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.

In this study, the material behavior in the T-shape microtube hydroforming (MTHF) of pure copper and stainless-steel SUS304 microtubes with an outer diameter of 500 µm and wall thickness of 100 µm was examined experimentally and numerically. This paper elucidates the basic deformation characteristics, the forming defects, and the forming limit as well as the effects of lubrication/friction and tube length. The hydroformability (bulge height) of the SUS304 microtube was shown to be higher than that of the copper microtube because of the high buckling resistance of SUS304. Good lubrication experimentally led to the high hydroformability of T-shape forming. The length of the microtube significantly affects its hydroformability. Friction resistance increases with increasing tube length and restricts the flow of the microtube material into the die cavity. By comparing the T-shape and cross-shape MHTF characteristics, we verified the hydroformability of the T-shape microtube to be superior to that of the cross-shape microtube theoretically and experimentally. In addition, the process window for T-shape MTHF had a narrower “success” area and wider buckling and folding regions than that for cross-shape MTHF. Furthermore, conventional finite element (FE) modeling without consideration of the grains was valid for MTHF processes owing to the many grains in the thickness direction.

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.

Warm deep drawing is an effective special deep drawing technique for improving the forming limits of difficult-to-form materials such as aluminum alloys, magnesium alloys, and stainless steels. This paper experimentally investigated the effect of a combined variable process path, which integrates a variable punch speed (VSPD) and a variable blank holder force (VBHF) path, on the warm deep drawing performance of an A5182 aluminum alloy sheet at 300 °C (where the strain rate sensitivity index m equals 0.11). Experiments demonstrated not only a reduction in the forming time and an improved wall thickness uniformity, but also an improvement in the forming limits. The significant improvement in the forming characteristics is discussed in terms of the theoretical three-dimensional process window (SPD-BHF-flange reduction ratio (ΔDR*) space) consisting of the fracture limit and flange wrinkling limit derived from deep drawing theory, and it was shown to be consistent with the experimental results. Finaly, the novel combined VSPD/VBHF process path successfully achieved deep drawing with a challenging drawing ratio (DR) of 3.3.