Additive manufacturing (AM) techniques can produce complex, high-value metal parts, with potential applications as critical parts, such as those found in aerospace components. The production of AM ...parts with consistent and predictable properties requires input materials (e.g., metal powders) with known and repeatable characteristics, which in turn requires standardized measurement methods for powder properties. First, based on our previous work, we assess the applicability of current standardized methods for powder characterization for metal AM powders. Then we present the results of systematic studies carried out on two different powder materials used for additive manufacturing: stainless steel and cobalt-chrome. The characterization of these powders is important in NIST efforts to develop appropriate measurements and standards for additive materials and to document the property of powders used in a NIST-led additive manufacturing material round robin. An extensive array of characterization techniques was applied to these two powders, in both virgin and recycled states. The physical techniques included laser diffraction particle size analysis, X-ray computed tomography for size and shape analysis, and optical and scanning electron microscopy. Techniques sensitive to structure and chemistry, including X-ray diffraction, energy dispersive analytical X-ray analysis using the X-rays generated during scanning electron microscopy, and X-Ray photoelectron spectroscopy were also employed. The results of these analyses show how virgin powder changes after being exposed to and recycled from one or more Direct Metal Laser Sintering (DMLS) additive manufacturing build cycles. In addition, these findings can give insight into the actual additive manufacturing process.
A simple solution processing method is developed to achieve a uniform and scalable stabilized lithium metal powder (SLMP) coating on a Li-ion negative electrode. A solvent and binder system for the ...SLMP coating is developed, including the selection of solvent, polymer binder, and optimization of polymer concentration. The optimized binder solution is a 1% concentration of polymer binder in xylene; a mixture of poly(styrene-co-butadiene) rubber (SBR) and polystyrene (PS) is chosen as the polymer binder. Results show that long-sustained, uniformly dispersed SLMP suspension can be achieved with the optimized binder solution. The uniform SLMP coating can be achieved using a simple “doctor blade” coating method, and the resulting SLMP coating can be firmly glued on the anode surface. By using SLMP to prelithiate the negative electrode, improvements in electrochemical performances are demonstrated in both graphite/NMC and SiO/NMC full cells.
•Scalable SLMP coating is developed with simple solution processing method.•A solvent and binder system (1% SBR+PS in xylene) is developed for SLMP coating.•Long-sustained, uniformly dispersed SLMP suspension has been achieved.•The effect of SLMP is demonstrated through graphite/NMC and SiO/NMC full cell.
The Hall-Petch relationship, according to which the strength of a metal increases as the grain size decreases, has been reported to break down at a critical grain size of around 10 to 15 nanometres
. ...As the grain size decreases beyond this point, the dominant mechanism of deformation switches from a dislocation-mediated process to grain boundary sliding, leading to material softening. In one previous approach, stabilization of grain boundaries through relaxation and molybdenum segregation was used to prevent this softening effect in nickel-molybdenum alloys with grain sizes below 10 nanometres
. Here we track in situ the yield stress and deformation texturing of pure nickel samples of various average grain sizes using a diamond anvil cell coupled with radial X-ray diffraction. Our high-pressure experiments reveal continuous strengthening in samples with grain sizes from 200 nanometres down to 3 nanometres, with the strengthening enhanced (rather than reduced) at grain sizes smaller than 20 nanometres. We achieve a yield strength of approximately 4.2 gigapascals in our 3-nanometre-grain-size samples, ten times stronger than that of a commercial nickel material. A maximum flow stress of 10.2 gigapascals is obtained in nickel of grain size 3 nanometres for the pressure range studied here. We see similar patterns of compression strengthening in gold and palladium samples down to the smallest grain sizes. Simulations and transmission electron microscopy reveal that the high strength observed in nickel of grain size 3 nanometres is caused by the superposition of strengthening mechanisms: both partial and full dislocation hardening plus suppression of grain boundary plasticity. These insights contribute to the ongoing search for ultrastrong metals via materials engineering.
The particle size distribution (PSD) and particle morphology of metal powders undoubtedly affects the quality of parts produced by additive manufacturing (AM). It is, therefore, crucial to accurately ...know the PSD and morphology of these powders. There exist several measurement techniques for these quantities, but since each method is based on different physical phenomena, which are sensitive to different aspects of a particle's shape and size, it is unclear how the measured PSDs and morphology compare to one another. In this study, five different techniques are used: sieve analysis, dynamic imaging analysis, laser diffraction analysis, X-ray computed tomography (XCT), and scanning electron microscopy. The first three are commonly used in the powder metallurgy field while the last two are laboratory-based tools capable of providing robust size and shape data. Nominally identical samples of stainless-steel powders were produced via riffling, and each technique was employed to measure effectively the same PSD and in some cases the morphology. In this paper, the differences among these measurement techniques are explored by a comparison of the measured results. Besides the random variations of the various measurement processes, the difference in the results is partly due to the fact that the particles are not perfectly spherical and that there are many multi-particles present. Each of these affect the principle of each method differently. Three-dimensional particle morphology and size data collected via XCT is used to provide insight regarding the discrepancies among other sizing and morphology measurement techniques.
(Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.)
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•Common sizing techniques are compared to more robust laboratory-based methods•Each sizing method quantifies different physical properties•Appropriate metrics are carefully chosen when comparing different techniques•3D XCT data provide insight into the discrepancies among other techniques•Complex morphologies present in gas atomized powders affect the measurement of size
One of the biggest challenges in the biocompatibility of implantable metals is the prevention of the stress shielding effect, which is related to the coupling of the bone-metal mechanical properties. ...This stress shielding phenomenon provokes bone resorption and the consequent adverse effects on prosthesis fixation. However, it can be inhibited by adapting the stiffness of the implant material. Since the use of titanium (Ti) porous structures is a great alternative not only to inhibit this effect but also to improve the osteointegration of orthopedic and dental implants, a brief description of the techniques used for their manufacturing and a review of the current commercialized implants produced from porous Ti assemblies are compiled in this work. As powder metallurgy (PM) with space holder (SH) is a powerful technology used to produce porous Ti structures, it is here discussed its potential for the fabrication of medical devices from the perspectives of both design and manufacture. The most important parameters of the technique such as the size and shape of the initial metallic particles, the SH and binder type of materials, the compaction pressure of the green form, and in the sintering stage, the temperature, atmosphere, and time are reviewed according to the bibliography reported. Furthermore, the importance of the porosity and its types together with the influence of the mentioned parameters in the final porosity and, consequently, in the ultimate mechanical properties of the structure are discussed. Finally, a few examples of the PM-SH application for the manufacturing of orthopedic implants are presented.
Because of its high specific capacity, silicon is regarded as the most promising candidate to be incrementally added to graphite‐based negative electrodes in lithium‐ion batteries. However, silicon ...suffers from significant volume changes upon (de‐)lithiation leading to continuous re‐formation of the solid electrolyte interphase (SEI) and ongoing active lithium losses. One prominent approach to compensate for active lithium losses is pre‐lithiation. Here, the “contact pre‐lithiation” of silicon/graphite (Si/Gr) negative electrodes in direct contact with passivated Li metal powder (PLMP) is studied, focusing on the pre‐lithiation mechanism in “dry state” and after electrolyte addition. PLMP is pressed onto the electrode surface to precisely adjust the degree of pre‐lithiation (25%, 50%, and 75%). By in situ XRD and ex situ 7Li NMR studies, it is proven that significant lithiation of Si/Gr electrodes occurs by direct contact to Li metal, that is, without electrolyte. After electrolyte addition, de‐lithiation of silicon and graphite is confirmed, resulting in SEI formation. The amount of Li metal highly impacts the presence and durability of the LixC and LixSi phases. Finally, the challenges for homogeneous pre‐lithiation and SEI formation are identified, and the impact of electrolyte addition is assessed by analysis of the lateral and in‐depth lithium distribution within the Si/Gr electrode.
Continuous re‐formation of the solid electrolyte interphase and ongoing active lithium losses of Si‐based materials are a serious challenge for implementing significant amounts of Si in state‐of‐the‐art graphite negative electrodes. Pre‐lithiation via Li metal powder is investigated electrochemically and analytically toward the lithiated phases and lithium distribution by revealing the impact of the electrolyte on the overall pre‐lithiation behavior.
► The effective porosity was measured and the total porosity was estimated. ► The total porosity shows a good correlation with the effective porosity. ► The exponential equation has been examined for ...porous concrete. ► A new model using Griffith’s fracture theory was proposed. ► Proposed model represents a significant improvement over the exponential equation.
As for many porous media, the strength of porous concrete is significantly affected by the porosity of its internal structure. This paper describes the development of a mathematical model to characterize the relationship between compressive strength and porosity for porous concrete by analyzing empirical results and theoretical derivations. The suitability of existing equations for porous concrete is assessed and a new model is then proposed. The new model, which was derived from Griffith’s theory, presents a better agreement with the experimental data for porous concrete. It is demonstrated that the proposed model could provide a better prediction of porous concrete compressive strength based on the material porosity.
Selective Laser Melting (SLM) is a rapidly developing and advanced manufacturing method for fabricating complex products. In SLM, the powder spreading process is crucial to ensure that the right ...amount of material can be fully melted by a certain laser energy input in order to minimise defects and achieve the desired microstructure. The packing density and homogeneity of the formed powder bed are of interest when comparing melting efficiency and quality of SLM processes with different metal powders or different spreading methods. Particle-based numerical studies are required for identifying the powder bed structure and particle dynamical behaviours which are affected by particle adhesion. In this work, experiments on powder packing density and repose angle for different particle size distributions are carried out. The discrete element method (DEM) model is validated and calibrated based on experimental results. The DEM is then used to examine the powder spreading process, focusing on the effects of particle adhesion and particle-based behaviours. Effects of spreader type, adhesion magnitude and particle size distribution are analysed. The results show that particle adhesion can reduce powder packing density and smoothness of the powder bed surface. Proper adhesion effects can improve powder bed homogeneity. Powder bed structure is determined not only by adhesion effects but also by particle rearrangement during spreading. Regarding spreader type, the roller can spread a better powder bed than rigid blade due to different particle contact force distributions and particle velocities in the powder pile and powder bed, which lead to different particle rearrangements and particle contact conditions.
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•Ni alloy powder spreading process in SLM is investigated by DEM.•Adhesion effects can reduce powder packing density and smoothness.•Rollers can pack powder more densely and smoothly than blades.•Adhesion effects in roller system are reduced compared with blade system.
The thermochemical properties of Al micropowder after exposure to microwave irradiation were investigated. The Al micropowder was exposed to microwave irradiation in air with a frequency of 2.85 GHz, ...a power density of 8 W/cm
, and a pulse duration of 25 ns and 3 µs. The thermochemical parameters of the irradiated metal powders were determined by the method of thermal analysis at the heating in air. It was found that an increase in the duration of microwave pulses and irradiation time leads to the thermal annealing of the metal particles, and the thermal processes of melting and sintering begin to dominate over non-thermal processes. The specific thermal effect of irradiated Al micropowder oxidation increases from 7744 J/g to 10,154 J/g in comparison with the unirradiated powder. The modeling of thermal heating processes of aluminum (Al) micropowder under the action of pulsed microwave radiation has been performed. It is shown that with an increase in the duration of microwave pulses and irradiation time, a significant heating of the Al micropowder occurs, leading to its melting and sintering. The results of modeling on the action of microwave radiation on the Al micropowder were compared with experimental results.
The consolidation of metal powders is a complex thermomechanical process, and the temperature has a significant effect on the density distribution in the compact. The consolidation process of metal ...powders with an average particle size of 10 μm, 25 μm, and 50 μm under hot isostatic pressure was simulated by finite element modeling. The distribution and evolution of the relative density after being hot isostatic pressing (HIP) under 1050 °C/130 MPa/4 h, 1150 °C/130 MPa/4 h, and 1250 °C/130 MPa/4 h conditions were simulated, respectively. The experimental data of HIP at 1050 °C/130 MPa/4 h were used to verify the modeling results via the geometric change in the container. The relative density difference between the simulated results and the experimental results at different positions was less than 2%. This methodology called "modeling prediction, experimental validation" can accelerate experimental discovery in an economic manner.
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