Impact of Defects in Powder Feedstock Materials on Microstructure of 304L and 316L Stainless Steel Produced by Additive
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ADDITIVE manufacturing (AM) is an emerging technique to create near-net shape parts in complex geometries for a variety of applications.[1,2] Several commercial methods are available to produce parts from a wide variety of materials, including structural metals, such as stainless steels. Though there are many different types of AM processes, most are linked by the common goal of localized melting and relatively rapid solidification of precursor material (typically 104 to 106 K/s) to create solid volumes of fused material in prescribed shapes. Each technique then has some unique processing parameters (i.e., method of powder distribution, powder size, laser power, and travel speed and pattern) which may weigh differently on the final part. Metal powder is a commonly used feedstock for processes such as directed energy deposition (i.e., the Optomec system used in this work), and powder bed fusion (i.e., the EOS system presented here).[3] Much of the current research efforts in the field of AM are devoted to understanding processing parameters and
BENJAMIN M. MORROW, THOMAS J. LIENERT, CAMERON M. KNAPP, JACOB O. SUTTON, MICHAEL J. BRAND, ROBIN M. PACHECO, VERONICA LIVESCU, JOHN S. CARPENTER, and GEORGE T. GRAY III are with the Los Alamos National Laboratory, P.O. Box 1663, MS G755, Los Alamos, NM 87545. Contact e-mail: [email protected] Manuscript submitted August 22, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
their effect on materials properties and, ultimately, part performance.[3–9] Relatively fewer studies characterize the initial powder feedstocks and effects they might have on the final parts. Those that do study feedstock sometimes note the size distribution and morphology of the feedstock as an important parameter in the AM process (such as Reference 10), or focus on changes as a function of powder reuse (such as Reference 11). There are a few studies following internal porosity from powder to final part, such as the works of Ng et al.,[12] and Cunningham et al.[13–15] Even so, the effects of defects present in virgin powder, especially with regard to powder chemistry, and their effect on the final microstructure remain largely unstudied. The present work examines AM powder feedstock particles designed for two different AM processes. AM parts were built using each process, and the final microstructures were characterized. Selected parts were heat treated after the build process to examine changes in microstructure due to temperature. During post-mortem investigation of AM parts, a fine dispersion of sub-micron particles was observed in the microstructure. Such particles were strongly correlated with ductile dimples on fracture surfaces. Post-mortem TEM of as-built and heat-treated specimens was used to characterize the particles. The general composition was measured, and it was revealed that the particle sizes and distributions were not greatly affected by post-build heat-treatment. Furthermore, these particles are not present in traditionally manufactured, wrought material. TEM studies are rarely performed on AM
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