| Powder bed fusion additive manufacturing promotes material efficiency as a unique selling point. Unfused particles are in effect recyclable after processing operation and reused [link]. Powder characteristics however tend to vary following different stages of the AM process and these variations could affect powder behaviour and final part quality. Specifically, feedstock quality must be tightly controlled due to their influence on powder flow and packing density. |  |
Besides in a production context, it is critical to understand the level of feedstock sensitivity on part manufacturing. Granulometry changes, particles morphology variability among other factors, affect powder behaviour variation.
Why should I care?
In SLM, powder bed characteristics are governed by morphology, granulometry, surface chemistry, packing density, rheology and thermal properties which are known to affect the behaviour of the feedstock and subsequent part forming procedure.
Powder particles average characteristics
Granulometry (particle size distribution)
The granulometry, (= particle size distribution: PSD), is used to quantify the proportion of particles of given sizes. PSD is a dynamic parameter. It may vary at various stages of the SLM process such as storage, recycling, processing [1] and it induces variation in powder behaviour.
Generally, commercial feedstocks for SLM follow a gaussian distribution and existing results have established preliminary powder granulometry requirements suitable for SLM processes: D90/D10 ≤19 and D50/D10≥10 and D90≤ Th_layer [2, 3].
Generally, commercial feedstocks for SLM follow a gaussian distribution and existing results have established preliminary powder granulometry requirements suitable for SLM processes: D90/D10 ≤19 and D50/D10≥10 and D90≤ Th_layer [2, 3].
 Cumulative PSD [27] |  Differential Gaussian powder size distribution [27] |
Morphology and internal pores
Pores in X22CrMoV particles [28] Powders produced from different atomisation techniques tend to vary in terms of their morphology, size and the occurrence of internal pores. In turn, these affects the packing density and flow properties of the feedstock and part porosity. Currently, qualified feedstock consists of mostly spherical particles with few irregular shapes.
Surface chemistry
Prolonged exposure of reactive powders to external environments, interstitial gas intrusions and the close proximity to heat irradiation during part forming trigger surface oxidation reactions [1, 4]. Another mode of contamination refers to the formation of hydroxides due to moisture adsorption at powders surfaces under relatively high humidity conditions [5].
How particles characteristics impact granular media behaviour
Packing density and surface area
Powder packing density, or fractional density, is a crucial feedstock parameter which determines how efficient powder particles arrange themselves with maximum particle-to-particle contact and minimum voids within the granular network. Key parameters affecting powder packing density are: size distribution, morphology (=shape), inter-particles forces, surface chemistry and flowability [6].
In PBF AM, powder particles are loosely arranged. The bed is randomly organised in a mixture of particle sizes and interstitial pores. This leads to relative packing densities typically in the range of ~40-60% of the parent material. Recoating issues can also disrupt the local packing density of the layer prior to radiation
In PBF AM, powder particles are loosely arranged. The bed is randomly organised in a mixture of particle sizes and interstitial pores. This leads to relative packing densities typically in the range of ~40-60% of the parent material. Recoating issues can also disrupt the local packing density of the layer prior to radiation
Generally, powder grades with a wide PSD and adequate amounts of fine particles tend to exhibit high packing densities. In powder metallurgy, PSD was believed to have the greatest influence on the packing behaviour [7]. Empirical models [8-11] illustrate different strategies to optimise powder packing for Gaussian and multimodal PSD. Across studies, it is found that packing efficiency is mainly improved by reducing voids in a coarse powder matrix by adding fine sized particles. These numerical approaches refer to fine particles size <10um. This may not be a practical approach in SLM due to health & safety issues linked with powder handling.
A wide PSD with added fine particles exhibit better packing behaviour than narrow PSD and had a predicted optimal packing density peaking at 96% [12]. Widened size distribution also increases its standard deviation which reduces the overall packing porosity by decreasing interstitial void sizes [13].
Thermal properties
Powder thermal properties such as absorptivity and conductivity are crucial parameters which affect laser absorption, melt formation, and other heat transfer related mechanisms in the process [14].
Compared to bulk solids, a cluster of powder particles has relatively high absorptivity. This is due to the inter-particles voids which promote penetration of the laser source into the powder under a multiple scattering effect [15, 16].
In contrast to thermal absorptivity, thermal conductivity is significantly reduced in powders compared to their bulk forms since porosity limits the number of particles contacts in the powder bed. The effective thermal conductivity of powder particles is mostly constrained by the gaseous medium among the voids.
Compared to bulk solids, a cluster of powder particles has relatively high absorptivity. This is due to the inter-particles voids which promote penetration of the laser source into the powder under a multiple scattering effect [15, 16].
In contrast to thermal absorptivity, thermal conductivity is significantly reduced in powders compared to their bulk forms since porosity limits the number of particles contacts in the powder bed. The effective thermal conductivity of powder particles is mostly constrained by the gaseous medium among the voids.
The powder density and local particle arrangement under the laser beam greatly affects thermal interaction between the light source and the deposited layer. In other words, the degree of laser absorption varies with the number of particles exposed under the irradiated beam area where higher thermal absorptivity is achieved when powder packing density increases.
In a bimodal powder grade [17], the higher concentration of fine particles under beam spot increases the probability of particles surfaces being irradiated and of material absorbing light. In a different study [7], increase in laser absorption is attributed to the refinement of particle size which allowed the enlargement of powder surface area exposed to the source.
However, thermal absorptivity values often fluctuate over different points along the powder bed due to the arbitrary dispersion of actual powders particles that generates variation in local particle packing. It was shown [18] that a higher relative density increases the thermal conductivity of metallic powders.
However, thermal absorptivity values often fluctuate over different points along the powder bed due to the arbitrary dispersion of actual powders particles that generates variation in local particle packing. It was shown [18] that a higher relative density increases the thermal conductivity of metallic powders.
relative density vs thermal conductivity [18]
Recoatability or flowability
Good powder flowability is essential in SLM as granular feedstock needs to be smoothly spread across the build area to create a homogeneous layer prior to consolidation. Powder with high cohesiveness impedes powder spreading and generates inhomogeneous regions: in turn this affects layer packing density, thermal and optical properties [19]. Powder cohesivity depends on its inter-particle forces and particles weight [20]. Fine powders tend to agglomerate given their inherent strong inter-particles attractive forces compared to coarse powders [14, 21].
The flowability of used powder [22] may deteriorate with recycling due to possible agglomeration and pre-sintering effects. Decrease in flowability is also tied to the coarsening of the PSD where pre-sintering creates powder clusters of irregular shaped.
Besides particles shape irregularities, a high amount of surface oxides is known to degrade powder flowability [23]. The effect of powder granulometry on oxide contamination is based on the particle size and on the proportion of coarse and fine particles. An increase in surface area is linked with decreasing particles size. This is why oxidation can occur more rapidly in fine powder particles that have higher specific surface values with respect to coarser sized powder grades.
The flowability of used powder [22] may deteriorate with recycling due to possible agglomeration and pre-sintering effects. Decrease in flowability is also tied to the coarsening of the PSD where pre-sintering creates powder clusters of irregular shaped.
Besides particles shape irregularities, a high amount of surface oxides is known to degrade powder flowability [23]. The effect of powder granulometry on oxide contamination is based on the particle size and on the proportion of coarse and fine particles. An increase in surface area is linked with decreasing particles size. This is why oxidation can occur more rapidly in fine powder particles that have higher specific surface values with respect to coarser sized powder grades.
Thermal absorptivity comparison between Gaussian and bimodal powder [24]
As compared to solidified oxides layers which are often hard and brittle, adsorbate films (born out of moisture adsorption) exhibit a viscous behaviour that also disrupts the flow of particles within the powder bed by favouring particles clustering [25].
Blending fine particles with coarse powder can help improve powder packing density via effective size mixing and percolation. Yet, the inclusion of fine particles can increase powder cohesion and inter-particles forces. Powder particles sizes below 30um tend to exhibit clustering behaviour [26]. Powder flowability becomes more restricted with decreasing powder particles size [7].
The addition of fine particles should then be weighted between achieving maximum packing density and flowability to optimise powder performance.
Blending fine particles with coarse powder can help improve powder packing density via effective size mixing and percolation. Yet, the inclusion of fine particles can increase powder cohesion and inter-particles forces. Powder particles sizes below 30um tend to exhibit clustering behaviour [26]. Powder flowability becomes more restricted with decreasing powder particles size [7].
The addition of fine particles should then be weighted between achieving maximum packing density and flowability to optimise powder performance.
References
[1] R.J. Hebert, Viewpoint metallurgical aspects of powder bed metal additive manufacturing, J. Mater. Sci. 51 (2016) 1165-1175
[2] G. Egger, P.E. Gygax, R. Glardon, N.P. Karapatis, Optimisation of powder layer density in selective laser sintering, 10th Solid free Fabr Symp (1999) 255-263
[3] A. B. Spierings, N. Herres, G. Levy, Influence of the particle size distribution on surface quality and mechanical properties in AM steel parts, Rapid Prototyp. J. 17 (2011) 195-202
[4] T. Starr, K. Rafi, B. Stucker, C. Scherzer, Controlling phase composition in selective laser melted stainless steels, Proc Solid Free Fabr Symp (2012) 439-446
[5] A. Hodgson, S. Haq, Water adsorption and the wetting of metal surfaces, Surf Sci Rep 64 (2009) 381-451
[6] R. M. German, Particle packing characteristics, Metal powder industries federation, Princeton N.J. 1989
[7] S. Liu, Z. Ha, Prediction of random packing limit for multimodal particle mixtures, Powder Technol. 126 (2002) 283-296
[8] J.E. Funk, D. Dinger, Predictive process control of crowded particulate suspensions, 1st ed, Springer US 1994
[9] C. C. Furnas, Grading aggregates I-Mathematical relations for beds of broken solids of maximum density, Ind. Eng. Chem. Res. 23 (1931) 1052-1058
[10] A.E.R. Westman, The packing of particles: empirical equations for intermediate diameter ratios, J. Am. Ceram. Soc. 19 (1936) 127-129
[11] A.H. Andreasen, J. Andersen, Relation between grain size and interstitial space in products of unconsolidated granules, Kolloid-Zeitschrift 50 (1930) 217-228
[12] G.P. Bierwagen, T.E. Sanders, Studies of the effects of particle size distribution on the packing efficiency of particles, Powder technol 10 (1974) 111-119
[13] G.T.Nolan, P.E. Kavanagh, Size distribution of interstices in random packings of spheres, Powder Technol. 78 (1994) 231-238
[14] M. Krantz, H. Zhang, J. Zhu, Characterisation of powder flow: static and dynamic testing, Powder Technol 194 (2009) 239-245
[15] P. Fischer, V. Romano, H.P. Weber, N.P. Karapatis, E. Boillat, R. Glardon, Sintering of commercially pure titanium powder with a Nd:YAG laser source, Acta Mater 51 (2003) 1651-1662
[16] A. Streek, P. Regenfus, H. Exner, Fundamentals of energy conversion and dissipation in powder layers during laser micro sintering, Phys Procedia (2013) 858-869
[17] C.D. Boley, S.A. Khairallah, AM Rubenchik, Calculation of laser absorption by metal powders in additive manufacturing, Appl Opt 54 (2015) 2477-2482
[18] M. Rombouts, L. Froyen, A.V. Gusarov, E.H. Bentefour, C. Glorieux, Photopyroelectric measurements of thermal conductivity of metallic powders, J. Appl Phys 97 (2005)
[19] A.B. Spierings, M Voegtlin, T Bauer, K Wegener, Powder flowability characterisation methodology for powder bed based metal additive manufacturing, Prog Addit Manuf 1 (2016) 9-20
[20] A. Castellanos, the relationship between attractive interparticle forces and bulk behaviour in dry and uncharged fine powder 2005
[21] A.B. Yu, J.S. Hall, Packing of fine powders subjected to tapping, Powder Technol 78 (1994) 247-256
[22] J. Clayton D. Millington-Smith, B Armstrong, The application of powder rheology in additive manufacturing, JOM 67 (2015) 544-548
[23] A. Strondl, O. Lyckfldt, H. Brodin, U. Ackelid, Characterisation and control of powder properties for additive manufacturing, JOM 67 (2015) 549-554
[24] I. Yadroitsev, A. Gusarov, I Yadroitsva, I SMurov, Single track formation in selective laser melting f metal powders, J. Mater Process Technol 210 (2010) 1624-1631
[25] V. Karde, C. Ghoroi, Fine powder flow under humid environmental conditions from the perspective of surface energy, Int. J. Pharm. 485 (2015) 192-201
[26] A. SImchi, the role of particle size on the laser sintering of iron powder, Metall. Mater. Trans B 35 (2004) 937-948
[27] J.H. Tan et al., Additive manufacturing 18 (2017) 228-255
[28] https://www.intechopen.com/books/new-trends-in-3d-printing/metal-powder-additive-manufacturing
[1] R.J. Hebert, Viewpoint metallurgical aspects of powder bed metal additive manufacturing, J. Mater. Sci. 51 (2016) 1165-1175
[2] G. Egger, P.E. Gygax, R. Glardon, N.P. Karapatis, Optimisation of powder layer density in selective laser sintering, 10th Solid free Fabr Symp (1999) 255-263
[3] A. B. Spierings, N. Herres, G. Levy, Influence of the particle size distribution on surface quality and mechanical properties in AM steel parts, Rapid Prototyp. J. 17 (2011) 195-202
[4] T. Starr, K. Rafi, B. Stucker, C. Scherzer, Controlling phase composition in selective laser melted stainless steels, Proc Solid Free Fabr Symp (2012) 439-446
[5] A. Hodgson, S. Haq, Water adsorption and the wetting of metal surfaces, Surf Sci Rep 64 (2009) 381-451
[6] R. M. German, Particle packing characteristics, Metal powder industries federation, Princeton N.J. 1989
[7] S. Liu, Z. Ha, Prediction of random packing limit for multimodal particle mixtures, Powder Technol. 126 (2002) 283-296
[8] J.E. Funk, D. Dinger, Predictive process control of crowded particulate suspensions, 1st ed, Springer US 1994
[9] C. C. Furnas, Grading aggregates I-Mathematical relations for beds of broken solids of maximum density, Ind. Eng. Chem. Res. 23 (1931) 1052-1058
[10] A.E.R. Westman, The packing of particles: empirical equations for intermediate diameter ratios, J. Am. Ceram. Soc. 19 (1936) 127-129
[11] A.H. Andreasen, J. Andersen, Relation between grain size and interstitial space in products of unconsolidated granules, Kolloid-Zeitschrift 50 (1930) 217-228
[12] G.P. Bierwagen, T.E. Sanders, Studies of the effects of particle size distribution on the packing efficiency of particles, Powder technol 10 (1974) 111-119
[13] G.T.Nolan, P.E. Kavanagh, Size distribution of interstices in random packings of spheres, Powder Technol. 78 (1994) 231-238
[14] M. Krantz, H. Zhang, J. Zhu, Characterisation of powder flow: static and dynamic testing, Powder Technol 194 (2009) 239-245
[15] P. Fischer, V. Romano, H.P. Weber, N.P. Karapatis, E. Boillat, R. Glardon, Sintering of commercially pure titanium powder with a Nd:YAG laser source, Acta Mater 51 (2003) 1651-1662
[16] A. Streek, P. Regenfus, H. Exner, Fundamentals of energy conversion and dissipation in powder layers during laser micro sintering, Phys Procedia (2013) 858-869
[17] C.D. Boley, S.A. Khairallah, AM Rubenchik, Calculation of laser absorption by metal powders in additive manufacturing, Appl Opt 54 (2015) 2477-2482
[18] M. Rombouts, L. Froyen, A.V. Gusarov, E.H. Bentefour, C. Glorieux, Photopyroelectric measurements of thermal conductivity of metallic powders, J. Appl Phys 97 (2005)
[19] A.B. Spierings, M Voegtlin, T Bauer, K Wegener, Powder flowability characterisation methodology for powder bed based metal additive manufacturing, Prog Addit Manuf 1 (2016) 9-20
[20] A. Castellanos, the relationship between attractive interparticle forces and bulk behaviour in dry and uncharged fine powder 2005
[21] A.B. Yu, J.S. Hall, Packing of fine powders subjected to tapping, Powder Technol 78 (1994) 247-256
[22] J. Clayton D. Millington-Smith, B Armstrong, The application of powder rheology in additive manufacturing, JOM 67 (2015) 544-548
[23] A. Strondl, O. Lyckfldt, H. Brodin, U. Ackelid, Characterisation and control of powder properties for additive manufacturing, JOM 67 (2015) 549-554
[24] I. Yadroitsev, A. Gusarov, I Yadroitsva, I SMurov, Single track formation in selective laser melting f metal powders, J. Mater Process Technol 210 (2010) 1624-1631
[25] V. Karde, C. Ghoroi, Fine powder flow under humid environmental conditions from the perspective of surface energy, Int. J. Pharm. 485 (2015) 192-201
[26] A. SImchi, the role of particle size on the laser sintering of iron powder, Metall. Mater. Trans B 35 (2004) 937-948
[27] J.H. Tan et al., Additive manufacturing 18 (2017) 228-255
[28] https://www.intechopen.com/books/new-trends-in-3d-printing/metal-powder-additive-manufacturing