Luleå Study Finds Powder Condition Changes Laser Absorption in Metal AM

Professor Alexander Kaplan at Luleå University of Technology has spent more than two decades studying how lasers interact with metal powders. A recent paper from his group tackles something that sounds almost too basic to publish: how much light does the powder actually absorb?

The answer matters more than it sounds. In laser powder-bed fusion, the machine delivers a set amount of energy through the laser. But the powder bed decides how much of that energy sticks. The rest reflects, scatters, or bounces between particles until it escapes. If you treat the powder bed like a flat metal surface, you get the wrong number.

Kaplan's team measured absorbance across 39 powder samples from 16 different materials. The list is broader than most AM labs test: several stainless steels, Inconel 718, AlSi10Mg, AlSi40, titanium, Ti-6Al-4V, Nitinol, chromium, copper, brass, iron ore, and a high-entropy alloy. They also varied powder condition: fresh versus used, different particle-size fractions, stored and oxidized samples, and mixtures.

The results show that powder geometry helps absorption. A rough, granular bed with satellite particles, oxide layers, and irregular contact points traps light longer than a flat surface would. The laser has more than one chance to interact with the material before energy escapes. That is why measured absorbance came out higher than flat-surface calculations predicted.

Wavelength is another variable. At the near-infrared wavelength around 1070 nm used by most fiber lasers, the study found larger differences between materials than at shorter wavelengths. Copper, which reflects strongly at 1070 nm, showed much higher absorbance in the visible range. That fits the industry's growing interest in green and blue lasers for copper L-PBF.

Particle size also shifts the number. Finer stainless steel powders absorbed about 6% more energy than coarser fractions of the same alloy. The explanation is straightforward: more surface area means more chances for the beam to interact with the material.

Powder history matters too. Stored titanium, used steel powders, oxidized copper, and aged AlSi10Mg all behaved differently from fresh material. The effects were not uniform. Oxidation changed copper's absorbance curve. Aging shifted AlSi10Mg's response. Recycled powders did not simply average out their ingredients.

These findings connect directly to what production teams see on the shop floor. Powder is stored, dried, sieved, reused, blended, and handled differently from one batch to the next. If each change alters energy coupling, then process parameters may need adjustment when powder condition shifts. That is why powder quality checks go beyond chemistry and particle size. Flowability, apparent density, oxygen content, and reuse limits are all necessary. The Luleå study adds another layer: the same powder may couple energy differently depending on its condition.

The paper does not claim to solve every reuse question. It measured absorbance, not part quality. But the measurements explain one mechanism through which powder variation enters the process. If you want to understand why a parameter change fixed a defect last week, powder absorption is a plausible part of the answer.