In arc welding, the intense heat needed to melt metal is produced
by an electric arc. The arc is formed between the actual work and an
electrode (stick or wire) that is manually or mechanically guided
along the joint. The electrode can either be a rod with the purpose
of simply carrying the current between the tip and the work. Or, it
may be a specially prepared rod or wire that not only conducts the
current but also melts and supplies filler metal to the joint. Most
welding in the manufacture of steel products uses the second type of
electrode.
Basic Welding Circuit :
The basic arc-welding circuit is illustrated in Fig. 1. An AC or DC
power source, fitted with whatever controls may be needed, is
connected by a work cable to the work piece and by a "hot"
cable to an electrode holder of some type, which makes an electrical
contact with the welding electrode.
An arc is created across the gap when the energized circuit and the
electrode tip touches the work piece and is withdrawn, yet still
with in close contact.
The arc produces a temperature of about 6500ºF at the tip.
This heat melts both the base metal and the electrode, producing a
pool of molten metal sometimes called a "crater." The
crater solidifies behind the electrode as it is moved along the
joint. The result is a fusion bond.
Arc Shielding :
However, joining metals requires more than moving an electrode
along a joint. Metals at high temperatures tend to react chemically
with elements in the air - oxygen and nitrogen. When metal in the
molten pool comes into contact with air, oxides and nitrides form
which destroy the strength and toughness of the weld joint.
Therefore, many arc-welding processes provide some means of covering
the arc and the molten pool with a protective shield of gas, vapor,
or slag. This is called arc shielding. This shielding prevents or
minimizes contact of the molten metal with air. Shielding also may
improve the weld. An example is a granular flux, which actually adds
deoxidizes to the weld.
Figure 2 illustrates the shielding of the welding arc and molten
pool with a Stick electrode. The extruded covering on the filler
metal rod, provides a shielding gas at the point of contact while
the slag protects the fresh weld from the air.
The arc itself is a very complex phenomenon. In-depth understanding
of the physics of the arc is of little value to the welder, but some
knowledge of its general characteristics can be useful.
Nature of the Arc :
An arc is an electric current flowing between two electrodes
through an ionized column of gas. A negatively charged cathode and a
positively charged anode create the intense heat of the welding arc.
Negative and positive ions are bounced off of each other in the
plasma column at an accelerated rate.
In welding, the arc not only provides the heat needed to melt the
electrode and the base metal, but under certain conditions must also
supply the means to transport the molten metal from the tip of the
electrode to the work. Several mechanisms for metal transfer exist.
Two (of many) examples include:
- Surface Tension Transfer® - a drop of molten metal
touches the molten metal pool and is drawn into it by surface
tension.
- Spray Arc - the drop is ejected from the molten metal at the
electrode tip by an electric pinch propelling it to the molten
pool. (Great for overhead welding!)
If an electrode is consumable, the tip melts under the heat of
the arc and molten droplets are detached and transported to the work
through the arc column. Any arc welding system in which the
electrode is melted off to become part of the weld is described as
metal-arc. In carbon or tungsten (TIG) welding there are no molten
droplets to be forced across the gap and onto the work. Filler metal
is melted into the joint from a separate rod or wire. More of the
heat developed by the arc is transferred to the weld pool with
consumable electrodes. This produces higher thermal efficiencies and
narrower heat-affected zones.
Since there must be an ionized path to conduct electricity across a
gap, the mere switching on the welding current with an electrically
cold electrode posed over it will not start the arc. The arc must be
ignited. This is caused by either supplying an initial voltage high
enough to cause a discharge or by touching the electrode to the work
and then withdrawing it as the contact area becomes heated. Arc
welding may be done with direct current (DC) with the electrode
either positive or negative or alternating current (AC). The choice
of current and polarity depends on the process, the type of
electrode, the arc atmosphere, and the metal being welded.
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