Fatigue crack growth properties of pulse current multipass MIG-weld of Al-Zn-Mg alloy

Multipass pulse current and continuous current MIG welding of 25 mm thick extruded section gf Al-Zn-Mg alloy (7039) have been carried out using Al-Mg (5183) filler wiser. During welding the pulse parameters such as the pulse frequency and duration are varied and their effects on dilution, zinc pick-up, precipitation behavoiur, microstructure refinement, porosity content and fatigue crack gorwth rate of the weld are studied. It is observed that the variation in pluse parameters affect the dilution, zinc pick-up, porosity content, amount of precipitate and microstructure refinement and consequently the fatigue crack gorwth rate of the weld. Zinc pick-up, fine; dendrite fraction, porosity content of the weld are correlated to the faigue crack gorwth rate of the weld. The characteristisc and fatigue crack growth rate properties of the pulse current weld are compared with those of continuous current MIG weld: The increase in zinc pick-up has been found to reduce the fatigue crack growth rate, whereas increase in finer dendrite fraction has been found to enhance the fatigue crack growth rate of the weld. However, it is observed that the increase in porosity content upto 3.4 vol. % slightly reduces the fatigue crack growth rate of the weld. It is also marked that the fatigue crack growth rate of the weld is comparatively slow in pluse current weld than in continuous current weld.