Processing Fine-Grained and Superplastic AZ31 Mg Tubes for Hydroforming

The microstructures and mechanical properties of the AZ31 Mg tubes fabricated by one-pass forward piercing tube extrusion operated at 250-400°C and 10^-2-10⁰ s^-1 are examined. The grain size is refined from the initial ∼75 μm grain size down to ∼1.5 μm. The room temperature tensile elongation along the extrusion direction also increases from ∼13% for the as-received billet up to 51%. The highest superplastic elongation of 610% was obtained as tensile loaded at 300°C and 2×10^-4 s^-1, and high strain rate superplasticity of 406% and 502% was achieved at 300°C and 400°C with a high strain rate of 1×10^-2 s^-1. Preliminary hydroforming or tube bulging at room temperature has demonstrated the feasibility. Hydrofoming at elevated temperature of 200°C or above should exhibit much more promising results, utilizing the capability of LTSP and HSRSP of the extruded tubes.