Hybrid additive manufacturing (hybrid-AM) has described hybrid processes and machines as well as multimaterial, multistructural, and multifunctional printing. The capabilities afforded by hybrid-AM are rewriting the design rules for materials and adding a new dimension in the design for additive manufacturing (AM) paradigm. This work primarily focuses on defining hybrid-AM in relation to hybrid manufacturing (HM) and classifying hybrid-AM processes. Hybrid-AM machines, materials, structures, and function are also discussed. Hybrid-AM processes are defined as the use of AM with one or more secondary processes or energy sources that are fully coupled and synergistically affect part quality, functionality, and/or process performance. Historically, defining HM processes centered on process improvement rather than improvements to part quality or performance; however, the primary goal for the majority of hybrid-AM processes is to improve part quality and part performance rather than improve processing. Hybrid-AM processes are typically a cyclic process chain and are distinguished from postprocessing operations that do not meet the fully coupled criterion. Secondary processes and energy sources include subtractive and transformative manufacturing technologies, such as machining, remelting, peening, rolling, and friction stir processing (FSP). As interest in hybrid-AM grows, new economic and sustainability tools are needed as well as sensing technologies that better facilitate hybrid processing. Hybrid-AM has ushered in the next evolutionary step in AM and has the potential to profoundly change the way goods are manufactured.

We report a water-based approach for making chemically reactive forms of hemp fibers and employing these materials to fabricate biocomposites for hydroponic applications. Our chemical strategy focused on coupling the lignin of hemp fibers and a bifunctional linker molecule (2-[(4-aminophenyl)­sulfonyl]­ethyl hydrogen sulfate) that contains an aromatic amine and a protected vinyl sulfone. The synthetic process started with converting the amine of this linker to an electrophilic diazonium ion, which then reacted with the electron-rich aromatic rings of the lignin within the hemp fibers to form the chemically reactive lignin. The other functional group of the linker was activated under basic conditions to yield the intermediate vinyl sulfone, which reacted with poly­(vinyl alcohol) via the Michael addition to yield the cross-linked hemp fiber composites in a 1 in. diameter quartz tube mold. The overall fabrication process was ecofriendly because only water was used as the solvent, and harmless inorganic salts were the only major byproducts. These hemp composites were durable and did not easily crumble under compressive mechanical tests. Fabrication experiments were performed with different weight ratios of bifunctional linkers to hemp fibers to evaluate the effect of this factor on the mechanical strength of resulting hemp composites. The compressive strength of these dry hemp composites was measured to increase from 0.91 to 1.81 MPa when this weight ratio was raised from 3: 40 to 3: 5. The hemp fiber composites fabricated using an intermediate weight ratio (3: 10) of the bifunctional linkers to hemp fibers were evaluated as a hydroponic growth medium. Properties of the hemp fiber composites, such as water holding capacity, carbon/nitrogen ratio, salinity, and acidity, were also evaluated to determine their suitability as plant growth media for hydroponic applications. The hemp fiber composites were demonstrated to be effective hydroponic growth media for Daikon radish and green peas in a 14-day growth period.

Additive manufacturing (AM) of metals often results in parts with unfavorable mechanical properties. Laser peening (LP) is a high strain rate mechanical surface treatment that hammers a workpiece and induces favorable mechanical properties. Peening strain hardens a surface and imparts compressive residual stresses improving the mechanical properties of a material. This work investigates the role of LP on layer-by-layer processing of 3D printed metals using finite element analysis. The objective is to understand temporal and spatial residual stress development after thermal and mechanical cancellation caused by cyclically coupling printing and peening. Results indicate layer peening frequency is a critical process parameter affecting residual stress redistribution and highly interdependent on the heat generated by the printing process. Optimum hybrid process conditions were found to exists that favorably enhance mechanical properties. With this study, hybrid-AM has ushered in the next evolutionary step in AM and has the potential to profoundly change the way high value metal goods are manufactured.

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