2005 Volume 23 Issue 1 Pages 53-64
Changes in arc voltage do not always mean fluctuations of arc length in aluminium alloy welding because cathode fall voltage increases with the absence of oxide. In addition, with the absence of oxide cathode spots become concentrated and the current density near the cathode surface becomes high so that the potential gradient there increases. After the breaking of a short circuit at electrode positive polarity cathode spots are formed on the centre of weld pool surface where little oxide exists. As a result, arc voltage becomes abnormally high in spite of the short arc length. Moreover, when polarity is switched from electrode positive to electrode negative right after the detachment of a droplet, cathode spots are formed on the surface of remained melt metal at the wire tip where little oxide exists so that arc voltage also becomes abnormally high. The arc becomes unstable when those abnormal arc voltages are included in the feedback signal for arc length control. Therefore, a digital filter is developed to obtain correct arc length information by eliminating those abnormal arc voltages from the feedback arc voltage.
The algorism of arc length control system is another important factor that affects arc stability. A new algorism is developed to monitor continually the feedback arc voltage of the present pulse cycle and to determine the right timing to terminate the base duration of the present pulse cycle. By using this algorism the average current and average voltage of every pulse cycle plot a line with a decreasing characteristic called pulse V-I characteristic. The inclination of the pulse V-I characteristic can be set freely and controlled accurately. Arc stability is improved by optimisation of the inclination in AC or DC pulsed MIG welding. Furthermore, low frequency modulated AC pulsed MIG welding process is developed by setting two pulse V-I characteristics and switching between them periodically at a low frequency.
