Advanced Welding Processes: Technologies and Process Control

114
Advanced welding processes
Table 7.2 Classification of transfer modes (modified version of IIW classification)
[115]
Transfer group Sub group Example
1.0 Free flight
1.1 Globular 1.1.1 Globular drop Low current GMAW
1.1.2 Globular repelled CO2 shielded GMAW
1.2 Spray 1.2.1 Projected GMAW above spray transition
1.2.2 Streaming Medium to high current GMAW
1.2.3 Rotating High current, extended stick-out
GMAW, plasma MIG
1.2.3 Explosive MMAW
2.0 Bridging transfer
1.2.4* Drop spray On transition current
(pulsed transfer) GMAW
2.1 Short circuiting Low current GMAW
2.2 Bridging without
interruption
3.0 Slag protected
transfer
Welding with filler wire addition
3.1 Flux wall guided SAW
3.2 Other modes SMAW, FCAW
* Note: author’s modification [116] to IIW classification.
7.5.1 Melting rate phenomena: GMAW
The melting rate, MR, is usually expressed as
blI
MR = a I +
a
2
(7.7)
where I is the current, l is the electrical stick-out or extension (Fig. 7.15), a
is the cross sectional area of the wire and a and b are constants. Measured values
of a and b for 1.2 mm plain carbon steel wire are a = 0.3 mm A –1 s –1 and b
= 5 ¥ 10 –5 A –2 s –1 ; for aluminium a = 0.75 mm A –1 s –1 and b is negligible.
The area term is absent in these figures since they apply to a fixed wire diameter.
The first term in equation (7.7) represents the arc heating effect whilst the
second term is due to resistive heating of the electrode. Melting rates are
affected significantly by the electrical resistance of the stick-out as shown
[117] in Fig. 7.16. The stick-out resistance depends on the electrode diameter/
cross sectional area, electrode resistivity, and the length of the extension.
DCEN (direct current electrode negative) operation increases the melting
rate [118] as shown in Fig. 7.17, but it is normally difficult to maintain a
stable arc and ensure adequate fusion with this mode of operation.