PSI - Issue 10

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

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2.3. Experimental procedure The tensile specimens were exposed to exfoliation corrosion environment in accordance to ASTM G34 specification immediately after all surfaces were cleaned with alcohol according to ASTM G1 specification. Table 2 presents the number of electron beam welded 2024-T3 aluminium alloy specimens which were exposed in exfoliation corrosion en vironment. A great range of corrosion exposure times has been chosen, from 2 h to 48 h. The corrosive solution con sisted of 4.0 M sodium chloride (NaCl), 0.5 M potassium nitrate (KNO 3 ) and 0.1 M nitric acid (HNO 3 ) diluted in 1 litre of distilled water; the solution volume was calculated per exposure area of the specimens which was approximately 20 ml/cm 2 , while the solution temperature was 25±2 o C. It was noticed that the specimens after exposure, exhibited sur face corrosion products that were mostly evident nearby the welded region. The corroded specimens were cleaned right after the corrosion exposure as it is described in the ASTM specification and subjected to tensile mechanical test.

Table 2. Investigated exfoliation corrosion (EXCO) exposure times and number of tensile specimens of the reference 2024-T3 alloy. Exposure time 0 h 2 h 4 h 8 h 12 h 24 h 48 h # specimens 3 3 2 2 2 2 2

Tensile mechanical tests were carried out in a servo-hydraulic Instron 100 kN loading frame according to the ASTM E8 specification, while the tests were carried out with a constant deformation rate of 1 mm/min. Data of axial force, displacement and axial strain where stored in a digital file during testing. Several mechanical properties were evaluated from the electron beam welded specimens, namely modulus of elasticity E , conventional yield stress R p0.2% , ultimate tensile strength R m , elongation at fracture A f and strain energy density W , based on the nominal thickness of the sheet.

3. Results and discussion

3.1. Microstructure analysis

An Olympus BX 41 M optical microscope was used to observe the grains in the different regions of the weld. The size as well as the morphology of the grains can be seen in Fig.1. By etching a mirror- polished sample in Keller’s reagent for 1 min at 25 o C, it was revealed the formation of the weld to the aluminum alloy. The pictures can efficiently describe the electron beam welded area. The weld seam can be seen in Fig.1a, the interphase of the fusion zone and heat affected zone can be seen in Fig.1b, while Fig.1c shows the solidified grains within the fusion zone.

(a)

(b) (c) Fig. 1. The grain structure in electron beam welded 2024 aluminum alloy

3.2. Mechanical tests results

The main purpose of this study is to assess the effect of corrosion exposure on the mechanical properties degradation of AA2024 electron beam welded specimens, concerning the fusion and heat affected zones that are created during the welding process. Electron beam welding of 2024-T3 is feasible, showing that ultimate tensile