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
3. Presence of oxides
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.
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