Advanced Welding Processes: Technologies and Process Control

7.1 Introduction
Due to its high operating factor and deposition rate, gas metal arc welding
(GMAW) has the potential to improve productivity over that obtained with
the GTAW and SMAW processes. Although the requirement to exploit the
economic benefits of the process has led to a clear trend towards greater use
of GMAW worldwide, it has in the past proved difficult to obtain reliable
quality. The main thrust of development has therefore been to improve control
and achieve more consistent quality.
In order to discuss the advances which have been made in this direction
it is necessary to reconsider the process fundamentals, in particular metal
transfer in GMAW and control of conventional GMAW.
7.2 Metal transfer in GMAW
The way in which material is transferred from the tip of the consumable
electrode into the weld pool has a significant influence on the overall
performance of GMAW: it affects process stability, spatter generation, weld
quality and the positional capabilities of the process. Phenomenological studies
of the mode of metal transfer have been carried out using high-speed cine or
stroboscopic cine and video techniques. [109] The various natural types of
transfer have been classified into groups. A simplified basic classification is
shown in Fig. 7.1.
In free-flight transfer an arc is maintained between the electrode and the
workpiece and the metal is transferred across the arc in the form of droplets.
The size of the droplets and frequency of transfer may vary considerably and
several subdivisions of free-flight transfer are necessary to accommodate
these variations. The common free-flight modes are:
∑ globular (drop and repelled);
∑ spray (drop projected streaming).
100
7
Gas metal arc welding