Superposing tensile stresses into single point incremental forming to affect martensitic transformation of SS304

Abstract

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.

Using tensile and compressive stress superposition during incremental forming to manipulate martensitic transformation in SS304

“…The resulting 𝛼'-martensite volume fractions were measured using an FMP30 Feritscope (Fischer Technology Inc.) and validated by electron backscatter diffraction. Further details regarding the material characterization experiments and Feritscope validation can be found in [3]. The Young's Modulus is approximately 170 GPa calculated from the stress-strain curve of uniaxial tension in the rolling direction (RD), and the Poisson's ratio is 0.33 based on the textbook value.…”

“…An overview of the constitutive modeling is described below and summarized in Table 1. Additional details, including experimental data, can be found in [3].…”

“…This model assumes isothermal conditions, and the maximum achievable volume fraction, 𝑐 max , is dictated by the temperature value. The transformation kinetics order from greatest to least with respect to equivalent plastic strain for SS304 is: shear, equibiaxial tension, uniaxial tension, and uniaxial compression [3]. Note that this order is dependent on the specific SS material and component form investigated [2].…”

“…Stress superposition is a used in manufacturing to increase material formability, decrease the required forming loads, and enable customization of components [2]. Alternatively, the stress superposition strategy can be adopted to manufacture functionally graded parts using stress state sensitive microstructure changes, e.g., martensite transformation [3], in metals. These heterogeneous parts are advantageous for applications requiring highly customized parts.…”

“…An isotropic, combined strain hardening law for constituent phases was identified to model SS304 and coupled with the Beese and Mohr 2011 martensitic transformation kinetics model [7]. This material model was implemented into Abaqus 2019 [8] for each process using a two-step approach [3]. The numerical results were validated by experimental results for all four IF processes.…”