Hydrogen pore formations in AlSi10Mg processed by SLM

SLM technology can produce AlSi10Mg components nearing 100% density [1]. Their mechanical properties are comparable and even better than that of parts produced by casting [2] due to their SLM-specific fine microstructure [3]. Here we review the physical mechanisms involved in formation of hydrogen pores during SLM.

Hydrogen pores in AlSi10Mg SLM samples [5] where powder shows moisture pick up

Gas pores footprint

Various porosity formations mechanisms occur in SLM of aluminium alloys. Imperfections are caused, for example, by un- melted powder or presence of oxides and generally have a statistically random geometric shape. Gas pores, by contrast, can be distinguished by their spherical shape [4]: the result of gas entrapment during solidification.

The availability of high power sources in SLM production machines favours higher build rate. A correlation is an increase of porosity. For instance, with a beam diameter up = 1 mm, a layer thickness =200 μm, laser power PL = 1000 W, scan speed vs = 300 mm/s, gas porosity is reported to increase by more than 10% [5].

As the laser beam scans across each layer, its energy is absorbed by the powder and forms a melt pool  that penetrates deeper than the layer thickness and bonds  with the previous layers.

Factors influencing pores formation: 1) moisture content

The moisture (H2O) [6 ] reacts with aluminum following these equations:
3H2O + 2 Al ==> Al2O3 +3 H2
H2 ==> 2 Hab
where Hab is the absorbed hydrogen in the melt.

These chemical  reactions occur  at  the liquid/solid interface of the melt front, where the particles are  melted and the liquid can be enriched with hydrogen.

Hydrogen pores nucleation and growth (controlled by diffusion [7] occurs if the local hydrogen content exceeds its maximum solubility in liquid aluminium at the melting temperature  (10x greater than in solid Al).

In this instance [5], given that the measured hydrogen content in the powder  is 50-fold higher than the solubility in the melt at melting temperature, even if only a part of that hydrogen contamination of the powder ends up in the melt pool, the nucleation and growth of the hydrogen pores starts in the melt pool.

In addition, aluminium gets supersaturated with hydrogen at the solidification front if H content remains greater than the hydrogen solubility in the solid phase.

Schematical overview of the interaction zone between laser radiation and powder [5]

Factors influencing pores formation: 2) melt currents/flows

The flows direction in the  melt pool  are influenced by the partial evaporation of alloying elements and Marangoni convection flows (themselves influenced by the presence of oxides or contaminants on surface of the melt pool).

In turn, these hydrodynamic currents, whose speed and intensity vary with temperature, can influence hydrogen intake in the liquid alloy. For instance a convection that redirects melt towards the inner centre of  the MP increase enrichment in H. Hydrogen recirculates in the melt pool and the probability of high value solubility increases in tune with the probability of pores nucleation at the solidification front. Whereas a current that will drive melt towards the surface will favour outgassing of Hydrogen and limit H-pores formation.

Factors influencing pores formation: 3) melt lifetime 

The time between the melting and the solidification, which can be influenced e by the scan speed, is a significant parameter. Since the material solidifies rapidly, the supersaturated hydrogen is trapped in the lattice. Therefore, a shorter melt lifespan generates lower H-pore density.

Low scan speeds favour outgassing of the pores from the melt pool.

As H-pores growth is controlled by diffusion, and given that the diffusion rate in the melt is significant higher than in the solid material, the melt lifespan also determines the final pores size.

Take-away

• The moisture in the powder particle surface as well as the dissolved hydrogen in the powder material leads to a H-supersaturated melt. 
  
• This results in nucleation and growth of hydrogen pores in the melt pool.
  
• The melt lifespan influences the probability of hydrogen pores formation and the pores size. The longer the melt pool lifespan, the easier it is for hydrogen to outgas in processing chamber without being trapped at the solidification front.
  

References
[1] Buchbinder, D., 2013. Selective Laser Melting von Aluminiumgusslegierungen. Shake Verlag, Aachen.
[2] Kempen, K., Thijs, L., Van Humbeeck, J., Kruth, J.-P., 2012. Mechanical properties of AlSi10Mg produced by Selective Laser Melting. Physics Procedia. 39, 439-446.
[3] Thijs, L., Kempen, K., Kruth, J.-P., Van Humbeeck, J., 2013. Fine-structured aluminium products with controllable texture by selective laser melting of pre-alloyed AlSi10Mg powder. Acta Materialia. 61, 1809 - 1819.
[4] Kempen, K., Thijs, L., Yasa, E., Badrossamay, M., Verheecke, W., Kruth, J. P., 2011. Microstructural analysis and process optimization for selective laser melting of AlSi10Mg. Proceedings Solid Freeform Fabrication Symposium. Texas, USA.
[5] Weingarten, C., Buchbinder, D., Pirch, N., Meiners, W., Wissenbach, K., Poprawe, R.,Formation and reduction of hydrogen porosity during Selective Laser Melting of AlSi10Mg, Journal of Materials Processing Technology (2015), http://dx.doi.org/10.1016/j.jmatprotec.2015.02.013
[6] Fromm, E., Gebhard, E., 1976. Gase und Kohlenstoffe in Metallen. Springer, Berlin.
[7] Atwood, R. C., Sridhar, S., Zhang, W., Lee, P. D., 2000 Diffusion-controlled growth of hydrogen pores in aluminium-silicon castings: on situ observation and modelling. Acta materialia. 48(2), 405-417.