Microstructures and Mechanical Properties of Al-Mg-Sc-Zr Alloy Additively Manufactured by Laser Direct Energy Deposition

An Al-Mg-Sc-Zr alloy was additively manufactured by laser direct energy deposition(DED)under different laser powers,and the microstructures and mechanical properties of the as-deposited samples were investigated.The samples showed a fully equiaxed grain structure with grain sizes of 2–30μm.Most of the blocky primary Al3(Sc,Zr)-precipitated phases(<5μm)were arranged along the grain boundaries.A small amount of fine granular secondary Al3(Sc,Zr)phases(<0.5μm)were precipitated owing to the cyclic heat treatment during the DED forming process.According to the EBSD(Electron backscatter diffraction)results,the texture index and strength of the sample were only slightly greater than 1,indicating that the material structure exhibited a certain but not obvious anisotropy.The sample in the horizontal direction had better yield strength,tensile strength,and elogation properties(399.87 MPa,220.96 MPa,9.13%)than that in the building direction(385.40 MPa,219.40 MPa,8.24%),although the sample in the〈XOZ〉plane had the finest equiaxed grains.The ductility of the〈XOZ〉sample deteriorated as the number of pores increased.

Parameter Optimization via Orthogonal Experiment to Improve Accuracy of Metakaolin Ceramics Fabricated by Direct Ink Writing

Kaolin/metakaolin-insulating ceramic components fabricated using direct ink writing(DIW)have important ap-plication prospects in architecture and aerospace.The accuracy of the entire process including the forming and sintering accuracy of ceramics greatly limits the application scope,and high-accuracy ceramic samples can meet the usage requirements in many scenarios.The orthogonal experiment was designed with four process parame-ters,including nozzle internal diameter,filling rate,printing layer height/nozzle internal diameter,and printing speed,to investigate the evolution of the DIW forming accuracy,sintering shrinkage rate and surface roughness of metakaolin-based ceramics with different process parameters.The influence of each process parameter and its mechanism were analyzed to obtain the DIW parameters for high-accuracy metakaolin ceramics.Multiple linear regression models between the dimensional change rate,surface roughness,and process parameters of the ceramic samples were established and validated.The results show that comprehensively considering the forming accuracy of the ceramic green bodies,sintering shrinkage rate and surface roughness,the optimal DIW process parameters were a 0.41 mm nozzle internal diameter,100%filling rate,50%printing layer height/nozzle inter-nal diameter,and a 15 mm/s printing speed.Multiple linear regression models were developed for the process parameters and the printing accuracy,sintering shrinkage rate and surface roughness.The error rates between the theoretical results obtained by substituting the optimal process parameters into the multiple linear regression models and the actual results obtained by printing the samples with the optimal parameters were extremely small,all less than 0.8%.This verified the correctness and predictability of the multiple linear regression models.This work provides a reference basis for rapid fabrication of high-accuracy ceramics via DIW and accuracy prediction with different process parameters.

Roadmap for Additive Manufacturing:Toward Intellectualization and Industrialization

With the rapid development of Additive Manufacturing(AM)technology in the past 30 years,AM has been shift-ing from prototyping to advanced manufacturing of functional components in industry.Intellectualization and industrialization of AM process and equipment could be the bottlenecks to the wide industrial applications of AM technology in the future,which have been highlighted in this paper,aiming at describing the technological research roadmaps for the next 5 to 10 years.According to the data flow in the process and value chains of AM technologies,state-of-art of design methodology,material,process&equipment,smart structures,and ap-plications in extreme scales and environments has been elaborated respectively.Some suggestions on potential challenges for research and development in AM technologies have been provided in each section,which would finally establish a critical technical platform for the future industrial innovation and entrepreneurship.

Quality Control:Internal Defects Formation Mechanism of Selective Laser Melting Based on Laser-powder-melt Pool Interaction:A Review

Selective laser melting(SLM)is a 3D printing technology with a high near-net-shape ability and forming accuracy.However,the inevitable internal defects significantly hinder its development.Therefore,it is essential to fully understand the causes of internal defects in SLM processing and minimize the defects to achieve quality control accordingly.This work reviews the recent studies on internal defects in SLM,presenting the main internal defects of SLM as impurities,lack of fusion,gas pores,and micro-crack.These internal defects occur on the various phenomena in the laser-powder-melt pool(LPMP)stage.The formation of SLM internal defects is mainly affected by oxidation,denudation,balling,spatter,and keyholes;here,balling,spattering,and the keyhole phenomenon are the main factors causing internal defects in LPMP.Hence,this paper focuses on reviewing the balling effect,spatter behavior,and keyhole phenomenon,introducing the action mechanism of the above three phenomena under different process conditions.Additionally,the spatter behavior when forming internal defects is proposed.This review also considers the correlation between the spatter behavior and keyhole phenomenon and makes an important contribution to understanding and reducing SLM internal defects.It presents a reliable opinion on real-time monitoring and machine intelligent learning for SLM processing in the future,as well as supporting a systematic thinking for the suppression of defect formation in SLM.

A Review on Discrete Element Method Simulation in Laser Powder Bed Fusion Additive Manufacturing

Laser powder bed fusion(LPBF)is a popular metal additive manufacturing technique.Generally,the materials employed for LPBF are discrete and particulate metal matters.Thus,the discontinuous behaviors exhibited by the powder materials cannot be simulated solely using conventional continuum-based computational approaches,such as finite-element or finite-difference methods.The discrete element method(DEM)is a proven numerical method to model discrete matter,such as powder particles,by tracking the motion and temperature of individual particles.Recently,DEM simulation has gained popularity in LPBF studies.However,it has not been widely applied.This study reviews the existing applications of DEM in LPBF processing,such as powder spreading and fusion.A review of the existing literature indicates that DEM is a promising approach in the study of the kinetic and thermal fluid behaviors of powder particles in LPBF additive manufacturing.

Material-structure Integrated Additive Manufacturing: Not a Simple Accumulation, But a Real Integration

Additive manufacturing(AM)/3D printing(3DP)technologies are evolving from single materials,limited structures,and mechanical property-oriented printing to multimaterial,innovative structures,and multifunction printing.Further,AM/3DP has rich scientific and technological connotations,covering materials,structures,processes,properties,and applications,which show a strong nonlinear coupling relationship with each other,thereby determining the service performance of the final printed components.Therefore,materials,structures,processes,and other elements no longer have a simple cumulative relationship for AM/3DP but a complex mapping relationship between them and the obtainable performance.Material-structure integrated AM/3DP and attendant systematic coordination of various coupling elements are promising strategies for coping with the emerging challenges of high-performance and multifunctional AM/3DP.

Additive Manufacturing towards Even Higher

Additive manufacturing(AM)/3D printing(3DP)has gradually evolved from a type of manufacturing technology to a manufacturing methodology,which has revolutionized the design,manufacturing,and production models of key components used in modern industries.Ad-ditive manufacturing technologies not only create complex components with unique structures,but also provide components with tailored high-performance.Driven by high-performance goals of 3D-printed compo-nents,additive manufacturing technologies are constantly experiencing innovation and development,including more intelligent printing equip-ment,more diverse and integrated printing processes,stronger and more reliable materials,and more intelligent and sustainable industrial appli-cations.

Advances in 3D Bioprinting

Three-dimensional(3D)bioprinting has emerged as a promising approach for engineering functional tissues and organs by layer-by-layer precise positioning of biological materials,living cells,and biochemical components.Compared with nonbiological printing,3D bioprinting involves additional complexities and technical challenges owing to the processing of living cells,such as the appropriate biomaterials that fulfill the requirements for both printability and functionality.In this review,we first introduce the development course of 3D bioprinting,highlighting innovative forms of living building blocks and advances in enabling techniques of 3D bioprinting.We then summarize the state-of-the-art advancements in 3D bioprinting for biomedical applications,including macroscale tissue or organ bioprinting,disease modeling,microphysiological systems,biobots,and bioprinting in space.Despite the rapid development of 3D bioprinting over the past decades,most 3D bioprinted tissue or organ constructs are still far from being suitable for clinical translation,and it is necessary for the field of bioprinting to shift its focus from shape mimicking towards functionality development.Therefore,we provide our perspectives on this burgeoning field with an emphasis on functional maturation post printing and translational applications at the bedside.

Keyhole-induced Porosity in Laser Manufacturing Processes: Formation Mechanism and Dependence on Scan Speed

Laser welding and laser-based powder-bed fusion additive manufacturing in the deep penetration(keyhole)mode are promising technologies for the synthesis of metal components.The significant potential of these technologies remains latent because of structural defects(porosity),which significantly degrade the structural integrity and performance of the end products.Practical strategies for reducing those defects are addressed through fundamental understanding of their formation.In this study,pore formation of hydrodynamic origin is investigated,including the dynamics and mechanisms of the formation based on the above mentioned technologies.The pore volume and frequency of pore appearance,depending on the amplitude and frequency of capillary vibrations,are considered.Physical analysis is performed to obtain the scanning velocity values for the maximum and zero amplitudes and the frequency of capillary waves.A comparison between calculated curves and experimental data confirms both the capillary origin of the pores and the estimated scanning speeds at which the parameters of the pores exhibit their maximum values or vanish.The results obtained may facilitate in the selection of the optimal scanning speed when designing a pore-free technology.

Additive Manufacturing of Liquid-Cooled Ceramic Heat Sinks:An Experimental and Numerical Study

With recent advances in power electronic packaging technologies,liquid-cooled ceramic heat sinks have been considered as a promising solution for further improving the performance of power electronic devices.In this study,several aluminum oxide heat sinks were fabricated and tested using the digital light processing-based ad-ditive manufacturing method,to verify their practical performance.The results showed that the complex cooling structures inside the heat sinks can be completely formed and exhibited high surface quality.The experimental thermal and hydraulic performances of the heat sinks were consistent with the numerically modeled predictions.Furthermore,by exploiting the advantages of additive manufacturing,a direct manifold microchannel(MMC)configuration was designed to reduce the vertical flow of the traditional MMC configuration and achieve an im-proved cooling efficiency.At a constant volumetric flow rate of 1 L/min,the direct MMC configuration achieved a 19.8%reduction in pressure drop and an 11.8%reduction in thermal resistance,as well as a more uniform temperature distribution.

Customized Design for Ergonomic Products via Additive Manufacturing Considering Joint Biomechanics

This paper presents a customized design method for ergonomic products via additive manufacturing(AM)con-sidering joint biomechanics.An ergonomic customized design model can be built based on kinesiology involving human joint biomechanics.Manifolds of the human bone can be reconstructed from X-rays,computed tomog-raphy(CT),magnetic resonance imaging(MRI),and direct 3D scanning.The conceptual and detailed design of customized products were implemented on ergonomic shoes and insoles.A lightweight lattice structure with vari-able porosity was generated via structural topology optimization for an ergonomic customized design.Notably,the upper surface of the custom-made insole may adhere perfectly to the plantar surface of the patient,resulting in a lower peak plantar pressure.Finite element analysis(FEA)can be employed to simulate the static or dynamic biomechanical characteristics.The conceptual ergonomic products were forwarded to the machine and fabricated via AM,driven by visual digital twin techniques.The experiments proved that a customized design suitability method for wearable ergonomic products via 3D printing is specifically tailored to the rehabilitation needs of individual customers,while consuming the least cost,time,and materials.

3D Printing Technology for Short-continuous Carbon Fiber Synchronous Reinforced Thermoplastic Composites:A Comparison between Towpreg Extrusion and In Situ Impregnation Processes

Three-dimensional(3D)printing of carbon fiber-reinforced thermoplastic composites(CFRTPs)provides an ef-fective method for manufacturing the CFRTPs parts with complex structures.To increase the mechanical per-formance of these parts,a 3D printing technology for short-continuous carbon fiber synchronous-reinforced thermoplastic composites(S/C-CFRTPs)has been proposed.However,the synchronous reinforcement that ex-isted only at particular positions led to a limited improvement in the mechanical performance of the 3D-printed S/C-CFRTP part,which made it challenging to meet the engineering requirements.To solve this problem,two methods for achieving synchronous reinforcement at all the positions of the 3D-printed S/C-CFRTP part are pro-posed.To determine a suitable printing process for the S/C-CFRTP part,a comprehensive comparison between the two methods was conducted through theoretical analysis and experimental verification,involving the print-ing mechanism,fiber content,impregnation percentage,and mechanical performance.The results indicated that the towpreg extrusion process was suitable for manufacturing the 3D-printed S/C-CFRTP part.Compared with the in situ impregnation process,the towpreg extrusion process led to a fiber content increase of approximately 7%and void rate reduction of approximately 6%,resulting in 19%and 20%increases in the tensile and flexural strengths of the 3D-printed S/C-CFRTPs,respectively.Additionally,an optimized process parameter setting for fabricating an S/C-CFRTP prepreg filament with excellent mechanical performance was proposed.The findings of this study can provide a new approach for further improving the mechanical performance of the 3D-printed advanced composites.

A Review of Residual Stress and Deformation Modeling for Metal Additive Manufacturing Processes

A metal additive manufacturing process results in a nearly net-shaped fabrication of parts directly from digital data.A local heat source melts the deposited material,and a part is built layer-by-layer.Residual stress and de-formation are critical issues experienced by additively manufactured parts.Modeling the additive manufacturing process provides important insights and can help determine an optimal build plan so as to minimize residual stress formation.Various approaches have been used for modeling of residual stresses,ranging from high-fidelity models to simplified models,for quicker results.This paper provides a state-of-the-art review of the approaches used to numerically model residual deformation and stresses in structures built using additive manufacturing.Fur-thermore,it describes the physical causes of residual-stress generation in an additively manufactured structure.

A Review on Distortion and Residual Stress in Additive Manufacturing

Additive manufacturing(AM)has gained extensive attention and tremendous research due to its advantages of fabricating complex-shaped parts without the need of casting mold.However,distortion is a known issue for many AM technologies,which decreases the precision of as-built parts.Like fusion welding,the local high-energy input generates residual stresses,which can adversely affect the fatigue performance of AM parts.To the best of the authors’knowledge,a comprehensive review does not exist regarding the distortion and residual stresses dedicated for AM,despite some work has explored the interrelationship between the two.The present review is aimed to fill in the identified knowledge gap,by first describing the evolution of distortion and residual stresses for a range of AM processes,and second assessing their influencing factors.This allows us to elucidate their formation mechanisms from both the micro-and macro-scales.Moreover,approaches which have been successfully adopted to mitigate both the distortion and residual stresses are reviewed.It is anticipated that this review paper opens many opportunities to increase the success rate of AM parts by improving the dimension precision and fatigue life.

Effects of Various Processing Parameters on Mechanical Properties and Biocompatibility of Fe-based Bulk Metallic Glass Processed via Selective Laser Melting at Constant Energy Density

The unique properties of bulk metallic glass(BMG)render it an excellent material for bone-implant applications.BMG samples are difficult to produce directly because of the critical cooling rate of molding.Advancements in additive manufacturing technologies,such as selective laser melting(SLM),have enabled the development of BMG.The successful production of materials via SLM relies significantly on the processing parameters;meanwhile,the overall energy density affects the crystallization and,thus,the final properties.Therefore,to further determine the effects of the processing parameters,SLM is performed in this study to print Fe-based BMG with different properties three dimensionally using selected processing parameters but a constant energy density.The printed amorphous Fe-based BMG outperforms the typical 316 L stainless steel(316 L SS)in terms of mechanical properties and corrosion resistance.Moreover,observations from nanoindentation tests indicate that the hardness and elastic modulus of the Fe-based BMG can be customized explicitly by adjusting the SLM processing parameters.Indirect cytotoxicity results show that the Fe-based BMG can enhance the viability of SAOS2 cells,as compared with 316 L SS.These intriguing results show that Fe-based BMG should be investigated further for orthopedic implant applications.

Mechanical Anisotropy of Selective Laser Melted Ti-6Al-4V Using a Reduced-order Crystal Plasticity Finite Element Model

In this study,a reduced-order crystal plasticity finite element(CPFE)model was developed to study the effects of the microstructural morphology and crystallographic texture on the mechanical anisotropy of selective laser melted(SLMed)Ti-6Al-4V.First,both hierarchical and equiaxed microstructures in columnar prior grains were modeled to examine the influence of the microstructural morphology on mechanical anisotropy.Second,the effects of crystallographic anisotropy and textural variability on mechanical anisotropy were investigated at the granular and representative volume element(RVE)scales,respectively.The results show that hierarchical and equiaxed CPFE models with the same crystallographic texture exhibit the same mechanical anisotropy.At the granular scale,the significance of crystallographic anisotropy varies with different crystal orientations.This indicates that the present SLMed Ti-6Al-4V sample with weak mechanical anisotropy resulted from the synthetic effect of crystallographic anisotropies at the granular scale.Therefore,combinations of various crystallographic textures were applied to the reduced-order CPFE model to design SLMed Ti-6Al-4V with different mechanical anisotropies.Thus,the crystallographic texture is considered the main controlling variable for the mechanical anisotropy of SLMed Ti-6Al-4V in this study.

Effects of Low-pressure Annealing on the Performance of 3D Printed CF/PEEK Composites

The interlayer bonding properties are normally unsatisfying for 3D printed composites owing to the layer-by-layer formation process.In this study,low-pressure annealing was performed on 3D printed carbon fiber reinforced polyether ether ketone(CF/PEEK)to improve the interlayer bonding strength.The effects of annealing parameters on the mechanical properties and microstructure were studied.The results showed that the interlaminar shear strength(ILSS)of CF/PEEK improved by up to 55.4%after annealing.SEM and𝜇-CT were also applied to reveal the reinforcing mechanism.This improvement could mainly be attributed to the increased crystallinity of the CF/PEEK after annealing.Additionally,annealing reduced the porosity of the printed CF/PEEK and improved the fiber-resin interface.This resulted in a reduction in the stress concentration areas during loading,thereby enhancing the interlayer bonding strength of CF/PEEK.

Numerical Investigation of the Thermal Distortion in Multi-Laser Powder Bed Fusion(ML-PBF)Additive Manufacturing of Inconel 625

Metal additive manufacturing,especially laser powder bed fusion(L-PBF),is increasingly being used to fabricate complex parts with fine features.Emerging L-PBF systems have large build volumes and several lasers that op-erate simultaneously.Hence,they can produce large and complex parts at reduced costs and short build times.However,the thermal distortion remains a critical challenge.Hence,a thorough understanding of the impact of multiple lasers on part distortion in multi-laser PBF(ML-PBF)is imperative.Although experimental investigation is possible,a more conducive approach is to design and create suitable predictive models to understand the impact of multiple lasers consolidating a part into layers.To fulfill this goal,in this study,a commercially available and widely used thermo-mechanical model,Netfabb,was used to investigate the effects of multiple lasers for com-plex scan patterns such as raster,spiral,and Hilbert on the temperature distribution and thermal distortion.The results show that the thermal distortion is minimal for the spiral scan pattern.Additionally,multiple lasers were found to decrease the build time(as expected)while maintaining or reducing the thermal distortion compared with their single-laser counterparts for all scan patterns(except Hilbert).Therefore,the newly developed ML-PBF predictive model is capable of providing critical insights into the effects of using multiple lasers,thereby opening new possibilities for the faster production of complex parts.In the future,small-scale computational models will be expanded to include large-scale parts,and probabilistic models will be developed to establish correlations.

Electrically Actuated Shape Recovery of NiTi Components Processed by Laser Powder Bed Fusion after Regulating the Dimensional Accuracy and Phase Transformation Behavior

To develop self-recovery intelligent components based on resistance heating and obtain satisfactory performance in practical applications,this study optimized the forming quality,dimensional accuracy,and phase transformation temperatures of Nickel-titanium(NiTi)alloys by controlling the process parameters.The tensile properties and shape-memory effects of the NiTi alloys prepared using the optimized process were clarified.The relationship between the change in temperature and the shape recovery process of the deformed structure under electrical excitation was investigated.The results show that the suitable processing window for ensuring the forming quality without noticeable distortion and macro cracks depends on the laser parameters.In both the X and Y directions,the measured dimensions increased with an increase in laser power and first decreased and then stabilized with an increase in scanning speed.The XRD results showed that all the as-built samples consisted of B2 austenite and B19’martensite phases and Ni3Ti.Mechanical tests suggested that excellent tensile properties with a tensile strength of 753.28 MPa and elongation of 6.81%could be obtained under the optimal parameters of 250 W and 1200 mm/s.An excellent shape-recovery rate of 88.23%was achieved under the optimal parameters.Subsequently,chiral lattice structures were successfully fabricated by laser powder bed fusion(LPBF)under the optimal parameters,and a shape-recovery rate of 96.7%was achieved under electrical actuation for a structure with a pre-compressed strain of 20%.This study also found that the temperatures at the grasp regions were always higher than those at other positions because of the generation of contact resistance at the grasp regions.This facilitates the rapid recovery of the structure at the grasp regions,which has important implications for the design iteration of NiTi smart components.

Broadband and High-Temperature-Resistant Microwave Absorber Using Additively Manufactured Ceramic Substrate

This paper presents an approach to achieve broadband absorption and temperature resistance using ceramic sub-strates.A specially formulated slurry suitable for additive manufacturing technology was developed to fabricate ceramic substrates with lattice structures.The lattice structure not only reduces the weight of the absorber but also facilitates the broadening of the absorption bandwidth.The experimental results demonstrate that the pro-posed structure exhibits absorption rates exceeding 88%within the frequency range of 19.9-30.41 GHz,with a relative absorption bandwidth of 41.8%under normal incidence.Furthermore,the absorber’s performance was assessed under high temperatures of up to 200℃,revealing absorption spectra that closely match the initially measured spectrum.Additive-manufactured ceramic lattice structures present a promising avenue for designing multifunctional broadband microwave absorbers capable of withstanding elevated temperatures.