Advanced Welding Processes: Technologies and Process Control

Gas metal arc welding 117
V = M + AI (7.8)
where V is the arc voltage, I is the current and M and A are constants. The
resultant relationship is illustrated in Fig. 7.19.
7.5.3 Control in conventional GMAW systems
Conventional wire feed systems are designed to maintain the feed speed
constant at a preset value irrespective of any variations in the arc behaviour.
Conventional GMAW power sources have for some time been designed with
constant-voltage (CV) characteristics in order to provide self-adjustment and
stabilization of the arc length. With these systems, if the arc length tends to
change, the current varies significantly and the burn-off behaviour acts in
such a way as to counteract the change in arc length. An increase in arc
length causes an increase in arc voltage and the power source output current
must reduce in order to meet the higher voltage demand; since melting rate
is current dependent, the reduced current will result in reduced melting and
since less wire is consumed the arc length is shortened. Shortening the arc
will produce an increase in current, increased melting and again the arc
length will return to its original value (Fig. 7.20).
7.5.4 Welding parameter selection: CV power source
Current is normally chosen according to the plate thickness being welded
and the travel speed required. The wire feed speed may be determined from
the burn-off curves as described above and the voltage is set to provide the
required current and satisfy the arc voltage–current characteristic. The operating
point for the process will be defined by the intersection of the power source
Arc voltage
25
20
15
10
5
0
50 70 90 110 130
Current
150 170 190
Actual values Linear regression Wire feed speed
7.19 1.0-mm wire dip transfer.
10
8
6
4
2
0