Technical Challenges and Future Environmentally Sustainable Applications for Multi-Material Additive Manufacturing for Metals

*Valentina Pusateri, Constantinos Goulas and Stig Irving Olsen*

## **Abstract**

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 decisionmaking 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.

**Keywords:** functionally gradient materials, topology optimization, alloys, scale-up, sustainability

## **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 [5–7]. 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, 11–17]. 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 *Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*

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]).


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:


## **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-bycase 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.

*Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*

#### **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 stateof-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 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].

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

*Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*

#### **Figure 2.** *Schematic representation of life cycle assessment (LCA).*

#### **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.*

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

#### **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.*

*Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*

#### **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.*

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].

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.

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].

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.

**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.*

## **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:

*Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*


For this qualitative forecast, upscaling factors and rule-of-thumb were discussed with MAM process operation experts, and retrieved from the literature [24, 37–41]. **Table 1** reports the expected changes in environmental performance by upscaling a metal additive manufacturing system from small to full scale.

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




*Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*

#### **Table 1.**

*Expected changes established by upscaling from lab-scale to full-scale MAM system and likely consequences of its environmental performance.*

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 [45–50]. 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 [48–50]. 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.

*Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*

#### **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** 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

**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.*

general, the fact that multiple different alloys are melted together adds complexity to the efficiency of the named trace elements recovery.

## **7. Conclusions**

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

*Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*

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.

## **Acknowledgements**

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.

## **Conflict of interest**

The authors declare no conflict of interest.

## **A. Appendix**

## **A1. Abbreviations**



#### **Table A1.**

*List of abbreviations used in the book chapter.*

## **A2. Life cycle assessment—results**

**Tables A2** and **A3** illustrate the characterized midpoint results with ReCiPe2016 (H) midpoint for holding ring and bathtub mold.


*Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*


#### **Table A2.**

*Characterized midpoint results with ReCiPe2016 (H) midpoint for holding ring.*


#### **Table A3.**

*Characterized midpoint results with ReCiPe2016 (H) midpoint for bathtub mold.*

## **Author details**

Valentina Pusateri<sup>1</sup> \*, Constantinos Goulas<sup>2</sup> and Stig Irving Olsen<sup>1</sup> \*

1 Division for Quantitative Sustainability Assessment (QSA), Department of Environmental and Resource Engineering, Technical University of Denmark, Kgs. Lyngby, Denmark

2 Faculty of Engineering Technology, Department of Design Production and Management, University of Twente, Enschede, The Netherlands

\*Address all correspondence to: valpu@dtu.dk and siol@dtu.dk

© 2023 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*

## **References**

[1] Priarone PC, Lunetto V, Atzeni E, Salmi A. Laser powder bed fusion (L-PBF) additive manufacturing: On the correlation between design choices and process sustainability. Procedia CIRP. 2018;**78**:85-90. DOI: 10.1016/j. procir.2018.09.058

[2] Krishnan M, Samandari H, Woetzel J, Smit S, Pacthod D, Pinner D, et al. The Net-Zero Transition - What it Would Cost, What it Could Bring. McKinsey Global Institute; 2022. [cited 2022 Nov 02]. Available from: https://policyc ommons.net/artifacts/2213603/the-netzero-transition/2970547/. CID: 20.500.12592/0d6rdj

[3] World Steel Association. World steel in figures [Internet]. 2020 [cited 2021 Dec 23]. Available from: https://www. worldsteel.org/en/dam/jcr:f7982217 cfde-4fdc-8ba0-795ed807f513/World% 2520Steel%2520in%2520Figures% 25202020i.pdf

[4] IEA. Industry – topics [cited 2022 Nov 29]. Available from: https://www. iea.org/topics/industry

[5] Henckens MLCM, Driessen PPJ, Worrell E. Metal scarcity and sustainability, analyzing the necessity to reduce the extraction of scarce metals. Resources, Conservation and Recycling. 2014;**93**:1-8

[6] Yokoi R, Nansai K, Hatayama H, Motoshita M. Significance of countryspecific context in metal scarcity assessment from a perspective of shortterm mining capacity. Resources, Conservation and Recycling. 2021;**166**:1- 9. DOI: 10.1016/j.resconrec.2020.105305

[7] Graedel TE, Erdmann L. Will metal scarcity impede routine industrial use? MRS Bulletin. 2012;**37**(4):325-331

[8] Owsianiak M, van Oers L, Drielsma J, Laurent A, Hauschild MZ. Identification of dissipative emissions for improved assessment of metal resources in life cycle assessment. Journal of Industrial Ecology. 2022; **26**(2):406-420

[9] Drielsma JA, Russell-Vaccari AJ, Drnek T, Brady T, Weihed P, Mistry M, et al. Mineral resources in life cycle impact assessment—defining the path forward. International Journal of Life Cycle Assessment. 2016;**21**(1): 85-105

[10] Bobba S, Carrara S, Huisman J, Mathieux F, Pavel C. Critical Raw Materials for Strategic Technologies and Sectors in the EU. Publications Office of the European Union; 2020. [cited 2022 Nov 15]. Available from: https://ec. europa.eu/docsroom/documents/42881. ISBN: 978-92-76-15336-8

[11] Priarone PC, Ingarao G, Lorenzo R, Settineri L. Influence of material-related aspects of additive and subtractive Ti-6Al-4V manufacturing on energy demand and carbon dioxide emissions. Journal of Industrial Ecology. 2016;**21**:191-202. DOI: 10.1111/ jiec.12523

[12] Böckin D, Tillman AM. Environmental assessment of additive manufacturing in the automotive industry. Journal of Cleaner Production. 2019;**226**:977-987

[13] Tang Y, Mak K, Zhao YF. A framework to reduce product environmental impact through design optimization for additive manufacturing. Journal of Cleaner Production. 2016;**20**(137): 1560-1572

[14] Bekker ACM, Verlinden JC. Life cycle assessment of wire + arc additive manufacturing compared to green sand casting and CNC milling in stainless steel. Journal of Cleaner Production. 2018;**177**:438-447. DOI: 10.1016/j. jclepro.2017.12.148

[15] Gao C, Wolff S, Wang S. Ecofriendly additive manufacturing of metals: Energy efficiency and life cycle analysis. Journal of Manufacturing Systems. 2021;**60**:459-472

[16] Yusuf SM, Cutler S, Gao N. The impact of metal additive manufacturing on the aerospace industry. Metals. 2019; **9**:1-35. DOI: 10.3390/met9121286

[17] Monteiro H, Carmona-Aparicio G, Lei I, Despeisse M. Energy and material efficiency strategies enabled by metal additive manufacturing – A review for the aeronautic and aerospace sectors. Energy Reports. 2022; **8**:298-305

[18] Grade2xl Home. [cited 2022 Nov 29]. Available from: https://www. grade2xl.eu/

[19] Priarone PC, Campatelli G, Montevecchi F, Venturini G, Settineri L. A modelling framework for comparing the environmental and economic performance of WAAM-based integrated manufacturing and machining. CIRP Annals. 2019;**68**(1): 37-40. DOI: 10.1016/j.cirp.2019.04.005

[20] Bandyopadhyay A, Heer B. Additive manufacturing of multimaterial structures. Materials Science & Engineering R: Reports. 2018; **129**:1-16

[21] Treutler K, Kamper S, Leicher M, Bick T, Wesling V. Multi-material design in welding arc additive manufacturing.

Metals (Basel). 2019;**9**(7):1-14. DOI: 10.3390/met9070809

[22] Mehrpouya M, Tuma D, Vaneker T, Afrasiabi M, Bambach M, Gibson I. Multimaterial powder bed fusion techniques. Rapid Prototyping Journal. 2022;**28**:1-19

[23] Aerosint Multi-material 3D printing. [cited 2022 Dec 7]. Available from: https://aerosint.com/

[24] Meltio Wire-laser metal 3D printing: Multi-metal laser metal deposition [cited 2022 Dec 12]. Available from: https:// meltio3d.com/

[25] Feenstra DR, Banerjee R, Fraser HL, Huang A, Molotnikov A, Birbilis N. Critical review of the state of the art in multi-material fabrication via directed energy deposition. Current Opinion in Solid State & Materials Science. 2021; **25**(4):1-12. DOI: 10.1016/j.cossms. 2021.100924

[26] Wei C, Li L. Recent progress and scientific challenges in multi-material additive manufacturing via laser-based powder bed fusion. Virtual and Physical Prototyping. 2021;**16**:347-371

[27] Horn M, Prestel L, Schmitt M, Binder M, Schlick G, Seidel C, et al. Multi-Material additive manufacturing – recycling of binary metal powder mixtures by screening. Procedia CIRP. 2020;**93**:50-55. DOI: 10.1016/j. procir.2020.04.098

[28] Hauschild MZ, Rosenbaum RK, Olsen SI. Life Cycle Assessment: Theory and Practice. Springer; 2018. DOI: 10.1007/978-3-319-56475-3

[29] Iso.org. n.d. ISO 14044:2006(en) Environmental management—Life cycle assessment—Requirements and guidelines. [cited 2022 Oct 02]. Available *Technical Challenges and Future Environmentally Sustainable Applications for Multi… DOI: http://dx.doi.org/10.5772/intechopen.109788*

from: https://www.iso.org/standard/ 38498.html

[30] Iso.org. n.d. ISO 14040:2006(en) Environmental management—Life cycle assessment—Requirements and guidelines. [cited 2022 Oct 02]. Available from: https://www.iso.org/standard/ 37456.html

[31] International Reference Life Cycle Data System (ILCD) Handbook - General guide for Life Cycle Assessment - Provisions and Action Steps. EUR 24378 EN. Luxembourg (Luxembourg): Publications Office of the European Union; 2010. JRC58190. DOI: 10.2788/94987

[32] Thonemann N, Schulte A, Maga D. How to conduct prospective life cycle assessment for emerging technologies? A systematic review and methodological guidance. Sustainability. 2020;**12**(3):1-23. DOI: 10.3390/ su12031192

[33] Hetherington AC, Borrion AL, Griffiths OG, McManus MC. Use of LCA as a development tool within early research: Challenges and issues across different sectors. International Journal of Life Cycle Assessment. 2014;**19**(1): 130-143

[34] Wender BA, Foley RW, Prado-Lopez V, Ravikumar D, Eisenberg DA, Hottle TA, et al. Illustrating anticipatory life cycle assessment for emerging photovoltaic technologies. Environmental Science & Technology. 2014;**48**(18):10531-10538

[35] Cooper DR, Gutowski TG. Prospective environmental analyses of emerging technology: A critique, a proposed methodology, and a case study on incremental sheet forming. Journal of Industrial Ecology. 2020; **24**(1):38-51

[36] Owsianiak M, Ryberg MW, Renz M, Hitzl M, Hauschild MZ. Environmental performance of hydrothermal carbonization of four wet biomass waste streams at industry-relevant scales. ACS Sustainable Chemistry & Engineering. 2016;**4**(12):6783-6791

[37] Liao J, Cooper DR. The environmental impacts of metal powder bed additive manufacturing. Journal of Manufacturing Science and Engineering. Transactions of the ASME. 2021;**143**(3): 1-11. DOI: 10.1115/1.4048435

[38] Ahn DG. Directed energy deposition (DED) process: State of the art. International Journal of Precision Engineering and Manufacturing – Green Technology. 2021;**8**:703-742. DOI: 10.1007/s40684-020-00302-7

[39] 3D Printing Industry. Fraunhofer opens "World's largest SLM facility". 2022 [cited 2022 Feb 18]. Available from: https://3dprintingindustry.com/news/ fraunhofer-opens-worlds-largest-slmfacility-aachen-germany-115961/

[40] Gibson I, Rosen DW, Stucker B. Additive Manufacturing Technologies. Springer; 2010. DOI: 10.1007/978-3- 030-56127-7

[41] Li G, Odum K, Yau C, Soshi M, Yamazaki K. High productivity fluence based control of directed energy deposition (DED) part geometry. Journal of Manufacturing Processes. 2021;**65**: 407-417

[42] Ingarao G, Priarone PC, Deng Y, Paraskevas D. Environmental modelling of aluminium based components manufacturing routes: Additive manufacturing versus machining versus forming. Journal of Cleaner Production 2018;**176**:261–275. Available from: https://doi.org/10.1016/j.jclepro.2017. 12.115

[43] Morrow WR, Qi H, Kim I, Mazumder J, Skerlos SJ. Environmental aspects of laser-based and conventional tool and die manufacturing. Journal of Cleaner Production. 2007;**15**(10): 932-943

[44] Ford S, Despeisse M. Additive manufacturing and sustainability: An exploratory study of the advantages and challenges. Journal of Cleaner Production. 2016;**137**:1573-1587

[45] Verhoef EV, Reuter MA, Scholte A, Dijkema GPJ. A dynamic LCA model for assessing the impact of lead free solder. In: Proceedings of the TMS Yazawa International Symposium: "Metallurgical and Materials Processing: Principles and Technologies". Vol. 2. 2003. pp. 1-19

[46] Reuter MA, Kojo IV. Challenges of metals recycling. Materia. 2012;**2**:50-57

[47] Reuter MA, van Schaik A. Strategic metal recycling : Adaptive metallurgical processing infrastructure and technology are essential for a Circular Economy. Annales des Mines - Responsabilité et environnement. 2016;**82**(2):62-66

[48] Verhoef EV, GPJ D, Reuter MA. Process knowledge, system dynamics, and metal ecology. Journal of Industrial Ecology. 2004;**8**:23-43

[49] Reuter MA, van Schaik A, Gutzmer J, Bartie N, Abadías-Llamas A. Challenges of the circular economy: A material, metallurgical, and product design perspective. 2019. DOI: 10.1146/annurev-matsci-070218-

[50] van Schaik A, Reuter MA. Materialcentric (aluminum and copper) and product-centric (cars, WEEE, TV, lamps, batteries, catalysts) recycling and DfR rules. In: Handbook of Recycling: State-of-the-Art for Practitioners, Analysts, and Scientists. Chapter 22.

Elsevier; 2014. pp. 307-378. DOI: 10.1016/B978-0-12-396459-5.00022-2 Section 2
