Comparison of characterization methods for differently atomized nickel-based alloy 625 powders☆
Graphical abstract
Introduction
Two commonly used atomization techniques to produce metal powder are (1) gas atomization (GA), which results in spherical particles and high packing density, and (2) water atomization (WA), which yields irregular particles that better retain shape and cost less [1,2]. Since the use of nickel superalloy components in aerospace, chemical, power plants and marine applications is growing [3], there is a need to explore and compare reliable powder characterization methods to ensure powder quality.
Powder molding and sintering and additive manufacturing (AM) are two main markets for metal powders [[4], [5], [6], [7], [8], [9]], each with similarities in their needs for powder quality control. It is known that powder characteristics, e.g. morphology, size and distribution; composition and impurities; flowability; possible recycling and reusing [[10], [11], [12], [13], [14], [15], [16], [17]], can directly affect processing parameters in various additive manufacturing technologies including selective laser melting (SLM) [18,19] and binder jet printing (BJP) [[20], [21], [22]] or powder metallurgy technology such as injection molding [8,9,23]. As mentioned above, atomization leads to different shapes, sizes and morphologies of powder particles which affect powder properties and utilization. Porosity also varies due to differing atomization; this is important to quantify as it can be used to predict internal defects and final part quality. Therefore, due to this variation, it is crucial to obtain powder characteristics prior to utilization and ensure similar quality between powder batches.
Generally, powder morphology is observed using optical microscopy (OM) or scanning electron microscopy (SEM) and powder particle size analysis is considered using laser analyzer, pycnometer and other techniques each having disadvantages. For many techniques, the metallography needed to prepare samples for analysis is time-consuming if statistical certainty is needed. For porosity analysis, since a very small fraction of particles have pores and only a fraction of those particles is cross-sectioned through a pore, the sample size required for optical analysis needed to be statistically significant becomes impractical and in most cases does not reflect the correct fraction of particles with pores. Potential contamination of the main chamber and required high vacuum for operation makes SEM sample preparation time-consuming. Particle size analysis instruments such as a laser analyzer require powder particles to be suspended in a solution which creates limitations for reactive and heavy metal powders. Compared to image analysis methods, pycnometery allows studying a great number of powder particles; however, this method does not provide a pore size distribution in the powder and cannot be utilized with a high level of confidence on alloys due to the uncertainty related to the theoretical density of the material [21,24,25]. Sieving is one of the most widely used methods to measure particle size due to its simplicity and low cost. Basically, a stack of sieves with decreasing mesh size from top to bottom is mechanically vibrated and, based on the particle size, each mesh will retain powder comprised of particles larger than the mesh size. Then, the mass of powder on each mesh is measured to report particle size distribution [26]. However, sieving will only reveal powder size distribution (PSD), not shape, porosity, or other properties of the powder.
Characterizing the properties of metallic powder used in an additive manufacturing system is a vital condition for research and industrial applications to be able to confidently select powder and produce consistent parts with predictable properties. In other words, ensuring consistent powder characteristics through characterization is vital to ensure repeatable manufacturing of parts. Determinating the most suitable particle analysis technique depends on a number of different considerations including the total mass, powder morphology, density, chemical composition, flow, and thermal of the particles to be characterized in addition to the size distribution technique to be used [[27], [28], [29], [30]]. The standards that were developed for sampling of powders for particle size analysis further verify the importance of sampling to conduct accurate particle sizing analyses. Basically, spherical metal powder is preferred in AM with a particle size distribution that is designed to facilitate good packing behavior, such that the final produced part is fully dense with good mechanical properties; however, irregular powders might be of interest as produced by water atomization due to their lower cost.
In this study, systematic comparisons between water and gas atomized alloy 625 powder in terms of composition, thermal behavior, morphology, size distribution, porosity in particles, and shape are carried out using both conventional techniques and a novel μCT approach. We introduce μCT as a technique to nondestructively characterize powder shape, size and morphology as well as pore analysis in three dimensions at a micron level spatial resolution. All this information is beneficial to better understand and adjust the use of WA and GA powders in powder metallurgy and additive manufacturing applications.
Section snippets
Materials and methods
Differently atomized nickel-based alloy 625 powders manufactured by air-melted water atomization (WA) and vacuum-melted argon gas atomization (GA) with a nearly identical nominal size range of 15–53 μm and 16–53 μm, respectively, were selected for this study. The WA powder was supplied by HAI Advanced Material Specialists, Inc. and the GA powders were supplied by Carpenter Technology Corporation. Characterization methods were utilized as follows.
Microscopy observations
OM and SEM micrographs of WA and GA powder particle morphology are shown in Figs. 1 and 2, respectively, and illustrate that the WA particles were angular and irregular in shape while the GA particles were spherical. In all powders, the individual powder particles had a dendritic microstructure typical of rapid solidification. Generally, powder morphology refers to the size and shape of powder particles. Both characteristics are known to play an important role in powder performance in AM such
Conclusion
In this study, powder characteristics of differently atomized powders were investigated. Shape, size and morphology of the powders were observed using optical microscopy, scanning electron microscopy and μCT. It was found that water atomization resulted in irregular shaped powder while gas atomization (vacuum-melted and argon-gas atomized), led to the formation of spherical powder. Phase and elemental analysis were carried out on the powders using X-ray diffraction and energy dispersive
Acknowledgements
This project was partially funded by the Air Force Research Laboratory under agreement number FA8650-12-2-7230 and by the Commonwealth of Pennsylvania, acting through the Department of Community and Economic Development, under Contract Number C000053981. This material is based upon work supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Office of Advanced Manufacturing, under contract number DE-AC05-00OR22725. CH would like to thank the Swanson School
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This manuscript has been co-authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).