Authors: Mouda Tung
Source: NCSU Libraries Repository (North Carolina State University Libraries) (article, 2019-07-31)
Field: Materials Science — Surfaces, Coatings and Films

In-situ Electron Imaging for the Electron Beam Melting Process. TUNG, MOUDA.In-situ Electron Monitoring for the Electron Beam Melting Process (Under the direction of Dr. Timothy Horn).Metal additive manufacturing process (AM) had been applied in the production of highly customized or geometrically complex components such as medical and aerospace applications.Yet some major issues in the robustness and stability of the process still persist in commercially available solutions, with limitations to its surface finish, tolerances, and repeatability.The ability to closely monitor the observable process signatures that links to defect formation is crucial to the ultimate goal of realtime parameter adjustments.Process-induced porosity defects such as lack of fusion, trapped gas, and keyhole formations, as well as warpage and deleterious phases in the resulting parts can all be mitigated through changes in melt pattern, beam focus, and beam current, etc. In-situ sensing methods used in EBM are limited to its vacuum and high temperature environment and active sensing methods used in selective laser melting (SLM) are not applicable.Visible and IR optics are the most common way to monitor the condition of the build, but these along with pyrometry measurements require access of a direct line of sight to the build platform, which a commercial ARCAM system does not provide.Even with the necessary modifications to support such monitoring hardware the optical path must be protected against metal vapor condensation, plus all available methods are off axis and are subjected to thermionic influence of the gun.The emerging electron imaging technique incorporated the principles associated with scanning electron microscopy (SEM), the same had been incorporated in the control system in the EB welding industry.Commercially available EB welding electron gun with a backscattered electron detector attachment, as well as modified ARCAM A1 heatshield were first experimented.The former demonstrated the ability to detect process-induced open porosity using a post-melt layer scan of the surface, while the latter benchmarked the resolution possible using a generic detector design with the least amount of intrusion compared with optical alternatives.The possibility of using such monitoring mechanism during the melting process of the EBM was eluded to in the following previous work, while the effect of plate temperature has been demonstrated to do little effect to the overall detector's functionality.The focus of this research is to first build a similar detector hardware and improve upon the design, as well as demonstrate the melt process monitoring capability while using the acquired information to assist the research into novel material AM parameter development.An electron detection and imaging system was developed around the ARCAM EBM system using data acquisition (DAQ) system form National Instruments and imaging software written in Python, and is demonstrated to be capable of performing post-melt layer imaging that is comparable to the ARCAM's LayerQam optical imaging system.The system is also employed during the melting process, where a DOE build for pure copper with known porosity-inducing parameters are used.The resulting process-induces open porosity of the surface are shown to be indicative of the porosity defects within the sample parts examined using optical microscopy. In situ ; Cathode ray ; Electron ; Materials science ; Process (computing) ; Physics ; Computer science ; Nuclear physics Electron and X-Ray Spectroscopy Techniques ; Advanced Electron Microscopy Techniques and Applications