The future of advanced manufacturing will include precise digital control of material, geometry and functionality in three dimensions to provide the next generation of customized, high-value, functionally-graded products [1]. With traditional subtractive processing providing the foundational process, hybrid additive manufacturing (as defined in this work) includes the integration of additive manufacturing into CNC machining systems. Leveraging common directed energy deposition (DED) technology, hybrid systems deposit material with wire or powder feedstock to create complex geometries [2–5]; subsequent machining renders superior dimensional accuracy and surface finish. Collaboratively, the integrated processes can fabricate structures through the incorporation of multiple manufacturing techniques to create products within a single, digitally-driven build chamber. The interleaving of the processes can occur at each layer or can be multiple layers depending on access and reachability of surfaces after an extended deposition cycle of N layers. Furthermore, repairs can be directly applied to existing structures through the subtraction of suspect material, 3D scan capture, deposition of new structure, 3D scan capture, and a final subtraction of material as necessary [4, 6–9] to finish with an intended geometry. Figure 1 illustrates the Okuma MU8000 Laser EX system equipped with both machining and additive deposition capability in the same five-axis gantry.
Starting in the 1990’s, DED has been investigated generally [2, 10–13] including with both metal and ceramic feedstock [14–21] in both powder or wire form, either of which is fed into a melt pool created by directed energy source such as laser, electric arc or electron beam [3]. Material is deposited in a selective manner in order to build up a three-dimensional object [22]. With powder-based systems, metal powder is blown by an inert gas to supply the melt pool with additional feedstock while providing protection from the otherwise oxidizing atmosphere [17]. The tool head includes the directed energy (laser beam in this example) coincident with the surface and gas-delivered powder. A unique advantage of blown-powder DED is the capability not only to change materials but also to vary the composition for functionally grading the structure with on-demand alloying with 3D spatial control [20, 23–26].
Hybrid manufacturing has been defined as the integration of multiple processes with layerwise access to a structure during fabrication to create a single material structure with improved dimensional accuracy or more complex multi-material and multicomponent structures [27–29]. Hybrid additive manufacturing in this paper is defined as the layerwise combination of AM with other traditional processes within a single build envelope [28] and in this case included: machining, directed energy deposition and the insertion of components. Hybrid DED with machining has been researched for decades [16, 27, 30–32]. By further enhancing the system with the integration of robotic placement within the build chamber, sub-components can be integrated into the fabricated structure by machining geometry-specific cavities, inserting components, and finally, capturing these sub-components within the structure with subsequent metal deposition. Motivations for embedding components within structures include modulation of density, improvement of impact energy management [33], or even the integration of embedded sensing and programmability for structural health diagnostics [34]. The inserted components can be fully encapsulated within the metal structure, assuming that the inserts can survive the subsequent high temperature metal deposition process. However, this requirement dramatically limits the range of components that can be integrated without introducing a thermal barrier or altering the process to minimize the exposure of the subcomponent to the high temperatures of the laser cladding process.
Process interruptions in additive manufacturing for the purpose of embedding components within polymer structures were first reported in the 90s [1, 35–44]. On the contrary, component integrations in metal additive processes have been less common due to the high temperatures required for melting different alloys - often destroying all but the simplest high temperature sensors. Of the successful attempts to embed components in metal objects; electron beam melting was used for a ceramic sensor [45]; sheet lamination was used and provides the lowest processing temperature of any metal additive manufacturing with ultrasonic welding of metal tape strips together [37, 46]; a combination of selective laser melting and directed energy deposition was used in a clever multi-process approach in [47]; and the earliest case in 2000 with a form of directed energy deposition referred to as shape deposition manufacturing [48] - prior to the standardization of terminology with ISO/ASTM 52900 [49]. Sensors were welded onto directed energy deposition structures in this case.
The current work advances the understanding of the digital manufacturing paradigm by using a commercially-available hybrid directed energy deposition to create a multi-material, multi-component demonstration to serve as a proof of concept. The process included: (1) machining a geometry-specific cavity in a 316L stainless steel substrate; (2) manually placing a ceramic insert into the cavity; (3) introducing a thermal barrier and providing a flush surface upon which the introduction of unmelted powder in the occupied cavity; (4) wetting the powder with available machining coolant to provide temporary powder stability and adhesion; and then subsequently (5) depositing metal to fully encapsulate the ceramic component and trapped powder within a larger steel structure. Future work will include introducing ceramic components with embedded thermocouples with windowed access to sensor terminals on the outer surface of the 3D printed metal structure. The introduction of ceramics into metal substrates could provide significant utility for a diversity of applications including impact energy management, thermal control, embedded sensors and electronics to name a few.