INFLUENCE OF PULSE CURRENT PARAMETERS ON THE PROPERTIES OF HEAT AFFECTED ZONE IN , MIG WELDING OF AI-Zn-Mg ALLOYS

Abstract

The effect of pulse frequency at the two different levels of an current on the mechanical properties of the heat affected zone (HAZ), in an AI-Zn-Mg alloy (Zn 4.0 -5. 07. , Mg 1.0 - 1.57. , Mn 0.2 - 0.7/. , Fe 0.7/. max, Cu 0.27. max, others .157. and rest aluminium) have been studied using the pulsed arc MIG welding process. A 1.6 0 filler wire of composition Al + 4.5/. Mg has been used in this investigation for making the welds. The pulse frequency has been varied at 25, 50 and 100 cycles/sec. Using the same filler wire the welding of the 'above mentioned alloy has also been carried, out at 0 cycle/sec. (equivalent to Conven-tional MIG), for the two different levels of mean current. "The micro structural features of the HAZ resulting from the various pulse frequencies have been studied under opti-cal and scanning electron microscope, and compared with the conventional one. A some what better morphology of the HAZ with respect to the grain size, has been marked in weld-ments produced by pulse arc welding process in comparision to the conventional one. The mechanical properties such as the hardness and notch tensile properties of heat affected zones have been estimated and found to be better in welds made under current pulsation mode as compared to the conventional ones. Further it has been noted that welds made at 100 cycles/sec. gives a significant improvement in the notch tensile properties of HAZ over the other freque ncy levels. The fractured surfaces of the notched specimen have been studied under scanning electron microscope. The fractured surface reveals that the crack after being initiated from the notch root in the HAZ migrates in the weld metal. However the crack is found to follow a longer path in the HAZ before reaching the weld metal in cases of welds made at 100 cycles/sec.