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Powder Bed Fusion - Laser Beam (PBF - LB) is an additive manufacturing (AM) technique enables flexibility in material design. Ti6Al4V, a proven AM alloy, is widely used in the biomedical industry as an implant material. However, its typically poor wear behavior hampers its use. Often, increasing surface hardness can improve wear resistance, but orthopedic implants require low stiffness to minimize stress-shielding. Although pushing the stiffness of a metallic implant material close to the bone is a difficult task, PBF - LB offers potential solutions. This study aims to develop a Ti-alloy using PBF - LB with a lower stiffness while having a harder surface compared to common PBF - LB Ti6Al4V. A two-step strategy was applied, combining powder mixing and laser rescanning. We demonstrate that by employing laser rescanning to a mixed powder of Ti6Al4V and 316L stainless steel, we can print a Ti alloy with a graded microstructure. The resulting Ti alloy possesses harder (>5 GPa) α' microstructure on the surface while having a relatively low bulk stiffness (90 GPa) from a β-matrix microstructure. These findings show the combination of powder mixing and laser rescanning is effective in tailoring the mechanical properties of Ti6Al4V alloy.

In this study, laser-material interactions during laser-powder bed fusion of WE43 magnesium alloy were characterized through numerical and experimental analyses. Various melting modes (i.e., conduction, transition, and keyhole) were induced through deposition of laser tracks at powers ranging from 80 to 130 W, and used as input parameters for a thermo-fluid model. Results of microscopy demonstrated good agreement between numerical and experimental measurements of melt pool depth, as well as a strong correlation between melt pool microstructure and the thermo-fluid conditions predicted by the model. Specifically, for conduction mode at 80 W, a predominance of cellular subgrains within the melt pool was consistent with the predicted steep thermal gradients, while for keyhole mode at 130 W, low thermal gradients correlated with high presence of equiaxed dendrites. Moreover, convection currents attributed to high recoil pressure in keyhole melt pools, were in agreement with locations of numerous subgrain boundaries having non-uniform morphologies, while under conduction, outward Marangoni flow led to a unique alignment of cellular subgrains and fewer subgrain boundaries. This study demonstrates the interplay among processing, thermal history, fluid flow and microstructure in WE43, and provides a basis for future design of microstructures for improved material properties.

The present work explored the potential of customizing the final part texture and mechanical properties of a biodegradable magnesium alloy (WE43, composition Mg-4 wt%Y-3 wt%Nd-0.5 wt%Zr) manufactured by laser beam powder bed fusion (PBF-LB). This was done by printing samples using two sets of laser scan strategies (670 and 900 rotation between consecutively scanned layers) and build directions (horizontal and vertically printed samples). Samples were characterized for density and microstructure, followed by in-depth texture analysis using both lab-based techniques and neutron diffraction measurements. The mechanical performance was evaluated through tensile testing. The findings in this work show that strong basal texture was generated in the build direction. This allowed for altering the mechanical strength of WE43, whereby horizontally built samples showed increased strength and Young's modulus under tensile loading in a direction normal to the basal texture. Laser scan strategy influences the overall texture, however with limited effect on the resulting mechanical properties for the two scan strategies under study. This study demonstrates the importance of sample design and build strategy for the resulting texture and final material properties.

Magnesium alloy WE43 is desirable for biomedical implants due to its biocompatibility, degradability, and mechanical properties closely matching those of natural bone. Processing of WE43 by additive manufacturing (AM), such as powder bed fusion laser beam (PBF-LB), can further its potential for biomedical applications through improved customization (i.e., patient-specific implants) and fabrication of complex geometries. Nevertheless, AM-fabricated Mg implants have yet to reach clinical implementation. To enable this transition, while contributing to the global drive for more sustainable manufacturing, the aim of this research was to investigate the effect of powder reuse during PBF-LB processing of WE43. Specifically, we focus on the influence on both powder properties as well as bulk density and mechanical properties of printed components. Results indicate that repeated reuse alters the powder morphology and particle size distribution (PSD), through the formation of satellites and agglomerates, along with a preferential consumption of smaller particles (<20 mu m) during processing. In turn, powder flowability increased, but both spreadability, and layer density decreased. Only minor compositional changes were observed in both powders and printed samples. Despite these changes in the powder feedstock, the effect on the density of printed samples was minimal, as only small variations in porosity were observed between the initial (0.43 %) and final printing cycles (0.61 %), thereby resulting in a limited effect on sample hardness. The impact on sample density remained minimal despite introducing out-ofspec powder to the feedstock as a worst-case scenario. These findings demonstrate the potential for reducing material wastage when processing biomedical alloy WE43 by PBF-LB and can be used to support powder handling recommendations to ensure quality and sustainability for future commercial applications.

Additive manufacturing for fabrication of patient-specific oral and maxillofacial implants enables optimal fitting, significantly reducing surgery time and subsequent costs. However, it is still common to encounter hardware- or biological-related complications, specifically when radiation treatment is involved. For mandibular reconstruction plates, irradiated patients often experience plate loosening and subsequent plate exposure due to a decrease in the vascularity of the irradiated tissues. We hypothesize that an acceleration of the bone ingrowth prior to radiation treatment can increase the survival of such plates. In this work, a new design of a mandibular reconstruction plate is proposed to promote osseointegration, while providing the necessary mechanical support during healing. In this regard, six different Triply Periodic Minimal Surface (TPMS) structures were manufactured using laser-powder bed fusion. Three-point bending and in-vitro cell viability tests were performed. Mechanical testing demonstrated the ability for all structures to safely withstand documented biting forces, with favorable applicability for the Gyroid structure due its lower flexural modulus. Finally, cell viability tests confirmed high cell proliferation rate and good cell adhesion to the surface for all TPMS structures. Overall, the new design concept shows potential as a viable option for plates with improved functionality and higher survival rate.

Magnesium (Mg) alloys are gaining significant interest in biomedical applications, particularly as biodegradable implants. Mg alloys naturally dissolve in the body, eliminating the need for a second surgery to remove implants after healing [1]. Additionally, Mg is biocompatible and has mechanical properties close to that of human bone, which reduces stress shielding and promotes better bone regeneration. However, the main challenge lies in controlling the rapid corrosion rate of Mg alloys, which can lead to premature degradation. Their corrosion behaviour must be tailored to allow controlled degradation in sync with bone healing. Current research focuses on tuning of microstructure and alloying strategies, including eliminating the use of rare-earth elements (REEs) in the most recent studies[3], while maintaining the alloy’s biodegradability.

Additive manufacturing has potential for increased corrosion resistance through design of unique microstructures, while enabling additional flexibility in designing complex, patient-specific implants [2]. AM of Mg alloys is challenging due to the narrow processing window available to maintain the Mg between its melting and boiling temperatures, especially considering that oxides are also present in the initial powder. The aim of the present study is to accelerate the development and parameter optimization for the laser powder bed fusion (L-PBF) processing of a novel Mg-Ca-Zn-based alloy.

A finite element code implemented on Python was used to model the laser interaction with the material. The melt pool shapes were calibrated with experimental single tracks on printed samples. In addition, the application of the CALPHAD method and ThermoCalc databases were employed to predict the nucleation of stable phases during the solidification process with the classical nucleation and growth theory (CNGT).

Steady state nucleation of three main phases (Mg-hcp, Ca₂Mg₆Zn₃ and MgZn) was obtained using two different interfacial energies calculation methods. Local thermal history of single tracks calculated with the FE model were used to run the CNGT code. Growth parameters were also calculated with a first non-supersaturated approach.

This work represents the initial steps toward developing a comprehensive layer-by-layer simulation. Currently, the model remains semi-empirical, highlighting the need for further refinement. Future improvements should focus on leveraging experimental data for validation rather than calibration, ensuring a more robust and predictive simulation framework.

Commercial purity titanium (cp-Ti) is considered for replacing Ti64 as an implant material in various applications, due to the potential toxicity associated with the release of Al and V ions. However, the mechanical properties of cp-Ti, particularly fatigue resistance, are inadequate for this purpose. In this study, cp-Ti grade 4 rods were processed using a combination of equal channel angular pressing and rotary swaging (ECAP/RS). Tensile and fatigue tests were conducted, along with detailed microscopy and evaluation of corrosion resistance and biocompatibility. An average yield strength of 1383 MPa was obtained while maintaining moderate ductility of 10 %. This represents the highest strength ever recorded for cp-Ti, even exceeding that of Ti64. Additionally, fatigue endurance limit increased by 43 % up to 600 MPa, almost obtaining that of Ti64. Strengthening mechanisms were attributed to the ultrafine-grained (UFG) microstructure generated by ECAP/RS, along with strong crystallographic texture and formation of sub-grain structure. Furthermore, the corrosion resistance and biocompatibility of cp-Ti were largely unaffected, potentially easing regulatory transition in future medical devices. Thus, these results demonstrate high potential of combined ECAP/RS processing to manufacture UFG cpTi grade 4 materials that prospectively allow for the substitution of questionable alloys and downsizing of medical implants.

Powder bed fusion – laser beam (PBF-LB) of Mg alloys provides new possibilities for the production of complex structures with optimized designs, both for weight reduction in aerospace applications, as well as for patient-specific implants in orthopedic applications. However, even though numerous studies have been carried out on the topic, the influence of the individual PBF-LB process parameters on the microstructure and resulting material properties of Mg alloys remains ambiguous. Thus, this study aims to investigate the influence of laser power on the surface roughness, microstructure and resulting key material properties, namely corrosion resistance and mechanical performance. Samples were produced by PBF-LB from gas atomized Mg-4%Y-3%Nd-0.5%Zr (WE43) alloy powder, using three different laser powers: 60 W, 80 W, and 90 W. Contrary to expectation, the 90 W samples exhibited the highest degradation rate, while 60 W samples had the lowest, despite the latter having highest surface roughness and large internal pores. The higher degradation rate for the 90 W samples was instead found to stem from the near-surface microstructure. The higher energy input and subsequently reduced grain size, resulted in an increased amount of second phase precipitates than for the 60 W samples, thereby increasing the tendency for pitting via microgalvanic corrosion. For the tensile strength and elongation at break, the opposite trend was observed. Here, a reduction in grain size and an increase in precipitates for the 90 W samples were found to be beneficial. In conclusion, a definite influence of laser power on the formation of microstructure was observed, ultimately impacting the resulting corrosion and tensile properties of WE43. Future work should investigate the influence of other PBF-LB process parameters, with the aim of establishing an optimum balance between corrosion resistance and mechanical properties.

INTRODUCTION: WE43 Mg alloy is one of the few biodegradable alloys currently used in clinical applications. However, most WE-based implants in use today are bulk forms (e.g., bone screws) and are conventionally manufactured (e.g., casting, extrusion). With the advancement of 3D printing technologies, such as Laser Powder Bed Fusion (LPBF), the fabrication of more intricate and porous implants has become feasible. The main challenge with Mg-based lattice implants though, is their excessive degradation rate. Literature reports varying results for WE43, with some lattices losing structural integrity within a day, and others maintaining it for up to 28 days [1-2]. This study aims to understand the underlying mechanisms for such discrepancies, primarily by investigating the influence of structure geometry, including unit cell configuration and relative density, as well as that of alloy microstructure.

METHODS: LPBF process optimization was first performed through variation of the laser power and scanning speed. Various structures, including Triply Periodic Minimal Surfaces (TPMS) and strut-based lattices, were designed and fabricated alongside bulk samples. These designs were chosen to explore the influence of geometry on both microstructure and corrosion behavior. Bulk corrosion testing was performed through hydrogen evolution measurements in phosphate-buffered saline (PBS) for two distinct durations: an initial short-term test of 2 hours, and a longer-term test of 3 days.  Furthermore, potentiodynamic polarization (PDP) and novel acoustic emission (AE) measurements were performed to identify underlying corrosion mechanisms at localized regions, including those attributed to the complex geometrical features of lattice structures.

RESULTS: High quality samples with densities as high as 99.6% were achieved. Short-term corrosion measurements on various samples indicated that strut-based lattice structures lost their structural integrity within the initial 2-hour period. In contrast, both the TPMS and bulk samples maintained their structural integrity after 3 days. AE measurements showed good correlation to PDP curves, clearly indicating the instance of surface breakdown and re-passivation, as shown in Fig. 1 for a bulk WE43 sample (black circles). Moreover, the AE amplitude remained at a constant level until the end of the test, indicating minimal cracking and structural damage; in agreement with immersion tests, where bulk-samples maintained integrity. In contrast, a higher occurrence of surface breakdown was seen in the case of strut-based lattices due to sharper edges compared to TPMS. This is also consistent with the increase in surface exposure to the medium and enhanced electrochemical pitting observed within the microstructure.

DISCUSSION & CONCLUSIONS: Structural geometry has a profound effect on the corrosion behavior of LPBF-fabricated WE43 Mg alloy. Both TPMS and bulk samples exhibit superior corrosion resistance compared to strut-based lattices whose inherent sharp edges and corners act as initiation sites for localized corrosion. AE and PDP measurements, together with microscopy, provide insight into the localized corrosion mechanisms and highlight the importance of optimizing strut thickness and orientation in lattices. Future investigations seek to design optimized lattice structures capable of striking a balance between mechanical performance and corrosion resistance.

REFERENCES: 1 M. Li, et al. (2021) Mater Sci Eng C 119:111623.2 Y. Li, et al. (2018) Acta Biomater 67:378–392