On the role of melt flow into the surface structure and porosity development during selective laser melting

In this study, the development of surface structure and porosity of Ti-6Al-4V samples fabricated by selective laser melting under different laser scanning speeds and powder layer thicknesses has been studied and correlated with the melt flow behaviour through both experimental and modelling approaches. The as-fabricated samples were investigated using optical microscopy (OM) and scanning electron microscopy (SEM). The interaction between laser beam and powder particles was studied by both high speed imaging observation and computational fluid dynamics (CFD) calculation. It was found that at a high laser power and a fixed powder layer thickness (20 μm), the samples contain particularly low porosity when the laser scanning speeds are below 2700 mm/s. Further increase of scanning speed led to increase of porosity but not significantly. The porosity is even more sensitive to powder layer thickness with the use of thick powder layers (above 40 μm) leading to significant porosity. The increase of porosity with laser scanning speed and powder layer thickness is not inconsistent with the observed increase in surface roughness complicated by increasingly irregular-shaped laser scanned tracks and an increased number of discontinuity and cave-like pores on the top surfaces. The formation of pores and development of rough surfaces were found by both high speed imaging and modelling, to be strongly associated with unstable melt flow and splashing of molten material.