Influence of the strain path changes on the formability of a zinc sheet

The formability of sheet metals is frequently characterized by the forming limit curve (FLC), which is determined by applying proportional loading paths until the onset of necking. However, metal forming processes usually involve strain path changes that can affect the limit strains. The purpose of the present work is to study the consequence of strain path changes on the formability of a 0.65 mm thick Zn-Cu-Ti sheet, by means of tests carried out in bi-linear stages. For this propose, tensile limit strains were determined by uniaxial deformation of biaxially pre-strained specimens. The FLC was first determined for six strain paths, obtained from hydraulic bulge and uniaxial tests with diverse specimen geometries. For the bi-linear strain paths, the tensile limit strains of the biaxially pre-deformed zinc sheet are significantly higher than the FLC of non pre-strained samples for the three orientations studied, revealing interdependence between the material's formability, the loading history and microstructure evolution.