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This work studied the effects of an on-line static magnetic field on the defects, microstructures, and mechanical properties of AlSi7Mg samples fabricated by laser powder bed fusion (LPBF). Process experiments were carried out on a self-developed LPBF equipment with an on-line static magnetic field generating system, where magnetic field intensity was adjustable from 0 to 0.3 T. With the action of static magnetic field, the relative density of samples increased from 96.9% to 98.6%. Furthermore, the solidification front of the columnar grain in the mushy zone was broken. With the increase of magnetic field intensity, the crystallographic orientation changed from strong to , and uniform distribution and the average grain was gradually refined from 8.35 to 7.22 μm. Based on the above optimisation, the ultimate tensile strength increased from (326.67 ± 5.31) MPa to (382.00 ± 2.45) MPa. Simultaneously, the elongation at break increased from 8.48% ± 0.20% to 11.78% ± 0.20%. In general, the reduction of pores, the refinement of grains and the increase of Mg2Si precipitates contributed to the simultaneous enhancement of strength and toughness together. This study could provide a new idea for laser additive manufacturing of excellent performance aluminum alloys.

In this work, a hot isostatic pressing (HIP) treatment was used to reduce the pores and improve the mechanical properties of binder jetting (BJ) fabricated 316L stainless steel. The relative density of the HIP treated samples sintered under vacuum, nitrogen, and argon atmospheres increased from 89.20∼90.76% to 94.02∼98.35%. The mean diameter of the internal closed pores decreased from 11.83∼14.70 μm to 0.94∼1.53 μm, and the internal porosity reduced from 5.47∼8.67% to 0.14∼0.35%. A remarkable enhancement of mechanical properties was achieved with the tensile strength by ∼15% and the elongation by ∼100% due to the reduction of porosity. The mechanical properties of the BJ 316L sintered under nitrogen and treated with HIP were even better than those of the direct powder HIP components due to the interstitial solution strengthening. These findings will be a valuable reference for optimising the HIP parameters to produce metal BJ components with superior mechanical properties.

Binder jetting (BJ) selectivity jets binders on powders to form green parts efficiently. The following sintering process is indispensable to consolidating as-printed loose parts, inevitably introducing shrinkage and grain growth. The pore morphology and grain size will change significantly in the sintering process, directly affecting the sintered parts' density and mechanical properties. However, there is no research on the microstructure evolution of BJ 316L in the sintering process to date, which is essential for the further application of BJ 316L. This study used a solid state sintering process to consolidate the BJ 316L green parts. The effect of vacuum sintering temperature (1300 °C–1400 °C) on the density, porosity, linear shrinkage, microstructure evolution, and tensile properties of the as-sintered parts was investigated. Results indicated that the sintering temperature significantly affects the properties of the sintered 316L parts. Notably, the grain size and tensile strength strongly correlate with porosity. The sintered 316L parts have a relative density of 92.0% and preferable mechanical properties (ultimate tensile strength of up to 473.7 MPa and elongation of up to 40.22%) after sintering at 1380 °C. The pores in the sintered BJ 316L were distinguished into three types according to pores' size and distribution characteristics. The potential influences of these three types of pores on the properties of the sintered BJ 316L were discussed. This work is engaged in providing a detailed reference for the densification and microstructure evolution of BJ 316L parts.

Laser beam intensity profile used in laser powder bed fusion (LPBF) process is generally Gaussian, which may not be the best to deliver site-specific microstructures. In this study, a flat-top laser with large-size laser spot and uniform intensity distribution is adopted in the LPBF process with Inconel 718 powder. Comparison investigations are conducted to the standard Gaussian laser. Firstly, the parameters for the flat-top laser are optimized by multi-physics simulation of single tracks. Then, the thermodynamic behaviors and morphologies of the molten pool are observed combined with experiments. A cubic sample is also fabricated utilizing double-laser system to reveal the different characteristics of the resultant microstructures. The results show that the flat-top laser can work at a relatively wider range of laser power without depression, which often occurs in the case of Gaussian laser. The molten pool induced by the flat-top laser has large width-depth ratio with a flat bottom boundary, and more moderate flow in the half bottom. These features combined with the temperature and flow fields of the flat-top laser provide advantageous conditions for directional epitaxial growth of columnar grains during multi-layer fabrication. The epitaxial growth is retained and can extends to 4 mm from the single-crystal baseplate. This demonstrates great potential of LPBF with the adopted flat-top laser to be applied in fabricating or repairing components with directional crystal structures or single-crystal structures.

During the multi-laser powder bed fusion (ML-PBF) process, the quality of the overlap region is essential to ensure the integrity between different laser scanning areas. Here, six different overlap region widths (ORWs) are selected to systematically investigate the effect of ORWs on densification, metallurgical defects, crystalline microstructure, and mechanical performance of the Hastelloy X samples processed by multi-laser powder bed fusion. ORWs are set to 0, 50, 100, 150, 200, and 250 μm, respectively. With an increment in ORW, the porosity gradually increases from 0.01% to 7.08%. Inversely, microcracks are reduced with increasing the ORWs due to the smaller width-to-depth ratio of the melt pools and the increased degree of recrystallisation. All the samples display equivalent ultimate tensile strength of ∼880 MPa, while the elongations at break of the overlap samples are 3.5%−10.5% lower than that of the single-laser processed one (28.4%). The inferior ductility is ascribed to the decreased densification in the overlap regions, of which the adverse effect can be minimised when an ORW of 150 μm is utilised. This work is anticipated to provide efficient reference and theoretical guidance for the large-size nickel-based superalloy components fabricated by ML-PBF.

Recent reports on the selective laser melting (SLM) process under a vacuum or low ambient pressure have shown fewer defects and better surface quality of the as-printed products. Although the physical process of SLM in a vacuum has been investigated by high-speed imaging, the underlying mechanisms governing the heat transfer and molten flow are still not well understood. Herein, we first developed a mesoscopic model of SLM under variable ambient pressure based on our recent laser-welding studies. We simulated the transport phenomena of SLM 316L stainless steel powders under atmospheric and 100 Pa ambient pressure. For typical process parameters (laser power: 200 W; scanning speed: 2 m∙s−1; powder diameter: 27 μm), the average surface temperature of the cavity approached 2800 K under atmospheric pressure, while it came close to 2300 K under 100 Pa pressure. More vigorous fluid flow (average speed: 4 m∙s−1) was observed under 100 Pa ambient pressure, because the pressure difference between the evaporation-induced surface pressure and the ambient pressure was relatively larger and drives the flow under lower pressure. It was also shown that there are periodical ripple flows (period: 14 μs) affecting the surface roughness of the as-printed track. Moreover, the molten flow was shown to be laminar because the Reynolds number is less than 400 and is far below the critical value of turbulence; thus, the viscous dissipation is significant. It was demonstrated that under a vacuum or lower ambient pressure, the ripple flow can be dissipated more easily by the viscous effect because the trajectory length of the ripple is longer; thus, the surface quality of the tracks is improved. To summarize, our model elucidates the physical mechanisms of the interesting transport phenomena that have been observed in independent experimental studies of the SLM process under variable ambient pressure, which could be a powerful tool for optimizing the SLM process in the future.

The formation of balling, porosity and cracking defects is a vital obstacle in selective laser melting of wrought Al alloys. However, it lacks systematic research on the origins of these imperfections. Herein, the formation mechanisms and avoidance methods of metallurgical defects in slective laser melting (SLM)-processed Al–Cu–Mg alloy were investigated by numerical simulation and microstructure characterization. Process optimization by altering laser energy density can effectively suppress balling and porosity, thus enhancing relative density. Cracking results from the stress concentration and columnar grains arise due to the rapid cooling process during SLM. The methods that promote the columnar-to-equiaxed grain transition, such as microalloying by Sc/Zr/Ti elements, co-incorporation of ceramic particles and introducing ultrasound, can effectively enhance the cracking resistance and mechanical properties of wrought Al alloys.

Federated learning (FL) allows a number of parties collectively train models without revealing private datasets. There is a possibility of extracting personal or confidential data from the shared models even-though sharing of raw data is prevented by federated learning. Secure Multi Party Computation (MPC) is leveraged to aggregate the locally-trained models in a privacy preserving manner. However, it results in high communication cost and poor scalability in a decentralized environment. We design a novel communication-efficient MPC enabled federated learning called CE-Fed. In particular, the proposed CE-Fed is a hierarchical mechanism which forms model aggregation committee with a small number of members and aggregates the global model only among committee members, instead of all participants. We develop a prototype and demonstrate the effectiveness of our mechanism with different datasets. Our proposed CE-Fed achieves high accuracy, communication efficiency and scalability without compromising privacy.

Additive manufacturing (AM) is a crucial aspect of contemporary science and engineering, enabling the layer-by-layer production of components [...]

Three-dimensionally printed materials show great performance and reliable stability in the removal of refractory organic pollutants in Fenton-like reactions. In this work, hierarchically porous zero-valent copper (3DHP-ZVC) was designed and fabricated via 3D printing and applied as a catalyst for the degradation of tetracycline (TC) through heterogeneous Fenton-like processes. It was found that the 3DHP-ZVC/H2O2 system could decompose over 93.2% of TC within 60 min, which is much superior to the homogeneous Cu2+/H2O2 system under similar conditions. The leaching concentration of Cu2+ ions in the 3DHP-ZVC/H2O2 system is 2.14 times lower than that in the Cu powder/H2O2 system in a neutral environment, which could be ascribed to the unique hierarchically porous structure of 3DHP-ZVC. Furthermore, 3DHP-ZVC exhibited compelling stability in 20 consecutive cycles. The effects of co-existing inorganic anions, adaptability, and pH resistance on the degradation of TC were also investigated. A series of experiments and characterizations revealed that Cu0 and superoxide radicals as reducing agents could facilitate the cycling of Cu(II)/Cu(I), thus enhancing the generation of hydroxyl radicals to degrade TC. This study provides new insights into employing promising 3D printing technology to develop high-reactivity, stable, and recycling-friendly components for wastewater treatment.

In this study, for the first time, a forward-rotating roller is proposed for the spreading of CF/PA12 composite powder in the selective laser sintering (SLS) process. The mesoscopic kinetic mechanism of composite particle spreading is investigated by utilizing the “multi-spherical” element within the discrete element method (DEM). The commercial software EDEM and the open-source DEM particle simulation code LIGGGHTS-PUBLIC are used for the simulations in this work. It is found that the forward-rotating roller produces a strong compaction on the powder pile than does the conventional counter-rotating roller, thus increasing the coordination number and mass flow rate of the particle flow, which significantly improves the powder bed quality. In addition, the forward-rotating pattern generates a braking friction force on the particles in the opposite direction to their spread, which affects the particle dynamics and deposition process. Therefore, appropriately increasing the roller rotation speed to make this force comparable to the roller dragging force could result in faster deposition of the composite particles to form a stable powder bed. This mechanism allows the forward-rotating roller to maintain a good powder bed quality, even at a high spreading speed, thus providing greater potential for the industry to improve the spreading efficiency of the SLS process.

Thermal fatigue is the main failure mode for chromium hot-work steels. In this study, pre-alloyed chromium hot-work steel powders with three different Cr, Mo, and V addition levels (low content (LH), medium content (MH), and high content (HH)) were used for selective laser melting (SLM). The microstructure and thermal fatigue properties of these SLM-processed materials were investigated. After thermal fatigue tests, LH possessed the lowest hardness (approximately 573 HV5) and longest crack length, MH possessed the highest hardness (approximately 688 HV5) and HH (with the hardness of approximately 675 HV5) possessed the shortest crack length. It can be concluded that the increase of V content in MH is the main reason for the refined grains which result in an enhanced hardness and thermal fatigue resistance compared to LH. The further increase of the Cr and Mo content in HH leads to the grain coarsening and hardness decreasing, which is supposed to degrade the thermal fatigue resistant properties according to the conventional theory. However, HH exhibited an enhanced thermal fatigue resistance compared to MH. That is because the higher stored energy in MH deteriorated its thermal fatigue resistance compared to HH.

Biomimetic functionalization is a pressing need for orthopedic implants to achieve reinforced osseointegration and accelerated osteogenesis, thereby obtaining a successful clinical outcome. To realize bioactivity, TiNb alloys with varying Nb contents (0–45 at.%) were additively manufactured using elemental powder blends. The regulation of the Nb contents could balance the surface constituent ratio of Nb2O5 and TiO2, enhancing the formation of the hydroxyl group and surface hydrophilicity. Moreover, the effect mechanism of Nb2O5 on immune modulation and bone regeneration was first put forward. The incorporation of Nb improved the viability of macrophages by inducing a macrophage polarization toward an anti-inflammatory M2-like state. The variation of morphological and up-regulated expression of anti-inflammatory factors led to a more favorable osteoimmune microenvironment for osteogenesis, promoting the osteogenic differentiation of osteoblasts in vitro. Considering the surface constituent and in vitro biological performance, 3D-printed Ti25Nb scaffolds were placed into the femoral condyle of the rabbit. The Micro-CT and histological analysis indicated that Ti25Nb scaffolds could accelerate bone regeneration and reinforce bone integration as relative to pure Ti. Overall, the Ti25Nb scaffold exhibited structural bionic ability, surprising immunomodulatory properties, superior bone regeneration and osseointegration capacity, showing exceptional potential in the treatment of bone defects.

Using numerical simulations and experimental tests, the temperature and velocity fields of the molten pool during selective laser melting (SLM) were investigated, where the laser scanning speed ranging from 2.5 m/s to 0.3 m/s was employed. Experiments for single tracks and part samples were conducted for verification. Three kinds of molten pool states were identified and investigated: unstable state, transition state and stable state. The unstable state is characterized by numerous balling defects, where the bulk density is severely deteriorated. The transition state is featured by the transition region where the melt velocity is relatively lower, and the molten pool is vulnerable to the necking defect. The molten pool with a depression region is identified as the stable state. A small depression is favorable for improving the surface quality of single track and the bulk density. However, exorbitant energy input will convert the depression into a keyhole. Additionally, a threshold of the scanning speed was found, where the bulk density peaked. Over the threshold, the density decreased continuously with the speed increasing. However, the density slightly decreased by 1.5% when the speed was below the threshold; this anomaly was ascribed to the residual pores induced by the recoil pressure. Keywords: Scanning speed, Selective laser melting, Temperature and velocity fields, Molten pool states, Threshold

Donor shortages for organ transplantations are a major clinical challenge worldwide. Potential risks that are inevitably encountered with traditional methods include complications, secondary injuries, and limited source donors. Three-dimensional (3D) printing technology holds the potential to solve these limitations; it can be used to rapidly manufacture personalized tissue engineering scaffolds, repair tissue defects in situ with cells, and even directly print tissue and organs. Such printed implants and organs not only perfectly match the patient’s damaged tissue, but can also have engineered material microstructures and cell arrangements to promote cell growth and differentiation. Thus, such implants allow the desired tissue repair to be achieved, and could eventually solve the donor-shortage problem. This review summarizes relevant studies and recent progress on four levels, introduces different types of biomedical materials, and discusses existing problems and development issues with 3D printing that are related to materials and to the construction of extracellular matrix in vitro for medical applications. Keywords: 3D printing, Implant, Scaffold, Biomedical material

The support material is necessary to prevent the collapse of the suspended structure when forming complex ceramic parts via layered extrusion forming (LEF, a type of additive manufacturing) method, and needs to be removed easily after sintering. In this work, salt-based slurries were prepared with magnesium sulfate monohydrate (MSM) and polyvinyl pyrrolidone (PVP) in order to solve the problem of extrusion and removal of the support material. The effect of the solid content on the rheological performance of slurries and the printing quality of specimens was studied. The results showed that the salt-based slurry containing 60 wt% MSM and 10 wt% PVP powder possessed shear-thinning behavior and the corresponding samples held sufficient bending strength and superior surface quality, which satisfied the requirements for the support structures in LEF method. After calcination at different temperature, the salt-based support structure could be easily removed, which offers an alternative support material choice for the LEF method. Keywords: Layered extrusion forming, Salt-based slurry, Magnesium sulfate, Rheology, Soluble support material

Laser powder bed fusion (LPBF) technology is beneficial for the fabrication of thermal conductive materials, integrating with the predesigned structure, which shows a great potential for high heat dissipation applications. Here, a Cu–Cr–Zr alloy with relative density of 98.53% is successfully prepared by LPBF after process optimization. On this basis, microstructure, phase identification, precipitates, mechanical and thermal properties are investigated. The results demonstrate that the surface morphology of microstructure is affected by laser energy density, the α-Cu is the main phase of the LPBF sample and the virgin powder, the size of Cr spherical precipitates in some areas is about 1 μm, and the tensile fracture mode is a mixed ductile–brittle mode. Furthermore, the Vickers hardness of the LPBF Cu–Cr–Zr sample is 70.7 HV to 106.1 HV, which is higher than that of LPBF Cu and a wrought C11000 Cu, and the difference in Vickers hardness of different planes reflects the anisotropy. Ultimately, the two types of Cu–Cr–Zr alloy heat sinks are successfully fabricated, and their heat transfer coefficients are positively correlated with the volume flow. The heat dissipation performance of the cylindrical micro-needle heat sink is better, and its maximum heat transfer coefficient is 3887 W/(m2·K).

In this work, a novel approach combining liquid deposition with selective laser melting (SLM) is used for fabricating reduced graphene oxide (RGO)/S136 metal matrix composites (MMCs). The grain sizes, crystallographic textures, phase compositions and mechanical properties can be tailored by controlling the RGO content in the RGO/S136 MMCs. The results show that the average grain size reaches its smallest size of 0.75 μm when 0.1 wt% RGO was added to the RGO/S136 MMCs. As the RGO content is increased from 0 wt% to 0.5 wt%, a continuous transition of the grains from the (001) orientation to the (101) and (111) orientations is observed. In addition, the cellular dendritic grains transform into equiaxed fine grains with increasing RGO content. The SLM-prepared RGO/S136 MMCs are dominated by high-angle grain boundaries (˃15°) and the martensite (bcc) phase. The hardness, ultimate tensile strength and yield strength of the SLM RGO/S136 MMCs exhibit trends that initially increase and then decrease, with maximum values of 580.6 HV, 535.3 MPa and 515.8 MPa, respectively. This paper highlights the possibility of controlling the RGO content to achieve the desired microstructural characteristics and mechanical properties of RGO/S136 MMCs fabricated by the SLM process. Keywords: Selective laser melting, S136 steel, Reduced graphene oxide (RGO), Microstructure, Mechanical properties