Effect of Solutionizing Heat Treatment and Hot Uniaxial Tensile Deformation on Ductility and Subsequent Age-hardening Characteristics of AA7075 Aluminum Sheet

Abstract

Aluminum alloy AA7075 in sheet form has been extensively used in the past for structural applications due to its higher strength to weight ratio compared to the other aluminum structural alloys. In recent years, it has attracted much interest in the automotive industry for making parts by hot stamping due to its higher potential for weight reduction and albeit its poor room temperature formability compared to the other softer lower strength aluminum alloys. Hot stamping involves heating the blank to the solution heat treatment (SHT) temperature, followed by hot stamping and in-die quenching to generate a supersaturated solid solute (SSSS). The formed part is then artificially aged to achieve a T6 peak-strength. The hot deformation behavior and final strength of AA7075 sheet is dictated by the microstructural evolution, during hot stamping, which in turn is governed by the pre-forming heat treatment variables such as cooling rate, holding time and intermediate deformation temperature. In the present work, the pre-forming heat treatment variables were systematically varied using a dilatometer based mechanical test system to establish a comprehensive understanding of the effect of process variables on microstructural evolution and corresponding mechanical properties of AA7075 in uniaxial tensile mode, and under solutionized and non-solutionized conditions. The relationships between formability (including strain heterogeneity, strain localization, void damage and fracture) and microstructural and texture characteristics (during and post deformation) of this alloy were investigated using in-situ uniaxial tension, ex-situ SEM (SEM-EBSD and SEM-fractography), X-ray diffraction and X-ray computed tomography methods under solutionized and non-solutionized conditions. Further, the effect of hot stamping process variables along with a typical pre-strain value on artificial aging response was also investigated. The solutionized samples performed better than the non-solutionized samples when deformed at 400 °C and showed considerably higher elongation compared to 300 °C. The non-solutionized samples at 450 °C produced better elongation compared to the solutionized samples. It was found that the arrangement of precipitates with respect to the grain boundaries played a critical role in plasticity, flow localization, damage development and fracture at the various deformation temperatures.