Superposing pre-stress on a SS304 sheet metal blank in biaxial tension and performing a single point incremental forming operation on the stretched blank is investigated experimentally. By applying a pre-stress to the sheet metal blank prior to incremental forming, the resulting microstructural change can be affected to obtain functionally graded materials according to the intended application. In austenitic stainless steels, this variation of the stress states alters the phase transformation, specifically the martensitic transformation kinetics, by influencing key process parameters, such as process force, temperature, and equivalent plastic strain. The phase transformation in truncated square pyramids is measured using magnetic induction. These measurements validate the effectiveness of the stress superposition method for achieving the desired mechanical properties based on altering the final microstructure of a simple geometry.

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In this paper, results for SS316 L microtube experiments under combined inflation and axial loading for single and multiloading segment deformation paths are presented along with a plasticity model to predict the associated stress and strain paths. The microtube inflation/tension machine, utilized for these experiments, creates biaxial stress states by applying axial tension or compression and internal pressure simultaneously. Two types of loading paths are considered in this paper, proportional (where a single loading path with a given axial:hoop stress ratio is followed) and corner (where an initial pure loading segment, i.e., axial or hoop, is followed by a secondary loading segment in the transverse direction, i.e., either hoop or axial, respectively). The experiments are designed to produce the same final strain state under different deformation paths, resulting in different final stress states. This difference in stress state can affect the material properties of the final part, which can be varied for the intended application, e.g., biomedical hardware, while maintaining the desired geometry. The experiments are replicated in a reasonable way by a material model that combines the Hill 1948 anisotropic yield function and the Hockett–Sherby hardening law. Discussion of the grain size effects during microforming impacting the ability to achieve consistent deformation path results is included.