Content uploaded by Shiren Wang
Author content
All content in this area was uploaded by Shiren Wang on Jul 20, 2021
Content may be subject to copyright.
Review
Eco-friendly additive manufacturing of metals: Energy
efficiency and life cycle analysis
Abstract
Additive manufacturing (AM) of metal materials has attracted widespread attention and is shifting the
conventional manufacturing landscape toward free-form processes. With increasing concerns about global
sustainability, eco-consideration is highly encouraged to be integrated into AM processes. This review
provides a comprehensive and timely discussion on the life cycle of metal parts fabricated through AM. The
energy consumption required for raw metal material extraction and subsequent AM processes is analyzed.
The eco-design and energy efficiency of metal AM are evaluated to reveal the role of manufacturing
methods, machine subsystems, and post-processing modes in the eco-integration. AM-induced supply chain
management, utilization, and recycling of the printed metal structure are also analyzed. Finally, a
comprehensive life cycle assessment regarding the environmental, social, and economic impacts of metal
AM is also addressed. Future directions of AM are also briefly discussed to provide insight and vision on
the emerging field of additive eco-manufacturing.
1
Introduction
1.1
Eco-Friendly additive manufacturing
Experts in climatology have expressed increasing concerns regarding the increasing rate and impact of global climate
change since the 20th century. The emission of greenhouse gases, primarily carbon dioxide and methane from fossil
fuels, further accelerates global warming [1
–
4]. Eco-friendly manufacturing has been considered an effective strategy to
diminish greenhouse gas emissions toward global sustainability. Eco-friendly manufacturing, also referred to as eco-
manufacturing, is critical for sustainable growth and development of the global industry [4
–
6]. Current industrial
manufacturing processes comprise 15 % of global energy consumption and 35
–
40% of global material consumption [7
]. Any reduction in the consumption of energy or resources in the industrial manufacturing sector will make progress
toward global sustainability. Increasing concerns of climate change, scarcity of natural resources, and resultant
Q1
Chongjie Gao, Sarah Wolff, Shiren Wang⁎ S.wang@tamu.edu
Department of Industrial and Systems Engineering, Department of Materials Science and Engineering, Texas A&M
University, College Station, TX, 77845, United States
⁎
Corresponding author.
i
The corrections made in this section will be reviewed and approved by a journal production editor.
Q2
Q3
Q4
Keywords:
Eco-manufacturing,Additive manufacturing,Energy-consumption,Metal structure
ecological impacts have boosted the popularity of eco-friendly manufacturing and paved a new way for future
industrial development.
Additive manufacturing (AM) is the process of fabricating three-dimensional structures from digital models through
computer-guided layer-by-layer deposition [8
–
10]. Several AM technologies for metallic structures have been
developed, including directed energy deposition (DED), powder bed fusion (PBF), binder jetting 3D printing (BJP),
bound metal deposition (BMD), and fused filament fabrication (FFF) [10
–
16]. Table 1 summarizes the feedstock
materials and the energy resources for metal AM.
PBF-based AM especially is receiving significant attention from the industry due to its high flexibility in material
selection [1]. Specifically, a high-energy beam is applied to selectively sinter a pre-deposited metal powder layer-by-
layer to obtain a three-dimensional structure [17
–
19]. Both laser and electron beams have been used as the energy
source for the sintering process in PBF [11,18]. The common PBF methods include selective laser sintering (SLS),
selective laser melting (SLM), and electric beam melting (EBM) [20]. Contrary to PBF, DED fabricates metal parts
through the melting or fusing of powders or wires as they are deposited onto the print bed [21,22]. The distinct
difference between PBF and DED is that the metal powder is deposited as needed in DED, however, a new layer of
metal powder has to be spread after one layer printing finished [11]. Compared with PBF, DED can achieve a higher
deposition rate due to its increased layer thickness [23]. Similar to powder-based DED, wire-based DED processes
feed metallic wires to the molten pool as a replacement of metallic powders [24]. The energy source to melt the metal
could be a laser, electric arc, electron beam, or plasma arc [25
–
27]. When the energy source utilized is an electric or
plasma arc, it is called wire and arc additive manufacturing (WAAM) [28]. BJP is another common process to
manufacture metal components using a polymer binding agent with metal powders to form a green structure layer-by-
layer, where the green part is then transferred to a furnace for sintering to remove the polymer binder and achieve a
dense form [22,29,30]. BMD, also called bound powder extrusion, is gaining popularity as a novel metal AM
technology based on fused extrusion deposition [13,15]. Particularly, metal rods are made from polymer-bound metal
powders and used as feedstock materials. Such feedstock materials avoid those loose metal powders and thus eliminate
the inherent risks associated with handling fine metallic particles [31
–
33]. These rods are heated and extruded to build
the green part layer-by-layer, similar to the fused- deposition model of traditional polymer AM. Once the printing
process is completed, the green part is transferred to a rinsing machine to remove the polymer binder and then sintered
to obtain a final product. Several metal materials have been developed commercially for this process, such as stainless
steel, tool steel, Inconel, copper, and titanium [10]. FFF is also a fused deposition process and the only difference
between FFF, and BMD is the form of the feedstock materials as the BMD process applies the polymer bounded metal
power rod and the FFF uses the filament. Because of the similar composition of the feedstock material and the working
mechanism of the printing process, these two processes would be analyzed together.
1.2
Life cycle analysis of metal additive manufacturing
alt-text: Table 1
Table 1
Summary of metal-based additive manufacturing materials.
Manufacturing methods Feedstock ma terial s Energy source
PBF Powder Laser, Electron beam
DED Powder, Wire Laser, Electron beam, Plasma
BJP Powder, Polymer Bin der Furnace
BMD/FFF
Rod of polymer-bound powder (BMD)
Furnace
Filament of polymer-bo und Power (FFF)
i
The table layout displayed in this section is not how it will appear in the final version. The representation below is solely
purposed for providing corrections to the table. To preview the actual presentation of the table, please view the Proof.
The metal AM is expanding in the industry while their societal impacts, such as safety, health, and employment
consequences, have not been fully understood [1,34
–
37]. It is well-known that sustainability could significantly impact
the environment, society, and economics, as shown in Fig. 1(A). Meanwhile, environment, society, and economics can
also interact with each other [38]. Life cycle assessment (LCA) is an effective tool for sustainability analysis and could
help to evaluate how the metal AM contributes to global sustainability [1,39,40]. Different from the conventional
methods that pay close attention to a single step of the product life, the LCA methodology evaluates all steps of product
life in terms of inputs and outputs, such as energy and raw material consumption, atmospheric emissions, soil, and
water waste, and land occupation [10,41
–
43]. The product life stages based on the LCA of metal include raw material
extraction, design, manufacturing, and distribution, use and maintenance, and recycle and waste management.
The life cycle of metal AM begins from the extraction of raw material and power production, as shown in Fig. 2. The
energy input and resultant environmental output, such as the atmosphere emission and waste, would be analyzed first.
Subsequently, those raw materials would be moved onto the design and manufacturing stage, where interactions
between the process and environments would also be studied. Similarly, the input and outputs for as-fabricated metal
parts would be studied for the following stages like distribution and storage, utilization, and end life of the printed
products. Categorizing metal AM into these elementary stages helps to quantify the contribution of each step to the total
environmental impact.
In this review, as shown in Fig. 3, we follow the elementary stages of metal AM to assess the required input and the
resultant environmental impacts from the energy consumption perspective. In the first section, the methods of raw
material extraction and preparation are discussed, and the energy consumption of wire and powder mining and
feedstock manufacturing are compared. Then the concept of eco-design for metal parts is introduced and the
methodologies of eco-design are discussed. In the manufacturing process, the energy consumption of the different
fabrication processes, different machine subsystems and different manufacturing modes are evaluated. Meanwhile, the
efficiency of wire-based and powder-based metal AM are studied. Subsequently, the energy efficiency and
environmental impacts of metal printed products are articulated. Finally, the whole framework of the LCA and the
alt-text: Fig. 1
Fig. 1
The sustainab ility relationship between the environment, society, and the economy [1].
alt-text: Fig. 2
Fig. 2
Diagram of the elementary stages of the addi tive manufactu ring of metal parts.
environmental, social and economic impacts are addressed. The prospects of integrating eco-consideration into future
metal AM are also discussed.
2
Energy efficiency of metal additive manufacturing
2.1
Metal material extraction and metal powder/wire feedstock manufacturing
Metal powders can be produced by a range of techniques including solid-state reduction, electrolysis, atomization,
chemical processes and mechanical comminution [44]. Among these methods, atomization and solid-state reduction are
the most popular methods. Metal atomization powder production typically includes three steps: melting the raw
materials, atomizing the intermediate products, and solidifying the resulting metallic powders [45
–
47]. The average
energy consumption for the atomization process depends on the material applied, as shown in Table 2 [11].
In solid-state reduction, the mined ore is selected and crushed at the primary stage, and then blended with carbon. The
resultant mixture goes through a continuous high-temperature furnace where a reduction reaction occurs to remove the
non-metallic section such as carbon and oxygen from the raw material. The major composition of the resulting product
is metal, which is further crushed and separated from all remaining non-metallic materials, resulting in pure metal
powders [1,48,49]. The oxide reduction and water atomization processes may harm the powder quality, such as particle
geometry, morphology, and chemical purity. In the metal AM process, the high-grade metal powder (above 99.5 %
purity) is typically used although it is not commonly found the atomized powder for pressing and sintering applications.
As a result, the combination of inert gas atomization and melting under vacuum is a promising production process for
alt-text: Fig. 3
Fig. 3
Schematic of the metal AM life cycle analysis based on inputs and outputs.
alt-text: Table 2
Table 2
Energy consumpti on of the gas atomization process for metal powder production [11].
Materia l Energy consumption (MJ/kg)
Ti-6Al-4V 7-31.7 (dependen t on the process parameters)
AlSi10 Mg 8.1
Nistelle 625 55.6
Tool steel 1.0
i
The table layout displayed in this section is not how it will appear in the final version. The representation below is solely
purposed for providing corrections to the table. To preview the actual presentation of the table, please view the Proof.
metal powders toward AM [50]. Although gas atomization is the standard method for powder production, it is
important to note that gas atomization is very expensive and time-consuming with low utilization efficiency of both
energy and resources [51]. The embodied energy of a material is commonly used to represent the energy consumption
during the production of metal powders, including extracting and processing of raw metal materials, fabrication,
transportation, and distribution. The embodied energy of metal powder materials is shown in Table 3. Copper and Zinc
powder show a low value of embodied energy. For metal wire production, the typical approach is to draw a wire from
a rod or bar through the reduction of a metal
’
s cross-section under tensile stress [52
–
54].
It is noted that the energy consumption for wire material processing is less than that of powder materials due to the
higher yield of wire material. Fig. 4 shows the energy consumption of feedstock processing (tool steel and titanium)
toward metal AM. The calculation for the BMD/FFF process is based on the assumption that 4 % content by weight
polymer binder is included in the feedstock material [51]. The feedstock material in BJP consumes slightly more energy
per kilogram than in the PBF and DED process because of the binder inclusion [51]. It is believed that the energy
consumption of the BMD/FFF process is similar to the BJP since both these two processes utilize a similar mixture of
polymer binder and metal powders as the feedstock materials, though the composition of the metal and polymer binder
may vary.
2.2
Eco-design and energy consumption of the manufacturing process
2.2.1
Eco-design methodology
alt-text: Table 3
Table 3
The emb odied energy of the metal p owder materials.
Materia l Energy consumption (MJ/kg) References
Alumin um 16 0-230 [11]
Cop per 33.0
–
64.5 [55]
Steel 78-97 [56]
Zinc 3 5.8
–
48.4 [55]
Nickel 113.5
–
193 .8 [55]
i
The table layout displayed in this section is not how it will appear in the final version. The representation below is solely
purposed for providing corrections to the table. To preview the actual presentation of the table, please view the Proof.
Q5
alt-text: Fig. 4
Fig. 4
The energy con sumption of feedstock processing toward metal AM [51].
The purpose of eco-design for metal AM is to minimize the negative environmental and social impacts while
maximizing the favorable economic impacts at the very early development stage [57]. Particularly, environmental issues
receive the most attention due to climate change. It is highly valuable to analyze and solve any potential environmental
issues during the early design phase before moving into the later development stages. The reason behind this is that the
initial concept of a product could have a far-reaching effect on its overall life cycle, including production, application,
disposal, and recycling. Therefore, the assessment, analysis, and elimination of adverse impacts on the environment
must be taken into account at the design stage to prevent complicated environmental issues. From the ecological
sustainability perspective, conceptual design and development could be considered the most important period to
quantify and improve the product. Eco-design also contributes to the minimization of environmental effects [58,59].
Eco-design methods for AM should primarily focus on two types of problems: design and ecological sustainability. Fig.
5 shows the three dimensions and aims of eco-design for AM processes.
From a fabrication standpoint, the product dimension, topology, generative design, and material selection need to be
investigated and optimized to further improve the usability of printed products. Environmental impacts such as CO 2
emission, energy or resource efficiency, and other ecological properties for metal AM must be considered when
ecological sustainability is concerned. Eco-design for AM aims to build a bridge to connect the manufacturing, design,
and recycling so that eco-friendly AM can be ensured at the primary stage of product development [57,60
–
63].
Several eco-design methodologies have been developed from various design perspectives. Gaha et al. proposed an eco-
design methodology based on creativity sessions for idea generation, where sufficient supports are required to validate
the environmental considerations of each decision before moving to the next stage in the design process [64]. Fig. 6
illustrates an example of an eco-design methodology for products based on feature-based modeling.
alt-text: Fig. 5
Fig. 5
Three dimension s and aims of eco-design for AM [57].
alt-text: Fig. 6
Fig. 6
Ponche proposed a novel methodology based on the consideration of four factors in product manufacturing: design
domain, final theoretical geometry of the product, corresponding realistic geometry, and estimation of the geometry [65
]. Kerbrat et al. suggested an alternative four-step methodology based on the numerical program generation, command
parameter extraction, process database construction and environmental impact assessment [66]. Each product
demonstrates its manufacturing path and tooling selection. The material removed from the product during
manufacturing depends on the desired final geometry of the product and the initial bulk shape, such as a wire, rod or
sheet. Currently, few approaches are available to precisely quantify the environmental impacts of the conventional
manufacturing process [67,68].
2.2.2
The energy consumption of the metal AM processing
The total energy consumption of the metal AM processing includes the primary energy in printing, such as laser or
electric arc to melt the metal materials, and the secondary energy used for machine and system operation [69]. The
majority of the total energy is consumed by the system loss and secondary energy for the AM machine. Equipment
such as heaters, chillers, pumps, motors, controllers, and washers used in fabrication primarily contributes to the overall
energy consumption. For instance, the energy consumption of the whole system is 5
–
10 times more than that of the
primary printing process [11]. Specifically, a PBF system with a 500-watt laser for printing consumes about 4,000
W to
power the system. Investigation of the relationship between secondary energy consumption and printing speed
indicated that secondary energy consumption did not improve dramatically while a higher printing speed reduced
energy consumption [8,20,70,71]. Fig. 7 shows the energy consumption for different metal AM processes, including
the post-treatment sintering process in BJP and BMD/FFF processes. Based on these data, BJP and BMD/FFF
consume much less energy than DED and PBF, while laser PBF is the least energy-efficient process. It is noticeable
that the energy consumption of DED using wire feedstock is much smaller than that of the PBF. That is because that
the deposition layer thickness of the PBF process is only 30
–
80 micrometers while the layer deposition thickness of
DED can reach more than 500
µ
m, which is much higher than that of PBF process. As a result, the printing speed of
the PBF is much lower than that of the wire-based DED [72,73]. Even though the power of the energy source of DED
equipment maybe higher than that of the PBF, the energy consumption of PBF is very high. The only difference
An eco-design meth odo logy based on feature-based modeling [64].
between BMD/FFF or BJP and PBF is the addition of the washing process to move the binder, which may take up to
24
h depending on the scale and layer thickness of the printed product. The energy consumption during the printing
process is estimated through the energy consumed to print a specific mass of the material (kWh/kg), alternatively
known as specific energy consumption (SEC) [11].
The deposition rate of wire-based electric beam DED (EBAM) ranges from 3.18 to 11.34
kg per hour, depending on
the selected materials and designed product specifications such as surface roughness and layer thickness. As a result, it
is the fastest metal AM process commercially available in the manufacturing market [27,74]. It was reported that the
wire-based laser DED demonstrated a very high deposition rate with 2.9
kg/h. However, powder-based laser DED tops
out around 2.27
kg per hour, much faster than the other powder-based PBF process for most cases [74]. The overall
printing speed and cost of the powder-based DED are close but still smaller than wire-based DED [22,51]. While PBF
methods like SLM, SLM and EBM, demonstrate low deposition rates of 0.01-0.2
kg/h [24,74,75]. Fig. 8 shows the
deposition rates of various metal AM processes. Overall, wire-based methods are much faster than powder-based
processes. On the other hand, wire-based metal printing tends to show a thicker layer in the deposition and rougher
surface in the printed part than powder-based printing does. Thus it is typically used for manufacturing large scale and
rough parts [51].
The overall manufacturing energy consumption of metal AM includes the energy consumed in mining, feedstock
manufacturing, printing, and post-processing stages. Fig. 9 illustrates the energy consumption of different AM
processes at each stage. It is interesting to note that the energy consumed in the mining and primary production toward
the wire feedstock is higher than that of powder feedstocks. Due to this reason, the energy consumption of the wire-
based AM is slightly higher than that of powder-based AM, although the printing energy of wire-based AM is much
smaller than the energy of powder-based fusion processes [51]. Improving or creating wire manufacturing technology
could considerably reduce the raw material requirement in the wire production and thus significantly reduce the energy
alt-text: Fig. 7
Fig. 7
The energy con sumption for different metal AM processes [51].
alt-text: Fig. 8
Fig. 8
The deposition rates of metal additive manufacturing methods [74,76].
consumption in the mining and primary production toward wire feedstocks. It has to be pointed out that the BMD/FFF,
BJP and PBF, in some cases, need the extra stages of washing and post-sintering in comparison with DED.
2.2.3
Energy consumption of different processing steps and printing system
In