PSI - Issue 60

M. Suresh Kumar et al. / Procedia Structural Integrity 60 (2024) 433–443 Suresh Kumar et al., / Structural Integrity Procedia 00 (2023) 000 – 000

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1. Introduction Al-Mg-Sc-Zr alloys such as AA5024 and AA5028 are being considered as potential materials for replacement of currently used 2000-series alloys in aerospace applications because of their higher specific strength, good thermal stability, corrosion resistance and superior damage tolerance and welding characteristics (Filatov et al. (2000); Wiley (1971); Satwell and Jensen (1990)). The useful combination of these properties has been attributed to the presence of nano-sized Al 3 Sc precipitates in 5024 and 5028 alloys. These alloys, however, are not amenable to stretch forming due to difficulty in the dissolution of Al 3 Sc precipitates at solutionizing temperatures (Jambu et al. (2002)). The alloys are also known to develop luder bands while shaping due to the dynamic strain aging phenomenon (Kumar et al. (2014)). Therefore, creep forming is usually the preferred method for fabrication of complex structures manufactured using these alloy sheets for aerospace applications. Creep forming (CF), also called creep age forming, essentially utilizes two metallurgical processes, namely, stress relaxation/creep and age hardening simultaneously. These processes bring about the requisite shape change and precipitation hardening, respectively, simultaneously (Holman (1989); Zhan et al. (2011)). The process is usually carried out in three stages. Firstly, the sheet/plate is deformed onto a tool with vacuum bagging techniques or mechanical loading methods. Subsequently, the sheet is heated for a specific period of time to allow stress relaxation/creep and precipitation to occur. The final stage involves the release of the sheet from the tool to affect the shape change. However, some spring back occurs due to elastic recovery depending on the material, temperature, and applied stress. The major influencing CF parameters are temperature, time, pressure, and initial material condition. For instance, the higher the CF temperature, the lower is the spring back, and vice versa. Higher temperatures can potentially alter the mechanical properties, while lower temperatures may affect the spring back. Therefore, it is vital to choose optimum CF parameters to obtain desired contours and also desirable mechanical properties. In this context, many authors conducted studies on several aluminum alloy systems to understand the effect of CF process parameters on spring back and mechanical properties. Yong Li et al. (2019) studied the effect of the initial temper condition of AA 2050 alloy on CF properties and reported that the T34 temper condition was optimum compared to the as quenched and T84 temper. Lei Chao et al. (2016) reported that the retrogression condition was optimum for AA 7075 alloy instead of solution and re-solution tempers. In another work on a similar alloy, multi-step heat treatment was reported to show excellent formability, good tensile strength, and exfoliation corrosion resistance (Jeshvaghani et al. (2012)). Gangloff et al. (1994) reported a significant improvement in fatigue crack growth resistance (FCGR) in AA 8090 alloy after being subjected to CF. Studies by Chi Liu et al. (2018) on alloy AA2524 alloy demonstrated that creep forming improved the tensile strength but resulted in deterioration of FCGR as a result of the CF process. Although many reports on CF of several aluminum alloys are available, limited literature exists on the Sc and Zr modified Al-Mg alloy system (Jambu et al. (2002); Zimemermann et al. (2018); Phillipp et al. (2019)). Jambu et al. (2002) conducted creep and stress relaxation tests at different temperatures on 5024 alloy and demonstrated that the alloy is amenable to CF. Zimmermann et al. (2018) identified optimum process parameters for 5024 alloy, but their studies were limited only to understanding the formability characteristics. With reference to alloy 5024, it is important to note that age hardening does not occur during CF. But, a few microstructural changes, such as depletion of Mg in the solid solution and precipitation of β phase are expected to occur during exposure to creep forming temperatures (Phillipp et al. (2019)). It is understood from the literature that CF parameters that produce good formability do not guarantee good mechanical properties. Also, static properties of the material can be within the specified limits after creep forming at optimum process parameters, but the fatigue properties can fall short. In view of this, in this present study, experiments were conducted on alloy AA 5024 to understand the effect of creep forming process temperature on spring back, mechanical properties, and high cycle fatigue and fracture behavior.