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 th ick extruded section gf Al-Zn-Mg alloy (7039) have been carried out us ing Al-Mg (5183) filler wiser. During welding the pulse parameters suc h as the pulse frequency and duration are varied and their effects on dilution, zinc pick-up, precipitation behavoiur, microstructure refine ment, porosity content and fatigue crack gorwth rate of the weld are s tudied. It is observed that the variation in pluse parameters affect t he dilution, zinc pick-up, porosity content, amount of precipitate and microstructure refinement and consequently the fatigue crack gorwth r ate of the weld. Zinc pick-up, fine; dendrite fraction, porosity conte nt of the weld are correlated to the faigue crack gorwth rate of the w eld. The characteristisc and fatigue crack growth rate properties of t he pulse current weld are compared with those of continuous current MI G weld: The increase in zinc pick-up has been found to reduce the fati gue crack growth rate, whereas increase in finer dendrite fraction has been found to enhance the fatigue crack growth rate of the weld. Howe ver, 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 comparat ively slow in pluse current weld than in continuous current weld.