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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with maintaining the mechanical performance and damage tolerance capabilities in high levels. Therefore, the im plementation of advanced welding technologies along with high-strength aluminum alloys is receiving nowadays significant attention to the aeronautical community. By replacing the riveting process with welding technologies, further reduction of the structural weight, higher joining rates and less amount of material waste can be achieved, as it was described by Wanjara et al (2010) . Fusion as well as solid-state welding methodologies are extremely promising regarding their exploitation in aeronautical aluminium alloys. Electron beam welding (EBW) is an efficient joining method for structural aeronautical materials, e.g. Lacki et al. (2011). Several industrial applications currently use this technology due to the high process quality such as high energy density, controllable beam size, etc. as reported by Sun et al. (1996). Wrought aluminium alloy 2024 is an aluminium - copper alloy and is widely used in aircraft applications due to the high strength to weight ratio, high specific mechan ical properties, damage tolerance capabilities, high corrosion resistance and good weldability, e.g. Preston et al. (2003). However, this alloy exhibits several difficulties during fusion welding, like large degraded fusion zones, solidification cracking, porosity as well as high solubility of hydrogen e.g. as pointed out in Matrukanitz et al. (1990). Above problems are escalated since the welded joints are also exposed to the atmospheric conditions that on the long run will result in surface degradation due to pitting and respective degradation mechanisms. The different zones in the welded joint, consisting of different phase transformations, may induce different kind of damage due to localized corrosion on the welded joints. Furthermore, the ageing condition (pointed out in the heat affected zones of the welded joints) has a significant role on the welding efficiency of the material since the ageing-induced phases transformations influence the mechanical properties of the alloy. In the present work the effect of corrosion exposure on the tensile mechanical performance of A2024 EBWed joints is investigated. The welded specimens were exposed to laboratory exfoliation corrosion solution for different exposure hours (up to 48 hours), so as to investigate the influence of the phase transformations as well as the susceptibility of the different formed welded zones on the corrosion.

2. Materials and methods

2.1. Material condition

The material used for the present investigation was a wrought aluminum alloy 2024-T3 of 3.2 mm nominal thick ness and in sheet form. The chemical composition (% wt.) of the alloy used is shown in Table 1. The tensile specimens were machined from the longitudinal (L) rolling direction of the material in accordance with ASTM E8 specification.

Table 1. Chemical composition of AA2024 (wt.%). Material Cu Mg Mn Fe

Si

Cr

Zn

Ti

Al

Composition 4.35% 1.50%

0.64%

0.50%

0.50%

0.10%

0.25% 0.15% Rem

2.2. Welding process

The Electron Beam Welding machine used to perform the welds for the current investigation is a TINIUS machine of Hellenic Aerospace Industry, Schimatari, Greece. Vacuum of 0.3 atm and specific energy / cone diameter was used. Laser power was adjusted at 3000 W, and the focal point position was on the top of the sheet. Spot size in focus was 0.4 mm. The welding speed was 1.8 m/min. To reduce the level of porosity, in the 2024 aluminium alloys weld ments the joint surfaces of the sheets were cleaned with acetone and gridded with a scouring pad to remove the surface oxides. The final cleaning has been done with ethanol to remove any surface debris according to ISO standard TR 17671-7. The welding direction was in parallel with the rolling direction and the weld has been chosen to be in the center of each specimen. Tensile specimens were machined from the welded material sheets according to ASTM E8 specification. The dimensions of the reduced cross-section were equal to 12.5 mm × 3.2 mm and the respective length of the specimens was equal to 50 mm. Rectangular pieces 300 mm x 200 mm were machined from the aluminium alloy sheets and were welded face to face.