In single point incremental forming of 304 austenitic stainless steel, the effects of the tool rotation rate on the forming limit, surface appearance, forming shape, martensite fraction, and microstructure were evaluated. When the tool rotation rate was relatively low, forming limit was drastically reduced with increasing tool rotation rate owing to intermittent swarf-like flash formation at the tool-contact surface. The amount of martensite formed increased with decreasing tool rotation rate or increasing elongation of the sheet. The evolution of the deformation microstructure and texture was similar to that in rolling. On the other hand, in the case of a relatively high rotation rate, the forming limit was improved with increasing tool rotation rate owing to continuous friction stirring and heating at the tool-contact surface. The forming accuracy was improved compared with that obtained with a low rotation rate.

We have developed a new hydroforming sealing technique with flat tools. This forming technique using seal punches with a flat end face enables the deformation of both ends of a tube during die closure while applying internal pressure to the tube. We studied the effects of various forming conditions on the sealing performance of this technique. The sealing performance was more stable when tube end faces were ground with a milling machine than when they were cut with a band saw. The sealing performance was better when the seal punch end faces were milled surfaces rather than textured surfaces. When the seal length was too large, tube ends tended to buckle. When the seal length was too small, the hydraulic fluid tended to leak, because the tube ends were pulled into the dies during hydroforming. The sealing pressure limit became higher with increasing axial force of the seal punches during die closure. However, that pressure became lower when the axial force was increased excessively. Effective hydroforming results were obtained when the axial stress applied to the tube ends after die closure was at least equivalent to the proof stress of the tube material. By this new hydroforming seal technique, we successfully produced full-length, flanged hydroformed parts and full-length, bent hydroformed parts with a rectangular cross section.