WELD PENETRATION CONTROL DURING GTA WELDING USING WELD POOL OSCILLATION SENSING

摘要

In this paper attention is given to on-line control of weld penetration during GTA welding by means of weld pool oscillation frequency measurements. Weld pool oscillations can be triggered by applying short current pulses. Due to changes in arc length as a result of the oscillation of the weld pool, the frequency of the oscillation can be measured from the arc voltage signal. Different modes of oscillation can be distinghuised. For each mode the oscillation frequency depends on the weld pool geometry, the density of the liquid metal and the surface tension. It appears that a dramatic change in oscillation frequency occurs when a transition takes place from partial to full penetration or vice versa. This phenomenon is used as a sensing tool for penetration control. By applying a feedback loop in the measuring and control system, a situation can be obtained where full penetration can be maintained by welding current adjustment. Experiments were carried out both in bead-on-plate welding as in orbital tube welding, which show that good results can be obtained. It appears that shielding gas composition plays an important role as it influences the electric field strength in the arc and thus the detectability of the changes in arc voltage/length.