Binder jet printing (BJP), one of the early metal 3D printing technologies, has distinct advantages over the other additive manufacturing (AM) processes to directly build 3D parts. Some of the advantages of BJP include printed parts free of residual stresses, a less amount of labor, without the starting build plate, and a higher powder reusability. However, the BJP technology has been adopted only in the very limited applications in prototyping due to its technical difficulty in achieving full-density parts. This work has concentrated in developing a new BJP protocol to attain full-density parts made of Stainless Steel (SS) 420 and 316L. The effect of the average particle size, mixture ratio, and sintering additives on the densities of green and sintered parts is investigated for SS420 and SS 316L powders. Multiple powders distinct in average particle sizes are mixed to improve the packing density. A systematic study of the binder burn-out procedure is conducted using thermogravimetric analysis, leading to a complete removal of binder phase without extensively oxidizing SS420 and SS 316L powder. The optimal sintering condition for promising powder mixtures is determined to maximize the final density with the addition of a small amount of boron compounds as sintering additives. The quality of the fully-sintered SS420 and SS 316L parts is evaluated using the various measurements including density, microstructure, hardness, and surface roughness. Relative densities up to 99.6% are obtained for both SS420 and SS 316L without pronounced structural distortion. After achieving the parts with a full density, we were able to prove the ability of printing a shell of a part instead of the entire solid part. After sintering, both printed samples have similar quality in term of powder consolidation. The shell-printing using BJP offers expediting the printing time and reducing the amount of binder phase used in printing process. Furthermore, we successfully developed a process to fabricate a completely enclosed serpentine channel with the length of 500mm and the width of 0.5mm in a 20mm x 10mm x 5mm block. This enclosed miniature internal structure fabrication is a unique accomplishment that is not easily achieved by any available AM technique. It offers potential applications such as fabricating columns for separation in gas chromatography and heat exchangers. Lastly, we employ BJP to print stainless steel parts (SS420) and sinter them in a reactive environment (e.g., oxygen), rendering the surfaces of the powder particles to be reacted and converted to electrically non-conducting ceramics (e.g., metal oxides). This metal/metal oxide hybrid structures exhibit several orders of magnitude higher electrical resistivity compared to the unreacted metal part. As a proof-of-the-concept demonstration, we have fabricated pin-fin-based miniature heating elements for rapid and energy-efficient heating and reaction applications.
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