Additive Manufacturing: Nano/Micro-mechanics and Length-scale Phenomena: LIVESTREAMED SESSION: Special Topics
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Additive Manufacturing Committee, TMS: Nanomechanical Materials Behavior Committee
Program Organizers: Meysam Haghshenas, University of Toledo; Robert Lancaster, Swansea University; Andrew Birnbaum, US Naval Research Laboratory; Jordan Weaver, National Institute of Standards and Technology; Aeriel Murphy-Leonard, Ohio State University

Thursday 2:00 PM
March 3, 2022
Room: 204B
Location: Anaheim Convention Center

Session Chair: Suveen Mathaudhu, Colorado School of Mines ; Sezer Ozerinc, Middle East Technical University

2:00 PM  
Harnessing Non Equilibrium Processing in Additive Manufacturing: Kathryn Small1; Markus Suddmans2; Anna Rawlings3; Ian Mccue4; Jaafar El-Awady2; Mitra Taheri2; 1John Hopkins University; 2Johns Hopkins University; 3U.S. Naval Research Laboratory; Johns Hopkins University; 4Northwestern University
    Laser-powder bed fusion (L-PBF) is an additive manufacturing (AM) technique that has gained popularity in recent years due to its potential for fast, low-waste manufacturing of complex components with narrow geometric tolerances. Its rapid solidification and fast cooling rates lead to microstructures that contain high residual stresses, anisotropic grain morphologies, intragranular misorientations, and unique, metastable dislocation cell structures. In addition to dislocation cell structures and residual stress variations, the grain and dendrite morphology in AM metals can differ based on the location within the build, owing to differences in number of remelting cycles and relative time at elevated temperature. This talk presents 1D geometries (e.g., single line traces) and 3D builds (e.g., cylinders) that show the impact of thermal environment and solidification pathway on the resulting microstructure. We reveal an increased microscale elastic strain level resulting from higher temperature gradients and faster cooling rates as well as a higher spatial correlation between elastic strain and dislocation density. These features have a direct impact on harnessing non-equilibrium structures.

2:20 PM  
Intentional and Unintentional Spatial Variation in Laser Powder Bed Fusion: Joy Gockel1; Cherish Lesko2; Anna Dunn2; Daniel Young2; Luke Sheridan3; 1Colorado School of Mines; 2Wright State University; 3Air Force Research Laboratory
    Additively manufactured material is built at the same time as the component. Changes in the processing conditions result in spatial differences in the microstructure and defects of the fabricated material. These differences will ultimately impact the mechanical properties of the final components. An understanding of the processing-structure-properties relationship can be used to purposefully tailor the properties within desired sections of a component using strategic processing modifications. However, unintentional variation in the material also results from aspects such as part geometry. Vickers microhardness is used to investigate the influence of processing and geometry on the properties of nickel superalloy 718 fabricated with laser powder bed fusion. Spatial measurements expose within sample variation that is related back to the microstructural features and processing conditions. The results from this work can be used to continue to understand the processing–structure-properties relationship and lead towards improved mechanical properties in additive manufacturing.

2:40 PM  
Multi-scale Dynamic Strain Measurement and Machine Learning Optimization to Uncover Solidification Dynamics of Ti-5553 L-PBF Melt Pools: Caleb Andrews1; Maria Strantza2; Nicholas Calta2; Tae Wook Heo2; Saad Khairallah2; Rongpei Shi2; Manyalibo Matthews2; Mitra Taheri1; 1Johns Hopkins University; 2Lawrence Livermore National Laboratory
    Laser powder bed additive manufacturing (L-PBF AM) is a technology which enables digital near-net fabrication of complex parts. However, the rapid solidification inherent to the L-PBF process generates residual strains across scales. Understanding how strain spans the micro-millimeter scale as a function of laser processing parameters can elucidate how to form strain free parts by optimizing the L-PBF process, or, create strain anisotropy at the microscale to tune mechanical properties. We demonstrate a high-resolution electron back scattering diffraction (HR-EBSD) framework to measure the absolute strain of Ti-5553 meltpools across their entire length scale with sub-micron resolution. Monte Carlo diffraction simulation is utilized to create a zero-strain point of reference, from which collected Kikuchi patterns can be correlated against, while a deep learning denoising autoencoder is employed to reduce noise from experimental data. This demonstrates a method to obtain the absolute strain tensor at the sub-grain scale without utilizing beamline techniques.

3:00 PM  
Strategies for Functional Grading Using Additive Manufacturing: Moataz Attallah1; 1University of Birmingham
     This talk focuses on the different strategies for functional grading using additive manufacturing and their influence on the mechanical properties. Different approaches for functional grading will be presented, including using lattice structures with variable porosity content, microstructural grading using process parameters control, and finally chemical grading through changing the alloy content along the build height. Functionally graded lattices were tailored to control the elastic modulus and the mechanical deformation behaviour. Digital Image Correlation measurements showed that functionally-graded lattices deform in an inwards fashion, whereas uniform lattices deform through generating a 45˚ deformation band. The talk also explores the use of laser scan strategy and process parameters in controlling the grain structure from uniform (columnar or equiaxed) into functionally graded grain structure in Ni-superalloys through controlling the solidification kinetics. Finally, chemical grading through the use of in-situ alloying will be presented, highlighting the potential opportunities and impact on the build performance.

3:20 PM Break

3:40 PM  
Investigations of Toughening Mechanisms in Nanoarchitected Polymers: Lucas Meza1; Zainab Patel1; 1University of Washington
    It is theorized that the remarkable toughness and damage tolerance of natural materials stems from their multiscale hierarchical architectures, but experimental quantification of toughening mechanisms at the micro- and nanoscale has historically been challenging. In this work, we demonstrate a novel method to both fabricate and mechanically test microscale crack propagation in nanoarchitected polymeric materials. Microscale beams with sub-micron features are created using two-photon lithography with unidirectional, helicoidal and herringbone architectures. Cracks are directly written into the beams, which are then tested in a 3-point bending configuration. We quantify the strain energy released during crack propagation in the different architectures, demonstrating enhanced energy dissipation mechanisms that emerge with increasing degrees of nanoscale structural complexity. This systematic investigation of toughening mechanisms imparts a fundamental understanding of the role of nanoscale architecture on crack propagation, energy dissipation, and toughening mechanisms.

4:00 PM  
Quasi-static Mechanical Properties of As-printed Thin Wall Inconel 718 Manufactured with Laser Powder Bed Fusion: Effects of Thickness and Hot Isostatic Pressing: Paul Paradise; Nikki Van Handel1; Samuel Temes1; Anushree Saxena1; Dhruv Bhate1; 1Arizona State University
    Applications such as heat exchangers rely on thin metal walls as crucial constituent elements of the structure. The design freedom enabled by additive manufacturing has led to an interest in exploiting this technology to further the performance of these components. However, there is limited data on how and why mechanical properties vary by wall thickness. This work focuses on elucidating the quasistatic tensile properties of as-printed, thin-wall Inconel 718 fabricated using laser powder bed fusion with wall thicknesses ranging from 0.3 – 2.0 mm to help aid in making the necessary design decisions of printing thin-walled structures. Special emphasis is placed on valid estimation of a section area for accurate stress calculations. Statistical analysis is conducted to identify the significance of Hot Isostatic Pressing, porosity, and surface roughness on the size dependency of tensile properties. Comparisons are made to sheet metal specimens to isolate process and microstructure contributions from geometry.

4:20 PM  
Investigating the Influence of Scan Strategy and Small-scale Geometrical Complexity on the Microstructure and Mechanical Properties of Thin Wall SLM IN718: Connor Varney1; Paul Rottmann1; 1University of Kentucky
    Additive manufacturing allows for a much wider range of geometries than conventional processing. This, however, comes at the cost of unoptimized microstructures generated from complex processing conditions including multiple thermal traversals with rapid, uncontrolled cooling rates dictated by scan strategy and part geometry. This results in as-built parts with a great deal of microstructural heterogeneity. Understanding the relationship between scan strategy and geometrical complexity (e.g. thin walls, through holes) with the properties and microstructure near those features is necessary to optimize build strategies. In this study a series of Inconel 718 test samples were printed via selective laser melting ranging from thin walls comprised solely of a single contour boundary to those built with contour+hatching scan strategies. Additional compact tension specimens were printed with and without intentional pores simulating lack-of-fusion defects. The microstructure (SEM, EBSD, microCT) and mechanical properties (tension, fatigue) were characterized and compared to bulk specimens.