Cryogenic friction-stir processing of ultrafine-grained Al-Mg-TiO2 nanocomposites

Submerged friction-stir processing under cryogenic conditions was employed to fabricate ultrafine-grained nanocomposites with enhanced mechanical characteristics. Al-Mg alloy sheet with 3 vol% TiO2 nanoparticles were processed under air (ambient temperature), a water-dry ice medium (~−25 °C), and liquid nitrogen. It is shown that a homogenous distribution of reinforcement particles throughout the metal matrix is attained at a rotational speed of 1400 rpm and a traverse velocity of 50 mm/min after 4 passes. In situ formation of Al3Ti and MgO nanophases during multi-pass processing is shown by transmission electron microscopy. Under the cryogenic cooling condition, ultrafine grains and cellular structures with sizes smaller than 1 μm and 200 nm are attained. It is shown that the formation of an ultrafine-grained structure is accompanied with significant improvement (150-200%) in the mechanical strength. The tensile yield strength of ~170 MPa, elongation of ~22% and Vickers hardness of ~165 HV are attained. A change in the fracture mode from ductile-brittle to fully ductile is presented when the cryogenic processing is employed. A relationship between the grain size and the fracture features is demonstrated. Effects of cooling conditions on the microstructure and mechanical properties of friction stir processed Al-based nanocomposites are addressed.