Reducing hydrogen pores formation in AlSi10Mg processed by Selective Laser Melting

Aluminium alloys are notoriously difficult materials to cast or weld. Their naturally high reactivity with the environment favours oxidation and moisture pick up that promotes pores formation during solidification.  Among other mechanisms, gas porosity need to be addressed. In casting, this can be done by adjusting the cooling rate [1]. In welding, it can be done by scanning the surface [2]. This post addresses a few ways to decrease hydrogen porosity formed during Selective Laser Melting.

Powder storage, handling and recycling conditions

During production, storage or handling of the powder, moisture is picked up at the surface of powder particles and can also decrease particles flowability [3] and hinder homogenous deposition of thin powder layers.

To minimise particles contact with atmosphere, Storage conditions have to be optimised to minimise oxidation for moisture pick up: storage under inert atmosphere or vacuum should be encouraged. In addition, closed loop recycling systems minimise contact with atmosphere.

When storage under inert atmosphere is not practical (i.e. for large powder quantities), three methods can reduce moisture contents [4]. 

Hydrogen pores in AlSi10Mg SLM samples built up with dried powder (left) [4].

External powder drying

For external drying, powder is placed in a furnace at constant temperature. It is shown [4] this may reduce the porosity by up to 35% at a drying temperature Theat=90°C and by >50% at Theat=200°C. The alloy’s sintering temperature limits the usable furnace temperature. No comments or investigations are made about potential aging of the material (Taging≥155C). 

Internal powder drying – single layer drying

After powder recoating, the layer is scanned at low power to remove excess moisture by allowing the H20 to react with laser radiation and break up into H and O elements into the building chamber before being extracted by the venting system.

Remelting

In addition, SLM samples built using double scans where each scan actually melts the material (remelting) showed larger densities. It is thought this method promoted outgassing of the hydrogen-pores and minimised total porosity.

Influence of the powder drying methods and process parameters on the hydrogen pore density (○ = not specified, + = increase, = decrease) - from Ref[4]

Summary

During SLM, dissolved moisture picked up by aluminium particles generates nucleation and growth of hydrogen pores in the melt pool. Hydrogen porosity in AlSi10Mg can be decreased by:

• minimising oxidation and moisture pick up during handling, processing, storage and recycling;
  
• internal (low power layer scan) and external (furnace) powder drying;
  
• and by modifying process parameters to include layer remelting using two successive high power scans.

References
[1] Kaufmann. J. G., Rooy, E. L. , 2004. Aluminum Alloy Castings: properties, processes, and applications. ASM International
[2] Haboudou, A., Peyre, P., Vannes, A. B., Peix, G., 2003. Reduction of porosity content generated during Nd:YAG laser welding of A356 and AA5083 aluminium alloys. Material Science & Engineering. 363, 40-52.
[3] Louvis, E., Fox, P., Sutcliffe, C. J., 2011. Selective Laser Melting of aluminium components. Journal of Materials Processing Technology. 211, 275-284.
[4] 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