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2. Literature Review
Robotic welding is the most predominant application of industrial robots in the
world [ref. 1]. Among the welding processes resistance spot welding and gas metal
arc welding (GMAW) are the most common applications. This chapter presents a
brief overview of robotic gas metal arc welding, focusing on the aspects relevant to
the programming and control of an integrated robotic welding cell.
2.1 GMAW process
2.1.1 Process description [refs. 2 and 3]
GMAW is a process that uses the heat generated by an electric arc, produced
between the end of a continuously fed welding electrode and the weld pool, to fuse
the joint (see Figure 2.1). The entire weld area, including the arc, molten pool and
electrode, is shielded by an externally supplied shielding gas. The shielding may be an
inert gas, such as argon or helium, or a mixture of an inert gas with oxygen (02)
and/or carbon dioxide (C02).
It should be noted that the composition of the shielding gas affects the way the
metal is transferred from the electrode and the stability of the process. A mixture of
argon with up to 20% CO2 and 5% 02 is normally used to ensure a robust process
performance [refs. 2,3,4]. (see Figure 2.2)
2.1.2 Modes of metal transfer
The way in which the metal is transferred from the consumable electrode tip to
the molten weld pool affects arc stability, spatter level, fume generation rate, bead
appearance and the positional capabilities of the process [ref. 3].
The modes of metal transfer are divided into two, namely: Free flight and Dip
[refs. 3,5,6].
In free flight, the metal is transferred from the filler wire to the workpiece in
discrete droplets through a continuously maintained arc (see Figure 2.3). Globular,
spray and pulse transfer are all sub-classifications of free-flight metal transfer [refs. 3,
7].
Globular transfer is characterised by low and irregular transfer rate of large
metal droplets. The transfer of metal droplets from the electrode to the molten pool is
dominated by gravitational forces, therefore limiting its applicability to the flat
position [refs. 2,5]. Another limitation of the globular transfer mode is the excessive
spatter generation due to either the splashing of liquid metal from the weld pool when
the large droplet is transferred or the overheating and explosive disintegration of the
large droplet when short circuited with the weld pool in a short arc (low voltage)
situation.
Spray transfer is characterised by small droplets (with equal or smaller
diameter to that of the filler wire) which are projected axially through the arc and
transferred to the molten pool in a high frequency stream.
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