I. Research Background: Limitations of Traditional Techniques and Exploration of New Technologies

The hexagonal close-packed crystal structure of magnesium alloys results in a limited number of independent slip systems, making it difficult to fabricate complex-shaped components using conventional casting or extrusion processes; moreover, the alloy is prone to corrosion‑induced failure. In recent years, Powder metallurgy and Additive manufacturing It has become a mainstream technology for magnesium alloy modification and near-net-shape forming, capable not only of refining grain size and tailoring phase composition but also of enabling the recycling and remanufacturing of scrap alloys.

Mg–Li alloys constitute an important subclass of lightweight magnesium alloys, with lithium significantly enhancing their ductility. In this study, two powder consolidation techniques—laser powder bed fusion (LPBF) and powder plasma sintering (PPS)—were applied to fabricate a recycled Mg–7.5Li–3Al–Zn alloy, with a particular focus on elucidating the relationships among processing parameters, microstructure, and corrosion performance.

II. Experimental Protocol: Material Preparation and Characterization System

1. Raw Material Preparation: Conventional Extrusion Alloy + [Key: Ultrasonic Atomization Powdering Technology]

Research selection Conventional casting + extrusion–annealed Mg-7.5Li-3Al-Zn alloy As a reference sample. Conducted on the extruded alloy. Ultrasonic atomization–assisted ultrafine pulverization for powder production : The alloy feedstock was heated to 1023 K at a rate of 150 K/min in an inert atmosphere using the rePowder equipment from Poland’s AMAZEET company, and held at this temperature for 2 minutes. The molten alloy was then processed with a Ti6Al4V ultrasonic horn operating at a frequency of 40 kHz and an amplitude of 15 μm; after 40 seconds of ultrasonic atomization, a metal powder was obtained. The powder was sieved through a 100-μm mesh and subsequently used for subsequent consolidation and forming.

Testing has confirmed, The ultrasonic atomization process did not result in the volatilization or oxidative loss of lithium. The lithium content of the powder is essentially consistent with that of the original alloy, and the powder particles are spherical and free of agglomeration, laying a solid raw-material foundation for subsequent forming processes.

2. Two Powder Sintering and Forming Processes

• Laser Powder Bed Fusion (LPBF) : A 1064 nm continuous-wave laser was used to fabricate specimens on a pure magnesium substrate under an inert, low-oxygen atmosphere; process parameters were: laser power of 15 W, scanning speed of 80 mm/s, and powder layer thickness of 25 μm.
• Pulsed Plasma Sintering (PPS) Under vacuum conditions, the powder is rapidly consolidated by holding at 350 °C and 50 MPa for 5 minutes.

3. Performance Characterization Techniques

The experiments comprehensively employed scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and micro‑X-ray computed tomography (MicroXCT) to analyze the microstructure, phase composition, porosity, and precipitate phases; mechanical properties were evaluated via Vickers hardness testing; relying on… Electrochemical testing, hydrogen evolution corrosion testing The corrosion resistance of the alloy was systematically evaluated in a 0.01 mol/L NaCl solution.

III. Core Experimental Results

1. Powder Particle Size and Phase Composition

The atomized powder exhibits a narrow particle-size distribution, with an average particle size of 70 μm, D10 = 48 μm, and D90 = 95 μm, covering a particle-size range that is well suited to the requirements of both LPBF and PPS processes. The powder consists primarily of the α(Mg) phase, the β(Li) phase, and a minor fraction of precipitate phases; following consolidation via different processing routes, the relative proportions of these two phases undergo significant changes:

2. Microstructure and Mechanical Hardness

All three alloys exhibit a dual-phase microstructure of α(Mg) + β(Li), yet their morphologies and defects differ markedly:

• Traditional extrusion alloy : The two-phase microstructure exhibits relatively large grain sizes and a uniform distribution, with the presence of a small amount of coarse Al-, Zn-, and Mn-rich precipitates.
• LPBF alloy : The grain size is significantly refined, and the β(Li) phase is uniformly dispersed within the α(Mg) matrix; There are numerous closed cells inside. , with a porosity of 3.51% and an average pore diameter of 28.3 μm, while a high-density stacking fault structure is simultaneously formed within the material.
• PPS alloy : The grain size is also refined, but remains distinct. Interface of non-recrystallized powder particles , the two-phase distribution within the particles is clearly defined.

Grain refinement and crystal defects directly enhance hardness; the hardness ranking is: LPBF alloy (130 HV₀.₂) > PPS alloy (114 HV₀.₂) > Extruded alloy (93 HV₀.₂) Furthermore, the number of coarse intermetallic precipitates in LPBF‑PPS alloys is significantly higher than that in conventionally extruded alloys.

3. Corrosion Performance: Verification Using Multiple Methods in Combination

Using a 0.01 mol/L NaCl solution as the corrosive medium, and employing a combination of open-circuit potential measurements, potentiodynamic polarization, electrochemical impedance spectroscopy, and hydrogen evolution tests, a consistent trend was observed: Corrosion resistance: Conventional extruded alloy > PPS alloy > LPBF alloy 。

Electrochemical parameters : The extruded alloy exhibits the most negative corrosion potential and the lowest corrosion current density, with a polarization resistance of 297 Ω·cm²; in contrast, the LPBF alloy has a polarization resistance of only 50 Ω·cm², indicating the highest corrosion activity.

Hydrogen evolution corrosion rate : The LPBF alloy exhibits the highest corrosion rate, followed by the PPS alloy, while the conventional extruded alloy shows the lowest corrosion rate.

Corrosion Morphology and Mechanism  

• Extruded alloy: Corrosion occurs between α(Mg) and β(Li). Microgalvanic corrosion Primarily, the corrosion product layer is dense.
• LPBF alloys: Internal porosity serves as a primary initiation site for corrosion; hindered solution exchange within the pores gives rise to autocatalytic reactions that accelerate corrosion, thereby becoming the principal cause of reduced corrosion resistance.
• PPS alloy: Corrosion preferentially occurs along Grain boundary of non-recrystallized grains Expansion, coupled with biphasic microgalvanic corrosion, significantly reduces the corrosion resistance of the interface.

IV. Mechanism Analysis and Discussion

The material ratio is tunable. : The cooling rate in LPBF is significantly higher than that in PPS, resulting in a substantial increase in the volume fraction of the β(Li) phase. Both powder consolidation processes allow for flexible control of the α(Mg)/β(Li) phase ratio, which constitutes a key advantage for optimizing the properties of magnesium–lithium alloys.

Organizational defects determine corrosion behavior. : Grain refinement itself enhances corrosion resistance, but Porosity in LPBF and the non-recrystallized interface in PPS Two types of defects serve as corrosion pathways, negating the grain‑refinement advantage. If LPBF‑fabricated alloys can be fully densified and PPS‑alloys can undergo complete recrystallization, the corrosion resistance of these alloys—produced by both processes—could surpass that of conventionally extruded materials.

Process adaptability : LPBF, owing to its rapid solidification, produces high‑density stacking faults and exhibits pronounced mechanical strengthening, making it well suited for fabricating complex components with superior mechanical properties; PPS offers high forming efficiency, but sintering parameters must be optimized to eliminate interparticle interfaces.

Recycling value : This study confirms that through Ultrasonic atomization powder production + powder consolidation Waste extruded magnesium–lithium alloys can be recycled and remanufactured, thereby broadening the pathways for the circular utilization of magnesium alloys.

V. Main Findings of the Study

Adopt Ultrasonic atomization technology It can efficiently produce spherical powders from the conventional extruded Mg-7.5Li-3Al-Zn alloy, with no loss of lithium during the preparation process, making it an excellent powder‑making method for the resource‑oriented recycling of magnesium alloys.

Both LPBF and PPS powder consolidation processes can significantly refine the alloy’s grain size and enable flexible control of the α(Mg) to β(Li) phase ratio, resulting in an alloy with a microhardness that markedly surpasses that of conventional extruded alloys.

Defects represent a critical weakness in corrosion performance: internal porosity in LPBF alloys and unrecrystallized grain boundaries in PPS alloys accelerate corrosion, resulting in lower corrosion resistance compared to conventional extruded alloys.

Powder metallurgy holds great promise for the preparation of fine-grained magnesium–lithium alloys and the recycling of spent magnesium alloys; going forward, it will suffice to optimize process parameters to achieve material… Complete densification and full recrystallization , thereby fully leveraging the performance advantages of powder consolidation processes.

Image and Text Guide

Table 1: Lithium concentrations (wt%) determined by atomic absorption spectroscopy (AAS) and zinc concentrations (wt%) determined by inductively coupled plasma optical emission spectroscopy (ICP-OES).        

Figure 1. Scanning electron microscope images of Mg-7.5Li-3Al-Zn alloy powder: a) overall view of the powder; b) image of an individual particle; c) microstructure of the powder; d) EDX analysis results for region A; e) magnified view of the area in b); f) magnified view of a localized region of the powder’s microstructure.

Figure 2. a) XRD patterns of Mg-7.5Li-3Al-Zn powder, b) PPS alloy, and c) LPBF‑fabricated alloy.

Table 2. Relative phase compositions (%) of the powder, PPS, and LPBF alloy, calculated from XRD measurements.

Figure 3. Power spectral density (PSD) obtained using a laser diffraction particle size analyzer.

Figure 4. SEM microstructural observations of a) the extruded alloy, b) the LPBF‑fabricated alloy, and c) the PPS alloy.

Figure 5. Microhardness distribution curves for extrusion‑formed, PPS‑based, and LPBF‑sintered alloys.

Figure 6. Bright-field transmission electron microscopy (TEM) images obtained from the alloys under investigation: a) and b) show LPBF‑fabricated alloys; c) and d) show PPS samples.

Figure 7. Planar microstructural images and their corresponding three-dimensional reconstructions, showing: a) the extrusion‑formed Mg‑7.5Li‑3Al‑Zn alloy; b) the alloy fabricated by LPBF; c) the porosity density distribution in the PPS alloy.

Figure 8. Pore distribution in LPBF alloys (pore volume fraction = 3.51%, average pore diameter = 28.3 μm, standard deviation = 18.2 μm, maximum pore diameter = 151.6 μm).

Table 3 Electrochemical parameters calculated using the Tafel method, based on potentiodynamic polarization test results obtained in a 0.01 M sodium chloride solution.

Table 4 Corrosion Rates Calculated by the Hydrogen Evolution Method.

Figure 9. Results of electrochemical measurements conducted in a naturally aerated 0.01 M sodium chloride solution: a) Ecorr values over a 1-hour period; b) Potentiodynamic polarization curves.

Figure 10. EIS measurement results obtained in a naturally aerated 0.01 M sodium chloride solution: a) Nyquist plot; b) EEC curve used for data fitting.

Table 5: Parameters obtained by fitting EIS data using the EEC model shown in Figure 8b

Figure 11. Characterization of the corroded surface of a conventionally extruded Mg-7.5Li-3Al-Zn alloy after immersion in 0.01 M sodium chloride for 1 hour: a) Surface covered with corrosion products; b) Surface without corrosion products; c) Surface without corrosion products (shown at higher magnification).

Figure 12. Characterization of the corroded surface of the Mg-7.5Li-3Al-Zn alloy fabricated by LPBF after immersion in 0.01 M sodium chloride for 1 hour: a) Surface covered with corrosion products; b) Surface without corrosion products; c) High-magnification image of the surface devoid of corrosion products.

Figure 13. Characterization of the corroded surface of the PPS Mg-7.5Li-3Al-Zn alloy after immersion in 0.01 M sodium chloride for 1 hour: a) Surface covered with corrosion products; b) Surface without corrosion products; c) Surface without corrosion products (shown at higher magnification).

Table 6 Polarization resistance values calculated based on the EIS results.