Integration of Circular Economy into Metal Additive Manufacturing: A Review of Ultrasonic Plasma Atomization for Producing Virgin and Recycled NiTi Powder

Additive manufacturing (AM) technologies based on sintering, such as Powder Bed Fusion (PBF), Direct Energy Deposition (DED), Binder Jetting (BJT), and Material Extrusion (MEX), enable the production of complex metallic components with reduced material waste and design flexibility. However, the intrinsic porosity, microstructural anisotropy, and mechanical properties of sintered AM metals significantly influence their machinability, affecting tool wear, surface integrity, and cutting forces. This review explores the key material characteristics affecting the machining performance of sintered AM metals, focusing on conventional processes such as turning, milling, and drilling. The impact of microstructure, density, and mechanical properties on machining outcomes is analyzed, along with the challenges posed by the unique properties of sintered materials. Additionally, post-processing strategies, including heat treatments and surface finishing techniques, are discussed as potential solutions to enhance machinability. The review concludes by identifying future research opportunities, particularly in optimizing AM process parameters and developing hybrid manufacturing approaches to improve the industrial applicability of sintered AM metallic materials. Although previous studies focus on individual AM technologies, this review takes a novel approach by systematically comparing the machinability of metallic materials produced via PBF, DED, BJT, and MEX. By identifying commonalities and differences among these sintering-based AM processes, this work provides a comprehensive perspective on their machining behavior and post-processing requirements, offering valuable insights for industrial applications.

Metal additive manufacturing (AM) processes are poised to transform the metal manufacturing industry, particularly in those areas where conventional manufacturing reaches its limitations in terms of both design freedom and manufacturing capabilities. Many metal AM systems are available today, including the powder-bed, powder-fed, and wire-fed processes based on laser, electron beam or plasma melting. At the same time, the variety of metal powder materials suitable for AM continues to expand. Currently there are 29 common metal powder materials available for AM, including stainless steels, aluminum, nickel, cobalt-chrome, and titanium alloys. The articles selected for this focus topic of JOM under Metal Powder for Additive Manufacturing are largely focused on metal powder for powder-bed fusion AM processes. The importance of metal powder characteristics in the powder-bed fusion AM processes has become increasingly recognized. How the powder flows and packs, can have a significant impact on powder bed formation, and hence the development of melt pools and microscopic homogeneity. Excessive variations in powder characteristics can lead to nonuniform layering, inconsistent bulk density, increased defects, undesired mechanical properties, and poor surface finish. As a result, it is essential to be able to identify the various powder characteristics that can ensure consistent and reliable performance, particularly when a lower cost, less spherical powder is intended for AM. In the first article, Slotwinski and Garboczi discuss the metrology needs for metal AM powders. The authors provide an informative overview of the current technical challenges and needs in characterizing metal powders for AM, processes based on laser, electron beam or plasma melting including recent efforts to standardize characterization methods in the ASTM International (ASTM) and the International Organization for Standardization (ISO), such as the recently released ASTM F3049, Standard Guide for Characterizing Properties of Metal Powders Used for Additive Manufacturing Processes. In the second article in this compilation, Clayton et al. show the necessity of appropriate metal powder characterization for AM through four case studies, and the inability of conventional characterization techniques to detect the subtle differences. These four case studies deal with (I) quantifying batch-to-batch variation in feedstocks, (II) the influence of different suppliers and manufacturing methods, (III) the effect of additives on feedstock properties, and (IV) process-relevant differences between fresh and used feedstocks. These are all important issues in metal AM. The third article by Strondl and co-workers is concerned with the characterization and control of powder properties for AM. The authors discuss the combined use of powder rheology and dynamic image analysis to characterize metal powders for AM. This study adds another useful case study to metal powder characterization for AM. In the fourth article in this sequence, Tang et al. report on the effect of powder reuse times on the AM of Ti-6Al-4V using an Arcam EBM A2 system (Arcam AB, Molndal, Sweden). Parts manufacturers are always both qualityand cost-conscious. In metal AM processes, the powder reuse times directly affect the affordability of the additively manufactured parts. Hence, it is necessary to identify the effect of powder reuse times on the AM process and the mechanical properties of the alloy thus fabricated. The powder composition, particle size distribution, apparent density, tap density, flowability, and particle morphology were studied as a function of powder reuse times and compared with respective properties of the virgin Arcam Ti-6Al-4V powder. Detailed tensile mechanical property data were produced from samples fabricated using Ti-6Al-4V powder that had been reused 16 times. The samples Ma Qian is the guest editor for the Powder Materials Committee the TMS Materials Processing and Manufacturing Division (MPMD), and coordinator of the topic Metal Powder for Additive Manufacturing (3D Printing) in this issue. JOM, Vol. 67, No. 3, 2015

Metallic powders for additive manufacturing (AM) processes are primarily produced by gas atomization, which consists of three steps: melting, atomization and cooling. In the present work, we report on the refurbishing of a laboratory-scale gas atomizer. The equipment facilitates small-scale atomization, useful for developing powders tailored specifically to metal AM processes (e.g. binder jetting, laser powder-bed fusion and direct energy deposition). The refurbished atomizer is operated by an in-house measurement and control system, fully equipped with pressure, oxygen, gas-flow and temperature sensors that allow the user to experiment with the input parameters, and thus, understand how they affect the physical and chemical properties of powders. In this paper, the working principle of the laboratory-scale gas atomizer is presented and the main characteristics of the newly refurbished equipment are described.

Metal additive manufacturing: Principles and applications

Binder jet metallic additive manufacturing (AM) is a popular alternative to powder bed fusion and directed energy deposition because of lower costs, elimination of thermal cycling, and lower energy consumption. However, like other metallic AM processes, binder jetting is prone to defects like porosity, which decreases the adoption of binder-jetted parts. Binder-jetted parts are sometimes infiltrated with a low melting temperature metal to fill pores during sintering; however, the infiltration is impacted by the part geometry and infiltration environment, which can cause infill nonuniformity. Furthermore, using an infiltration metal creates a complicated multiphase microstructure substantially different than common wrought materials and alloys. To bring insight to the binder jet/infiltration process toward part qualification and improved part quality, spatially dependent ultrasonic wave speed and attenuation techniques are being applied to help characterize and map porosity in parts made by binder jet AM. In this paper, measurements are conducted on binder-jetted stainless steel and stainless steel infiltrated with bronze samples. X-ray computed tomography (XCT) is used to provide an assessment of porosity.

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

This comprehensive review introduces occupational (industrial) hygienists and toxicologists to the seven basic additive manufacturing (AM) process categories. Forty-six articles were identified that reported real-world measurements for all AM processes, except sheet lamination. Particles released from powder bed fusion (PBF), material jetting (MJ), material extrusion (ME), and directed energy deposition (DED) processes exhibited nanoscale to submicron scale; real-time particle number (mobility sizers, condensation nuclei counters, miniDiSC, electrical diffusion batteries) and surface area monitors (diffusion chargers) were generally sufficient for these processes. Binder jetting (BJ) machines released particles up to 8.5 µm; optical particle sizers (number) and laser scattering photometers (mass) were sufficient for this process. PBF and DED processes (powdered metallic feedstocks) released particles that contained respiratory irritants (chromium, molybdenum), central nervous system toxicants (manganese), and carcinogens (nickel). All process categories, except those that use metallic feedstocks, released organic gases, including (but not limited to), respiratory irritants (toluene, xylenes), asthmagens (methyl methacrylate, styrene), and carcinogens (benzene, formaldehyde, acetaldehyde). Real-time photoionization detectors for total volatile organics provided useful information for processes that utilize polymer feedstock materials. More research is needed to understand 1) facility-, machine-, and feedstock-related factors that influence emissions and exposures, 2) dermal exposure and biological burden, and 3) task-based exposures. Harmonized emissions monitoring and exposure assessment approaches are needed to facilitate inter-comparison of study results. Improved understanding of AM process emissions and exposures is needed for hygienists to ensure appropriate health and safety conditions for workers and for toxicologists to design experimental protocols that accurately mimic real-world exposure conditions. ABBREVIATIONS ABS : acrylonitrile butadiene styrene; ACGIH® TLV® : American Conference of Governmental Industrial Hygienists Threshold Limit Value; ACH : air change per hour; AM : additive manufacturing; ASA : acrylonitrile styrene acrylate; AVP : acetone vapor polishing; BJ : binder jetting; CAM-LEM : computer-aided manufacturing of laminated engineering materials; CNF : carbon nanofiber; CNT : carbon nanotube; CP : co-polyester; CNC : condensation nuclei counter; CVP : chloroform vapor polishing; DED : directed energy deposition; DLP : digital light processing; EBM : electron beam melting; EELS : electron energy loss spectrometry; EDB : electrical diffusion batteries; EDX : energy dispersive x-ray analyzer; ER : emission rate; FDM™ : fused deposition modeling; FFF : fused filament fabrication; IAQ : indoor air quality; LSP : laser scattering photometer; LCD : liquid crystal display; LDSA : lung deposited particle surface area; LOD : limit of detection; LOM : laminated object manufacturing; LOQ : limit of quantitation; MCE : mixed cellulose ester filter; ME : material extrusion; MJ : material jetting; OEL : occupational exposure limit; OPS : optical particle sizer; PBF : powder bed fusion; PBZ : personal breathing zone; PC : polycarbonate; PEEK : poly ether ether ketone; PET : polyethylene terephthalate; PETG : Polyethylene terephthalate glycol; PID : photoionization detector; PLA : polylactic acid; PM1 : particulate matter with aerodynamic diameter less than 1 µm; PM2.5 : particulate matter with aerodynamic diameter less than 2.5 µm; PM10 : particulate matter with aerodynamic diameter less than 10 µm; PSL : plastic sheet lamination; PVA : polyvinyl alcohol; REL : recommended exposure limit; SDL : selective deposition lamination; SDS : safety data sheet; SEM : scanning electron microscopy; SL : sheet lamination; SLA : stereolithography; SLM : selective laser melting; SMPS : scanning mobility particle sizer; SVOC : semi-volatile organic compound; TEM : transmission electron microscopy; TGA : thermal gravimetric analysis; TPU : thermo polyurethane; UAM : ultrasonic additive manufacturing; UC : ultrasonic consolidation; TVOC : total volatile organic compounds; TWA : time-weighted average; VOC : volatile organic compound; VP : vat photopolymerization

While there have been many advancements in additive manufacturing (AM) technologies for metal products, there has not been a great deal of attention paid toward developing an understanding of the relative sustainability performance of various AM processes for production of aerospace components, such as wire feed and powder bed fusion processes. This research presents a method to calculate and compare quantitative metrics for evaluating metal AM process on a basis of sustainability performance. The process-level evaluation method encompasses a triple bottom line analysis for low volume part production. A representative aerospace titanium alloy (Ti-6Al-4V) component is considered for the study and the production of the part is modeled using direct energy deposition (DED) as the representative wire feed AM process and selective laser melting (SLM) as the representative powder bed AM process. The results indicate that DED has a superior sustainability performance to SLM, mainly due to the relatively slower deposition rate and higher cost of material for SLM than DED. This research provides decision makers an approach method and a demonstrated case study in comparing DED and SLM AM processes. This understanding reveals advantages between the two options and offers avenues of future investigation for these technologies for further development and larger scale use.

Recently, the metal Additive Manufacturing (AM) process has seen a substantial surge in its usage than other processes, especially indirect routes to obtain metal parts, including Rapid Sand Casting (RSC) using Selective Laser Sintering (SLS) and Binder Jetting (BJ). Further, it is observed that the SLS process has lost importance due to the dominance of the BJ. Thus, literature and commercial AM systems are investigated to determine the causes. This study comprises a Systematic Literature Review (SLR) and Bibliometrix using the Biblioshiny web application for patternless sand mold and core production using RSC. This article aims to obtain research articles via SLR for the SLS and BJ AM process, followed by the Bibliometrix using the Biblioshiny web application. The SLR divided the RSC domain into the AM and the Sand Casting (SC). The authors considered three databases, Scopus, Web of Science (WoS), and EBSCO, and applied the SLR, resulting in 148 articles. Subsequently, the authors analyzed 131 relevant articles via Biblioshiny, which led to valuable insights for future research. The bibliometrix and content analysis revealed seven clusters: RSC processes, its optimization, comparison, multi-material compatibility, environmental impact, mold coating, and applications. The RSC via BJ exhibited dominance in the research and commercial market over SLS, one reason SLS lost its importance. The authors proposed the revival of the SLS AM process by employing affordable CO2 laser and linear motor flying optics coupled with recent advancements, including incorporating a hybrid AM approach, adaptive slicing, and mold design for AM (DfAM).

This article provides an overview of metal additive manufacturing (AM) processes and describes sources of failures in metal AM parts. It focuses on metal AM product failures and potential solutions related to design considerations, metallurgical characteristics, production considerations, and quality assurance. The emphasis is on the design and metallurgical aspects for the two main types of metal AM processes: powder-bed fusion (PBF) and directed-energy deposition (DED). The article also describes the processes involved in binder jet sintering, provides information on the design and fabrication sources of failure, addresses the key factors in production and quality control, and explains failure analysis of AM parts.

Metal additive manufacturing (AM) techniques such Direct Energy Deposition (DED), Powder Bed Fusion (PBF), and Wire Arc Additive Manufacturing (WAAM) enable the production of complex metal components built at rapid rates. Because of the complexity of the process, including high thermal gradients, residual stress, and parameter optimization, these techniques pose significant challenges necessitating the need for advanced computational modeling. A powerful technique to reduce or, in some cases, eliminate these challenges at a much lower cost compared to trial-and-error experiments, is Finite Element Analysis (FEA). This study provides a comprehensive review of the FEA techniques being used and developed to model metal AM processes focusing on the thermal, mechanical, and coupled thermo-mechanical models in DED, PBF, and WAAM. Key topics include heat transfer, residual stress and distortion prediction, microstructure evolution and parameter optimization. Recent advancements in FEA have improved the accuracy of AM process simulations, reducing the need for costly experimental testing, though there is still room for improvement and further development of FEA in metal AM. This review serves as a foundation for future work in the metal AM modeling field, enabling the development of optimized process parameters, defect reduction strategies and improved computational methodologies for high-fidelity simulations.

Metal additive manufacturing (AM) encapsulates the myriad of manufacturing processes available to meet industrial needs. Determining which of these AM processes is best for a specific aerospace application can be overwhelming. Based on the application, each of these AM processes has advantages and challenges. The most common metal AM methods in use include Powder Bed Fusion, Directed Energy Deposition, and various solid-state processes. Within each of these processes, there are different energy sources and feedstock requirements. Component requirements heavily affect the process determination, despite existing literature on these AM processes (often inclusive of input parameters and material properties). This article provides an overview of the considerations taken for metal AM process selection for aerospace components based on various attributes. These attributes include geometric considerations, metallurgical characteristics and properties, cost basis, post-processing, and industrialization supply chain maturity. To provide information for trade studies and selection, data on these attributes were compiled through literature reviews, internal NASA studies, as well as academic and industry partner studies and data. These studies include multiple AM components and sample build experiments to evaluate (1) material and geometric variations and constraints within the processes, (2) alloy characterization and mechanical testing, (3) pathfinder component development and hot-fire evaluations, and (4) qualification approaches. This article summarizes these results and is meant to introduce various considerations when designing a metal AM component.

Metal additive manufacturing (AM) is capable of producing complex parts, using a wide range of functional metals that are otherwise very difficult to make and involve multiple manufacturing processes. However, because of the involvement of thermal energy in the fabrication of metallic AM parts, residual stress remains one of the major concerns in metal AM. This residual stress has negative effects on part quality, dimensional accuracy, and part performance. This study aims to carry out a comprehensive review and analysis of different aspects of residual stress, including the causes and mechanisms behind the generation of residual stress during metal AM, the state-of-the-art measurement techniques for measuring residual stress, various factors influencing residual stress, its effect on part quality and performance, and ways of minimizing or overcoming residual stress in metal AM parts. Residual stress formation mechanisms vary, based on the layer-by-layer deposition mechanism of the 3D printing process. For example, the residual stress formation for wire-arc additive manufacturing is different from that of selective laser sintering, direct energy deposition, and powder bed fusion processes. Residual stress formation mechanisms also vary based on the scale (i.e., macro, micro, etc.) at which the printing is performed. In addition, there are correlations between printing parameters and the formation of residual stress. For example, the printing direction, layer thickness, internal structure, etc., influence both the formation mechanism and quantitative values of residual stress. The major effect residual stress has on the quality of a printed part is in the distortion of the part. In addition, the dimensional accuracy, surface finish, and fatigue performance of printed parts are influenced by residual stress. This review paper provides a qualitative and quantitative analysis of the formation, distribution, and evolution of residual stress for different metal AM processes. This paper also discusses and analyzes both in situ and ex situ measurement techniques for measuring residual stress. Microstructural evolution and its effect on the formation of residual stress are analyzed. Various pre- and post-processing techniques used to countermeasure residual stress are discussed in detail. Finally, this study aims to present both a qualitative and quantitative analysis of the existing data and techniques in the literature related to residual stress, as well as to provide a critical analysis and guidelines for future research directions, to prevent or overcome residual stress formation in metal AM processes.

Metal additive manufacturing (AM) technologies are classified into two groups according to the consolidation mechanisms and densification degrees of the as-built parts. Densified parts are obtained via a single-step process such as powder bed fusion, directed energy deposition, and sheet lamination AM technologies. Conversely, green bodies are consolidated with the aid of binder phases in multi-step processes such as binder jetting and material extrusion AM. Green-body part shapes are sustained by binder phases, which are removed for the debinding process. Chemical and/or thermal debinding processes are usually devised to enhance debinding kinetics. The pathways to final densification of the green parts are sintering and/or molten metal infiltration. With respect to innovation types, the multistep metal AM process allows conventional powder metallurgy manufacturing to be innovated continuously. Eliminating cost/time-consuming molds, enlarged 3D design freedom, and wide material selectivity create opportunities for the industrial adoption of multi-step AM technologies. In addition, knowledge of powders and powder metallurgy fuel advances of multi-step AM technologies. In the present study, multi-step AM technologies are briefly introduced from the viewpoint of the entire manufacturing lifecycle.

Layering deposition methodology in metal additive manufacturing (AM) and the influence of different processing parameters, such as energy source level and deposition speed, which can change the melt pool condition, are known to be the important influencing factors on properties of components fabricated via AM. The effect of melt pool conditions and geometry on properties and quality of fabricated AM components has been widely studied through experimental and simulation techniques. There is a need for better understanding the influence of solidified melt pool topography on characteristics of next deposition layer that can be applied to complex surfaces, especially those with sparse topographical features, such as those that occur in AM deposition layers. Topography of deposited layers in metal additive manufacturing is a significant aspect on the bonding condition between the layers and defect generation mechanism. Characterization of the topography features in AM deposition layers offers a new perspective into investigation of defect generation mechanisms and quality evaluation of AM components. In this work, a feature-based topography study is proposed for the assessment of process parameters’ influence on AM deposition layers topography and defect generation mechanism. Titanium alloy (Ti6Al4V) samples deposited on steel substrate, by direct energy deposition (DED) AM technique at different process conditions, were used for the assessment. Topography datasets and analysis of shape and size differences pertaining to the relevant topographic features have been performed. Different AM process parameters were investigated on metallic AM samples manufactured via direct energy deposition (DED) and the potential defect generation mechanism was discussed. The assessment of the topography features was used for correlation study with previously published in-situ monitoring and quality evaluation results, where useful information was obtained through characterization of signature topographic formations and their relation to the in-situ acoustic process monitoring, as the indicators of the manufacturing process behavior and performance.