The fabrication of Mg-Zn-Y-Nd minitubes via cyclic extrusion compression and hot drawing for vascular stent applications

Abstract

Precision magnesium alloy minitubes are key raw materials used in the manufacture of magnesium alloy vascular stents. However, the poor room-temperature formability of magnesium alloys limits the production of high-performance precision minitubes through conventional cold drawing methods. In this study, a combined process of cyclic extrusion compression (CEC) and hot drawing was proposed to manufacture the Mg alloy minitubes. The results show that CEC produces a fine grain with an average size of approximately 1.96 µm, and providing a favorable microstructure for subsequent hot drawing. As the temperature rises from 25 °C to 250 °C, the deformation mechanism of CEC processed ZE21B magnesium alloy transitions from basal slip and twin to prismatic and pyramidal slips, further activating two {10$\overline{1}$2} tensile twin variants, thereby, the cross-section reduction rate (CRR) during one pass increases from 21.45 % to 46.24 %. The minitubes with a wall thickness of 0.14 mm are fabricated using only four drawing passes and one final annealing via hot drawing at 250 °C, whereas conventional cold drawing requires six passes and five annealing passes. Moreover, the final hot drawn minitubes containing two twin variants induce an S-shaped stress-strain curve with ultimate tensile strength (UTS) of 273.5 ± 6.6 MPa, an elongation (EL) of 22.7 ± 0.2 %, its yield ratio (YR) is approximately 0.5 indicating better suitability for subsequent stent deformation. Meanwhile, a high density of twin boundary suppresses preferential grain boundary corrosion, result in a uniform corrosion morphology and improve the corrosion resistance. This study offers theoretical foundation and a novel processing route for the manufacture of biodegradable Mg alloy stents.