Arc-Welding Fundamentals
The Lincoln Electric Company, 1994.
Arc welding is one of several fusion processes for joining metals. By applying intense heat,
metal at the joint between two parts is melted and caused to intermix - directly, or more
commonly, with an intermediate molten filler metal. Upon cooling and solidification, a
metallurgical bond is created. Since the joining is an intermixture of metals, the final weldment
potentially has the same strength properties as the metal of the parts. This is in sharp contrast to
non-fusion processes of joining (i.e. soldering, brazing
etc.) in which the mechanical and physical properties of
the base materials cannot be duplicated at the joint.
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 Fig. 1 The basic arc-welding circuit
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 workpiece 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
workpiece and is withdrawn, yet still with in close contact.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 deoxidizers 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 Fig. 2 This shows how the coating on a
gas at the point of contact while the slag protects the coated (stick) electrode provides a
fresh weld from the air. gaseous shield around the arc and a slag
covering on the hot weld deposit.
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:
1. Surface Tension Transfer® - a drop of molten metal touches the molten metal pool and is
drawn into it by surface tension.
2. 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 of
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.
