Evaluation of microstructure and mechanical properties of aluminum AA7022 produced by friction stir extrusion

Friction stir extrusion (FSE) is a process in which a sample is produced through the heat generated by the friction between die and materials, and the extrusion pressure. The extruded sample could be wire, tube, rod or another form. The wired samples were produced of aluminum alloy AA7022 at different rotational speeds and extrusion forces, and impacts of these two parameters were studied. The samples produced at higher rotational speeds and lower forces had a far better surface quality and less surface cracks were seen on them. The optical microscopy and scanning electron microscopy (SEM) were used to examine the microstructural properties of samples and the mechanical properties of specimens were studied by micro hardness and compression tests. Using X-ray diffraction technique and Rietveld method, the average crystallite size and micro-strain and dislocation density of samples were calculated and the yield strength of each sample was estimated by Taylor Equation. The average crystallite size and grain size were increased and the mechanical properties and dislocation density were decreased with the rise of rotational speed. Increasing the extrusion force to a certain level caused increase in yield strength of material. The experimental yield strength of compression test and Taylor's yield strength were compared with each other and had a good conformity.