Bead Formation in Pulsed TIG Welding

Study on Pulsed Arc Welding (Report 4)

Abstract

Pulsed TIG welding phenomena of mild steel, stainless steel and aluminum alloy, are investigated with respect to the welding current waveform, supplied from analogue transistor controlled power source. The pressure of TIG arc in pulsed or steady current are measured carefully, taking account of response characteristic of pressure measurement system used. Weld pool behaviour is observed by high speed photography.
Results obtained are summarized as follows;
(1) In pulsed TIG welding, use of pulsed current having an excessive amplitude of current causes weld defects such as humping or tunnel bead even if the average current is kept constant.
(2) Occurrence of humping bead in pulsed TIG welding is dependent on pulse frequency applied. It is found out that there exists the critical pulse frequency (humping frequency) f_c. At this frequency, humping occurs in minimum amplitude of current at the same average current level.
(3) Humping frequency f_c becomes higher as the welding speed increases if the average current is kept constant. The humping frequency of mild steel is observed to be higher than that of stainless steel or aluminum alloy under same welding condition.
(4) TIG arc pressure in sinusoidal current wave varies in accordance with current waveform. Amplitude and average value of pressure fluctuation are almost kept constant in the frequency range up to 300 Hz.
(5) In pulsed TIG welding, molten pool is drived to oscillate by arc pressure fluctuation due to change in welding current. The intrinsic frequency of weld pool can be found out. It depends on both pool geometry and physical properties of materials such as surface tension, fluidity and density.