10
BOC Smootharc TIG 200 DC Operating Manual
2.0 Gas tungsten arc welding (GTAW/TIG)
2.3 Process variables
Process variable
Explanation
Usage
DCEN
Narrow bead,
deep penetration
Nozzle
Ions
Electrons
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
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
Wide bead,
shallow penetration
Nozzle
Ions
Electrons
The DCEP (reverse polarity) are different from the DCEN in
following ways:
• High heat is produced on the electrode rather on the
base metal
• The heat melts the tungsten electrode tip
• The base metal remains relatively cool compared to
using straight polarity
• Relatively shallow penetration is obtained
• An electrode whose diameter is too large will reduce
visibility and increase arc instability
• 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)
2.1 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
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.
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.
Schematic of the TIG welding process
Tungsten electrode
Collet
Workpiece
Shielding gas
Arc
TIG filler rod
Weld pool
