2.3. Surface treatments
Prior to anodizing, the specimens were initially degreased manually by diestone followed by degreasing in alkaline bath (pH ∼ 9) at 60 ± 5 °C during 3 min followed by water rinsing for 1–2 min. For pickling process, specimens were submerged in aqueous solution of H2SO4 (10–15% by weight), HNO3 (20–30% by weight) and ferric-sulphate Fe2(SO4)3 (20–30% by weight) for 3 min at 32 ± 2 °C followed by 1–2 min water rinsing. Anodizing was carried out in chromic acid CrO3 solution (55 g/L) at 40 ± 2 °C for 50 min followed by water rinsing.
The thickness of anodic film was measured by optical microscopy and was also confirmed by SEM inspection. The average thickness of film produced by the process is measured to be about 3 μm.
2.4. Surface measurements
Besides the observations made by SEM, to characterize the effect of each surface treatment quantitatively, topography measurements were carried out with the help of a contact profilometer coupled with motorized table. To do so rectangular specimens of dimension 40 × 40 mm2 were machined with the help of a shaper to give same surface roughness ‘Ra’ values as generated for fatigue specimens machined by lathe. After machining, the specimens were subjected to different surface treatments. Several measurements adjacent to each other were done with zone size of 4 × 4 mm2 each with transverse and longitudinal resolutions of 5 μm in each direction. The characterization of pits was done with a specific program developed for this purpose. From surface topography data, a plane orientation correction was made using the least square method and pits were initially neglected in this step. Then, surface profile, calculated from machining condition, is subtracted from the real surface measured by profilometer to define the depth of pits.
2.5. Fatigue testing
Axial fatigue tests have been performed at 10 Hz in ambient conditions at stress ratio ‘R’ of 0.1 according to ASTM E 466 [15]. All tests were conducted under load controlled condition using a 100kN servo-hydraulic MTS machine. The nominal maximum cyclic stress was set at a value that was expected to result in a fatigue life of between 104 and 106 cycles and tests were stopped if the specimen did not fail at 1.2 × 106 cycles.
3. Results and discussions
3.1. Characterizing the effects of pre-treatments
Microscopic examination was made after degreasing and pickling process to verify if these pre-treatments affect the surface topography of the specimen. By comparing micrographs Fig. 3a and b before and after degreasing process, we concluded that degreasing process did not change the surface topography of the specimens. Moreover, EDS analysis showed that the constituent particles; i.e. Al7Cu2Fe, Al2CuMg and Mg2Si were always present after the degreasing treatment. On the contrary, after the pickling solution exposure, numbers of pits were observed at the surface of the specimen. The pickling process was found to attack the constituent particles resulting in pits formations (Fig. 3c and d). During SEM examination of the pickled specimens, it was observed that some particles were completely dissolved while others were partially dissolved leaving behind a trace (Fig. 3d). This phenomenon has also been reported earlier [16]; Birbilis et al. have shown that nature of the constituent particle had an influence on the pitting process. EDS analysis of some partially dissolved particles showed that these were particles rich in Cu and Fe; i.e. Al7Cu2Fe and Al2CuMg.