Powder production for additive manufacturing
From gas atomization to plasma atomization to the perfect degree of sphericity.
1. Introduction: The Foundation of Every Masterpiece
Metal 3D printing (especially PBF and Binder Jetting) is only as good as the powder that is poured into the machine. If the powder is lumpy, uneven or contaminated, the finished component will have pores, mechanical failure or block the machine's coater.
But how do you actually turn a hard block of titanium or stainless steel into microscopic, perfectly round (spherical) powder balls? This white paper explains the high technology of metal powder atomization.
What makes a good AM powder?
1. Sphericity (roundness): The rounder the particles, the better they flow like water (flowability) and form an extremely smooth, dense layer in the powder bed.
2. Particle size distribution (PSD): PBF usually requires powder between 15 and 45 µm. A good mix of large and small balls ensures a high packing density, as small balls slip into the gaps between large ones.
2. Gas Atomization (GA)
This is the global standard process for mass production of AM powders (Stainless Steel, Aluminum, Inconel).
- The process: The raw metal is melted in an induction furnace. The melt flows down through a tiny tube. At the end of the tube (the nozzle), the liquid metal is hit by a high-pressure gas jet (usually argon or nitrogen) and atomized with enormous force.
- The solidification: The atomized liquid metal droplets fall down in a large tower (atomization tower). As they fall, they cool and solidify into round solid particles before they touch the ground.
- Advantages: High yield, relatively inexpensive, easily scalable.
- Disadvantages: So-called “satellites” are often created – tiny powder particles that fuse with larger particles and make them out of round (potato-shaped). In addition, argon gas can be trapped in the melt, which later leads to unwanted gas pores in the component during 3D printing.
3. Plasma Atomization (PA)
This premium process is often used for extremely reactive metals with a high melting point, especially titanium (Ti6Al4V). Pioneers here are companies like AP&C (a GE Additive subsidiary).
- The Process: Instead of atomizing a jet of liquid metal, the metal is fed into the chamber in the form of a wire. There, three plasma torches (approx. 10,000 °C) hit the tip of the wire. The metal vaporizes/melts instantly and is torn into perfect, tiny droplets.
- Advantages: The powder is extremely round (perfect sphericity), free of gas pores and contains almost no satellites. It flows excellently in the 3D printer.
- Disadvantages: Production is extremely expensive and energy-intensive.
4. Rotating electrode (PREP)
PREP stands for Plasma Rotating Electrode Process.
- The Process: A solid rod made of the desired metal rotates at extremely high speed (up to 20,000 rpm). A plasma arc melts the tip of the rod. The centrifugal force throws away the finest droplets, which cool in flight.
- Advantages: Highest purity (no contact of the melt with the crucible), no gas inclusions.
5. Classification and Sieving
No matter which process is used: at the end a mixture of particles from 1 µm to 200 µm falls out of the tower.
Since PBF printers only require powder of approx. 15-45 µm (DED systems use coarser powder, 45-106 µm), the powder must be carefully sieved or sorted using air classification. The powder that is too coarse or too fine is (if possible) melted down again. This low yield of exactly the "right" fraction (often only 30 to 50%) explains the high price per kilo of AM special powders.
6. Conclusion: Quality has its price
An attempt to save costs in 3D printing by buying cheap, "out-of-round" or poorly classified metal powder (or even welding powder) immediately results in porous components, poor dimensional accuracy and, in the worst case, destroyed coater lips. Atomization is its own high-tech industry, the perfection of which forms the basis for the success story of metal 3D printing.