The quality of parts manufactured by laser powder bed fusion is closely related to the uniformity and density of the powder bed.In this work,the discrete element method is used to simulate the powder spreading process by different spreader geometries with rough substrate surfaces.The results indicate that reducing the spreader inclination angle significantly increases the number of force chains,enhances compaction,and consequently improves the quality of the powder bed.Studies also show that opti-mizing the bottom structure of the spreader can effectively reduce exposed areas.An arc-shaped structure promotes particle packing and filling,improving the powder distribution characteristics.A narrow spreader significantly affects the packing density of the powder bed at low layer gaps,whereas a wide spreader is relatively less constrained.At high spreading speeds,the spreader with an inclination angle of 135° produces the highest quality of the powder bed.R1000 performs excellently at larger layer gaps.The above findings provide valuable guidance for optimizing powder spreading strategies in the laser powder bed fusion process.

Additive manufacturing of Ti-6Al-4V porous dental implants has become a prominent advancement in the field of prosthetic dentistry, providing enhanced osseointegration and biomechanical long-term stability. To further enhance the biomechanical properties, this study designed different porous structures with axial gradient porosity, investigated their stress distribution, and permeability behavior. The implant–bone mechanical interaction was studied using finite element analysis (FEA). To observe the actual mechanical performance and biological characteristics of implants, the experimental analysis was performed on the laser powder bed fusion LPBF-fabricated specimens together with cytocompatibility tests. The FEA results showed that a Gyroid structure with axial gradient porosity of 40–80

Controlling the quality of the powder bed is critical for guaranteeing component quality in laser powder bed fusion (LPBF). In this work, the discrete element method is used to examine how substrate surface morphology, including the roughness and texture angle, affects powder bed quality. The results indicate that the bed quality is more sensitive to changes in surface roughness than texture angle. Powder coverage can be improved by increasing the texture angle. The force analysis reveals that on rough surfaces, the contact force acting on the substrate has strong fluctuations. The particle-substrate contact force under the piles has an increasing-decreasing trend with the distance from the scraper increasing. In addition, the in-situ re-coating technique at a proper gap increment can effectively fill the depressions generated from the rough surface, achieving more uniform and dense powder beds. The findings provide a theoretical basis for optimizing powder-spreading strategies in LPBF process.

Motivated by the PEACOC project on metal recovery from solid wastes, an innovative magnetic density separation (MDS) process has been developed for granular material sorting. It has intrinsic advantages over the existing industrialized MDS by eliminating fluid turbulence and particle jamming problems. The new MDS applies an inclined planar magnet and a horizontal basin containing a static magnetic fluid as the separation medium. A particle sliding phenomenon is identified as a feature of the process which could help the separation. The MDS successfully sorted shredded PCBAs to concentrate valuable metals and sorted shredded wires to reduce metallic contaminants in the plastic fractions. A pilot scale facility is introduced to show the design to achieve continuous production and to reduce the consumption of ferrofluid.

Solid waste sorting is an important pre-treatment in recycling to improve the efficiency of material recovery and reduce costs. Motivated by the PEACOC project on metal recovery from solid wastes, an innovative magnetic density separation (MDS) process has been developed for solid waste sorting. It has intrinsic advantages over conventional gravity separation technologies and the previously industrialized MDS process. The new MDS process applies an inclined planar magnet and a horizontal basin containing a static magnetic fluid as the separation medium. A particle sliding phenomenon is identified as a feature that could help the separation. Experiments have been carried out to demonstrate the role of the MDS in concentrating valuable metals in shredded PCBAs and reducing metallic contaminants in plastic fractions of shredded wires. A pilot scale facility is introduced to show the design to achieve continuous production and to reduce the consumption of ferrofluid.

Deterministic optics fabrication using sub-aperture tools has been vital for manufacturing precision optical surfaces, industrial robotic polishing, which is a more economical and intelligent method is required in modern fabrication process. However, the challenge of robotic polishing lies in the widely used spiral and raster paths, which may leave excess waviness from the tool path, and the unavoidable constant removal layer is added to obtain positive dwell time which cause low polishing accuracy. The waviness can be removed by either using smoothing tools sequentially or randomizing the tool path. However, process efficiency and accuracy are not well considered in the existing tool-path planning. A density adaptive path based on a stacked rotation convolution model to ensure polishing accuracy and efficiency while avoiding waviness generation is proposed in this study, and then the dwell time is calculated by anti-aliasing space-variant deconvolution. The robotic polisher experimental results confirm that the root mean square (RMS) of the final surface figure has been successfully reduced and stabilized at 7.355 nm, and the convergence effect at unit wavelength has been significantly augmented with an improvement of 367%, reducing the measurement from 28% to 131%; in addition, no obvious mid-spatial frequency (MSF) peak was generated in the PSD analysis of density adaptive path polishing results. Henceforth, the polishing accuracy, efficiency, and MSF error of robotic polishing can be greatly enhanced. (c) 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Powder bed fusion additive manufacturing has been applied to the fabrication of functionally graded materials. A new design that allows the material composition to change along the direction perpendicular to the powder spreading has been reported in the literature. Based on this design, this work examines the quality of the graded spread powder layer with two powders, which have a large difference of density. The results reveal that during the spreading of graded powders, the volume of particles on the heavy powder side is deposited less than that on the light powder side, indicating that heavy particles diffuse to the light powder side. This diffusion is affected by the spreading speed, but not much by the layer gap. Large spreading speed causes more significant deviation. The results also show that particle size affects diffusion, indicating that decreasing the particle size of the heavy powder may be a solution to reduce diffusion.

The mechanical properties and permeability properties of artificial bone implants have high-level requirements. A method for the design of trabecular-like porous structure (TLPS) with mixed porosity is proposed based on the study of the mechanical and permeability characteristics of natural bone. With this technique, the morphology and density of internal porous structures can be adjusted, depending on the implantation requirements, to meet the mechanical and permeability requirements of natural bone. The design parameters mainly include the seed points, topology optimization coefficient, load value, irregularity, and scaling factor. Characteristic parameters primarily include porosity and pore size distribution. Statistical methods are used to analyze the relationship between design parameters and characteristic parameters for precise TLPS design and thereby provide a theoretical basis and guidance. TLPS scaffolds were prepared by selective laser melting technology. First, TLPS under different design parameters were analyzed using the finite element method and permeability simulation. The results were then verified by quasistatic compression and cell experiments. The scaling factor and topology optimization coefficient were found to largely affect the mechanical and permeability properties of the TLPS. The corresponding compressive strength reached 270–580 MPa; the elastic modulus ranged between 6.43 and 9.716 GPa, and permeability was 0.6 × 10−9–21 × 10−9; these results were better than the mechanical properties and permeability of natural bone. Thus, TLPS can effectively improve the success rate of bone implantation, which provides an effective theory and application basis for bone implantation.

Aims at keyhole instability and poor surface-quality in welding of aluminum alloy, a novel laser technology has been studied on welding of Al 6061. The novel laser beam consists of a ring part and a center part. Different power ratio of ring laser beam to center laser beam and different kind of laser beam welding experiments have been conducted, respectively. The keyhole stability and surface quality were analyzed by using a high-speed camera and a confocal sensor accordingly. Additionally, the energy absorption rate was studied by analyzing the penetration and fusion area of cross-section. Experimental results showed that the ring laser beam provided great stability on the center laser-induced keyhole in high-speed dual-mode ring/center laser welding of aluminum. The stability effects can be improved by increasing the ring power in dual-mode ring/center laser welding of aluminum alloy, and the stabilized effect would contribute to the high-surface quality on welding of aluminum alloy. Furthermore, the combination of ring laser and center laser (Gaussian laser) greatly improved the welding efficiency due to its synergistic effects on process status in the ARM laser welding process. This work provided a basic research to both scientist and engineer on the mechanism of ARM laser welding aluminum alloy.

In the process of artificial metal bone implantation, the mismatch of elastic modulus and bad permeability behavior lead to obvious stress shielding and poor bone tissue ingrowth. For these problems, this paper proposed a new design method of the variable porosity porous structure based on the stress line and Voronoi diagram. In the design process, the principal stress line was determined by mechanical finite element analysis, which controlled the seeds distribution of the 3D Voronoi diagram, making local porosity con-forms to the stress distribution. In addition, the designed samples were fabricated through laser powder bed fusion (L-PBF) technology with TC4 powders. The mechanical charac-teristics were evaluated by static mechanical simulation with ABAQUS and compression tests, and the permeability characteristics were analyzed by fluid simulation with COMSOL and in vitro and vivo test. The research results showed that the designed structure obtained the relatively low elastic modulus and higher mechanical strength, the designed structure has closer permeability characteristics to the reconstructed natural bone, and was conducive to cell proliferation. So the porous structure based on the stress line and Voronoi diagram might have good application potential in the bionic design of bone implants.(c) 2023 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

The optimization of the powder bed fusion (PBF) parameters is always an important part of a new alloy forming process, especially in the field of high-entropy alloys (HEAs). In this work, based on the extraction of molten pool feature, a kind of efficient process design method for pores suppression is proposed and eliminates the pores generated during the PBF process for a new Nb30Mo10Ta30Ti20Ni10 HEA whose tensile strength and elongation can reach 1.46 GPa and 5.5 %, respectively, after the optimization process. The high strength of this HEA is not only due to the high Archimedes' density but also the high grain boundary density & rho;, which causes a proportion of the heat transfer interface of the molten pool. Therefore, as a guide for the PBF process, the proposed pores suppression process design method is benefit for finding the performance limits of HEAs, which are always hidden by non-optimal processes.

With the increasing demand for bone repair, the bionic bone scaffolds have become a research hotspot. A sub-regional design method of the bionic bone scaffolds, using macrostructural topology, is proposed in this paper, aiming to provide a functionally enhanced region division method for the gradient design. The macrostructural topology was carried out by the bi-directional evolutionary structural optimization (BESO), dividing the predefined design domain into sub-region A and sub-region B. Subsequently, a combined probability sphere model and a distance-to-scale coefficient mapping model are established to implement the graded porosification based on the Voronoi tessellation. This approach takes geometric and mechanical continuity into fully account and assures a reasonable distribution of characteristic parameters, yielding to improve the mechanical strength under specific stress conditions. Finally, the scaffolds were fabricated by the laser powder bed fusion (LPBF) process using the Ti-6Al-4V powder. The results of compression tests are satisfactory, showing that the as-built specimens implement sub-regional functionality. The apparent elastic modulus and the ultimate strength range, respectively, between 1.50 GPa and 7.12 GPa (for the first module) and between 38.55 MPa and 268.03 MPa (for the second module), which conform to the required level of natural bone, providing a possibility for clinical application.

The shape variation of the laser beam is evidently observed in the laser powder bed fusion (LPBF) process because of changes in laser incidence angle and misalignment between the build plate and the laser focus plane.This issue is particularly relevant in large-scale LPBF systems where the laser beam needs to scan a large build area.However, most LPBF modeling studies assume vertical laser radiation.The heat transfer, melt pool, and solidification evolution due to the laser shape variation have not been well addressed and quantified.In the present study, the temperature distribution, melt pool geometry and flow dynamics are captured via numerical modelling, and the grain morphology is characterized under various laser incidence angles.The results show that the melt pool depth becomes shallower, and the width is near the beam size as the laser beam becomes more elongated.The beam shape variation can affect the liquid flow pattern with increasing incidence angle, resulting in a larger vortex at the front of the melt pool and a smaller vortex at the rear of the melt pool.The thermal gradient increases and the solidification rate decreases as the laser incident angle becomes larger.The present study enhances the understanding of multi-physics in the LPBF process.

Ionic rare earth ore is a type of featured rare earth ore in China. Its mining process suffers from a long leaching cycle and considerable consumption of leaching agents. Improving mining efficiency requires a sound physical understanding of the leaching process. In this study, the CFD-based numerical model is used to analyze the physical process of leaching through porous media formed by particles. The simulation results indicate that a lower packing porosity and smaller particles packed granular porous medium result in much larger energy dissipation during seepage, and the energy dissipation increases with seepage velocity. It is found that when the seepage velocity increases to a certain high value, the energy dissipation exceeds the value predicted by Darcy’s law, which is mainly caused by liquid turbulence. Additionally, the effect of particle shape is examined. The results show that the granular medium composed of prolate particles causes larger energy dissipation than oblate particles, and spherical particles play the least role. This phenomenon may result from the particle shape affecting the area of the frontal contact surface between particles and liquid. The results provide new insights into the fundamental understanding of percolation and seepage behaviors in the ion-adsorption-type rare earth ore leaching process.

The hot-core heavy reduction rolling (HHR2) technology is a solidification end reduction technique to enhance the quality of steel. In the current study, the Cellular Automata (CA) calculation is applied to investigate the casting process before HHR2 and the HHR2 dynamic recrystallization (DRX) process. The experimental data and calculation data are compared to verify the accuracy of the method and models. Afterward, based on the actual casting process, the solidification structure in the thickness direction of a billet is calculated, and the influence on the HHR2 grain size of various conditions, such as rolling speed, deformation temperature, and reduction strain, is simulated. Furthermore, based on the fixed HHR2 process, the influence of different superheats, casting speeds, and cooling conditions on the core-surface temperature schedule and the grain size before and after HHR2 deformation is calculated. Finally, the process optimization trend to produce high-carbon bearing steel billet is obtained.

Recently, laser powder bed fusion (LPBF) has shown great potential in advanced manufacturing. However, the rapid melting and re-solidification of the molten pool in LPBF leads to the distortion of parts, especially thin-walled parts. The traditional geometric compensation method, which is used to overcome this problem, is simply based on mapping compensation, with the general effect of distortion reduction. In this study, we used a genetic algorithm (GA) and backpropagation (BP) network to optimize the geometric compensation of Ti6Al4V thin-walled parts fabricated by LPBF. The GA-BP network method can generate free-form thin-walled structures with enhanced geometric freedom for compensation. For the GA-BP network training, an arc thin-walled structure was designed and printed by LBPF and measured via optical scanning measurements. The final distortion of the compensated arc thin-walled part based on GA-BP was reduced by 87.9% compared with PSO-BP and mapping method. The effectiveness of this GA-BP compensation method is further evaluated in an application case using new data points, and the result shows that the final distortion of the oral maxillary stent was reduced by 71%. In summary, the GA-BP-based geometric compensation proposed in this study can better reduce the distortion of thin-walled parts with higher time and cost efficiencies.

Recently, laser powder bed fusion (LPBF) has shown great potential in advanced manufacturing. However, the rapid melting and re-solidification of the molten pool in LPBF leads to the distortion of parts, especially thin-walled parts. The traditional geometric compensation method, which is used to overcome this problem, is simply based on mapping compensation, with the general effect of distortion reduction. In this study, we used a genetic algorithm (GA) and backpropagation (BP) network to optimize the geometric compensation of Ti6Al4V thin-walled parts fabricated by LPBF. The GA-BP network method can generate free-form thin-walled structures with enhanced geometric freedom for compensation. For the GA-BP network training, an arc thin-walled structure was designed and printed by LBPF and measured via 3D scanning measurements, resulting in point cloud data sets of more than 470,000 data points. The final distortion of the compensated arc thin-walled part based on GA-BP was reduced by 72.7%, which is better than the compensation method based on BP and mapping. The effectiveness of this GA-BP compensation method is further evaluated in an application case using new data points, and the result shows that the final distortion of the oral maxillary stent was reduced by 71%. In summary, the GA-BP-based geometric compensation proposed in this study can better reduce the distortion of thin-walled parts with higher time and cost efficiencies.

Compared with large amount of work on the powder layer characteristics in powder-bed-based additive manufacturing (AM) under different operating conditions and powder properties, the significant effects of powder surface chemistry on the quality of the powder layer were less investigated. To further enhance people's insights in this aspect so as to obtain superior powder layer for subsequent process, Ti-6Al-4 V powders with different surface chemistries (e.g., the as received powder without any treatment, the moistened powder with surface H2O molecule, the exsiccated powder with grown nano-scaled oxides film) were firstly prepared, where the surface chemistry state of each powder was characterized and corresponding flowability and spreadability were measured in physical experiments. On this basis, systematic numerical investigations on the evolution of different surface chemistry induced powder layer characteristics during spreading were conducted. The results demonstrate that the surface moistened powder would deteriorate powder layer quality due to the strong cohesion effect between particles. However, the increase of the surface roughness on exsiccated powder could facilitate a greater deposition rate compared to that of the as-received powder during spreading, and therefore result in a powder layer with more pronounced apparent density. The mechanism lies in that high friction from the rough surface film could restrict the scattered motion behavior of sticky particles caused by the breakdown of cluster structure during the blade movement, and thus facilitate the most prominent deposition rate of this kind of particles during spreading. Nevertheless, the simultaneous increase of interactions within exsiccated powder also diminishes the boundary effect with blade and substrate, resulting in abnormal elevation of powder layer thickness, enlargement of internal pore volume, and depressed packing fraction correspondingly. Consequently, the powder with grown surface film is not ideal for printing, and the chemical reactions on powder that causes intensive film roughness should be prevented in powder storage and treatment process.

Particle scale modelling of the process physics involved in laser powder bed fusion (LPBF) is a recent research hotspot, and many efforts have been made in the literature. However, a comprehensive review of the physics in LPBF and the effects of key variables such as powder- and operation-related parameters, and the mechanisms of defects formation is still lacking. This paper aims to offer a state-of-the-art review on multi-physics related to metal powder recoating and further melting and solidification process. The studies on powder bed recoating explored by discrete element method are presented first, including model theory and validation, effects of process parameters, and physics of particle flow and size segregation. Then powder melting approach based on computational fluid dynamics and the involved phenomena such as melt pool dynamics, keyhole dynamics, defects formation mechanisms of pores, and balling and spattering are described. The needs for future research are also discussed.