J.G. Morris

Dynamic strain aging in aluminum alloys

Abstract The athermal component of flow stress, as it is normally determined, is frequently in error for polycrystalline alloys of aluminum. The reason is that a transient occurs in the flow stress-deformation temperature curve for these alloys the character of which is a result of dynamic strain aging. The intensity of dynamic strain aging is a function of the initial structural state of the alloy as well as of its chemical composition. Dynamic strain aging tends to interfere with the normal tendency of an aluminum alloy when worked to form a distinct dislocation cell structure; the tendency to produce a non-cellular array of dislocations increases with an increase in the intensity of dynamic strain aging. Dynamic strain aging contributes significantly to the mechanical strengthening of aluminum alloys. Thermomechanical processing procedures which control this phenomenon allow unusual mechanical properties to be developed in aluminum alloys.

An investigation of serrated yielding in 5000 series aluminum alloys

Abstract The effect of different thermal treatment temperatures (from 472 to 783 K) on the characteristics of serrated yielding of three commercial aluminum alloys, AA5052, AA5754 and AA5182, was investigated. In the high temperature treatment range, the stress drop (Δ σ ) decreases with increasing thermal treatment temperature. This is the result of an increase in grain size as the thermal treatment temperature increases and can be explained from the solute–dislocation interaction model. For these alloys without any thermal heat treatment after cold rolling and those with a 472 K heat treatment after cold rolling, there is a critical strain before the onset of serrated yielding. The critical strain is larger for those after 472K heat-treatment as compared to those without heat-treatment. This result can be explained by the precipitation of Mg atoms from the solid solution when heat-treated at 472 K.

Comparison of recrystallization and recrystallization textures in cold-rolled DC and CC AA 5182 aluminum alloys

The recrystallization and recrystallization textures in cold-rolled direct chill cast (DC) and continuous cast (CC) AA 5182 aluminum alloys were investigated. The recrystallization behavior of cold-rolled DC and CC AA 5182 aluminum alloys was evaluated by tensile properties. The evolution of recrystallization textures in cold-rolled DC and CC AA 5182 aluminum alloys was determined by X-ray diffraction. The results showed that the recrystallization temperature of cold-rolled DC AA 5182 aluminum alloy was somewhat lower than that of cold-rolled CC AA 5182 aluminum alloy. The resulting recrystallization textures of cold-rolled AA 5182 aluminum alloy were characterized by the strong R orientation and the cube orientation with strong scattering about the rolling direction towards the Goss orientation. CC AA 5182 aluminum alloy showed slightly weaker recrystallization textures than DC AA 5182 aluminum alloy.

Kinetics of the formation of the β fiber rolling texture in continuous cast AA 5xxx series aluminum alloys

Abstract The texture evolution in cold rolled continuous cast AA 5xxx series aluminum alloys was investigated by X-ray diffraction. The volume fraction of the β fiber component was calculated by integration of the ODF intensities within 15.5° of the center position of the β fiber. The relationship between the volume fraction of the β fiber component and true strain follows the Avrami equation.

Lattice rotation of the cube orientation to the β fiber during cold rolling of AA 5052 aluminum alloy

Abstract The hot bands and annealed hot bands of AA 5052 aluminum alloy were cold rolled to different reductions ranging from 0% to 92%. The ODFs of cold rolled samples were determined by X-ray diffraction. The rotation path of the cube orientation to the β fiber was determined based on the variation in the orientation distribution function with reduction, and the stability of orientations on the β fiber was discussed.

Texture, microstructure and formability of SC and DC cast Al-Mg alloys

Abstract Texture, microstructure and formability were studied in direct chill (DC) cast and strip cast (SC) Al–Mg alloys (AA5754 and AA5182) and were found to largely depend on the production processes (DC versus SC) and chemical compositions. DC hot bands are usually fully recrystallized and thus have strong recrystallization textures while SC hot bands have deformation structures and thus strong deformation textures. It is difficult to obtain strong recrystallization textures in SC hot band materials with elevated temperature annealing. However, annealed SC5754 hot bands have stronger recrystallization texture components than annealed SC5182 hot bands. This is due to the fact that a stronger dynamic precipitation occurs in SC5182 hot bands during annealing. As a result, annealed SC5754 hot bands have small 90° earing while SC5182 hot bands have both 90° earing and 45° earing after annealing. Cross rolling of materials with a deformation structure causes a significant texture change in the final cold rolled samples while it causes no texture change in materials with a recrystallized structure. Compared to samples after straight rolling, cross rolling reduces the volume fraction of the deformation texture components (especially the copper component) with little change in recrystallization texture components. Al–Mg alloys are strong and difficult to work. However, the workability can be significantly improved after annealing at an elevated temperature.

Microstructure and texture evolution of Al during hot and cold rolling

The evolution of microstructure and texture of commercial purity Al during hot and cold rolling has been studied. The results show that the dynamic restoration mechanism for Al rolled to a total equivalent strain of 2.66 at a mean equivalent strain rate of 14.4 s-1 at 510 °C is essentially dynamic recrystallization (DRX), whereas for those materials deformed to lower strains at lower strain rates at this temperature, the restoration mechanism is mainly dynamic recovery (DRV). This is confirmed by examining the microstructures, textures, and substructures of the material developed during hot rolling as well as by comparing the results with those developed during cold rolling and annealing. The texture analysis using orientation distribution functions (ODFs) indicates that the dynamically recrystallized material has a random orientation distribution, whereas dynamically recovered materials have a developed deformation texture with a predominantD component and a Cu component. The substructure observation by transmission electron microscopy (TEM) indicates that the subgrains in the dynamically recrystallized material are completely dynamically recovered, which is very similar to those subgrains in cold-rolled material. However, the annealed material shows a completely different substructure. By studying all of these structural features, which are closely associated with the dynamic restoration mechanism, it is confirmed that Al undergoes DRX in the present work, which is different from either DRV or static recrystallization (SRX).

Effect of initial texture on texture evolution in cold-rolled AA 5182 aluminium alloy

AA 5182 aluminium alloy with a strong cube texture was cold rolled to different reductions along the following directions: firstly, the original rolling direction; secondly, at angles of 22.5 and 45° to the original rolling direction. The evolution of texture in the cold-rolled samples with different initial textures was then investigated by X-ray diffraction. The texture evolution was quantified by mathematical formulae of texture volume fractions and rolling true strain. The results show that initial texture has a strong influence on the evolution of rolling texture. AA 5182 aluminium alloy with an initial rotated-cube (r-cube) (45° normal direction r-cube) texture exhibits the fastest rate of formation of the β fibre. The rate of formation of the β fibre decreases as the initial texture changes from the r-cube texture to the cube texture. The relationship between the rate of formation of the β fibre (k β value in the mathematical formula for the volume fraction of the β fibre) and the initial texture (M cube and M r-cube: the volume fractions of the cube and r-cube components respectively) can be expressed as k β  = 0.37 − 0.03(M cube /5.41 − M r − cube /5.64).

The effect of structure on the mechanical behavior and stretch formability of constitutionally dynamic 3000 series aluminum alloys

Abstract For a better understanding of the forming mechanism, microstructural changes occurring during processing and their effects on mechanical behavior and stretch formability were studied. The features studied in stripcast and d.c. cast Al-1.25 Mn alloys (3003) (where the composition is given in approximate weight per cent) were (1) the cast structure, (2) the degree of solute supersaturation of the matrix, (3) the size and distribution of second-phase particles, (4) the grain size and (5) the degree of preferred grain orientation produced in materials by selected processing procedures. The structural variables were obtained by a thermomechanical process which indluced (a) preheat treatment, (b) cold rolling and (c) annealing. Concerning mechanical behavior, the main interests were the tensile yield strength, ultimate tensile strength, uniform and non-uniform elongation, strain-hardening exponent (n value) and strain rate sensitivity parameter (m value), all of which are known to have significant influence of formability. Forming limit diagrams were determined by the use of a punch stretch test. In this study the limitation of n and m values in controlling exclusively the limit strains in stretch formibility tests is pointed out and further the significance of microstructure in obtaining high limit strains is indicated. Any quantitative mathematical model of formability must incorporate certain microstructural features of the material.

The effect of structure on the strain rate sensitivity of 3003 AlMn alloy

The effect of initial structure on the strain rate sensitivity of 3003 commercial aluminum alloy was studied as a function of strain at temperatures of 25° C and −196° C. The strain rate sensitivity was found to be critically dependent on the initial structure, test temperature and strain, and thus on the instantaneous structure. The change of flow stress with an increase in strain rate resulted in positive, K > 0[K = (dσ/ d logdote)T], negative, K < 0, and insensitive, K = 0, strain rate effects depending on the initial structure and the test temperature used. The parameter K was found to increase as the strain increased. To explain the variation in the character of K with structure, strain and test temperature, structural arguments are invoked associating the observed strain rate effects with a stress/strain induced solute disposition instability. A model is proposed relating the strain rate effects to the inherent stability of the initial structure and to the dynamic deformation conditions.