Abstract
A non-isothermal ageing process was proposed for an Al-Zn-Mg-Cu alloy aiming to accommodate the slow heating/cooling procedure during the ageing of large components. The evolution of microstructure and microchemistry was analyzed by using transmission electron microscopy, high-angle annular dark field imaging, and energy dispersive spectrometry. The age-hardening of the alloy was examined to evaluate the strengthening behavior during the non-isothermal process. The corrosion behavior was investigated via observing the specimens immersed in EXCO solution (solution for Exfoliation Corrosion Susceptibility test in 2xxx and 7xxx series aluminum alloys, referring ASTM G34-01). Secondary precipitation was observed during the cooling stage, leading to increased precipitate number density. The distribution of grain boundary precipitates transits from discontinuous to continuous at the cooling stage, due to the secondary precipitation’s linking-up effect. The solutes’ enrichment on grain boundary precipitates and the depletion in precipitate-free zones develops during the heating procedure, but remains invariable during the cooling procedure. The corrosion in NIA (Non-isothermal Ageing) treated specimens initiates from pitting and then transits to intergranular corrosion and exfoliation corrosion. The transition from pitting to intergranular corrosion is very slow for specimens heated to 190 °C, but accelerates slightly as the cooling procedure proceeds. The transition to exfoliation corrosion is observed to be quite slow in all specimens in non-isothermal aged to over-aged condition, suggesting a corrosion resistance comparable to that of RRA condition.
Highlights
The Al-Zn-Mg-Cu series of aluminum alloys have been widely used for fabrication of bearing components, including wing spars, stringers, bulkheads, and so on, in the aircraft industry, for the past a few decades [1,2,3,4,5,6,7]
HA120, which is of under-aged condition, The treatment during the cooling stage, revives the selective corrosion of grain boundaries (GBs), and leads to exhibits high susceptibility to intergranular corrosion (IGC) and exfoliation corrosion (EFC)
The enrichment of Cu in grain boundary precipitates (GBPs), as well as the depletion of Zn and Mg in precipitate free zones (PFZs), becomes evident when the specimen was put into procedures with elevated temperature, which is consistent to that noted in previous studies [41,42]
Summary
The Al-Zn-Mg-Cu series of aluminum alloys have been widely used for fabrication of bearing components, including wing spars, stringers, bulkheads, and so on, in the aircraft industry, for the past a few decades [1,2,3,4,5,6,7]. Besides optimizing the composition [17,18,19], different ageing processes have been established to balance mechanical performances and LC resistance to meet comprehensive requirements from aircraft fabrication [20,21,22,23,24]. The development of T77 ageing has presented an effective way for obtaining satisfactory LC resistance without compromising mechanical performance [23,24,25], which has greatly promoted the application of 7150 and 7055 Al alloys [1,2,3,4]. The initiating and the developing of LC is believed related to the microstructure that established during the ageing. Comparison in microstructure and the corrosion process with that in the T6 and RRA (Retrogression Re-Aging) condition was systematically carried out to evaluate the particularity of the NIA treating
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