Seyyed Mohamad Hasan Karparvarfard

The influences of the forging process and micro-arc oxidation (MAO) coating on the corrosion behavior of ZK60 wrought magnesium alloys exposed to salt spray and constant stress corrosion conditions were investigated. The microstructure of the ZK60 Mg alloy specimens forged under different temperatures (i.e., 250, 300, and 450 °C) was characterized using metallography, EBSD, and SEM. It was demonstrated that the ZK60 alloy forged at 300 °C (i.e., ZK60EF-300) had finer grain and uniformly distributed β-phase and, thus, better corrosion resistance than the ZK60 forged at 450 °C. At the lower forging temperature (250 °C) twins formed in the ZK60 alloy, which accelerated the corrosion of the ZK60E-250 specimen. The MAO coating provided robust corrosion protection for all the ZK60 wrought Mg alloy substrates. The salt spray corrosion test results showed that when the MAO coating broke down at certain weak sites, the corrosion performance of the coated Mg alloy was predominantly determined by the alloy substrate. The stress corrosion behaviors of the uncoated and MAO-coated ZK60 alloy specimens were also investigated under a constant load of 80 MPa in 3.5 wt.% NaCl solution. The MAO coating was found to improve the stress-corrosion resistance of the ZK60 alloy pronouncedly.

Proportional and non-proportional multiaxial fatigue tests are conducted on the closed-die forged ZK60 extrusion. The shear strain amplitude was kept constant at 0.5% for all the tests, while two different axial strain amplitudes of 0.4% and 0.7% were considered. At the higher strain amplitude (0.7%) significant difference was observed between the torque amplitudes of proportional and non-proportional tests, whereas the axial load amplitude responses remained the same regardless of the phase angle shifts. It is likely that as the phase angle changes from 0-90, the twin volume fraction at the peak shear strain decreases resulting in higher torque responses. On the other hand, at the lower strain amplitude, i.e. 0.4%, where twinning is not active, phase angle does not show any effect on the shear response. An energy-based fatigue model is employed that effectively explains the different damage contributions by the axial and torsional loadings at different strain amplitudes, and accurately predicts the proportional and non-proportional multiaxial fatigue lives.