Additive Manufacturing of Metals: Complex Microstructures and Architecture Design: Microstructure Evolution in Structural Metals
Sponsored by: TMS Additive Manufacturing Committee
Program Organizers: Yu Zou, University of Toronto; Hang Yu, Virginia Polytechnic Institute And State University

Wednesday 2:00 PM
November 4, 2020
Room: Virtual Meeting Room 6
Location: MS&T Virtual

Session Chair: Mathieu Brochu, McGill University; Wei Xiong, University of Pittsburgh

2:00 PM  Invited
Laser Powder Bed Fusion of Single-crystalline-like Stainless Steel 316L; From Samples to Parts: Xianglong Wang¹; Jose Alberto Muniz Lerma¹; Oscar Sanchez Mata¹; Mohammad Attarian Shandiz¹; Mathieu Brochu¹; ¹McGill University
    Laser powder bed fusion (LPBF) process is one of the most popular additive manufacturing techniques for producing near-net shape metallic components. The process features steep thermal gradients and therefore enables the production of highly texturized samples. Efforts have recently been made towards crystallographic texture control via LPBF, which also include the fabrication of single-crystalline microstructure. The present study will explore the possibility of achieving single-crystalline or single-crystalline-like microstructure on different geometries produced by LPBF. The mechanical properties of these single-crystalline or single-crystalline-like samples will also be investigated.

2:30 PM
Additive Manufacturing of Pure Magnesium: Bandar AlMangour; ¹
    The low density and high biocompatibility of Mg-based materials make them suitable for lightweight structural and biomedical applications. In this study, selective laser melting (SLM), an emerging additive manufacturing process, was used to process pure Mg under various laser energy densities (η). The densification behavior, microstructure evolution, and microhardness were evaluated. Both the peak temperature gradients within the molten pool and the molten pool dimensions increased with increasing η, and an opposite trend was observed for the cooling rate. Low η generated low operating temperature and short liquid lifetime, resulting in poor wettability and large amount of porosity chain and balling phenomena. However, the increase in η generated melt pool instability, which resulted in extensive evaporation, cracks, and porosity, and was accompanied by an increase in the grain size due to the lower cooling rate.

2:50 PM
Fabrication of High Temperature High Strength Austenitic Steels by Laser Powder-bed Fusion: Sebastien Dryepondt¹; Peeyush Nandwana¹; Kinga Unocic¹; Patxi Fernandez-Zelaia¹; Ying Yang¹; Yousub Lee¹; Fred List¹; ¹Oak Ridge National Laboratory
    The extremely fast cooling rates during laser powder-bed fusion (LPBF) results in unique microstructural features for austenitic steels such as 316L. For example, the formation of sub-grain cellular structures with high dislocation density leads to superior tensile properties at room temperature with great ductility and yield strength. These cellular structures are, however, only stable at temperatures below ~600慢 for the LPBF 316L steel. One solution to improve the stability of the cells is to pin the cellular walls with fine precipitates. A high temperature Fe-25Cr-20Ni-1.4Nb-0.2C steel (HK30Nb) was fabricated by LPBF. Fine NbC precipitates were found at the cellular walls leading to high yield strength at temperatures up to 900慢. The creep lifetime of the LPBF HK30Nb steel at 700-800慢 was ~3 times higher than the creep lifetime of cast HK30Nb, highlighting the alloy microstructure stability. The design of advanced high temperature austenitic steels fabricated by LPBF will be discussed.

3:10 PM
The Structure of Cellular Features in Additively Manufactured 316L: Richard Fonda¹; Joseph Aroh²; Jerry Feng¹; David Rowenhorst¹; ¹Naval Research Laboratory; ²Carnegie Mellon University
    Additive manufacturing of 316L stainless steel by laser powder bed fusion produces a fine cellular structure within the grains. These cellular structures have been associated with enhancements in the as-built properties, but their origins and characteristics are not well understood. The cells exhibit a variety of morphologies and orientations on a polished 2D surface; cells range in aspect ratio from equiaxed to very elongated and often exhibit multiple cell orientation domains within a single grain. We have systematically characterized these cellular features to reveal their three-dimensional crystallography and orientation. We will discuss the characteristics of these cellular structures and how they relate to the local crystallography, local crystal growth direction, thermal gradient, and overall build direction.

3:30 PM
Secondary Orientation Preference of Ni-based Superalloy Single Crystals Produced via Electron Beam Melting: Patxi Fernandez-Zelai¹; Michael Kirka¹; Yousub Lee¹; Andres Marquez Rossy¹; Sebastien Dryepondt¹; ¹Oak Ridge National Laboratory
    Additive manufacturing (AM) enables site specific microstructure control which can be used to optimize the design of high performance components. In recent years a few groups have reported on the fabrication of single crystal Ni-based superalloys via powder bed electron beam melting. While the build direction exhibits a [001] preference, typical for cubic metals, these works all report a strong [011] secondary orientation preference normal to the line scan direction. The physical explanation for this anomalous observation is not yet well understood. In this talk we present an experimental study resulting in the fabrication of single crystals from two commercially available Ni-based superalloy powders. A number of experiments were performed to perturb the printing dynamics to understand the secondary orientation preference. We find that there are two driving forces controlling the secondary orientation; one which preserves epitaxial crystallographic growth and a second driven by the line scan sequence.