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TY  - THES
AU  - Magaji-Mehta, Neeta
TI  - "Development of a test method for testing stress corrosion cracking of 7000 series aluminium alloys for automotive applications"
PB  - Rheinisch-Westfälische Technische Hochschule Aachen
VL  - Dissertation
CY  - Aachen
M1  - RWTH-2023-11557
SP  - 1 Online-Ressource : Illustrationen
PY  - 2023
N1  - Veröffentlicht auf dem Publikationsserver der RWTH Aachen University 2024
N1  - Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023
AB  - In recent years, the Al-Zn-Mg-(Cu) aluminium alloys, also known as the aluminium 7xxx series, have drawn considerable attention in the automotive industry owing to its high strength-to-weight ratio, making it a favored candidate for light-weight automobile construction. Nonetheless, a prominent challenge associated with these alloys is their high risk of susceptibility to stress corrosion cracking (SCC), which poses a significant risk of spontaneous failures in the service life of an automobile. Stress corrosion cracking is a complex phenomenon, involving the simultaneous effect of a susceptible material, minimum threshold stress, and a corrosive medium. Understanding the effects of SCC with respect to automobile poses a serious challenge due to the difficulty in predicting the influence of critical factors, such as stress conditions and environmental variations in the complex automotive lifecycle. This work aims to scientifically analyse the critical influencing factors, with a primary focus on their synergistic interactions occurring in the complex automotive lifecycle. This knowledge is then incorporated into designing a novel accelerated SCC test, called the MBSC test, which can test the SCC susceptibility of the 7xxx alloys specifically for the automotive application. In this thesis, a novel design approach, called Design for Six Sigma (DFSS), was the primary design methodology. The DFSS is a customer-centric product development methodology that enables the designing of complex products or processes systematically. The potential use of this methodology for the development of future corrosion test methods was assessed in this thesis. Four alloys, each with varying contents of Zn, Mg and Cu, and varying temper treatments were studied in this work, hence providing a range of SCC susceptibility for testing. The effects of various factors that are critical to SCC were clustered into environmental and mechanical parameters and individually analysed. The significant environmental parameters were investigated through Design of Experiment. The significant mechanical parameters were experimentally investigated on the three chosen mechanical specimens and compared to the specimen stresses occurring in critical in-service load conditions. Finally, an outdoor exposure test was carried out, the results of which were used to validate the Mercedes Benz Stress Corrosion Test (MBSC). The results showed that using the DFSS was advantageous in structuring the complex analysis of the automotive lifecycle and efficiently implementing it in the designing of the MBSC Test. A key benefit of using this approach was the ability to study the influencing parameters as an integrated whole rather than studying them individually under carefully controlled parameters, which is typically observed in most scientific studies. A significant outcome of this thesis was highlighting the potential of DFSS method as a standardized framework for the development of future corrosion test methods, particularly in the context of SCC tests. The results of the individual and synergistic effects from the above investigations served as the basis for the development of a new test method, the Mercedes Benz Stress Corrosion Test (MBSC). Studying the influence of environmental parameters revealed critical parameters such as test temperature and range of temperature variation in the MBSC corrosion test cycle had a significant influence on the SCC susceptibility. Moreover, anions and cations, NH4+ and NO3- in the salt solution, had a significant influence on the intergranular corrosion of the tested alloys. This in turn acted as accelerator for initiation points for SCC, thus increasing SCC susceptibility. Similarly, qualitative analysis of the various mechanical specimens showed the significant influence of mechanical parameters such as load propagation mode, stress distribution and stress magnitude on the SCC susceptibility of the tested alloys. Optimal environmental and mechanical parameters for the MBSC test were derived through this analysis. The results obtained from the MBSC test were consequently validated by showing a high correlation to the outdoor exposure test results.With the help of the DFSS method, a novel test was created to the needs of the automotive industry which can represent the SCC corrosion susceptibility of 7xxx Al alloys during the service life of a vehicle. The MBSC allows an exact differentiation between SCC susceptible and non-susceptible 7xxx Al alloys. It should be emphasized that the MBSC test can also identify only slightly susceptible alloys, which is not possible with ASTM G47-98, a widely used standard test for SCC susceptibility. The MBSC has a high significance for the automotive industry, as it allows a differentiated selection of 7xxx aluminium alloys for future automotive designing.
LB  - PUB:(DE-HGF)11
DO  - DOI:10.18154/RWTH-2023-11557
UR  - https://publications.rwth-aachen.de/record/974704
ER  -