Metal powder certificates: reading the numbers that matter
Every batch of metal powder arrives with a certificate, and in most shops it is filed without being read. That is a mistake with a delay built into it: the figures on that sheet decide how the powder spreads, how it melts, how much oxygen ends up in the part, and whether the batch now being qualified will behave like the batch qualified last year.
Additive manufacturing arguments tend to be about machines. The machine is the capital item, so it gets the attention, the trade show floor space and the procurement scrutiny. Powder gets a purchase order. This is backwards in a specific and measurable way: a qualified process on a stable machine will drift when the powder changes, and the document that tells you whether the powder has changed is the certificate nobody reads.
What the standards cover
Two documents do most of the work. ISO/ASTM 52900 fixes the terminology for additive manufacturing, which matters more than terminology usually does, because the field grew several vocabularies in parallel and suppliers still use them inconsistently. ASTM F3049, a standard guide for characterising properties of metal powders used in additive manufacturing processes, is the one that bears on the certificate: it sets out which properties are worth measuring and which methods are recognised for measuring them.
The important word in that title is guide. A guide tells you what good practice looks like; it does not oblige a supplier to report any particular property on any particular sheet. What appears on a certificate is therefore a commercial decision, constrained by what the customer asked for. A thin certificate is not non-compliance. It is an unasked question.
Reading the sheet
Chemistry comes first and is the part most often checked, usually against the alloy specification. It is also the part least likely to be wrong. The figures that cause trouble in production are the physical ones.
Particle size distribution is reported as a distribution, not a number, and the shape of that distribution is what determines whether a layer spreads evenly. Two batches with the same nominal range can behave differently if one carries more fines. The fines are also the fraction that behaves worst in handling and best at absorbing moisture and oxygen, which is why they appear in two separate arguments.
Flow behaviour is reported by a named method, and the method must be named, because the numbers are not comparable across methods. A flow figure without its method is decoration. Apparent and tap density sit next to it and describe how much metal is actually present in a spread layer, which is the quantity the melt parameters were developed against.
Oxygen and nitrogen content are the figures that reward attention over time. They tend to rise with reuse, they are influenced by storage and handling, and in the alloys where they matter they matter to mechanical properties rather than to appearance. A supplier reporting them on every batch is a supplier whose process you can track.
What is missing, and why
Three things are routinely absent from powder certificates and routinely needed. The first is particle morphology: whether the particles are spherical, satellited or irregular is visible in an image and invisible in a size distribution, and it affects spreading. The second is moisture, which is a storage and transport property rather than a manufacturing one and therefore belongs to whoever handled the drum last. The third is any statement about reuse: certificates describe virgin powder, and the material in a machine after several builds is a blend whose history exists only in the shop's own records.
That last gap is the one that invalidates qualification work. A process qualified on virgin powder and run on a blend is a process running on an undocumented material, and no supplier certificate will ever cover it. The record has to be kept locally, which means somebody has to decide what a batch is, how many reuse cycles it may see, and what is measured when it is topped up.
Practical sequence
- Specify the properties you want reported, by name and by method, in the purchase order rather than hoping for them on the sheet.
- Keep the certificates in the same system as the build logs, so a batch number can be traced from a part back to a sheet without an archaeology project.
- Fix a reuse policy in writing — cycles, sieving, top-up ratio — and treat the resulting blend as a material with its own identity.
- Re-run the qualification when the supplier changes the atomisation route, not when the part fails.
None of this is exotic metallurgy. It is document discipline applied to a consumable, and it is the difference between a process that holds and a process that is re-tuned every few months by somebody who does not know why.