The cold metal transfer (CMT) based wire arc additive manufacturing (WAAM) technique has limited wear applications compared to other manufacturing methods due to its lower wear resistance. However, integrating friction stir processing (FSP) with WAAM can enhance mechanical or wear performance, making it a promising and reliable manufacturing process for aerospace, automotive, and marine applications. In this work, CMT-WAAM technique has been used to fabricate a bimetallic wall of aluminium alloys (ER4043/ER5356), employing a bidirectional depositional strategy. The impact of FSP on the interface layer of the WAAM wall, serving as a post-processing treatment on the wall's surface, is studied in terms of microstructure and microhardness. Additionally, a pin-on-disk wear test is performed on the interface layer of both WAAM and FSP-treated WAAM walls under loads of 20N, 30N, and 40N. Results from optical and Field emission scanning electron microscopy (FESEM) microstructures revealed the grain refinement in the stirred zone, with a higher wt. % of Mg through Energy Dispersive Spectrometer (EDS) analysis. X-ray diffraction (XRD) identifies various intermetallic compounds, including Al12Mg17, Al3.21Si0.47, and Mg2Si, with higher peak intensities in the FSP-treated wall. Electron Backscatter Diffraction (EBSD) analysis indicates a decrease in the average grain size of stirred area, which is ~72.40 µm in unstirred area and ~5.57 µm in stirred area due to the local strain concentration effect during FSP. Grain refinement during FSP leads to an increase in average hardness by 35.9%. The wear rate and coefficient of friction (COF) get reduced, credited to high-temperature dynamic recrystallization and continuous grain recovery in FSP. Scanning electron microscope (SEM) analysis of worn surfaces highlights abrasion, delamination, and adhesion as significant wear mechanisms.