Experimental study of the effects of pin geometry, advancing speed and D/d ratio on the mechanical and microstructural properties of 6061 aluminum alloy under the friction stir processing

The aim of this research is to investigate the effect of different pin geometries, the ratio of shoulder diameter to pin diameter, and advancing speed on the mechanical and microstructural properties of the specimens fabricated from 6061 aluminum sheet by friction stir processing. Cylindrical, frustum and prisms with triangular section (in three sizes), square and hexagonal cross-sections pins were prepared. The diameter of the shoulder was considered 18 and the diameter of the peripheral circle of all the pins was considered 6 mm. Advancing speeds of 14, 20, and 28 mm/min and rotational speed of 1000 rpm were considered. The smallest grain size was obtained using a pin with square cross-section. As the advancing speed increased, the average grain size decreased and its lowest value was observed at the advancing speed of 28 mm/min. In addition, the best mechanical properties were observed in the specimens fabricated by square cross-section pin and frustum pin. As the advancing speed increased, the ultimate strength of all specimens and the yield stress of most specimens increased. The highest hardness was observed in the specimens fabricated by square cross-section pin and the lowest hardness was observed using cylindrical pin. Also, in specimens fabricated by triangular cross-section pins, by decreasing the ratio of the shoulder diameter to the pin diameter, the ultimate strength and hardness increased and the elongation decreased.