Materials aspects of age-forming of Al-xCu alloys

The age-forming technique has been utilized in the manufacturing process of integrally stiffened structures to improve performance and to significantly reduce fabrication costs of aerospace structures. To better evaluate the technique and to simulate the overall mechanical behavior of the manufacturing parts, using Al-xCu alloys as model materials, we studied the associated materials aspects - age hardening during the age-forming process, particularly the stress-aging effects on precipitation and on the resulting yield strength of aluminum alloys. We found that a relatively high stress, e.g. > 100 MPa or > YS (yield strength of the material prior to aging treatment) in Al-4Cu alloy, was required to generate significant plastic deformation. While the kinetics of the stress-age-hardening was not much different than that of the conventional one, there were some special features of the process. Different from the situation of the plastic deformation prior to the aging (the T8 temper) when precipitates are formed heterogeneously on dislocations, application of an elastic stress during aging may significantly affect the orientation distribution of plate-shaped precipitates. We examined the aging conditions (stress, temperature and alloy concentration) for the formation of the aligned precipitate structures in single crystal and cube-textured polycrystalline Al-xCu alloys. These observations were, more or less, reflected in the results of our computer simulations of the concomitant precipitation processes under unidirectional stresses. Both mechanical testing of Al-xCu alloys and computer simulations of dislocation-slip processes through unshearable particles indicated that the alignment of the plate-shaped precipitates led to lower yield stresses of the stress-aged specimens when compared with the free-stress-aged specimens where randomly oriented plates (on their habit planes) were formed. (C) 2001 Elsevier Science B.V. All rights reserved.