Warm Aging of Pre-aged AA6013 Sheet and Its Relevance to Room Temperature and Warm Forming Applications—Experimental and Modeling Analyses

Abstract

Warm forming has been shown to be an effective way to increase the formability of various aluminum sheet alloys. If applied to precipitation hardenable alloys, aging and/or coarsening may occur during heating and/or forming processes. As such, there is a potential to leverage the warm forming process to artificially age precipitation hardenable alloys. In this study, the aging characteristics of a pre-aged Al-Mg-Si-Cu alloy (designated AA6013-PA and comprising a 4 h, 100 °C pre-aging cycle following solutionization) were examined using: (i) room temperature tensile and (ii) bendability experiments, as well as (iii) elevated temperature tensile and formability experiments in the temperature range of 220-280 °C. In the pre-aged condition, the alloy exhibited very high work hardening, elongation and bendability. It was found that T6-like properties can be obtained with short duration secondary aging of the AA6013-PA starting temper (410 s at 235 °C), followed by a paint bake cycle (30 min at 177 °C). In such heavily aged conditions, the bendability is found to be moderate and consistent with a T6 temper. Bendability of the pre-aged alloy is shown to be enhanced by reducing the extent of aging, which may be beneficial for applications requiring energy absorption during crash, for example. In the pre-aged condition, AA6013 displayed a 55% improvement in room temperature formability, relative to the T6 condition, for near plane-strain loading using a Nakazima punch, although neither temper exhibited formability improvements through warm forming. The aging kinetics of AA6013-PA were determined from mechanical test data and used to calibrate an aging kinetics model. A strong obstacle model was used to relate precipitation state to yield strength, for artificial aging in the range 220-280 °C. The incremental strength increase due to exposure to an automotive paint cure cycle was also modeled and found to yield accurate results. Warm deformation, as well as room temperature deformation, were shown to enhance the paint bake response of the alloy, reducing the time to the peak aged condition by up to 14% for artificial aging at 177 °C.