Abstract: The original scheduling program of process control for arc starting system is improved. Console for controlling arc starting process and routine for collecting temperature signals are programmed. Experiments for aluminum alloys 2219 and LF6 are conducted via variable polarity plasma arc welding technique. Temperature history for the measured points near the molten pool are obtained using infrared thermoscope through trial and error. Critical values for the measured temperature history are utilized as the signals for formation of the keyhole weld pool, on the basis of which the computer controls the signal for the workpiece to travel. It is shown that the improved arc starting system reduces operation complexity and increases automation and efficiency. Moreover, temperature of the meas-ured points reaches the critical value and then decreases sharply at the moment of keyhole formation, and the measured critical value for a given point changes little despite repetitive meas-urements. Besides, the arc starting stage transits stably to the main welding stage which realizes the establishment of stable keyhole pool, thus feasibility and reliability of the improved arc starting system are experimentally verified.

Key words: Aluminum alloy, Critical temperature, Keyhole weld pool, Variable polarity vertical-up plasma arc welding