Additive Manufacturing of Metals: Microstructure, Properties and Alloy Development: Processing and Characterization
Program Organizers: Prashanth Konda Gokuldoss, Tallinn University of Technology; Jurgen Eckert, Erich Schmid Institute of Materials Science; Zhi Wang, South China University of Technology
Wednesday 2:00 PM
October 12, 2022
Room: 302
Location: David L. Lawrence Convention Center
Session Chair: Jonah Klemm-Toole, Colorado School of Mines
2:00 PM
Laser Powder Bed Fusion Process: Effect of Laser Remelting/Scanning/Pulsing/Shaping: Prashanth Konda Gokuldoss1; 1Tallinn University of Technology
The laser-based powder bed fusion process (LBPF) is gaining rapid attention due to its ability to process a wide spectrum of materials with intricate shapes and complex geometries directly from 3D CAD data with improved/similar properties as their conventional counterparts. The main drawbacks in this SLM process are the presence of both internal (dislocation density, stacking faults, twins, etc.) and external (porosity, unmelted particles, etc.) that may lead to premature failure in these SLM processed materials. Both these internal and external defects were controlled by the process parameters including the laser power, laser scan speed, etc. It has been demonstrated that laser pulsing and or laser shaping also influence the microstructure and properties. Accordingly, this abstract will focus on the effect of laser re-melting/scanning/shaping/pulsing on the formation of microstructures and in turning their properties, which can be effectively used to tune the properties of LPBF fabricated materials.
2:20 PM
Design New Feedstock Materials for Additive Manufacturing Using a Commercial Alloy Powder Mixture: Daozheng Li1; Wei Xiong1; 1University of Pittsburgh
Additive manufacturing (AM) is a powerful tool for alloy design and prototyping due to its ability to rapidly process complicated shapes with minimized wastes. However, the rapid heating and cooling cycle readily introduces texture with large columnar grains causing anisotropic properties and large residual stress, which significantly limits the application of the AM process over the traditional manufacturing methods. Therefore, there is an urgent need of discovering new alloys suitable for AM to reduce anisotropy in microstructure and properties. Through the ICME modeling and high-throughput characterization, a mixture of Stainless Steel 316L and Inconel 718 was designed using the laser powder bed fusion. After homogenization at high temperature, experimental observation confirms the expected phenomenon of grain refinement. Such a microstructure refinement due to a hybrid effect of entropy, residual stress, and Zener pinning particle indicates a pathway of using commercial powder mixture to design new alloys for AM.
2:40 PM
Tuning the Microstructure and Mechanical Properties of AlSi10Mg Alloy via In-situ Heat-treatments in Laser Powder Bed Fusion: Federico Bosio1; Chinmay Phutela1; Alya Alhammadi1; Nesma Aboulkhair1; 1Technology Innovation Institute
Together, advanced technologies and metallic materials rapidly progress in the field of Laser Powder Bed Fusion (L-PBF) additive manufacturing. Aluminium alloys are one of the most attractive metal systems processed with high reliability, being used in high-value applications for demanding industrial sectors. Although L-PBF drastically shortens the design-to-manufacturing time, post-processing operations on metal parts, including heat-treatments, are often time-consuming. Therefore, in this study, we assessed the viability of conducting in-situ heat-treatments during the additive production of AlSi10Mg parts. A build plate heated at set temperatures up to 500 ºC was employed to tune the microstructure of AlSi10Mg alloy, like in conventional post-process heat-treatments. For comparison, we also processed a batch of samples at similar temperatures followed by conventional solution heat-treatment. Furthermore, we assessed the effect of the heated build plate and printing time on the meso- and micro- structure of AlSi10Mg samples, correlating these with the mechanical properties. The experimental results showed that increasing the printing time at 220 ºC results in an in-situ direct aging heat-treatment with an actual increment of hardness and alloy strength. Printing at 300 ºC induces a significant Al lattice relaxation and consequent in-situ stress relieving on the as-built parts. Lastly, in-situ solution heat-treatment is possible when using build plate temperatures at 500 ºC, although a reduction in ductility occurs as compared to conventionally solution heat-treated counterparts.
3:00 PM
Exceptional Ductility Induced by The Intrinsic Grain Boundary Engineering: Lin Gao1; Wenhao Lin1; Zhongshu Ren1; Ma Ji1; Tao Sun1; 1University of Virginia
Developing effective strategies to fabricate structural materials with superior mechanical performance is attractive in metal additive manufacturing (AM). We applied a wiggle deposition pattern in wire laser directed energy deposition of 316L stainless steel, which enables the grain boundary (GB) engineering during the solidification process. The as-deposited sample exhibits exceptional ductility without compromise of strength. Through multi-physics simulation and operando near-infrared imaging, we discover that the wiggle deposition induces highly dynamic melt flow and oscillating thermal gradient in the melt pool, which are responsible for the generation of GB protrusions consisting of multiple subgrains. This unique microstructure is found to promote intergranular deformation, therefore improving the tensile properties of the sample. We believe the fundamental principle of tailoring GB structures via perturbing the solidification can be readily implemented in other metal AM processes involving high-energy-density heat sources to further push the classical strength–ductility envelope of conventional and emerging alloys.
3:20 PM Break
3:40 PM
From Chemistry at the Scale of Printing to Bulk Quantitation: A Powerful Tool for Characterizing Additive Manufacturing Materials: Jonathan Putman1; Peyton Willis1; Madeline Martelles2; Ellen Williams1; 1Exum Instruments; 2University of Tulsa
This work showcases a new technology for characterizing additive manufacturing metal materials, Laser Ablation Laser Ionization Time of Flight Mass Spectrometry (LALI-TOF-MS). LALI-TOF-MS is a powerful technique capable of trace-level quantitation for virtually the entire periodic table. The LALI ionization source eliminates many challenges associated with other techniques, and TOF-MS provides a complete mass spectrum for each laser spot. LALI-TOF-MS is capable of quantifying both trace and major elements, as well as many interstitial elements like carbon, nitrogen, and oxygen. Part of this work compares quantified results obtained for additive manufacturing metal materials with their certified values. With a spatial resolution ranging from 5-200 um, LALI-TOF-MS also provides chemical data at the same scale as the melt pool in many additive manufacturing techniques. Chemical mapping experiments demonstrate this capability and reveal localized heterogeneities in elemental and oxide composition between individual particles of metal powder feedstocks.
4:00 PM
Measurement and Classification of SLM Feedstock Powders by X-Ray Microscopy and Machine Learning: Daniel Sinclair1; Eshan Ganju1; Hamidreza Torbati-Sarraf1; Nikhilesh Chawla1; 1Purdue University
Selective laser melting (SLM) has received attention as a transformative metal 3D printing method in commercial, industrial, and defense applications. However, SLM parts are susceptible to reduced reliability due to defects retained from irregular powder feedstock. As sources of feedstock are diversified to supply expanded manufacturing, granulometry methods are needed which distinguish sources of irregularity. In recycled or composite powder blends, for example, standard stereography conflates fused and elongated particles. In this work, we studied an irregularly shaped, recycled AA7050 feedstock powder with titanium additives (AA7050-RAM2). Lab-scale x-ray microscopy was coupled with automated watershed segmentation to quantify the shapes and sizes of particles. Novel 3D shape factors were developed to describe nonstandard geometries based on concavity and convexity. Additionally, automated methods of classification were compared, using a combination of algorithmic clustering and the Random Forests machine learning algorithm, to maximize efficiency and capture a wide range of powder morphologies.
4:20 PM
Visualization of Metallic Alloy Microstructural Evolution under Additive Manufacturing Conditions: Oliver Hesmondhalgh1; Amy Clarke1; 1Colorado School of Mines
Additive manufacturing (AM) is highly customizable and can be used for the economic production of complex structures. 3D printed metals often contain coarse columnar grains, which may result in directional properties that may be deleterious of advantageous. To achieve the strict specifications needed for some structural applications, control of the microscopic structure and the promotion of grain refinement are crucial. Recent experimental advances allow for the visualization of dynamic solidification events in metals at unprecedented length- and timescales. This work presents the in-situ/ex-situ characterization of microstructural evolution during AM, with the aim of advancing our current understanding of microstructure development under AM conditions, and provides insights into alloy design approaches for AM.
4:40 PM
Origin of Epitaxy Loss in Laser Powder Bed Fusion: Prosenjit Biswas1; Ji Ma1; 1University of Virginia
Although the epitaxial growth from the substrate dominates the initial grain development during laser powder bed fusion, epitaxial solidification is lost quickly after a small transition region on the order of <1 to several layer thickness. Moreover, the width of the transition region varies with processing conditions and substrate texture. This study examines the texture transition mechanism and the condition that leads to a larger transition region. Two identical SS316L samples with different printing speeds were printed on a (110) SS316L Single Crystal substrate to ensure uniform initial grain development. The misorientation analysis from the EBSD map shows that the misorientation angle gradually accumulates with the distance from substrate and once it reaches 12~15 degrees, large discontinuous jump in misorientation occurs to form completely new grain. The misorientation angle is related to the growth direction of cellular solidification features and the rate of accumulation is inversely proportional to scan speed.
5:00 PM
Effect of Thermo-mechanical Treatment on Mechanical Performance of Cold Spray Additively Manufactured Deposits: Ahmad Nourian1; Sinan Muftu1; 1Northeastern University
Cold spray technique has exhibited great potential as an additive manufacturing approach for producing complex geometries and restoring damaged components. In this process, the powder feedstock is accelerated to supersonic velocities. Solid-state bonding is formed due to severe plastic deformation in the particles impacting on the underlying material. In this study, the microstructural and mechanical properties of aluminum 6061 deposits were investigated as free-standing material and repaired specimens. The properties of the deposits as free-standing material were evaluated by preparing specimens from thick deposits. Repaired specimens were also prepared by spraying the powder on notched specimens. Different thermal and thermo-mechanical post-spray treatments were applied on the deposits. The effect of different post-spray treatments on mechanical performance of the deposits were evaluated by conducting three-point bending, tensile, as well as fatigue tests. Results showed that the mechanical performance of deposits has been improved significantly by applying post-spray treatments.
5:20 PM
Surface Nanotextured Powders for 3D Printing of High Reflectivity Metals: Ottman Tertuiliano1; Philip Depond2; Andrew Lee3; David Doan3; X. Gu3; Manyalibo Matthews2; Wei Cai3; Adrian Lew3; 1University of Pennsylvania; 2Lawrence Livermore National Laboratory; 3Stanford University
The widespread application of additive manufacturing is hindered by an inability to control the complex interaction between the energy source and feedstock. Here we develop a process to introduce nanoscale features to the surface of copper (Cu) powders which increases powder absorptivity by ~70% during laser powder bed fusion. The absorptivity increase enables printing of pure Cu structures with densities >90% using low laser powers down to 100W. The nanotextured powders uniquely improve absorptivity and printing conditions at low energy density, reducing machine damage induced by high reflectivity metals. Ray tracing simulations show the measured improved absorptivity is mostly described by increased local absorptivity at the nanoscale features of etched surfaces. High speed video of single-layer hatches is performed to observe modified powder dynamics. The approach taken here demonstrates a generalized approach to modifying the absorptivity and printability of metal powders by only changing the surface texture of the feedstock.