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1. Structure-Property Relationships of Differently Heat-Treated Binder Jet Printed Co-Cr-Mo Biomaterial

   https://doi.org/10.1016/j.mtcomm.2023.107716  

   Khademitab, M ; Rodriguez_de_Vecchis, P ; Staszel, P ; Vaicik, M_K ; Chmielus, M ; Mostafaei, A ( November 2023 , Materials Today Communications)

   This investigation systematically examines the influence of sintering temperature and aging treatment on the density, microstructure evolution, phase formation, and mechanical properties of a binder jet printed Co-Cr-Mo biomedical alloy. Sintering at 1380 °C for 2 h yielded a near-fully dense part (99.1%) with favorable mechanical properties (up to 325 HV0.1 hardness and up to 693 MPa ultimate tensile strength). The grain size remained unchanged after aging at 800 °C for 24 h (89 ± 21 µm). Aging resulted in increased microhardness and tensile strength due to phase formation (Cr23C6, CrMo, and ε phase), but a significant decrease in ductility. Consequently, the sintered and aged specimen exhibited higher hardness (522 HV0.1), yield strength (641 MPa), and ultimate tensile strength (854 MPa) compared to cast Co-Cr-Mo alloy. Biocompatibility testing with fibroblasts showed a cell viability of 95 ± 2%, indicating that binder jet printing did not affect the biocompatibility of the Co-Cr-Mo alloy. Exemplary printed parts including hip-joint, partial denture, and small-scale knee joint were successfully demonstrated. This study highlights the comparable properties of binder jet Co-Cr-Mo alloy to the cast alloy, affirming its potential for biomedical applications. 

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2. Aspects of sinter-cracking in binder jet 3D printed parts

   Reid Carazzone ( February 2022 , Progress in Additive Manufacturing 2020)

   Shamsaei ‪Nima, Seifi ‪Mohsen (Ed.)

   Binder jet three-dimensional (3D) printing is a scalable, potentially low-cost additive manufacturing route able to process materials not attainable to other techniques, especially nonweldable materials. It relies on postprocess sintering to achieve final properties but encounters problems with distortion and cracking during sintering. The present work seeks to understand how part design geometry and 3D printing build orientation influence cracking during sintering, with the goal of mitigating the problem. In situ monitoring experiments reveal how sinter-cracks initiate and grow in 3D-printed notched panel specimens during densification. Different design geometries and build directions are tested to identify sinter-crack-resistant regimes. 

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3. Characterization of Electroless Nickel-Phosphorous Plating Roughness and Thickness on Binder-Jetted 420 Stainless Steel Infiltrated with Bronze

   Grizzle, Andrew ; Mutunga, Eva ; Elliot, Amy ; Tyagi, Pawan ( January 2022 , Additive manufacturing)

   "Binder jetting is an economical and rapid additive manufacturing process that offers vast opportunities to combine a variety of materials, yielding interesting and useful properties. However, binder jetted parts, which can involve at least one hard and one soft material, can be more susceptible to corrosion and wear compared to conventional single alloy components produced by laser sintering or other high-temperature processes. This paper discusses the electroless nickel coating on 420 Stainless Steel and Bronze Binder-Jetted Composites(BJC). Electroless nickel, a well-known coating to provide high corrosion resistance and hardness, was attempted on BJC. To produce high-quality smooth electroless nickel coatings, we attempted the Taguchi Design of Experiments. Our design of experiment involved important factors, such as the surface preparation methodology prior to electroless nickel coating. During electroless nickel coating, we investigated the role of phosphorus content, temperature, and time in the production of smooth deposition. Optical microscopy was performed for qualitative and quantitative analysis. We also performed SEM to investigate the microstructure of different electroless coatings on BJC. Interestingly, all the combinations of parameters used in the electroless nickel coating produced different microstructures. We found that surface preparation was a critical factor in determining the smoothness of the film. We also showed that the dependent on the Ni solution’s phosphorus level and temperature. Our research ng insights for improving the usefulness of a wide variety of BJC by various coatings." 

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4. In-situ monitoring of sintering and analytical modeling of densification and shrinkage in binder jetted 316L stainless steel

   https://doi.org/10.1016/j.mtla.2024.102131  

   Jamalkhani, Mohammad ; Deng, Zhifang ; Sossong, Dominick ; Dashtgerd, Iman ; Martiska, Greg ; Mostafaei, Amir ( May 2024 , Materialia)

   In binder jetting, shrinkage and deformation occur during the sintering step, both of which are affected by the green density of the binder jetted materials. The study innovatively introduces a cost-effective, practical, and in-process monitoring system for visualizing shrinkage and deformation on larger samples than conventionally observed using small-scale specimens in dillatometry equipment. The powder characteristics and binder jet printing process itself influence the initial green density. The comprehensive analysis of powder flowability and packing density, densification behavior, and shrinkage reveals that the consolidated parts using virgin powder (with a green density of 55%) can achieve a relative density above 99.9% with an anisotropic shrinkage in the Z>X>Y direction. In contrast, the used or recycled powder exhibits a lower green density of ∼48%, higher shrinkage rate in all three dimensions, and a decreased degree of anisotropy. Using in-process imaging and experimental data on the grain size attained through optical microscopy and electron backscatered diffraction imaging, the material's shear and bulk viscosities were determined. The formation of delta-ferrite and its impact on densification were discussed in the context of solid-state and supersolidus liquid phase sintering. The model relied on the continuum sintering theory formulated by Skorohod and Olevsky. The strain evolution from the in-situ imaging of sintering process is correlated with porosity based on the used feedstock and applied sintering temperatures. The outcomes of this study offer valuable perspectives on anisotropic sintering mechanisms, bridging the knowledge gap regarding the relationships between structures produced through binder jetting and subsequent sintering of materials. 

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5. Binder Jetting Additive Manufacturing: Powder Packing in Shell Printing

   https://doi.org/10.3390/jmmp7010004  

   Miao, Guanxiong ; Moghadasi, Mohammadamin ; Li, Ming ; Pei, Zhijian ; Ma, Chao ( February 2023 , Journal of Manufacturing and Materials Processing)

   Shell printing is an advantageous binder jetting technique that prints only a thin shell of the intended object to enclose the loose powder in the core. In this study, powder packing in the shell and core was investigated for the first time. By examining the density and microstructure of the printed samples, powder packing was found to be different between the shell and core. In addition, the powder particle size and layer thickness were found to affect the powder packing in the shell and core differently. At a 200 µm layer thickness, for the 10 µm and 20 µm powders, the core was less dense than the shell and had a layered microstructure. At a 200 µm layer thickness, for the 70 µm powder, the core was denser and had a homogeneous microstructure. For the 20 µm powder, by reducing the layer thickness from 200 µm to 70 µm, the core became denser than the shell, and the microstructure of the core became homogeneous. The different results could be attributed to the different scenarios of particle rearrangement between the shell and core for powders of different particle sizes and at different layer thicknesses. Considering that the core was denser and more homogeneous than the shell when the proper layer thickness and powder particle size were selected, shell printing could be a promising method to tailor density and reduce anisotropy.

    

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