Powder-based Additive Manufacturing Technology Research Articles

Immunotoxic, genotoxic, and endocrine disrupting impacts of polyamide microplastic particles and chemicals

Due to their exceptional properties and cost effectiveness, polyamides or nylons have emerged as widely used materials, revolutionizing diverse industries, including industrial 3D printing or additive manufacturing (AM). Powder-based AM technologies employ tonnes of polyamide microplastics to produce complex components every year. However, the lack of comprehensive toxicity assessment of particulate polyamides and polyamide-associated chemicals, especially in the light of the global microplastics crisis, calls for urgent action. This study investigated the physicochemical properties of polyamide-12 microplastics used in AM, and assessed a number of toxicity endpoints focusing on inflammation, immunometabolism, genotoxicity, aryl hydrocarbon receptor (AhR) activation, endocrine disruption, and cell morphology. Specifically, microplastics examination by means of field emission scanning electron microscopy revealed that work flow reuse of material created a fraction of smaller particles with an average size of 1–5 µm, a size range readily available for uptake by human cells. Moreover, chemical analysis by means of gas chromatography high-resolution mass spectrometry detected several polyamide-associated chemicals including starting material, plasticizer, thermal stabilizer/antioxidant, and migrating slip additive. Even if polyamide particles and chemicals did not induce an acute inflammatory response, repeated and prolonged exposure of human primary macrophages disclosed a steady increase in the levels of proinflammatory chemokine Interleukin-8 (IL-8/CXCL-8). Moreover, targeted metabolomics disclosed that polyamide particles modulated the kynurenine pathway and some of its key metabolites. The p53-responsive luciferase reporter gene assay showed that particles per se were able to activate p53, being indicative of a genotoxic stress. Polyamide-associated chemicals triggered moderate activation of AhR and elicited anti-androgenic activity. Finally, a high-throughput and non-targeted morphological profiling by Cell Painting assay outlined major sites of bioactivity of polyamide-associated chemicals and indicated putative mechanisms of toxicity in the cells. These findings reveal that the increasing use of polyamide microplastics may pose a potential health risk for the exposed individuals, and it merits more attention.

Sustainable Powder-Based Additive Manufacturing Technology

A thorough exploration of sustainability in powder-based additive manufacturing (AM) is presented. This review focuses particularly on the design of sustainable alloys for AM. Environmental, economic, and social aspects of sustainability are covered. The importance of life cycle assessment (LCA) in evaluating environmental impact is discussed. LCA tools are used to analyse factors such as energy consumption, waste management, and air pollution, providing a comprehensive view of AM’s environmental footprint. Additionally, the economic dimension of sustainability is addressed through life cycle costing analysis. Production costs, energy use, and waste management are scrutinised, showcasing AM’s potential cost savings. Social life cycle assessment is introduced to assess societal impacts, focusing on worker welfare, community engagement, and overall societal well-being. A forward-looking concept of predicting sustainability before printing, using a product sustainability index, is presented. The approach emphasises environmentally responsible material selection, considering factors such as global warming potential in alloy design. This study offers a holistic approach to designing sustainable alloys and optimising AM processes through a sustainable materials science paradigm to establish the relationship amongst processing, microstructure, properties, sustainability, and performance.

Wire-Feeding Based Additive Manufacturing of the Ti–6Al–4V Alloy. Part II. Mechanical Properties

Currently, the interest in the application of metal additive manufacturing (AM), also known as 3D printing, is grown massively in the various fields of the industry and surgery. AM has significant multiple advantages compared to traditional subtractive technologies for making highly customized parts with complex geometries without causing noteworthy extra costs. Now, several powder-based AM technologies for metals’ 3D printing are in progress, in particular, selective laser sintering (SLS), selective laser melting (SLM), and electron-beam melting (EBM). In the past few decades, increasing research and developments are devoted to the wire-feeding-based 3D printing production of parts made of the Ti–6Al–4V alloy, which is widely investigated in different fields such as aerospace, automotive, energy, and marine industries as well as the prosthetics and the production of orthopaedic implants. Due to the feasibility of economical producing large-scale metal components with relatively high deposition rate, low machinery cost, high material efficiency, and shortened lead-time compared to powder-based AM, wire-feeding-based AM (WFAM) is attracting significant attention in the industry and academia owing to its ability for the production of the large components of the medium geometric complexity. In recent years, three options of WFAM are intensively researched, which differ by the wire-melting heating sources: wire + arc additive manufacturing (WAAM); wire-laser AM (WLAM), and wire electron-beam additive manufacturing (WEBAM). The purpose of the present review is systematic analysis of the mechanical properties of the Ti–6Al–4V alloy samples 3D-printed by WFAM with various heating melting sources, namely, arc, laser, and electron beam. Particularly, considering the literature data for the period of 2013–2020, such important properties as yield strength, tensile strength, elongation, and hardness are analysed for the samples in the as-printed and post-processed conditions.

Preparation of iron oxide–coated aramid fibres for improving the mechanical performance and flame retardancy of multi jet fusion–printed polyamide 12 composites

ABSTRACT Multi jet fusion (MJF), a powder-based additive manufacturing technology, is suitable for fabricating fibre-reinforced polymer composites. However, the types of reinforcement fibres applied in MJF are limited due to the incompatibility between the fibres and polymer. Herein, a simple and cost-effective surface modification method for Fe3O4-coated aramid fibres (Fe3O4@AF hybrids) is proposed to fabricate polyamide 12 (PA12) composites by MJF. The optimal fibre content for Fe3O4@AF/PA12 composites is 6 wt%, which resulted in 10.83% and 16.05% higher ultimate tensile strength and Young’s modulus, respectively, compared with AF/PA12. The addition of Fe3O4@AF hybrids also improved the flame retardancy of AF/PA12 by reducing the peak heat release rate (PHRR) and postponing the temperature at PHRR. This practical surface modification method can be potentially applied to other reinforcement fibres for fabricating functional polymer composites by other powder-based manufacturing technologies.

Wire Arc Additive Manufactured Mild Steel and Austenitic Stainless Steel Components: Microstructure, Mechanical Properties and Residual Stresses.

Wire arc additive manufacturing (WAAM) is an additive manufacturing process based on the arc welding process in which wire is melted by an electric arc and deposited layer by layer. Due to the cost and rate benefits over powder-based additive manufacturing technologies and other alternative heat sources such as laser and electron beams, the process is currently receiving much attention in the industrial production sector. The gas metal arc welded (GMAW) based WAAM process provides a higher deposition rate than other methods, making it suitable for additive manufacturing. The fabrication of mild steel (G3Si1), austenitic stainless steel (SS304), and a bimetallic sample of both materials were completed successfully using the GMAW based WAAM process. The microstructure characterization of the developed sample was conducted using optical and scanning electron microscopes. The interface reveals two discrete zones of mild steel and SS304 deposits without any weld defects. The hardness profile indicates a drastic increase in hardness near the interface, which is attributed to chromium migration from the SS304. The toughness of the sample was tested based on the Charpy Impact (ASTM D6110) test. The test reveals isotropy in both directions. The tensile strength of samples deposited by the WAAM technique measured slightly higher than the standard values of weld filament. The deep hole drilling (DHD) method was used to measure the residual stresses, and it was determined that the stresses are compressive in the mild steel portion and tensile in austenitic stainless steel portion, and that they vary throughout the thickness due to variation in the cooling rate at the inner and outer surfaces.

Incorporating Metal Precursors towards a Library of High-resolution Metal Parts by Stereolithography

Additive manufacturing (AM) of three-dimensional (3D) metals with intricate fine features and complex geometry at the macro- and micro- scale for energy generation and storage has attracted great interest recently. Fabrication of metals using state-of-art powder-based AM technology with high resolution and tunable microstructure remains a challenge. In this work, a new method for achieving sophisticated metallic structure via the digital light process (DLP) of metal precursors containing photo-resin followed by thermal heating and chemical reduction process is presented. Thermal decomposition of 3D-printed metal compound and chemical reduction makes it promising to synthesize a morphology-tunable and geometrically complex metal, alloy, metal oxide, or multi-material component. Two potential applications of 3D-printed copper using this method are demonstrated in the electrochemical field as oxygen evolution reaction (OER) catalyst support and current collector. NiFe hydroxide@3D-printed copper can drive a 100 mA/cm2 stably with a low energy barrier in an alkaline solution. Galvanostatic cycling profile of DLP Copper/electrolyte/Li at current densities of 2 mA/cm2, 4 mA/cm2 and 10 mA/cm2 can be conducted stably for a capacity of 20 mA·h/cm2. This work is the first to realize metal-based printing with ultra-high resolution thorough photopolymerization.

Influences of the scanning strategy on surface roughness in selective laser melting

Selective laser melting (SLM) is a powder-based additive manufacturing technology that can be used to fabricate high-density components with complex geometry. Several studies have investigated the process parameters that affect surface quality. However, most researchers have ignored the importance of the scanning strategy. In this study, the Taguchi method was used to investigate the relation between warpage and fundamental parameters (laser power, scanning speed, overlap, and scanning angle) to fabricate stable and undistorted specimens. Moreover, several scanning strategies (offset scanning, line scanning, meander scanning, meander scanning with hatch vector, and lightning scanning) were applied to explore the influences on surface quality. The results revealed that meander scanning and lightning scanning generated consistent specimens without large deformation. The process parameters, such as an increased 45° scanning direction and 30% overlap, optimized the surface quality. A lower scanning speed (500 mm/s) could generate lower Ra of specimens, with the exception of lightning scanning with an increased 45° scanning direction. This study may contribute to the growing understanding of the scanning strategy in SLM.

Optimization of the production processes of powder-based additive manufacturing technologies by means of a machine learning model for the temporal prognosis of the build and cooling phase

The increase in additive manufacturing production volume in recent years has led, not only, to a need for an increased level of productivity during the build job, but also the optimization of other crucial steps in each respective technologies process chain. With HP’s Multi-jet Fusion technology relatively recent entry (2014) into a seemingly stagnant scene of industrial scale powder-based additive manufacturing, it presented itself as an ideal candidate for post-processing optimization using methods of machine learning due to its faster print time. 66% of the Multi-jet Fusion production process, from build job preparation and nesting to delivery of finished parts is comprised of its cooling process step. This cooling process step can take anywhere from 3 to 30 h, depending on a number of factors. While speeding up cooling does not come into question when processing semi-crystalline polymers, such as polyamides, knowing the necessary cooling time with relatively high accuracy, becomes crucial. In this study, a machine learning model was created to predict a significantly more accurate cooling time using a number of build job parameters. The optimized cooling model was trained using the measured cooling times of varying build jobs as output and build job height, number of layers, packing density, number of parts and room temperature as inputs. The machine learning model predicts significantly more accurate cooling times than the manufacturer predictions. Furthermore, as with all machine learning models, it was shown that an increased number of data, resulted in more accurate predictions. The implementation of the optimized cooling model at BMW’s AM production facility leads to increased transparency, leaner production and a higher overall economic viability of AM technologies.

Multi-physics modelling of molten pool development and track formation in multi-track, multi-layer and multi-material selective laser melting

Selective laser melting (SLM) is a promising powder-based additive manufacturing technology due to its capability to fabricate metallic components with complex geometries. While most previous investigations focus on printing with a single material, recent industry-orientated studies indicate the need for multi-material SLM in several high-value manufacturing sectors including medical devices, aerospace and automotive industries. However, understanding the underlying physics in multi-material SLM remains challenging due to the difficulties of experimental observation. In this paper, an integrated modelling framework for multi-track, multi-layer and multi-material SLM is developed to advance the in-depth understanding of this process. The main novelty is in modelling the molten pool evolvement and track morphology of multiple materials deposited on the same and across different layers. Discrete element method (DEM) is employed to reproduce the powder deposition process of multiple materials in different deposition patterns, with particle size distribution imported from a particle size analyser. Various phenomena including balling effect, keyhole depression, and lack of fusion between layers are investigated with different laser energy inputs. As a result of the different thermal properties, several process parameters including energy density and hatch spacing are optimised for different powder materials to obtain a continuous track profile and improved scanning efficiency. The interface between two layers of different materials is visualised by simulation; it was found that the phase migration at the interface is related to the convection flow inside the molten pool, which contributes to the mixing of the two materials and elemental diffusion. This study significantly contributes to the challenging area of multi-material additive manufacturing by providing a greater in-depth understanding of the SLM process from multi-material powder deposition to laser interaction with powders across multiple scanning tracks and different building layers than can be achieved by experimentation alone.

Towards digital metal additive manufacturing via high-temperature drop-on-demand jetting

Drop-on-demand jetting of metals offers a fully digital manufacturing approach to surpass the limitations of the current generation powder-based additive manufacturing technologies. However, research on this topic has been restricted mainly to near-net shaping of relatively low melting temperature metals. Here it is proposed a novel approach to jet molten metals at high-temperatures (>1000 °C) to enable the direct digital additive fabrication of micro- to macro-scale objects. The technique used in our research – “MetalJet” - is discussed by studying the ejection and the deposition of two example metals, tin and silver. The applicability of this new technology to additive manufacturing is evaluated through the study of the interface formed between the droplets and the substrate, the inter-droplets bonding, the microstructure and the geometrical fidelity of the printed objects. The research shows that the integrity of the samples (in terms of density as well as metallurgy) varies dramatically in the two investigated materials due to the different conditions that are required to melt the interface of the stacked droplets. Nevertheless the research shows that by a careful choice of the jetting strategy and sintering treatments 3D structures of various complexity can be formed. This research paves the way towards the next generation metal additive manufacturing where various printing resolutions and multi-material capabilities could be used to obtain functional components for applications in printed electronics, medicine and the automotive sectors.

Modelling of the capillarity effect in Multi Jet Fusion technology

Multi Jet Fusion is a powder-based Additive Manufacturing technology patented by Hewlett-Packard Inc. It is characterised by the use of lamps instead of lasers to heat and melt polymers and by fusing and detailing agents that are jetted on the polymeric particles to modify and to control their heat absorption and thus selectively melt them. The high production rate and excellent mechanical properties of the manufactured parts, even in comparison with Laser Sintering, together with the overall product quality make this technology effective for a production of small series of end-parts rather than functional prototypes.In the present paper, the so-called capillarity effect is investigated. Capillarity derives from the interaction between the detailing and the fusing agents during the layer-based part building and it determines a deviation of the upper planar geometries when close to the border edges between the molten material and the powder bed.A model for the estimation of the capillarity effect is here proposed by adopting the free liquid meniscus theory. A benchmark geometry was designed to be affected by the capillarity effect and then manufactured by the MJF process. Values of the contact angle and of the characteristic length of the capillary, which are necessary to implement the analytical model, were obtained by experimental measurements made on the benchmark geometry.As a result the capillarity effect showed a dependence on the border edge orientation. The comparison between calculated shapes of the plane affected by the capillarity effect through the analytical model was in accordance with the experimental measurements thus allowing a reliable prediction to be made.

Effect of Various Infrared Heaters on Sintering Behavior of UHMWPE in Selective Inhibition Sintering Process

Selective inhibition sintering (SIS) is an emerging powder-based additive manufacturing technology that creates polymer or metal based parts through adhesion of layer-by-layer from three-dimensional computer-aided design model. Replacement of costly laser system in selective laser sintering tremendously reduces the cost of manufacturing. SIS attempts to incorporate low cost heaters to achieve efficient sintering for production of high quality parts. However, SIS demands uniform heating of each layer for effective sintering. The present study focused on examining the heating characteristics of three different types of infrared heaters with respect to various layer thickness and determining the optimal distance between the heating surface and the powder bed. Experiments are conducted using the low-cost heaters to obtain uniform distribution of heat energy across the ultra-high molecular weight polyethylene (UHMWPE) powder surface. The thermal and optical images are captured to observe the temperature distribution on the powder and the surface roughness. Tensile and compressive specimens were fabricated and their corresponding strength was determined and surface roughness was measured to study the surface characteristics of the parts.

Repairing Automotive Dies With Directed Energy Deposition: Industrial Application and Life Cycle Analysis

Powder-based additive manufacturing technologies are developing rapidly. To assess their applicability, comparison of performance and environmental impacts between additive technologies and conventional techniques must be performed. Toyota manufactures over two million aluminum four-cylinder engines in the U.S. each year via die casting. The dies used in this process are traditionally repaired via tungsten inert gas (TIG) welding and only last an average of 20.8% of the number of cycles of the original die life before another repair is needed. A hybrid repair process involving machining away the damaged areas and then rebuilding them additively via powder-blown directed energy deposition (DED) has been developed. The die repaired via DED resulted in the same life as the original die. The use of DED repair eliminated the need for emergency repairs and nonscheduled downtime on the line because the DED repaired dies last for as many cycles as the original die before another repair is needed. Life cycle analyses were conducted comparing the traditional welding repair process to the DED repair process. The results show that the DED repair process results in significantly less damage to the assessed impact categories except for ionizing radiation. Therefore, it can be concluded that the DED repair process could lessen most environmental impacts compared to traditional welding repair. Further work toward increasing energy and material efficiencies of the method could yield further reductions in environmental impacts.

Powder-scale multi-physics modeling of multi-layer multi-track selective laser melting with sharp interface capturing method

As a promising powder-based additive manufacturing technology, selective laser melting (SLM) has gained great popularity in recent years. However, experimental observation of the melting and solidification process is very challenging. This hinders the study of the physical mechanisms behind a variety of phenomena in SLM such as splashing and balling effects, and further poses challenges to the quality control of the products. Powder-scale computational models can reproduce the multi-physics process of SLM. In this study, we couple the Finite Volume Method (FVM) and Discrete Element Method to model the deposition of powder particles, and use the FVM to model the melting process, both with ambient air. In particular, a cutting-edge sharp surface capturing technique (iso-Advector) is incorporated into the Volume of Fluid Model to reconstruct the interface between different phases during the melting process. Iso-Advector is then used to capture and reconstruct the interface between molten material and ambient air, which is further used as a solid boundary for spreading the next powder layer. As such, 3D geometrical data is exchanged between these two stages repeatedly to reproduce the powder spreading-melting process of SLM incorporating different scan paths on multiple powder layers. To demonstrate the effectiveness of the powder-scale multi-physics modeling framework, typical scenarios with different fabrication parameters (Ti–6Al–4V powder) are simulated and compared with experimental observations available in literature.

Numerical simulation of multi-component evaporation during selective electron beam melting of TiAl

Powder-based additive manufacturing technologies such as selective laser melting (SLM) and selective electron beam melting (SEBM) hold great promise for producing high-value metal components with customized geometries. Development of scanning strategies and prediction of optimal process parameters thereby remain key factors for the fabrication of high-quality products without local inhomogeneities in alloy composition or porosity.In this contribution, the relation between energy input, evaporation and residual porosity is numerically investigated on the titanium aluminide alloy Ti-48Al-2Cr-2Nb. The numerical model is based on a lattice Boltzmann method and includes hydrodynamics, thermodynamics and multi-component evaporation. Simulation results show that the spatial distribution of alloying elements within the final part is dominated by the advection of melt which is driven by surface tension and evaporative recoil and influenced by the random arrangement of powder particles. Regarding evaporation losses, the line energy appears to be of central importance, as it strongly affects peak temperatures during processing. For a given melt strategy there is a trade-off between porosity and loss of aluminum due to evaporation. It is demonstrated that significant reductions in evaporation losses can be achieved by application of a suitable beam scanning strategy. The presented numerical findings are consistent with experimental data.

Powder-Bed Stabilization for Powder-Based Additive Manufacturing

The most successful additive manufacturing (AM) technologies are based on thelayer-by-layer depositionof a flowable powder. Although considered as the third industrial revolution, one factor still limiting these processes to become completely autonomous is the often necessary build-up of support structures. Besides the prevention of lateral shifts of the part during the deposition of layers, the support assures quality and stability to the built process. The loose powder itself surrounding the built object, or so-called powder-bed, does not provide this sustenance in most existent technology available. Here we present a simple but effective and economical method for stabilizing the powder-bed, preventing distortions in the geometry with no need for support structures. This effect, achieved by applying an air flow through the powder-bed, is enabling an entirely autonomous generation of parts and is a major contribution to all powder-based additive manufacturing technologies. Moreover, it makes powder-based AM independent of gravitational forces, which will facilitate crafting items in space from a variety of powdery materials.

Preparation of monosized metal powders by pulsated orifice injection: Y. Kuroki et al (Tohoku University, Sendai, Japan)

Drop-on-demand jetting of metals offers a fully digital manufacturing approach to surpass the limitations of the current generation powder-based additive manufacturing technologies. However, research on this topic has been restricted mainly to near-net shaping of relatively low melting temperature metals. Here it is proposed a novel approach to jet molten metals at high-temperatures (>1000 °C) to enable the direct digital additive fabrication of micro- to macro-scale objects. The technique used in our research – “MetalJet” - is discussed by studying the ejection and the deposition of two example metals, tin and silver. The applicability of this new technology to additive manufacturing is evaluated through the study of the interface formed between the droplets and the substrate, the inter-droplets bonding, the microstructure and the geometrical fidelity of the printed objects. The research shows that the integrity of the samples (in terms of density as well as metallurgy) varies dramatically in the two investigated materials due to the different conditions that are required to melt the interface of the stacked droplets. Nevertheless the research shows that by a careful choice of the jetting strategy and sintering treatments 3D structures of various complexity can be formed. This research paves the way towards the next generation metal additive manufacturing where various printing resolutions and multi-material capabilities could be used to obtain functional components for applications in printed electronics, medicine and the automotive sectors.