Investigation On Susceptibility To The Localized Corrosion Of Aluminium Alloy7050Thick Plate

Abstract:Aluminium alloy7050thick plate is an important aeronautic material, and has been widely used as the main structural components in aeroplanes like frames, longerons and wallboard, etc. However, the aluminium alloy7050thick plate in service is usually susceptible to exfoliation corrosion and stress corrosion cracking (SCC) due to the effect of aggressive medium and tensile stress. To improve the resistance to localized corrosion of the aluminium alloy7050thick plate and to meet the production need for the big airplane project, the corrosion behavior of the alloy7050in a solution containing Cl-1ion was studied using immersion tests, electrochemical tests and complementary techniques like optical microscopy, scanning electron microscopy, transmission electron microscopy and scanning Kelvin probe force microscopy. Also, the influences of solution heat treatment, quenching, pre-stretching and two-step ageing on the microstructure and corrosion resistance, and the corrosion anisotropy of the thick plate were investigated. The main conclusions are as follows:(1)The localized corrosion initiated from coarse second phases in the aluminium alloy7050plate. For example, the residual submicron Al2CuMg phase had an anodic character and its Volta potential relative to the surrounding Al matrix was about-100～-500mV, while primary Al7Cu2Fe phase had a cathodic character and its Volta potential difference with respect to the matrix ranged between+100and+600mV; the nature of the isolated Al2CuMg phase changed with the increase of immersion time, which led to the decrease in rate of corrosion and little emergence of corrosion micro-cracks; clustering of the isolated Al2CuMg and Al7Cu2Fe phases which have different natures will accelerate corrosion attack and produce micro-cracks. With the increase of single-stage solution temperature, the volume fraction of the residual phases in the aluminium alloy7050plate reduced gradually resulting in low corrosion susceptibility, while the volume of the recrystallized grains increased with solution temperature which induced the decrease of both the strength and the corrosion resistance; the optimal solution process was470℃/2.5h for single-stage solution. Through multi-stage solution heat treatments, the aluminium alloy7050thick plate can obtain high strength and high resistance to localized corrosion; the optimal two-stage solution process was450℃/1.5h+480℃/0.5h; Step-to-step solution heat treatment can obtain the highest KISCC value (about17.8MPa-m1/2).(2)The corrosion morphologies of the aluminium alloy7050thick plate were determined by the coverage ratio of η-MgZn2precipitates at grain boundaries. The coverage ratio of η-MgZn2precipitates at grain boundaries and sub-grain boundaries increased one after another with prolonging quenching transfer time, leading to the corrosion morphologies of the aluminium alloy7050thick plate transformed gradually from intragranular corrosion to large intergranular corrosion (IGC) network and fine IGC network. Overageing treatment can promote the diffusion of copper element at grain boundary precipitates (GBPs) and hence effectively reduced the high sensitivity to IGC of the7050alloy. After the copper content in GBPs had reached about10at.%(an equilibrium state), however, the IGC susceptibility of the aluminium alloy7050thick plate will show a modest decrease with the increase of ageing degree.(3)The pre-stretching amount should be controlled in about2%. With the increase of pre-stretching deformation degree, the GBPs size, neighbour distance and PFZ width in the alloy7050plate first increased slightly and then decreased which led to change of the GBPs distribution from interrupted distribution to continuous distribution and the gradual decrease in SCC resistance.(4) At high temperature stage of two-step ageing (ageing treatment first at a relatively low temperature then at an elevated temperature), the hardness value of the alloy7050plate first increased and then declined with secondary ageing time. Meanwhile, the corrosion morphologies of the alloy7050plate changed from intergranular to intragranular with increase of ageing degree. This made the weight loss rate of the alloy increase, but the corrosion rate and propagation rate of cracks reduced with ageing degree. Overall, the corrosion resistance of the aluminium alloy plate was improved with the increase of ageing degree. The optimal secondary ageing process was163℃/12h.(5)The aluminium alloy7050-T7451plate with a thickness of120mm showed a high anisotropy of localized corrosion. The resistance to exfoliation corrosion of the alloy plate first reduced and then increased slightly from surface to center. For the same thickness layer of the alloy plate, the SCC susceptibility to different directions relative to the rolling direction was assessed by SSRT technique, which showed the susceptibility order, S direction> L direction> T direction. The result showed that the smaller the grain aspect ratio, the better the corrosion resistance to SCC was.