PSI - Issue 10

E. Dovletoglou et al. / Procedia Structural Integrity 10 (2018) 73–78 E. Dolvetoglou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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strength of the welded joints can be as high (almost at 70%) as the reference material, Alexopoulos et al. (2016). The wide range of different exfoliation corrosion times allow to understand the degradation of AA2024-T3 electron beam welds. In several works, e.g. Kamoutsi et al. (2006), showed that for high corrosion exposure times the mechanical properties and especially tensile ductility of non-welded 2024 aluminum alloys are significantly decreased. Alexopoulos et al. (2016) showed that no essential stress decrease of the tensile flow curves is noticed for AA2024 up to 2 h exposure time while elongation at fracture is continuously decreasing with increasing corrosion exposure time. After 24 h corrosion exposure time, almost 15 % stress decrease due to the corrosion-induced surface pitting formation was noticed that subsequently decrease the effective thickness of the specimen. The effect of exfoliation corrosion on the electron beam welded AA2024 specimens is summarized in the flow curves of Fig.2. It can be noticed that even for the short corrosion exposure times and up to 2 hours an essential decrease in ductility is evident while the tensile strength remains almost unaffected . This ductility decrease for the very short exposure times, where no significant surface attack is evident, is attributed to the hydrogen embrittlement phenomenon . Up to 4 h corrosion exposure time, both ductility and ultimate tensile strength have been significantly decreased. Nevertheless , for higher corrosion exposure times and up to 48 h, the ductility tends to decrease in lower rates than the tensile stress that exhibited an essential decrease in stress from 338 MPa to 260 MPa.

Fig. 2. Typical experimental tensile flow curves for electron beam welded (EBW) AA2024-T3 specimens for different exposure times to exfoliation corrosion solution.

3.3. Effect on the mechanical properties Conventional yield stress and elongation at fracture for different corrosion exposure times are summarized and presented in the next sections. The corrosion-induced degradation of the mechanical properties is described as average values and respective standard deviation derived from the specimens used during the process. Conventional yield stress results are presented in Fig.3 for the six different exposure times in exfoliation corrosion environment, based on the nominal cross-section of the specimens. The non-corroded welded specimens exhibited 232 MPa conventional yield stress while after 2 hours EXCO, yield stress was decreased by approximately 8% (213 MPa). Higher exposure time to corrosion solution (e.g. 4 h and so forth) essentially decrease the mechanical properties with the extreme 48 hours of exposure having a 30% stress drop. Elongation at fracture after exposed to corroded environment can be seen in Fig.4: Tensile ductility of 2 h corroded welded presented a 4% drop from the initial value of non-corroded specimen. Higher decrease was noticed for addi tional exposure time, e.g. from 4 h and up to 12 h EXCO exposure. For the extreme exposure times at the exfoliation corrosion, e.g. from 24 h up to 48 h, elongation at fracture is being decreased by 22 and 24%, respectively. 3.4. Fractography Fig.5 shows the tensile fracture location of electron beam welded joints. The cross-sections of the 2024-T3 speci mens after been exposed to different times of EXCO were revealed by etching a mirror polished sample in Keller’s