allow for functionally graded materials employing hybrid AM/sintering process chains, this technology is highly relevant for manufacturing geometrical gradients in materials. Micro components made from metamaterials with topology optimized unit cells can thus be manufactured employing vat photopolymerization and sees usage ranging from medical components through energy applications.
1.5.2 CAD Design and Shrinkage Compensation
The hybrid process chain, in its entirety, is as illustrated in Figure 1.6. The CAD body of the component is designed specifically for the process chain. This is done in order to compensate for shrinkage during the sintering step. Shrinkage is typically up to 30% but can be higher for specialty feedstocks. The shrinkage behavior of small‐sized parts (<50 × 50 × 50 mm3) is quite uniform and predictable. Larger sized components, however, require accurate multi‐physics simulation models for shrinkage behavior. The shrinkage typically is considered to be isotropic for binder jetting and vat photopolymerization [95]. For filament extrusion, the shrinkage is normally anisotropic even for smaller parts with a larger shrinkage in the normal direction to the layers. This is due to a flow phenomenon where the powdered filler has a tendency to concentrate in the very core of the individual extruded strands of filament, leaving a skin layer with a little amount of particles in the skin layer. As such, the powdered filler concentration in the normal direction to the layering of the part exhibits alternating laminae of high and low particles. During sintering, the low concentration zones see more matrix material removal during debinding and thus higher shrinkage.
1.5.3 Additive Manufacture
For powder‐bed binder jetting and vat photopolymerization, certain industrially available systems are tailored for the hybrid AM process chain [96]. Examples are the Desktop Metal and ExOne (metals) system for binder jetting and the Lithoz system for vat photopolymerization (ceramics/metals). Given that these systems are tailored to the process chain, the workflow from CAD body design, through debinding to sintering, is offered as a bundled solution that minimizes the efforts needed for successful adoption. For experimental binder/matrix materials, the readily available industrial systems are less suited, as these are qualified to operate solely in a palette of materials offered by the systems provider. For exotic materials, a custom system is needed that allows for an open parameter setup in order to optimize the build for the feedstock chosen. Such solutions are not readily available and require an initial effort in setting up a custom AM system by the adopter, raising the barrier, therefore tremendously.
Figure 1.6 Schematic of the hybrid metal AM process chain through sintering. DfAM refers to Design for AM and VPAM refers to Vat‐photo‐polymerization AM.
Source: A part of the figure is adapted in accordance with the Creative Commons license and is a copyright of Holo Inc. [94]. © 2020 John Wiley & Sons.
For filament extrusion hybrid AM, there is an abundance of capable entry‐level systems that allow for a free selection of filament [97]. The powder‐loaded filaments for hybrid AM are offered by renowned providers (e.g., BASF) and often accompanied with technical white‐papers, aiding the operator of the system in achieving a successful AM build, debind, and sintering. The matrix polymer is often the same polymer as general‐purpose feedstock (e.g., polylactic acid), and the powdered filler is often stainless steel (e.g., 316L). Take due notice that some filaments are loaded with metal particles for aesthetic reasons only. Filaments with bronze or copper are often employed to give the additively manufactured part an aesthetic sculpted appearance that will oxidize and wither through time due to exposure to the elements. These materials are not suitable for hybrid AM. The build of a component with a suitable filament for hybrid AM manufacture requires different processing conditions than those of a standard filament. The extrusion temperature is generally increased to aid flowability, and since the extrusion pressure/force is higher due to the altered rheology of the feedstock, a direct drive extruder drive mechanism is favored. Retrofitting a dual traction extruder drive system is oftentimes offered as a drop‐in replacement for entry‐level filament AM system.
1.5.4 Debinding and Sintering
The debinding process serves to remove the matrix material of the green body part. Once removed, the component has reached an intermediate state, commonly known as the brown part. This, at large, is achieved by one of two methods. The most convenient is through a burn‐out cycle where the matrix material is debound by decomposing and outgassed from the green body part. This is in particular convenient as it can be integrated into a combined debind and sintering cycle in a programmable oven. This duration of this combined cycle is over multiple hours and in cases up to 2 days. For some specific feedstocks, a separate debinding method is added as an additional link in the process chain. This is introduced out of necessity, to ensure that the component integrity is maintained throughout the sintering cycle. This step involves a chemical dissolving of the binder by exposure to a solvent and is carried out as pertains to one or multiple of the following phenomena; Accumulation of residuals (e.g., soot) as a result of thermal debinding that can form impurities in the sintered component. Mitigation of crack formation during outgassing in case of thermal debinding. The possibility of infiltration of a functional component after debinding (a flux agent or introduction of a matrix component). Solvent‐based debinding is rarely applied at a large scale for industrial applications and is primarily seen applied when producing exotic components from novel materials.
1.5.5 Functionally Graded Components in Sintered Components
Given the limitations for powder‐bed and vat photopolymer systems, functionally graded components that are built from multiple materials are difficult to manufacture by hybrid AM/sintering as feedstock will be cross‐contaminated in‐situ. As a result, hereof, functional gradients in hybrid AM/sintered components are primarily obtained through geometry. By application of flexure mechanisms, from topology optimization and from graded unit cells in cellular structured metamaterials, it is though possible to derive novel functionality. Select examples are such as negative Poisson's ratio [98], negative thermal expansion [99], acoustic properties [100], and bioinspired structures [101] and bioinspired hierarchical microstructures [102]. The high fidelity of small features that can be manufactured using a hybrid AM/sintering approach, as described in Section 1.5.1, renders these technologies particularly interesting for such topology optimized metamaterials. Extrusion‐based hybrid AM/Sintering offers a novel opportunity to sinter multi‐material components as a result of the inherent capabilities of this process. This opens a possibility to combine widely different particle loaded filaments to fabricate green bodies that form, for example, metallic/ceramic composites that can be subsequently sintered [103]. This method does set demands for the dissimilar materials to have an overlap in sintering temperature interval and thermal expansion coefficients.
1.6 Conclusions
Additive manufacturing facilitates new ways to build functionality into metal components. The unpreceded geometrical freedom that these manufacturing technologies bring forth gives rise to new ways of considering geometry induced functionality through lattice structures, metamaterials, flexures, and topology optimization. Equally contingent is the ability to transition material composition throughout a component, either by‐layer in sandwiched gradients or in‐layer through a by pointwise material deposition. It is worthwhile to notice that despite AM offering certain new degrees of design freedom, a process inherent
