CNC Incremental Stretch-Expanding of Perforated Plates.

Abstract

The process of CNC incremental forming in which a blank is formed to fit on a tool-envelope surface made by relative motion of a tool around the blank. has the advantage of not only adapting to small-lot production of thin shell parts but also improvement of the forming limit by preventing plastic localization, such as necking and subsequent fracture. Although many cases in which plastic localization on the process is prevented have been reported, most of these are based on solid sheet (sheet metal without holes) alone. Here we report the possibility of CNC incremental stretch-expanding of perforated plates so as to clarify the effect. Aluminum sheets perforated with a periodic equilateral triangular pattern of circular holes were stretch-expanded to conical shells with various half-apex angles, α, and the deformation behavior was examined experimentally. It was found that when e is larger than about 45°, conical shells can be formed from a perforated plate by means of the process with a single tool-path, despite containing the necking around the holes. Furthermore, experimental results showed that the global deformation modes of the perforated plate are similar to those of the solid sheet, because the strain distributions at the center of the ligament were approximately equal to those obtained from the Sine law. The necking pattern found in the process can be explained on the basis of the geometrical relationship between the global deformation mode and the array of holes.