Page 20 - Welding Robots Technology, System Issues, and Applications

Welding Robots
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the arc welding process. Although there’s a huge number of welding processes,
usually suited for a particular type of application, arc welding is used in nearly all
applications in the metal manufacturing industry. The two most common types of
arc welding processes are the gas shielded tungsten arc welding (GTAW) and the
gas shielded metal arc welding (GMAW) processes.

The gas shielded tungsten arc welding process (GTAW), also known as tungsten
inert gas (TIG), is a welding process where the arc is created between a non-
consumable electrode and the work metal. The process is shielded from
contamination by the atmosphere using an inert gas, usually argon or a mixture of
gases. The intense heat, generated by the electric arc produced by an electric
current in the 50 to 700 A range, melts the work metal and allows the joining as the
metal solidifies. Since the electrode is non-consumable the welding can be
performed without the addition of filler metal, but in some cases a filler metal is
used depending on the requirements established for the particular join.

The gas shielded metal arc (GMAW), also known as MIG (Inert Gas Metal) /
MAG (Active Gas Metal) welding process, uses the heat of the electric arc to melt
the consumable electrode wire and the metallic components to be welded. Figure
1.3 illustrates the welding principle. The fusion is carried out under the protection
of an inert gas (argon or helium), or mixture of an inert gas with much cheaper
gases like oxygen or carbon dioxide (CO 2), in order to prevent the pernicious
contamination with some gases of the atmosphere (oxygen, nitrogen and
hydrogen). Applying a high current to the electrode causes its tip to melt
transferring in this way metal to the work-piece. The electrode is fed automatically
to the arc using a coil that unfolds at a controlled speed. The rate at which the
electrode is fed is known as wire feed rate, and is one parameter of fundamental
importance for controlling this welding process. Depending on the magnitude of
the electrode current and voltage, along with the type of gas and size of the
electrode, four different types of metal transfer modes can be obtained: spray,
short-circuiting, globular and pulsed transfer.

A complete description of these and other current welding processes will be
presented in Chapter 2. Nevertheless, the brief description above makes it easy to
conclude that a good quality weld relies on the welder’s experience and skill. The
experienced and skilled manual welder is able to select the welding process
parameters based on similar cases previously encountered. In particular, he is able
to:

1. Select the type of shielding gas, the type and diameter of wire to use, and
the initial current and voltage settings more suitable for the case in hand.
2. Adjust continuously the process variables by looking to the molten pool or
by listening to the sound produced by the arc.
3. Maintain the torch in the correct position with precision and stability,
which is fundamental for a good and constant weld.