Influence of powder morphology on component density
Why satellites, agglomerates and trapped argon gas in the powder grain lead to fatal pores.
1. Introduction: When the sand gets stuck in the gearbox
In metal 3D printing, everything focuses on the laser power or scanning speed. But the invisible protagonist is the powder itself. The physical shape of each of the millions of powder grains (the powder morphology) determines whether the finished component has a flawless density of 99.9% or whether it is full of tiny, strength-reducing pores.
This white paper delves deep into the micro-world of powder grains and explains why "round" does not always mean "good".
How is metal powder for AM produced?
Most powders for PBF-LB are produced by gas atomization. Liquid metal is forced through a nozzle and shredded by an ice-cold argon gas jet. The flying droplets solidify in the drop tower into more or less round balls.
2. Sphericity: The perfect sphere
Ideally, every grain of powder would be a perfect ball. Balls roll perfectly on top of each other (like in a ball pit). This means:
- High flowability: The powder can be spread by the recoater in an absolutely flat, smooth 30 micrometer layer. It doesn't get stuck.
- High packing density (apparent density): The balls are arranged extremely densely. There is hardly any air between them. When the laser melts, there is enough material to fill the volume and the risk of microscopic voids (pores) in the melt pool decreases dramatically.
3. The ugly reality: satellites and agglomerates
The reality of gas atomization is different. Under the scanning electron microscope (SEM), the powder often looks more like popcorn than billiard balls.
- Satellites: If a small, already solidified grain of powder collides with a larger, still liquid drop in the drop tower, it sticks to it. A grain with small "satellites" is created.
- Irregular shapes: Some drops solidify as elongated flakes or ovals.
Such non-round particles get stuck together (high cohesion). The powder flows poorly (poor “Hall Flow” value). The coater then often creates grooves in the powder bed. The packing density is low (too much air between the grains), which leads to "lack of fusion" (bonding errors) and massive porosity in the finished printed part.
4. Porous particles: The danger from within
Hollow grains are even more dangerous than non-round grains. During atomization it can happen that the liquid metal includes argon gas. The solidified powder grain is perfectly round on the outside, but inside there is a gas bubble (argon inclusion).
When the laser melts this grain of powder in the 3D printer, the enclosed argon expands explosively. A macroscopic pore (gas porosity) inevitably arises in the component, which can hardly be closed even by hot isostatic pressing (HIP), since argon cannot diffuse out of the crystal lattice.
5. Conclusion: Quality assurance begins in the laboratory
An AM component can never be better than the powder it was made from. Today, certified powder laboratories scan every new batch using dynamic image analysis (e.g. Camsizer) and measure the sphericity of tens of thousands of particles per second. Anyone who saves money here and buys cheap, irregular powder will pay the price later during the CT test through high rejection rates.