Effect of pulse on pulse frequency on welding process and welding quality of pulse on pulse MIG welding-brazing of aluminum alloys to stainless steel

Lap joining of 6061 aluminum alloy (Al alloy) to 304 stainless steel (SS) by pulse on pulse metal inert gas (MIG) welding-brazing process using 4043 Al-Si filler wire was conducted to study the effect of pulse on pulse frequency on welding process and welding quality. The results show that for high-energy pulse, the effect of pulse on pulse frequency on arc characteristics and behavior of metal transfer is significant. However, for low-energy pulse, this effect is insignificant. The external oscillation frequency of the molten pool induced by variation of arc force and impingement of metal droplets is closer to its natural oscillation frequency when the pulse on pulse frequency is 5.3 Hz; the weld pool is more intensively oscillated. This phenomenon results in the formation of fine and uniform equiaxed grain structures, and thin brazed interface layers, which leads to excellent mechanical properties. The thickness of brazed interface layers varied from about 5 to 9 mu m, and the thinner the layer, the better the mechanical properties of Al alloy/SS joints are. When pulse on pulse frequency is less than or equal to 5.3 Hz, the joints fractured at heat-affected zone of 6061 Al alloy with highest strength of 117.4 MPa up to 94.7 % of the tensile strength of Al alloy. However, the joints fractured at the brazed interface layer with much lower strength when the pulse on pulse frequency is larger than 5.3 Hz.