Advanced Welding Processes: Technologies and Process Control

Gas metal arc welding 113
is the current density and d v is the vapour density. The vaporization force
usually only becomes significant at higher currents or when low-vapourpressure
elements are present.
7.3.5 Surface tension
Surface tension plays a very important role in metal transfer; in free-flight
transfer it is the principal force which prevents droplet detachment and, in
dip transfer, it is the major force which pulls the droplet into the weld pool.
A simple static analysis of the drop-retaining force in globular transfer would
suggest that the force is given by
F st = 2pr wsf (r w/c) (7.6)
where r w, is the wire diameter, s is the surface tension and f(r w/c) is a
function of wire diameter and the constant of capillarity c. For large droplets,
the value of this equation approximates to 2pr w.
Calculation of the magnitude of the force due to surface tension is, however,
complicated by the significant temperature dependence and the dramatic
influence of certain surface-active elements (for example at the melting point
of steel its surface tension will be reduced by around 30% by a concentration
of 0.1% oxygen and the effect of small amounts of sulphur in changing the
surface tension/temperature gradient has already been discussed in Chapter
6). Values of 300 dyn for aluminium and 600 dyn for steel have been calculated,
however, for globular transfer with a 1.6 mm diameter wire.
7.4 Summary: metal transfer phenomena
Metal transfer phenomena may be classified as free-flight or dip and within
the free-flight mode several alternative transfer types may be observed. A
classification which embraces these phenomena has been devised by the
International Institute of Welding [115] and this is illustrated in Table 7.2.
The mode of metal transfer is influenced by a balance of forces which will
depend on the operating parameters for the process. Gravitational,
electromagnetic and surface tension are the most significant forces controlling
metal transfer. In conventional GMAW, the level of these forces and the
resultant transfer behaviour is determined by the physical properties of the
system (material and the shielding gas), but is controlled to a significant
extent by the welding current.
7.5 Control of conventional GMAW
Mean current determines the transfer mechanism of the process as described
above and also controls the melting rate of the filler wire.