In Laser Metal Fusion processes great attention must be paid in choosing the technology’s most suitable metal powder, to ensure the best quality standard of the final items.
Physical-chemical properties of the powder, such as chemical composition, size and shape of the particles, thermal conductivity, etc. play a fundamental role in regulating and modifying their behavior, not only during the layering phase, but especially during the laser’s light absorption/reflection phase.
These properties have to be joined with the expected mechanical properties of the desired item; acting mainly on the chemical composition of the powders, and on post-treatment heating, it is possible to modify the microstructure of the bulk material and then obtain the mechanical performance required for the proper use of the piece.
In the Laser Metal Fusion process a laser source is selectively melting a thin layer of metal powder, typically about 15 to 50 nm thick (1mm = 1000nm); a process that evolves from the bottom to the top, layer by layer.
This is why it’s important to handle a metal powder that can be properly layered by the mechanical systems inside the laser working area.
Powders’ physical properties such as morphology, granulometry, apparent density and flowability define how easily it is possible to proceed with a compact and homogeneous powder layering.
It describes the geometric shape of the particles and it highly depends on the productive technology adopted (atomization, grinding, electrochemical processes).
When dealing with the Laser Metal Fusion technology a strong preference goes to powders obtained through gas-atomizing processes, because they allow to obtain highly spherical particles.
Morphology and granulometry of the particles contribute to define the powder flowability, therefore its layering capability (the easiness of being layered).
Every single powder particle can be assimilated to a sphere of which we can measure the diameter.
With the Laser Metal Fusion technology we prefer to use powders with unimodal distribution and typical grain size of -45 +10 or -63 +15 microns, depending on the characteristics and chemical composition of the powders and the type of the machine used.
Free-flow apparent density:
It defines the grams of powder within a stated volume, it is measured through careful analysis and is expressed as g/cm3.
In Laser Metal Fusion powders, with unimodal distribution and high sphericity, the free-flow apparent density is between 45-60% of the solid material density.
It cannot be considered a direct property of the material since it results both from the material’s physical properties but also from some external factors as the system adopted to measure it, usage, stocking, and process conditions.
For Laser Metal Fusion applications the granulometric distribution requested is the one that avoids the most particles to be smaller than 10-15 mm. Even though this particles can increase the material’s free-flow apparent density and the tap density, they reduce the powder flowability.
THE ATOMIZING PROCESS
The atomization process essentially consists in the nebulization (breaking up) of a liquid material into very small droplets; any liquid material can then be transformed into powder by the atomization process.
The powders obtained by nitrogen atomization, are characterized by a high sphericity and a smooth surface of the particles with a limited amount of satellites* (mainly related to the turbulence generated within the atomizer); these features allow, after appropriate sieving, to obtain a high flowability powder that can thus be taken as raw material (feedstock) for Laser Metal Fusion processes.
*Satellites are small particles adhering to larger particles during the solidification phase in the atomization process; generally a big presence of satellites indicates an atomizing process that has not been optimized completely.
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