Weld characteristics of pulse current multipass MIG-welded Al-Zn-Mg alloy

Multipass pulse current MIG welding of 25 mm thick extruded section Al-Zn-Mg alloy (7039) was carried out using Al-Mg (5183) filler wire. During welding the pulse parameters such as the pulse frequency and duration were varied as 25 to 100 Hz and 4.5 to 8.5 ms respectively and their effect on weld characteristics, denoted as chemical composition, microstructure and porosity content of the weld, were studied. It was observed that at a given pulse duration the increase in pulse frequency from 25 to 50 Hz enhanced the finer dendrite fraction and reduced the zinc pick-up of the weld followed by a decrease and increase in them respectively with a further increase in pulse frequency to 100 Hz. It was also found that an increase in pulse frequency (fixed pulse duration), increase in pulse duration or reduction in 'pulse-off' time (fixed pulse frequency) reduced the porosity content of the weld considerably. It has been demonstrated that careful selection of pulse characteristics can reduce porosity levels to below those observed in continuous current MIG welds.