. Geometry
Fig. 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 five basic types of
weld joints are the butt joint, lap joint, corner joint, edge joint, and 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.
Fig. 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.
4. 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 defectfree, 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.
4.1. Heat-affected zone
Fig. The HAZ of a pipe weld, with the blue area being the metal most affected by the
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. Conversely, a low diffusivity leads to slower cooling and a
larger HAZ. The amount of heat injected 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.[29][30] To calculate the heat input for arc welding procedures, the following
formula can be used:
where Q = heat input (kJ/mm), V = voltage (V), I = current (A), and S = welding
speed (mm/min). The efficiency is dependent on the welding process used, with
shielded metal arc welding having a value of 0.75, gas metal arc welding and
submerged arc welding, 0.9, and gas tungsten arc welding, 0.8.
4.2. 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.[32] 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.[1,8]
4.3. 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.
4.3.1. 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.[34] 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.[1,2]
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.[6]
4.3.2. 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.[1]
5. 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 was done in the
following decades, and today researchers continue 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 International Space Station, which will likely rely heavily
on welding for joining in space the parts that were manufactured on Earth.[5]
6. Safety
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 personal protective equipment 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 welding 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, translucent 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, which in some cases can lead to
medical conditions like metal fume fever. 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 fumes and various gases, most
commonly carbon dioxide, ozone and heavy metals, that can prove dangerous
without proper ventilation and training. 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.[10]
7. 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
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,[42] 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.[8]
8. Welding Glossary
throat The perpendicular distance between two lines each parallel to a line
joining the outer toes one being tangent at the weld face and the
other being through the furthermost point of fusion penetration.
Air-arc cutting Thermal cutting using an arc for melting the metal and a stream of
air to remove the molten metal to enable a cut to be made.
A gas welding technique in which the flame rightward welding
test A block of metal consisting of one or more beads or runs fused
together for test purposes. It may or may not include portions of
parent metal.
test A test specimen that is composed wholly of weld metal over the
portion to be tested.
A lengthening or deflection of a DC welding arc caused by the
Arc blow
interaction of magnetic fields set up in the work and arc or cables.
The fan-shaped flame associated with the atomic-hydrogen arc.
Arc fan
The voltage between electrodes or between an electrode and the
Arc voltage
work, measured at a point as near as practical to the work.
Arc welding in which molecular hydrogen, passing through an arc
Atomicbetween two tungsten or other suitable electrodes, is changed to its
atomic form and then re-combines to supply the heat for welding
A welding sequence in which short lengths of run are (Back-step
Retrogression of the flame into the blowpipe neck or body with rapid
self extinction.
A piece of metal or other material placed at a root (Temporary
Backing bar
backing)(These terms are applied only to the welding of pipes or
A piece of metal placed at a root and penetrated by (Permanent
Backing strip
Block sequence A welding sequence in which short lengths of the (Block welding)
A cavity generally over 1.6 mm in diameter, formed by entrapped
gas during solidification of molten metal.
A device for mixing and burning gases to produce a flame for
welding, brazing, bronze welding, cutting, heating and similar
Fusing of the electrode wire to the current contact tube by sudden
Burn back
lengthening of the arc in any form of automatic or semi-automatic
metal-arc welding using a bare electrode.
The linear rate of consumption of a consumable electrode.
Burn off rate
A localised collapse of the molten pool due to (Melt through)
Burn through
Arc welding using a carbon electrode or electrodes.
An intermittent weld on each side of a joint (usually fillet welds in T
and lap joints) arranged so that the welds lie opposite to one another
along the joint.
flux Metal-arc welding in which a flux-coated or flux containing electrode
is deposited under a shield of carbon dioxide.
Metal-arc welding in which a bare wire electrode is used the arc and
CO2 welding
molten pool being shielded with carbon dioxide.
Concave fillet
A fillet weld in which the weld face is concave (curved inwards).
The more luminous part of a flame, which is adjacent to the nozzle
A weld extending along the entire length of a joint.
A fillet weld in which the weld face is convex (bulbous).
Coupon plate A test piece made by adding plates to the end of a joint to give an
extension of the weld for test purposes. (Note: this term is usually
used in the shipbuilding industry.)
A longitudinal discontinuity produced by fracture. Cracks may be
longitudinal, transverse, edge, crater, centre line, fusion zone
underhead, weld metal or parent metal.
A depression due to shrinkage at the end of a run where the source
Crater pipe
of heat was removed.
A flat plate to which two other flat plates or two bars are welded at
right angles and on the same axis.
An electrode with a covering that aids the production of such an arc
that molten metal is blown away to produce a groove or cut in the
Cutting oxygen Oxygen used at a pressure suitable for cutting.
The removal of the surface defects from ingots, blooms, billets and
slabs by means of a manual thermal cutting.
A method of metal-arc welding in which fused particles of the
Dip transfer
electrode wire in contact with the molten pool are detached from the
electrode in rapid succession by the short circuit current, which
develops every time the wire touches the molten pool.
The projected distance between the two ends of a drag line.
Serrations left on the face of a cut made by thermal cutting.
Drag lines
Electron-beam Thermal cutting in vacuum by melting and vaporising a narrow
section of the metal by the impact of a focused beam of electrons.
Excessive metal protruding through the root of a fusion weld made
from one side only.
Face bend test A bend test in which a specified side of the weld Normal bend test.
(The side opposite that containing the root or )
The carbon-rich zone, visible in a flame, extending around and
beyond the cone when there is an excess of carbonaceous gas.
a fusion weld, other than a butt, edge or fusion spot weld, which is
Fillet weld
approximately triangular in transverse cross-section.
Flame cutting Oxygen cutting in which the appropriate part of the material to be cut
is raised to ignition temperature by an oxy-fuel gas flame.
Flame snap-out Retrogression of the flame beyond the blowpipe body into the hose,
with possible subsequent explosion.
Flame washing A method of surface shaping and dressing of metal by flamecutting
using a nozzle designed to produce a suitably shaped cutting
oxygen stream.
A safety device fitted in the oxygen and fuel gas system to prevent
any flashback reaching the gas supplies.
Floating head A blowpipe holder on a flame cutting machine which, through a
suitable linkage, is designed to follow the contour of the surface of
the plate, thereby enabling the correct nozzle-to-workpiece distance
to be maintained.
Free bend test A bend test made without using a former.
In fusion welding. The depth to which the parent metal has been
The part of the parent metal which is melted into the weld metal.
Fusion zone
An auxiliary device designed for temporarily cutting off the supply of
gas to the welding equipment except the supply to a pilot jet where
Gas envelope The gas surrounding the inner cone of an oxy-gas flame.
A cavity generally under 1.6 mm in diameter, formed by entrapped
Gas pore
gas during solidification of molten metal.
Gas regulator A device for attachment to a gas cylinder or pipeline for reducing
and regulating the gas pressure to the working pressure required.
bend A bend test made by bending the specimen round a specified
Heat affected The part of the parent metal which is metallurgically affected by the
heat of welding or thermal cutting but not melted. (Also known as the
zone of thermal disturbance).
Hose protector A small non-return valve fitted to the blow-pipe end of a hose to
resist the retrogressive force of a flashback.
Included angle The angle between the planes of the fusion faces of parts to be
Slag or other foreign matter entrapped during welding. The defect is
usually more irregular in shape than a gas pore.
Incomplete root
Failure of weld metal to extend into the root of a joint.
A continuous or intermittent channel in the surface of a weld,
running along its length, due to insufficient weld metal. The channel
filled groove
may be along the centre or along one or both edges of the weld.
A series of welds at intervals along a joint.
The void left after metal has been removed by thermal cutting.
Lack of fusion Lack of union in a weld.(Between weld metal and parent metal,
parent metal and parent metal or between weld metal and weld
A gas welding technique in which the flame is (Forward welding)
The width of a fusion face in a fillet weld.
Thermal cutting by melting using the heat of an arc between a metal
electrode and the metal to be cut.
Arc welding using a consumable electrode.
Metal transfer The transfer of metal across the arc from a consumable electrode to
the molten pool.
Inert-gas welding using a consumable electrode (inert-gas metal-arc
A gas regulator in which the gas pressure is reduced to the working
pressure in more than one stage.
Nick-break test A fracture test in which a specimen is broken from a notch cut at a
predetermined position where the interior of the weld is to be
Arc welding in which the arc is visible.
circuit In a welding plant ready for welding, the voltage between two output
terminals which are carrying no current.
An imperfection at a toe or a root of a weld caused by metal flowing
on to the surface of the parent metal without fusing it.
Thermal cutting in which the ignition temperature is produced by an
electric arc, and cutting oxygen is conveyed through the centre of an
electrode, which is consumed in the process.
Oxygen lance A steel tube, consumed during cutting, through which cutting oxygen
passes, for the cutting or boring of holes.
Oxygen lancing Thermal cutting in which an oxygen lance is used.
Packed lance An oxygen lance with steel rods or wires.
Weld metal protruding through the root of a fusion weld made from
one side only.
A weld made by filling a hole in one component of a workpiece so as
Plug weld
to join it to the surface of an overlapping component exposed
through the hole.
A group of gas pores.
Powder cutting oxygen cutting in which powder is injected into the cutting oxygen
stream to assist the cutting action.
Powder lance An oxygen lance in which powder is mixed with the oxygen stream.
Oxygen used at a suitable pressure in conjunction with fuel gas for
raising to ignition temperature the metal to be cut.
Stress remaining in a metal part or structure as a result of welding.
welding stress
Reverse bend A bend test in which the other than that specified for a face bend
test is in tension.
A gas welding technique in which the flame is (Backward welding)
Root (of weld) The zone on the side of the first run farthest from the welder.
The portion of a fusion face at the root which is not bevelled or
Root face
Run-off-plate(s) A piece, or pieces, of metal so placed as to enable the full section of
of weld to be obtained at the end of the joint.
Run-on-plate(s) A piece, or pieces, of metal so placed as to enable the full section of
weld metal to be obtained at the beginning of a joint.
The removal of the surface defects from ingots, blooms, billets and
slabs by means of a flame cutting machine.
MIG - welding
Seal weld
Sealing run
Side bend test
Skip sequence
Slot lap joint
Spray transfer
Stack cutting
Striking voltage
Test piece
Test specimen
Thermal cutting
TIG - welding
test specimen
Touch welding
A weld, not being a strength weld, used to make a (sealing weld)
The final run deposited on the root side of a fusion (backing run)
A shallow groove caused by contraction of the metal along each
side of a penetration bead.
A bend test in which the face of a transverse section of the weld is in
A welding sequence in which short lengths of run are (skip welding )
A configuration in a joint or joint preparation which may lead to the
entrapment of slag.
A joint between two overlapping components made by depositing a
fillet weld round the periphery of a hole in one component so as to
join it to the other component exposed through the hole.
Metal transfer which takes place as globules of diameter
substantially larger than that of the consumable electrode from
which they are transferred.
The thermal cutting of a stack of plates usually clamped together.
An intermittent weld on each side of a joint (usually fillet welds in T
and lap joints) arranged so that the welds on one side lie opposite
the spaces on the another side along the joint.
The minimum voltage at which any specified arc may be initiated.
Metal-arc welding in which a bare wire electrode or electrodes are
used; the arc or arcs are enveloped in a flux, some of which fuses to
form a removable covering of slag on the weld.
Gas welding in which a carburizing flame is used to melt the surface
of the parent metal which then unites with the metal from a suitable
filler rod.
Retrogression of the flame into the blowpipe neck or body the flame
remaining alight. Note: This manifests itself either as "popping" or
"squealing" with a small pointed flame issuing from the nozzle orifice
or as a rapid series of minor explosions inside.
Components welded together in accordance with a specified welding
procedure, or a portion of a welded joint detached from a structure
for test.
A portion detached for a test piece and prepared as (Test coupon)
The parting or shaping of materials by the application of heat with or
without a stream of cutting oxygen.
Inert-gas welding using a non-consumable electrode (inert-gas
tungsten-arc welding)
The boundary between a weld face and the parent metal or between
weld faces.
A potion so cut in two straight lengths of pipe joined by a butt weld
as to produce a tongue containing a portion of the weld. The cuts
are made so that the tongue is parallel to the axis of the pipes and
the weld is tested by bending the tongue round a
Metal-arc welding using a covered electrode, the covering of which
is kept in contact with the parent metal during welding.
Weld junction
An inclusion of tungsten from the electrode in TIG-welding.
A gas regulator in which the gas pressure is reduced to the working
pressure in two stages.
An irregular groove at a toe of a run in the parent metal, or in
previously deposited weld metal, due to welding.
The boundary between the fusion zone and the heat affected zone.
A specified course of action followed in welding including the list of
materials and, where necessary, tools to be used.
The order and direction in which joints, welds or runs are made.
The manner is which the operator manipulates an electrode, a
blowpipe or a similar appliance.
An elongated or tubular cavity formed entrapped gas during the
solidification of molten metal.
9. Bending
Bending is a common manufacturing method to process sheet metal. It is usually
done on a bend press (or break press), but also swing-bending-machines are used.
Typical products that are made like this are electrical enclosures.
In engineering mechanics, bending (also known as flexure) characterizes the
behavior of a structural element subjected to a lateral load. A structural element
subjected to bending is known as a beam. A closet rod sagging under the weight of
clothes on clothes hangers is an example of a beam experiencing bending.
Bending produces reactive forces inside a beam as the beam attempts to
accommodate the flexural load: in the case of the beam in Figure 1, the material at
the top of the beam is being compressed while the material at the bottom is being
stretched. There are three notable internal forces caused by lateral loads (shown in
Figure 2): shear parallel to the lateral loading, compression along the top of the
beam, and tension along the bottom of the beam. These last two forces form a
couple or moment as they are equal in magnitude and opposite in direction. This
bending moment produces the sagging deformation characteristic of compression
members experiencing bending.
The compressive and tensile forces shown in Figure 2 induce stresses on the beam.
The maximum compressive stress is found at the uppermost edge of the beam while
the maximum tensile stress is located at the lower edge of the beam. Since the
stresses between these two opposing maxima vary linearly, there therefore exists a
point on the linear path between them where there is no bending stress. The locus of
these points is the neutral axis. Because of this area with no stress and the adjacent
areas with low stress, using uniform cross section beams in bending is not a
particularly efficient means of supporting a load as it does not use the full capacity of
the beam until it is on the brink of collapse. Wide-flange beams (I-Beams) and truss
girders effectively address this inefficiency as they minimize the amount of material in
this under-stressed region.
9.1.Simple or Symmetrical Bending
Beam bending is analyzed with the Euler-Bernoulli beam equation. The classic
formula for determining the bending stress in a member is:
simplified for a beam of rectangular cross-section to:
σ is the bending stress
M - the moment at the neutral axis
y - the perpendicular distance to the neutral axis
Ix - the area moment of inertia about the neutral axis x
b - the width of the section being analyzed
h - the depth of the section being analyzed
This equation is valid only when the stress at the extreme fiber (i.e. the portion of the
beam furthest from the neutral axis) is below the yield stress of the material it is
constructed from. At higher loadings the stress distribution becomes non-linear, and
ductile materials will eventually enter a plastic hinge state where the magnitude of the
stress is equal to the yield stress everywhere in the beam, with a discontinuity at the
neutral axis where the stress changes from tensile to compressive. This plastic hinge
state is typically used as a limit state in the design of steel structures.
9.2.Complex or Unsymmetrical Bending
The equation above is, also, only valid if the cross-section is symmetrical. For
unsymmetrical sections, the full form of the equation must be used (presented
Complex Bending of Homogeneous Beams
The complex bending stress equation for elastic, homogeneous beams is given as
where Mx and My are the bending moments about the x and y centroid axes,
respectively. Ix and Iy are the second moments of area (also known as moments of
inertia) about the x and y axes, respectively, and Ixy is the product of inertia. Using
this equation it would be possible to calculate the bending stress at any point on the
beam cross section regardless of moment orientation or cross-sectional shape. Note
that Mx, My, Ix, Iy, and Ixy are all unique for a given section along the length of the
beam. In other words, they will not change from one point to another on the cross
section. However, the x and y variables shown in the equation correspond to the
coordinates of a point on the cross section at which the stress is to be determined.
10. Sheet metal
Sheet metal is simply metal formed into thin and flat pieces. It is one of the
fundamental forms used in metalworking, and can be cut and bent into a variety of
different shapes. Countless everyday objects are constructed of the material.
Thicknesses can vary significantly, although extremely thin pieces of sheet metal
would be considered to be foil or leaf, and pieces thicker than 1/4 inch or a
centimeter can be considered plate.
10.1. Introduction
Sheet metal is generally produced in sheets less than 6 mm. by reducing the
thickness of a long work piece by compressive forces applied through a set of rolls.
This process is known as rolling and began around 1500 AD. Sheet metals are
available as flat pieces or as strip in coils. It is characterized by its thickness or gauge
of the metal. The gauge of sheet metal ranges from 30 gauge to about 8 gauge. The
higher the gauge, the thinner the metal is. There are many different metals that can
be made into sheet metal. Aluminum, brass, copper, cold rolled steel, mild steel, tin,
nickel and titanium are just a few examples of metal that can be made into sheet
metal. Sheet metal has applications in car bodies, airplane wings, medical tables,
roofs for building and many other things.
10.2. Processes
Fig. Forming metal on a pressbrake
A main feature of sheet metal is its ability to be formed and shaped by a variety of
processes. Each process does something different from the metal giving it a different
shape or size.
10.3. Stretching
Stretching is a process where sheet metal is clamped around its edges and stretched
over a die or form block. This process is mainly used for the manufacture of aircraft
wings, automotive door and window panels.
10.4. Drawing
Drawing forms sheet metal into cylindrical or box shaped parts by using a punch
which presses the blank into a die cavity. Drawing process can also be utilised to
create arbitrary shapes with the help of soft punch.
10.5. Deep Drawing
Deep Drawing is a type of Drawing process where the depth of the part is greater
than its diameter. Deep drawing is used for making automotive fuel tanks, kitchen
sinks, 2 piece aluminum cans, etc.
10.6. Cutting
Cutting sheet metal can be done in various ways from hand tools called tin snips up
to very large powered shears. With the advances in technology, sheet metal cutting
has turned to computers for precise cutting.
Most modern sheet metal cutting operations are now based either on CNC Lasers
cutting or multi-tool CNC punch press.
CNC laser involves moving a lens assembly carrying a beam of laser light over the
surface of the metal. Oxygen or nitrogen is fed through the same nozzle through
which the laser beam exits. The metal is heated and then burnt by the laser beam,
cutting the metal sheet. The quality of the edge can be mirror smooth, and with a
precision of around 0.1mm can be obtained. Cutting speeds on thin (1.2mm) sheet
can be as high as 25m a minute. Most of the laser cutting systems use a CO2 based
laser source with a wavelength of around 10um, some more recent systems use a
YAG based laser with a wavelength of around 1um.
Punching is performed by moving the sheet of metal between the top and bottom
tools of a punch. The top tool (punch) mates with the bottom tool (die), cutting a
simple shape (e.g. a square, circle, or hexagon) from the sheet. An area can be cut
out by making several hundred small square cuts around the perimeter. A punch is
less flexible than a laser for cutting compound shapes, but faster for repetitive
shapes (for example, the grille of an air-conditioning unit). A typical CNC punch has a
choice of up to 60 tools in a "turret" that can be rotated to bring any tool to the active
punching position. A modern CNC punch can take 600 blows per minute.
A typical component (such as the side of a computer case) can be cut to high
precision from a blank sheet in under 30 seconds by either punch or laser.
10.7. Bending and Flanging
Bending and flanging imparts stiffness to a sheet metal part or to form various
shapes, such as 3 piece aluminum cans[1]. See Bending (metalworking).
10.8. Punching and Shearing
During punching or shearing, the sheet metal is cut by using a punch and die.
10.9. Spinning
Spinning is used to make axis-symmetric parts by applying a work piece to a rotating
mandrel with the help of rollers or rigid tools. Spinning is used to make rocket motor
casings and missile nose cones and satellite dishes for example.
10.10. Press Forming
This is a form of bending, used for long and thin sheet metal parts. The machine that
bends the metal is called a pressbrake. The lower part of the press contains a V
shaped groove. This is called the die. The upper part of the press contains a blade
that will press the sheet metal down into the v shaped die, causing it to bend. There
are several techniques used here, but the most common modern method is "air
bending". Here, the die has a sharper angle than the required bend (typically 85
degrees for a 90 degree bend) and the upper tool is precisely controlled in its stroke
to push the metal down the required amount to bend it through 90 degrees. Typically,
a general purpose machine has a bending force available of around 25 tonnes per
metre of length. The opening width of the lower die is typically 8 to 10 times the
thickness of the metal to be bent (for example, 5mm material could be bent in a
40mm die) the inner radius of the bend formed in the metal is determined not by the
radius of the upper tool, but by the lower die width. Typically, the inner radius is equal
to 1/6th of the V width used in the forming process. The press usually has some sort
of backstop to position the material in the jaws of the machine. The backstop can be
computer controlled to allow the operator to make a series of bends in a component
to a high degree of accuracy. Simple machines control only the backstop, more
advanced machines control the position and angle of the stop, its height and the
position of the two reference pegs used to locate the material. The machine can also
record the exact position and pressure required for each bending operation to allow
the operator to achieve a perfect 90 degree bend across a variety of operations on
the part.
10.11. Roll Forming
A continuous bending operation for producing open profiles or welded tubes with long
lengths or in large quantities, see Roll forming.
Fig. Roll Forming Stand
Roll forming is a continuous bending operation in which a long strip of metal is
passed through consecutive sets of rolls, or stands, each performing only an
incremental part of the bend, until the desired cross-section profile is obtained. Roll
forming is ideal for producing parts with long lengths or in large quantities.
A variety of cross-section profiles can be produced, but each profile requires a
carefully crafted set of roll tools. Design of the rolls starts with a flower pattern,
which is the sequence of profile cross-sections, one for each stand of rolls. The roll
contours are then derived from the profile contours. Because of the high cost of the
roll sets, simulation is often used to validate the designed rolls and optimize the
forming process to minimize the number of stands and material stresses in the final
10.11.1. Rolling
Fig. profile rolling (to manufacture a cone)
Rolling is a fabricating process in which the metal, plastic, paper, glass, etc. is
passed through a pair of rolls. There are two types of rolling process, flat and profile
rolling. In flat rolling the final shape of the product is either classed as sheet (typically
thickness less than 3 mm, also called "strip") or plate (typically thickness more than 3
mm). In profile rolling, the final product may be a round rod or other shaped bar
such as a structural section (beam, channel, joist etc). Rolling is also classified
according to the temperature of the metal rolled. If the temperature of the metal is
above its recrystallization temperature then the process is termed as hot rolling, If the
temperature of metal is below its recrystallization temperature the process is termed
as cold rolling.
Other processes also termed as 'hot bending' are induction bending, whereby the
section is heated in small sections, and dragged into a required radius (see 'steel
bending services'[1] for examples of all bending processes)
Heavy plate tends to be formed using a press process, and is termed forming, rather
than rolling.
11. Molding Process
11.1. Introduction
A press, or a machine press is a tool used to work metal (typically steel) by
changing its shape and internal structure.
A forge press reforms the workpiece into a three dimensional object—not only
changing its visible shape but also the internal structure of the material. A stronger
part results from this process than if the object was machined.
Bending is a typical operation performed and occurs by a machine pressing, or
applying direct pressure, to the material and forcing it to change shape. A press
brake is a typical machine for this operation.
An easy to understand type of machine press is a set of rollers. Metal is fed into the
rollers, which are turning to pull the material through. The space between the rollers
is smaller than the unfinished metal, and thus the metal is made thinner and/or wider.
Another kind of press is a set of plates with a relief, or depth-based design, in them.
The metal is placed between the plates, and the plates are pressed up against each
other, deforming the metal in the desired fashion. This may be coining or embossing
or forming. A punch press is used for forming holes.
Progressive stamping is a manufacturing method that can encompass punching,
coining, bending and several ways of modifying the metal, combined with an
automatic feeding system. The feeding system pushes a coil of metal through all of
the stations of a progressive stamping die. Each station performs one or more
operations until a finished part is made per the requirements on the print. The final
operation is a cutoff operation, which separates the finished part from the carrying
web. The carrying web, along with metal that is punched away in previous
operations, is considered scrap metal.
A Press Brake is a special type of machine press that bends sheetmetal into shape.
Fig. Press Brake
A good example of the type of work a press brake can do is the backplate of a
computer case. Other examples include brackets, frame pieces and electronic
enclosures just to name a few. Some press breaks have CNC controls and can form
parts with accuracy to a fraction of a millimeter. These machines can be dangerous
considering the knife-edge bending dies and powerful 100+ ton bending force.
However in the hands of a skilled operator the machine presents minimum hazard.
Machine presses are used extensively around the world for shaping all kinds of
metals to a desired shape. A typical toaster (for bread) has a metal case that has
been bent and pressed into shape by a machine press.
Also remember that machine presses have a high hazardous level , so safety
measures must always be taken. Injuries in a press may be permanent, since there
are over 100s tons on top of a limb. Bimanual controls (both hands need to be on the
buttons to make the press work) are a very good way to prevent accidents. Also light
sensors that keep the machine from working if the operator is in range of the die (tool
that goes inside the press to shape metal), or any limbs is in range.
11.2. Types of Presses
Mechanical Press
Pneumatic Press
Knuckle-joint Press
Hydraulic Press
Fine blanking Press
Forging Press (Hammers)
9. References
1.B. H. Amstead, P. F. Ostwald, M. L. Begeman, Manufacturing Process,
2.ASM International (2003). Trends in Welding Research. Materials Park,
Ohio: ASM International. ISBN 0-87170-780-2
3.Blunt, Jane and Nigel C. Balchin (2002). Health and Safety in Welding and
Allied Processes. Cambridge: Woodhead. ISBN 1-85573-538-5.
4.Cary, Howard B. and Scott C. Helzer (2005). Modern Welding Technology.
Upper Saddle River, New Jersey: Pearson Education. ISBN 0-13-113029-3.
5.Hicks, John (1999). Welded Joint Design. New York: Industrial Press. ISBN
6.Kalpakjian, Serope and Steven R. Schmid (2001). Manufacturing
Engineering and Technology. Prentice Hall. ISBN 0-201-36131-0.
7.Lincoln Electric (1994). The Procedure Handbook of Arc Welding.
Cleveland: Lincoln Electric. ISBN 99949-25-82-2.
8.Weman, Klas (2003). Welding processes handbook. New York: CRC Press
LLC. ISBN 0-8493-1773-8.
9.ASM International (2003). Trends in Welding Research. Materials Park,
Ohio: ASM International. ISBN 0-87170-780-2
10.Blunt, Jane and Nigel C. Balchin (2002). Health and Safety in Welding and
Allied Processes. Cambridge: Woodhead. ISBN 1-85573-538-5.
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