Particle refinement and fine-grain formation leading to enhanced mechanical behaviour in a hypo-eutectic Al–Si alloy subjected to multi-pass friction stir processing

The present research aims at studying the synergetic effect of pre-process thermal treatment and multi pass friction stir processing (FSP) on microstructure and mechanical properties of a hypo-eutectic Al–Si automotive alloy. The effect of FSP with and without pre-process thermal treatment is studied up to 3 passes and it is found that both the size of silicon particles and aluminium grains become finer with increasing FSP passes. Maximum particle and grain refinement is observed in the material subjected to 3 passes of FSP with pre-process thermal treatment. The thermal treatment prior to FSP led to spheroidization of silicon particles which influenced the particle and grain refinement during FSP to a great extent. The effect of microstructural modification by FSP on the mechanical properties has been studied by micro-hardness and tensile testing. The 3 pass FSPed material without thermal treatment showed maximum ductility of 36.5%. However, maximum tensile strength of 251MPa is obtained in the 3 pass FSPed material with pre-process thermal treatment. The various mechanisms involved in enhancing the mechanical properties are explained by means of fractography results and the microstructural evolution during multiple FSP passes.