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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)

   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 aremore »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.
3. Fabrication of Cu-CNT Composite and Cu Using Laser Powder Bed Fusion Additive Manufacturing

   https://doi.org/10.3390/powders1040014  

   Ladani, Leila ; Razmi, Jafar ; Sadeghilaridjani, Maryam ( December 2022 , Powders)

   Additive manufacturing (AM) as a disruptive technique has offered great potential to design and fabricate many metallic components for aerospace, medical, nuclear, and energy applications where parts have complex geometry. However, a limited number of materials suitable for the AM process is one of the shortcomings of this technique, in particular laser AM of copper (Cu) is challenging due to its high thermal conductivity and optical reflectivity, which requires higher heat input to melt powders. Fabrication of composites using AM is also very challenging and not easily achievable using the current powder bed technologies. Here, the feasibility to fabricate pure copper and copper-carbon nanotube (Cu-CNT) composites was investigated using laser powder bed fusion additive manufacturing (LPBF-AM), and 10 × 10 × 10 mm3 cubes of Cu and Cu-CNTs were made by applying a Design of Experiment (DoE) varying three parameters: laser power, laser speed, and hatch spacing at three levels. For both Cu and Cu-CNT samples, relative density above 90% and 80% were achieved, respectively. Density measurement was carried out three times for each sample, and the error was found to be less than 0.1%. Roughness measurement was performed on a 5 mm length of the sample to obtain statisticallymore »significant results. As-built Cu showed average surface roughness (Ra) below 20 µm; however, the surface of AM Cu-CNT samples showed roughness values as large as 1 mm. Due to its porous structure, the as-built Cu showed thermal conductivity of ~108 W/m·K and electrical conductivity of ~20% IACS (International Annealed Copper Standard) at room temperature, ~70% and ~80% lower than those of conventionally fabricated bulk Cu. Thermal conductivity and electrical conductivity were ~85 W/m·K and ~10% IACS for as-built Cu-CNT composites at room temperature. As-built Cu-CNTs showed higher thermal conductivity as compared to as-built Cu at a temperature range from 373 K to 873 K. Because of their large surface area, light weight, and large energy absorbing behavior, porous Cu and Cu-CNT materials can be used in electrodes, catalysts and their carriers, capacitors, heat exchangers, and heat and impact absorption.« less
4. A printability assessment framework for fabricating low variability nickel-niobium parts using laser powder bed fusion additive manufacturing

   https://doi.org/10.1108/RPJ-01-2021-0024  

   Zhang, Bing ; Seede, Raiyan ; Whitt, Austin ; Shoukr, David ; Huang, Xueqin ; Karaman, Ibrahim ; Arroyave, Raymundo ; Elwany, Alaa ( July 2021 , Rapid Prototyping Journal)

   Purpose There is recent emphasis on designing new materials and alloys specifically for metal additive manufacturing (AM) processes, in contrast to AM of existing alloys that were developed for other traditional manufacturing methods involving considerably different physics. Process optimization to determine processing recipes for newly developed materials is expensive and time-consuming. The purpose of the current work is to use a systematic printability assessment framework developed by the co-authors to determine windows of processing parameters to print defect-free parts from a binary nickel-niobium alloy (NiNb5) using laser powder bed fusion (LPBF) metal AM. Design/methodology/approach The printability assessment framework integrates analytical thermal modeling, uncertainty quantification and experimental characterization to determine processing windows for NiNb5 in an accelerated fashion. Test coupons and mechanical test samples were fabricated on a ProX 200 commercial LPBF system. A series of density, microstructure and mechanical property characterization was conducted to validate the proposed framework. Findings Near fully-dense parts with more than 99% density were successfully printed using the proposed framework. Furthermore, the mechanical properties of as-printed parts showed low variability, good tensile strength of up to 662 MPa and tensile ductility 51% higher than what has been reported in the literature. Originality/value Although many literature studies investigatemore »process optimization for metal AM, there is a lack of a systematic printability assessment framework to determine manufacturing process parameters for newly designed AM materials in an accelerated fashion. Moreover, the majority of existing process optimization approaches involve either time- and cost-intensive experimental campaigns or require the use of proprietary computational materials codes. Through the use of a readily accessible analytical thermal model coupled with statistical calibration and uncertainty quantification techniques, the proposed framework achieves both efficiency and accessibility to the user. Furthermore, this study demonstrates that following this framework results in printed parts with low degrees of variability in their mechanical properties.« less
5. An Experimental Study on the Dynamics of Binder Drops Impacting on a Powder Surface in Binder Jetting Additive Manufacturing

   https://doi.org/10.2514/6.2023-0466  

   Pakulniewicz, Zachary ; Liu, Yang ( January 2023 , AIAA SCITECH 2023 Forum)

   Binder jetting additive manufacturing (AM) is an innovative form of 3D printing that generates complex and advanced structures of various materials by jetting binder drops onto a powder bed. The drops on the bed cure the powder to form structures in a quick and efficient manner. However, the method suffers several flaws including manufacturing inconsistencies and coarse resolution of structures. These flaws may be explained by complex interactions between the binder drop and the powder during the printing process. Therefore, a better understanding of these interactions will be instrumental in the development of binder jetting for fabricating multipurpose, higher quality functional structures. In this study, these complex interactions are analyzed during the impact and subsequent processes. The impact dynamics of binder drops on a powder surface were examined by using a custom impingement rig under various test conditions (i.e., different impact velocities and binder viscosities). A high-speed imaging system was used to capture the transient details of the drop-powder interactions. This study concludes that an increase in drop impact velocity results in a greater range of particle ejection. A lower drop viscosity results in a higher dry spread of particles while a higher drop viscosity results in a greater numbermore »of binder-encapsulated particles. Across all cases, binder drops absorb particle granules at a rate inverse to their viscosity.« less