17-12-2012, 06:32 PM
Arc welding - an overview
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History of welding
Methods for joining metals have been known for thousands of years, but for most of this
period the only form of welding was forge welding by a blacksmith.
A number of totally new welding principles emerged at the end of 19th century;
sufficient electrical current could then be generated for resistance welding and arc
welding. Arc welding was initially carried out using carbon electrodes, developed by
Bemados, and was shortly followed by the use of steel rods. The Swede Oskar Kjellberg
made an important advance when he developed and patented the coated electrode. The
welding result was amazing and formed the foundation of the ESAB welding company.
Distortion
All fusion-welding methods produce the weld by moving a molten pool along the joint;
when the heated metal cools, the shrinkage introduces residual stresses and distortion in
the welded structure. The stresses produce longitudinal and rotational distortion.
Longitudinal distortion. "Shortens" the weld, but may in many cases not be a serious
problem. An example of this type of distortion is a welded beam that can be bent if the
weld is not located symmetrically (in the centre of gravity of the cross section). If more
than one weld is used, they must be symmetrical.
Rotational distortion. The rotational distortion (see Figure 1.13) can be minimised by
making the weld bead symmetrical about the neutral axis or by having a parallel-sided
single pass weld, as with electron beam welding. A stiff section can also prevent this
type of distortion from appearing.
Spray arc
At high current, the resulting magnetic forces are directed downwards which helps the
droplet to be released from the surface tension at the electrode. The droplet transfer is
characterised by a stream of small droplets.
Short arc
At lower current it has the opposite effect. The magnetic forces are smaller and are also
directed upwards. The droplet hanging at the tip of the electrode tends to increase in size
and the process runs the risk of being unstable. A way to overcome this problem is to
keep the arc length so short that the droplets will dip into the pool before they have
grown too much. Surface tension will then start the transfer of the melted material and
the tail of the droplet will be constricted by the magnetic forces, the so-called "pinch
effect".
No metal is transferred in the form of free droplets across the arc gap. The stability of
the short circuiting transfer is very sensitive to variations in the shielding gas, the
chemical composition of the electrode and the properties of the power source and wire
feed system.
Magnetic arc blow
The force or 'arc blow' that arises when the magnetic field around the arc is not
completely symmetrical, is a well-known problem with arc welding. In critical cases, it
can result in a defective weld.
The weld pool, and thus the weld bead, can be deflected towards one side, producing
a defective weld.
If the arc is deflected along the joint, the width of the bead and the penetration can be
affected.
The protection provided by molten slag or gas can be affected, resulting in the formation
of pores.
The problem becomes worse, and more noticeable, as the welding current increases,
as this results in a corresponding rapid increase in all the electromagnetic forces in
and around the arc.