Porosity in metal components made using Selective Laser Melting

Finding laser processing parameters for high density is the cornerstone of your parameters development process. Without high density, it is futile to minimise surface roughness or investigate mechanical properties. 


In the process of finding suitable high density processing parameters, scientific journal papers report four main causes for porosity.

These are:  

1. Insufficient melting;
2. Key hole effect;
3. Presence of oxides and oxidation;
4. Scattering of condensate particles.

How to avoid these?

1. Insufficient melting

Insufficient melting of powder particles can arise from a) a mismatch between the layer thickness and the laser track depth or b) mismatch between the scanning hatch distance and the laser tracks width or c) insufficient laser energy density to generate melting. Note that laser track width varies from the theoretical beam spot size as a function of input energy density, a combination of laser power and scanning speed, as well as the powder used. As a rule of thumb, aim for an overlap in the xy plan (parallel to the substrate) and in the building z direction equivalent to 30/50% of the actual laser beam track width and depth respectively. In addition, the scanning strategy can play a role [8,9]. For insufficient melting, optical microscopes images taken after metallography usually show powder particles are trapped in the pores.

2. Keyhole effect

The key-hole effect is mostly due to very high density laser processing parameters. It’s observed in welding technology (see right handside [2,3,4,5]) and is due to the combination vaporisation and deep V-shape melt pool at the bottom of which gas/vapour typically get trapped.

3. Presence of oxides

This is largely material dependent. The more sensitive to O2 levels the material is, the more susceptible your process is to oxidation-based pores. Titanium and aluminium alloys are particularly prone to oxidation. You can assume there is an oxides layer on the powder particles before you start machining. Oxides layer usually have higher melting temperature than their parent compound. For instance melting temperature of pure aluminium is 660*C whereas the oxides (Al2O3) melting temperature is 2072*C [1, 6, 7]. To get rid of these, the oxide layer present on the powder particles need to be evaporated and/or “disturbed” using suitably high energy parameters.

4. Scattering of condensate particles during processing

Condensate scattering occurs when the laser density is such that both melting and vaporisation occurs, producing recoil pressure and ejecting material over the rest of the components. Similarly to what occurs during laser cutting by vaporisation, the tiny condensate – molten droplets – get redeposited and the successive layer is deposited on tope of these and remelted. However, depending on its size or composition, the condensate particle might get partially molten and create pores or defects. To avoid these, the best way is to work in “melting-only” mode during SLM [ 8,9]. 

To sum up...

Pores can arise from four different mechanisms or a combination of these: under-melting (powder particles in pores), over-melting (redeposited spatter and melt pool & build instability due to onset of vaporisation or very hot, liquid and reactive melt), presence of oxides, and key hole effect (onset of vaporisation gas trapped in deep melt pool). To avoid these, it is required to choose suitable scanning strategy, ensure suitable laser track width and depth with respect to hatching distance and layer thickness, and make sure you work in pure melting regime to avoid defects due to vaporisation.

References
[1]         E. Louvis, P. Fox, and C. J. Sutcliffe, “Selective laser melting of aluminium components,” J. Mater. Process. Technol., vol. 211, no. 2, pp. 275–284, Feb. 2011.
[2]         M. Courtois, M. Carin, P. Le Masson, S. Gaied, and M. Balabane, “A new approach to compute multi-reflections of laser beam in a keyhole for heat transfer and fluid flow modelling in laser welding,” J. Phys. D. Appl. Phys., vol. 46, no. 50, p. 505305, Dec. 2013.
[3]         M. Courtois, M. Carin, P. Le Masson, and S. Gaied, “A two-dimensional axially-symmetric model of keyhole and melt pool dynamics during spot laser welding,” Rev. Métallurgie, vol. 110, no. 2, pp. 165–173, Apr. 2013.
[4]         M. Courtois, M. Carin, S. Gaeid, and P. L. E. Masson, “Heat and fluid flow modeling of keyhole formation in laser welding,” in COMSOL conference, 2012.
[5]         L.-E. Loh, C.-K. Chua, W.-Y. Yeong, J. Song, M. Mapar, S.-L. Sing, Z.-H. Liu, and D.-Q. Zhang, “Numerical investigation and an effective modelling on the Selective Laser Melting (SLM) process with aluminium alloy 6061,” Int. J. Heat Mass Transf., vol. 80, pp. 288–300, Jan. 2015.
[6]         E. O. Olakanmi, “Selective laser sintering/melting (SLS/SLM) of pure Al, Al–Mg, and Al–Si powders: Effect of processing conditions and powder properties,” J. Mater. Process. Technol., vol. 213, no. 8, pp. 1387–1405, Aug. 2013.
[7]         B. Zhang, H. Liao, and C. Coddet, “Effects of processing parameters on properties of selective laser melting Mg–9%Al powder mixture,” Mater. Des., vol. 34, pp. 753–758, Feb. 2012.
[8]         X. Su and Y. Yang, “Research on track overlapping during Selective Laser Melting of powders,” J. Mater. Process. Technol., vol. 212, no. 10, pp. 2074–2079, Oct. 2012.
[9]         M. Khan, N. A. Sheikh, S. H. I. Jaffery, L. Ali, and K. Alam, “Numerical Simulation of Meltpool Instability in the Selective Laser Melting ( SLM ) Process,” Lasers Eng., vol. 28, pp. 319–336, 2014.