Lightweight aluminium alloys are frequently deployed in the aerospace industries because of their strength to weight ratio, durability, availability, price, corrosion resistance and recyclability. Fusion welding is not suitable for joining dissimilar aluminium alloys from the 2XXX and 7XXX series, as it often leads to defects in the weld joints, including porosity, lack of fusion, incomplete penetration, and crack formation, such as hot cracking and stress corrosion cracking.

Refill friction stir spot welding, however, is a solidstate joining process. The process utilizes a non-consumable tool consisting of two independently controlled rotating parts—a probe and a shoulder. During the joining process, the material stays in a solid state, preventing the formation of fumes, porosity, and spatter. RFSSW also eliminates defects associated with the melting and so-lidification of metals.

The study investigates the joining of AA 2024-T3 and AA 7475-T361 using Refill Friction Stir Spot Welding (RFSSW). Joints were created at various tool rotational speeds, with all other parameters kept constant. Following visual inspection, samples were prepared for microstructural analysis, which included the steps of cutting, mouldings, grinding, polishing, and etching. During the microstructural examination, defects and flaws were identified.

Subsequently, hardness tests were conducted across different weld zones. The results revealed significant variations in hardness within the weld zones of samples produced at different tool rotational speeds.The hardness of the SZ significantly improved and microstructural defects decreased with increase in tool rotational speed from 1800rpm to 2200rpm. By highlighting the substantial effect of different tool rotational speeds on weld zone hardness, this thesis provides important insights into the field of joining using RFSSW.

The findings enhance our understanding of the welding process and offer practical guidance for improving joint quality. Future research could investigate the influence of additional weld-ing parameters on the microstructure and hardness of RFSSW joints. Additionally, conducting tensile and fatigue tests is necessary to gain a deeper understanding of failure mechanisms, which would aid in further refining the welding process.