Materials Science Forum Vols. 519-521

Abstract: The process of work hardening in aluminum alloys is important from the viewpoint of formability and the prediction of the properties of highly deformed products. However the complexity of the strengthening mechanisms in these materials means that one must carefully consider the interaction of dislocations with the detailed elements of the microstructure and the related influence of the elements on dislocation accumulation and dynamic recovery. In addition, it is necessary to consider the influence of the work hardening process at various levels of plastic strain. This permits the possibility of designing microstructure for tailored plastic response, e.g. not simply designed for yield strength but also considering uniform elongation, spring-back, ductility etc. This presentation will explore the concept of identifying the various interactions which govern the evolution of the work hardening and their possible role in alloy design.

Abstract: TEM, Kikuchi diffraction analyses, EBSD, neutron diffraction and hardness measurements have been applied in a study of commercial purity aluminum (AA1200) cold rolled to strains 2 and 4 and afterwards recovered by a heat treatment for 2h at temperatures up to 220 °C. The deformation microstructure is a lamellar structure delineated by dislocation boundaries and high angle boundaries ( ) parallel to the rolling plane. The macrotexture is a typical rolling texture which is composed of individual texture components present as micrometer- and submicrometre-sized volumes. In the lamellar structure, correlations have been established between microstructural parameters and the local texture, showing for example that the density of high angle boundaries and the stored energy vary locally. The local variations affect the annealing behaviors in a way that some regions coarsen faster than others, leading to a recovered structure which is heterogeneous.

Abstract: We introduce a simulation procedure for through-process texture and anisotropy prediction, in particular for AA5182 sheet production from hot rolling through cold rolling and annealing. The FEM package ‘T-Pack’ based on the software LARSTRAN served as a process model. It was combined with physics based microstructure models for deformation texture (GIA), work hardening (3IVM), nucleation texture (ReNuc), and recrystallization texture (StaRT). The terminal sheet texture was used for a FEM simulation of cup drawing. A new concept of interactively updated texture based yield locus predictions was employed. The simulation predictions were compared to experimental data. The procedure can be applied to a wide variety of Aluminum alloys.

Abstract: Necking under in-plane plane strain tension along the transverse direction (TD) is numerically simulated for two sheets: one with very high Cube (HC) and the other with low Cube (LC). To do so, the EBSD measurement is performed in the TD-ND (normal direction) section for the sheets. The EBSD map (grain orientations and their spatial distributions) is directly implemented into the crystal plasticity based finite element code. More specifically, the measured orientations are assigned to elements in the mesh according to their positions. The values of the material parameters in the crystal plasticity model are determined by curve-fitting numerical simulations of uniaxial tension in the rolling direction (RD) to corresponding experimental data. The effect of spatial grain orientation distribution on necking is emphasized. It is found that both the global averaged texture and its spatial distribution are important to the onset of necking. The predicted results are in good agreement with experimental observations.

Abstract: Forming limit diagram (FLD) is a measure of the formability of a sheet material. The major-minor strain pairs that are closest to the neck on multiple specimens of various strain paths are utilized to construct a boundary between safe and unsafe zones. The challenge to obtain the FLD is the determination of incipient necking. Three approaches to determine the limit strains have been investigated and compared in this research in order to establish the optimal one for implementation: (1) commonly used Bragard criterion ( 1)e Br with periodic grids; (2) tracking the region of large local strains from strain history to locate the instance when critical major strain ( 1)e cr happens; (3) post-processing of strain history to locate the inflection in the major strain rate curve 1 max (e&&) at the onset of localization. The last criterion of inflection in strain rate 1 max (e&&) carries both a numerical and a physical meaning towards developing an understanding of flow localization, formability and fracture.

Abstract: Finite element modelling of sheet-forming operations, such as pressure-ram-forming, (PRF™) requires knowledge of forming limits under biaxial strain conditions. In this work, elliptical bulge tests have been used to evaluate the forming limits of an aluminum bodystock alloy, X309, that is used for PRF™ applications. Limiting dome heights have been determined as a function of pressure-rate and temperature. All tests have been done with the rolling direction, RD, of the sheet aligned with the major axis of the bulge.

Abstract: The objective of this study was to investigate and compare the influence of Mn and Fe additions on the fracture behaviour of AA6000 series alloys in under, peak and overaged conditions. Testing was completed under uniaxial tension and the microstructures of the alloys were observed using optical and Transmission Electron Microscopy. Alloys with very low levels of both Mn and Fe underwent a transition from transgranular to intergranular fracture and a reduction in strain-tofracture when heat treated from the under aged (UA) to peak aged (PA) condition. Increasing Mn and Fe content prevents this transition in fracture mode such that the stain-to-fracture is similar in the UA and PA states. Despite the change in fracture mode, when comparing the strain-to-fracture for a given ageing condition, increasing Fe systematically reduces the strain-to-fracture. Conversely, increases in Mn systematically increase the strain-to-fracture for a given ageing condition.

Abstract: The influence of surface defect geometry on the localization and failure behaviour of AA6111 sheet has been investigated through experimentation and numerical modelling. A series of uniaxial tensile samples were produced with idealized top and bottom surface defects (i.e. grooves), located either symmetrically or asymmetrically on the opposing surfaces. The symmetric arrangement corresponds to the “groove-like” initial imperfection of the classical Marciniak- Kuczyński (M-K) model. Experimental results indicate that both the symmetry of the defects and their wavelength have a profound effect on the resulting mode of localization and failure as well as on the limit strains. Specifically, symmetric surface defects are seen to induce localization and failure through simple necking, whereas asymmetric defects tend to promote macroscopic, throughthickness shearing. Furthermore, asymmetric surface defect geometries are found to produce lower limit strains in the AA6111 sheet under study for defect wavelengths below about 1.5 mm, while the reverse is true when defect wavelengths are above 1.5 mm. Finite element method (FEM) modelling simulations are also presented, demonstrating that the experimentally-observed trends in localization and failure behaviour can be replicated using a mixed isotropic-kinematic hardening implementation of the Gurson-Tvergaard-Needleman (GTN) material model.

Abstract: This paper presents results from quasi-static and high rate tensile testing of three aluminum sheet alloys, AA5754, AA5182 and AA6111, all of which are candidates for replacing mild steel in automotive bodies. Tests were performed at quasi-static rates using an Instron apparatus and at strain rates of 600 to 1500 s-1 using a tensile split Hopkinson bar. Additionally, an in-depth investigation was performed to determine the levels of damage within the materials and its sensitivity to strain rate. The constitutive response of all of the aluminum alloys tested showed only mild strain rate sensitivity. Dramatic increases in the elongation to failure were observed with increases in strain rate as well as greater reduction in area. Additionally, the level of damage was seen to increase with strain rate.