Textures in Strip-Cast Aluminum Alloys: Their On-Line Monitoring and Quantitative Effects on Formability. Final Technical Report

摘要

Aluminum sheets produced by continuous casting (CC) provide energy and economic savings of at least 25 and 14 percent, respectively, over sheets made from conventional direct chill (DC) ingot casting and rolling. As a result of the much simpler production route in continuous casting, however, the formability of CC aluminum alloys is often somewhat inferior to that of their DC counterparts. The mechanical properties of CC alloys can be improved by controlling their microstructure through optimal thermomechanical processing. Suitable annealing is an important means to improve the formability of CC aluminum alloy sheets. Recrystallization of deformed grains occurs during annealing, and it changes the crystallographic texture of the aluminum sheet. Laboratory tests in this project showed that this texture change can be detected by either laser-ultrasound resonance spectroscopy or resonance EMAT (electromagnetic acoustic transducer) spectroscopy, and that monitoring this change allows the degree of recrystallization or the ''recrystallized fraction'' in an annealed sheet to be ascertained. Through a plant trial conducted in May 2002, this project further demonstrated that it is feasible to monitor the recrystallized state of a continuous-cast aluminum sheet in-situ on the production line by using a laser-ultrasound sensor. When used in conjunction with inline annealing, inline monitoring of the recrystallized fraction by laser-ultrasound resonance spectroscopy offers the possibility of feed-back control that helps optimize processing parameters (e.g., annealing temperature), detect production anomalies, ensure product quality, and further reduce production costs of continuous-cast aluminum alloys. Crystallographic texture strongly affects the mechanical anisotropy/formability of metallic sheets. Clarification of the quantitative relationship between texture and anisotropy/formability of an aluminum alloy will render monitoring and control of its texture during the sheet production process even more meaningful. The present project included a study to determine how the anisotropic plastic behavior of a continuous-cast AA 5754 aluminum alloy depends on quantifiable texture coefficients. Formulae which show explicitly the effects of texture on the directional dependence of the q-value (a formability parameter) and of the uniaxial flow stress, respectively, were derived. Measurements made on a batch of as-received AA 5754 hot band and its O-temper counterpart corroborate the validity of these formulae. On the other hand, these measurements also indicate that some microstructure(s) other than texture could play a significant role in the plastic anisotropy of the AA 5754 alloy. For the q-value of a set of O-temper samples of this alloy, the additional microstructure that affects plastic anisotropy was shown to be grain shape. A formula that captures both the effects of crystallographic texture and grain shape on the q-value of the O-temper material was derived. A simple quadratic plastic potential that delivers this q-value formula was written down. Verification of the adequacy of this plastic potential, however, requires further investigations.