Gas contamination effects in varible polarity plasma arc welded aluminum

A series of variable polarity plasma arc welds were made on 0.25in. (0.635-cm) thick 2219 aluminum sheet using carefully measured contaminant levels of oxygen, hydrogen, nitrogen and methane in both the arc and shielding gases. Porosity, asymmetric undercutting, incomplete fusion and fine extrusions at the edge of the weld were found to be associated with various levels of contamination. Many of the phenomena can be interpreted by noting that little or no true sputtering took place during the reverse polarity cycle, rather the surface tension of the weld pool was altered by even low fluxes of contaminant gases.