In this study, the influence of internal pressure and axial compressive displacement on the formability of small-diameter ZM21 magnesium alloy tubes in warm tube hydroforming (THF) was examined experimentally and numerically. The deformation behavior of ZM21 tubes, with a 2.0 mm outer diameter and 0.2 mm wall thickness, was evaluated in taper-cavity and cylinder-cavity dies. The simulation code used was the dynamic explicit finite element (FE) method (FEM) code, LS-DYNA 3D. The experiments were conducted at 250 °C. This paper elucidated the deformation characteristics, forming defects and forming limit of ZM21 tubes. Their deformation behavior in the taper-cavity die was affected by the axial compressive direction. Additionally, the occurrence of tube buckling could be inferred by changes of the axial compression force, which were measured by the load cell during the processing. In addition, grain with twin boundaries and refined grain were observed at the bended areas of tapered tubes. The hydroformed samples could have a high strength. Moreover, wrinkles, which are caused under a lower internal pressure condition, were employed to avoid tube fractures during the axial feeding. The tube with wrinkles was expanded by a straightening process after the axial feed. It was found that the process of warm THF of the tubes in the cylinder-cavity die was successful.

A stent is employed to expand a narrowed tubular organ, such as a blood vessel. However, the persistent presence of a stainless steel stent yields several problems of late thrombosis, restenosis and chronic inflammation reactions. Biodegradable magnesium stents have been introduced to solve these problems. However, magnesium-based alloys suffer from poor ductility and lower than desired fatigue performance. There is still a huge demand for further research on new alloys and stent designs. Then, as fundamental research for this, AZ31 B magnesium alloy has been investigated for the effect of equal-channel angular pressing on the fatigue properties. ECAP was conducted for one pass and eight passes at 300 °C using a die with a channel angle of 90°. An annealed sample and ECAP sample of AZ31 B magnesium alloy were subjected to tensile and fatigue tests. As a result of the tensile test, strength in the ECAP (one pass) sample was higher than in the annealed sample. As a result of the fatigue test, at stress amplitude σ_a = 100 MPa, the number of cycles to failure was largest in the annealed sample, medium in the ECAP (one pass) sample and lowest in the ECAP (eight passes) sample. It was suggested that the small low cycle fatigue life of the ECAP (eight passes) sample is attributable to severe plastic deformation.

Stent is employed to expand the narrowed tubular organ such as blood vessel. The persistent presence of a metallic stent yields several problems like late thrombosis, restenosis and chronic inflammation reactions. Biodegradable magnesium stents have been introduced to solve these problems. However, there is still a huge demand in further research on new alloys and stent designs. Then, as fundamental research for this, effect of microstructure change on mechanical properties and corrosion behavior of Equal-Channel Angular Pressed AZ31B magnesium alloy was investigated. As a result of tensile test at room temperature of ECAPed sample, the tensile strength was 316 MPa, which was slightly improved by about 1.08 times compared to the annealed sample. As a result of the corrosion test, corrosion resistance of the ECAPed sample was lower than the annealed sample. It was suggested that stress relieving annealing (aging) after ECAP is beneficial for improving corrosion resistance due to the reduction of crystal defects, and at the same time, it is a method that can further improve ductility.

A stent is a medical device that acts as a scaffold to internally support weak or narrowed blood vessels. Conventional stents are generally manufactured from biocompatible grades of stainless steel and remain semi-permanently in the body with the possible risks of restenosis and throm-bosis. Magnesium, which has excellent biocompatibility and biodegradability, has recently been attracting attention as a candidate material for bioabsorbable stents that gradually disappear over time. A significant challenge to the application of magnesium to stent technology is the dif-ficulty in controlling its corrosion rate in-vivo. Following implantation in a blood vessel the stent will be repeatedly subjected to tensile and compressive stresses due to the pulsatile blood flow induced by the beating of the heart. This paper investigates the effects of stress and heat treatment on the corrosion behaviour of pure magnesium. Corrosion testing and grain-size measurements were conducted on pure mag-nesium samples which were subjected to tensile strain, compressive strain and repeated tensile-compressive cyclic stress following heat treatment processing. It was observed that mass loss of specimens with refined crystal grains was decreased by the application of strain or cyclic stresses and annealing near the recrystallization temperature region. Conversely, heat treatment in the high-temperature region increased mass loss in specimens with coarsened grains. It is thus sug-gested that grain refinement improves corrosion resistance while grain coarsening decreases re-sistance.