Failure mechanism during incremental sheet forming of a commercial purity aluminum alloy

Incremental sheet forming (ISF) is known for many advantages over conventional forming, such as part flexibility, low tooling cost and higher formability. Previously various studies have explored the failure mechanism experimentally and numerically. However, limited studies have been performed on the micro-mechanics of failure during ISF process. This study involves failure mechanism during incremental sheet forming of a commercial purity aluminum alloy (AA1050). During experiments, failure (fracture) was observed in truncated cone specimens by increasing the wall angle from 71 degrees to 72 degrees. The mechanism behind failure was explored with detailed microstructural characterization and finite element (FE) simulations. Various samples were taken from the successfully formed and failed cones for microstructural analysis. The electron back-scatter diffraction (EBSD) based microtexture measurements associate this failure with more grain fragmentation and necking or localized deformation in the failed cone. Further, the X-ray diffraction (XRD) based residual stress measurements indicate high positive (tensile) hydrostatic stresses (similar to 157 MPa) on the outer surface of the failed (fractured) cones. Finite element analysis (FEA) with Gurson-Tvergaard-Needleman (GTN) damage model was used for fracture prediction. The FEA results also support the residual hydrostatic stress evolution in the cones, albeit at a slightly different wall angle. Based on microstructural and finite element analysis it is concluded that deformation becomes inhomogeneous when the failure occurs during ISF process.