3D Printing for Energy Applications. Группа авторов
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 design constraints apply and must be thoroughly understood before embarking on the implementation of an AM infrastructure for functionally graded manufacturing of metal components. Some processes such as vat photopolymerization and laser PBF induce issues as relates to keeping process materials separated such that excessive waste is not generated during build operations. For direct energy deposition, this issue is less pronounced as segregated waste powder collection can, to a large extent, remedy cross‐contamination. Most favorable is the pointwise deposition by means of multi‐material filament extrusion and subsequent sintering. A hybrid approach where no mixed waste product is generated, yet at the cost of having a lower component fidelity as extrusion‐based AM, is limited in minimum feature size and minimum layer height. It is crucial that the benefits of AM, including geometrical freedom, digitally driven, highly agile manufacturing is weighted against the process limitations, including low throughput, process‐dependent design, and manufacturing paradigms that must be learned. Further, post‐processing of AM parts is often performed as an art form by manual fit and finishing. It can be regarded as the secret of additive manufacturing that nonchalantly by the populace is deemed a fully digital process when the underlying modality is that of a high degree of post‐processing in order to present a functional component that meets design tolerances.
The overview of recent advances in metal AM is an excellent barometer for the evolution of existing and new applications in the energy sector. The three primary drivers in favor of AM components are geometric/material functionality, lead‐time reduction, and sustainable production. The geometric functionality applied toward heat‐exchangers, topology‐optimized turbine blades, nuclear core‐reactors, etc., has the potential to be improved by the addition of material gradient functionality. Nuclear power components, steam/gas turbines components, fuel cell components, oil & gas are the major sectors in which a steady growth of critical functional metal AM components is expected over the coming decades, with an increase in available materials. Metal AM applications driven by lead‐time reduction and sustainable production are expected to have an exponential growth in the nuclear, oil & gas sectors specifically for on‐demand and on‐site production. Consolidated designs produced with AM are increasingly proving to be an efficient solution for the energy industries complex needs. Steadily the capabilities are evolving toward much higher degrees of design freedom, toward better capabilities of multi‐material manufacturing in highly dissimilar materials and for the confident and resolute, the capabilities that currently present themselves for introducing a new function to components that have the potential to have an encompassing implication to how new technology can be brought to market.
Acknowledgment
This work was supported by the Poul Due Jensen Foundation.
References
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