A metal powder size interval is a useful starting description, but it is not a complete feedstock specification. Two lots with the same nominal interval can have different fines, coarse tails, satellites, internal pores, surface chemistry, and spreading behavior. Those differences can matter when the powder moves through a hopper, forms a layer, enters a spray torch, or is compacted into a green body.
This guide connects particle size and morphology to practical powder-processing questions. It distinguishes measurements that are often grouped under “flowability” and explains why the right powder for one machine may be unsuitable for another. Numerical size intervals below are illustrative purchasing examples, not confirmed Basis Materials stock fractions or process approvals.
Read the distribution, not only the interval
Descriptions such as 15–45 µm, 45–105 µm, or 53–150 µm often describe a nominal classified fraction. Ask how the fraction was produced and what acceptance limits apply to material outside it. A sieve specification and a laser-diffraction distribution do not describe particle size in exactly the same way. The measurement method, sampling, and distribution basis belong beside the numbers.
D10, D50, and D90 are cumulative distribution percentiles. D50 is the diameter below which half of the measured distribution lies, using the stated basis; D10 and D90 correspond to the tenth and ninetieth percentiles. They are not the smallest, average, and largest individual particles. A volume-based D50 should not be directly compared with a number-based median without considering the conversion and assumptions.
NIST's particle-size characterization guide discusses distribution descriptors and measurement interpretation. For procurement, record the method, dispersion conditions where relevant, and whether the result is based on number, volume, or mass. Three percentiles summarize a distribution but can conceal different tail shapes or multiple particle populations. [1]
Morphology changes particle interactions
Spherical and irregular particles
Near-spherical particles can reduce some opportunities for mechanical interlocking, while irregular particles present different contact and packing geometries. However, roundness is not the only variable. Size, surface roughness, cohesion, contamination, and the imposed motion can change the observed behavior. A spherical powder can still spread poorly or fail to discharge through a particular opening.
Use a defined image-analysis metric when shape is a controlled requirement. A two-dimensional circularity value is not identical to three-dimensional sphericity, and either value depends on the measurement method and analysis choices. Specify how many particles or fields are examined and how agglomerates or touching particles are classified.
Satellites and agglomerates
Satellites are smaller attached particles that change the apparent surface and contact behavior of a larger particle. Agglomerates can also appear as large objects in a size measurement without behaving like a single dense sphere. Their significance depends on attachment strength, population, machine motion, and the application. A single attractive microscope field cannot establish the lot-wide frequency.
Ask for representative observations rather than an image selected only to show the best particles. Define whether a morphology assessment reports satellites, irregular particles, fused multi-particles, or other categories. If the powder is reused, apply the same method to fresh and reused samples before drawing conclusions about change.
Internal porosity
A smooth outside does not reveal the inside of a particle. Internal pores may require cross-sectioning, density-related methods, or X-ray computed tomography, depending on the question and resolution. NIST researchers compared three-dimensional and two-dimensional characterization of titanium-alloy powders and showed why size, shape, and internal porosity need complementary measurements. [2]
Do not equate every powder pore with a pore in the final part. Melting, gas transport, solidification, and process conditions influence whether defects remain or develop. Conversely, a low powder-pore population cannot guarantee a dense part if the deposition or consolidation process is poorly matched.
Flowability is a response under a defined test
A flow result describes behavior under particular geometry, stress, and motion. A funnel test, a rotating drum, and a shear test impose different conditions. Passing one test does not automatically establish reliable recoating, hopper discharge, or pneumatic feeding. Compare like with like and identify the test relevant to the process failure being investigated.
ASTM B213 addresses flow using the Hall funnel, while other methods serve powders or conditions that the Hall method does not represent. Record the method and report a nonflow result honestly rather than converting it into a convenient numerical score. A supplier and buyer should agree on how results are compared between lots. [3]
Fines can increase the influence of surface forces relative to particle weight. Moisture, surface films, electrostatic effects, and handling can modify powder interactions as well. That does not mean removing all fines is always the best choice: the distribution also affects packing, melting, and usable yield. Optimize for the application rather than for the most favorable isolated flow number.
Spreadability is especially important for a powder bed. A powder that exits a funnel acceptably may still form streaks or an uneven layer under a recoater. NIST's powder-metrology laboratory treats flow, spreading, particle distribution, and layer density as related but distinct characterization questions. [4]
Apparent density, tap density, and particle density
Apparent density measures powder mass per bulk volume in a defined loose-filled condition. Tap density measures the bulk state after a prescribed tapping procedure. Neither is the density of the solid alloy itself. Their difference reflects rearrangement under the method, but it does not independently prove that a powder will spread well or compact to a specific green density.
ASTM B527 defines a tap-density method for metal powders and compounds. Use a consistent method, container, and sample preparation when comparing lots. If a derived ratio is used as a screening indicator, keep the underlying measurements and avoid treating the ratio as a machine-independent process qualification. [5]
Particle density and internal porosity require a different interpretation from the voids between particles. A powder bed contains interparticle spaces even when each particle is internally dense. Separating these scales helps explain why a high bulk packing result does not establish low internal gas porosity, and why both can matter to process development.
Oxygen, nitrogen, and the condition of the surface
Size and morphology do not replace chemistry. Finer fractions generally provide more surface area per unit mass for similar particle shapes, making surface condition and handling important. Oxygen and nitrogen sensitivity depend on the alloy and process. Specify the elements relevant to the application rather than assuming that a nominal alloy designation includes the required interstitial limits.
Ask whether chemical results represent the delivered classified fraction or a different sample of the production lot. Surface analysis and bulk elemental analysis answer different questions. NIST's metal-powder characterization work combines methods for chemistry, crystalline phases, shape, and density, illustrating the value of a complementary measurement plan. [6]
For reused powder, document collection, sieving, blending, exposure, and reuse history. A blend of virgin and reused material is not characterized solely by the virgin certificate. Sampling should represent the powder actually returned to the machine, including any segregation introduced during transport or handling.
How VIGA, EIGA, and PREP fit into the specification
Atomization influences the starting particle population, but classification and handling help determine the delivered fraction. VIGA combines induction melting with gas atomization; EIGA supplies melt from an induction-heated electrode without a conventional melting crucible; PREP uses a rotating electrode and centrifugal droplet ejection.
Basis Materials has PREP, VIGA, and EIGA equipment, as confirmed by the company. That does not establish that every alloy, size fraction, or quantity can be produced using all three routes. Discuss the particular composition and required powder characteristics, then qualify the supplied lot. The existing PREP, VIGA, and EIGA guide explains the operating principles in more detail.
Do not use the process label as a substitute for measured morphology, chemistry, or pores. An electrode route still depends on electrode quality and atmosphere control; a crucible-based route requires suitable melt-contact materials. The relevant comparison is the demonstrated delivered powder, not a universal hierarchy of atomization names.
Match particle characteristics to the process
| Application | Characteristics to discuss | Qualification evidence |
|---|---|---|
| Powder bed fusion | Distribution tails, spreading, satellites and chemistry | Machine-relevant layer behavior and qualified build results |
| Directed energy deposition | Feeder response, distribution, shape and composition | Stable feeding and deposition under the intended settings |
| Thermal spray | Injection, heating response, size and morphology | Torch-specific feeding and coating assessment |
| Pressing and sintering | Packing, compressibility, surface chemistry and green integrity | Compaction and sintering trials with dimensional assessment |
For additive manufacturing, start with the machine and process qualification rather than assuming that a commonly quoted interval is approved. Layer thickness, feeding architecture, energy input, atmosphere, and alloy behavior interact. ASTM F3049 provides a guide to relevant powder-characterization techniques; it is not a universal acceptance specification for every AM machine. [7]
For thermal spray and related feeding processes, evaluate particle transport and heating in the actual equipment. For conventional powder metallurgy, irregularity can sometimes serve a different purpose from the flow and spreading priorities of a powder bed. A powder optimized for one use should not be labeled unsuitable in general simply because its shape differs from an AM feedstock.
Sampling is part of the specification. Particles can segregate during container filling, transport, or repeated withdrawal, so a convenient scoop from the top may not represent the whole lot. Agree on sample collection and subdivision before comparing certificates. Preserve enough material for a repeat measurement when a distribution tail or morphology result is disputed.
A powder RFQ that supports a useful comparison
Specify alloy composition, required fraction, distribution method and basis, tail limits where needed, morphology criteria, relevant chemistry limits, quantity, and packaging. Include the process and machine context and identify whether the feedstock is for first-time development or an already qualified production route.
Request lot-linked evidence for the delivered fraction: distribution, representative morphology, flow or spreading measurements relevant to the use, apparent or tap density when useful, and chemistry. If internal pores are important, agree on a method and its resolution. Define sampling and acceptable variation rather than requesting a favorable generic data sheet.
Explore metal powders for additive manufacturing and high-entropy alloy materials, or contact Basis Materials to discuss a custom composition and particle fraction. The aim is a powder matched to the application and supported by an agreed inspection plan.
Frequently asked questions
Does D90 mean that no larger particles exist?
No. It is a percentile on a specified distribution basis. Coarse-tail acceptance needs its own definition and measurement.
Does spherical powder always flow better?
No. Shape is one influence among size, surface condition, cohesion, and the imposed test or machine motion.
Can a Hall flow result qualify a powder bed?
It can be useful screening evidence, but it does not replace spreading or build qualification in the intended machine.
Related guides in this series
- How High-Entropy Alloys Are Manufactured: Arc Melting vs. Powder Metallurgy
- Manufacturing Custom Refractory Metal Parts: W, Mo, Ta, and Nb
Technical references
- NIST, Particle Size Characterization, Recommended Practice Guide. Distribution and method interpretation.
- Garboczi et al., Three-dimensional particle size, shape, and internal porosity characterization. Additive Manufacturing 44 (2021), 102060.
- ASTM B213, Flow Rate of Metal Powders Using the Hall Flowmeter Funnel. Official method scope.
- NIST, Additive Manufacturing Powder Metrology Laboratory. Flow and spreading metrology context.
- ASTM B527, Tap Density of Metal Powders and Compounds.
- Slotwinski et al., Characterization of Metal Powders Used for Additive Manufacturing. NIST research, 2014.
- ASTM F3049, Guide for Characterizing Properties of Metal Powders Used for Additive Manufacturing Processes. Use the applicable edition and agreed test methods.