Abstract: The effects of single-wire unitary pulsed metal inert-gas (MIG),synergic pulsed MIG and double-wires tandem MIG welding processes on the microstructures and properties of high strength Al-Cu alloy welds were investigated.The results showed that the enhanced stirring action of the liquid metals in weld pool due to peak-current fluctuating periodically under single-wire synergic pulsed MIG welding decreased temperature gradients and increased heterogeneous nuclei within the region of constitutional supercooling just ahead of the solid-liquid interface.As a result,the microstructure of the weld was refined.The strength and ductility of the weld increased.The high heat inputs and rapid welding speed of double-wires MIG welding enhanced the formation of coarse equiaxed dendrites in the weld and increased the eutectic constituents at grain or dendrite boundaries.The strength,hardness and ductility of the weld decreased.