Estudo do efeito de pulsos elétricos no auto-reparo da liga pré-deformada AA7075 (Al-Zn-Mg-Cu): comportamento mecânico e evolução microestrutural

Abstract

Over the past few years, research into self-healing materials has gained increasing scientific attention. In this class of bioinspired materials, engineers and scientists rely on biological mechanisms to design new materials with unique characteristics. Advances in this sense are widely noted in several classes of materials. Specifically for metallic materials, processing by electrical pulses, EPT (Electropulsing Treatment) is a new approach to self-healing that is increasingly important in technology. Applying a controlled current of sufficiently high density but with a low-temperature rise generates an electron flow with an energy capable of modifying the microstructure, which may promote stress relief, the reduction of microvoids and microcracks, or even the recrystallization. Given the importance of techniques that increase the useful life of aeronautical components, the purpose of this work is to establish a methodology with the development of appropriate parameters for the self-healing process via electrical pulses in the aluminum alloy AA 7075 (Al-Zn-Mg -Cu). Compositional and phase analysis was performed via X-ray fluorescence (XRF) and X-ray diffraction (XRD). The mechanical behavior and generation of crystalline defects were previously evaluated in specimens machined according to the ASTM E8 standard under interrupted tensile tests at deformations of 60%, 70%, 80%, 90%, and 95% relative to rupture, increasing yield stresses and reduction of ductility. The most hardened samples (90 and 95% deformed) were treated via EPT with peak current (Ip) of 400 A, with a current density that varied from 23.5 A/mm2, initially for specimens of cross-sectional area of 17 mm2, and 50 A/mm2 for specimens with a reduced section area of 8 mm2. Experimentally, a specimen was tested with an Ip of 500 A and a current density of 29.4 A/mm2; These samples used 0 A base current (I b ), peak time (T p ) and base time at 0.1 s (T b ). in a total treatment time of 7s, 14s and 21s in cycles of 35, 70 and 105 respectively. The most critical condition, referring to samples deformed to 95% of rupture, treated by EPT (I p of 400 A) and current density of 50 A/mm2 with 7s, 14s, and 21s, was analyzed via residual stresses by XRD, proving the relief of residual stresses promoted by EPT due to of the application period, agreeing with the behavior under tension. The microstructural analysis considered the notched samples to generate defects. A peak current Ip of 400 A was used with a current density of 80 A/mm2, with a base time (tb) of 0.1 seconds and peak residence time varying from 0.1 to 0.3 seconds. The results showed that there was microstructural evidence in the total and partial closure of microcracks in micrographs observed via field emission scanning electron microscopy (SEM-FEG), thus corroborating the results obtained in mechanical tests and via x-ray residual stress.