Abstract: In this study, the two types of aluminium alloys namely Al-Zn-Mg and Al-Zn-Mg-Sc were developed by casting route. The cast plates were subjected to double passes friction stir processing (FSP) by in-situ vertically fitted milling machine. The tool design and process parameters were found to affect the microstructural and mechanical properties of these aluminium alloys. Therefore, the experimental samples were collected from the stirring zone and selected for mechanical testing. The FSP caused intense plastic deformation, material mixing, and thermal exposure, resulting in significant microstructural refinement, densification, and homogeneity of processed zone. It has been observed that the mechanical properties improved as a result of the complex interactions due to FSP, thermo-mechanical effects, dissolution, coarsening and re-precipitation of the strengthening precipitates in these two experimental aluminium alloys. It can be concluded that the strengthening effect of the Al-Zn-Mg and Al-Zn-Mg-Sc alloys after FSP are associated with the fine grain strengthening, subgrain strengthening and precipitation strengthening of Al3Sc and MgZn2 precipitates. Finally, the resultant properties have been evaluated by FSP parameters such as rotational speed of 1000 rpm and traverse speed of 70 mm/min and specified tool design. In addition, the detrimental effect of high Zn content (~8.20 wt.%) and high Sc content (~0.87 wt.%) aluminium alloy may be attributed to the vaporization of Zn at the time of the FSP. The experimental samples were collected from double passes FSPed zone and critically examined for optical microscopy (OM), scanning electron microscopy (SEM), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Vicker’s hardness testing, tensile testing and fractographic analysis.