Blowholes on Pulsed MAG welding

Blowholes and their formation of Pulsed MAG welding was investigated. Among welding parameters such as arc length, average current, wire extension, welding speed and shielding gases, arc length was the most important factor. The average size of blowholes was as small as 0.62 mm. As results of analysing gas components of the blowholes by means of gas chromatograph, Ar, H2, and CH4 were detected and CO2 and CO and O2 were not in the blowholes. H2 and CH4 were also detected in blank test pieces. From SEM observation, the shape of the blowhole was like a sphere, the bottom of which was flat and the upper side was uneven like a corn having a slag spot at the top. The slag composition in the blowhole was Al, Si, Mn, Ca, La and Ce etc., and differed from the slag on the weld bead. From these results, the mechanism of blowhole formation in Pulsed MAG welding was considered as follows. Disturbance of the weld pool accompanied with spattering forms gas bubbles of the shielding gas in the pool. Small bubbles tend to be dragged into the bottom of the pool by whirling action, and be confirmed there to form blowholes close to the fusion line. CO2 in the blowholes are deoxidized to form the slag at the top of the blowholes.