ARC WELDING PROCESS
Welding Arc Characteristics
The most common high-intensity arc is probably the welding arc.
These arcs vary in brightness and in ultraviolet radiation content, primarily as a function of arc current, shielding gas, and the metals being welded.
There are a variety of different welding arc processes and cutting processes which vary in their ultraviolet and visible light output.
The following pages summarize the principal techniques and the standard nomenclature used by the American Welding Society (AWS).
Although arc currents vary from approximately 50 amperes up to nearly 1,000 amperes for different processes, there is no one process that covers this entire range of currents.
For instance Gas Tungsten Arc Welding (GTAW) on soft metals such as aluminum may use only 50 amperes; however, a very high-powered plasma cutting (PAC) torch may exceed 1,000 amperes.
Arc welding requires a large current, generally of a relatively low voltage after the arc has been struck.
The arc is struck between an electrode and the work piece -- the base metal.
The electrode may have either a largely non-consumable metallic tip or it may be a consumable rod of carbon or a consumable metal rod or wire.
In some processes, a separate wire or rod – a welding rod (a rod of filler material that is not an electrode and should not be confused with rod-shaped electrodes used in shielded metal arc welding) may be used to supply filler metal.
Welding does not necessarily require the addition of filler metal from a consumable electrode or welding rod.
Fusion of two metal surfaces can be produced with only the high temperature of the arc.
Some welding process may employ automatic wire feed systems and be totally automated.
In other semi-automatic operations, the welder must advance arc along the work piece, but the wire is fed automatically.
Many electric arc welding processes make use of a direct current (DC) rather than alternating current. In any direct current arc, the specification of polarity can be very important.
If the electrode is a cathode and is negative (dcen), the AWS refers to this as direct current straight polarity, or DCSP.
If the welding electrode is an anode or electrode positive (dcep), this is referred to as direct current reverse polarity (DCRP). In the normal DCSP condition, the base metal being bombarded by the electrons is hotter than the electrode and a deep, “penetrating” weld is produced.
In contrast, DCRP produces a wider, shallower weld and the electrode is hotter than the base metal. An ac arc will produce intermediate characteristics of DCRP and DCSP welds since the arc polarity reverses every half cycle.
Carbon-arc welding (CAW) and carbon-arc (CAC) were the first arc welding and cutting processes. They were developed near the close of the nineteenth century.
These processes, although uncommon today, are still employed in some special applications.
A carbon electrode is typically the cathode and the base metal is the anode.
The intense heat of the arc melts the surfaces of the base metal to be joined.
Often a separate filler rod (the “welding rod”) is also used. In air-carbon-arc cutting (AAC), a high-pressure stream of air at about 550 kPa blows away the molten metal through the kerf.
The kerf is the slot cut in a metal plate.
The carbon electrodes are generally coated with copper to increase current capacity. AAC is one of the more common arc cutting and gouging processes.
Shielded-Metal-Arc Welding (SMAW) evolved from CAW when it was realized that a consumable electrode – eliminating any need for a welding rod, could replace the carbon electrode.
To reduce oxidation, the electrode wire is coated with materials such as fluorides, oxides, carbonates, metal alloys, and binders to stabilize the arc, to produce gases to shield the weld from oxygen and atmospheric contaminants, and to introduce metal alloy to weld. SMAW is used principally with nickel and ferrous base metals.
The electrodes are typically 2 to 6mm (3/32 to ¼ inch) in diameter and are controlled by the welder in a clamp-type electrode holder. Because of the rod shape of the electrode, SMAW is sometimes referred to as stick welding.
The arc is struck by the welder when he briefly touches the electrode to the work piece and withdraws it to an optimum gap.
A very experienced welder can advance the rod and maintain an optimum arc gap that produces a reasonably stable optical emission for short periods of time.
But the optical radiation emitted from this type of arc when most welders hold the stick will fluctuate substantially with time.
During World War II a dramatically different type of welding process – originally called heliarc welding or tungsten-inert-gas (TIG) welding – was developed.
Now properly termed gas-tungsten-arc welding (GTAW), this process was developed in the aircraft industry to permit effective welding of aluminum and magnesium alloys.
GTAW employs a non-consumable tungsten electrode and often a separate welding rod of filler metal.
The lack of any flux meant that slag did not have to be removed from the weld as is required in SMAW.
An Inert gas shield is provided through a concentric gas nozzle surrounding the electrode.
Because of the requirements for compressed gas, a specialized welding gun, and more sophisticated current regulation equipment, this process is found most often in heavy industry.
Helium was used predominantly at first (hence the term heliarc), whereas argon is now far more common as the inert gas.
Helium, because of its higher ionization temperature, produces a hotter arc and is still preferred (despite it’s high cost) for specialized applications where a deeper penetrating arc is desired. Regardless of the shielding gas used, GTAW is generally regarded as the process which produces the highest quality conventional weld.
Gas tungsten arc cutting (GTAC) would probably use the same arc producing equipment as GTAW, but is run to permit burn-through of the bas metal.
As in other arc cutting (AC) procedures, the arc is largely buried in the base metal and the optical radiation emitted is thereby greatly reduced. GTAC is not commonly used AC Processes.
Gas-Metal-Arc welding (GMAW) is one form of metal-inert-gas (MIG) welding.
GMAW was an outgrowth of the development of GTAW.
As in the GTAW process, an inert shielding gas such as argon, helium, or CO2 enshrouds the arc, but the GMAW electrode is consumable wire.
GMAW is a much faster process than GTAW.
Since metal is being transferred from the electrode and deposited in the weld, the nature of this transfer can greatly affect the arc characteristics and resultant weld.
Specialized GMAW current modes are used to achieve specific forms of metal transfer.
Spray transfer GMAW with an argon-shielded, high-current, DCRP arc produces a fine “spray” of metal droplets at rates of hundreds per second.
In spray transfer there is little apparent “sputtering” of the arc because of the smooth transfer of metal.
This results in a rather stable emission of optical radiation.
In the pulsed arc (GMAW-P), the spray of droplets is produced primarily during high-current pulses, although a steady arc sustaining current exists between pulses.
In the buried arc process, CO2-rich gas mixtures are used to inhibit spray transfer and crater in the steel with lest optical radiation emitted.
The short-circuiting arc (GMAW-S) process for welding thin sections also produces a train of high-current pulses resulting from a controlled short at least every 20 ms.
The various GMAW variations probably account for the largest volume of industrial welding.
This is surely true if one includes the sister process, FCAW.
These two types of MIG welding are considered the most effective of filler-type welding methods.
Gas metal arc cutting (GMAC) uses the GMAW welding machine to achieve burn-through, but this is not a common AC process.
Flux-cored-arc welding (FCAW) is a variation of metal-inert-gas (MIG) welding where the electrode wire is replaced by cored wire – a fine electrode tubing filled with flux.
The flux may produce the shielding gas (self shielding); however, external gas shielding (often CO2), as in the GMAW arrangement is frequently used.
The power supplies, guns, and electrode feed rolls are essentially the same as those used in GMAW. Cored electrodes are most commonly 1.6mm (1/16 inch) in diameter, although electrode diameters of 2.4 mm (3/32 inch) are also used.
A more recently developed welding process – plasma arc welding (PAW) – resulted from progress in plasma physics during the 1950’s and 1960’s.
Although requiring more sophisticated and more costly equipment, PAW features a more stable, more concentrated arc that permits faster welds of higher quality than most competing processes.
A pilot arc of argon introduced through an orifice inside a nozzle assembly reaches very high temperatures and ionizes a blanket of shielding gases to produce a second, larger plasma.
The larger plasma forms the tight, “transferred” welding arc that exists between a tungsten electrode and the base metal.
Welding rod is sometimes used in the GTAW process.
PAW techniques are generally of three variations;
the melt-in-mode, the keyhole mode for a very penetrating arc, and the needle arc for low currents.
The PAW arc, although rich in ultraviolet emission because of its high temperature, does not always emit high levels of optical radiation since it is often buried to a considerable extent in the base metal.
Plasma arc cutting (PAC) is a common AC process.
At high currents of 600 to 1000 A, PAC is used to cut very thick plate steel in excess of 2 cm in thickness.
The elongated high-velocity jet arc that can be achieved in specialized plasma arc cutting nozzles makes this possible.
The high-velocity jet forces molten metal through the kerf.
Water injection is used to cool the work piece. Sometimes UV absorbing dyes are added to this water.
The high noise levels created by the plasma arc have lead to the development of a water muffler that is a shield of flowing water that enshrouds the arc.
The PAC power supply can be quite expensive due to the high open-circuit voltages required to maintain a high arc voltage.
Gas mixtures for PAC make use of argon with hydrogen or hydrogen with nitrogen.
As in other AC processes the arc is largely occluded from view by the base metal.
Plasma arc spraying (PSP) – a surface treatment process – is one process where the intense plasma arc may be completely exposed.
This results in exceedingly high levels of ultraviolet and visible radiation in the vicinity of this equipment
