Process-induced degradation of the powder feedstock in additive manufacturing may have a negative influence on the final properties of built components. Consequently, it may lower the cost-effectiveness of powder bed additive manufacturing, which relies on recycling of the nonconsumed powder. This is especially the case for production of high-performance aero engine components where high material and process reliability is required. This study comprises a detailed investigation on the degradation of Alloy 718 powder during multicycle electron beam melting (EBM). The surface-sensitive analysis methods, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), were combined with scanning electron microscopy (SEM) to depict the differences in surface morphology, and surface composition of powder samples exposed to varying numbers of re-use cycles. The results show a significant change in surface characteristics after exposing the powder to the process and the environment in the build chamber. The virgin powder is covered mainly by a relatively thin and homogeneous oxide layer. The re-used powder, however, has undergone transformation to a heterogeneous oxide layer, rich in thermodynamically stable Al-rich oxide particulates, which started already during the first build cycle. Significant growth of the Al-rich oxide occurs via selective oxidation of Al under the conditions in the build chamber, including both pickup of oxygen from the process atmosphere and redistribution of initial surface-bound oxygen from less-stable products like Ni-oxide and/or hydroxide.
Constructed wetlands are increasingly applied for industrial wastewater treatment. However, current knowledge of the stress responses of helophytes to selected toxicants such as dichromate is limited. The goal of the experiments presented here was to characterize the physiological response of Juncus effusus to different concentrations of dichromate dependent upon the growth and constitution of the plants. The growth parameters, shoot length, and dry weight already were strongly affected at low dichromate concentrations of approximately 34 microM. Concentrations of 340 microM impaired chlorophyll fluorescence and a decrease in chlorophyll a started at concentrations higher than 170 microM dichromate. The concentrations of chlorophyll b and carotenoids also were influenced negatively. Thus, the reduction of the pigment content started before any obvious influence on the chlorophyll fluorescence. The highest concentration of dichromate, which caused no permanent inhibition of growth and photosynthesis, was found to be 17 microM K2Cr2O7. In principle, J. effusus is suitable for constructed wetlands to treat chromium-containing wastewater. Because the stress resistance of J. effusus is limited, the maximum concentration of dichromate in the treated wastewater should not exceed 34 microM. The growth parameters, shoot length, and dry weight were sensitive to much lower dichromate concentrations and did react more quickly than the biochemistry-related parameters chlorophyll fluorescence and pigment concentration. Therefore, the example of Juncus effusus shows that the use of only biochemical parameters to define concentration limits for the treatment of dichromate-containing wastewater can lead to incorrect conclusions and result in disturbed long-term operation of the system.
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