Plasma Arc Welding
Introduction:
Plasma welding a modern high quality welding process which is very similar
to TIG as the arc is formed between a pointed tungsten electrode and the
workpiece. Plasma welding has greater energy concentration and can permit
higher welding speeds or less distortion. Additionally plasma welding
greater torch standoff. Plasma welding also has improved arc stability.
Out of position welding is simpler with plasma welding.
WHAT IS PLASMA?
Plasma is commonly known as fourth state of matter after solid, liquid
and gas. This is an extremely hot substance which consists of free
electrons, positive ions, atoms and molecules. It conducts electricity.
How it works:
By positioning the electrode within the body of the torch, the plasma
arc can be separated from the shielding gas envelope. Plasma is then forced
through a fine-bore copper nozzle which constricts the arc. There are
three operating modes which can be produced by varying bore diameter and
plasma gas flow rate:
•Microplasma: 0.1 to 15A.
•Medium current: 15 to 200A.
•Keyhole plasma: over 100A.
The plasma arc is usually operated with a DC, drooping characteristic
power source. Because its unique operating features are results of the
special torch arrangement and separate plasma and shielding gas flows,
a plasma control console can be added on to a normal TIG power source.
Full plasma systems are also available. The plasma arc is not stabilised
with sine wave AC. Arc reignition is difficult when there is a long electrode
to workpiece distance and the plasma is constricted, extreme heating of
the electrode during the positive half-cycle causes balling of the tip
which can disturb arc stability. Special-purpose switched DC power sources
are available. By misbalancing the waveform to reduce the duration of
electrode positive polarity, the electrode is kept passably cool to maintain
a pointed tip and achieve arc stability.
Although the arc is initiated using HF, it is first formed between the
electrode and plasma nozzle. This 'pilot' arc is held within the body
of the torch until required for welding then it is transferred to the
workpiece. The pilot arc system ensures dependable arc starting and, as
the pilot arc is maintained between welds, it obtains the need for HF
which may cause electrical interference.
Electrode
The electrode used for the plasma process is tungsten-2%thoria and the
plasma nozzle is copper. The electrode tip diameter is not as critical
as for TIG and should be maintained at around 30-60 degrees. The plasma
nozzle bore diameter is critical and too small a bore diameter for the
current level and plasma gas flow rate will lead to excessive nozzle erosion
or even melting. Large bore diameter should be carefully used for the
operating current level.
Because too large a bore diameter, may give problems with arc stability
and maintaining a keyhole.
Plasma and shielding gases
The normal combination of gases is argon for the plasma gas, with argon
plus 2 to 5% hydrogen for the shielding gas. Helium can be used for plasma
gas but because it is hotter this reduces the current rating of the nozzle.
Helium's lower mass can also make the keyhole mode more difficult.
Applications:
Microplasma welding:
Microplasma was traditionally used for welding thin sheets (down to 0.1
mm thickness), and wire and mesh sections. The needle-like stiff arc minimises
arc wander and distortion. Although the alike TIG arc is widely used,
the newer transistorised (TIG) power sources can produce a very stable
arc at low current levels.
Medium current welding:
When used in the melt mode this is a substitute to normal TIG.
The advantages are:
1-Deeper penetration (from higher plasma gas flow).
2-Greater tolerance to surface contamination including coatings (the electrode
is within the body of the torch).
The major disadvantage lies in the bulkiness of the torch, making manual
welding more difficult. In mechanised welding, greater attention must
be paid to maintenance of the torch to ensure consistent performance.
Keyhole welding:
This has several advantages which can be exploited: deep penetration and
high welding speeds. Compared with the TIG arc, it can penetrate plate
thicknesses up to l0mm, but when welding using a single pass technique,
it is more usual to limit the thickness to 6mm. The normal methods is
to use the keyhole mode with filler to ensure smooth weld bead profile
(with no undercut). For thicknesses up to 15mm, a vee joint preparation
is used with a 6mm root face. A two-pass technique is employed and here,
the first pass is autogenous with the second pass being made in melt mode
with filler wire addition.
As the welding parameters, plasma gas flow rate and filler wire addition
(into the keyhole) must be carefully balanced to maintain the keyhole
and weld pool stability, this technique is only suitable for mechanised
welding. Although it can be used for positional welding, usually with
current pulsing, it is normally applied in high speed welding of thicker
sheet material (over 3 mm) in the flat position. When pipe welding, the
slope-out of current and plasma gas flow must be carefully controlled
to close the keyhole without leaving a hole.