AZ91D, a die‐casted alloy, offers several benefits (better saltwater corrosion resistance, suitable castability, and good atmospheric stability) over other grades of magnesium alloy. However, it cannot overcome the challenging requirements of advanced applications in its current form. Extensive research in coating technologies is crucial in enhancing the performance of AZ91D alloy in highly corrosive environments (coastal and marine, industrial, farm, etc.). The high corrosion resistance of AZ91D for various applications is achievable through the deposition of different materials. Developments in coating materials and techniques, with their influence on the corrosion protection behavior of AZ91D alloy, over the last decade (2012–2022), are focused on in this study. It is observed that composite, biodegradable, and self‐healing coatings are gaining popularity. The transition in the coating technology, from conventional to self‐healing coating materials and from conventional to environment‐friendly coating techniques, is a vital part of this discussion. This review further explores how different additives, pre‐treatments, post‐treatments, and corrosion inhibitors affect the corrosion performance of coatings. This study helps select the optimum coating material for AZ91D magnesium alloy to fulfil a specific application requirement and guides toward technological developments in this segment.
The surface properties of the AZ91D alloy are altered using surface mechanical attrition treatment (SMAT), a promising method of severe surface deformation, where the role of process parameters is crucial. In this study, specimens are SMATed using ≈3 and ≈10 m s−1 ball velocities (maintaining a constant percentage coverage). The SMATed specimens show higher twin density near the surface, which is reduced gradually, and twin thickness is increased with increasing depth. Further, high‐velocity balls cause more twin density and better grain refinement (≈32 nm grain size at the surface). The higher ball velocity helps form a considerably thicker gradient layer (≈3500 μm) with higher hardness (≈1.98 GPa) and compressive residual stress (≈281 MPa) within a shorter SMAT duration (≈10 min). Ball velocity also influences nanomechanical properties such as nanohardness, creep resistance, strain rate sensitivity (SRS), etc. The non‐SMATed alloy's SRS is about 0.037–0.040. The gradient microstructure affects SRS. The SRS value near the SMATed surface (where the reduced grain size plays a dominating role) is about 0.018–0.027; however, it drops suddenly to ≈0.01 (with a slight increase in depth), and subsequently, it rises with an increased distance in the SMATed layer (where twins play a dominating role).
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