Page 131 - Welding of Aluminium and its Alloys

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MIG welding

7.1 Introduction

The metal arc inert gas shielded process,EN process number 131,also known
as MIG, MAGS or GMAW, was first used in the USA in the mid 1940s. Since
those early days the process has found extensive use in a wide range of indus-
tries from automotive manufacture to cross-country pipelines. It is an arc
welding process that uses a continuously fed wire both as electrode and as
filler metal, the arc and the weld pool being protected by an inert gas shield.
It offers the advantages of high welding speeds, smaller heat affected zones
than TIG welding, excellent oxide film removal during welding and an all-
positional welding capability. For these reasons MIG welding is the most
widely used manual arc welding process for the joining of aluminium.

7.2 Process principles

The MIG welding process, illustrated in Figs. 7.1 and 7.2, as a rule uses direct
current with the electrode connected to the positive pole of the power
source, DC positive, or reverse polarity in the USA. As explained in
Chapter 3 this results in very good oxide film removal. Recent power source
developments have been successful in enabling the MIG process to be also
used with AC. Most of the heat developed in the arc is generated at the
positive pole, in the case of MIG welding the electrode, resulting in high
wire burn-off rates and an efficient transfer of this heat into the weld pool
by means of the filler wire. When welding at low welding currents the tip
of the continuously fed wire may not melt sufficiently fast to maintain the
arc but may dip into the weld pool and short circuit.This short circuit causes
the wire to melt somewhat like an electrical fuse and the molten metal is
drawn into the weld pool by surface tension effects. The arc re-establishes
itself and the cycle is repeated. This is known as the dip transfer mode of
metal transfer. Excessive spatter will be produced if the welding parame-
ters are not correctly adjusted and the low heat input may give rise to lack-

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