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Gas Metal Arc Welding
(1) Gas metal arc welding ( GMAW or MIG welding) is an electric arc welding
process which joins metals by heating them with an arc established between a
continuous filler metal (consumable) electrode and the work. Shielding of the
arc and molten weld pool is obtained entirely from an externally supplied gas or
gas mixture, as shown in figure 10-44.
The process is sometimes referred to as MIG or CO2 welding. Recent
developments in the process include operation at low current densities and
pulsed direct current, application to a broader range of materials, and the use
of reactive gases, particularly CO2, or gas mixtures. This latter
development has led to the formal acceptance of the term gas metal arc welding
(GMAW) for the process because both inert and reactive gases are used. The term
MIG welding is still more commonly used.
Watch a video of a vertical mig weld being made here:
Vertical Mig Welding
(2) MIG welding is operated in semiautomatic, machine, and automatic modes.
It is utilized particularly in high production welding operations. All
commercially important metals such as carbon steel, stainless steel, aluminum,
and copper can be welded with this process in all positions by choosing the
appropriate shielding gas, electrode, and welding conditions.
b. Equipment.
(1) Gas metal arc welding equipment consists of a welding gun, a power
supply, a shielding gas supply, and a wire-drive system which pulls the wire
electrode from a spool and pushes it through a welding gun. A source of cooling
water may be required for the welding gun. In passing through the gun, the wire
becomes energized by contact with a copper contact tube, which transfers current
from a power source to the arc. While simple in principle, a system of accurate
controls is employed to initiate and terminate the shielding gas and cooling
water, operate the welding contactor, and control electrode feed speed as
required. The basic features of MIG welding equipment are shown in figure 10-45.
The MIG process is used for semiautomatic, machine, and automatic welding.
Semiautomatic MIG welding is often referred to as manual welding.
(2) Two types of power sources are used for MIG welding: constant current and
constant voltage.
(a) Constant current power supply. With this type, the welding current
is established by the appropriate setting on the power supply. Arc length
(voltage) is controlled by the automatic adjustment of the electrode feed rate.
This type of welding is best suited to large diameter electrodes and machine or
automatic welding, where very rapid change of electrode feed rate is not
required. Most constant current power sources have a drooping volt-ampere output
characteristic. However, true constant current machines are available. Constant
current power sources are not normally selected for MIG welding because of the
control needed for electrode feed speed. The systems are not self-regulating.
(b) Constant voltage power supply. The arc voltage is established by
setting the output voltage on the power supply. The power source will supply the
necessary amperage to melt the welding electrode at the rate required to
maintain the present voltage or relative arc length. The speed of the electrode
drive is used to control the average welding current. This characteristic is
generally preferred for the welding of all metals. The use of this type of power
supply in conjunction with a constant wire electrode feed results in a
self-correcting arc length system.
(3) Motor generator or dc rectifier power sources of either type may be used.
With a pulsed direct current power supply, the power source pulses the dc output
from a low background value to a high peak value. Because the average power is
lower, pulsed welding current can be used to weld thinner sections than those
that are practical with steady dc spray transfer.
(4) Welding guns. Welding guns for MIG welding are available for
manual manipulation (semiautomatic welding) and for machine or automatic
welding. Because the electrode is fed continuously, a welding gun must have a
sliding electrical contact to transmit the welding current to the electrode. The
gun must also have a gas passage and a nozzle to direct the shielding gas around
the arc and the molten weld pool. Cooling is required to remove the heat
generated within the gun and radiated from the welding arc and the molten weld
metal. Shielding gas, internal circulating water, or both, are used for cooling.
An electrical switch is needed to start and stop the welding current, the
electrode feed system, and shielding gas flow.
(a) Semiautomatic Guns. Semiautomatic, hand-held guns are usually
similar to a pistol in shape. Sometimes they are shaped similar to an
oxyacetylene torch, with electrode wire fed through the barrel or handle. In
some versions of the pistol design, where the most cooling is necessary, water
is directed through passages in the gun to cool both the contact tube and the
metal shielding gas nozzle. The curved gun uses a curved current-carrying body
at the front end, through which the shielding gas is brought to the nozzle. This
type of gun is designed for small diameter wires and is flexible and
maneuverable. It is suited for welding in tight, hard to reach corners and other
confined places. Guns are equipped with metal nozzles of various internal
diameters to ensure adequate gas shielding. The orifice usually varies from
approximately 3/8 to 7/8 in. (10 to 22 mm), depending upon welding requirements.
The nozzles are usually threaded to make replacement easier. The conventional
pistol type holder is also used for arc spot welding applications where filler
metal is required. The heavy nozzle of the holder is slotted to exhaust the
gases away from the spot. The pistol grip handle permits easy manual loading of
the holder against the work. The welding control is designed to regulate the
flow of cooling water and the supply of shielding gas. It is also designed to
prevent the wire freezing to the weld by timing the weld over a preset interval.
A typical semiautomatic gas-cooled gun is shown in figure
10-46.
(b) Air cooled guns. Air-cooled guns are available for applications
where water is not readily obtainable as a cooling medium. These guns are
available for service up to 600 amperes, intermittent duty, with carbon dioxide
shielding gas. However, they are usually limited to 200 amperes with argon or
helium shielding. The holder is generally pistol-like and its operation is
similar to the water-cooled type. Three general types of air-cooled guns are
available.
1. A gun that has the electrode wire fed to it through a flexible
conduit from a remote wire feeding mechanism. The conduit is generally in the 12
ft (3.7 m) length range due to the wire feeding limitations of a push-type
system. Steel wires of 7/20 to 15/16 in. (8.9 to 23.8 mm) diameter and aluminum
wires of 3/64 to 1/8 in. (1.19 to 3.18 mm) diameter can be fed with this
arrangement.
2. A gun that has a self-contained wire feed mechanism and electrode
wire supply. The wire supply is generally in the form of a 4 in. (102 mm)
diameter, 1 to 2-1/2 lb (0.45 to 1.1 kg) spool. This type of gun employs a
pull-type wire feed system, and it is not limited by a 12 ft (3.7 m) flexible
conduit. Wire diameters of 3/10 to 15/32 in. (7.6 to 11.9 mm) are normally used
with this type of gun.
3. A pull-type gun that has the electrode wire fed to it through a
flexible conduit from a remote spool. This incorporates a self-contained wire
feeding mechanism. It can also be used in a push-pull type feeding system. The
system permits the use of flexible conduits in lengths up to 50 ft (15 m) or
more from the remote wire feeder. Aluminum and steel electrodes with diameters
of 3/10 to 5/8 in. (7.6 to 15.9 mm) can be used with these types of feed
mechanisms.
(c) Water-cooled guns for manual MIG welding similar to gas-cooled types with
the addition of water cooling ducts. The ducts circulate water around the
contact tube and the gas nozzle. Water cooling permits the gun to operate
continuously at rated capacity and at lower temperatures. Water-coded guns are
used for applications requiring 200 to 750 amperes. The water in and out lines
to the gun add weight and reduce maneuverability of the gun for welding.
(d) The selection of air- or water-cooled guns is based on the type of
shielding gas, welding current range, materials, weld joint design, and existing
shop practice. Air-cooled guns are heavier than water-cooled guns of the same
welding current capacity. However, air-cooled guns are easier to manipulate to
weld out-of-position and in confined areas.
c. Advantages.
(1) The major advantage of gas metal-arc welding is that high quality welds
can be produced much faster than with SMAW or TIG welding.
(2) Since a flux is not used, there is no chance for the entrapment of slag
in the weld metal.
(3) The gas shield protects the arc so that there is very little loss of
alloying elements as the metal transfers across the arc. Only minor weld spatter
is produced, and it is easily removed.
(4) This process is versatile and can be used with a wide variety of metals
and alloys, including aluminum, copper, magnesium, nickel, and many of their
alloys, as well as iron and most of its alloys. The process can be operated in
several ways, including semi- and fully automatic. MIG welding is widely used by
many industries for welding a broad variety of materials, parts, and structures.
d. Disadvantages.
(1) The major disadvantage of this process is that it cannot be used in the
vertical or overhead welding positions due to the high heat input and the
fluidity of the weld puddle. Advances in welding power supplies, wire, gases, and wire
feeders have made the use of short arc, pulse arc, and flux-core for out of
position welding feasible and advantageous for high quality, high production
welding. These processes are now routinely used in shipyards and production shops
world wide.
(2) The equipment is complex compared to equipment used for the shielded
metal-arc welding process.
e. Process Principles.
(1) Arc power and polarity.
(a) The vast majority of MIG welding applications require the use of direct
current reverse polarity (electrode positive). This type of electrical
connection yields a stable arc, smooth metal transfer, relatively low spatter
loss, and good weld bead characteristics for the entire range of welding
currents used. Direct current straight polarity (electrode negative) is seldom
used, since the arc can become unstable and erratic even though the electrode
melting rate is higher than that achieved with dcrp (electrode positive). When
employed, dcsp (electrode negative) is used in conjunction with a "buried" arc
or short circuiting metal transfer. Penetration is lower with straight polarity
than with reverse polarity direct current.
(b) Alternating current has found no commercial acceptance with the MIG
welding process for two reasons: the arc is extinguished during each half cycle
as the current reduces to zero, and it may not reignite if the cathode cools
sufficiently; and rectification of the reverse polarity cycle promotes the
erratic arc operation.
(2) Metal transfer.
(a) Filler metal can be transferred from the electrode to the work in two
ways: when the electrode contacts the molten weld pool, thereby establishing a
short circuit, which is known as short circuiting transfer (short circuiting arc
welding); and when discrete drops are moved across the arc gap under the
influence of gravity or electromagnetic forces. Drop transfer can be either
globular or spray type.
(b) Shape, size, direction of drops (axial or non-axial), and type of transfer
are determined by a number of factors. The factors having the most influence
are:
1. Magnitude and type of welding current.
2. Current density.
3. Electrode composition.
4. Electrode extension.
5. Shielding gas.
6. Power supply characteristics.
(c) Axially directed transfer refers to the movement of drops along a line
that is a continuation of the longitudinal axis of the electrode. Non-axially
directed transfer refers to movement in any other direction.
(3) Short circuiting transfer.
(a) Short circuiting arc welding uses the lowest range of welding currents
and electrode diameters associated with MIG welding. This type of transfer
produces a small, fast-freezing weld pool that is generally suited for the
joining of thin sections, out-of-position welding, and filling of large root
openings. When weld heat input is extremely low, plate distortion is small.
Metal is transferred from the electrode to the work only during a period when
the electrode is in contact with the weld pool. There is no metal transfer
across the arc gap.
(b) The electrode contacts the molten weld pool at a steady rate in a range
of 20 to over 200 times each second. As the wire touches the weld metal, the
current increases. It would continue to increase if an arc did not form. The
rate of current increase must be high enough to maintain a molten electrode tip
until filler metal is transferred. It should not occur so fast that it causes
spatter by disintegration of the transferring drop of filler metal. The rate of
current increase is controlled by adjustment of the inductance in the power
source. The value of inductance required depends on both the electrical
resistance of the welding circuit and the temperature range of electrode
melting. The open circuit voltage of the power source must be low enough so that
an arc cannot continue under the existing welding conditions. A portion of the
energy for arc maintenance is provided by the inductive storage of energy during
the period of short circuiting.
(c) As metal transfer only occurs during short circuiting, shielding gas has
very little effect on this type of transfer. Spatter can occur. It is usually
caused either by gas evolution or electromagnetic forces on the molten tip of
the electrode.
(4) Globular transfer.
(a) With a positive electrode (dcrp), globular transfer takes place when the
current density is relatively low, regardless of the type of shielding gas.
However, carbon dioxide (CO2) shielding yields this type of transfer
at all usable welding currents. Globular transfer is characterized by a drop
size of greater diameter than that of the electrode.
(b) Globular, axially directed transfer can be achieved in a substantially
inert gas shield without spatter. The arc length must be long enough to assure
detachment of the drop before it contacts the molten metal. However, the
resulting weld is likely to be unacceptable because of lack of fusion,
insufficient penetration, and excessive reinforcement.
(c) Carbon dioxide shielding always yields non-axially directed globular
transfer. This is due to an electromagnetic repulsive force acting upon the
bottom of the molten drops. Flow of electric current through the electrode
generates several forces that act on the molten tip. The most important of these
are pinch force and anode reaction force. The magnitude of the pinch force is a
direct function of welding current and wire diameter, and is usually responsible
for drop detachment. With CO2 shielding, the wire electrode is melted
by the arc heat conducted through the molten drop. The electrode tip is not
enveloped by the arc plasma. The molten drop grows until it detaches by short
circuiting or gravity.
(5) Spray transfer.
(a) In a gas shield of at least 80 percent argon or helium, filler metal
transfer changes from globular to spray type as welding current increases for a
given size electrode. For all metals, the change takes place at a current value
called the globular-to-spray transition current.
(b) Spray type transfer has a typical fine arc column and pointed wire tip
associated with it. Molten filler metal transfers across the arc as fine
droplets. The droplet diameter is equal to or less than the electrode diameter.
The metal spray is axially directed. The reduction in droplet size is also
accompanied by an increase in the rate of droplet detachment, as illustrated in
figure
10-47. Metal transfer rate may range from less than 100 to several hundred
droplets per second as the electrode feed rate increases from approximately 100
to 800 in./min (42 to 339 mm/s).
(6) Free flight transfer.
(a) In free-flight transfer, the liquid drops that form at the tip of the
consumable electrode are detached and travel freely across the space between the
electrode and work piece before plunging into the weld pool. When the transfer
is gravitational, the drops are detached by gravity alone and fall slowly
through the arc column. In the projected type of transfer, other forces give the
drop an initial acceleration and project it independently of gravity toward the
weld pool. During repelled transfer, forces act on the liquid drop and give it
an initial velocity directly away from the weld pool. The gravitational and
projected ties of free-flight metal transfer may occur in the gas metal-arc
welding of steel, nickel alloys, or aluminum alloys using a direct current,
electrode-positive (reverse polarity) arc and properly selected types of
shielding gases.
(b) At low currents, wires of these alloys melt slowly. A large spherical
drop forms at the tip and is detached when the force due to gravity exceeds that
of surface tension. As the current increases, the electromagnetic force becomes
significant and the total. separating force increases. The rate at which drops
are formed and detached also increases. At a certain current, a change occurs in
the character of the arc and metal transfer. The arc column, previously
bell-shaped or spherical and having relatively low brightness, becomes narrower
and more conical and has a bright central core. The droplets that form at the
wire tip become elongated due to magnetic pressure and are detached at a much
higher rate. When carbon dioxide is used as the shielding gas, the type of metal
transfer is much different. At low and medium reversed-polarity currents, the
drop appears to be repelled from the work electrode and is eventually detached
while moving away from the workpiece and weld pool. This causes an excessive
amount of spatter. At higher currents, the transfer is less irregular because
other forces, primarily electrical, overcame the repelling forces. Direct
current reversed-polarity is recommended for the MIG welding process. Straight
polarity and alternating current can be used, but require precautions such as a
special coating on the electrode wire or special shield gas mixtures.
(c) The filler wire passes through a copper contact tube in the gun, where it
picks up the welding current. Some manual welding guns contain the wire-driving
mechanism within the gun itself. Other guns require that the wire-feeding
mechanism be located at the spool of wire, which is some distance from the gun.
In this case, the wire is driven through a flexible conduit to the welding gun.
Another manual gun design combines feed mechanisms within the gun and at the
wire supply itself. Argon is the shielding gas used most often. Small amounts of
oxygen (2 to 5 percent) frequently are added to the shielding gas when steel is
welded. This stabilizes the arc and promotes a better wetting action, producing
a more uniform weld bead and reducing undercut. Carbon dioxide is also used as a
shielding gas because it is cheaper than argon and argon-oxygen mixtures.
Electrodes designed to be used with carbon dioxide shielding gas require extra
deoxidizers in their formulation because in the heat of the arc, the carbon
dioxide dissociates to carbon monoxide and oxygen, which can cause oxidation of
the weld metal.
(7) Welding parameters. Figures 10-48 through
10-54 show the relationship between the voltage and the current levels, and
the type of transfer across the arcs.
f. Welding Procedures.
(1) The welding procedures for MIG welding are similar to those for other arc
welding processes. Adequate fixturing and clamping of the work are required with
adequate accessibility for the welding gun. Fixturing must hold the work rigid
to minimize distortion from welding. It should be designed for easy loading and
unloading. Good connection of the work lead (ground) to the workpiece or
fixturing is required. Location of the connection is important, particularly
when welding ferromagnetic materials such as steel. The best direction of
welding is away from the work lead connection. The position of the electrode
with respect to the weld joint is important in order to obtain the desired joint
penetration, fusion, and weld bead geometry. Electrode positions for automatic
MIG welding are similar to those used with submerged arc welding.
(2) When complete joint penetration is required, some method of weld backing
will help to control it. A backing strip, backing weld, or copper backing bar
can be used. Backing strips and backing welds usually are left in place. Copper
backing bars are removable.
(3) The assembly of the welding equipment should be done according to the
manufacturer’s directions. All gas and water connections should be tight; there
should be no leaks. Aspiration of water or air into the shielding gas will
result in erractic arc operation and contamination of the weld. Porosity may
also occur.
(4) The gun nozzle size and the shielding gas flow rate should be set
according to the recommended welding procedure for the material and joint design
to be welded. Joint designs that require long nozzle-to-work distances will need
higher gas flow rates than those used with normal nozzle-to-work distances. The
gas nozzle should be of adequate size to provide good gas coverage of the weld
area. When welding is done in confined areas or in the root of thick weld
joints, small size nozzles are used.
(5) The gun contact tube and electrode feed drive rolls are selected for the
particular electrode composition and diameter, as specified by the equipment
manufacturer. The contact tube will wear with usage, and must be replaced
periodically if good electrical contact with electrode is to be maintained and
heating of the gun is to be minimized.
(6) Electrode extension is set by the distance between the tip of the contact
tube and the gas nozzle opening. The extension used is related to the type of
MIG welding, short circuiting or spray type transfer. It is important to keep
the electrode extension (nozzle-to-work distance) as uniform as possible during
welding. Therefore, depending on the application, the contact tube may be
inside, flush with, or extending beyond the gas nozzle.
(7) The electrode feed rate and welding voltage are set to the recommended
values for the electrode size and material. With a constant voltage power
source, the welding current will be establish by the electrode feed rate. A
trial bead weld should be made to establish proper voltage (arc length) and feed
rate values. Other variables, such as slope control, inductance, or both, should
be adjusted to give good arc starting and smooth arc operation with minimum
spatter. The optimum settings will depend on the equipment design and controls,
electrode material and size, shielding gas, weld joint design, base metal
composition and thickness, welding position, and welding speed.
10-13. FLUX-CORED ARC WELDING (FCAW)
a. General.
(1) Flux-cored, tubular electrode welding has evolved from the MIG welding
process to improve arc action, metal transfer, weld metal properties, and weld
appearance. It is an arc welding process in which the heat for welding is
provided by an arc between a continuously fed tubular electrode wire and the
workpiece. Shielding is obtained by a flux contained within the tubular
electrode wire or by the flux and an externally supplied shielding gas. A
diagram of the process is shown in figure
10-55.
(2) Flux-cored arc welding is similar to gas metal arc welding in many ways.
The flux-cored wire used for this process gives it different characteristics.
Flux-cored arc welding is widely used for welding ferrous metals and is
particularly good for applications in which high deposition rates are needed. At
high welding currents, the arc is smooth and more manageable when compared in
using large diameter gas metal arc welding electrodes with carbon dioxide. The
arc and weld pool are clearly visible to the welder. A slag coating is left on
the surface of the weld bead, which must be removed. Since the filler metal
transfers across the arc, some spatter is created and some smoke produced.
b. Equipment.
(1) The equipment used for flux-cored arc welding is similar to that used for
gas metal arc welding. The basic arc welding equipment consists of a power
source, controls, wire feeder, welding gun, and welding cables. A major
difference between the gas shielded electrodes and the self-shielded electrodes
is that the gas shielded wires also require a gas shielding system. This may
also have an effect on the type of welding gun used. Fume extractors are often
used with this process. For machines and automatic welding, several items, such
as seam followers and motion devices, are added to the basic equipment. Figure 10-56
shows a diagram of the equipment used for semiautomatic flux-cored arc welding.
(2) The power source, or welding machine, provides the electric power of the
proper voltage and amperage to maintain a welding arc. Most power sources
operate on 230 or 460 volt input power, but machines that operate on 200 or 575
volt input are also available. Power sources may operate on either single phase
or three-phase input with a frequency of 50 to 60 hertz. Most power sources used
for flux-cored arc welding have a duty cycle of 100 percent, which indicates
they can be used to weld continuously. Some machines used for this process have
duty cycles of 60 percent, which means that they can be used to weld 6 of every
10 minutes. The power sources generally recommended for flux-cored arc welding
are direct current constant voltage type. Both rotating (generator) and static
(single or three-phase transformer-rectifiers) are used. The same power sources
used with gas metal arc welding are used with flux-cored arc welding. Flux-cored
arc welding generally uses higher welding currents than gas metal arc welding,
which sometimes requires a larger power source. It is important to use a power
source that is capable of producing the maximum current level required for an
application.
(3) Flux-cored arc welding uses direct current. Direct current can be either
reverse or straight polarity. Flux-cored electrode wires are designed to operate
on either DCEP or DCEN. The wires designed for use with an external gas
shielding system are generally designed for use with DCEP. Some self-shielding
flux-cored ties are used with DCEP while others are developed for use with DCEN.
Electrode positive current gives better penetration into the weld joint.
Electrode negative current gives lighter penetration and is used for welding
thinner metal or metals where there is poor fit-up. The weld created by DCEN is
wider and shallower than the weld produced by DCEP.
(4) The generator welding machines used for this process can be powered by an
electric rotor for shop use, or by an internal combustion engine for field
applications. The gasoline or diesel engine-driven welding machines have either
liquid or air-cooled engines. Motor-driven generators produce a very stable arc,
but are noisier, more expensive, consume more power, and require more
maintenance than transformer-rectifier machines.
(5) A wire feed motor provides power for driving the electrode through the
cable and gun to the work. There are several different wire feeding systems
available. System selection depends upon the application. Most of the wire feed
systems used for flux-cored arc welding are the constant speed type, which are
used with constant voltage power sources. With a variable speed wire feeder, a
voltage sensing circuit is used to maintain the desired arc length by varying
the wire feed speed. Variations in the arc length increase or decrease the wire
feed speed. A wire feeder consists of an electrical rotor connected to a gear
box containing drive rolls. The gear box and wire feed motor shown in figure 10-57
have form feed rolls in the gear box.
(6) Both air-cooled and water-cooled guns are used for flux-cored arc
welding. Air-cooled guns are cooled primarily by the surrounding air, but a
shielding gas, when used, provides additional cooling effects. A water-cooled
gun has ducts to permit water to circulate around the contact tube and nozzle.
Water-cooled guns permit more efficient cooling of the gun. Water-cooled guns
are recommended for use with welding currents greater than 600 amperes, and are
preferred for many applications using 500 amperes. Welding guns are rated at the
maximum current capacity for continuous operation. Air-cooled guns are preferred
for most applications less than 500 amperes, although water-cooled guns may also
be used. Air-cooled guns are lighter and easier to manipulate.
(7) Shielding gas equipment and electrodes.
(a) Shielding gas equipment used for gas shielded flux-cored wires consists
of a gas supply hose, a gas regulator, control valves, and supply hose to the
welding gun.
(b) The shielding gases are supplied in liquid form when they are in storage
tanks with vaporizers, or in a gas form in high pressure cylinders. An exception
to this is carbon dioxide. When put in high pressure cylinders, it exists in
both liquid and gas forms.
(c) The primary purpose of the shielding gas is to protect the arc and weld
puddle from contaminating effects of the atmosphere. The nitrogen and oxygen of
the atmosphere, if allowed to come in contact with the molten weld metal, cause
porosity and brittleness. In flux-cored arc welding, shielding is accomplished
by the decomposition of the electrode core or by a combination of this and
surrounding the arc with a shielding gas supplied from an external source. A
shielding gas displaces air in the arc area. Welding is accomplished under a
blanket of shielding gas. Inert and active gases may both be used for flux-cored
arc welding. Active gases such as carbon dioxide, argon-oxygen mixture, and
argon-carbon dioxide mixtures are used for almost all applications. Carbon
dioxide is the most common. The choice of the proper shielding gas for a
specific application is based on the type of metal to be welded, arc
characteristics and metal transfer, availability, cost of the gas, mechanical
property requirements, and penetration and weld bead shape. The various shielding gases are
summarized below.
1. Carbon dioxide. Carbon dioxide is manufactured from fuel gases
which are given off by the burning of natural gas, fuel oil, or coke. It is also
obtained as a by-product of calcining operations in lime kilns, from the
manufacturing of ammonia and from the fermentation of alcohol, which is almost
100 percent pure. Carbon dioxide is made available to the user in either
cylinder or bulk containers. The cylinder is more common. With the bulk system,
carbon dioxide is usually drawn off as a liquid and heated to the gas state
before going to the welding torch. The bulk system is normally only used when
supplying a large number of welding stations. In the cylinder, the carbon
dioxide is in both a liquid and a vapor form with the liquid carbon dioxide
occupying approximately two thirds of the space in the cylinder. By weight, this
is approximately 90 percent of the content of the cylinder. Above the liquid, it
exists as a vapor gas. As carbon dioxide is drawn from the cylinder, it is
replaced with carbon dioxide that vaporizes from the liquid in the cylinder and
therefore the overall pressure will be indicated by the pressure gauge. When the
pressure in the cylinder has dropped to 200 psi (1379 kPa), the cylinder should
be replaced with a new cylinder. A positive pressure should always be left in
the cylinder in order to prevent moisture and other contaminants from backing up
into the cylinder. The normal discharge rate of the CO 2 cylinder is
about 10 to 50 cu ft per hr (4.7 to 24 liters per min). However, a maximum
discharge rate of 25 cu ft per hr (12 liters per min is recommended when welding
using a single cylinder. As the vapor pressure drops from the cylinder pressure
to discharge pressure through the CO 2 regulator, it absorbs a great
deal of heat. If flow rates are set too high, this absorption of heat can lead
to freezing of the regulator and flowmeter which interrupts the shielding gas
flow. When flow rate higher than 25 cu ft per hr (12 liters per min) is
required, normal practice is to manifold two CO 2 cylinders in
parallel or to place a heater between the cylinder and gas regulator, pressure
regulator, and flowmeter. Excessive flow rates can also result in drawing liquid
from the cylinder. Carbon dioxide is the most widely used shielding gas for
flux-cored arc welding. Most active gases cannot be used for shielding, but
carbon dioxide provides several advantages for use in welding steel. These are
deep penetration and low cost. Carbon dioxide promotes a globular transfer. The
carbon dioxide shielding gas breaks down into components such as carbon monoxide
and oxygen. Because carbon dioxide is an oxidizing gas, deoxidizing elements are
added to the core of the electrode wire to remove oxygen. The oxides formed by
the deoxidizing elements float to the surface of the weld and become part of the
slag covering. Some of the carbon dioxide gas will break down to carbon and
oxygen. If the carbon content of the weld pool is below about 0.05 percent,
carbon dioxide shielding will tend to increase the carbon content of the weld
metal. Carbon, which can reduce the corrosion resistance of some stainless
steels, is a problem for critical corrosion application. Extra carbon can also
reduce the toughness and ductility of some low alloy steels. If the carbon
content in the weld metal is greater than about 0.10 percent, carbon dioxide
shielding will tend to reduce the carbon content. This loss of carbon can be
attributed to the formation of carbon monoxide, which can be trapped in the weld
as porosity deoxidizing elements in the flux core reducing the effects of carbon
monoxide formation.
2. Argon-carbon dioxide mixtures. Argon and carbon dioxide are
sometimes mixed for use with flux-cored arc welding. A high percentage of argon
gas in the mixture tends to promote a higher deposition efficiency due to the
creation of less spatter. The most commonly used gas mixture in flux-cored arc
welding is a 75 percent argon-25 percent carbon dioxide mixture. The gas mixture
produces a fine globular metal transfer that approaches a spray. It also reduces
the amount of oxidation that occurs, compared to pure carbon dioxide. The weld
deposited in an argon-carbon dioxide shield generally has higher tensile and
yield strengths. Argon-carbon dioxide mixtures are often used for
out-of-position welding, achieving better arc characteristics. These mixtures
are often used on low alloy steels and stainless steels. Electrodes that are
designed for use with CO2 may cause an excessive buildup of
manganese, silicon, and other deoxidizing elements if they are used with
shielding gas mixtures containing a high percentage of argon. This will have an
effect on the mechanical properties of the weld.
3. Argon-oxygen mixtures. Argon-oxygen mixtures containing 1 or 2
percent oxygen are used for some applications. Argon-oxygen mixtures tend to
promote a spray transfer which reduces the amount of spatter produced. A major
application of these mixtures is the welding of stainless steel where carbon
dioxide can cause corrosion problems.
(d) The electrodes used for flux-cored arc welding provide the filler metal
to the weld puddle and shielding for the arc. Shielding is required for sane
electrode types. The purpose of the shielding gas is to provide protection from
the atmosphere to the arc and molten weld puddle. The chemical composition of
the electrode wire and flux core, in combination with the shielding gas, will
determine the weld metal composition and mechanical properties of the weld. The
electrodes for flux-cored arc welding consist of a metal shield surrounding a
core of fluxing and/or alloying compounds as shown in figure 10-58.
The cores of carbon steel and low alloy electrodes contain primarily fluxing
compounds. Some of the low alloy steel electrode cores contain high amounts of
alloying compounds with a low flux content. Most low alloy steel electrodes
require gas shielding. The sheath comprises approximately 75 to 90 percent of
the weight of the electrode. Self-shielded electrodes contain more fluxing
compounds than gas shielded electrodes. The compounds contained in the electrode
perform basically the same functions as the coating of a covered electrode used
in shielded metal arc welding. These functions are:
1. To form a slag coating that floats on the surface of the weld metal
and protects it during solidification.
2. To provide deoxidizers and scavengers which help purify and produce
solid weld-metal.
3. To provide arc stabilizers which produce a smooth welding arc and
keep spatter to a minimum.
4. To add alloying elements to the weld metal which will increase the
strength and improve other properties in the weld metal.
5. To provide shielding gas. Gas shielded wires require an external
supply of shielding gas to supplement that produced by the core of the
electrode.
(e) The classification system used for tubular wire electrodes was devised by
the American Welding Society. Carbon and low alloy steels are classified on the
basis of the following items:
1. Mechanical properties of the weld metal.
2. Welding position.
3. Chemical composition of the weld metal.
4. Type of welding current.
5. Whether or not a CO2 shielding gas is used.
An example of a carbon steel electrode classification is E70T-4 where:
1. The "E" indicates an electrode.
2. The second digit or "7" indicates the minimum tensile strength in
units of 10,000 psi (69 MPa). Table 10-12,
below, shows the mechanical property requirements for the various carbon steel
electrodes.
3. The third digit or "0" indicates the welding positions. A "0"
indicates flat and horizontal positions and a "1" indicates all positions.
4. The "T" stands for a tubular or flux cored wire classification.
5. The suffix "4" gives the performance and usability capabilities as
shown in table
10-13. When a "G" classification is used, no specific performance and
usability requirements are indicated. This classification is intended for
electrodes not covered by another classification. The chemical composition
requirements of the deposited weld metal for carbon steel electrodes are shown
in table
10-14. Single pass electrodes do not have chemical composition requirements
because checking the chemistry of undiluted weld metal does not give the true
results of normal single pass weld chemistry.
The classification of low alloy steel electrodes is similar to the
classification of carbon steel electrodes. An example of a low alloy steel
classification is E81T1-NI2 where:
1. The "E" indicates electrode.
2. The second digit or "8" indicates the minimum tensile in strength
in units of 10,000 psi (69 MPa). In this case it is 80,000 psi (552 MPa). The
mechanical property requirements for low alloy steel electrodes are shown in table 10-15.
Impact strength requirements are shown in table
10-16.
3. The third digit or "1" indicates the welding position capabilities
of the electrode. A "1" indicates all positions and an "0" flat and horizontal
position only.
4. The "T" indicates a tubular or flux-cored electrode used in flux
cored arc welding.
5. The fifth digit or "1" describes the usability and performance
characteristics of the electrode. These digits are the same as used in carbon
steel electrode classification but only EXXT1-X, EXXT4-X, EXXT5-X and EXXT8-X
are used with low alloy steel flux-cored electrode classifications.
6. The suffix or "Ni2" tells the chemical composition of the deposited
weld metal as shown in table 10-17
below.
The classification system for stainless steel electrodes is based on the
chemical composition of the weld metal and the type of shielding to be employed
during welding. An example of a stainless steel electrode classification is
E308T-1 where:
1. The "E" indicates the electrode.
2. The digits between the "E" and the "T" indicates the chemical
composition of the weld as shown in table 10-18
below.
3. The "T" designates a tubular or flux cored electrode wire.
4. The suffix of "1" indicates the type of shielding to be used as
shown in table
10-19 below.
(8) Welding Cables.
(a) The welding cables and connectors are used to connect the power source to
the welding gun and to the work. These cables are normally made of copper. The
cable consists of hundreds of wires that are enclosed in an insulated casing of
natural or synthetic rubber. The cable that connects the power source to the
welding gun is called the electrode lead. In semiautomatic welding, this cable
is often part of the cable assembly, which also includes the shielding gas hose
and the conduit that the electrode wire is fed through. For machine or automatic
welding, the electrode lead is normally separate. The cable that connects the
work to the power source is called the work lead. The work leads are usually
connected to the work by pinchers, clamps, or a bolt.
(b) The size of the welding cables used depends on the output capacity of the
welding machine, the duty cycle of the machine, and the distance between the
welding machine and the work. Cable sizes range from the smallest AWG No 8 to
AWG No 4/0 with amperage ratings of 75 amperes on up. Table 10-20
shows recommended cable sizes for use with different welding currents and cable
lengths. A cable that is too small may become too hot during welding.
c. Advantages. The major advantages of flux-cored welding are reduced
cost and higher deposition rates than either SMAW or solid wire GMAW. The cost
is less for flux-cored electrodes because the alloying agents are in the flux,
not in the steel filler wire as they are with solid electrodes. Flux-cored
welding is ideal where bead appearance is important and no machining of the weld
is required. Flux-cored welding without carbon dioxide shielding can be used for
most mild steel construction applications. The resulting welds have higher
strength but less ductility than those for which carbon dioxide shielding is
used. There is less porosity and greater penetration of the weld with carbon
dioxide shielding. The flux-cored process has increased tolerances for scale and
dirt. There is less weld spatter than with solid-wire MIG welding. It has a high
deposition rate, and faster travel speeds are often used. Using small diameter
electrode wires, welding can be done in all positions. Some flux-cored wires do
not need an external supply of shielding gas, which simplifies the equipment.
The electrode wire is fed continuously so there is very little time spent on
changing electrodes. A higher percentage of the filler metal is deposited when
compared to shield metal arc welding. Finally, better penetration is obtained
than from shielded metal arc welding.
d. Disadvantages. Most low-alloy or mild-steel electrodes of the
flux-cored type are more sensitive to changes in welding conditions than are
SMAW electrodes. This sensitivity, called voltage tolerance, can be decreased if
a shielding gas is used, or if the slag-forming components of the core material
are increased. A constant-potential power source and constant-speed electrode
feeder are needed to maintain a constant arc voltage.
e. Process Principles. The flux-cored welding wire, or electrode, is a
hollow tube filled with a mixture of deoxidizers, fluxing agents, metal powders,
and ferro-alloys. The closure seam, which appears as a fine line, is the only
visible difference between flux-cored wires and solid cold-drawn wire.
Flux-cored electrode welding can be done in two ways: carbon dioxide gas can be
used with the flux to provide additional shielding, or the flux core alone can
provide all the shielding gas and slagging materials. The carbon dioxide gas
shield produces a deeply penetrating arc and usually provides better weld than
is possible without an external gas shield. Although flux-cored arc welding may
be applied semiautomatically, by machine, or automatically, the process is
usually applied semiautomatically. In semiautomatic welding, the wire feeder
feeds the electrode wire and the power source maintains the arc length. The
welder manipulates the welding gun and adjusts the welding parameters.
Flux-cored arc welding is also used in machine welding where, in addition to
feeding the wire and maintaining the arc length, the machinery also provides the
joint travel. The welding operator continuously monitors the welding and makes
adjustments in the welding parameters. Automatic welding is used in high
production applications.
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