The evolution of microstructure and composition homogeneity induced by borders in laser powder bed fused Inconel 718 parts

Highlights: • Homogenous microstructure can be achieved through novel design in fabrication. • Regulating the thermal gradient reduces the percentage of porosity. • Deeper melt pools can be obtained by controlling the rate of heat conduction. • Heat transfer optimization leads to uniform distribution of strengthening phases. • Higher hardness and precipitation were observed in samples fabricated with border. -- Abstract: Laser powder bed fusion (LPBF) is a robust metal additive manufacturing technology which can produce precise parts with high complexity and freedom. In a recent work by the authors, a significant number of microstructural defects was observed on the surface of the LPBF-fabricated parts, which could account for the poor tensile strength of the parts. It was revealed that the regions near the surface of the part are more affected by the convection heat transfer mode compared to the areas located away from the surface. In this study, for the first time, a novel technique is proposed to address the issue of microstructure inconsistency in the LPBF-fabricated Inconel 718 parts. This approach is based on the design and fabrication of a border surrounding the main part during the LPBF fabrication. It is hypothesized that such a border, if designed appropriately, would mitigate the undesirable convention heat transfer mode in the areas near the surface of the LPBF-fabricated parts and may result in microstructure homogeneity. To this aim, cubic samples surrounded by borders at two different gap distances of 0.5 mm and 2 mm were fabricated to control the rate of heat dissipation and thus control the microstructure along the cross section and reduce surface defects such as microcracks and porosities. Reference cubic samples were fabricated without borders to compare them with the samples fabricated with borders. Scanning electron microscope (SEM) and optical microscope were used to study the changes in microstructure, X-ray diffraction (XRD) was used to perform compositional analysis, and Vickers hardness test was conducted to study the hardness properties. For the sample fabricated with a smaller gap, the smaller melt pools overlapped to form deeper melt pools and led to the formation of more columnar grain structures elongated parallel to the build direction. The fabrication of borders around the sample also helped in controlling the heat transfer of the sample and thus reducing the surface porosities and defects. A higher level of secondary strengthening phases (γ′, γ″) was seen to precipitate into the matrix for the samples fabricated with borders. Vickers hardness values were higher for the samples fabricated with borders than that of the reference sample.