08
09
EN
TIG 185 DC. Operating manual.
TIG 185 DC. Operating manual.
2. Gas Tungsten Arc Welding (GTAW/TIG).
Shielding gas is directed into the arc area by the welding torch and a
gas lens within the torch distributes the shielding gas evenly over the
weld area. In the torch the welding current is transferred to the tungsten
electrode from the copper conductor. The arc is then initiated by one of
several methods between the tungsten and the workpiece.
During TIG welding, the arc can be initiated by several means:
Scratch start
With this method, the tungsten electrode is physically scratched on the
surface of the workpiece and the arc is initiated at the full amperage
set by the operator. The incidence of the tungsten melting at the high
initiation amperage is high and tungsten inclusions in the weld metal are
quite common.
High frequency start
During High Frequency start, the arc will ‘jump’ towards the workpiece
if a critical distance is reached. With this method, there is no incidence
of tungsten inclusions happening. High Frequency is only available on
certain types of machines and it can affect nearby electronic equipment.
Lift Arc™
During this method of arc initiation, the tungsten is actually touching the
workpiece. This occurs at very low amperage that is only sufficient to
pre-heat, not melt the tungsten. As the tungsten is moved off the plate,
the arc is established. With this method, there is little chance of tungsten
inclusion occurring.
2.3 Process variables
DCEN
When direct-current electrode-negative (straight polarity) is used:
→ Electrons strike the part being welded at a high speed
→ Intense heat on the base metal is produced
→ The base metal melts very quickly
→ Ions from the inert gas are directed towards the negative electrode at
a relatively slow rate
→ Direct current with straight polarity does not require post-weld
cleaning to remove metal oxides
2.1 Introduction
The Tungsten Inert Gas, or TIG process, uses the heat generated by an
electric arc struck between a non-consumable tungsten electrode and
the workpiece to fuse metal in the joint area and produce a molten
weld pool. The arc area is shrouded in an inert or reducing gas shield to
protect the weld pool and the non-consumable electrode. The process
may be operated autogenously, that is, without filler, or filler may be
added by feeding a consumable wire or rod into the established weld
pool.
2.2 Process
1
Shielding gas,
2
Arc,
3
TIG filler rod,
4
Weld pool,
5
Collet,
6
Tungsten Electrode,
7
Workpiece
Schematic of the TIG welding process
1
2
3
4
5
6
7
Direct or alternating current power sources with constant current output
characteristics are normally employed to supply the welding current.
For DC operation the tungsten may be connected to either output
terminal, but is most often connected to the negative pole. The output
characteristics of the power source can have an effect on the quality of
the welds produced.
Use of DCEN
For a given diameter of tungsten electrode, higher amperage can be
used with straight polarity. Straight polarity is used mainly for welding:
→ Carbon steels
→ Stainless steels
→ Copper alloys
The increased amperage provides:
→ Deeper penetration
→ Increased welding speed
→ A narrower, deeper, weld bead
DCEP
The DCEP (reverse polarity) is different from the DCEN in the following
ways:
→ High heat is produced on the electrode rather than on the base metal
→ The heat melts the tungsten electrode tip
→ The base metal remains relatively cool compared to straight polarity
→ Relatively shallow penetration is obtained
→ An electrode whose diameter is too large will reduce visibility and
increase arc instability
Use of DCEP
→ Intense heat means a larger diameter of electrode must be used with
DCEP
→ Maximum welding amperage should be relatively low (approximately
six times lower than with DCEN)
2.4 Shielding gas selection
Brass
Cobalt-based alloys
Copper nickel (Monel)
Deoxidised copper
Nickel alloys (Inconel)
Mild steel
Magnesium alloys
0.5% Molybdenum
Silicon bronze
Stainless steel
Titanium alloys
With argon, the arc is stable and there is little smoke.
Argon provides a stable, easy-to-control arc.
Argon gives a stable, easy-to-control arc. Also used for welding copper nickel to steel.
Helium is preferred as it helps greatly in counteracting thermal conductivity of copper. A mixture of 75% helium and 25%
argon (Alushield Heavy) produces a stable arc, less heat than an arc produced with helium alone.
Argon produces a very stable arc. Helium is recommended for automatic welding at high speeds
For manual welding, argon is recommended. Successful welding depends on the skill of the welder. Helium is preferred for:
→ high speed automatic welding
→ where deeper penetration than with argon is required
→ small HAZ
Argon recommended with continuous high frequency AC. Produces good arc stability and good cleaning action
Pure argon or helium is recommended. For good welding ductility, welding must be carried out in a draught-free area.
Argon decreases internal tension in base metal and in the weld since there is less penetration with this gas compared to
helium.
Argon is the most commonly used gas for stainless steel. Helium can be used if better penetration is required.
Argon produces a stable arc. Helium is recommended for high speed welding.
DCEN – Narrow bead, deep penetration
DCEP – Wide bead, shallow penetration
Nozzle
Nozzle
Ions
Ions
Electrons
Electrons
