Evaluating the Effect of Processing Parameters on Porosity in Electron Beam Melted Ti-6Al-4V via Synchrotron X-ray Microtomography
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Electron beam melting (EBM) is one of the subsets of direct metal additive manufacturing (AM), an emerging manufacturing method that fabricates metallic parts directly from a three-dimensional (3D) computer model by the successive melting of powder layers. This family of technologies has seen significant growth in recent years due to its potential to manufacture complex components with shorter lead times, reduced material waste and minimal post-processing as a “near-net-shape” process, making it of particular interest to the biomedical and aerospace industries. The popular titanium alloy Ti-6Al-4V has been the focus of multiple studies due to its importance to these two industries, which can be attributed to its high strength to weight ratio and corrosion resistance. While previous research has found that most tensile properties of EBM Ti-6Al-4V meet or exceed conventional manufacturing standards, fatigue properties have been consistently inferior due to a significant presence of porosity. Studies have shown that adjusting processing parameters can reduce overall porosity; however, they frequently utilize methods that give insufficient information to properly characterize the porosity (e.g., Archimedes’ method). A more detailed examination of the result of process parameter adjustments on the size and spatial distribution of gas porosity was performed utilizing synchrotron-based x-ray microtomography with a minimum feature resolution of 1.5 µm. Cross-sectional melt pool area was varied systematically via process mapping. Increasing melt pool area through the speed function variable was observed to significantly reduce porosity in the part.
The authors acknowledge America Makes for providing funding for this research under the project entitled “A Database Relating Powder Properties to Process Outcomes for Direct Metal AM,” Award Number FA8650-12-2-7230, and the National Science Foundation for providing funding under Grant CMMI-1335298. They would also like to thank Dr. Xianghui Xiao and the rest of the 2-BM beamline staff at the Advanced Photon Source for assisting in the acquisition of the synchrotron tomography data, and North Carolina State and Dr. Ron Aman for assisting with the sample fabrication.
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