The Influence of Warm Forming on Corrosion Susceptibility and Mechanism of AA7075 Alloy

Abstract

This study focuses on the influence of warm forming on corrosion behaviour of AA7075 aluminium alloy. The microstructure evolution under different warm forming temperatures is characterized by using analytic electron microscopy. Further, the influence of plastic deformation introduced by forming on the corrosion behaviour of the alloy is investigated by correlative corrosion testing in acidified sodium chloride solution and quasi-in-situ electron microscopy, with the aim of establishing possible correlation between the corrosion initiation and propagation processes and the microstructure features in the alloy. It is observed that warm forming at a 30% strain level at 320℃ introduces significant dislocation structures in grain interior and discrete sub-grain boundaries (SGBs). This is because the low misorientation dislocation wall (DW) structures tend to arrange themselves into SGBs during the warm forming process. This DW to SGB transfer occurs more frequently when the warm forming temperature was increased to 420℃. Due to the higher mobility of dislocations at higher temperatures, a network of DW/SGB emerged in the alloy deformed at 420℃ as a result of frequent dislocation annihilation. Coarse second phases, such as constituent intermetallic Al7Cu2Fe and dispersoids, underwent little change except for breaking up during warm forming at the selected temperatures. Grain boundary precipitates Mg(ZnCu)2 also remained unchanged after deformation at both 320℃ and 420℃. The η phase precipitates formed on the SGBs were found to have high Cu content. A Zn-rich layer was found along HAGBs in the alloys after warm forming. Also, Zn- and Mg-rich thin layers were observed on the SGBs and DWs introduced by warm forming. The elemental segregation on the DWs and SGBs is caused by the interaction between dislocation movement and solid solute atoms in the early stage of deformation. It is found the grain structure changes, specifically the distribution of dislocations, dislocation walls and sub-grain boundaries have significant influence on the corrosion behaviour of AA7075 alloy. The corrosion propagation mainly proceeds along the high-angle grain boundaries (HAGB) and results in intergranular corrosion (IGC) in a reference group alloy that was exposed at 320℃ without deformation. A comparison of the corrosion behaviour between the alloy exposed at 320℃ without deformation and the alloy deformed at 320℃ revealed different types of corrosion, i.e. intergranular corrosion and intragranular corrosion respectively. The presence of high population of DW/SGB in the alloy deformed at 320℃ plays an essential role in the development of the intragranular corrosion. Further, the high deformation temperature of 420℃ led to more severe intragranular corrosion, resulting in corrosion network due to preferential dissolution of the DW/SGB network. The presence of a Zn-rich layer along HAGB was found to be a critical factor in supporting the continuous dissolution along the HAGB. Also, Zn- and Mg-rich thin layers on the SGBs is believed to promote intragranular corrosion propagation through galvanic coupling with the adjacent alloy matrix. The η phase precipitates with high Cu content on the SGBs may change the electrochemical properties of the precipitates. Further, it was observed that the Mg(ZnCu)2 GBPs remained intact in the early stage of corrosion due to the noble electrode potential associated with the high content of Cu, but de-alloying occurred on the GBPs once they are disconnected from the alloy matrix.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Xiaorong Zhou (Supervisor) & Michele Curioni (Supervisor)