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Through additive manufacturing (AM), it is now possible to produce functionally gradient materials (FGM) by depositing different metal alloys at a specific location to locally improve mechanical properties and enhance product performance. Despite recent developments, however, there are still some important trade-offs to consider and inherent challenges that must be addressed. These include limitations to the volume, size, and range of materials used and a data-driven strategy to drive decision-making and automation. Additionally, many potential advantages exist in environmentally sustainable terms of multi-material additive manufacturing (MM-AM). In particular, for products that require a complex design, high value, and low production volume, material and energy use can be reduced significantly. However, there are significant uncertainties in terms of environmental impact and applications of MM-AM that need to be addressed during the initial stage of the technology development to understand its potential future environmental performance improvements.
Division for Quantitative Sustainability Assessment (QSA), Department of Environmental and Resource Engineering, Technical University of Denmark, Denmark
Constantinos Goulas
Faculty of Engineering Technology, Department of Design Production and Management, University of Twente, The Netherlands
Stig Irving Olsen*
Division for Quantitative Sustainability Assessment (QSA), Department of Environmental and Resource Engineering, Technical University of Denmark, Denmark
*Address all correspondence to: valpu@dtu.dk and siol@dtu.dk
1. Introduction
There is agreement among all the countries of the world that we need to become more sustainable. The Paris Agreement on climate change provides the basis for an urgent need for global response to the threat of climate change and the 2030 Agenda for Sustainable Development addresses sustainability more broadly. A core aim is to increase the ability of countries to reduce and tackle the impacts of climate change, by supporting developing and the most vulnerable countries in joining the global effort. Manufacturing has an elevated environmental impact as it contributes with roughly 98% of the annual total direct carbon dioxide (CO2) emissions, and the industrial sector alone (among energy, transportation, and building sectors) accounts for approximately a quarter of the global carbon emissions [1]. One of the major contributors of the industry sector is the steel production that represents 8% of the global CO2 emission share [2]. In particular, in 2018 about 1.8 Gt steel was produced worldwide [3], this corresponds to roughly 2.1 Gt direct CO2 emissions worldwide [4] and represents 8% share of the global CO2 emissions [2]. Additionally, the increasing demand of ore mining for manufacturing [3, 5] has posed attention to the sustainable extraction and management of abiotic resources [5, 6, 7]. In a global perspective, the supply horizon and scarcity of elements have been considered an important indicators for the severity of increased extraction although newer methods focusing on environmental dissipation are being developed [8]. Nevertheless, it has also been argued that resource availability is very dependent on sociopolitical issues, opportunity costs, fixed stocks, etc., and therefore not well evaluated using an environmental tool such as life cycle assessment (LCA) [9]. From the perspective of the European economy, the European Commission monitors and evaluates the critical resources for the European economy based on their economic importance and supply risk [10]. In that report, they also evaluate the potential significance of supply risks in different industries, that is, 3D printing. They recommend diversifying the materials supply, especially titanium, and minor alloying elements such as scandium and niobium should be in focus. Additionally, the possibilities of recycling and reuse should be further investigated [10].
In the literature, the overall sustainability of conventional or additive manufacturing appeared to be product application or production technique dependent [1, 11, 12, 13, 14, 15, 16, 17]. In this context, the European project, Grade2XL, aims to investigate the potential of multi-material wire arc additive manufacturing (MM-WAAM) for large objects relative to strength, durability, and sustainability of engineering structures [18].
MM-WAAM is a variant of the wire arc additive manufacturing process (WAAM) that allows for the fabrication of complex parts with multiple materials. In WAAM, an arc is used to melt a wire consumable, which is then deposited layer by layer to build up the part [14, 19]. In multi-material WAAM, multiple wire consumables are used in the same part, allowing for the creation of multi-material parts with unique properties and improved performance.
From both a sustainability or recycling, and technical point of views, MM-WAAM still faces various challenges. The primary reason is that the materials are not easily differentiated into the different waste streams, thus reducing the recycling quality, and layering different alloys constitute a challenge [20, 21].
2. Current state of the technology for multi-material additive manufacturing
Multi-material additive manufacturing can produce highly complex products with improved functional behavior [20, 22], often at a reduced total cost. There are several available additive manufacturing methods able to achieve multi-material additive manufacturing with metals, each with its own set of advantages and limitations. The most common methods include:
Laser Powder Bed Fusion: This method involves using a high-powered laser or electron beam to melt and fuse metal powders on a powder bed layer by layer. Even though typically suited for single material builds, certain modules exist that enable the selective distribution of different types of material powder at different locations in a given layer (e.g., Aerosint multi-material printing bundle [23]).
Binder Jetting: Binder Jetting involves a printer head which deposits a liquid binder onto layers of powder. The binder binds the powder together, forming a solid part. The type of binder or powder being used, can be changed locally and in this way it is possible to create multi-material parts.
Sheet Lamination: Sheet Lamination is an additive manufacturing method in which thin sheets of material (usually supplied via a system of feed rollers) are bonded together layer-by-layer to form a single piece that is cut into a 3D object using a CNC machine. Ultrasonic Consolidation is an AM method belonging to the sheet lamination family that enables multi-material printing by joining dissimilar metal sheets in solid state using ultrasonic vibrations.
Directed Energy Deposition: This method involves using a high-powered laser or other heat source (Arc, electron beam) to deposit and fuse metallic material in powder or wire form directly onto a build platform. By moving the heat source and changing the type of material being deposited, it is possible to create multi-material parts. A recent commercialized multi-material DED solution was presented by Meltio [24], which enables the coaxial feeding of two different wire feedstock into a focus point of six individual lasers. MM-WAAM, the main technology studied in the Grade2XL project belongs to the DED family, and for this reason, we will analyze MM-WAAM further.
DED (and MM-WAAM) has some unique advantages [25], which make it a very attractive manufacturing method for large-multi-material components. These advantages include:
High rate of material deposition, reaching up to 5 kg/h deposition rate per deposition unit.
Ability to use widely available feedstock in the form of conventional welding wire and/or coarse metal powder, which is low cost compared to LPBF powders.
Ability to produce components of large size without significant increase in equipment investment cost, since there is no need for a defined chamber in protected atmosphere. As a motion system, an industrial robot can be used, which enables to reach dimensions greater than 1 m.
Ability to deposit different materials where it is needed in the component, without major process disruptions. The way the second material is delivered is versatile; it can be introduced by a second robot, tandem torch, or cold wire/powder feeder.
Using multiple types of wire or blown powder as feedstock allows for the production of complex functional gradients, multi-material layers, and even composite materials.
3. Technical challenges and future applications for multi-material additive manufacturing for metals
For all Metal Additive Manufacturing (MAM) methods, their technical challenges currently hinder broad adoption of the technology in the industry. Some challenges are specific to each MAM method and some are common. The common challenges are related to the fact that a monolithic component is built with different materials, which is rather uncommon in the industry. Mixing and melting different materials together in a single component causes the risk of creating intermetallic phases and interfaces in the material that may exhibit undesired behavior, which needs to be studied case-by-case for every combination and mixing level. This of course means that in case the multi-material part needs to be certified, new protocols will need to be developed to account for these variations. Additionally, the alloys currently used are not designed to be mixed, so the elements present in each alloy are not necessarily designed to be compatible with other alloys in the same component. Non-material related but still very important is the limitation that MAM has compared to the single material counterpart in terms of scalability. Since MAM processes become increasingly complicated to accommodate multiple materials, the production workflows also become complicated, raising the costs.
The MAM techniques have some extra challenges. In LPBF, for example, the use of multiple materials becomes a complex matter that influences the whole process chain. From separating single material powders, reclaiming them, as well recycling the multi-material component remain challenging. However, there are more difficulties such as complexity in selective material deposition, issues with co-processing and material interface formation and developing materials and process modeling [26, 27]. For DED, there are some specific challenges as well, including the low resolution in terms of where the materials are mixed, which also results in extensive post-processing requirements. Additionally, in the case blown powder is used, a large proportion of the material used is not ending up in the part, but is lost or contaminated and not reused, which reduces the efficiency of the method.
Multi-material wire arc additive manufacturing (WAAM) can be used to fabricate a wide range of complex parts with unique properties and improved performance. Because it allows for the use of multiple materials in the same part, it opens new possibilities for the design and manufacture of parts with functional and performance characteristics that cannot be achieved using traditional manufacturing processes. Some examples of parts that can be made with multi-material WAAM include:
Structural components with tailored mechanical properties: by combining materials with different mechanical properties, such as stiffness, strength, and toughness, it is possible to create structural components with tailored properties that are optimized for specific applications. For example, multi-material WAAM could be used to create aircraft components with high strength and low weight, or automotive or railway components with high stiffness and good fatigue resistance. For example, in Figure 1, the MM-WAAM produced bogie consists of a combination of steels with different strengths, which optimizes the mechanical behavior of the component while it minimizes the material that is used for its production.
Functionally graded materials: by carefully controlling the composition and location of the different materials in a part, it is possible to create functionally graded materials with graded properties. For example, a part could be designed with a stiff core and a compliant outer layer, or with a high-conductivity region and a low-conductivity region. This can improve the performance and functional properties of the part.
Complex shapes with internal features: because the WAAM process allows for the deposition of material layer by layer, it is possible to create complex parts with internal features that would be difficult or impossible to fabricate using other manufacturing processes. For example, multi-material WAAM could be used to create parts with internal channels, passages, or voids that are required for specific applications.
Figure 1.
Topology optimized, MM-WAAM produced bogie for the railway industry. Combination of different steels yields superior mechanical behavior of the component. Image courtesy of RAMLAB BV.
The potential applications of multi-material WAAM are vast and will continue to expand as the technology matures and advances. Since the potential of the technology is high, it is very important to assess the environmental consequences of producing parts with MM-WAAM and other MAM methods instead of traditional methods.
4. Life cycle assessment for multi-material additive manufacturing for metals
It is important to assess the sustainability impact of anthropogenic activities and technologies in order to understand what contributes most and how to reduce the impacts. To this purpose, life cycle assessment (LCA) was applied to WAAM, which is an emerging technology still under development and optimization. LCA is a state-of-the-art methodology for assessing multiple environmental impacts of a system over time and space throughout its lifecycle from cradle to grave, that is, from extraction of the materials through production and use or operation of the system till its end-of-life (EoL) [28]. It is further an ISO-standardized methodology [29, 30].
Many claim several benefits of additive manufacturing compared to conventional manufacturing [14, 15, 17, 19]. For instance, reduction of waste, energy, or fuel consumption during the use of product and reduction of cost due to optimization of shapes, lightweight design, and shorter time from ordering a product until you receive it. However, to what extent are these claims valid? Which quantifiable trade-offs are important? As explained, LCA takes a life cycle thinking perspective; this means that all the processes required to deliver the function of a product or activity from the raw material production to the disposal of it are included in the assessment (see Figure 2) [31].
Figure 2.
Schematic representation of life cycle assessment (LCA).
Figure 3 represents a generic overview of all life cycle processes considered for Grade2XL products produced with WAAM. The two boxes with dotted lines represent avoided activities as incineration provides heat and power additionally to burning of waste, and with recycling of metal scrap is possible to avoid the production of a certain amount of virgin material. In this study, the modeling approach taken was attributional and system expansion was used to assess processes that provided a second service in addition to the main one (e.g., incineration) [31].
Figure 3.
System boundaries considered in cradle-to-grave LCA of Grade2XL products. Processes related to energy consumption, transportation, and capital goods were included in the model, but they are not represented here. The two boxes with the dotted line are avoided products.
A further advantage of LCA is the holistic perspective of the comprehensive coverage of environmental issues. Rather than focusing exclusively on climate change, which currently receives generally most attention, LCA covers a broad range of environmental issues and impacts. For instance, it usually includes among others: freshwater use, land occupation and transformation, toxic impacts on human health, and depletion of non-renewable resources. In this way, the major trade-offs between impacts are addressed and burden-shifting can be avoided.
For all the reasons previously mentioned, the LCA framework is currently used to compare the sustainability of all Grade2XL products to the same objects fabricated with the traditional manufacturing processes. Figure 4 illustrates the preliminary results of the cradle-to-grave LCA of the production of a holding ring for spherical turbine inlet valves in hydraulic power plant produced with WAAM or casting. The software and database used were SimaPro 9.4.0.2 and ecoinvent 3.8, respectively. The functional unit was defined as “enabling the production of an average amount of GWh for 10 years in France.” The specific value of the average amount is confidential and thus not disclosed here. All the equivalent processes were excluded from the calculation of the environmental impact. Figure 4 illustrates the impact score for multiple environmental issues considered in the assessment (see also Table A3 in the Appendix).
Figure 4.
Internally normalized impact score with ReCiPe2016 (H) midpoint for holding ring produced by conventional manufacturing (CM, columns on the left) and WAAM (columns on the right). The full name of the impact categories in the x-axis is reported in Appendix A.2.
It is clear the quite better environmental performance of WAAM over conventional processes, except for the impact category “ionizing radiation”. In order to understand the reason why the impact score is visibly higher for WAAM for this impact category, a simple hot-spot analysis was developed by doing a process contribution analysis for WAAM. Figure 5 illustrates the processes that contribute to the impact score for each impact category of the holding ring produced with WAAM. In general, the steel and bronze wires and the energy used during manufacturing are clearly the major contributors to both type of manufacturing processes. In particular, in the impact category “ionizing radiation” the major contribution comes from the energy consumption during WAAM operation. The negative contribution of the recycled material to the total environmental impact can be explained by the chosen modeling method, in which the recycled material is avoiding the extraction and production of primary material [28, 31].
Figure 5.
Process contribution analysis with ReCiPe2016 (H) midpoint for holding ring fabrication by WAAM. The full name of the impact categories in the x-axis is reported in Appendix A.2.
Similarly, a Life Cycle Assessment was done for a bathtub mold. Figure 6 shows the internally normalized results of the LCA of the production applying WAAM or conventional manufacturing (i.e., casting and Nickel vapor deposition in sealed vessel). In this case, the functional unit was defined as “enabling the production of 10,000 polyurethane bathtubs without surface defects in the Netherlands.” All the equivalent processes between the two systems were excluded from the calculation of the environmental impact. Figure 6 clearly shows a lower impact score of WAAM over conventional processes, except for the impact category “ionizing radiation”. It is worth to mention that for “Terrestrial/Freshwater/Marine ecotoxicity” the WAAM alternative’s impact score is 1% of the conventional alternative explaining why it does not show in Figure 6. It is possible to see the numerical results more precisely at Table A2 in the Appendix.
Figure 6.
Internally normalized impact score with ReCiPe2016 (H) midpoint for bathtub mold fabricated by conventional manufacturing (CM, columns on the left) and WAAM (columns on the right). The full name of the impact categories in the x-axis is reported in Appendix A.2.
In this case, as well, a simplified hot-spot analysis was done for WAAM by analyzing its process contribution for the bathtub mold. Figure 7 presents the processes that contribute to the impact score for each environmental impact category of the bathtub mold manufactured with WAAM. It is clear that, in general, the steel wires are the major contributors. In particular, in the impact category “ionizing radiation” the major contribution comes from the energy consumption during WAAM operation. The negative contribution of the recycled material to the total environmental impact as for the holding ring is linked to the fact that system expansion is considered [28, 31].
Figure 7.
Process contribution analysis with ReCiPe2016 (H) midpoint for the fabrication of a bathtub mold by WAAM. The full name of the impact categories in the x-axis is reported in Appendix A.2.
Overall, for both products considered their LCA showed the highest impact score for the impact category “ionizing radiation” due to the elevated electricity use during WAAM. However, except for this impact category WAAM appears to be a better alternative than the conventional manufacturing route.
5. Expected changes on the environmental performance by upscaling and optimizing a metal additive manufacturing (MAM)
Considering sustainable development of new and future technologies is essential. However, future applications of different technologies involve many uncertainties both related to the markets, to technology upscaling, etc. [32]. Assessing this with LCA in a so-called prospective LCA, entails issues of unknown future applications (i.e., aim, functionality, system boundaries), industrial scales (compared to lab scale), and large inventory data gaps comprising issues of data availability and data quality, altogether increasing the level of LCA uncertainty [32, 33]. However, it is important to address those aspects during an initial stage of the technology development to understand potential future environmental performance improvements of a technology [33, 34]. [32] reviewed 44 case studies of prospective LCA and developed a framework for facing the challenges mentioned and [35] developed a prospective assessment of the environmental impact of incremental sheet forming (ISF). [36] took an approach mainly looking at the expected changes of environmental performance by upscaling and process optimization. Taking this approach, an individual metal additive manufacturing (MAM) system from lab- to full-scale production was qualitatively analyzed in order to anticipate the potential influence of upscaling. In particular, it was assumed that the upscaled system would be fully optimized, automated, and continuously operating for large metal objects production with a low annual volume production demand. Below is presented a more detailed list of assumptions:
Medium or large objects;
Same technology different products (thanks to build-in flexibility);
Produce many different objects every year;
All types of metal additive manufacturing techniques are considered (e.g., directed energy deposition [DED], sheet lamination, electron beam melting [EBM], laser powder bed fusion [LPBF], binder jetting);
Fully optimized;
Industrial scale;
Automated;
Low volume production demand of the same part per year;
Not improvement in manufacturing energy efficiency;
Not considered downtime;
Continuously working system;
For this qualitative forecast, upscaling factors and rule-of-thumb were discussed with MAM process operation experts, and retrieved from the literature [24, 37, 38, 39, 40, 41]. Table 1 reports the expected changes in environmental performance by upscaling a metal additive manufacturing system from small to full scale.
Model parameter
Forecasted change when up-scaled from lab-scale to full-scale
Differences induced by upscaling and expected consequences on environmental performance
Factor
Motivation
Process capability (yield)
Increase by a factor 2 to 10 (considering kg product/h)
For some laser-based directed energy deposition (DED) and laser powder bed fusion (LPBF) techniques, it has been already possible to increase massively the deposition rate in laboratory with industrial scale manufacturing systems (e.g., for WAAM from 0.5 to 5 kg/h, for EBM from 2 to 20 kg/h)
Larger need of metals per year is forecasted to enlarge the impacts associated to the MAM system on climate change, resource depletion, and various toxicity- and non-toxicity-related impact categories due to the need for more material. Of course, this will also depend on what the products substitute
Material input for construction of MAM full-scale system for unit part produced
Decrease by a factor 1 to 10 (in relation to process outputs)
The factor would depend on the upscaling approach considered since it is possible to have: a) increased electricity use and deposition rate, or b) higher number of 3D printing systems. In the former case, the amount of necessary material for MAM equipment for unit part produced would decrease proportionally to the deposition rate (see process capability above). However, this is not applicable to all MAM techniques (e.g., LPBF). In the second case, the needed material input for construction would be the same as for now. In general, since an increase in process capability is forecasted, the material for capital goods per unit part produced will likely decrease proportionally with the deposition rate
Smaller need of metals and crude oil per unit part produced is forecasted to decrease the impacts on climate change, resource depletion, and various toxicity- and non-toxicity-related impact categories due to the less need for fabrication of supplementary parts and equipment
Material input for construction of post-treatment MAM full-scale system for unit part produced
Decrease by a factor 1 to 5 (in relation to process output)
The factor depends on several aspects: a) heat treatment temperature, and b) geometry of the part(s). There is a difference if the heat treatment is done locally or the part does not fit into an available furnace and the producer uses a portable heat treatment service. Moreover, once MAM will be optimized the need for subtractive manufacturing processes (e.g., milling, grinding) will be decreased
Electricity use for unit part produced
Decrease by a factor 2 to 10 (in relation to process output)
The manufacturing processes are assumed to not have any improvement in energy efficiency. However, the electricity use is dependent on the process capability, thus if the latter is improved for the production of one part, then also the electricity use can be reduced to the same extent
Climate change, ionizing radiation, and toxic-impacts on humans and freshwater ecosystems are projected to stay the same due to reduced emissions of fossil CO2, NOx, SO2, and metals stemming from lower electricity use for unit part
Process gas use for unit part produced
Decrease by a factor 2 to 10 (in relation to process output)
The increase in efficiency of process gas use is related to a) impurities differences in the part, and b) process capability. For instance, if it is known how to manipulate well the production of a specific product, the process gas use could be reduced up to 40%
Climate change, fossil depletion, water consumption and toxic-impacts on humans and freshwater ecosystems are projected to be minimized due to lower emissions of fossil CO2, NOx, SO2, and vapor due to reduced process gas use for unit part
Metal scrap for unit part produced
Increase by a factor 1.5 (base plate excluded and in relation to process output)
The bigger the part, the more a distortion would have an impact on the tolerance of the as-built part. For manufacturing one part and increase its tolerance, the amount of material which has to be grinded off later (more scrap) will also increase. Except the surface can remain in the as-built state, then we get an increased safety by thicker parts. Moreover, if more than one part is manufactured, the tolerance would be optimized and minimized. Since a scenario with large product is assumed, a factor of 1.5 in relation to the part weight was assumed. This means if the part weight is doubled, the scrap is three times as high
Reduced metals scraps production per unit of manufacturing system is forecasted to relatively decrease the impacts on climate change, resource depletion, and various toxicity- and non-toxicity-related impact categories due to the need for less waste treatment processes and less need for raw materials
Fumes and powder dispersion during manufacturing for unit part produced
No change (in relation to process output)
Generally, it is expected that machines would be encased, as standardized machines in the market, with the possibility of an effective fume extraction. Thus, the emission outside the machine would be minimized (near zero)
Fumes and powder dispersion during manufacturing for unit part is forecasted to not change so the impacts would be on climate change, resource depletion, and various toxicity- and non-toxicity-related impact categories, also thanks to special working garments and an improved filtration system (e.g., hoods)
Table 1.
Expected changes established by upscaling from lab-scale to full-scale MAM system and likely consequences of its environmental performance.
Overall, upscaling is expected to result in a reduction of environmental impact of MAM per unit part produced. The larger manufacturing system configuration will result in an increased material consumption, but this is forecasted to not significantly affect the environmental performance. The main benefits from MAM system upscaling stem primarily from an improved process capability. This would influence positively also the input/consumption of electricity and process gas use for unit part produced. Moreover, it is expected that an advancement of MAM system optimization can achieve a lower production of metal scrap and an improved handling of welding fumes or powder dispersion. On the whole, this could reduce the current environmental impact associated with a MAM system from 1.5 to 10 times. The numbers would depend on several factors (e.g., MAM technology, product shape, etc.). Overall, these numbers should be considered carefully, also because they were obtained through a qualitative investigation of future expected in environmental performance due to improvement of the technology, and there are several underlying assumptions within this analysis and uncertainties were not quantified.
6. Future environmentally sustainable applications for multi-material additive manufacturing for metals
On one hand, when planning product fabrication for additive manufacturing, often the focus is on design shape complexity (e.g., “solid-to-cavity ratio” [1, 19, 42, 43]), and lightweight and strength (e.g., “strength-to-weight ratio” [44]). In particular, there are studies illustrating the potential of multi-material additive manufacturing to enhance product strengths and prolong its lifetime [20, 21] which are beneficial aspects in relation to circular economy. On the other hand, there is a general lack of argument relating to how product recyclability design can or should be applied to multi-material additive manufacturing. This topic has been discussed relative to the metal manufacturing sector, life cycle assessment (LCA) and circular economy but only in generic terms addressing the complexity of production of clean recycled materials [45, 46, 47, 48, 49, 50]. Currently, there is a decrease in quality of metal stocks as a result of nowadays use of complex alloys and the recovery practice of those metals. That happens even though the technology and contemporary infrastructures would allow to undertake this challenge and maintain the quality. Thus, to bring forward industrial ecology concepts into businesses, the metallurgic constraints of metals recovery should complement policy development and product design taking into account costs feasibility aspects [48, 49, 50]. Indeed, all systems and technologies starting from product design to metal recovery are interconnected and could support in addressing this challenging task [47] but there is a need for tools or frameworks to understand and help with this task. Particularly interesting in this context is the concept of the metal wheel introduced by [48]. The first illustrates metals linkages in geology, showing the capacity of current metallurgical technologies for the recovery of trace elements in their (primary or secondary) feed.
Here we applied this framework to investigate design for recyclability and resource efficiency relative to alloys used for multi-material Wire-arc Additive Manufacturing (WAAM). Indeed, being aware of those during product design and process optimization can support the prevention of negligent recycling and develop product design for recycling and resource efficiency [49].
Figure 8 illustrates the Metal Wheel for the small holding ring (described earlier) made of three different types of wire: high-strength alloy and hot rolled steel, stainless steel martensitic, and bronze.
Figure 8.
Metal Wheel for a small holding ring adapted from [49]. In the small circles are represented with different colors, the trace compounds. When they are green, they can be mainly recovered; in yellow, they can be recovered in the alloys or lost if they are directed to incorrect stream or scrap; in red, the elements are lost as they are not compatible with carrier metal.
Here it is clear that the majority of elements present high chances to be lost if they end up in the wrong scrap stream (i.e., yellow circles). However, in stainless steel, Nickel and Chromium would be mainly recovered, and in bronze both zinc and lead would be primarily retrieved.
Figure 9 shows the Metal Wheel applied to investigate potential for design for recycling and resource-efficiency for a bathtub mold (described above) produced with two wires: high-alloyed ferritic martensitic stainless steel, high strength alloy, and hot rolled steel.
Figure 9.
Metal Wheel for a bathtub mold adapted from [50]. In the small circles are represented with different colors, the trace compounds. When they are green, they can be mainly recovered; in yellow, they can be recovered in the alloys or lost if they are directed to incorrect stream or scrap; in red, the elements are lost as they are not compatible with carrier metal.
Figure 9 shows that also in this case, only a few elements can be recovered (see Nickel and Chromium in stainless steel alloy). The others show high probability of being dispersed if they finish in the wrong scrap stream. On the other hand, in general, the fact that multiple different alloys are melted together adds complexity to the efficiency of the named trace elements recovery.
The manufacturing sector, especially metal, as a significant contributor to greenhouse gas emissions, needs to adopt a lower emission strategy. Additionally, there is an increased demand for circular economy strategies aiming to retain the value of the material for as long as possible meaning either reducing use, prolonging lifetime, or optimizing recycling of materials. This chapter aims to address the potential future sustainability of metal additive manufacturing. At the same time, there are common technical challenges, such as the possibility of obtaining undesired behavior of parts originated by mixing several materials, which might not be compatible, in a single component due to lack of standardization.
Most MAM technologies are currently still at small scale or lab scale. Through a qualitative evaluation, upscaling to industrial scale will probably reduce the environmental impacts of the technology. The results from the environmental life cycle assessments already show an environmental benefit of multi-material additive manufacturing in relation to conventional manufacturing processes. The main impacts from MAM originate from metal production, which is why there is also significant benefit to harvesting by recycling the metals after the manufactured product’s end. In particular, recycling multi-material objects may constitute challenges with reduced quality of metal stocks if individual alloying elements are dissipated due to not being recoverable from the recycled metal base. The compatibility and recoverability should be considered early in the design stage.
The authors thank for helpful comments to Sami Kara, Martin Schmitz Niederau, and Michael Zwicky Hauschild for providing useful insights on assumptions and principles for the upscaling of a metal additive manufacturing system. This work was supported by the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 862017. This publication reflects only the author’s view and the Commission is not responsible for any use that may be made of the information it contains.
Tables A2 and A3 illustrate the characterized midpoint results with ReCiPe2016 (H) midpoint for holding ring and bathtub mold.
Impact category
Unit
Casting
WAAM
Global warming
kg CO2 eq
5.94E+03
3.07E+03
Stratospheric ozone depletion
kg CFC11 eq
2.16E-03
1.43E-03
Ionizing radiation
kBq Co-60 eq
5.50E+02
1.06E+03
Ozone formation, Human health
kg NOx eq
1.33E+01
6.23E+00
Fine particulate matter formation
kg PM2.5 eq
1.55E+01
5.61E+00
Ozone formation, terrestrial ecosystems
kg NOx eq
1.36E+01
6.37E+00
Terrestrial acidification
kg SO2 eq
2.16E+01
1.13E+01
Freshwater eutrophication
kg P eq
6.51E+00
3.02E+00
Marine eutrophication
kg N eq
7.85E-01
2.73E-01
Terrestrial ecotoxicity
kg 1,4-DCB
1.06E+05
2.27E+04
Freshwater ecotoxicity
kg 1,4-DCB
5.51E+02
2.41E+02
Marine ecotoxicity
kg 1,4-DCB
7.68E+02
3.24E+02
Human carcinogenic toxicity
kg 1,4-DCB
4.71E+03
1.34E+03
Human non-carcinogenic toxicity
kg 1,4-DCB
1.20E+04
5.18E+03
Land use
m2a crop eq
3.65E+02
1.12E+02
Mineral resource scarcity
kg Cu eq
3.83E+02
5.73E+01
Fossil resource scarcity
kg oil eq
1.56E+03
8.12E+02
Water consumption
m3
8.77E+01
6.19E+01
Table A2.
Characterized midpoint results with ReCiPe2016 (H) midpoint for holding ring.
Impact category
Unit
CM
WAAM
Global warming
kg CO2 eq
3.40E+04
5.48E+03
Stratospheric ozone depletion
kg CFC11 eq
1.95E-02
3.22E-03
Ionizing radiation
kBq Co-60 eq
3.33E+03
5.13E+03
Ozone formation, human health
kg NOx eq
1.22E+02
9.39E+00
Fine particulate matter formation
kg PM2.5 eq
1.20E+02
5.95E+00
Ozone formation, terrestrial ecosystems
kg NOx eq
1.23E+02
9.55E+00
Terrestrial acidification
kg SO2 eq
3.34E+02
1.45E+01
Freshwater eutrophication
kg P eq
3.52E+01
4.04E+00
Marine eutrophication
kg N eq
2.03E+00
3.82E-01
Terrestrial ecotoxicity
kg 1,4-DCB
1.47E+06
1.09E+04
Freshwater ecotoxicity
kg 1,4-DCB
4.63E+04
3.38E+02
Marine ecotoxicity
kg 1,4-DCB
5.67E+04
4.38E+02
Human carcinogenic toxicity
kg 1,4-DCB
5.16E+03
7.97E+02
Human non-carcinogenic toxicity
kg 1,4-DCB
2.64E+05
6.26E+03
Land use
m2a crop eq
1.73E+03
1.64E+02
Mineral resource scarcity
kg Cu eq
1.30E+03
2.13E+01
Fossil resource scarcity
kg oil eq
8.44E+03
1.57E+03
Water consumption
m3
4.24E+02
1.20E+02
Table A3.
Characterized midpoint results with ReCiPe2016 (H) midpoint for bathtub mold.
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Written By
Valentina Pusateri, Constantinos Goulas and Stig Irving Olsen
Submitted: 12 December 2022Reviewed: 03 January 2023Published: 10 February 2023
Open access peer-reviewed chapter
