Metal Powder Part 2

Powder Attributes

Characteristics of a metallic powder that influence its suitability and behavior in additive manufacturing, including particle size, particle-size distribution, surface oxide, and individual particle chemistry.

Particle Size

The characteristic dimension of an individual powder particle.

Particle-Size Distribution (PSD)

The distribution of particle sizes present within a powder population.

Why is particle-size distribution important in AM?

The range and distribution of particle sizes influence how a powder behaves during handling and additive manufacturing.

D10

The particle diameter below which 10% of the powder particles are smaller.

D50

The particle diameter below which 50% of the powder particles are smaller.

D90

The particle diameter below which 90% of the powder particles are smaller.

Meaning of D10–D90 Powder Range

A particle-size range described by the diameter below which 10% of particles fall and the diameter below which 90% of particles fall.

Additional PSD Percentiles

Some powder specifications use additional particle-size distribution values such as D2, D50, and D98.

Typical Shape of an Overall Manufactured Powder-Size Distribution

The source describes the overall particle-size distribution produced during powder manufacturing as resembling a Gaussian distribution.

Sieving of Metal Powder

The separation of a manufactured powder population into specified particle-size ranges after powder production.

Why is manufactured powder sieved?

Powder manufacturing produces a broad range of particle sizes, so sieving is used to separate the powder into the size ranges required by customers or processes.

Particle Morphology

The shape and geometric characteristics of individual powder particles.

Sphericity

The degree to which a powder particle approaches a spherical shape.

Why is spherical powder often preferred for PBF and DED?

Spherical powder is commonly preferred, although the source emphasizes that the ability of the powder to flow and spread evenly is more important than sphericity itself.

Flowability

The ability of powder particles to flow during handling and delivery.

Spreadability

The ability of a powder to spread evenly when forming a layer.

Why are flowability and spreadability important?

Powder must be capable of flowing and spreading evenly to produce suitable and homogeneous powder layers in processes that depend on powder deposition.

Is high sphericity always the most important powder requirement?

No. The source states that the ability to flow and spread evenly is more important, and sphericity is less important for some AM processes.

Satellite

A small powder particle adhered to a larger powder particle.

Powder Particle Porosity

Void space contained within an individual powder particle.

Tap Density

The density of a compacted container of powder expressed as a percentage of the density of the fully dense alloy.

Low Tap Density

A condition in which a compacted powder occupies substantially more volume than the equivalent fully dense metal; some irregular or spongy direct-reduced powders can have very low tap density.

Powder Packing

The arrangement of powder particles within a volume or deposited layer.

Powder Packing Characteristics

The ability of a powder population to arrange and compact within a given volume, influenced by its particle characteristics.

Surface Oxide

A thin oxide layer present on the surface of a metallic powder particle, particularly important for reactive alloys.

Reactive Alloy Powder

Metal powder, such as aluminum-, magnesium-, or titanium-alloy powder, for which oxidation is an especially important consideration.

Surface-Area-to-Volume Ratio

The amount of particle surface area relative to particle volume; this ratio increases as particle diameter decreases.

Particle Size vs. Surface-Area-to-Volume Ratio

As powder particle diameter decreases, its surface-area-to-volume ratio increases.

Why do smaller reactive-alloy powders generally contain more oxygen than larger powders?

The oxide layer is generally of approximately constant thickness, so smaller particles have a greater surface-area-to-volume ratio and therefore a greater amount of oxide relative to their volume.

Particle Size–Oxide Relationship

Smaller particle diameter → higher surface-area-to-volume ratio → greater relative oxide content → generally higher oxygen content.

Why can very fine powder with very low oxygen content be difficult or expensive to obtain?

Smaller powders naturally have a higher relative oxide contribution, making the combination of very small particle size and very low oxygen content difficult to achieve.

Powder Oxygen Pickup

An increase in the oxygen content of powder during additive-manufacturing processing, handling, recovery, or reuse.

Powder Reuse

The recovery and subsequent use of powder remaining after an additive-manufacturing build.

Why is starting oxygen content important when powder will be reused?

If the starting oxygen level is already close to the maximum permitted value, additional oxygen pickup during processing and reuse can quickly cause the powder to exceed specification.

Powder Scrappage Due to Oxygen

Powder may need to be discarded when oxygen pickup raises its oxygen content above the maximum allowed specification.

Can out-of-specification high-oxygen powder simply be blended with low-oxygen powder to meet specification?

The source states that many specifications prohibit blending powder above the maximum allowed oxygen level with powder below the limit merely to create an in-specification blend.

Powder Size vs. PBF Layer Thickness

In powder bed fusion, powder particle diameter is constrained by layer thickness because particles larger than the layer can interfere with recoating and layer homogeneity.

Oversized Particle in Powder Bed Fusion

A powder particle larger than the intended layer thickness that can interfere with the recoater and reduce layer homogeneity.

Recoater

A component in powder bed fusion that distributes powder to create a new powder layer.

Layer Homogeneity

The uniformity of a deposited powder layer.

Why can particles larger than the PBF layer thickness be problematic?

They can interfere with the recoater and negatively affect the homogeneity of the deposited layer.

Why can excessively small powder particles also be problematic in PBF?

The source states that particles that are too small can be displaced by the melt pool.

Particle Size as an AM Process Variable

Powder size must be compatible with the way a particular AM process delivers, spreads, melts, binds, or consolidates its feedstock.

Why might smaller powders be preferred in binder jetting and polymer ME/MJ?

Smaller powder sizes can improve resolution, surface finish, and sintering time.

Why might larger powders be preferred in binder jetting and polymer ME/MJ?

Larger powder sizes can offer advantages in cost, shrinkage control, and handling safety.

Why might a blend of particle sizes be used?

A blend of particle sizes can be selected to optimize competing considerations such as sintering time and shrinkage.

Green State

The condition of a printed powder-based part before binder removal.

Brown State

The condition of a powder-based part after binder removal but before final sintering.

Green Strength

The strength of a printed part in its green condition, allowing it to retain its shape before later processing.

Brown Strength

The strength of a part after binder removal that helps it retain its shape before or during subsequent sintering.

Why can less-flowable powder sometimes be useful in binder jetting or polymer ME/MJ?

The source states that powders that do not flow as well can provide better green and brown strength, helping parts retain their shape during sintering.

Powder Chemistry

The chemical composition of powder particles, including the target alloy chemistry and impurity or oxygen content.

Individual Particle Chemistry

The chemical composition of each individual powder particle rather than only the average composition of the entire powder batch.

Powder Characterization

The evaluation of powder characteristics such as particle size, PSD, morphology, oxide content, chemistry, porosity, density, flow, and spreading behavior to determine suitability for processing.

Powder Attribute–Processing Relationship

Powder characteristics influence how feedstock flows, spreads, packs, and interacts with the additive-manufacturing process.