The 6XXX aluminum alloys are magnesium and silicon alloys that are widely used in several applications for their attractive mechanical properties. However, there are some problems associated with the welding of aluminum alloys. Aluminum alloys are difficult to weld due to their high thermal conductivity and high thermal expansion. The weldability of aluminum alloys varies significantly, depending on its chemical composition. In this paper, an experimental investigation studies the effect of heat treatment on mechanical properties of 6063 aluminum alloy for a single pass of Activated Tungsten Inert Gas (ATIG) welding. A thin layer of flux was applied to the welding area. Using ATIG, using the edge preparation, time and power consumption, shielding gas amount and wire quantity can be considerably reduced. Moreover, the number of joints per shift can be increased. No degradation in the microstructure and mechanical properties of the ATIG welds have been observed compared to those produced by conventional TIG welding. The optimal ageing parameters (temperature-time) have been determined to improve the mechanical properties. Taguchi method used to optimize the ageing parameters to improve the mechanical properties.