The corrosion behavior of the tungsten inert gas (TIG) welded Mg–3Nd–3Gd–0.2Zn–0.5Zr alloy with different post-weld heat treatments was systematically investigated. The results show that the corrosion resistance of the sand-cast base material (BM) was inferior to that of the fusion zone (FZ), which was attributed to the larger grain size and exacerbated galvanic corrosion caused by coarser Mg₃(Nd, Gd) eutectic phases and numerous β precipitates. It is found that post-weld solid-solution (T4) treatment could significantly enhance the corrosion resistance of the joint due to the dissolution of the cathodic second phases and the denser protective film abundant in RE oxides generated in corrosive solution. The precipitation of nanosized phases and Zn–Zr clusters would slightly increase the susceptibility to localized corrosion of the peak-aged (T6) joint. As the main corrosion products, MgO and Mg(OH)₂ are distributed throughout the whole corrosion film, while RE oxides and RE hydroxides are mainly distributed in the inner layer, which can be explained by inward oxidation and replacement reactions between RE elements and MgO/Mg(OH)₂. Based on the composition and structure of the corrosion product film, a physical model has been proposed for depicting the microstructure evolution associated with the corresponding corrosion behavior of the joints. This work could promote the applications of welded Mg–RE alloy joint in some corrosion environments.

Additive manufactured Mg-RE alloys usually show exceptional mechanical properties, which is mainly attributed to their refined grains in previous studies. Since Mg-RE series are typical age-hardenable alloys, this study focuses on the aging behavior of wire arc additive manufactured Mg-9Gd-3Y-0.5Zr (GW93K) alloy and compares it with the as-cast counterpart, providing a new insight into the strengthening mechanism of additive manufactured alloys. It was revealed that both the refined equiaxed α-Mg grains and small-sized (only 5~10 nm) β′ precipitates with an extremely high number density (~2.53 × 10⁴ µm⁻²) should be considered for the strengthening mechanisms of the deposited alloy. The promoted precipitation behavior is facilitated by the dislocation pile-ups formed under multiple thermal cycles and a high cooling rate during deposition. As a result, the deposited alloy at peak-aged state exhibits better comprehensive properties of UTS=392 MPa and EL=3.3%, which is 19% and 18% higher than that of the cast sample, individually.

Surface oxide layers play a significant role in forming secondary oxidation inclusions during the casting process. In this study, three typical Mg-RE alloys (Mg-3Nd (NZ30K), Mg-3Nd-3Gd (EV33) and Mg-3Nd-4Y (WE43A)) are selected. Their surface oxide layers formed during the solidification are characterized in detail, and the corresponding oxidation mechanisms are discussed. The results reveal that RE elements obviously influence the characteristics of surface oxide layers, which depends on their ability to purify the formed MgO in the melt via the reaction (2RE + 3MgO = 3Mg + RE₂O₃). On the one hand, as Nd and Gd do not easily displace MgO already formed in the melt, the loose oxide layers in NZ30K and EV33 alloys are mainly composed of MgO matrix with embedded RE-rich oxide particles. On the other hand, due to the strong ability of Y to purify MgO in the melt, the oxide layer of WE43A alloy becomes a denser and thinner Y₂O₃ oxide layer. Note that the differences in surface oxide layers well explain the different secondary inclusions that occur in three typical Mg-RE alloys during the casting process.