Metal Powder

What is Metal Powder

Metal powder is a collection of discrete metal or alloy particles produced deliberately as an engineering feedstock, not as a by-product. Particle diameters in industrial use span roughly three orders of magnitude, from sub-micron carbonyl iron near 1 to 8 microns, through the 10 to 45 micron range used in fine sintering and laser printing, up to coarse 100 to 200 micron powders used in directed energy deposition and conventional pressing. A powder is therefore not a single material but a statistical population of particles, and its behavior in a press, a furnace, or a printer is governed by the distribution of sizes and shapes as much as by the underlying metallurgy.

The defining commercial fact about metal powder is that two lots with identical chemistry can behave completely differently. A 316L stainless powder that is irregular and coarse compacts well in a die but will not spread in a powder bed fusion machine; a spherical, narrow-cut 316L flows and prints cleanly but compacts poorly and costs several times more. For this reason the powder spec sheet always pairs a chemistry table with a physical-property table covering particle-size distribution, apparent density, tap density, and flow rate. Buying powder by alloy name alone is the single most common procurement error in this category.

Powder metallurgy as an industrial discipline is old. Sponge-iron and electrolytic copper powders were produced at industrial scale from the early twentieth century, and self-lubricating bronze bearings made by press-and-sinter date to the 1920s and 1930s in the automotive industry. The modern expansion was driven first by structural press-and-sinter parts for automobiles, then by metal injection molding from the 1980s for small complex parts, and most recently by metal additive manufacturing, which created demand for highly spherical, gas- and plasma-atomized powders in tight size cuts and drove the writing of dedicated standards such as ASTM F3049 and ISO/ASTM 52907.

The economic logic of powder routes is net-shape or near-net-shape production. Pressing and sintering, metal injection molding, and additive manufacturing all build a part close to final geometry, minimizing machining scrap and enabling features that are difficult or impossible to machine: internal porosity for self-lubrication, controlled density, fine complex geometry, and conformal cooling channels. Powder also enables alloys and composites that cannot be made by conventional melting, including immiscible systems, tungsten heavy alloys, and metal-matrix composites, because the constituents are blended in the solid state.

Four families of physical metrics dominate powder quality across all downstream processes: particle-size distribution, particle shape and sphericity, flow and packing behavior (apparent density, tap density, Hall or Carney flow), and interstitial chemistry, principally oxygen and nitrogen content. These four families recur throughout this guide because they, rather than nominal alloy designation, determine process yield, part density, and final mechanical properties.

Production Routes and Powder Types

The production route fixes a powder's shape, surface chemistry, size range, and cost long before any downstream process sees it. The major industrial routes are atomization (gas, water, plasma, and centrifugal), chemical reduction, electrolysis, and the carbonyl process, plus mechanical milling for brittle or composite materials. Atomization, especially water and gas, produces the large majority of tonnage. The table below summarizes the engineering character of each route.

Gas atomization melts the alloy and pours it through a nozzle where a high-velocity jet of inert gas, usually argon or nitrogen, breaks the molten stream into droplets that solidify in free flight. The droplets are nearly spherical with clean surfaces and low oxygen, and the size cut can be tuned fine, with a meaningful fraction below 20 microns. This combination of sphericity, low oxygen, and controllable distribution makes gas atomization the dominant route for additive manufacturing and metal injection molding feedstock. The trade-offs are higher cost, inert-gas consumption, and a wide native distribution that must be sieved or classified into the narrow cuts each process needs.

Water atomization replaces the gas jet with a high-pressure water stream. The much faster quench and higher energy give high throughput at low cost, which is why water-atomized iron and low-alloy steel powders dominate press-and-sinter tonnage. The penalty is irregular, ligamented particle shape and a higher surface oxide from the water contact. Irregular shape is actually an advantage in die compaction because particle interlocking raises green strength, so water atomization and press-and-sinter are a natural pairing, but the same shape and oxide make water-atomized powder a poor fit for powder bed fusion.

Plasma atomization and the related plasma rotating electrode process feed a wire or rotating bar into a plasma torch, producing extremely spherical, satellite-free particles with very low contamination. The high cost confines these routes mainly to reactive and high-value metals, above all titanium and titanium alloys for aerospace and medical implants, where AP&C, 6K Additive, and Tekna are established suppliers. Carbonyl processing decomposes iron or nickel carbonyl gas into very fine spherical particles down to a few microns, the standard route for the fine iron powder used in metal injection molding, soft magnetic cores, and microwave-absorbing materials.

Chemical reduction reduces an oxide, such as mill scale or ore, with hydrogen or carbon to yield porous sponge particles; reduced iron and tungsten powders follow this route. Electrolysis deposits dendritic, high-purity copper, iron, or silver powder from solution, valued where purity and high green strength matter. Mechanical milling in attritors or ball mills fractures brittle materials and blends composites, and is the route for many hard-metal, master-alloy, and mechanically alloyed powders. Many production powders are blends: a base atomized powder mixed with lubricant, graphite, and alloying additions to hit a target MPIF grade.

Particle Size by Process

The most consequential procurement decision is matching the particle-size cut to the downstream process. Each process has a working window driven by how it spreads, feeds, or compacts powder, and ordering the wrong cut wastes the lot even when the alloy is correct. The table below gives the typical cuts; treat them as starting points and confirm the exact window with the machine or tooling supplier.

Laser powder bed fusion, sold as SLM or DMLS, spreads a layer typically 20 to 50 microns thick and melts it with a fine laser spot, so it needs a fine cut, most often 15 to 45 microns, with 20 to 53 microns also common. The powder must be spherical and free-flowing so the recoater lays a dense, void-free layer. Excessive fines below about 5 to 10 microns hurt flow and raise safety and recoating problems, which is why a controlled D10 is part of the spec.

Electron beam melting preheats and lightly sinters the powder bed under vacuum, which lets it run a coarser 45 to 106 micron cut. The coarser powder reduces the electrostatic "smoke" or powder repulsion that plagues fine powders under an electron beam, and the preheat relaxes residual stress. Directed energy deposition blows powder through a nozzle into a melt pool and needs coarse, very free-flowing 50 to 150 micron powder so the feed is steady and the nozzle does not clog. Binder jetting prints a binder onto a powder bed and is the most size-tolerant of the additive routes, roughly 10 to 100 microns, and often benefits from a bimodal blend that packs denser before sintering.

Metal injection molding mixes very fine powder with a polymer binder, molds it, debinds it, and sinters to high density. Fine powder, a median near 10 microns and D90 of 20 to 30 microns, is essential because sintering rate scales steeply with surface area; specifications often require that 80 percent of particles sit below 22 microns. Press-and-sinter structural PM takes the opposite approach, using coarser, often water-atomized and irregular powder below 150 microns whose interlocking gives the pressed green part enough strength to handle before sintering. Thermal spray spans a wide window, 5 to 90 microns, tuned to the spray process so particles melt or soften on impact without vaporizing.

Characterization Standards

Metal powder is one of the most heavily standardized materials in procurement because its behavior is so sensitive to physical character. Three bodies write the dominant standards: ASTM International (B-series for powders, F42 for additive manufacturing), ISO (TC 119 for powder metallurgy and TC 261 for additive manufacturing), and the Metal Powder Industries Federation, MPIF, whose Standard 35 fixes press-and-sinter material grades. A reputable supplier states which standard and revision each reported number was measured against; a number with no method behind it is not comparable across suppliers.

Hall and Carney flow. The Hall flowmeter funnel (ASTM B213, ISO 4490) times how long 50 grams of powder take to drain through a calibrated 2.5 millimeter orifice, reported in seconds per 50 grams; lower is more free-flowing. Powders that will not flow through the Hall funnel are measured with the larger-orifice Carney funnel (ASTM B964). Flow is the single most predictive property for powder bed fusion recoating and for die-fill uniformity in pressing, which is why it sits near the top of every spec sheet.

Apparent and tap density. Apparent density (ASTM B212, ISO 3923-1) is the mass of loosely poured powder per unit volume; tap density (ASTM B527, ISO 3953) is the higher density after standardized vibration. The ratio of tap to apparent density is the Hausner ratio, a fast index of flow: roughly below 1.25 is free-flowing, above 1.4 is cohesive. These densities set the powder volume needed to fill a die or build chamber and feed directly into yield and cost calculations.

Particle-size distribution. Sieve analysis (ASTM B214, ISO 4497) reports mass fractions retained on a stack of sieves and remains common for coarser PM powders, while laser diffraction (ASTM B822, ISO 13320) is standard for fine additive and MIM powders and reports the distribution as percentiles: D10, D50 (the median), and D90, the diameters below which 10, 50, and 90 percent of the volume lies. A practical rule of thumb for AM powder is a D90-to-D10 ratio near five, with D10 kept above roughly 6 microns to limit agglomeration and protect flow.

For additive manufacturing specifically, ASTM F3049 is the umbrella guide for characterizing AM metal powders, and ISO/ASTM 52907 specifies feedstock requirements and test methods for metal powders in AM, including sampling and the qualification of reused powder. Oxygen and nitrogen, critical for reactive alloys, are measured by inert-gas fusion (ASTM E1019). For finished press-and-sinter parts, MPIF Standard 35 fixes the material designation codes and minimum mechanical properties that link a powder grade to a qualified part property.

Key Specification Parameters

A complete powder spec sheet pairs a chemistry table with a physical-property table. Different suppliers list ten to twenty fields, but a manageable set of parameters drives almost every selection decision: particle-size distribution, particle shape and sphericity, apparent and tap density, flow rate, oxygen and nitrogen content, and bulk chemistry against the alloy grade. Each is explained below.

Particle-size distribution (PSD) is reported as D10, D50, and D90 from laser diffraction, or as sieve fractions for coarser powder. The median D50 sets the working window, while the D10 and D90 tails control flow and packing: too many fines (low D10) hurt flow and raise dust-explosion and safety risk, while too many coarse particles (high D90) cause poor surface finish and lack of fusion in printing. Always specify all three percentiles, not just a nominal mesh range, and state the measurement method because laser and sieve results are not interchangeable.

Particle shape and sphericity are reported qualitatively (spherical, rounded, irregular, dendritic, spongy) and increasingly quantitatively via image analysis as an aspect ratio or circularity number. Sphericity follows directly from the production route and governs flow and packing. Spherical gas- and plasma-atomized powder spreads and packs well for additive and MIM; irregular water-atomized powder compacts well for pressing. Satellites, small particles fused to larger ones during atomization, degrade flow and are a recognized quality defect in AM powder.

Apparent density, tap density, and flow together describe how the powder behaves in bulk. Apparent density (ASTM B212) and tap density (ASTM B527) set the volume of powder needed; Hall flow (ASTM B213) in seconds per 50 grams predicts recoating and die-fill behavior. The Hausner ratio, tap over apparent density, condenses flow into one number. A free-flowing AM powder typically shows a Hall flow on the order of 15 to 30 seconds per 50 grams and a Hausner ratio below about 1.25, though exact figures are alloy and cut dependent and must be read from the certificate.

Oxygen and nitrogen are the interstitial elements that most affect mechanical performance, and they tighten as the alloy gets more reactive. Indicative oxygen limits include below roughly 0.13 percent for aerospace Grade 23 (ELI) Ti-6Al-4V and below about 0.20 percent for standard Grade 5, below about 0.03 percent for many nickel superalloys, and looser limits for stainless and tool steels. Oxygen rises with each reuse cycle in additive manufacturing as new surface oxide forms, so reused-powder specs and reuse limits should reference oxygen alongside PSD and flow. Demand a certificate of analysis with measured oxygen per lot for reactive alloys.

Bulk chemistry verifies that the powder meets the named alloy grade. Common workhorse grades and their roles include the following:

• 316L / 304L stainless steel: corrosion-resistant general purpose, the most widely used powder across PM, MIM, and AM.
• 17-4PH stainless: precipitation-hardening, high strength, common in MIM and AM hardware.
• Ti-6Al-4V (Grade 5 / Grade 23): aerospace and medical, high strength-to-weight, tightly oxygen-controlled.
• IN718 / IN625 nickel superalloy: high-temperature strength for turbine and energy parts.
• AlSi10Mg: lightweight casting-grade aluminum for AM heat exchangers and brackets.
• Maraging / tool steel (18Ni300, H13): tooling and high-hardness inserts.
• CoCrMo: wear- and corrosion-resistant, dental and orthopedic implants.

Beyond these, the spec sheet should list moisture content, sieve oversize limits, any flow additive or lubricant in PM blends, lot traceability, and packaging (sealed metal cans or pails under inert gas for reactive alloys). For regulated industries the relevant quality system, AS9100 for aerospace or ISO 13485 for medical, is part of the specification, not an extra.

Selection Decision Factors

To convert the preceding chapters into a purchase order, work the decision sequence below in order. Most powder selection errors come not from a single wrong number but from deciding chemistry first and discovering too late that the physical character is wrong for the process. These steps double as a fixed RFQ template.

1. Downstream process first: Identify the process (LPBF, EBM, DED, binder jetting, MIM, press-and-sinter, or thermal spray) before anything else, because it fixes the required particle-size cut, shape, and flow class per Chapter 3.
2. Alloy and grade: Specify the alloy to a recognized designation (for example 316L, Ti-6Al-4V Grade 23, IN718) with the full chemistry tolerance band, not just a trade name.
3. Particle-size distribution: State D10, D50, and D90 (or sieve cut) and the measurement method. Pin the D10 to protect flow and the D90 to protect surface finish and fusion.
4. Shape and production route: Decide spherical gas or plasma atomized versus irregular water atomized based on whether flow and packing or green-strength compaction matters. Limit satellites and oversize.
5. Interstitial chemistry: Set maximum oxygen and nitrogen for reactive alloys (titanium, aluminum, nickel) and require a per-lot certificate of analysis measured to ASTM E1019.
6. Standards and quality system: Name the characterization standards (ASTM B-series, ISO, F3049, ISO/ASTM 52907) and, for regulated work, the quality system (AS9100, ISO 13485) and any MPIF Standard 35 grade for finished PM parts.
7. Reuse and traceability: For AM, define virgin-to-reused blending ratio, reuse limits, and the re-test plan (oxygen, flow, PSD) against control limits. Require lot numbers and full traceability.
8. Lot-to-lot consistency and total cost: Weigh purchase price against yield, scrap from out-of-spec lots, qualification cost, and the cost of re-qualifying a new supplier. Powder that prints first-time-right is cheaper than nominally cheaper powder that fails builds.

One commonly overlooked dimension is handling, safety, and serviceability. Fine reactive metal powders, especially aluminum, titanium, magnesium, and zirconium, are combustible dusts governed by NFPA 484, requiring inert handling, grounding and bonding, explosion venting, and HEPA collection; many are also inhalation hazards. On the supply side, evaluate sealed inert packaging, minimum order and lead time, technical support for qualification, batch-blending services, and whether the supplier can certify recycled-powder lots. Established producers cover distinct niches: Hoganas, Rio Tinto Metal Powders, and JFE lead water-atomized iron and steel for press-and-sinter, while Sandvik (Osprey), Carpenter Additive, GKN Powder Metallurgy, Linde (formerly Praxair Surface Technologies), AP&C, 6K Additive, and Tekna supply gas- and plasma-atomized powders for MIM and additive manufacturing. Shortlist by alloy, atomization route, certified standards, and consistency rather than headline price.