Olaf Engler

Texture control by thermomechanical processing of AA6xxx Al–Mg–Si sheet alloys for automotive applications—a review

Abstract The properties of Al-alloys for car body applications are largely controlled by microstructure and crystallographic texture of the final sheets. In this paper, the impact of texture on formability and, in particular, on surface appearance of the sheets is reviewed. The paper summarizes the principles of microstructure and texture evolution during the main steps of the thermomechanical processing of age-hardenable Al–Mg–Si sheets (6 xxx series alloys). The most important parameters that may be used to modify the textures and hence to improve the resulting properties are outlined.

Effect of intermediate annealing on texture, formability and ridging of 17%Cr ferritic stainless steel sheet

Abstract In order to improve the formability of ferritic stainless steel (FSS) sheets, the crystallographic texture of the sheets was modified by way of an intermediate annealing (IA) during the cold rolling. In the as-received hot band pronounced through-thickness texture gradients with a strong rotated cube orientation in the sheet center layer were observed. This strong initial texture has an impact on the texture and the resulting properties, including the undesired appearance of ridging, in the final recrystallized sheet. Instead of forming the {111}//ND γ-fiber texture typical of recrystallized low-carbon steel sheets, the recrystallization texture was notably shifted towards {334}〈4 8 3〉. Introduction of an intermediate anneal during the cold rolling led to a weaker, more typical bcc rolling texture with less pronounced texture gradients. Accordingly, recrystallization gave rise to the characteristic γ-fiber texture. The evolution of the different rolling textures is interpreted by means of texture simulations based on the Taylor deformation theory. The recrystallization textures are explained in terms of a preferred nucleation of recrystallization at the preexisting grain boundaries and a subsequent growth selection. The improvement in sheet formability through the IA is demonstrated by means of analysis of the R-values. Ridging is assessed through surface profile measurements.

Deformation and texture of copper-manganese alloys

Abstract Manganese is the only element that can be dissolved in copper up to 12% without markedly changing the stacking fault energy (SFE). Hence, investigation of the deformation behaviour of Cu–Mn alloys may yield insight into the influence of alloying elements apart from the (usual) decrease in SFE. The development of the crystallographic texture is analysed in various Cu–Mn alloys that were deformed either in compression or by cold rolling. The texture development is simulated by means of a novel materials-dependent Taylor-deformation model. The spatial arrangement of the rolling texture orientations in the as-deformed microstructure is studied by an orientation-sensitive etching technique. Despite the constant SFE, with increasing Mn content the rolling textures change from the pure metal (copper) type towards the alloys (brass) type. This effect is attributed to the altered deformation behaviour as well as the increased tendency to form shear bands due to the large increase in strength of the Cu–Mn alloys.

Randomization of the annealing texture in aluminum 5182 sheet by cross-rolling

Abstract In order to obtain a randomization of the annealing texture in aluminum AA5182 sheet, the cold-rolling texture is modified through introduction of an additional step of cross-rolling during the deformation. The β-fiber orientations that form during the preceding normal cold-rolling are unstable under cross-rolling conditions. Rather, preferred orientations with {011}//ND (α-fiber) develop. A proper control of the respective reduction degrees before and after the cross-rolling leads to the formation of strong orientation densities along the α-fiber. After recrystallization annealing, the cross-rolled samples depict a very weak, almost random texture. This texture randomization is explained by a suppression of both the nucleation and the subsequent growth of the {001}〈100〉 cube orientation which prevails in normally rolled and recrystallized Al sheets. Because of the resulting homogeneous planar anisotropy (R≈1) of the randomly textured sheets, a better sheet formability was obtained than in conventional sheets with a pronounced cube texture.

Influence of deformation variables on the formation of copper-type shear bands in Al–1Mg

Abstract The formation of copper-type or grain-scale shear bands in Al–1Mg during plane strain compression (PSC) is described as a function of strain, strain rate and deformation temperature. Shear banding is observed to decrease dramatically in strength over a small range of the Zener–Hollomon parameter ( Z ) at Z approximately equal to 1.0×10 14 s −1 . This sharp transition in shear band formation is seen to correlate exactly with a sharp increase in dynamic recovery during deformation, as inferred from flow stress measurements, hardness measurements and microstructural analysis of as-deformed specimens. The observed correlation of shear band formation and dynamic recovery during deformation provides further insight into the basic mechanisms of shear band formation. The influence of shear bands on deformation texture is also investigated and is seen to affect the relative intensities of, and precise position along, the β-fibre.

Evolution of the cube texture in high purity aluminum capacitor foils by continuous recrystallization and subsequent grain growth

The evolution of the cube-texture in high purity aluminum foils which were produced with a high level of cold reduction and a low finishing temperature is tracked by analysis of microstructure and crystallographic texture. The formation of the cube-texture proceeds in two consecutive steps. (1) During the first stages of annealing, the deformed substructure is transformed into a very fine grained recrystallized microstructure through extended recovery reactions (continuous recrystallization); the rolling texture is retained. (2) Subsequently, the cube-orientation becomes prominent by discontinuous grain growth under consumption of the continuously recrystallized microstructure.

On the influence of orientation pinning on growth selection of recrystallisation

Abstract With the help of techniques for local orientation determination it has been proved that the recrystallisation textures of Al-alloys evolve by a preferred formation of some orientations at characteristic nucleation sites and a subsequent growth selection of distinct orientations out of this spectrum of nucleus orientations. As an alternative to the standard explanation of a favoured growth of grains with a special orientation relationship to the deformed matrix (e.g. 40° 〈111〉), recently the concept of “orientation pinning” has been proposed, which considers that the growth selection is determined by special orientation relationships being disfavoured (low-angle and twin grain boundaries). The present paper reviews the two rivalling theories of oriented nucleation and oriented growth to explain recrystallisation textures and addresses the new approach of orientation pinning. New EBSD-results on commercial purity Al which substantiate a growth selection of the exact cube-orientation out of the nucleus spectrum available in the cube-bands and the R-orientation out of the nucleus spectrum of the grain boundaries are presented. A semi-quantitative model to simulate the effect of orientation pinning on the evolution of recrystallisation textures is introduced. The results are discussed in view of growth selection and the 40° 〈111〉 orientation relationship.

Deformation and recrystallization of hexagonal metals: modeling and experimental results for zinc

A polycrystal approach that divides the grains into small cells and accounts for local interactions in a self consistent way is used to calculate deformation and texture evolution of hexagonal zinc. As this model incorporates local effects, it predicts intragranular deformation and gives a description of the deformed microstructure in terms of misorientation between elements and variation in stored energy. This provides information which can be used as a basis for simulating recrystallization processes. The grains are composed of parallelepipedic cells, and a Monte Carlo algorithm is used for simulating static recrystallization. Nucleation and boundary mobility depend on the misorientation between cells and on the local variation in stored energy. The model is applied to simulate the kinetics of static recrystallization and the associated change in crystallographic texture in zinc polycrystals. Experimental results obtained by deforming zinc in plane strain compression compare well with the predictions and are consistent with a mechanism where nucleation occurring in highly deformed domains controls the recrystallization kinetic.

Moderation of the recrystallization texture by nucleation at copper-type shear bands in Al-1Mg

Abstract The critical dependency of copper type shear band formation on deformation variables in Al-1Mg has been described (Acta mater., 49 (2001) 2739). The influence of copper type shear bands on recrystallization behaviour during post-deformation annealing in Al-1Mg is now discussed. Local orientation measurements and orientation images of grains nucleating at shear bands have been made using the electron back-scatter diffraction (EBSD) technique. Bulk recrystallization texture is investigated from X-ray diffraction (XRD) measurements. Shear bands are shown to be potent nucleation sites during annealing and a weak, almost random, texture associated with nucleation at shear bands has been inferred. Most significantly, shear band formation is found to have a pronounced moderating influence on the strength of the potentially dominant cube component of the recrystallization texture.

Recrystallisation textures in copper-manganese alloys

Manganese is the only alloying element that does not lower the (normalised) stacking fault energy (SFE) of copper-alloys. Therefore, analysis of the recrystallisation behaviour of Cu-Mn alloys may yield information on the impact of solute atoms on the recrystallisation, independent of the influence of the SFE. For this purpose the progress of recrystallisation is tracked in various Cu-Mn alloys by hardness measurements and X-ray texture investigations. To support the results obtained by macrotexture analysis, and in particular to trace the origin of the recrystallisation texture orientations, local orientation measurements are performed by means of electron back scattering diffraction (EBSD). The recrystallisation textures as well as data on the recrystallisation kinetics of the present Cu-Mn alloys are compared to results obtained in other binary Cu-alloys.