During forming, many alloys develop surface finishes with undesirable features such as orange-peel, banding, or roping. In addition to being cosmetically unacceptable, heterogeneous surface deformation also initiates strain localization that induces necking, tearing, or wrinkling in the part. Surface heterogeneities also produce localized variations in the friction as well as premature failures that cannot be predicted by simulation. Formability simulations are typically based on phenomenological constitutive relations that assume the response to an imposed macroscopic strain is distributed homogeneously at the microstructural level up to the onset of localization. This is predicated on the overly simple notion that, during deformation, the evolution of surface roughness is also homogeneous. Improvements in the property data have been achieved through incorporating revised plasticity, kinematic hardening models, and the results from numerous studies examining the influence of material parameters (e.g., grain size, grain orientation). However, significant inconsistencies still occur between numerically predicted formability and what is observed experimentally. One likely source of these discrepancies is that the influence of the surface heterogeneities is either minimized or greatly simplified to reduce the required computational time. One can consider a deformed surface as a composite of the topographical characteristics produced by each mechanism that was active during the deformation process. So, careful analyses of those topographical characteristics can reveal substantial information about the nature of the physical mechanisms involved. More importantly, they can determine how changes in microstructure influence the homogeneity of the surface deformation and the onset of strain localization. The objective of these studies is to improve the reliability of the predicted formability by evaluating the complex relationships between material properties, plastic deformation, and performance limiting parameters. These analyses integrate controlled deformation of relevant alloys in a range of multi-axial strain paths and strain levels, high-resolution topographical measurement techniques, and new analysis methods that minimize the statistical uncertainty in the measurement data.
This project involves the measurement of surface roughness, the best statistical way to represent the geometry of roughened surfaces, and research into tying evolving crystallographic texture with multiaxial plastic strain to the measured roughness.