Understanding the mechanisms and parameters affecting the structural corrosion of sheet gauge 7xxx alloys

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Bhaskaran, Ganesh
Singh, Preet M.
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The light-weighting of automotive structures has been an effective method of increasing vehicle fuel efficiency. In the past, the use of aluminum alloys was limited to hang-on parts and outer skin applications to reduce weight. However, due to the high strength-to-weight ratio of 7xxx series alloys, exciting alternative options for replacing high-strength steels in load-bearing or structural applications have emerged. These alloys derive their strength through the precipitation hardening mechanism, which involves the formation of nanosized precipitates after a specific heat treatment sequence. However, one of the main roadblocks to the penetration of 7xxx alloys in structural applications like rockers and bumpers is concerns regarding structural forms of corrosion, such as intergranular corrosion (IGC) and stress corrosion cracking (SCC). These can be attributed to the microstructure features of the grain boundary, specifically the precipitate and adjoining precipitate-free zone. Most published structural corrosion work on 7xxx alloys has been associated with plate gauge applications with thicknesses greater than 10mm. However, for automotive applications, sheet type with thicknesses of 1.5 to 2.8mm is desired, as it enables weight reduction without loss of dent resistance while meeting stiffness requirements. Moreover, the difference in the manufacturing process between sheet and plate leads to a distinct microstructure. The objective of this study is to examine the impact of processing sequence, alloy composition, and joining methods on grain boundary microstructure, as well as to understand their potential impact on the structural forms of corrosion in sheet gauge 7xxx alloys. Due to the complex grain boundary microstructure and corrosion property relationship, an efficient research scheme was needed to minimize or isolate the secondary effects. The research scheme was designed in such a manner that when one of the variables of interest was changed, the rest of the processing parameters were kept constant. The research is divided into three sections. In the first part, two different processing sequences were utilized to alter the grain size of the material. They were recovery anneal of final gauge material and reduction of the percentage of cold work on hot-rolled material. The grain boundaries were characterized using scanning transmission electron microscopy (STEM) to determine the width of the precipitate-free zone, the size and continuity of grain boundary precipitates between the finer and coarser grain materials. The samples were then subjected to IGC and SCC tests to determine the impact of microstructure on the corrosion mechanism. In the second part, the effect of Cu and Zn/Mg ratio on the resulting microstructure of high-solute 7xxx alloys and their impact on corrosion resistance was evaluated, with a specific focus on the effect of testing environment and constituent particles on localized corrosion resistance. In the third part, the microstructure changes, and their impact on the corrosion properties of commonly used automotive joining methods, such as resistance spot welding (RSW) and self-pierce rivet (SPR), were evaluated for the dissimilar aluminum alloy joint of 7075-T6 to 5182-O. The gradient microstructure of the joint sections was characterized and evaluated for corrosion resistance. The study revealed that the three factors (processing, composition, joining) uniquely influenced the size, continuity, and composition of the grain boundary precipitates and adjoining precipitate-free zone, which in turn affected the IGC and SCC performance of the materials. The results also showed that the grain boundary features that control the corrosion mechanism vary uniquely, depending on the environment. The findings of this research have significant implications for the development of 7xxx sheet gauges, particularly in the areas of alloy design, manufacturing process selection, and end application. The study will provide valuable insights into improving the corrosion resistance of these alloys, which will enable them to compete more effectively with ultra-high strength press-hardened steel in automotive structural applications. Ultimately, this research has the potential to contribute to the development of the next generation of high-performance aluminum alloys.
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