In the present work, the AA7075-O was subjected to friction stir welding. The rotating and welding speeds were 630 rpm and 32 mm/min, respectively. The diameters of shoulder and pin were 30 and 5.7 mm, respectively. The length of threaded pin was 4.8 mm. The tensile, bending, and hardness tests were carried out on the friction stirwelded specimens. The results show that the strength of friction stir-welded sample was about 15 % (i.e., 25 MPa) higher than that of base metal. By contrast, the ductility of base metal was as much as twice that of friction stirwelded one. According to the obtained hardness profile, the maximum hardness was obtained for the stirred zone with a continuous decrease toward the base metal. This resulted in the formation of cracks after bending specimens prepared from weldment, while there was no sign of crack in the bent specimens taken from base metal. Besides, there are microstructural evolutions from the weldment toward the base metal. These include the variations in the size and distribution of precipitates. The precipitates were coarser in the heat-affected zone, while they were finer in the stirred zone.
In this research, plasma electrolytic oxidation (PEO) coatings were prepared on 7075 Al alloy in a silicate-based solution with Na2MoO4 additive using a unipolar waveform at constant current density. The coatings displayed micro-pores, micro-cracks, pancake-like and crater-like features, and also solidified molten oxide particles on the surface. The coatings were majorly composed of Al2O3 (γ, δ, and α), SiO2 (amorphous), and MoO3 phases, which confirms the incorporation of molybdenum in the case of additive-containing coatings. Molybdenum species were transported through cracks, channels, and micropores, as the ready access pathways into the coating and partly sealed the coating pores. The EIS technique was used to evaluate the long-term corrosion performance of the coatings up to 168 h of immersion in 3.5 wt.% NaCl solution. The results showed that the barrier action of the PEO coatings was highly enhanced by adding Na2MoO4 due to the higher resistance that alumina achieved to chlorine absorption and also its higher stability by the incorporation of MoO3. The coating formed in the presence of 5 g L−1 Na2MoO4 showed the highest thickness and the lowest porosity percent (15.15%), which provided the highest corrosion performance at long immersion times.
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