Effect of particles size distribution and packing density on the formation of balling defects during SLM of In718

[1]

Laser-powder bed fusion (L-PBF) is an additive manufacturing technology that involves complex physical processes. Beam absorptance, heat transfer and molten metal flowS, phase transformation, and thermal stress and distortion combine to influence the final build quality and properties [1-3].

Balling is a defect that can occur when the molten pool created during selective laser melting (SLM=L-PBF) becomes discontinuous and breaks into separated islands. In this post, we report and discuss how the powder particle arrangement impact the bead geometry and formation of balling defects during SLM.

powder particle distributions [1]

melt pool shape with positively skewed powder particle distribution [1]

melt pool shape with negatively skewed powder particle distribution [1]

Effect of particle size distribution on balling defects
In the case of powder particles skewed towards smaller diameters (PSD+), the molten pool shape appears smoother than for that of powder particles skewed towards larger particles (PSD-). PSD+ powder natually contains a large fraction of smaller particles that are more likely to be melted completely due to their smaller volume/mass. In contrast, larger particles are more likely to be partially melted and to form a molten pool with rougher, irregular edges [4,5].

melt pool shape of powder bed density =38% [1]

Effect of packing density
Research shows that a higher powder packing density produces dense(r) parts with smoother as-built surfaces [2, 6]. A relatively lower packing density also seems to enhance fluid convections driven downwards by gravity [7]. These strong downward flows tend promote melt pool hydrodynamic instabilities and result in balling defect formation.
Increasing packing density from 38% to 45% eliminates the discontinuous molten pool and produces a smoother surface contour.

melt pool shape of powder bed density =45% [1] - (all processing conditions fixed)

Key takeaways
The powder particle arrangement has a significant effect on the formation of balling defect:

• Powder containing a large fraction of smaller particles generates a melt pool with smoother contours compared to powder containing a high quantity of larger particles.
• A higher packing density is found to reduce the likeliness of forming such defect. 

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References
[1] Y.S. Lee and W. Zhang, Mesoscopic simulation of heat transfer and fluid flow in laser powder bed additive manufacturing, 2015, http://sffsymposium.engr.utexas.edu/
[2] Korner C, Bauereiss A, Attar E (2013) Fundamental consolidation mechanisms during selective beam melting of powders. Model Simul Mater Sci Eng 21(8):085011.
[3] Klassen A, Bauereiss A, Korner C (2014) Modelling of electron beam absorption in complex geometries. J Phys D-Appl Phys 47(6):065307.
[4] Spierings A, Levy G (2009) Comparison of density of stainless steel 316L parts produced with selective laser melting using different powder grades. In Proc of the Annual Int Solid Freeform Fabrication Symp. University of Texas at Austin, Austin, pp 342-353
[5] Liu B, Wildman R, Tuck C, Ashcroft I, Hague R (2011) Investigation the effect of particle size distribution on processing parameters optimisation in selective laser melting process. In Proc of the Annual Int Solid Freeform Fabrication Symp, University of Texas at Austin, Austin. pp 227-238
[6] Spierings A, Herres N, Levy G (2011) Influence of the particle size distribution on surface quality and mechanical properties in am steel parts. Rapid Prototyping J 17(3):195-202.
[7] Attar E (2011) Simulation of selective electron beam melting processes. PhD thesis, University of Erlangen-Nuremberg, Erlangen, Germany