Microstructural inhomogeneity and biaxial stretching limits in aluminium alloy AA6016

Abstract The ductility of sheet metals in the regime where both principal in-plane strains are positive is strongly dependent on macroscopic inhomogeneity of the sheet. This can be clearly seen when the grain size becomes large compared with the sheet thickness. However, even with quite small grain sizes there is a tendency in aluminium alloys for grains of similar orientation to occur in colonies, and this complicates any simple correlation between grain size and limit strains in stretching. Sheets of the alloy AA6016 Al–Mg–Si were heat treated to give a wide difference in grain size, and the microstructure characterised using electron back scattered diffraction (EBSD) electron microscopy, which can reveal the presence of orientation colonies. Marciniak driving blank tests were used to reveal the development of dimensional inhomogeneity and the limit strains in balanced biaxial tension. An approach using the Marciniak–Kuczynski model with yield functions derived from texture data and the ‘defect’ associated with the measured inhomogeneity of crystallographic texture gives first-order predictions of the limit strains in this material, although issues of texture evolution and adequate quantification of inhomogeneity need further consideration.

[1]  R. Becker,et al.  Effects of strain localization on surface roughening during sheet forming , 1998 .

[2]  On the directionality of strain localization when stretching aluminum alloy sheets in biaxial tension , 1986 .

[3]  Frédéric Barlat,et al.  Crystallographic texture, anisotropic yield surfaces and forming limits of sheet metals , 1987 .

[4]  A. Needleman,et al.  Sheet Necking—I. Validity of Plane Stress Assumptions of the Long-Wavelength Approximation , 1978 .

[5]  W. T. Roberts,et al.  Effect of Grain anisotropy on limit strains in biaxial stretching: part i. influence of sheet thickness and grain size in weakly textured sheets , 1981 .

[6]  P. Houtte,et al.  The Incorporation of Texture-Based Yield Loci Into Elasto-Plastic Finite Element Programs , 1995 .

[7]  F. J. Humphreys.  Review Grain and subgrain characterisation by electron backscatter diffraction , 2001 .

[8]  Z. Marciniak,et al.  Influence of the plastic properties of a material on the forming limit diagram for sheet metal in tension , 1973 .

[9]  P. Bate.  The prediction of limit strains in steel sheet using a discrete slip plasticity model , 1984 .

[10]  H. Wenk,et al.  Texture and Anisotropy , 2004 .

[11]  Brent L. Adams,et al.  Influence of surface texture on orange peel in aluminum , 1998 .

[12]  J. Bryant,et al.  Analysis of ridging in aluminum auto body sheet metal , 1998 .

[13]  P. Bate.  Texture inhomogeneity and limit strains in aluminium sheet , 1992 .

[14]  P. B. Mellor,et al.  Predictions of limit strains in sheet metal using a more general yield criterion , 1978 .

[15]  R. Hill,et al.  On discontinuous plastic states, with special reference to localized necking in thin sheets , 1952 .

[16]  On the role of texture development in the forming limits of sheet metals , 1996 .

[17]  R. Knutsen,et al.  A microtexture based analysis of the surface roughening behaviour of an aluminium alloy during tensile deformation , 1999 .

[18]  Z. Marciniak,et al.  Limit strains in the processes of stretch-forming sheet metal , 1967 .

[19]  W. T. Roberts,et al.  Effects of grain anisotropy on limit strains in biaxial stretching: part ii. sheets of cubic metals and alloys with well-developed preferred orientations , 1981 .