THE INFLUENCE OF WELDING PARAMETERS ON PROPERTIES OF AI-Zn-Mg ALLOY WELDS PRODUCED BY PULSED TIG WELDING WITH COLD WIRE FEEDING

Abstract

The effect of pulse frequency at three different parameters on the mechanical properties of the weld metal, in an Al-Zn-Mg alloy (Zn=4.5%, Mg=1.25 %, Mn=0 . 45 %, Fe = J . 7% max, rest Aluminium) , have been studied, using the pulsed TIG process with cold fed wire (dia 1.2 inn, Mg=4.9%, Mn .7%, Cr=0.15%, Ti=0.15%, rest Al), in this investigation. The pulse frequency has been varied at 0.5,2.5,4.0,5.5 and 7.5 Hz. Using the same filler wire the welding of the above mentioned alloy has also been carried out at 0 Hz (continuous TIG), for three different levels of current. The microstructural features of the weld metal resulting frcrn the various, pulse frequencies have been studied under optical and stereo microscope and cc*npared with the conventional one. No appreciable change in cast structures has been marked there in. But the effect of pulse frequency cn the width of recrystallised region at HAZ was severe. Increasing pulse frequency fran 0.5 to 4.0 Hz tend to reduce the width of recrystallised region at HAZ while a further increase in pulse frequency increases it. The porosity estimation was also done but it was found in the range where it had insignificant effect on the mechanical properties. The mechanical properties such as the hardness and tensile strength of the weld metal have been estimated. It has been observed that the increase in pulse frequency upto 4.0 Hz show a relative increase in hardness of weld deposit followed by a decrese in it with further. increase in pulse frequency up to 7.5 Hz. The increase in pulse frequency shows a minor increase in tensile strength and elongation of the weld in case of welds having no flaw. But in case of welds with flaw the tensile properties were found to vary linearly with size of flaw, with no significant effect of welding parameters on it. For these welds plain strain fracture toughness values were also calculated which shamed that for smaller a/w radio (Area fraction flaw) there is a need to change the equation for calculating K. For other specimens the variation in welding parameters was not found to affect the KC values.