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To investigate the microstructural changes that occur in stainless steel (SS) 304 during single point incremental forming (SPIF), experiments and finite element (FE) simulations were conducted for a truncated square pyramid geometry. Results from material characterization experiments for four stress states, i.e., uniaxial tension, equibiaxial tension, shear, and uniaxial compression, were combined to construct a material model based on the constituent phases and transformation kinetics. The material model was implemented into numerical analyses, where a two-step FE approach was utilized to predict martensite transformation in SPIF with increased computational efficiency. Validation experiments showed good agreement with the martensite transformation predictions from the FE simulations. The four locations along the pyramid wall revealed varying martensite volume fractions because of the differing stress states of bending, stretching, and shear that the blank is subjected to during SPIF, which can affect the microstructure. The stress state can be defined in terms of the stress triaxiality and Lode angle parameter. The FE results indicate that stress triaxiality impacted the martensitic transformation kinetics in SS304 more than the Lode angle parameter for SPIF for this particular material and geometry. Thus, distinct stress states in incremental forming can affect the martensitic transformation locally and, when used strategically, achieve functionally graded materials. This is pertinent to industrial applications requiring custom components, e.g., trauma fixation hardware for medical applications. 

Abstract In this paper, finite element analyses were conducted to investigate the stress and strain states resulting from varying the deformation of stainless steel 316L under biaxial loading. To that end, a biaxial specimen geometry was designed in collaboration with the US National Institute of Standards and Technology (NIST) to achieve large and uniform strain values in the central pocket region. Special care was taken to ensure that the specimen design could be readily manufactured with available resources. Simultaneously, the specimen design criteria required an acceptable strain uniformity in a sufficiently large pocket section to allow for accurate deformation and austenite to martensite phase fraction measurements. This demonstrates the concept of altering the final material properties through stress superposition. Numerical results show that nearly linear curves were observed in the strain path plots. The minimum uniform deformation area for the 4:1 case had a radius of ∼1 mm, which is sufficient for experimental analyses, e.g., digital imaging correlation and electron beam backscatter diffraction. As an application for such heterogeneous materials, patient specific trauma fixation hardware, which are surgically implanted to set broken bones during healing, require high strength in areas where screws are located, i.e., martensite phase, yet low weight elsewhere. 

Abstract 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.