Influence of reinforcing particles on microstructure and mechanical properties of friction stir processed aluminium alloy AA7075‐T651

Abstract

This paper emphasis the improvement of mechanical properties of AA7075‐T651 using friction stir processing through localized surface modification by adding nano boron carbide particles. The reinforcement techniques such as the groove and blind hole methods were used by changing reinforcements of nano boron carbide and a matrix of AA7075‐T651 surface composites volume percentages (2 %, 4 %, and 6 %) along with tool rotational speed and processing speeds. Optical microscopy, scanning electron microscope and x‐ray diffraction analysis were used to examine the particle dispersion for the surface composites and to correlate with the enhanced mechanical properties. Results revealed that high input parameters have given grain coarsening and precipitate agglomeration and low input parameters provide poor nugget metal consolidation and no vertical material flow. The L9 orthogonal Array designed and optimized the process parameters for enhancing the surface properties of processed samples. Mechanical properties like ultimate tensile strength, yield strength, hardness, percentage of elongation and impact strength were evaluated for the groove friction stir processing method and blind‐hole friction stir processing methods. From the results, it has been observed that the blind‐hole technique resulted in higher hardness and the homogenous dispersion of nano boron carbide particles in the stir zone than the groove method. Consequently, for blind‐hole friction stir processing, grey relational analysis (GRA) and particle swarm optimization (PSO) approaches were proposed to optimise process parameters. From the compared optimization results between grey relational analysis and particle swarm optimization, particle swarm optimization approach was shown the best optimization results. Successively, the optimum condition in the respective experimentation is accomplished. Based on these observation and results, final validation tests were carried by changing the volume percentages of reinforcement keeping tool rotation speed and tool processing speed as constant. It is apparent that dynamic recrystallization in aluminium alloy at the processed zone due to presence of heterogeneous nucleation sites with nano boron carbide particles.