Powder recyclability in Selective Laser Melting and Electron Beam Melting

Powder characteristics and chemical composition is critical in ensuring repeatable process and suitable mechanical properties of metal components built using AM. One of the major advantages of SLM and EBM is the reusability of the powder. However, few studies have been carried out to assess how recycling particles after high power machining affect the components properties. Let's have a look at preliminary studies. 

The powder properties (size distribution, shape, composition) significantly affects its processing behaviour and the final qualities of components built using AM techniques. The powder outside the localised melting area can be recycled. This contributes to economic viability and sustainability of  these technologies. To ensure reliable and repeatable production, it is necessary to assess and control/limit variation in powder stock properties. The first step is to understand how high power and high-temperature AM processes affect the recycled powder characteristics. We sum up the changes occurring in Ti64 and IN718 powders used and recycled during Electron Beam Melting and Following Selective Laser Melting respectively:

New (left) and recycled (right) Ti64 powder processed by EBM [2].

Following Electron Beam Melting of Ti6Al4V powder (recycled only) [1]:

• Large O2 pick up
• No significant change in powder composition
• Carbon increase (still within ASM4911 standard)
  
• Aluminium decrease (still within ASM4911 standard)
  
• Hydrogen decreasing 
  
• No change in particle size distribution (45-110um)
  
• No general morphology change (apart from occasional aggregates)
  
• Maximum cycles: 12
  

Following Electron Beam Melting of Ti6Al4V powder (recycled +5%new) [2]:

• appearance of smaller size particles in small quantity,
  
• lower flowability, 
  
• some change in morphology due to pre-sintering, 
  
• no change in final microstructure, 
  
• o2 pick up, 
  
• no change in composition

New (left) and recycled (right) IN718 powder processed using SLM [2].

Following Selective Laser Melting (EOS M270) of IN718 powder (recycled +5%new) [2]:

• disappearance of smaller particles, 
  
• better flowability, 
  
• no change in particles morphology, 
  
• no change in processed microstructure, 
  
• slight porosity increase, possibly connected to o2 pick up, 
  
• no change in yield strength and ultimate tensile strength, 
  
• decrease in ductility and impact toughness still within scatter band: possibly connected to o2 pick up, 
  
• O2 pick up, 
  
• no change in composition

Variation of O2 content in Ti64 powder processed by EBM. ref[1]

Key takeaways

Machining has a clear impact on powder. Vaporisation of low melting temperature compounds occurs but their quantities tend to remain within aerospace standards. Higher pressure in the build chamber seems effective in preventing such vaporisation.

Ultimately and most importantly, oxygen pick up (hence powder susceptibility to oxidation?) seems to be the limiting factor for powder recyclability in both SLM and EBM.

More systematic reviews are needed to verify these trends and predict specific numbers of cycles.

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References
[1]         V. P. R. Niñerola, “Powder recyclability in Electron Beam Melting for aeronautical use,” Aircr. Eng. Aerosp. Technol., vol. 87, no. 2, 2015.
[2]         a. Strondl, O. Lyckfeldt, H. Brodin, and U. Ackelid, “Characterization and Control of Powder Properties for Additive Manufacturing,” Jom, 2015.