Spatter and oxidation reactions during SLM

Understanding the events that take place during metal melting and resolidification is a key success factor of metal SLM technology. Investigation into spatter generation reveals that surface oxides enriched in the most volatile alloying elements of the material are present in the residue.

Research shows that the melting regime and the melt pool stability depend on the presence of oxides that might form during SLM. Considering the O2 partial pressure present in SLM build chambers, it is likely that the high temperatures reached by the melt pool could trigger the formation of oxide films.[1]

Thin (nm range) protective oxide layers found on metals such as stainless steel and titanium alloys, can be stirred by the melt pool dynamics [2] and may have a negligible effect on the SLM production. However, the process may not completely disrupt or vaporise thicker oxides layers. These oxide residues tend to lower wetting ability, induce balling and deteriorate the melt pool stability [2]. 

For typical energy inputs used in SLM (10E5 to 10E7 W/cm2), some metallic compounds may evaporate to a certain extent [1,3].  Little has been studied on material evaporation during SLM, but research on laser welding reports that the evaporation affects laser absorption, giving rise to the re-deposition of fumes inside the building chamber and contributes to the ejection of laser spatter from the melt pool.[4–7]

As it originates directly from the melt pool, spatter can provide important information regarding the oxidation reactions that occur during SLM. Here we summarise findings for three widely used materials.

316L steel [8] – surfaces oxides – no bulk oxides

New powder particles: 
Single fcc austenitic phase, alloying elements in full solid solution with no intergranular precipitates (characteristic of rapid cooling rate during gas atomisation)

Spatter particles: 
coarse microstructure with mainly equiaxed grains, grain boundary segregation Cr, Mo, and Mn – coarser grain + segregation: slower cooling rate?

Surface of spatter particles:
preferential Mn + Si oxides, 

New powder particle (316L stainless steel) [8]

Spatter particle (316L stainless steel) [8]

Surface of spatter particle (316L stainless steel) [8]

---

AlSi10Mg [8] – surface oxides – no bulk oxides

New powder: 
non homogeneous: outer shell with core, no intermetallic compounds (Mg2Si), Si-rich areas in the core, 

Spatter particles: 
larger than particle, homogeneous structure, no intermetallic compounds distinguishable, no oxide in the bulk, surface Mg-rich oxides, 

New powder particle (AlSi10Mg) [8]

Scatter particle (AlSi10Mg) [8]

---

Ti64 [8] -  no surface or bulk oxides 

New and spatter particles:
compositional analysis (EDS) shows all elements in full solid solution, no surface oxides

New powder particle (Ti64) [8]

Scatter particle (Ti64) [8]

The compositional analysis of the oxides found on the laser spatter shows selective oxidation of mainly Mn (and Si) in 316L stainless steel and Mg in Al-Si10-Mg.

Key take-aways

• Regardless of the material, condensate particles are larger than the starting pre-alloyed powders with a spherical morphology. 
  
• Selective oxidation occurs at the surface of the condensate particles of 316L and Al-Si10-Mg.
  
• No oxides were observed in the bulk microstructure of the spatter particles.
  
• The analysis suggests that the formation of surface oxides is underpinned by surface enrichment of the most volatile element present in the alloy. If these elements also have great affinity to oxygen, as in the case of Mn, Si, and Mg, thick oxides layers (up to several um) can be formed. 
  
• The laser spatter formed during the processing of Ti-6Al-4V contains no oxides, likely because the alloy has no alloying elements with high volatility.
  
• Promoting efficient conduction melting will decrease oxidation occurring and spatter(waste).
  

References
1. S. Das: Adv. Eng. Mater., 2003, vol. 5, pp. 701–11.
2. E. Louvis, P. Fox, and C.J. Sutcliffe: J. Mater. Process. Technol., 2011, vol. 211, pp. 275–84.
3. F. Verhaeghe, T. Craeghs, J. Heulens, and L. Pandelaers: Acta Mater., 2009, vol. 57, pp. 6006–12.
4. M.J. Zhang, G.Y. Chen, Y. Zhou, S.C. Li, and H. Deng: Appl. Surf. Sci., 2013, vol. 280, pp. 868–75.
5. D.K. Low, L. Li, and P. Byrd: J. Mater. Process. Technol., 2003, vol. 139, pp. 71–76.
6. S. Li, G. Chen, S. Katayama, and Y. Zhang: Appl. Surf. Sci., 2014,vol. 303, pp. 481–88.
7. A.F.H. Kaplan and J. Powell: J. Laser Appl., 2011, vol. 23, pp. 1–8.
8 MARCO SIMONELLI, CHRIS TUCK, NESMA T. ABOULKHAIR, IAN MASKERY, IAN ASHCROFT, RICKY D. WILDMAN, and RICHARD HAGUE, A Study on the Laser Spatter and the Oxidation Reactions During Selective Laser Melting of 316L Stainless Steel, Al-Si10-Mg, and Ti-6Al-4V, METALLURGICAL AND MATERIALS TRANSACTIONS A DOI: 10.1007/s11661-015-2882-8