Critical analysis of necking and fracture limit strains and forming forces in single-point incremental forming

Abstract

Single-Point Incremental Forming (SPIF) is an emerging manufacturing process especially suitable to pro-duce small batches of metal parts. Moreover, the enhanced formability of metal sheets deformed by SPIFmakes this technology useful to those industrial applications requiring high deformation levels. In thissense, the precise setting of limit strains in SPIF in relation to the conventional formability limits ofthe material, as well as the influence of the process parameters on these strains, are essential variablesto understand how and how much can be deformed the metal sheets in real production. On the otherhand, the forming force in SPIF is an essential variable, especially for the design of dedicated equipmentor for the safe use of adapted machinery. This paper revisits failure in SPIF by means of an experimentalanalysis of the influence of process parameters, such as the tool diameter, the spindle speed and the stepdown, on the formability in SPIF (spifability) of AISI 304 metal sheets, studied in the light of circle gridanalysis. The work also involves the independent determination of conventional formability limits bynecking and fracture under laboratory conditions by using stretching tests (Nakazima tests), in conjunc-tion with stretch-bending tests performed in order to quantify the influence of the bending induced bythe tool radius. Failure strains are experimentally obtained and compared in stretch-bending and SPIFtests, being the failure mode discussed in each case. Finally, the axial forming force evolution wasrecorded with the aim of analyzing the range of process parameters that would guarantee the safely uti-lization of the non-dedicated process equipment.