Additive Manufacturing Strategy Research Articles

A new strategy for metal additive manufacturing using an economical water-atomized iron powder for laser powder bed fusion

We develop a pre-alloyed water-atomized (WA) iron powder for laser powder bed fusion (LPBF)-based 3D printers; such a powder has never been used for 3D printing. The powder features low levels of nano-sized Cu-Ni-Mo; these alloying elements are placed into the concavities of the iron powder. When these powders were used for LPBF printing, the processing window for the volumetric energy density ranged from 93 to 130 J/mm3. However, careful control of laser power (150 – 225 W) and scan speed (500 – 900 mm/s) was essential. In terms of production, a maximum relative density of 99.67% was achieved upon application of an energy density of 108 J/mm3 (175 W, 700 mm/s). An average hardness of 217.1 ± 4.7 HV was obtained, exhibiting stable standard deviations and confirming excellent mechanical homogeneity. Energy-dispersive X-ray spectroscopy mapping revealed excellent chemical homogeneity; this translated to good mechanical homogeneity because of the uniform mixing of the nano-sized alloying elements. The printed tensile bars were stable and of a relatively high aspect ratio. Tensile strength measurements revealed that samples prepared using the partially pre-alloyed iron powder were better than those fabricated from simple raw WA iron powder; the former samples exhibited an ultimate tensile strength (UTS) of 460.91 MPa with elongation of 6.98% and the latter a UTS of 197.15 MPa with elongation of 0.45%. Therefore, the pre-alloyed WA iron powder developed in this study can successfully be used in LPBF and presents a more cost-effective alternative to traditional gas atomized iron powders.

Isotropic 3D printing using material extrusion of thin shell and post-casting of reinforcement core

Material extrusion (ME) can be used to manufacture a range of three-dimensional shapes by stacking extruded polymer layers with a heated nozzle. While the process is suitable for customized production in small quantities, the use of ME-printed features is often limited with regard to mechanical purposes due to the intrinsic anisotropic strength shortcoming, as parts can easily failure against a load in the transverse direction. In addition, the production speed for multiple parts is another major drawback preventing real competition with other mass manufacturing technologies. In this paper, we introduce an additive manufacturing strategy of shell pre-printing and core post-casting (referring as shell-core printing) that effectively enhances the isotropic strength and the production speed. Shell-core printing involves 3D printing a thin shell of the three-dimensional shape using the conventional ME method, followed by the injection and curing of a reinforcement resin inside the core. We show that the transverse strength of the layered shell can be reinforced by the core and that the reinforcement can be more effective when the shell-core interface roughness is effectively removed. Using an additional surface-polishing step between the pre-printing and the post-casting, the degree of isotropy with regard to the strength can be enhanced significantly from 0.4 to nearly 1, while the overall production time can also be reduced by half that in the conventional ME process. We also investigate the tensile behavior of the shell-core printed specimen and experimentally verify that the characteristics in general follow the classical theory of the rule of mixtures. In addition, we find that many different fracture modes exist in shell-core printed parts depending on the adhesive strength at the shell-core interface. We provide a theoretical criterion for the interfacial bonding energy which determines the different fracture behaviors and experimentally validate this in a mode II fracture test. • We introduce a shell-core printing strategy, material extrusion of 3D shell and casting of reinforcement material in the core. • Shell-core printing can improve the degree of isotropy in strength from 0.4 to nearly 1. • Total production time of shell-core printing can also be reduced by half that in general material extrusion 3D printing. • Shell-core Interface topography and adhesive strength determine the fracture behavior of the printed shell-core products.

Metal Knitting: A New Strategy for Cold Gas Spray Additive Manufacturing.

Cold Spray Additive Manufacturing (CSAM) is an emergent technique to produce parts by the additive method, and, like other technologies, it has pros and cons. Some advantages are using oxygen-sensitive materials to make parts, such as Ti alloys, with fast production due to the high deposition rate, and lower harmful residual stress levels. However, the limitation in the range of the parts’ geometries is a huge CSAM con. This work presents a new conceptual strategy for CSAM spraying. The controlled manipulation of the robot arm combined with the proper spraying parameters aims to optimize the deposition efficiency and the adhesion of particles on the part sidewalls, resulting in geometries from thin straight walls, less than 5 mm thick, up to large bulks. This new strategy, Metal Knitting, is presented regarding its fundamentals and by comparing the parts’ geometries produced by Metal Knitting with the traditional strategy. The Metal Knitting described here made parts with vertical sidewalls, in contrast to the 40 degrees of inclination obtained by the traditional strategy. Their mechanical properties, microstructures, hardness, and porosity are also compared for Cu, Ti, Ti6Al4V, 316L stainless steel, and Al.

A laser-shock-enabled hybrid additive manufacturing strategy with molten pool modulation of Fe-based alloy

A novel hybrid laser additive manufacturing (LAM) process based on laser shock modulation of molten pool (LSMMP) is proposed in this work. By using a combination of experiment and simulation, the residual stress, heat transfer, and mass transfer behaviors are comprehensively studied. Also, microstructure evolution behavior is analyzed. The relationship between the convection behavior, the evolution of microstructure and the enhancement of residual stress induced by LSMMP is established. Compared with the traditional post-treatment process, the LSMMP assisted LAM process exhibits higher efficiency in residual stress control with the same pulsed laser energy input. The hidden mechanism in microstructure evolution and residual stress enhancement is expected to be possible related to intensified molten pool convection, uniform solute distribution and improved cooling rate induced by LSMMP. The hybrid LAM process strategy based on LSMMP provides a new approach for the application of short pulse laser in hybrid manufacturing.

A Learn-and-Control Strategy for Jet-Based Additive Manufacturing

In this article, we develop a predictive geometry control framework for jet-based additive manufacturing (AM) based on a physics-guided recurrent neural network (RNN) model. Because of its physically interpretable architecture, the model’s parameters are obtained by training the network through back propagation using input–output data from a small number of layers. Moreover, we demonstrate that the model can be dually expressed such that the layer droplet input pattern for (each layer of) the part to be fabricated now becomes the network parameter to be learned by back-propagation. This approach is applied for feedforward predictive control in which the network parameters are learned offline from previous data and the control input pattern for all layers to be printed is synthesized. Sufficient conditions for the predictive controller’s stability are then shown. Furthermore, we design an algorithm for efficiently implementing feedback predictive control in which the network parameters and input patterns (for the receding horizon) are learned online with no added lead time for computation. The feedforward control scheme is shown experimentally to improve the RMS reference tracking error by more than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\boldsymbol{30\%}$</tex-math></inline-formula> over the state of the art. We also experimentally demonstrate that process uncertainties are compensated by the online learning and feedback control.

Adoption of additive manufacturing for sustainable operations in the era of circular economy: Self-assessment framework with case illustration

Additive manufacturing (AM) is a revolutionary technological advancement that has the potential to contribute to sustainability in operations. The environmental benefits offered by AM have motivated firmsto the adoption of this technology. However, the real-world examples of successful applications of AM are limited. This study aims to explore and prioritise the challenges faced during the adoption of AM to achieve circular economy goals. The fuzzy Delphi method is used to finalise the barriers with the help of experts’ opinions. Then, using the Best Worst Method, the prioritisation of barriers has been done. Finally, the Graph Theory Matrix approach is used for the quantification of barriers’ intensity for a case company in the Indian automotive manufacturing industry.The study uses TOE (Technology, Organisation, Environment) theory and Innovation Resistance theory in the context of AM adoptionbarriers to achieving circular economy goals. This study will help decision-makers to tackle their ambivalence towards the feasibility of implementing AM to achieve sustainability. The proposed self-assessment framework will help in evaluating barriers' intensity while implementing AM. Insights from the study will also be useful for developing effective strategies for AM.

Material extrusion for ceramic additive manufacturing with polymer-free ceramic precursor binder

Additive manufacturing of dense ceramic products has been challenging because of the high polymeric content of the ceramic feedstock materials. Several studies have been conducted to increase the ceramic content in the feedstock, but the binder and additives in it have remained in the polymeric content. A novel strategy for highly dense additive manufacturing using a sol-gel-based ceramic slurry without polymeric additives is presented. Alumina, which is the most widely used fine ceramic, was chosen as a representative ceramic material. The proposed sol-gel solution enabled a high solid loading of approximately 50 vol% without any polymeric dispersant while satisfying the requirements for material extrusion process, such as extrudable viscosity (<100 Pa.s) and self-sustainable yield stress. This novel sol-gel binder system evolves into aluminum oxide nanoparticles during the sintering process, eventually fusing to the alumina particles. The as-printed green body possessed high alumina content and 66% of theoretical density, which was higher than that obtained via the conventional molding methods. A reduced linear shrinkage of less than 16% and a high density of 99.5% of the theoretical value were also achieved. This research would present a practical strategy for ceramic additive manufacturing as an emerging fabrication process.

A combination strategy for additive manufacturing of AA2024 high-strength aluminium alloys fabricated by laser powder bed fusion: Role of hot isostatic pressing

Hot cracks and pores remain major obstacles for the practical application of high-strength aluminium alloys fabricated by laser powder bed fusion (LPBF). A combination manufacturing strategy, integrating functional treatment of feedstock, inoculation treatment and hot isostatic pressing (HIP) post-processing, was used to tackle this issue. Crack-free AA2024 alloys with fine equiaxed grains were manufactured by LPBF via incorporating TiC and TiH2 grain-refining particles. However, when the build rate increases from 0.36 mm3/s to 2.16 mm3/s, the relative density decreases from 97.0% to 93.3%, resulting in a decline in the ultimate tensile strength (UTS) and elongation (El) of as-built samples from 395 ± 20 MPa and 10.2 ± 2.1% to 347 ± 17 MPa and 3.5 ± 1.3%, respectively. By healing pores and controlling precipitates, HIP reduces the properties difference between high-speed and low-speed printed alloys. After HIP treatment, the typical network eutectics that segregated at the grain boundaries evolve into spherical θ-Al2Cu dispersoids without any obvious grain coarsen. T-Al20Cu2Mn3 and Al18Ti2Mg3 intermetallic phases with twin substructure appear in the samples, which is beneficial to dispersion strengthening of alloys. The high-speed printed samples exhibit an excellent UTS of 462 ± 23 MPa and an El of 12.6 ± 2.4%, which is comparable to those of low-speed printed samples. Thus, for critical applications that require high productivity, HIP is a promising post-processing solution to minimise the deterioration of mechanical properties caused by porosity in high-speed printed alloys.

Recent advances in 3D printing of tough hydrogels: A review

Over the past two decades, the development of hydrogels with high mechanical strength and toughness shows both fundamental and practical significances. It is challenging yet highly desired to process these tough hydrogels into structural elements with desired architectures, which are important for real applications. Three-dimensional (3D) printing, as an emerging additive manufacturing strategy, demonstrates great potentials for engineering tough hydrogels with versatile applications. This paper aims to review the recent progress on 3D printing of tough hydrogels and their relevant applications. We give an overview on the advantages and requirements of tough hydrogel systems for specific 3D printing technologies, followed by detailed summary of the network structures, mechanical performances, toughening mechanisms, and representative applications of the printed tough hydrogels. Finally, recent advances of 4D printing of tough hydrogels are discussed as a prospective direction to devise soft actuators and robots.

Additive Manufacturing Strategies for Personalized Drug Delivery Systems and Medical Devices: Fused Filament Fabrication and Semi Solid Extrusion.

Novel additive manufacturing (AM) techniques and particularly 3D printing (3DP) have achieved a decade of success in pharmaceutical and biomedical fields. Highly innovative personalized therapeutical solutions may be designed and manufactured through a layer-by-layer approach starting from a digital model realized according to the needs of a specific patient or a patient group. The combination of patient-tailored drug dose, dosage, or diagnostic form (shape and size) and drug release adjustment has the potential to ensure the optimal patient therapy. Among the different 3D printing techniques, extrusion-based technologies, such as fused filament fabrication (FFF) and semi solid extrusion (SSE), are the most investigated for their high versatility, precision, feasibility, and cheapness. This review provides an overview on different 3DP techniques to produce personalized drug delivery systems and medical devices, highlighting, for each method, the critical printing process parameters, the main starting materials, as well as advantages and limitations. Furthermore, the recent developments of fused filament fabrication and semi solid extrusion 3DP are discussed. In this regard, the current state of the art, based on a detailed literature survey of the different 3D products printed via extrusion-based techniques, envisioning future directions in the clinical applications and diffusion of such systems, is summarized.

Effect of laser intensity profile on the microstructure and texture of Inconel 718 superalloy fabricated by direct energy deposition

Epitaxial growth of dendrites often results in a strong texture, which is a common phenomenon in a laser additive manufacturing process. In this study, pulsed lasers with flat-top and Gaussian intensity profiles were used to modulate the molten pool morphology and the solidification texture with a unidirectional scanning strategy. The melt pool is planar for the flat-top mode and it turns arc-shaped for the Gaussian mode. Long columnar grains are uniformly distributed and a strong fiber texture with a weak cube texture is obtained for the flat-top mode sample due to a uniform heat flux through many deposition layers. In contrast, large grains and fine grains are alternately arranged for Gaussian mode sample. The large grains exhibit a strong rotated cubic texture with a V-pattern in the overlapping area of the melt pool due to side-branching from the curved melt pool boundary; while the fine grains exhibit a weak fiber texture in the center of melt pool with a less restriction of the other two crystallographic axes perpendicular to the build direction. After heat treatment, the intensity of the textures of both samples is reduced. More recrystallized grains with twin boundaries are found under Gaussian mode due to the accumulated residual strain. This work manifests a promising method for texture customization by manipulating the heat flux strategy for laser additive manufacturing.

3D Printing of Ceramic Biomaterials

• Five additive manufacturing strategies for printing bioceramics are described herein. • Printing bioceramics lead to highly porous, bioactive microstructures as favored by tissue-engineered scaffolds, grafts, and implant coatings. • The strengths and weaknesses of these 3D printed constructs depend on the factors like specific application goal, additive manufacturing method, and choice of materials. Bioceramics are a popular class of materials used in biomedical applications due to their mechanical stability and biocompatibility. They exist in a variety of fields including hip joints for orthopedics, tooth fillings for dentistry, and scaffolds for tissue engineering; however, the standard processes currently used to manufacture these ceramic products can be time-consuming and costly. In response, current literature alternatively proposes additive manufacturing (3D printing) strategies to fabricate bioceramic materials in a cost-effective and efficient manner. Herein, we briefly cover five common processes and materials used in additive manufacturing bioceramics: fused deposition modeling, material jetting, binder jetting, powder bed fusion, and vat photopolymerization. Furthermore, we discuss the potential of these 3D printed ceramic structures when applied to different biomedical technologies such as bone tissue scaffolds and structural implants.

Design strategies for bi-metallic additive manufacturing in the context of wire and arc directed energy deposition

While the recent research in manufacturing bi-metallic structures using wire and arc directed energy deposition focuses on feasibility studies to understand material compatibility, the study presented here encircles a larger design framework, integrating deposition process, intermediate surface conditions, and mutual deposition (hatching) direction. The investigation addresses the issues associated with manufacturing actual bi-metallic structures and shades light on the intrinsic mechanism responsible for process-induced anisotropy and resulting interfacial strength. Interfacial shear tests of the candidate bi-metallic pair of low-carbon steel-316L stainless steel reveal that the intermediate machining before deposition of the second material, if deposited with low dilution and low heat input has a limited effect on the final build quality. The deposition direction influences the isotropy, i.e., strength improves when the shear force acts transversely to the deposition direction of the second material. Interlocking promoted by favorable deposition direction, as revealed by fractography, combined with metallurgical bonding, enhances the interfacial strength. Despite element migration and hardness spike in the interface up to 500 HV due to martensitic needles, a defect-free bi-metallic structure was possible with shear strengths over 400 MPa. The investigation contributes to the fundamental understanding of the approaches suitable for the deposition of sound bi-metallic structures.

Reinforcement Strategies for Additive Manufacturing in Construction Based on Dynamic Fibre Winding: Concepts and Initial Case Studies

Whereas entire houses are reported to be built by means of 3D Concrete Printing (3DCP), automated integration of reinforcement is still a vastly unresolved challenge and - undoubtfully - a crucial requirement for widespread adoption of 3DCP in construction practice. The disruptive technology of 3DCP opens up the chance to overcome insufficiencies of conventional steel reinforcement and its rationalised orthogonal use. This paper discloses a field of options for future construction by exploring possible reinforcement strategies for different kinds of cement-based Additive Manufacturing techniques using continuous textile fibres. A methodology is introduced to identify, compare and assess new processes for integration of fibre reinforcement based on robotic winding. Two novel approaches will be described regarding Large Particle 3D Concrete Printing and Shotcrete 3d Printing. Respective case studies will be presented in order to give a basic proof of concept.

Additive manufacturing of cartilage-mimetic scaffolds as off-the-shelf implants for joint regeneration

Biomimetic scaffolds that provide a tissue-specific environment to cells are particularly promising for tissue engineering and regenerative medicine applications. The goal of this study was to integrate emerging additive manufacturing and biomaterial design strategies to produce articular cartilage (AC) mimetic scaffolds that could be used as ‘off-the-shelf’ implants for joint regeneration. To this end alginate sulfate, a sulfated glycosaminoglycan (sGAG) mimic, was used to functionalize porous alginate-based scaffolds and to support the sustained release of transforming growth factor-β3 (TGF-β3). Covalent crosslinking dramatically improved the elasticity of the alginate/alginate sulfate scaffolds, while scaffold architecture could be tailored using a directional freezing technique. Introducing such an anisotropic architecture was found to promote mesenchymal stem cell (MSC) infiltration into the scaffold and to direct the orientation of the deposited extracellular matrix, leading to the development of cartilage tissue with a biomimetic zonal architecture. In vitro experiments also demonstrated the capacity of the sulfated scaffolds to both enhance chondrogenesis of MSCs and to control the release of TGF-β3, leading to the development of a tissue rich in sGAG and type II collagen. The scaffolds were further reinforced with a 3D printed poly(lactide-co-ϵ-caprolactone) (PLCL) framework, leading to composite implants that were more elastic than those reinforced with polycaprolactone, and which better mimicked the bulk mechanical properties of native cartilage tissue. The ability of this composite scaffold to support chondrogenesis was then confirmed within a dynamic culture system. Altogether, these findings demonstrate the potential of such biomimetic scaffolds as putative ‘single-stage’ or ‘off-the-shelf’ strategies for AC regeneration.

Polycaprolactone usage in additive manufacturing strategies for tissue engineering applications: A review.

Polycaprolactone (PCL) has been extensively applied on tissue engineering because of its low-melting temperature, good processability, biodegradability, biocompatibility, mechanical resistance, and relatively low cost. The advance of additive manufacturing (AM) technologies in the past decade have boosted the fabrication of customized PCL products, with shorter processing time and absence of material waste. In this context, this review focuses on the use of AM techniques to produce PCL scaffolds for various tissue engineering applications, including bone, muscle, cartilage, skin, and cardiovascular tissue regeneration. The search for optimized geometry, porosity, interconnectivity, controlled degradation rate, and tailored mechanical properties are explored as a tool for enhancing PCL biocompatibility and bioactivity. In addition, rheological and thermal behavior is discussed in terms of filament and scaffold production. Finally, a roadmap for future research is outlined, including the combination of PCL struts with cell-laden hydrogels and 4D printing.

Selectively doped piezoelectric ceramics with tunable piezoelectricity via suspension-enclosing projection stereolithography

The macroscopic piezoelectric properties of piezoelectric materials can be regulated via localized control of microstructures to introduce site-specific electric displacement vectors, but much of the current research is centered around design and manipulation of meso-scale architectural units that are inherently limited in achieving high functionality. In this study, we propose a stereolithography-based additive manufacturing (AM) strategy to spatially tune the properties of piezoelectric ceramics at a grain-microstructural scale through selectively incorporating dopants into the ceramic materials for tailoring the grain development. The effects of different doping parameters (including ceramic solid loading, dopant type, and dopant concentration) on the microstructures and properties of printed piezoelectric ceramics are investigated. The thermodynamics and kinetics of different doping additives in the dopant-ceramic interaction are experimentally and numerically studied to enable location-specific inhibition of microstructures. Our results indicate that a doping concentration of 2 wt% promoted the homogeneity of local grain growth, resulted in a higher compressive strength and lower porosity, and improved dielectric permittivity and piezoelectric voltage constant in printed piezoelectric ceramics. Moreover, our results suggest that thermochemically stable particles (e.g., ZrO2) with a high melting point and a low vapor pressure exhibited micro-scale diffusion behaviors, in contrast to millimeter-scale diffusion behaviors of common doping additives (e.g., ZnO), which are more suitable as a locally incorporated dopant for achieving location-specific property tuning. Test cases of selectively doped piezoelectric components in predefined patterns highlight the potential of the proposed approach in creating novel piezoelectric materials with programmable location-specific properties.

Coordinated Heat and Feed Printing Strategy for Wire and Arc Additive Manufacturing of Metal-Cored Wires

Coordinated Heat and Feed Printing Strategy for Wire and Arc Additive Manufacturing of Metal-Cored Wires

Microstructure and hardness comparison of as-built inconel 625 alloy following various additive manufacturing processes

Experimental examples of as-built microstructures and associated microindentation hardnesses (HV) for a wide range of metal additive manufacturing (AM) processes for Inconel 625 alloy are presented in this research paper. These processes included cold spray, laser powder bed fusion, wire arc additive manufacturing, electron beam directed energy deposition, laser hot wire, binder jetting, laser power directed energy deposition, electron beam melting, and laser wire directed energy deposition. Microstructures ranged from impact bonded, flattened 625 alloy grains containing microdendrites and dislocations, having a microindentation hardness of HV 590 for cold spray, to large, equiaxed grains for binder jet-printed components having a microindentation hardness of HV 180. Electron and laser beam melt-related processes, including wire and powder feeds, produced varying sizes of columnar grains with parallel and cellular arrays of microdendrites, having microindentation hardnesses ranging from HV 304 to HV 228. Delta phase precipitate needles were also produced in laser-powered directed energy deposition microstructures along with columns of gamma double-prime precipitates for electron beam melting. The results demonstrate a range of AM processes applications and strategies to produce as-built and surface-modified Inconel 625 alloy products and components. These results demonstrate differences in the microstructure from the various AM processes and considerations for use in end use applications.

Design Strategy for Additive Manufacturing Ti-Al-Fe alloys with Calculation of Phase Diagram Method.

Calculation of Phase Diagram (CALPHAD) method was used to develop an additive manufacturing (AM) titanium alloy design strategy for new alloys in the Ti-Al-Fe system. This design strategy includes five key criteria addressing AM-processing suitability, mechanical properties and heat treatment capability. The methods for evaluating the design criteria were determined considering both physical implications and computational efficiency. Available experimental results agree well with CALPHAD predictions in the design map. CALPHAD method in this work can be applied for high-throughput early-stage assessments in large composition space for new AM titanium alloy development.