Influence of Rotational Speed on the Microstructure and Mechanical Response of ECO-AZ31 Magnesium Tubes Processed by Friction Stir Extrusion from Recycled Chips

ECO-AZ31 magnesium alloy modified with 1 wt.% CaO, was investigated for the production of thinwalled tubes by Friction Stir Extrusion (FSE), employing recycled chips through solid-state consolidation. The study focused on the influence of tool rotational speed, ranging from 500 to 1000 rpm, on the consolidation quality, microstructure and mechanical properties of the extruded tubes. The results indicate that low rotational speeds lead to poor material consolidation and surface defects due to insufficient heat generation, whereas high rotational speeds caused overheating and excessive grain growth. In contrast, intermediate rotational speeds were found to be optimal, as they resulted in defect-free tubes with a wall thickness of ~500 μm, exhibiting a refined microstructure (achieving grain sizes in the range of 5–7 μm in the inner region) and improved microhardness (with values close to 80 HV). Microstructural analysis revealed a heterogeneous microstructure throughout the tube wall thickness, associated with the thermal and deformation gradients. At intermediate rotational speeds, {10 1 2} tensile twinning was activated as the dominant mechanism of deformation accommodation, partially reducing dislocation accumulation and softening the texture, leading to a localized decrease in hardness despite the refined grain size. These results demonstrate that the combined effect of heat generation, plastic deformation, dynamic recrystallisation, and twinning determines the final tube properties, confirming FSE as a sustainable and energy-efficient route for producing thin-walled tubes from recycled machining chips.