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.

Microstructure and Mechanical Properties of FeCoNiCrAl x High-entropy Alloys by Selective Laser Melting

In this study,the thermal analysis theory of selective laser melting(SLM)was introduced,and different high-entropy alloy(HEA)specimens were prepared using the SLM technology.The effects of different powder sizes,elemental contents,and process parameters on the microstructure and mechanical properties of FeCoNiCrAl x HEA specimens fabricated using SLM were analyzed.Moreover,hardness and tensile tests of these high-entropy alloys were performed.The results showed that with increasing laser power and hatch spacing,the hardness of the specimens initially increased and subsequently decreased;it also increased with increasing scanning speed.The FeCoNiCrAl 0.5 HEA specimens prepared using fine powder exhibited better tensile properties,followed by FeCoNiCrAl 0.8 HEA.However,the FeCoNiCrAl 0.5 HEA prepared using coarse powder exhibited the poorest tensile properties.A comparison of the tensile properties of the specimens at different heights revealed that the specimens formed at the middle height exhibited improved tensile properties.

Study of the Mechanical Properties of 3D-printed Onyx Parts: Investigation on Printing Parameters and Effect of Humidity

Recently,an increasing number of parts have been produced using additive manufacturing technology.They are no longer simply prototypes but structural parts whose mechanical characteristics must be known before printing.One of the weaknesses of 3D printing is the significant variability in the dimensions and geometrical and mechanical properties of the printed parts.These properties depend on specific printing parameters and environmental conditions.This study aims to determine the influence of two printing parameters,namely,the orientation and positioning of the parts on the printing platform and the influence of humidity on the mechanical properties of the parts.The studied samples were fabricated with onyx using a Markforged X7 printer.The results showed that onyx could be considered an isotropic material to a certain extent because its mechanical properties do not vary sufficiently according to the orientation angle on the printing platform;a maximum deviation of 10%was observed between the different orientations.In contrast to the orientation,the positioning(flat or XY,on-edge or XZ,and upright or ZX)of the workpieces significantly influenced the mechanical properties.Positioning on the edge allowed the Young’s modulus to be up to 50% greater than that of flat and upright positioning.The study of the sensitivity to humidity revealed that a specimen absorbs approximately 2%of the humidity and loses up to 65%of its Young’s modulus after 165 days of exposure,significantly influencing the mechanical properties of the parts.Consideration should be given to this aging of onyx when using printed parts as structural parts.

Effects of a Perimeter on the Post-failure Behavior of Fiber-reinforced Polymer Composite Lattices

The post-failure behavior of a fiber-reinforced polymer composite lattice was experimentally studied using a beam structure.Anisotropic topology optimization was conducted to maximize the structural stiffness and partial latticing to improve toughness.Subsequently,an infill structure was generated from the optimized results using a phase field approach.The perimeter of the two-dimensional beam structure was generated from a binary solution of the optimized results.Optimized composite lattice structures were obtained using three-dimensional printing.Three-point bending tests demonstrated that the perimeter enhanced the toughness of the composite lattice.The perimeter prevented shear band failure and improved the load-carrying capability,even after maximum loading.

Advanced Additive Remanufacturing Technology

Additive remanufacturing technology,as one of the key technologies of remanufacturing engineering,can realize the integrated repair of the structure and function of high value-added key metal parts of large and complex equipment,which can significantly reduce the use and maintenance costs,save labor and time costs.It applies to the on-site repair and remanufacturing of key parts in the aerospace,energy and chemical industry,heavy haul machinery,and other fields,as well as the on-site rapid repair of parts in special environments such as tunnels,open seas,and space.Additive remanufacturing technology can promote the reform of the maintenance and support mode of weapons and equipment and become the research hotspot of major military-developed countries.This paper expounds on the connotation and characteristics of additive remanufacturing technology and introduces its evolution process.The research achievements of the author in the development of additive remanufacturing platforms,material design,and process optimization were summarized.Given the problems(such as control shape,control performance,and control position)in the additive remanufacturing process,the author puts forward solutions and looks forward to the future development direction of additive remanufacturing technology.

Microstructure and Mechanical Properties of an Ultrahigh-strength Titanium alloy Ti-4.5Al-5Mo-5V-6Cr-1Nb Prepared Using Laser Directed Energy Deposition and Forging:A Comparative Study

The application of titanium alloys in aerospace put forward the requirement for higher strength.Additive manu-facturing is a promising method for the efficient and economical processing of titanium alloys.However,research on the additive manufacturing of ultrahigh-strength titanium alloys is still limited.The mechanisms of microseg-regation for high alloying elements and poor plasticity are still not clear.In this study,an ultrahigh-strength titanium alloy Ti-4.5Al-5Mo-5V-6Cr-1Nb(TB18)was prepared using two methods:laser direct energy deposi-tion(LDED)and forging.The LDEDed alloy contains three zones with similar grain morphologies but different microstructure.The microsegregation of the alloy is limited due to the rapid solidification and almost eliminated after the thermal cycle and solution treatment.With stress relief treatment,the LDEDed alloy exhibits anisotropic mechanical properties.After solution and aging treatments,its ultimate strength is enhanced;however,its plas-ticity is relatively lower than that of the wrought alloy with equally high strength.The excellent balance of the strength and plasticity of the wrought alloy can be ascribed to the formation of𝛼WGB and multiscale𝛼laths,which provides enlightenment for optimizing the properties of the LDEDed alloy.

3D Printed Antennas for 5G Communication:Current Progress and Future Challenges

With the advent of 5G and future trends for communication systems moving to millimeter wave(MMW)and higher frequencies,antennas will be required to have high gain,wide bandwidth,and low losses.3D printing realizes structures by sequential stacking layer-by-layer,which enables the manufacturing of antennas with ar-bitrary shapes in a cheaper,faster,and flexible manner.This study provides a review of current state-of-the-art 3D printed antennas for different frequencies.First,an overview of 3D printing technology is presented.A huge number of 3D printed antennas,categorized by their material composition,have been described,including poly-mer,metallic,ceramic,composite material,and multi-material integrated antennas.Finally,the main challenges and prospects are discussed to provide insight into how 3D printing can be further progressed in antenna manu-facturing.

Design and 3D Printing of Graded Bionic Metamaterial Inspired by Pomelo Peel for High Energy Absorption

Light-weight,high-strength metamaterials with excellent specific energy absorption(SEA)capabilities are sig-nificant for aerospace and automobile.The SEA of metamaterials largely depends on the material and structural design.Herein,inspired by the superior impact resistance of pomelo peel for protecting the pulp and the elevated SEA ability of a functionally graded structure,a graded bionic polyhedron metamaterial(GBPM)was designed and realized by 3D printing using a soft material(photosensitive resin)and a hard material(Ti-6Al-4V).Guided by compression tests and numerical simulations,the elevated SEA ability was independent of the materials.The fluctuation region appeared in hard-material-fabricated bionic polyhedron metamaterial(BPMs)and was absent in soft-material-fabricated BPMs in the stress-strain curves,resulting in the growth rate of the SEA value of the soft-material-fabricated GBPM being enhanced by 5.9 times compared with that of the hard-material-fabricated GBPM.The SEA values of soft-and hard-material-fabricated GBPM were 1.89 and 44.16 J/g,which exceed those of most soft-and hard-material-fabricated metamaterials reported in previous studies.These findings can guide the design of metamaterials with high energy absorption to resist external impacts.

A Direct Toolpath Constructive Design Method for Controllable Porous Structure Configuration with a TSP-based Sequence Planning Determination

The inherent capabilities of additive manufacturing(AM)to fabricate porous lattice structures with controllable structural and functional properties have raised interest in the design methods for the production of extremely in-tricate internal geometries.Current popular methods of porous lattice structure design still follow the traditional flow,which mainly consists of computer-aided design(CAD)model construction,STereoLithography(STL)model conversion,slicing model acquisition,and toolpath configuration,which causes a loss of accuracy and manufac-turability uncertainty in AM preparation stages.Moreover,toolpath configuration relies on a knowledge-based approach summarized by expert systems.In this process,geometrical construction information is always ignored when a CAD model is created or constructed.To fully use this geometrical information,avoid accuracy loss and ensure qualified manufacturability of porous lattice structures,this paper proposes a novel toolpath-based con-structive design method to directly generate toolpath printing file of parametric and controllable porous lattice structures to facilitate model data exchange during the AM preparation stages.To optimize the laser jumping route between lattice cells,we use a hybrid travelling salesman problem(TSP)solver to determine the laser jumping points on contour scans.Four kinds of laser jumping orders are calculated and compared to select a minimal laser jumping route for sequence planning inside lattice cells.Hence,the proposed method can achieve high-precision lattice printing and avoid computational consumption in model conversion stages from a geomet-rical view.The optical metallographic images show that the shape accuracy of lattice patterns can be guaranteed.The existence of“grain boundaries”brought about by the multi-contour scanning strategy may lead to different mechanical properties.

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.

3D Printing of Stretchable Strain Sensor Based on Continuous Fiber Reinforced Auxetic Structure

Stretchable strain sensors play a key role in motion detection and human-machine interface functionality,and deformation control.However,their sensitivity is often limited by the Poisson effect of elastic substrates.In this study,a stretchable strain sensor based on a continuous-fiber-reinforced auxetic structure was proposed and fabricated using a direct ink writing(DIW)3D printing process.The application of multi-material DIW greatly simplifies the fabrication process of a sensor with an auxetic structure(auxetic sensor).The auxiliary auxetic struc-ture was innovatively printed using a continuous-fiber-reinforced polydimethylsiloxane composite(Fiber-PDMS)to balance the rigidity and flexibility of the composite.The increase in stiffness enhances the negative Poisson’s ratio effect of the auxetic structure,which can support the carbon nanotube-polydimethylsiloxane composite(CNT-PDMS)stretchable sensor to produce a significant lateral expansion when stretched.It is shown that the structural Poisson’s ratio of the sensor decreased from 0.42 to−0.33 at 20%tensile strain,and the bidirectional tensile strain increases the sensor sensitivity by 2.52 times(gage factor to 18.23).The Fiber-PDMS composite maintains the excellent flexibility of the matrix material.The auxetic sensor exhibited no structural damage af-ter 150 cycles of tension and the signal output exhibited high stability.In addition,this study demonstrates the significant potential of auxetic sensors in the field of deformation control.

Simultaneous Optimization of Carbon Fiber Allocation and Orientation by IFM-GA

This paper proposes an individual fitness method genetic algorithm(IFM-GA)for carbon fiber-reinforced plastic(CFRP).The strength of CFRP depends on the carbon fiber allocation and orientation.Waste carbon fiber is generated if this design is inappropriate.Consequently,CFRPs are less cost-effective.It is necessary to optimize the allocation and orientation as design variables to solve this problem.The problem involves combinatorial optimization.The genetic algorithm(GA)is suitable for combinatorial optimization.However,it is difficult to obtain an optimal solution using the GA owing to the large number of combinations.Hence,the IFM-GA is developed in this study.It is a GA-based method with a different fitness calculation.The GA calculates the fitness of each design,whereas the IFM-GA calculates the fitness of each design element.As a result,the IFM-GA yields a higher-stiffness design than the GA.To conclude,the IFM-GA can enable optimum fiber allocation and orientation,whereas the GA cannot.

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.

Novel Class of Additive Manufactured NiTi-Based Hierarchically Graded Chiral Structure with Low-force Compressive Actuation for Elastocaloric Heat Pumps

Elastocaloric refrigeration is the most promising green solid-state refrigeration technology to replace conventional vapor compression refrigeration.The development direction of the elastocaloric component that acts as a key part of the elastocaloric refrigeration system contains a large elastocaloric effect,low stress hysteresis,high heat exchange performance,and small driving loads.The first two indices can be realized by material modification;however,the last two are more dependent on a novel porous structure design.However,the conventional porous structure is confronted with some critical challenges,including inhomogeneous stress,a significant hysteresis area,and deformation instability under the alternating cyclic loading.In this study,a NiTi-based elastocaloric structure model with chirality feature and gradient design as innovative elements was presented,bio-inspired by the structure of the plant tendrils.A quantitative optimization for the NiTi-based elastocaloric structure was performed using the finite element analysis(FEA)method.Strain and martensite volume fraction(MVF)fields during the loading and unloading processes were predicted and evaluated.The simulated results indicated that increasing the thickness gradient g_(1) of the strip or decreasing the diameter gradient g_(2) of the structure was beneficial to achieving more homogeneous strain and martensite distribution,simultaneously with higher energy storage efficiency and specific surface area.In addition,these NiTi-based chiral structures with different structural parameters were fabricated by laser powder bed fusion(LPBF).At the optimized structure parameters of g_(1)=2 and g_(2)=1.11,the LPBF-fabricated NiTi-based chiral structure could achieve an adiabatic temperature change ΔT_(ad) of 2.3 K,driving force of as low as 149.11 N,and|ΔT_(ad)/F|of as high as 15.42 K/kN at a recoverable compressive strain of 10%.

Development and Evaluation of Multiscale Fiber-reinforced Composite Powders for Powder-bed Fusion Process

The development of multiscale fiber-reinforced composite powders is an effective way to improve the mechanical properties and functionality of additively manufactured parts.Herein,a novel thermally induced precipitation process is proposed for preparing multiscale fiber-reinforced powders.A systematic evaluation was conducted to explore the main factors influencing powder morphology,powder flow,and microstructure.In the powder-forming mechanism,the polymer matrix is coated on the microfiber,and a film of carbon nanotubes covers the powder surface,which is promoted by heterogeneous nucleation.The composite powders comprised polyamide 12,carbon fiber(CF),and carbon nanotubes,which have been successfully applied in powder bed fusion processes including selective laser sintering(SLS).Smooth flow and powder deposition were observed,and the composite components obtained via SLS were well-fabricated using the optimized process parameters.A CF loading ratio of up to 66.7 wt%and homogeneous fiber distribution within the matrix were successfully achieved.

Additive Manufacturing of Composites: Toward Light-weight, Functionalization, and Intellectualization

Advanced composites are critical materials for the development of high-end equipment,as we say one generation of materials and one generation of equipment.Following materials such as aluminum,steel,and titanium,composite materials will become one of the four major materials in the field of aeronautics and astronautics.Usage of advanced composites is expected to exceed 50%in new commercial airplanes to significantly improve the level of lightweight.Thus,innovative devel-opment of high-end equipment in aerospace,rail traffic,biomedical and other fields also have an urgent demand for high-performance and multi-functional composites.

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.

High-fidelity Modeling of Multilayer Building Process in Electron Beam Powder Bed Fusion:Build-quality Prediction and Formation-Mechanism Investigation

High-fidelity simulations of powder bed fusion(PBF)additive manufacturing have made significant progress over the past decade.In this study,an efficient two-dimensional frame was developed for simulating the electron beam PBF process with hundreds of tracks for the direct prediction of the build quality.The applicable parameter range of the developed model was determined by comparing the heat transfer with that in three-dimensional cases.Subsequently,powder deposition and selective melting were coupled for a continuous simulation of the multilayer process.Three powder deposition models were utilized to generate random powder particles,and their effects on the packing structure and the resultant simulated build quality were investigated.The predicted build quality was validated using experimental results from independent studies.By reproducing the building process,the defect development mechanism in a multilayer process was revealed for the coalescence behaviors of randomly distributed powder particles,which also confirmed the importance of simulation at the high-fidelity powder scale.The effects of key process parameters during multilayer and multi-track processes on the build quality were systematically investigated.In particular,the formation statuses of all tracks during the simulated building process were recorded and analyzed statistically,which provided crucial information on the printing process for understanding the building mechanism or performing uncertainty analysis.

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.

Additive Manufacturing of Large-scale Metal Mesh with Core-shell Composite Structure for Transparent Electromagnetic Shielding/glass Heater

Transparent electromagnetic(EM)shielding glass with a metal mesh has significant potential for application in different fields of EM radiation and anti-EM interference light-transmitting observation windows.In particular,a transparent EM-shielding glass with a large-aspect-ratio metal mesh can effectively alleviate the contradictory problems of shielding effectiveness and light-transmission performance constraints.However,the fabrication of high-aspect-ratio metal meshes on glass substrates has problems such as high cost,complex processes,low efficiency,small area,and easy damage issues,which limit their application in the field of high-performance,transparent EM-shielding glass.Therefore,this paper proposes a composite additive manufacturing process based on electric-field-driven microjet 3D printing and electroplating.By fabricating metal meshes with an Ag-Cu core-shell structure on a glass substrate,EM-shielding glass with high shielding efficiency and light transmission can be manufactured without increasing the aspect ratio of the metal meshes.The prepared Ag-Cu composite metal mesh has excellent optoelectronic properties(period 250𝜇m,line width 10𝜇m,90.1%transmission at 550 nm visible light,square resistance 0.21Ω/sq),efficient electrothermal effect(3 V DC voltage can reach 189°C steady-state temperature),stable EM-shielding effectiveness(average shielding effectiveness 23 dB at X-band),and acceptable mechanical and environmental stability(less than 3%change in square resistance after 150-times adhesion test and less than 6%and 0.6%change in resistance after 72 h in acid and alkali environments,respectively).This method provides a new solution for the mass production of high-performance large-area transparent electric heating/EM-shielding glass.