ABSTRACT
The properties of Extra Low Interstitials (ELI) Ti6Al4V components fabricated via the laser-based powder bed fusion (L-PBF) process are prone to variation, particularly throughout a powder reuse regime. Interstitial pick-up of interstitial elements within the build chamber during processing can occur, most notably, oxygen, nitrogen, and hydrogen, which can impair the mechanical properties of the built component. This study analyses ELI Ti6Al4V components manufactured by the L-PBF process when subjected to a nine-stage powder reuse sequence. Mechanical properties are reported via hardness measurement and tensile testing. Results showed that from 0.099 wt.% to 0.126 wt.% oxygen content, the mean hardness and tensile strength increased from 367.8 HV to 381.9 HV and from 947.6 Mpa to 1030.7 Mpa, respectively, whereas the ductility (area reduction) reduced from around 10% to 3%. Statistical analysis based on the empirical model from Tabor was performed to determine the strength-hardness relationship. Results revealed a significant direct relationship between tensile strength and Vickers hardness with a proportionality constant of 2.61 (R-square of 0.996 and p-value of 6.57 × 10-6).
ABSTRACT
It is known that ZnO is an n-type semiconductor with photocatalytic performances under ultraviolet light irradiation. Constructing a superior structure for a modified electron band has been one of the major research goals for photocatalytic ZnO. Here we report a new technical route for making nano-ZnO coatings with a porous topographic morphology. The coatings were fabricated by plasma spraying the mixture of suspension and solution liquid precursors. Pre-loading of ZnO and Zn powders in the precursor was carried out for the purpose of tailoring the structure of the coatings. The coatings in micron thicknesses showed a porous skeleton and a fluffy top layer consisting of ultrafine ZnO grains. Photocatalytic testing by measuring the degradation of methylene blue revealed significantly enhanced activities of the coatings deposited using the ZnO/Zn loaded precursor. The hybrid-structured ZnO coatings exhibited a narrowed band gap and modified oxygen defects as compared to those deposited from the single liquid feedstock. The results shed light on a one-step easy thermal spray fabrication of polytropic nanostructured functional coatings by employing solid powder-loaded liquid as the starting feedstock.
ABSTRACT
Nickel-based Inconel 718 is a very good candidate for selective laser melting (SLM). During the SLM process, Inconel 718 develops a complex and heterogeneous microstructure. A deep understanding of the microstructural features of the as-built SLM material is essential for the design of a proper post-process heat treatment. In this study, the microstructure of as-built SLM Inconel 718 was investigated at different length scales using optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Electron backscatter diffraction (EBSD) was also used to analyze the grain morphology and crystallographic texture. Grains elongated in the build direction and crossing several deposited layers were observed. The grains are not constrained by the laser tracks or by the melt pools, which indicates epitaxial growth controls the solidification. Each grain is composed of fine columnar dendrites that develop along one of their <100> axes oriented in the direction of the local thermal gradient. Consequently, prominent <100> crystallographic texture was observed and the dendrites tend to grow to the build direction or with occasional change of 90° at the edge of the melt pools. At the dendrite length scale, the microsegregation of the alloying elements, interdendritic precipitates, and dislocations was also detected.
