The Effect of Scan Length on the Structure and Mechanical Properties of Electron Beam-Melted Ti-6Al-4V
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TRODUCTION
FOR powder bed fusion additive manufacturing (AM) techniques, such as selective laser melting (SLM) or electron beam melting (EBM), widely accepted material property information for metals does not yet exist. In these metal AM techniques, the multitude of process variables that have a direct effect on the properties of the consolidated material adds significant complication to a sound experimental design. Outside the characteristics of the feedstock material, the process variables typically manipulated are the power of the directed energy source and the scanning speed. The EBM process employed by Arcam ABÒ utilizes scanning strategies in which parameters change throughout a build as a function of the geometry of the part.[2,3] A strong understanding of the EBM process utilized on the Arcam ABÒ systems is critical to the review of current literature. While a complete, detailed description of EBM theory and the Arcam A2X is too lengthy to describe here, some relevant details will be provided as these are pertinent to the design of experiments and are typically not provided in previously reported work. The operation of all Arcam ABÒ systems is similar, although improvements have been made between each generation of machine. The general process of EBM for Ti-6Al-4V starts with using the electron beam to preheat a stainless steel build plate to a set temperature of approximately 970 K (700 °C) in a vacuum [1]
WESLEY EVERHART, JOSEPH DINARDO, and CHRISTIAN BARR, Engineers, are with the Kansas City National Security Campus, Honeywell FMT, Kansas City, MO, 64147. Contact e-mail: [email protected] Manuscript submitted May 10, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
environment. The build plate is lowered by 50 lm, and a raking mechanism dispenses a layer of powder. The electron beam is then used to preheat and sinter this newly deposited layer of powder. Next, the electron beam melts two concentric contours of each part being fabricated. Finally, the beam is used to in-fill these contour areas by hatching with a beam raster parallel with the x or y directions and alternating orthogonally with each layer. Then, the build plate is lowered another 50 lm and the process repeats. Upon completion of the build, the build chamber is back filled with ultra-high-purity helium and allowed to cool to below 370 K (100 °C) before the chamber door can be opened to the ambient room environment. The settings for beam power and speed utilized during the hatch sequence are subject to vary within a build and are largely dependent on the scan length that the beam will travel and thus are a function of the dimensions of the part being manufactured. The scan length dependency of these parameters is designed to maintain a stable and equal-sized melt pool regardless of the scan length.[2,3] The EBM control software performs the calculation of these settings automatically for each layer in the process. Figure 1 displays a flow diagram illustrating the general procedure. The beam current, iBeam, is calculated utilizing a function depe
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