Abstract
The 3rd generation of aluminum–lithium (Al–Li) alloys provides a desirable combination of high mechanical properties and low density compared to their traditional counterparts, exempt of lithium. Therefore, providing a reliable joint between new and conventional aluminum alloys is crucial for making hybrid structures. The present study focuses on improving the mechanical properties of dissimilar AA2198/AA2024 joints using different heat treatments before and after welding. Tensile tests paired with digital image correlation (DIC) techniques and micro-hardness maps were performed to document the macro-scale and local mechanical behavior of the joints. As-welded joints demonstrated a similar yield strength, 30% lower than that of the base metals in T3 and T8 metallurgical states. As-welded joints failed at the AA2198 side in the heat affected zone (HAZ), parallel to the thermo-mechanically affected zone (TMAZ), region experiencing intense strain concentration and minimal hardness values. Post welding-heat treatments (PWHT) was found to successfully strengthen HAZ on the AA2198 side, without abnormal grain growth in the nugget and impairing the hardness properties on AA2024 side. This improvement in local mechanical properties on the AA2198 side was related to the re-precipitation of dissolved T1 (Al2CuLi) and θ (Al2Cu) during welding as characterized by differential scanning calorimetry (DSC) and microscopy analyses. However, PWHT joint variants demonstrated a reduction in total elongation and ultimate tensile strength due to intense strain localization on the AA2024 retreating side compared to a much more homogeneous strain distribution in the as-welded joints.
Highlights
The 3rd generation of aluminum–lithium (Al–Li) alloys is ideal for the transport industry when weight reduction is a priority
In order to investigate the effect of pre-welding heat treatment and post-welding thermomechanical treatments on the tensile performance of FSWed dissimilar joints of AA2198 and AA2024, four categories of samples were produced as shown in Figure 4: (i) T3 as-welded joints (AW-T3), (ii) T8 as-welded joints (AW-T8), (iii) T8 post-welding heat treatment (PW-T8), and (iv) 3% pre-stretching followed by T8 post-welding heat treatment (PSPW-T8)
It may be related to different recrystallization mechanisms, and it requires more texture investigation with help electron backscatter diffraction (EBSD) to address it
Summary
Faculty of Engineering Technology, ß 2021 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis Group. The dissimilar welding of AA2024 and AA2198 is challenging owing to the local evolution of the microstructure and mechanical properties due to severe thermomechanical stress inherent to the FSW process. The post-weld heat treatment (PWHT) has been targeted in the present research to improve the joint mechanical properties. The combination of solution and aging treatment resulted in the joint ductility impairment, which was correlated to coarse dI (Al3Li) precipitates, producing high-stress concentration during plastic deformation. Zhang et al.[23] reported that the combination of solution and aging treatment of Al–Li alloys could decrease the ductility due to grain coarsening in the NZ and TMAZ. This research addresses the challenge of optimizing heat treatment for dissimilar joints made of AA2198 and AA2024, in order to improve the weakest as-FSWed regions without impairing the rest of the joint
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