Performance Analytics for Electron Beam Powder Bed Fusion of Ti-6Al-4V
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Date
2021-09-21
Authors
Shanbhag, Gitanjali
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
Titanium alloys are widely used in biomedical, aerospace, marine, energy and chemical industries because of their unique properties. Ti-6Al-4V, an α+β alloy, offers desirable properties such as high strength-to-weight ratio, low coefficient of thermal expansion, metallurgical stability, excellent corrosion resistance, high tissue and bone integration and excellent biocompatibility. The advent of electron beam powder bed fusion (EB-PBF) additive manufacturing (AM) has made it possible to fabricate titanium alloy components directly from three-dimensional computer aided design (CAD) models. AM is an innovative manufacturing process that offers near-net shape fabrication of highly complex components, resulting in a much-needed reduction in lead-time, waste, and cost. The increasing demand for manufacturing functional and custom-designed components via EB-PBF is the driving force for achieving a deep understanding of the process-structure-properties relationship. This thesis specifically aims to advance the scientific body of knowledge in understanding the interconnected relationships between the powder properties, bulk powder behavior, in-situ performance, and part properties by (i) understanding the effect of reuse on powder properties and the EB-PBF process, (ii) understanding the tensile behavior and pore space characteristics of EB-PBF components, and (iii) understanding the in-situ powder cake properties and its effects on the surface topography, geometric dimensional deviations and de-powdering, in the EB-PBF process.
Upon robust investigation of the effect of reuse on powder properties and the EB-PBF process, it was found that there is an increase in measured response in powder size distribution, tapped density, Hausner ratio, Carr index, basic flow energy and specific energy, dynamic angle of repose, oxygen and nitrogen content, while the bulk density remained largely unchanged. The morphology of the powder showed extensive physical changes with powder reuse as a result of the powder recovery process, tumbling process, as well as the high temperature conditions leading to overheating and smelting of particles and satellites. Powder characteristics such as the flow properties (basic flow energy and specific energy) and packing properties (tap density, Hausner ratio and Carr index) deteriorated with increasing the degree of powder reuse as a result of the mechanical interlocking and friction between particles. Lastly, the chemical composition (oxygen and nitrogen concentration) remained below the limits outlined by ASTM F2924-14, however, a gradual increase with increasing degree of powder reuse, was observed. From the trends, it was concluded that the powder may be deemed unusable after 5-6 reuse cycles, according to the ASTM F2924-14 standard. Based on the observations, a unified powder quality score called the EB-PBF Suitability Factor was established to help compare the degree of deterioration of the reused powder.
Upon in-depth analysis of the tensile behavior and pore characteristics of the EB-PBF components, it was found that changes in specimen geometry, specimen size, build orientation, and the internal porous defects have significant effects on the tensile properties. The horizontally-built specimens had higher yield and tensile strength, but lower elongation compared to vertically built specimens. Furthermore, the horizontally-built specimens had the highest variability in layer-wise pore fraction, with occurrence of large unevenly-distributed defects. As such, a single tensile specimen size and geometry may not accurately represent the mechanical properties of all features of a component, which has a significant impact on part qualification criteria. It was observed that all vertically-built specimens only displayed pores < 200 µm, whereas all horizontally-built specimens had additionally much larger pores (up to 1200 µm). The pore space < 100 µm was very similar across specimens manufactured in both orientations. Additionally, an increase in cross-sectional area led to an increase in the yield, tensile strength, and elastic modulus. The subsurface pore population was observed to be significantly less for the horizontal specimens having the largest and smallest cross-sectional area whereas it was quite similar for all vertical specimens. An increase in surface area to volume ratio, led to a decrease in the yield and tensile strength; indicating that cylindrical specimens with a smaller diameter and flat specimens with a larger width and/or smaller thickness will have decreased performance. The average solid fraction of the specimens had no influence on any measured tensile properties. Furthermore, with an increase in maximum pore size, the elongation of the specimen decreased. The % elongation was linked to the pore equivalent diameter such that specimens that showed an absence of pores > 500 µm had higher elongation values.
Upon detailed experimental characterization of the powder cake properties and its effects on surface topography, geometric dimensional deviations, and de-powdering, it was found that an increase in preheating temperature led to a linear increase in packing density, contact size ratio, coordination number, effective thermal conductivity of the powder cake, and surface roughness of manufactured parts. Logarithmic regression equations were established from the empirically-derived thermal conductivity data and the measured surface roughness data. These equations can be used to predict the powder cake properties and the surface roughness of parts when using preheating temperatures between 650 °C and 730 °C. The current study shows that a decrease of 80 °C in the preheating temperature led to a 13% and 18% decrease in the Ra and Sa values, respectively with a mere change of 3% in the layer-wise density and 12% in the effective thermal conductivity. Therefore, a decrease in the preheating temperature can improve surface roughness without dramatic changes in the packing density of the powder cake. Furthermore, it was found that a decrease in the preheating temperature led to a decrease in partially-fused powder particles onto the solidified structure, leading to better de-powdering and increased geometric fidelity of manufactured parts.
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Keywords
additive manufacturing, electron beam powder bed fusion, Ti-6Al-4V