I. Research Background: Pain Points in the Development of Biodegradable Iron-Based Alloys

At present, magnesium-, zinc-, and iron-based alloys are the most widely used biodegradable metals: magnesium alloys tend to generate excessive gas during degradation, while iron-based alloys generally exhibit… The degradation rate is relatively slow. This issue limits its clinical application. To optimize performance, researchers adjusted the manganese–carbon ratio to enhance the properties of the iron alloy: a composition containing approximately 32 wt% manganese and 1.2 wt% carbon can stably form a non‑magnetic austenitic phase, meeting the requirements for MRI examinations; however, the high manganese content is prone to evaporative loss during high‑temperature processing, imposing stringent demands on the fabrication process.

Meanwhile, the fabrication route emerges as a critical factor influencing alloy performance. While conventional casting and forging processes are well-established, laser powder bed fusion (LPBF), as a leading additive manufacturing technique, enables the customization of patient‑specific implants and the fabrication of porous structures, thereby effectively accelerating material degradation; in recent years, it has been increasingly applied to the development of biomedical ferrous alloys. However, systematic comparative studies on the differences in alloy microstructures, corrosion behavior, and biocompatibility between these two processing methods remain scarce—this constitutes the central focus of the present investigation.

II. Sample Preparation: Two Processes + Multiple Parameter Comparisons

This study uniformly adopted Fe-Mn-C The base alloy system comprises two major experimental groups: a conventional fabrication group and an LPBF additive manufacturing group, while… The focus is on preparing LPBF‑specific alloy powders using plasma‑assisted ultrasonic atomization technology. 。

1. Preparation of Raw Material Powder

��  Plasma-assisted ultrasonic atomization for powder production This is the core preparation technology for the additive manufacturing feedstock in this study. The research first uses iron–manganese alloy billets produced by conventional casting or forging as the base material, and employs… Poland’s AMAZEMET rePowder plasma-assisted ultrasonic atomization equipment The material is processed into alloy powder. The resulting powder exhibits near‑spherical particles with a particle size distribution centered between 40 and 70 μm, yielding an average particle diameter of approximately 53 μm, which meets the requirements of the LPBF process. Due to the high vapor pressure of manganese, some manganese evaporates during the atomization process; therefore, the initial feedstock was deliberately enriched in manganese content to ensure that both the as‑produced powder and the final alloy product maintain a stable manganese concentration of around 32%. Analysis reveals a uniform distribution of iron and manganese within the powder, with a thin, discontinuous layer of manganese oxide coating the particle surfaces.

2. Preparation of Block Specimens

Traditional Crafts Group The alloy raw materials were melted in an induction furnace at 1700°C, cast into ingots by gravity casting, and then subjected to hot forging and water‑quenching annealing to obtain standard specimens, which served as blank controls.

LPBF Additive Manufacturing Group : Using the AconityMINI device, with fixed layer thickness, scan spacing, and other basic parameters set, Process parameters of the two groups And ensure that the total input energy remains constant:

• LPBF 200/400: Laser power 200 W, scanning speed 400 mm/s
• LPBF 350/700: Laser power 350 W, scanning speed 700 mm/s

According to inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis, the carbon and manganese contents of all samples were essentially identical, thereby eliminating the influence of chemical composition on the experimental results.

III. Microstructure: Differences in processing lead to significant variations in microstructural features.

The study employs characterization techniques such as XRD, EBSD, and SEM to analyze the phase composition, grain structure, and grain boundary characteristics of various samples.

• Phase composition : All specimens consist of a single austenitic phase (γ phase), with no magnetic martensite or other secondary phases, exhibiting excellent MRI compatibility. Only LPBF specimens fabricated from pure elemental powders develop oxide inclusions, confirming… Alloy powder + ultrasonic atomization powder production It is the preferred solution for ensuring phase purity.
• Grain morphology : Traditional cast and forged alloys are Equiaxed grains ; The alloy prepared by LPBF exhibits a typical Columnar grains , accompanied by dendritic structures ranging from the nanoscale to the micrometer scale.
• Grain Orientation and Grain Boundaries : The two LPBF specimen types exhibit distinct grain‑orientation‑difference strengths, with the 350/700 parameter set showing a higher orientation difference; the three specimen classes have comparable fractions of high‑angle grain boundaries, which do not account for the primary differences in mechanical performance. Instead, **coincident lattice site (CLS) grain boundaries—particularly ∑3 boundaries—emerge as the key distinguishing factor**: the number of ∑3 twin boundaries in conventional alloys is approximately six times that observed in LPBF specimens.

IV. Corrosion and Degradation Performance: Grain Boundaries Govern the Degradation Rate

The study conducted potentiodynamic polarization tests and static immersion experiments in Hank’s solution, which simulates a physiological environment, and in lactic acid solution, which simulates a muscular environment, while employing XPS analysis to elucidate the corrosion mechanisms.

• Overall Corrosion Pattern : The corrosion rate ranking is as follows LPBF 350/700 > LPBF 200/400 > Conventional cast and forged alloys . Traditional alloys exhibit the highest corrosion potential and the best corrosion resistance; however, the LPBF process significantly accelerates alloy degradation, and high‑power, high‑scan‑speed processing parameters further increase the corrosion rate. Lactic acid immersion tests also confirm that the concentrations of iron and manganese ions leached from LPBF specimens are substantially higher than those from conventionally manufactured specimens.
• The Crucial Role of Grain Boundaries : At low Σ3 values, the CLS grain boundary structure is stable and can impede the penetration of corrosive media, The higher the proportion of grain boundaries, the greater the alloy’s corrosion resistance. . In LPBF specimens, the scarcity of grain boundaries is the primary reason for their accelerated degradation; meanwhile, the nano‑oxide particles inherent in the powder and the micro‑porosity generated during additive manufacturing further exacerbate corrosion.
• Initial Corrosion Mechanism (XPS Analysis) A mixed iron–manganese oxide film rapidly forms on the alloy surface, with manganese oxides predominating in the surface layer. Manganese has a lower reduction potential, resulting in an oxide film of poor stability and weaker passivation compared to the oxide layer on pure iron, ultimately accelerating electrochemical corrosion.

V. Biocompatibility: All parameters met the standards, with only slight differences observed in cell spreading.

The study employed human osteoblasts (MG-63 cells) to conduct in vitro biocompatibility testing, simulating orthopedic implantation conditions:

1. Cytotoxicity : The proportion of dead cells in all samples was below 10%, with an extremely small difference compared to the blank glass control group. None of the three alloy types exhibited significant cytotoxicity. , with good cell adhesion capability.
2. Cell morphology and spreading : On the surfaces of conventional alloy and LPBF 350/700 specimens, cell spreading is robust, with well‑developed pseudopodia; in contrast, cells on LPBF 200/400 specimens are smaller in size and exhibit slightly reduced spreading, though this does not compromise short‑term biocompatibility.

Overall, both traditional fabrication methods and LPBF‑additive‑manufactured iron–manganese alloys meet the biocompatibility requirements for medical implant materials.

VI. Core Research Findings

1. An iron–manganese–carbon alloy containing 32% manganese and 1.2% carbon can form a non‑magnetic austenitic microstructure after being prepared via either of two processing routes, thereby meeting the requirements of clinical MRI diagnostics. Plasma-assisted ultrasonic atomization technology High-quality spherical alloy powders can be produced, which is a critical enabler for the stable operation of the LPBF process.
2. The fabrication process profoundly alters the alloy’s microstructure: conventionally cast and forged alloys, rich in stable Σ3 twin boundaries, exhibit the lowest corrosion rates; in contrast, LPBF‑additive manufacturing, characterized by a reduced density of Σ3 boundaries and the presence of oxides and porosity, shows a markedly increased degradation rate, which can be further tuned by adjusting laser power and scanning speed.
3. Corrosion behavior is primarily driven by the interplay of grain boundary types and nano‑oxide particles; the unstable manganese‑based oxide film on the alloy surface serves as a key initiating factor for the onset of corrosion.
4. The biocompatibility of the iron–manganese alloys prepared by both processes was satisfactory, with no cytotoxicity. LPBF‑fabricated iron–manganese alloys fully possess the application potential for medical biodegradable implants. , which can be used for the development of personalized orthopedic and maxillofacial bone repair implants.

Research Summary

This study has established a complete logical chain linking “fabrication process — microstructure — degradation performance — biocompatibility.” Traditional iron–manganese alloys exhibit excellent corrosion resistance, making them well suited for implantable applications requiring long-term mechanical support; meanwhile, leveraging… Plasma-assisted ultrasonic atomization for powder production Iron–manganese alloys produced via LPBF additive manufacturing exhibit controllable degradation rates and enable personalized shaping, aligning seamlessly with the current trend toward precision medicine and providing crucial experimental evidence for optimizing the fabrication processes and facilitating the clinical translation of bioresorbable metallic implants.

Image and Text Guide

Table 1 Carbon and Manganese Contents (in weight percent, with iron as the balance).

Figure 1. Fe–Mn powders prepared by the plasma‑assisted ultrasonic method: (a) SEM/SE images of the powder deposited on a graphite tape; (b) Cross-sectional image of the powder, with EDS chemical analysis results; (c–e) The elemental distributions of Fe, Mn, and O are shown separately.

Figure 2. XRD patterns of Fe–Mn alloys subjected to different processing techniques: (a) LPBF fabricated from alloy powder; (b) LPBF fabricated from pure elemental powders; (c) Cast and forged alloys. The inset shows the EDS elemental distribution map of Mn.

Figure 3. Microstructures of Fe–Mn alloys prepared by different methods and their corresponding GROD images: (a) EBSD IPF–Z maps of Fe–Mn alloys fabricated by LPBF, processed using 200/400 parameters; (b) GROD image; (c) EBSD IPF–Z maps of Fe–Mn alloys fabricated by LPBF, processed using 350/700 parameters; (d) GROD image; (e) EBSD IPF–Z maps of cast and forged Fe–Mn alloys; (f) GROD image.

Figure 4. (a) Potentiodynamic curve; (b) The relationship between corrosion rate in lactic acid and Fe–Mn behavior as a function of the production route.

Figure 5. (a) Overlay of EBSD IPF Z‑axis and band‑contrast images; (b) Image showing CLS grain boundary band contrast with red markings.

Figure 6. Frequency of occurrence of specific types of Σ grain boundaries.

Figure 7. EBSD IPF–Z grain boundary distribution map: (a) In the LPBF FeMn alloy processed with 200/400 parameters, HAGBs (15°–180°) are shown in blue; LAGBs (1°–5° and 5°–15°) are represented in black and red, respectively. (b) LPBF Fe-Mn alloy processed with 350/700 parameters; (c) Casting and forging of Fe-Mn alloys; (d) Grain boundary proportion histogram.

Figure 8. Oxide particle sizes measured from ECCI images of Fe–Mn alloy processed by LPBF: (a) 200/400 process parameters; (b) 350/700 process parameters; (c) The relationship between the number of oxides per μm² and the LPBF process parameters.

Figure 9. Example of XPS analysis results for the LPBF 350/700 in the initial state: (a) Fitted Fe 2p3/2 spectrum; (b) Mn 2p3/2 XPS spectrum; (c) XPS depth-profiling curves for all elements; (d) XPS depth‑profile curves for metal ions only (excluding oxygen). The inset provides a visual illustration of the initial oxidation process at the interface between the oxide layer and the metal layer, along with thickness values derived from these depth profiles.

Figure 10. Adhesion of MG-63 cells to different surfaces after 24 hours of culture: (a) MG-63 cell density and viability on the control glass surface, cast and forged samples, LPBF (200/400) samples, and LPBF (350/700) samples; (b-e) MG-63 cells were separately adhered to (b) Compare with the glass surface, (c) Castings and forged samples, (d) LPBF (200/400) samples and (e) Representative fluorescence microscopy images of LPBF (350/700) samples. Live cell nuclei are stained blue with Hoechst 33342, whereas the nuclei of dead cells—those with compromised plasma membranes—are stained red with propidium iodide.

Figure 11. Scanning electron micrographs of MG-63 cells on the following samples: (a) Against the glass surface; (b) Casting and forging surfaces; (c) LPBF (200/400); (d) LPBF (350/700).