Continuous Wire Welding Theory; Introduction; Methods - Selco Unistep 3500 Instrucciones De Uso

Porosity
Cause Grease, varnish, rust or dirt on the workpieces to
be welded.
Solution Clean the workpieces carefully before welding.
Cause Grease, varnish, rust or dirt on the filler material.
Solution Always use quality materials and products.
Keep the filler metal always in perfect condition.
Cause Incorrect arc length.
Solution Decrease the welding voltage.
Cause Humidity in the welding gas.
Solution Always use quality materials and products.
Ensure the gas supply system is always in perfect
condition.
Cause Insufficient shielding gas.
Solution Adjust the gas flow.
Check that the diffuser and the gas nozzle of the
torch are in good condition.
Cause The weld pool solidifies too quickly.
Solution Decrease the travel speed while welding.
Pre-heat the workpieces to be welded.
Increase the welding current.
Hot cracks
Cause Incorrect welding parameters.
Solution Decrease the welding voltage.
Use a smaller diameter electrode.
Cause Grease, varnish, rust or dirt on the workpieces to
be welded.
Solution Clean the workpieces carefully before welding.
Cause Grease, varnish, rust or dirt on the filler metal.
Solution Always use quality materials and products.
Keep the filler metal always in perfect condition.
Cause Incorrect welding mode.
Solution Carry out the correct sequence of operations for
the type of joint to be welded.
Cause Pieces to be welded have different characteristics.
Solution Carry out buttering before welding.
Cold cracks
Cause Humidity in the filler metal.
Solution Always use quality materials and products.
Keep the filler metal always in perfect condition.
Cause Particular geometry of the joint to be welded.
Solution Pre-heat the pieces to be welded.
Carry out post-heating.
Carry out the correct sequence of operations for
the type of joint to be welded.
For any doubts and/or problems do not hesitate to contact
your nearest customer service centre.
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6 CONTINUOUS WIRE WELDING THEORY

6.1 Introduction

A MIG system consists of a direct current power source, wire
feeder, wire spool, torch and gas.
The current is transferred to the arc through the fusible elec-
trode (wire connected to positive pole); in this procedure the
melted metal is transferred onto the workpiece through the arc
stream. The automatic feeding of the continuous filler material
electrode (wire) is necessary to refill the wire that has melted
during welding.

6.1.1 Methods

In MIG welding, two main metal transfer mechanisms are
present and they can be classified according to the means by
which metal is transferred from the electrode to the work-
piece. The first one, defined "SHORT-ARC", produces a small,
fast-solidifying weld pool where metal is transferred from the
electrode to the workpiece only for a short period when the
electrode is in contact with the weld pool. In this timeframe,
the electrode comes into direct contact with the weld pool
generating a short circuit that melts the wire which is there-
fore interrupted. The arc then turn on again and the cycle is
repeated (Fig. 1a).
Another mechanism for metal transfer is called the "SPRAY-
ARC" method, where the metal transfer occurs in the form of
very small drops that are formed and detached from the tip of
the wire and transferred to the weld pool through the arc stream
(Fig. 1b).
MIG manual welding system
Fig. 1a
Fig. 1b
SHORT cycle (a) and SPRAY ARC welding (b)