Upsetting using a mildly wedged upper die was attempted in order to produce a non-axisymmetric part with an oval cross section. The advantage of this upsetting method was that it required no container and caused low forging load. The mild wedge on the upper die promotes dividing material flow in the single direction. The ratio of the major axis to the minor axis of the oval cross section was 1.49 after the compression ratio of 70% in upsetting. The fracture limit of the material was improved by using a mildly wedged upper die with a round top.

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Biaxial loading experiments for a seamless steel tube subjected to prestraining are carried out using a servo-controlled multiaxial tube expansion testing machine. This testing apparatus is capable of applying arbitrary biaxial stress paths to the central section of a tubular specimen by controlling axial forces and internal hydraulic pressure. The tubular specimens are loaded via linear and bilinear stress paths. Contours of plastic work of the as-received material are measured by linear stress path experiments. Bilinear stress path experiments are performed to investigate the effect of axial compressive prestraining on the stress-strain curves in the subsequent biaxial loading. In addition, small uniaxial tensile specimens are machined from the axial and hoop directions of the mother tube wall, and tension-compression reverse loading experiments are performed to quantitatively evaluate the Bauschinger effect of the test material. It is experimentally found that the Bauschinger effect observed in uniaxial reverse loading and biaxial reloading can be reproduced by kinematic hardening models and that the associated flow rule is almost satisfied with the yield locus used in the kinematic hardening models. The material model will be useful in improving the accuracy of numerical analyses of the cold-working processes for fabricating steel tubes.

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A new extrusion system for producing tubes with greatly twisted fins was proposed. In the proposed extrusion system, the metal flow was controlled by rotating a mandrel and a flow guide during extrusion to produce fins with various twisted shapes. The characteristics of the twist for fins were investigated by carrying out extrusion tests for a variety of rotation speeds and rotary directions of the mandrel and the flow guide. The specific angle of twist of the fin was increased by increasing the rotation speed of the mandrel and the flow guide. A tube with greatly twisted fins was obtained by rotating the mandrel and the flow guide in the same direction.

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