Effect of SiC nano particles on grain stability of friction stir processed AA7075

Abstract

Friction stir processing (FSP) is a new technique to produce fine-grained microstructure with enhanced microstructure characteristics and mechanical properties compared to the base metal (BM). However, microstructures are not stable under elevated temperature. The microstructure of the friction stir processed (FSPed) metallic substrate can be improved by reinforcing the matrix with secondary phase particles. With this motivation, aluminium alloy plate of typically AA7075-T651 is reinforced with nano-particles of silicon carbide (SiCNP) and the composites were processed by FSP at the tool-rotation speed (TRS) of 1000 rpm and the tool-traverse speed (TTS) of 25 mm/min with six passes. The grain stability of the composite was studied by heat-treating to temperatures range 450 °C–500 °C for various durations range 1 h 30 min to 4 h 30 min. Micro and macrostructure of heat-treated samples are characterized using an scanning electron microscopy (SEM) and optical microscope (OM). Results showed that even after 4 h 30 min of samples subjected to heat treatment, the grains structures of the FSPed composites were stable at 450 °C. Further, for higher temperatures, i.e. 475 °C and 500 °C, composite grain structures at the stir zone (SZ) were stable up to 3 h. In comparison, abnormal grain growth (AGG) was observed in the six-pass FSPed aluminium alloy plate of AA7075-T651 without addition of SiCNP and heat-treated for 450 °C for 1 h 30 min. Thus, these experimental investigations indicate that grain boundary pinning by SiCNP has significantly increased the stability of the grains under elevated temperature and results in grain refinement with average grain size range 1.5–2 µm at the SZ of the FSPed composites.