The document compares three arc welding processes: shielded metal arc welding, gas metal arc welding (MIG), and gas tungsten arc welding (TIG). All three use direct current electricity to create an arc between an electrode and workpiece. They also all use some form of shielding and filler material matched to the workpiece. Shielded metal arc welding uses a coated electrode for electrical current and filler material, with the coating forming a protective gas shield. MIG uses an external gas shield and continuous wire feeding electrode. TIG uses a non-consumable tungsten electrode and inert gas shield, allowing for welding of all positions and alloys with a high quality, slow weld.

1. Arc welding
A Comparison Between Shielded
Metal, Gas Metal, and Gas Tungsten
Arc Welding

2. The similarities of the Three
Processes
• Each process uses direct current electricity
to create an arc between an electrode and
the work piece.
• Each process uses some form of shielding to
protect the weld from contaminates.
• Each process uses a filler material, which
has similar chemical composition to the
work piece.

3. Shielded Metal Arc Welding
This form of arc welding was developed in the early
20th century. Its primary use is in the welding of steel. (Ref
3) Shielded metal arc welding is performed by striking an
arc between a coated-metal electrode and the work piece.
Once the arc is established the work piece becomes molten
at the location of the arc. The Electrode wire serves two
functions. First is as a medium for the electrical current.
Second, the filler material used to form the joint. The
coating, which surrounds the electrode, forms a gas that
helps to shield the weld. Figure 7 illustrates how the
shielded arc welding works. (Ref 1)

4. Shielded Metal Arc Welding

5. From the figure a slag can be seen. This slag
combines with the electrode coating and forms a protective
barrier for the weld. This barrier protects the weld from the
outside air and keeps out contaminates, which will make the
weld weak.

6. Gas Metal Arc Welding
Gas metal arc welding (GMAW), also call metal inert
gas (MIG) or metal active gas (MAG) welding, shields the
weld with an external gas such as argon, helium, carbon
dioxide, or other gas mixtures. (Ref 6) GMAW is used to weld
all the commercially important metals, including steel,
aluminum, cooper, and stainless steel. The filler metal
selection has chemical composition that is closely matched to
the base material being welded. The process can be used in
any position, including flat, horizontal, vertical, and overhead.
(Ref 5) The following figure illustrates a detailed diagram of
how GMAW torch works.

7. Gas Metal Arc Welding

8. It is usually recommended for use with direct
current electrode positive polarity. The electrode is
continuous feeding wire, which protrudes through the
torch. The arc is automatically controlled with only
semiautomatic operation needed. The torch positioning,
guidance, and travel speed are the only parameters, which
the operator controls. GMAW uses a relatively low heat
and therefore can be used on metals with thicknesses less
than one quarter inch. Since the operations are mostly
automatic this welding process is usually easier to learn as
compared to other welding processes. (Ref 5)

9. Gas Tungsten Arc Welding
In gas tungsten arc welding (GTAW), a tungsten
electrode is used in place of the metal electron used in shielded
metal arc welding. A chemically inert gas, such as argon,
helium, or hydrogen, is used to shield the metal from
oxidation. The heat fro the arc formed between the electrode
and the metal melts the edges of the metal. Metal for the weld
may be added by placing a bare wire in the arc or the point of
the weld. This process can be used with nearly all metals and
produces a high quality weld. However, the rate of welding is
quite slow. (Ref 3) The following figure illustrates the process
of gas tungsten arc welding.

10. Gas Tungsten Arc Welding

11. The tungsten electrode has a high melting point that
makes it virtually non-consumable. This gives the gas
tungsten arc welding specific advantages:
•Welding can be done in any position.
•The weld is usually equal to the base metal in composition.
•Flux is not used; therefore, finished welds do not require
cleaning of corrosive residue.
•Smoke or fumes are not present to obscure vision.
•Distortion of the base metal is minimal because the heat is
concentrated in a small area.
•No splatter is produced because metal is not transferred
across the arc.

12. Another form of gas tungsten arc welding is called
plasma arc welding (PAW). PAW is the next step in GTAW. It
uses a restricted arc that is squeezed through a copper nozzle.
The results being a restricted arc which is longer, thinner, and
more focused. The constricting process greatly increases the
arc voltage and the amount of ionization that takes place. The
raising arc temperature increases the hottest area of the
plasma, extending it to the outside of the nozzle down toward
the work piece. This results in a more concentrated heat
source at a higher temperature that greatly increases the heat
transfer efficiency, allowing for faster travel speeds. (Ref 4)

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