Microstructure, Mechanical and Corrosion Behaviour of AA7075 Aluminium Alloy Friction Stir Welds

Friction stir welding (FSW) is emerging as an alternative technique for joining high strength aluminium alloys as it eliminates the problems during fusion welding. In this work, the effect of post weld treatments (PWHT), viz., peak aging (T6) and retrogression and reaging (RRA) on the microstructure, mechanical properties, pitting corrosion and stress corrosion cracking (SCC) resistance of AA7075 aluminium alloy friction stir welds has been studied. An attempt also has been made to change the chemical composition of the weld nugget by adding boron carbide (B₄C) nano powder with the aid of the FSW. Hardness and tensile properties were found to be better in PWHT – T6. Pitting corrosion and SCC resistances were improved in PWHT-RRA condition with negligible loss of strength when compared to PWHT-T6. RRA promotes coarse precipitation of the equilibrium phase η in the grains and sub grain boundaries, while maintaining a fine distribution of η' in the grain interiors. The increased strength and hardness in the peak aged (T6) condition was attributed to the presence of semi-coherent intermediate η' (MgZn₂). With the addition of B₄C nano powder to the weld nugget, hardness, tensile properties, pitting corrosion resistance and SCC resistance were further improved significantly when compared to the unreinforced weld nugget. PWHT-RRA treatment on the welds with B₄C nano powder addition resulted in improved hardness of weld nugget which is attributed to the uniform distribution of strengthening precipitates in the matrix and powder strengthening. Pitting corrosion resistance, Tensile strength and SCC resistance was improved significantly in B₄C added welds after RRA treatment when compared to the same welds without B₄C addition.