Gas pores footprint
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% .
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
3H2O + 2 Al ==> Al2O3 +3 H2
H2 ==> 2 Hab
where Hab is the absorbed hydrogen in the melt.
Hydrogen pores nucleation and growth (controlled by diffusion  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 , 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.
Factors influencing pores formation: 2) melt currents/flows
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
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.
- 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.
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