STUDIES ON INFLUENCE OF PULSE PARAMETERS ON FCGR AND FRACTURE TOUGHNESS BEHAVIOUR OF PULSE CURRENT MIG WELDED Al-Zn-Mg ALLOY

Abstract

Pulsed current MIG welding of extruded section (50x10 mm) of Al-Zn-Mg alloy was carried out by using 1.6 mm diameter Al-Mg filler wire and commercial argon as shielding gas. During pulsed current welding the pulse parameters such as the pulse frequency and pulse duration were varied. The weld groove was filled by single pass. During pulsed current welding the mean current was kept constant and the variation in the peak and base current as well as the pulse time and pulse-off time were measured using a digital oscilloscope. For a comparative study some welds were also prepared by conventional continuous current MIG welding process, where the welding current was kept same as mean current of the pulse current welding. The microstructure' and porosity content of the weld were studied at the transverse section of weld joint under an optical microscope. Chemical analysis of the weld deposit was carried out to study the zinc pick-up in it. The zinc pick-up of the weld was'confirmed by measurement of dilution: The zinc pick-up of weld was correlated to the pulse parameters. Tensile properties of the weld joint were studied and also correlated to pulse parameters. The fracture toughness J and fatigue crack growth rate of the weld were also studied and correlated to the pulse parameters. The microstructure of the pulse current weld has been found comparatively finer than that of continuous current weld. The porosity content of the weld also found to vary with the pulse parameters. The zinc pick-up and porosity content of the weld with the change in pulse parameters are found to affect the tensile properties of the weld. The change in tensile properties influenced the J1~ as well as fatigue crack growth rate of the weld. The fatigue crack growth rate has also been found to be affected by the porosity content of weld deposit. Under certain pulse parameters the fatigue crack growth rate and fracture toughness of the weld are found to be superior to those of ocnventional continuous current weld. (