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1. Discrete-Element Simulation of Powder Spreading Process in Binder Jetting, and the Effects of Powder Size

   https://doi.org/10.1115/MSEC2021-63351  

   Clares, Ana Paula ; Manogharan, Guha ( June 2021 , Discrete-Element Simulation of Powder Spreading Process in Binder Jetting, and the Effects of Powder Size)

   null (Ed.)

   Binder Jetting has gained particular interest amongst Additive Manufacturing (AM) techniques because of its wide range of applications, broader feasible material systems, and absence of rapid melting-solidification issues present in other AM processes. Understanding and optimizing printing parameters during the powder spreading process is essential to improve the quality of the final part. In this study, a Discrete Element Method (DEM) simulation is employed to evaluate the powder packing density, flowability, and porosity during powder spreading process utilizing three different powder groups. Two groups are formed with monoidal size distributions (75–84 μm and 100–109 μm), and the third one consisting of a bimodal distribution (50 μm + 100 μm).

   A thorough investigation into the effects of powder size distribution during the powder spreading step in a binder jetting process is conducted using ceramic foundry sand. It was observed that coarser particles result in higher flowability (62% decrease in repose angle) than finer ones due to the cohesion effect present in the latter. A bimodal size distribution yields the highest packing density (8% increase) and lowest porosity (∼12% reduction) in the powder bed, as the finer particles fill in the voids created between the coarser ones. Findings from this study are directly applicable to binder-jetting AM process, and also offer new insights for AM powder manufacturers.

    

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2. A Numerical Study on the Keyhole Formation During Laser Powder Bed Fusion Process

   https://doi.org/10.1115/1.4044100  

   Shrestha, Subin ; Kevin Chou, Y. ( October 2019 , Journal of Manufacturing Science and Engineering)

   The dynamic phenomenon of a melt pool during the laser powder bed fusion (LPBF) process is complex and sensitive to process parameters. As the energy density input exceeds a certain threshold, a huge vapor depression may form, known as the keyhole. This study focuses on understanding the keyhole behavior and related pore formation during the LPBF process through numerical analysis. For this purpose, a thermo-fluid model with discrete powder particles is developed. The powder distribution, obtained from a discrete element method (DEM), is incorporated into the computational domain to develop a 3D process physics model using flow-3d. The melt pool formation during the conduction mode and the keyhole mode of melting has been discerned and explained. The high energy density leads to the formation of a vapor column and consequently pores under the laser scan track. Further, the keyhole shape resulted from different laser powers and scan speeds is investigated. The numerical results indicated that the keyhole size increases with the increase in the laser power even with the same energy density. The keyhole becomes stable at a higher power, which may reduce the occurrence of pores during laser scanning. 

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3. Understanding Melt Pool Behavior of 316L Stainless Steel in Laser Powder Bed Fusion Additive Manufacturing

   https://doi.org/10.3390/MI15020170  

   Zhang, Zilong ; Zhang, Tianyu ; Sun, Can ; Karna, Sivaji ; Yuan, Lang ( February 2024 , Micromachines)

   In the laser powder bed fusion additive manufacturing process, the quality of fabrications is intricately tied to the laser–matter interaction, specifically the formation of the melt pool. This study experimentally examined the intricacies of melt pool characteristics and surface topography across diverse laser powers and speeds via single-track laser scanning on a bare plate and powder bed for 316L stainless steel. The results reveal that the presence of a powder layer amplifies melt pool instability and worsens irregularities due to increased laser absorption and the introduction of uneven mass from the powder. To provide a comprehensive understanding of melt pool dynamics, a high-fidelity computational model encompassing fluid dynamics, heat transfer, vaporization, and solidification was developed. It was validated against the measured melt pool dimensions and morphology, effectively predicting conduction and keyholing modes with irregular surface features. Particularly, the model explained the forming mechanisms of a defective morphology, termed swell-undercut, at high power and speed conditions, detailing the roles of recoil pressure and liquid refilling. As an application, multiple-track simulations replicate the surface features on cubic samples under two distinct process conditions, showcasing the potential of the laser–matter interaction model for process optimization.

    

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4. Phase-Field Modeling of Wetting and Balling Dynamics in Powder Bed Fusion Process

   https://doi.org/https://doi.org/10.1063/5.0046771  

   Li, L. ; Li, J.-Q. ; Fan, T.-H. ( April 2021 , Physics of fluids)

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   In a powder bed fusion additive manufacturing (AM) process, the balling effect has a significant impact on the surface quality of the printing parts. Surface wetting helps the bonding between powder and substrate and the inter-particle fusion, whereas the balling effect forms large spheroidal beads around the laser beam and causes voids, discontinuities, and poor surface roughness during the printing process. To better understand the transient dynamics, a theoretical model with a simplified 2D configuration is developed to investigate the underlying fluid flow and heat transfer, phase transition, and interfacial instability along with the laser heating. We demonstrate that the degree of wetting and fast solidification counter-balance the balling effect, and the Rayleigh-Plateau flow instability plays an important role for cases with relatively low substrate wettability and high scanning rate. 

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5. In situ / operando synchrotron x-ray studies of metal additive manufacturing

   https://doi.org/10.1557/mrs.2020.275  

   Sun, Tao ; Tan, Wenda ; Chen, Lianyi ; Rollett, Anthony ( November 2020 , MRS Bulletin)

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   Additive manufacturing (AM) comprises a group of transformative technologies that are likely to revolutionize manufacturing. In particular, laser-based metal AM techniques can not only fabricate parts with extreme complexity, but also provide innovative means for designing and processing new metallic systems. However, there are still several technical barriers that constrain metal AM. Overcoming these barriers requires a better understanding of the physics underlying the complex and dynamic laser–metal interaction at the heart of many AM processes. This article briefly describes the state of the art of in situ / operando synchrotron x-ray imaging and diffraction for studying metal AM, mostly the laser powder-bed fusion process. It highlights the immediate impact of operando synchrotron studies on the advancement of AM technologies, and presents future research challenges and opportunities. 

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