Advanced Welding Processes: Technologies and Process Control

Gas metal arc welding 123
The mean current for a rectangular waveform will be given by
I
m
I t + I t
=
t + t
p p b b
p b
(7.11)
where I m is the mean current and t b is the background time. Since detachment
is controlled by the pulse amplitude and duration, the mean current may be
reduced to well below the transition current simply by increasing the
background time or reducing the pulse frequency. In practice, controlled
drop spray transfer may be achieved at currents down to 50 A with a 1.2 mm
diameter steel wire (transition current ª240 A).
Using the simple equations above, it is possible to predetermine the operating
parameters for the process as shown in Fig. 7.25. The procedure is as follows:
m min –1
400
350
300
Pulse current (A) 450
250
200
0 1 2 3 4 5 6 7 8
Pulse duration (ms)
(a)
12
11
10
9
8
7
6
5
4
3
2
1
0
200
150
100
50
0
50 100 150 200 250 300 350
Mean current (A)
(b)
mm/sec
WFS (m min –1 )
0
0 50 100 150 200 250 300
Frequency (Hz)
7.25 Pulse parameter prediction for 1.2 mm wire in argon/5% CO 2: A
pulse parameters; B burn-off rate and C frequency versus wire feed
speed.
16
14
12
10
8
6
4
2
T = 1/ F – T
l
b p
b
lT T lT
=
T
( + ) –
p b p p
b
(c)