shortcomings of conventional PAW process, the ‘controlled pulse
key-holing’ strategy is proposed and the experimental system is
developed [28,29].
As shown in Fig. 23, the keyhole sensor is developed through
measuring the efflux plasma voltage. A simple sensor consisting of
a resistor and a capacitor is used to detect the electrical potential
or the voltage between the workpiece and the measuring bar. The
larger the keyhole size, the higher the intensity of the efflux plasma,
thus, the larger the efflux plasma voltage. There is a correlation of
Fig. 23. The experimental set-up of the controlled pulse keyholing PAW [29].
the keyhole diameter at backside with the measured efflux plasma
voltage signal [29].
Fig. 24 shows the specially designed welding current waveform
and keyhole signal. At the dropping stage of welding current
from the peak level to the base level, two sub-stages of current
decreasing with different slopes of K1 and K2 are added. At instant
t1, the peak current IP is applied, and the system keeps detecting
the signal from the efflux sensor. The signal of the efflux plasma
voltage is around zero before the keyhole is established. At instant
t2, the keyhole is established and the efflux plasma voltage exceeds
a certain value. To avoid the keyhole size expands too much, the
Fig. 22.