Surface roughness and pore evolutions in multi-layer laser powder bed fusion of extra-low interstitial Ti-5Al-2.5Sn powder: A numerical study

In this paper, the three-dimensional discrete element method (DEM) and computational fluid dynamics (CFD) coupled approach was used to numerically reproduce the whole process of laser powder-bed-fusion (L-PBF) additive manufacturing (AM) of extra-low interstitial (ELI) Ti-5Al-2.5Sn powder. The effects of key parameters such as scanning strategy and hatch spacing (h) on the surface roughness (Ra) and pores during multi-layer printing are systematically investigated by characterizing the molten pool characteristics and thermal behavior upon laser motion; and the melt volume in this duration is quantified by the volume of fluid (VOF) method to demonstrate inter-layer interactions. The results show that Ra can be categorized according to the scanning directions. Along the scanning direction, the Ra is affected by the heat accumulation effect and increases as the h decreases. In this case, the Ra caused by the Marangoni effect can be reduced by increasing the melt volume at the end of the track through the layer rotation. The Ra perpendicular to the scanning direction is caused by the ripple-like surface formed by track overlap and decreases as the h decreases. For defects, the pores formed by shrinkage due to insufficient melting or by lack of fusion (LoF) due to incomplete track overlap decrease with the decrease of h. The LoF pores caused by weak inter-layer metallurgical bonding are affected by the surface morphology of the previous layer, which is increased as the h increases. The layer rotation can also reduce such LoF pores. On this basis, a quality control chart suitable for actual production is established.