Customized heat treatment is essential for enhancing the mechanical properties of additively manufactured metallic materials, especially for alloys with complex phase constituents and heterogenous microstructure. However, the interrelated evolutions of different microstructure features make it difficult to establish optimal heat treatment processes. Herein, we proposed a method for customized heat treatment process exploration and establishment to overcome this challenge for such kind of alloys, and a wire arc additively manufactured (WAAM) Mg-Gd-Y-Zn-Zr alloy with layered heterostructure was used for feasibility verification. Through this method, the optimal microstructures (fine grain, controllable amount of long period stacking ordered (LPSO) structure and nano-scale β′ precipitates) and the corresponding customized heat treatment processes (520 °C/30 min + 200 °C/48 h) were obtained to achieve a good combination of a high strength of 364 MPa and a considerable elongation of 6.2%, which surpassed those of other state-of-the-art WAAM-processed Mg alloys. Furthermore, we evidenced that the favorable effect of the undeformed LPSO structures on the mechanical properties was emphasized only when the nano-scale β′ precipitates were present. It is believed that the findings promote the application of magnesium alloy workpieces and help to establish customized heat treatment processes for additively manufactured materials.

Based on wire arc additive manufacturing (WAAM) technology, AZ31 magnesium alloy in bulk was successfully fabricated, and its microstructure as well as mechanical properties in different planes were observed and analyzed. The AZ31 magnesium alloy has a similar microstructure in the building direction (Z) and travel direction (X), both of which are equiaxed grains. There are heat-affected zones (HAZs) with coarse grains below the fusion line. The second phase is primarily composed of the Mg₁₇Al₁₂ phase, which is evenly distributed in different directions. In addition, the residual stress varies in different directions. There is no significant difference in the hardness of the AZ31 alloy along the Z and X directions, with the average hardness being 68.4 HV and 67.9 HV, respectively. Even though the specimens’ ultimate tensile strength along the travel direction is higher in comparison to that along the building direction, their differences in elongation and yield strength are smaller, indicating that the anisotropy of the mechanical properties of the material is small.

Due to its lightweight structure, magnesium alloys have been widely used in the automotive and aerospace industries. However, the corrosion resistance of magnesium alloy is a key factor limiting their further application. Rare earth alloying is an economical and effective method to improve the corrosion resistance of magnesium alloys. In this work, Mg–2Y–2Al (wt%) alloy was prepared by conventional casting method. After 168 h of immersion in 3.5 wt% NaCl solution, the corrosion rates determined by hydrogen evolution test and mass loss experiment were 0.57 mm/y and 1.01 mm/y, respectively. The results provide a basic understanding of the synergistic effect of Al and Y on enhancing the corrosion resistance of magnesium alloys. The co-addition of Al and Y is conducive to the formation of a stronger protective film on the alloy surface and effectively inhibits the corrosion of alloys. It provides important insights for the future development of high-performance and corrosion-resistant magnesium alloys.

The difficulty in enhancing both tensile strength and ductility is limiting the development of high-performance Mg alloys. The “plastic deformation + electropulsing (EP) treatment” is an effective process for modifying the microstructure and enhancing the mechanical properties of metals. In this work, the influence of the current density of EP treatment on the microstructure and tensile property evolution of the as-extruded Mg-6Al-1Zn alloy was systematically investigated. The microstructure of the as-extruded sample was predominantly composed of an α-Mg matrix and a minor quantity of the β-Mg₁₇Al₁₂ phase on grain boundaries. After EP treatments, the microstructure underwent recrystallization, resulting in the formation of fine recrystallized grains. Meanwhile, the distribution and volume fraction of the β-Mg₁₇Al₁₂ phase demonstrated minor changes. After the 60 cycles of EP with a current density of 1050 A·mm⁻² for a duration of 60 s, the tensile strength and elongation of the as-extruded Mg-6Al-1Zn alloy improved from 260 ± 2.6 MPa and 22 ± 1.3% to 319 ± 3.6 MPa and 23 ± 1.1%, respectively. The results prove the effectiveness of EP treatment in tailoring recrystallization via changing current density.

A series of Mg-Y-Ni alloys with different volume fractions of long-period stacking-ordered (LPSO) phase were prepared, by controlling the alloy composition, heat treatment, and single-pass extrusion, to assess the influence of increasing LPSO phase volume fraction on the hydrogen absorption and desorption properties of the extruded alloys. The LPSO phase volume fraction in the alloys increased with increasing solute concentration, from ~24% LPSO in Mg₉₇Y₂Ni₁ (at.%) to ~60% LPSO in Mg₉₃Y₄Ni₃ (at.%) up to ~92% LPSO in Mg₉₁Y₅Ni₄ (at.%). The most refined microstructure was obtained in the alloy with highest volume fraction of LPSO phase. After 100 s at 300 °C, the Mg₉₁Y₅Ni₄ alloy absorbed 4.6 ± 0.2 wt.% H while the Mg₉₇Y₂Ni₁ and Mg₉₃Y₄Ni₃ alloys each absorbed 3.8 ± 0.2 wt.% H. After 10,000 s at 300 °C, all three alloys had absorbed a maximum of 5.3 ± 0.2 wt.% H with no further significant difference in hydrogen absorption kinetics. The Mg₉₁Y₅Ni₄ alloy desorbed 1.8 ± 0.2 wt.% H after 100 s at 300 °C against a vacuum while the Mg₉₇Y₂Ni₁ and Mg₉₃Y₄Ni₃ alloys desorbed 0.8 ± 0.2 wt. H and 0.6 ± 0.2 wt.% H, respectively. After 10,000 s at 300 °C, the Mg₉₁Y₅Ni₄ and Mg₉₇Y₂Ni₁ alloys completely desorbed 5.2 ± 0.2 wt.% H and 5.4 ± 0.2 wt.% H, respectively, but the Mg₉₃Y₄Ni₃ alloy desorbed only 3.7 ± 0.2 wt.% H. Hydrogen absorption and desorption kinetics were fastest in the Mg₉₁Y₅Ni₄ alloy with the highest LPSO volume fraction, but no consistent trend with LPSO phase volume fraction was observed with the Mg₉₃Y₄Ni₃ alloy, which showed the slowest absorption and desorption kinetics. The hydrogen pressures corresponding to metal–hydride equilibrium did not vary with LPSO phase volume fraction or alloy composition, indicating that the (de)hydrogenation thermodynamics were not significantly changed in any of the alloys. Hydrogen absorption experiments with thin foils, made of extruded Mg₉₁Y₅Ni₄ alloy with the highest LPSO phase fraction, demonstrated that the LPSO structures decompose into Mg phase, Mg₂Ni phase, lamellar Mg/Mg-Y structures, and YH_x particles. This study shows that hydrogen kinetics can be impacted in Mg-Y-Ni alloys by controlling the LPSO phases using common metallurgical techniques.