The role of (super) powders in SLM

Did you think any powder would do the job? Did you really?

As industrial AM increases exponentially, so does the need for metal powder production. Not all commercial powders, however, are made equal. As production techniques vary from supplier to supplier (and quality from batch to batch), so do final powder composition and physical characteristics.

As the required composition naturally influences the type of process used to manufacture powder, in turn, the physical characteristics depend directly of the powder processing route [1]: water-, central- or gas-atomisation, milling, jar-milling, type of feeding stock, process parameters, sieving and classification steps all influence powder quality. 

Particle shape and layer density

Since the industrial development of Selective Laser Melting, research has been carried out (and duly forgotten!) to identify optimal powder characteristics. Powder controls, together with the melting mechanisms, the aspect and quality of your finished product.

Assuming a similar composition, literature shows the non-negligible impact of particles’ physical characteristics such as size, shape and size distribution on the finished components. For instance, aluminium particles’ shape influences how much oxidation each particle can readily pick up [2]. In addition to non-uniform illumination, non-uniform oxides distribution generates 1) non-uniform particle melting and 2) pores. This is undesirable for the development of multiple layer parts. In contrast, SLS/SLM processing of aluminium powders with spherically shaped particles results in the formation of a homogeneous and dense layer [2]. They facilitate high layer packing density since they exhibit low interparticle friction and high mobility [4].

Particle size and size distribution

Particles’ size and size distribution drive powder flowability. Fine particles (ø: 0.1-5µm) tend to form clusters and prevent uniform recoating during SLM. Large particles (ø: 90-120µm) reduce the maximum layer packing density available [6] (different from typical tap or apparent densities).  A mix of small and larger particles is best suited for SLM, where the smaller particles percolate through the larger particles and suitably fill the void to achieve higher density [7] in thin layers used in SLM [8].

McGeary [6] showed in his experiments that a bi-modal mixture of powders with a size ration of 1:7 can increase the powder density by about 30%, if the amount of fine particles reaches 30%. If the amount of fine particles (ø: ≤5µm), is too high, the agglomeration of particles can eliminate their positive effects of filling up voids.

Investigating readily available powders, Karapatis [8] found that critical criteria are D10, D50 and D90. D10 represents the particle diameter corresponding to 10% cumulative (from 0 to 100%) undersize particle size distribution. In other words, if particle size D10 is 7.8um, we can say 10% of the particles in the tested sample are smaller than 7.8 micrometer, or the percentage of particles smaller than 7.8 micrometer is 10%.

He found the following criteria must be fulfilled for a maximum recoated thin layer density of 60%:

• D10 < layer thickness; ie 10% of the particles are smaller than the layer thickness;
• D90 < layer thickness, ie 90% of the particles are smaller than the layer thickness;
• D50 ≥ 10×D10, ie 50% of the particles are 10 times coarser than the 10% finer grains;
• D90 ≤ 19×D10, ie the coarsest particles are in a 1:19 ratio with finer particles.

Combined with the recoating systems (hard/stiff blades, rubber blades, fibre brush, roller) that can compact the thin layers, particle size and size distribution controls recoating homogeneity, real layer thickness and layer packing density. Such factors are known to directly affect the surface roughness, SLM efficiency and repeatability. They can make the difference between easy build and repeatable components properties or technological hassle. Given powder costs, it’s worth keeping this in mind!

References
[1] I. Chang and Y. Zhao, Advances in Powder Metallurgy - Properties, Processing and Applications. Woodhead Publishing, 2013.
[2] E. O. Olakanmi, “Selective laser sintering/melting (SLS/SLM) of pure Al, Al–Mg, and Al–Si powders: Effect of processing conditions and powder properties,” J. Mater. Process. Technol., vol. 213, no. 8, pp. 1387–1405, Aug. 2013.
[4] N. P. Karapatis, G. Egger, P. Gygax, and R. Glardon, “Optimization of Powder Layer Density in Selective Laser Sintering,” in Proc. Of the 9th Solid Freeform Fabrication Symposium, 1999, pp. 255–264.
[5] E. Louvis, P. Fox, and C. J. Sutcliffe, “Selective laser melting of aluminium components,” J. Mater. Process. Technol., vol. 211, no. 2, pp. 275–284, Feb. 2011.
[6] R. K. Mcgeary, “Mechanical Packing,” J. Am. Ceram. Soc., vol. 58, no. 1931, 1955.
[7] R. M. German and S. J. Park, Mathematical relations in particulate materials processing. John Wiley & Sons, Inc, 2008, p. 419.
[8] P. Karapatis, “A sub-process approach of selective laser sintering,” Ecole Polytechnique Federale de Lausanne, 2002.