Formation Mechanism of Pores Inside Structure Fabricated by Metal-Based Additive Manufacturing

The powder bed fusion (PBF) technique is a metal-based additive manufacturing (AM) method in which metal powder is deposited on a substrate and melted by selective laser-beam irradiation. Given that the process and parameters of metal-based AM are complicated, there are various problems in high-precision fabrication. One of these is that although metal-based AM can be used for fabrication of high-density parts, pores can easily form inside the fabricated structure owing to process instabilities. Pore formation degrades the mechanical strength of the fabricated structure. Therefore, this study investigated the pore formation mechanism inside a structure fabricated by PBF. Pore suppression by controlling the substrate temperature was also evaluated. Small- and large-sized pores with diameters of 10 μm and more than 50 μm, respectively, were found. Furthermore, differences in pore formation in the cross-section of the fabricated structure were observed owing to a variation in the volume-specific energy density and substrate temperature. At a substrate temperature of 25°C, the number of pores decreased more at the upper position than at the lower position owing to repeated melting and solidification under the laser-beam irradiation. At a substrate temperature of 200°C, the number of pores decreased significantly more than at 25°C. Furthermore, as the substrate temperature increased, the wettability of the molten metal improved, resulting in smaller contact angles of the fabricated structure in the single-line track. In PBF, multiple lines are fabricated in each layer. At low substrate temperatures, interstices were formed between the lines owing to the low wettability of the molten metal. These interstices acted as the origins of pores when the next layer was fabricated. Heating the substrate made the surface of the structure smooth owing to the high wettability of the molten metal and a reduction in the number of pores. Therefore, the formation of large pores could be reduced by controlling the substrate temperature.