A comprehensive study on microstructure and tensile behaviour of a selectively laser melted stainless steel

316L stainless steel samples have been prepared by selective laser melting (SLM) using a pulsed laser mode and different laser powers and scanning patterns. The as-fabricated samples were found to be dominated by clusters of nano-sized γ needles or cells. TEM imaging shows that these needles contain a high population of dislocations while TEM-EDX analysis reveals high chemical homogeneity throughout the as-fabricated samples as evidenced by the fact that there is even no micro-/nano-segregation at interfaces between neighbouring γ needles. The good chemical homogeneity is attributed to the extremely high cooling rate after SLM (>106 °C/s) and the formation of Si- and Mn-oxides that distribute randomly in the current samples. The laser-processed samples show both superior strength and ductility as compared with conventionally manufactured counterparts. TEM examination on the deformed specimens reveals a significantly high density of dislocations and a great number of twinning within nano-needles, suggesting that the plastic deformation has been governed by both gliding of dislocations and twinning deformation, which is believed to be responsible for the simultaneous acquisition of superior strength and ductility. Finally, laser power shows a much more dominant role than laser scanning pattern in porosity and grain size development for the SLM-processed 316L stainless steel samples

Influence of laser processing strategy and remelting on surface structure and porosity development during selective laser melting of a metallic material

316L samples were fabricated by selective laser melting (SLM) with different laser powers and scanning strategies/patterns. The porosity distribution and surface structures of the as-fabricated samples were characterized using optical microscopy and scanning electron microscopy. This combined with a mathematical modeling of the SLM process aims to understand the formation mechanism of pores in a newly built layer and the role of remelting of previous layers on internal porosity development. It is shown that the surface structure and the formation of pores in a newly built layer are mainly associated with melt flow behavior, but the formation of pores within bulk samples, particularly those at interlayer interfaces, were largely dictated by the extent of remelting of previous layers during SLM. Laser melting of a powder layer tends to develop rough surfaces and open pores on the uppermost layer. With laser remelting of a newly built layer, the sample surfaces become much smoother and the pores within the uppermost layer can be completely eliminated. During SLM processing, sufficient remelting of previous layers leads to development of good bonding at the interlayer interfaces, whereas less extent of remelting of previous layers results in an increased number of pores at the interlayer interfaces. Laser power or energy density shows a much more dominant role than the laser scanning strategy in porosity development, which is attributed to the fact that laser power or energy density shows greater influence on the extent of remelting as compared with the latter. The mechanism on how remelting affects the evolution of pores is also demonstrated through modeling