WORKPIECE CLEANING DURING VARIABLE POLARITY PLASMA-ARC WELDING OF ALUMINUM

Variable Polarity Plasma Arc welding has proved to be extremely succes sful in welding aluminum alloys despite their adherent refractory oxid e. This success has been attributed to removal of the oxide during the reverse polarity cycle. In situ optical spectroscopy is used to measu re the amount of hydrogen and oxygen in the plasma arc with a minimum detectable limit of less than 100 ppm. It was found that the amount of contamination is independent of surface preparation and torch speed. Using this information, it is proposed that the predominant mechanism for reverse polarity cleaning in aluminum is dielectric breakdown of t he surface oxide ahead of the torch rather than by ion sputtering.