Threshold tool-radius condition maximizing the formability in SPIF considering a variety of materials: Experimental and FE investigations

In the current study, a new level of understanding on the influence of using small tool radii on the formability (θmax) is identified for single point incremental forming (SPIF). The relative value of tool radius and blank thickness (i.e., R/TB, where R is the tool radius and TB is the blank thickness) was varied over a range (from 1.1 to 3.9), and a formability diagram in the R/TB-θmax space was obtained. The formability was observed to show an inverse V-type pattern which revealed that there is a critical radius of tool (Rc) that maximizes the formability in SPIF. Further, this radius which was found to be independent of the material type (or property) is a function of blank thickness related as, Rc≈2.2TB. This radius was termed as threshold radius. The formability, in agreement with general opinion in the literature, was noticed to increase with the decrease in the tool radius above the threshold value. However, contrarily it reduced with the decrease in the tool radius below the threshold value. In fact, undue surface cutting and metal squeezing was detected when the tests were performed with pointed tools, i.e., below threshold radius. This unstable deformation, which according to the FE analyses was found to be an outgrowth of in-plane compression under the tool center, increasingly weakened the material by inducing corresponding increase in damage (quantified by stress triaxiality) with the decrease in the tool radius. On the other hand, the damage was also observed to increase due to decrease in compression with the increase in the tool radius above the threshold value. This revealed high compression with low damage constitutes the most conducive condition that maximizes the formability in SPIF, which is realized when R≈2.2TB.