No abstract

A systematic matrix with 25 samples, using five different point distances and five laser exposure times, depositing 75-μm-thick layers of Alloy 718 has been studied. The work has concentrated on defects formed, hardness of the deposits, and the microstructure. Relatively large amount of defects, both lack of fusion and porosity, was found in several of the specimens in the deposits. The defects were never possible to fully eliminate, but a significant decrease, mainly in the lack of fusion, was seen with increasing laser exposure time. The gas porosity on the other hand was not affected to any larger degree, except for the lowest laser energy input, where a slight increase in porosity was seen. A small increase in hardness was noted with increasing laser energy input. The width of the deposited beads increased with increasing laser energy, while the depth of deposits was more or less constant. However, for the lowest combination of point distance and laser exposure time, quite deep and narrow beads were formed. A comparison was made with deposition of 50-μm-thick layers, with quite similar laser energy input, but with some variation in detailed deposition parameters. It was found that the 75-μm-thick layers contained less lack of fusion, particularly for small point distances. The amount of porosity was also less, but that did not vary with deposition parameters.

Effects of build layout and orientation consisting of (a) height from the build plate (Z-axis), (b) distance between samples, and (c) location in the build plate (X-Y plane) on porosity, NbC fraction, and hardness in electron beam melted (EBM) Alloy 718 were studied. The as-built samples predominantly showed columnar structure with strong ˂001˃ crystallographic orientation parallel to the build direction, as well as NbC and δ-phase in inter-dendrites and grain boundaries. These microstructural characteristics were correlated with the thermal history, specifically cooling rate, resulted from the build layout and orientation parameters. The hardness and NbC fraction of the samples increased around 6% and 116%, respectively, as the height increased from 2 to 45 mm. Moreover, by increasing the height, formation of δ-phase was also enhanced associated with lower cooling rate in the samples built with a greater distance from the build plate. However, the porosity fraction was unaffected. Increasing the sample gap from 2 to 10 mm did not change the NbC fraction and hardness; however, the porosity fraction increased by 94%. The sample location in the build chamber influenced the porosity fraction, particularly in interior and exterior areas of the build plate. The hardness and NbC fraction were not dependent on the sample location in the build chamber.

No abstract

No abstract

Understanding the effect of powder feedstock alterations during multicycle additive manufacturing on the quality of built components is crucial to meet the requirements on critical parts for aerospace engine applications. In this study, powder recycling of Alloy 718 during electron beam melting was studied to understand its influence on fracture behavior of Charpy impact test bars. High resolution scanning electron microscopy was employed for fracture surface analysis on test bars produced from virgin and recycled powder. For all investigated samples, an intergranular type of fracture, initiated by non-metallic phases and bonding defects, was typically observed in the regions close to or within the contour zone. The fracture mode in the bulk of the samples was mainly moderately ductile dimple fracture. The results show a clear correlation between powder degradation during multi-cycle powder reuse and the amount of damage relevant defects observed on the fracture surfaces. In particular, samples produced from recycled powder show a significant amount of aluminum-rich oxide defects, originating from aluminum-rich oxide particulates on the surface of the recycled powder.

No abstract

No abstract