An official website of the United States government
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.
more »
« less
Using tensile and compressive stress superposition during incremental forming to manipulate martensitic transformation in SS304
Abstract Single point incremental forming (SPIF) is a flexible manufacturing process that has applications in industries ranging from biomedical to automotive. In addition to rapid prototyping, which requires easy adaptations in geometry or material for design changes, control of the final part properties is desired. One strategy that can be implemented is stress superposition, which is the application of additional stresses during an existing manufacturing process. Tensile and compressive stresses applied during SPIF showed significant effects on the resulting microstructure in stainless steel 304 truncated square pyramids. Specifically, the amount of martensitic transformation was increased through stress superposed incremental forming. Finite element analyses with advanced material modeling supported that the stress triaxiality had a larger effect than the Lode angle parameter on the phase transformation that occurred during deformation. By controlling the amount of tensile and compressive stresses superposed during incremental forming, the microstructure of the final component can be manipulated based on the intended application and desired final part properties.
more »
« less
- Award ID(s):
- 1757371
- PAR ID:
- 10519349
- Publisher / Repository:
- IOP Publishing
- Date Published:
- Journal Name:
- IOP Conference Series: Materials Science and Engineering
- Volume:
- 1307
- Issue:
- 1
- ISSN:
- 1757-8981
- Page Range / eLocation ID:
- 012006
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Uniaxial tension is a universal material characterization experiment. However, studies have shown that increased formability can be achieved with simultaneous bending and unbending of the material. This so-called continuous bending under tension process is an example of bending stress superposition to a uniaxial tension process. In this research, experiments are conducted on stainless steel 304 to investigate the effects of bending stress superposition on the austenite to martensite phase transformation. Two vortex tubes are mounted to the carriage of the machine and used to decrease the temperature in a localized region of the specimen to evaluate two temperature conditions. The in-situ strain and temperature fields are captured using 3D digital image correlation and infrared cameras. The deformation induced α′ -martensite volume fraction is measured at regular intervals along the deformed gauge length using a Feritscope. The number of cycles that the rollers traverse the gauge length, corresponding to the strain level, is also varied to create five conditions. The deformed specimens revealed heterogeneous martensite transformation along the gauge length due to the non-uniform temperature fields observed for each test condition. Decreasing the temperature and increasing the number of cycles led to the highest amount of phase transformation for this bending-tension superposed process. These results provide insight on how stress superposition can be applied to vary the phase transformation in more complex manufacturing processes, such as incremental forming, which combines bending, tension, and shear deformation.more » « less
-
We studied the compressive-tensile yield asymmetry (CTYA) and its sensitivity to standard post-processing treatments for a 17-4 PH stainless steel processed with selective laser melting (SLM). Quasistatic tensile and compression tests at ambient temperatures reveal a consistent CTYA for all tested conditions, with compressive yield strengths exceeding tensile values. In the as-printed state, yield asymmetry (Δσ) is ∼113 MPa. Stress-relieving at 300 °C results in only a marginal decrease in asymmetry (Δσ ∼109 MPa), suggesting that the residual stresses generated during SLM have a negligible effect on the observed CTYA. Our analyses indicate that “dynamic softening” due to a stress-assisted austenite-to-martensite transformation governs the yield behavior similar to that observed in transformation-induced plasticity (TRIP)-assisted steels containing mechanically unstable retained austenite. This interpretation is further supported by the increased asymmetry, Δσ ∼127 MPa, observed in a solution-treated and aged specimen, which has a slightly higher retained austenite volume fraction (24 % vs. 21 %). Direct aging at 482 °C of the as-printed steel with a ferritic microstructure causes severe embrittlement. Brittle fracture occurs under tensile stresses well below the yield strength. This pronounced loss of ductility most likely arises from a strong <001> fiber texture along the build direction developed during SLM, which is known to promote cleavage-type fracture on {001} planes. A solution treatment at 1000 °C for 1 h, austenitizes the microstructure completely, and subsequent quenching produces a predominantly martensitic structure with a much weaker crystallographic texture, which restores a favorable strength-ductility balance after aging.more » « less
-
Daehn G., Cao J. (Ed.)Controlling the microstructure of components is of interest to achieve optimal final part properties, i.e., materials by design. The manufacturing process itself can affect a material’s characteristics by changing the microstructure. For example, past research has shown that austenite to martensite phase transformation in stainless steel occurs during deformation. Temperature is known to have a significant influence on this phenomenon. In this paper, the effect of temperature on the austenitic to martensite phase transformation in SS 316L under uniaxial tension is investigated. Both a cooling system and a heat exchanger were employed in a uniaxial tension experimental setup to control the temperature. Tensile specimens were strained to fracture at four temperatures of −15, 0, 10, and 20 °C. Digital imaging correlation (DIC) and a thermal imaging camera were used for tests at 0 °C and above to capture strain and temperature data, respectively. Strain and temperature data could not be obtained at −15 °C due to the DIC paint flaking during testing. X-ray diffraction was used to measure the volume fraction of martensite in both the as-received and the tensile-tested materials.more » « less
-
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.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1088/1757-899X/1307/1/012006
