Stress corrosion behavior and microstructure analysis of Al-Zn-Mg-Cu alloys fabricated by CMT wire arc additive manufacturing with different post-treatments

Abstract

In this study, Al-Zn-Mg-Cu alloys components were fabricated by cold metal transfer (CMT) wire arc additive manufacturing (WAAM), and the stress corrosion cracking (SCC) behavior of the alloys with different post-treatments was studied by combining in-situ electrochemical impedance spectroscopy (EIS) with slow strain rate tensile. The hot-rolling process was employed to close the internal pores of the WAAM products, following by single-stage aging (T6) to further improve the mechanical properties of the products. The elongation loss and SCC susceptibility index of T6 sample are 38.0% and 43.0%, respectively, which exceed those of the hot-rolled sample at 35.5% and 37.0%. Furthermore, the corrosion fracture of the T6 sample exhibits intensive secondary cracks, indicating an increased SCC susceptibility. The stress corrosion process is classified into four stages based on the trend of the EIS: (Ⅰ) Initial stage of corrosion; (Ⅱ) Corrosion products accumulation stage; (Ⅲ) Corrosion products coverage stage; (Ⅳ) Approaching fracture stage. The hot-rolled samples have reduced SCC susceptibility due to the presence of coarse grain boundary precipitates (GBPs) which can act as the irreversible traps to capture hydrogen. The ultimate tensile strength and elongation of T6 sample reached 568.3 ± 11.5 MPa and 9.1 ± 0.7%, respectively. However, the small-sized GBPs and wider precipitates-free zone reduced its SCC resistance. This study found that the hot-rolling process can fully exploit the mechanical property potential of Al-Zn-Mg-Cu alloys and improve their SCC resistance, which is an important guiding significance for the practical application of WAAM technology in Al-Zn-Mg-Cu alloys.