Environmental impact assessment of an innovative strategy based on an additive and subtractive manufacturing combination

Accepted Manuscript Environmental impact assessment of an innovative strategy based on an additive and subtractive manufacturing combination

Van Thao Le, Henri Paris, Guillaume Mandil PII:

S0959-6526(17)31373-2

DOI:

10.1016/j.jclepro.2017.06.204

Reference:

JCLP 9954

To appear in:

Journal of Cleaner Production

Received Date:

11 April 2017

Revised Date:

13 June 2017

Accepted Date:

23 June 2017

Please cite this article as: Van Thao Le, Henri Paris, Guillaume Mandil, Environmental impact assessment of an innovative strategy based on an additive and subtractive manufacturing combination, Journal of Cleaner Production (2017), doi: 10.1016/j.jclepro.2017.06.204

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Environmental impact assessment of an innovative strategy based on an additive and subtractive manufacturing combination Van Thao Le, Henri Paris*, Guillaume Mandil Univ. Grenoble Alpes, CNRS, G-SCOP, 38000 Grenoble, France *Corresponding author. E-mail addresses: [email protected] (V.T. Le); [email protected] (H. Paris); [email protected] (G. Mandil)

Abstract Recently, the combination of additive and subtractive manufacturing technologies is gaining significant attention from both the industrial and academic sectors. Due to consolidated advantages of individual techniques, this combination provides the capability of producing innovative products, and opens new perspectives for developing new remanufacturing/manufacturing strategies. In this paper, an innovative strategy is proposed. The strategy aims to manufacture a new part directly from an EoL part (or existing part), using a sequence of additive and subtractive manufacturing processes. The existing part is transformed into the new part without involving the material recycling phase. This paper particularly focuses on the environmental assessment of the proposed strategy. For this purpose, a methodology based on the life cycle assessment method is developed. The environmental trade-offs between the proposed strategy and the conventional strategy are also discussed through the case study. The conventional strategy uses conventional processes (such as material recycling, casting and machining) to manufacture the new part, whereas the proposed strategy is based on combining electron beam melting (EBM) and CNC machining processes. The results show that the proposed strategy becomes more environmentally friendly when the material volume of existing part reused increases more than 60%. The proposed methodology can help designers and manufacturers to select the most suitable strategy to manufacture new parts from existing parts with minimum environmental impacts. Keywords: Life cycle assessment; End-of-life product; Additive manufacturing; CNC machining; Remanufacturing.

Nomenclature EoL

End-of-life

LDD

Laser Direct Deposition

CNC

Computer Numerical Control

DMD

Direct Material Deposition

CAD

Computer-Aided Design

SLM

Selective Laser Melting

AM

Additive Manufacturing

DALM

Direct Additive Laser Manufacturing

FFF

Fused Filament Fabrication

CLAD

Construction Laser Additive Deposition

EBM

Electron Beam Melting

LCA

Life Cycle Assessment

DMLS

Direct Metal Laser Sintering

LCI

Life Cycle Inventory

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1. Introduction

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The performance of a manufacturing system is generally assessed by considering the manufacturing attributes such as cost, time, quality and flexibility. However, to deal with environmental impacts and natural resource depletion, the production system today must be balanced from economic, environmental and social points of view (Bashkite et al., 2014; Östlin et al., 2009). These aspects should be considered simultaneously and equally in the design and manufacturing stages. In this context, many environmental policies and regulations, for example ELV (End-of-Life Vehicle directives) and WEEE (Waste Electrical and Electronic Equipment directives) as presented by (Gehin et al., 2008), have been applied to restrict the quantity of landfilled waste. They push manufacturing companies to produce their products in a cleaner and greener manner by using energy and resources efficiently. The increase of end-of-life (EoL) products is also a major issue, which causes an increase of environmental impacts and the burden on landfill facilities. Thus, the development of efficient strategies able to recover EoL products while taking into consideration the environmental aspect is very important. In the past three decades, additive manufacturing (AM) has attracted increasing attention of researchers both in the academic and industrial sectors (Esmaeilian et al., 2016). AM techniques offer the ability to build complex parts including internal structures directly from the CAD models, using layer-by-layer construction method (Gibson et al., 2010). In comparison with conventional manufacturing processes, such as casting and machining, AM is being considered as a “cleaner production” technique, which does not generate chips and waste. This technique presents the main sustainability advantages as follows:  Firstly, the design freeform offered by AM allows producing complex parts that are difficult to manufacture by machining. This also opens new prospects for topological optimization in design of innovative products and lightweight components (Huang et al., 2016). As a result, AM technique has potential for reducing energy and resource consumptions during the manufacturing process. Compared with AM, the manufacture of lightweight components (e.g., aircraft components) by machining always presents a significant amount of chips - about 80% of used raw material (Huang et al., 2016; Serres et al., 2011). The design freedom of AM also allows the redesign of products. This means that several products made of an assembled mechanism can be replaced by an integrated component. Thus, production costs and quality problems resulting from assembling operations can be reduced (Ford and Despeisse, 2016; Thompson et al., 2016).  Secondly, AM techniques have potential for extending product life through technical approaches, such as repairing and remanufacturing of components, or rapid manufacturing of components for replacing in products (Ford and Despeisse, 2016).  Thirdly, AM technique has significant feasibility to reduce environmental impacts by reducing scrap and CO2 emissions generated during the manufacturing process (Huang et al., 2013). Gebler et al. (2014) estimated by 2025 that AM techniques have potential to reduce 130.5 – 525.5 Mt of CO2 emissions and 2.54E18 – 9.30E18 J of the total primary energy supply.

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 Finally, AM technique offers shorter and simpler supply chains, as well as more localized production due to the capability of manufacturing spare parts on-demand. As a result, the transport and inventory of waste are reduced (Chen et al., 2015; Ford and Despeisse, 2016). However, AM techniques also present a number of limitations, such as limited materials available, long production time and poor surface quality. In addition, residual stresses and thermal deformation of parts are other concerns that have to be taken into account (Vayre et al., 2012). On the other hand, CNC machining - a typical subtractive manufacturing process, is usually used to manufacture components with high levels of surface finish and dimensional accuracy. This technology also enables a relatively short production time. Nevertheless, due to tool accessibility, the manufacture of complex geometries (including internal structures) by this technique is very difficult or impossible (Karunakaran et al., 2010; Zhu et al., 2013). Recently, the combination of additive and subtractive manufacturing technologies is considered as a technical promising solution for designers and manufacturers to develop innovative manufacturing strategies (Caligiana et al., 2017; Flynn et al., 2016; Francia et al., 2017). Due to the consolidated benefits of individual techniques, this solution makes it easy to manufacture parts including internal features with desirable accuracy. Zhu et al. (2013) proposed an iAtractive framework, which is able to manufacture high accuracy plastic parts including internal structures, using the combination of subtractive (i.e., CNC machining), additive (i.e., fused filament fabrication, FFF) and inspection processes. The iAtractive framework can also generate different strategies to produce new plastic parts directly from existing components (Newman et al., 2015; Zhu et al., 2017). Karunakaran et al. (2010) demonstrated that using combined additive gas metal arc welding and CNC machining processes, which takes the best features of additive and subtractive processes, enables manufacturing metallic components in a low-cost manner. This process combination is adequate for most engineering applications. Manogharan et al. (2016) introduced a hybrid method combining electron beam melting (EBM) or direct metal laser sintering (DMLS) with CNC machining to manufacture mechanical parts. In their work, the near-net shape of complex parts was built via AM process (i.e., EBM or DMLS) and the part accuracy was achieved by CNC machining. The authors showed that their approach is more economically attractive in manufacturing of expensive and harder machining materials. Furthermore, combining additive and subtractive processes has successfully been used for remanufacturing of high value EoL components. Wilson et al. (2014) demonstrated the effectiveness of using laser direct deposition (LDD) and CNC machining technologies in remanufacturing turbine blades. Rickli et al. (2014) combined direct material deposition (DMD) technique with CNC machining in an additive remanufacturing system. This system was able to restore high value EoL cores to original specifications. Navrotsky et al. (2015) used selective laser melting (SLM) for repairing gas turbine burner tips. Their results also demonstrated the potential of SLM technique for building new features on existing components. Although the consolidated advantages of combining subtractive and additive manufacturing processes have been demonstrated, it is necessary to assess this process combination in terms of environmental impacts (resource consumptions and emissions such as greenhouse gases, toxic substances, and so on). This 3

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assessment will help manufacturers to select the best strategy between this process combination and the conventional processes (e.g., recycling of materials, forging and machining). In fact, the manufacture of parts by AM processes (e.g., EBM, SLM and DMD) requires metallic powder as raw material input. The production of metallic powder consumes a significant amount of resources and energy, and generates corresponding environmental impacts. Thus, energy and resource consumptions in the powder production process (such as gases in the atomization process) should be taken in consideration. However, limited research has been focused on the environmental assessment of combining additive and subtractive processes. Many studies have evaluated energy consumption and environmental impacts at the unit process level for both machining and AM processes. Concerning machining processes, Kara and Li (2011) proposed an empirical model to characterize the relationship between specific energy consumption (SEC) and material removal rate (MRR) for turning and milling. The SEC (kJ/cm3) is the energy consumption by machine-tools for removing 1 cm3 of materials. This model can predict the energy consumption of machining process with an accuracy of more than 90%. In the framework of CO2PE!, Kellens et al. (2012a, 2012b) developed a life cycle assessment (LCA) oriented methodology for systematic inventory analysis of manufacturing unit processes. The methodology consists of two approaches - those are the screening and in-depth approaches, which allow obtaining the life cycle inventory (LCI) data more completely, accurately and in a relatively rapid manner. Concerning AM processes, Le Bourhis et al. (2013) presented a method based on LCA for direct additive laser manufacturing (DALM) process. To assess environmental impacts of the process, the authors considered not only electrical energy consumptions, but also fluid and raw material consumptions. However, the general consumption of materials and energy was divided into different flows and calculated separately. Finally, the total energy consumption was converted to environmental impact factors. Baumers et al. (2010, 2016) estimated that the energy consumption rate of EBM process for the build of specified Ti6Al-4V parts was about 17 (kWh/kg). However, the energy consumption in EBM process depends on not only the part volume, but also other elements, for example the build height of parts. For more information, a good review and comprehensive analysis of published works on the energy consumption of machining and AM processes can be found in previous works (Huang et al., 2016; Ingarao, 2017). A number of research have compared additive and subtractive technologies from resource consumption and environmental perspectives. Morrow et al. (2007) compared the SEC and air emissions between traditional processes (e.g., casting and machining) and DMD process in the cases of steel tool production. The authors concluded that DMD could reduce emissions and energy consumption for remanufacturing valuable tools compared to the production of new ones by machining. Moreover, DMD process was more environmentally friendly and more energy-efficient to manufacture molds, which present a significant amount of material to be removed by machining. On the contrary, CNC milling is still more environmentally friendly than DMD process for manufacture of molds with less material volume to be removed in machining. Serres et al. (2011) used LCA method to assess and compare CLAD (construction laser additive deposition) and machining in terms of environmental impacts. The authors used a life cycle inventory as large as possible. To develop the comparison, the manufacture of a specified mechanical Ti-6Al-4V part was used as the functional unit. The authors showed that the CLAD is much more interesting than CNC 4

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machining to produce the parts with 70% of environmental impacts that are reduced. More recently, Paris et al. (2016) compared the cumulative energy that is required for the manufacture of a titanium based turbine using CNC machining and EBM processes. They suggested that AM process is preferable when the material volume (chips) to be removed in CNC machining is important. Huang et al. (2016) analyzed five case studies of aircraft components manufactured by metal-based AM techniques (e.g., EBM, DMLS and SLM). The authors mentioned that, for each case study, AM methods led to significantly lower energy consumption compared to conventional manufacturing pathways. This is due to the ability of AM to manufacture lightweight parts (using topological optimization for instance). Tang et al. (2016) also presented a framework for environmental impact analysis of AM process (i.e., binder jetting) and compared it with CNC machining. They proved that the binder jetting process consumes less energy and generates fewer CO2 emissions to produce a topologically optimized part than CNC milling. As a conclusion, the above literature survey demonstrates two points: Firstly, it is possible to develop an alternative strategy for efficient material reuse based on the combination of additive and subtractive manufacturing technologies. Secondly, LCA is the appropriate method to assess and compare environmental impacts of these alternatives. In this paper, taking into account the benefits of combined additive and subtractive manufacturing processes, an “innovative strategy” is first proposed. This strategy allows transforming EoL metallic parts (or existing parts) into new parts directly, avoiding the material recycling phase. The obtained parts are then intended for another product; namely, the existing parts will have a new life and new usage in their life cycle. The paper particularly focuses on the assessment of the proposed strategy and compares it with the conventional manufacturing strategy in terms of environmental impacts. The LCA method is also used to calculate the environmental impacts. The article is organized as follows. In Section 2, an overview of the innovative strategy is presented. Sections 3 and 4 particularly focus on the environmental assessment of the strategy based on LCA method; and compare it with the conventional strategy in the environmental aspect. Section 3 describes the goal and scope definition, as well as system boundaries of the assessment method. Section 4 presents the method to calculate environmental impacts through the case study. Section 5 is intended for discussing the trade-offs between the innovative and conventional strategies based on the results obtained in Section 4. Finally, Section 6 presents conclusions and future work. 2. Presentation of the innovative strategy Traditionally, the EoL parts (or existing parts) are recycled into raw materials (workpieces). The desired parts (or final parts) are then manufactured from workpieces using conventional processes, such as machining and grinding. Based on combining additive and subtractive technologies, the strategy proposed in this paper reuses the material of existing parts directly to produce the final parts, avoiding the material recycling stage (Le et al., 2015). The technological feasibility of metal-based AM techniques (i.e., DMD, SLM and EBM) to develop the proposed strategy has been demonstrated in previous published works (Dutta and Froes, 2015; Liu et al., 2014; Mandil et al., 2016). In these works, the authors showed that new features built on the existing part 5

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by metal-based AM techniques have a strong bonding with the existing part. The part obtained by this way also has mechanical characteristics comparable to those of the parts manufactured by conventional processes (e.g., casting, forging and machining). Fig. 1 presents the general workflow to manufacture the final part from the existing part. It consists of three major stages: pre-processing, processing and post-processing. In the pre-processing stage, the existing part is cleaned analyzed in terms of material quality and geometry. Thereafter, its actual geometry and dimensions are achieved by a system of measurement and scanning. The available information and the CAD models of the existing and final parts are used for the processing stage.

Fig. 1. The general procedure to manufacture the final part from the existing part.

In the processing stage, the process planning for combined additive and subtractive processes is designed based on the knowledge of additive and subtractive processes, technological requirements, and available resources (such as AM machines, machine-tools, and so on) (Le et al., 2017a, 2017b, 2017c). The heat treatment may be required in this stage to achieve expected quality (e.g., fully dense part, good microstructures and mechanical properties), or to reduce residual stresses and thermal deformation of the parts after AM processes. The inspection operations are placed in the way to rehabilitate the sequence. This ensures the quality of final part and avoids material waste. The post-processing stage consists of final inspections and additional operations (such as labeling and so on). The final inspection operations ensure all required specifications of the final part are respected. 3. Environmental assessment methodology The study goal is to assess the environmental impacts of the proposed strategy by using life cycle assessment (LCA) method. The proposed strategy is also compared with the conventional manufacturing strategy (which uses conventional processes such as material recycling and machining to produce parts) in the environmental dimension. For this purpose, the following case study (that considers the manufacture of a mechanical part from an existing part) is used. To perform the assessment and comparison of environmental impacts between two strategies, the functional unit is defined as follows: “manufacture of a defined final part from an identified existing part.” 6

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In the case study, the mass of the final part is assumed bigger than that of the existing part. The material of the part is Ti-6Al-4V alloy. The chemical composition of Ti-6Al-4V alloy is given in Table 1. Table 1. Chemical compositions of the Ti-6Al-4V alloy. Element Wt. %

Al

V

C

Fe

O

N

H

Ti

5.5-6.75

3.5-4.5

< 0.08

< 0.25

< 0.2

< 0.05

< 0.01

Base

Fig. 2 presents the LCA boundaries for the environmental impact assessment of the part manufacture according to the proposed strategy (Fig. 2a) and the conventional strategy (Fig. 2b). The red arrows present the flow of part material. The blue arrows present the flow of electricity energy and resource (argon, water and oil) consumptions.

Fig. 2. LCA boundaries for the environmental assessment: the innovative (a) and conventional (b) strategies.

Generally, the environmental assessment is released considering the entire process cycle, which consists of five principal stages, such as extraction of raw materials, production, transport, use, and EoL stage. However, this paper particularly focuses on the assessment of environmental impacts generated from the manufacturing processes of parts and associated transport. In addition, the case study uses an EBM machine model A1 of Arcam to perform additive operations in the innovative strategy, and a 3-axis CNC machine (Fadal VMC 4020) for subtractive operations both in the innovative and conventional strategies. Furthermore, the cleaning of existing parts and the heat treatment (Fig. 1) are not included in the assessment process. 7

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Fig. 2a presents the phases to manufacture the final part directly from the existing part according to the innovative strategy. Firstly, the existing part is machined (i.e., roughing operations) on the CNC machine to obtain a horizontal planar surface for depositing materials in EBM process. The part after roughing operations is called the common part, which presents the existing part material reused to manufacture the final part. Thereafter, new features are built on the common part by EBM process. This enables to achieve a near shape of the final part. Titanium powder used in additive operations is produced by the gas atomization process (Yolton and Froes, 2015). Semi-finishing and finishing operations are then released on the CNC machine to achieve the final part with required technological specifications. The chips produced in roughing, ½ finishing and finishing operations (including supports for building parts in EBM), as well as material waste in the atomization process are recycled into raw materials by the “4C process” - Cold Crucible Continuous Casting process (Durand, 2005). Due to the mass of final part is bigger than that of the existing part, an additional amount of titanium taken from another existing part is also required. The raw material obtained from the material recycling phase is used to produce titanium powder in the next manufacturing cycle. In the case study, we assume that the transport of chips and titanium powder are performed by the same type of lorry. Fig. 2b describes the manufacture of final part according to the conventional strategy. The same technique “4C process” is used to produce workpieces from the existing part, chips generated in roughing, ½ finishing and finishing operations of the previous manufacturing cycle, and an additional amount of titanium taken from another existing part. After that, the final part is achieved from a workpiece by CNC machining (i.e., roughing, ½ finishing and finishing operations). We also assume that the transport of chips and workpieces in this case is performed by the same type of lorry as in the innovative strategy. 4. Life cycle inventory (LCI) and calculation method To assess environmental impacts, the inputs and outputs of each unit process in two strategies (Fig. 2) are calculated. Thereafter, all unit processes and two strategies are modeled in the SimaPro software (version 8.0.4.30). In the case study, we assume that the existing part mass is fixed at a value of 1 kg. The mass of chips in ½ finishing and finishing operations in both the innovative and conventional strategies is also fixed at a same value of 0.04 kg (i.e., Mfinishing chip (a) = Mfinishing chip (b) = 0.04 kg). The parameters used to calculate energy and material consumptions in these strategies are summarized in Table 2. Table 2. Parameters used to calculate energy and resource consumptions. Common parameter

Symbol

Mass of the existing part

Me = 1 (fixed)

kg

Mass of the final part

Mf

kg

Mass of the common part

Mc

kg

Additional titanium mass required for manufacture of the final part

Madd

kg

Mass of the chips in machining

Mchip

kg

Axial depth of cut in milling operations (Fig. 7)

ap

mm

Radial depth of cut in milling operations (Fig. 7)

ae

mm

8

Unit

Feed rate in milling operations

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mm/min

Cutting speed in milling operations

Vc

m/min

Feed per tooth in milling operations

fz

mm/tooth

Diameter of the cutting tool

D

mm

Number of teeth of the cutting tool

z

-

Cutting time in machining operations

tcutting

s

The material removal rate in machining

MRR

cm3/s

The specific energy consumption in machining

SEC

kJ/cm3

Volumetric mass density of materials



g/cm3

Thermal capacity of materials

Cp

J/g.K

Parameter used in the innovative strategy (a) Mass of the chips in the roughing operations

Mroughing chip (a)

kg

Mass of the chips in the ½ finishing and finishing operations

Mfinishing chip (a) = 0.04 (fixed)

kg

Height of the common part according to the build direction in EBM

Hc

mm

Height of the final part according to the build direction in EBM

Hf

mm

Height of the plate according to the build direction in EBM

Hplate = 10

mm

Total height of the part build in EBM

Htotal build

mm

Mass of powder required to build the part in EBM

Mpowder

kg

Mass of material waste in atomization process

Mwaste

kg

Mass of the chips in the roughing operations

Mroughing chip (b)

kg

Mass of the chips in the ½ finishing and finishing operations

Mfinishing chip (b) = 0.04 (fixed)

kg

Mass of the workpiece

Mw

kg

Parameter used in the conventional strategy (b)

4.1. Material recycling process In this study, the direct recycling process, “4C process” (Durand, 2005), is used to recycle chips and material waste in the innovative strategy (Fig. 2a) and to produce workpieces in the conventional strategy (Fig. 2b). This process takes place under high vacuum pressure. The melted metal is solidified in a copper crucible and cooled by water that circulates within the crucible wall. This process allows 100% of titanium chips and waste to be recycled into raw materials or workpieces. In addition, the workpiece obtained by this technique is dense and directly suitable for machining (Durand, 2005). The material and energy consumptions to recycle 1 kg of titanium are shown in Table 3. In the innovative strategy, chips and material waste generated from milling and atomization phases, as well as an additional amount of material taken from another existing part (Madd = Mf – Me) are recycled into raw material, which is then used for powder production. The total mass of titanium to be recycled is calculated by Eq. 1. 𝑀𝑡𝑖𝑡𝑎𝑛𝑖𝑢𝑚 𝑐ℎ𝑖𝑝𝑠 𝑎𝑛𝑑 𝑤𝑎𝑠𝑡𝑒 = 𝑀𝑎𝑑𝑑 + 𝑀𝑟𝑜𝑢𝑔ℎ𝑖𝑛𝑔 𝑐ℎ𝑖𝑝 (𝑎) + 𝑀𝑓𝑖𝑛𝑖𝑠ℎ𝑖𝑛𝑔 𝑐ℎ𝑖𝑝 (𝑎) + 𝑀𝑤𝑎𝑠𝑡𝑒

(1)

In the conventional strategy, the workpiece is produced directly from the existing part, chips generated from machining operations and an additional amount of materials (Madd = Mf – Me). The mass of workpiece is calculated by Eq. 2: 9

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𝑀𝑤 = 𝑀𝑒 + 𝑀𝑎𝑑𝑑 + 𝑀𝑟𝑜𝑢𝑔ℎ𝑖𝑛𝑔 𝑐ℎ𝑖𝑝 (𝑏) + 𝑀𝑓𝑖𝑛𝑖𝑠ℎ𝑖𝑛𝑔 𝑐ℎ𝑖𝑝 (𝑏)

(2)

Table 3. Electricity energy and resource consumptions in the titanium recycling process and the gas atomization (Paris et al., 2016). Process

4.08 (kWh)

Atomization: for production of 1 kg of titanium powder 6.6 (kWh)

Water

155 (l)

155 (l)

Argon

- (in a vacuum)

3.5 (m3)

1 (kg)

1.03 (kg)

Element

Recycling of 1 kg of titanium waste

Electricity

Titanium

4.2. Powder production Several techniques exist to produce metallic powder, such as plasma rotating electrode process, gas atomization and plasma atomization. However, the gas atomization process is mainly used (Yolton and Froes, 2015). The gas atomization principle consists of melting materials in a crucible. The melted material is flowed through a nozzle under the gravity effect. The material flow is then spitted into fine droplets by argon jets. The droplets are solidified thanks to a convective exchange during their displacement in the atomization room. For titanium-based alloys, this process allows the powder production with a high efficiency - about 97% of titanium used at the beginning of process is transformed into titanium powder (Khatim, 2011). The material and energy consumptions to obtain 1 kg of titanium powder are given in Table 3. In the case study, the mass of powder required to manufacture the final part can be calculated by Eq. 3: 𝑀𝑝𝑜𝑤𝑑𝑒𝑟 = 𝑀𝑓 + 𝑀𝑓𝑖𝑛𝑖𝑠ℎ𝑖𝑛𝑔 𝑐ℎ𝑖𝑝 (𝑎) ‒ 𝑀𝑐

(3)

4.3. Energy consumption in EBM process Additive operations in the proposed strategy are performed on an EBM machine, model A1 of Arcam (Fig. 3). Detailed information and the work principle of EBM machines have been presented in previous works (Béraud et al., 2017; Mandil et al., 2016). Fig. 4 presents temperature evolution at the bottom surface of the build plate during four main build steps (i.e., vacuum, heating, melting and cooling). These steps are briefly described as follows:  Firstly, a 316L stainless steel plate (with 210 mm x 210 mm x 10 mm of dimensions) is placed on the build table of the machine. Titanium powder is loaded in two powder hoppers and dispersed onto the build plate by the powder rake. Thereafter, the vacuum is performed until the build environment pressure reaches 10-5 (mbar). Note that in this study the common part achieved from the existing part is also positioned on the build plate, and the starting build surface is the top surface of common part.

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Fig. 3. (a) EBM machine schemas and (b) the build of a layer, adapted from (Körner, 2016; Smith et al., 2016).

 Following the vacuum step, the heating is performed. The build plate, the common part and powder filled around are heated by electron beam until temperature at the top surface of the common part reaches initial build temperature (e.g., 750°C in the case of Ti-6Al-4V alloy). At that time the build of first layer is started. The build of parts is performed layer by layer until the total build is complete. The process of each layer is introduced in Fig. 3b. Firstly, the current powder layer (50 µm of thickness) is heated and presintered (Fig. 3b-1). This step allows layer temperature to be kept at a constant value of 750°C. Thereafter, all cross-section contours of parts are melted and materials inside cross-sections are fully melted (Fig. 3b2 and Fig. 3b-3). Once the current layer is fully built, the build table is lowered an increment of 50 µm for building the next layer.  When the total build of parts is complete, the slow cooling process taking under vacuum is executed until temperature at the bottom surface of the build plate reaches 100°C. From this moment, the powder block including built parts can be taken out from the machine. The built parts are then treated in the postprocessing stage.

Fig. 4. The temperature evolution at the build plate bottom during the total build of EBM process.

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In EBM process, it is considered that there is no material waste because unused powder will be reused in the next manufacture of parts. Thus, consumed powder quantity comprises the powder used to build the parts and supports (Vayre et al., 2013). The electricity energy consumption in EBM process is the total energy used in four steps (Fig. 4). The energy consumed in the vacuum step is considered as a constant value. The vacuum is performed in about 1 hour 15 minutes, and consumes about 1.78 (kWh) of electricity (Table 4). The energy consumed in the heating phase is also considered as a constant if the same build plate is used. In the normal EBM build, where a stainless steel plate (with 210 mm x 210 mm x 10 mm of dimensions) was used, the heating phase takes place for 40 minutes and consumes about 2.02 (kWh) of electricity (Table 4). However, in this study it should also take into account energy consumed to heat the common part and powder filled around it, as illustrated in Fig. 5.

Fig. 5. Heating of the common part, powder and the stainless steel plate in EBM.

In fact, the porosity ratio of titanium powder presents a value about of 0.39. The thermal capacity and density of materials (Ti-6Al-4V and stainless steel) depend on temperature of materials (Arce, 2012). However, to simply calculate energy consumptions for heating the common part and powder, we assume that powder and the solid part have same physical properties. Thus, the common part and the powder located on the stainless steel plate are considered as a titanium plate, which has the same surface area of the stainless steel plate. The thermal capacity of Ti-6Al-4V (Cp(Ti-6Al-4V)) and stainless steel plate (Cp(316L steel)) are also fixed at 0.553 (J/g.K) and 0.44 (J/g.K), respectively. Based on these assumptions, the ratio (REtitanium/steel) between the energy required to heat the common part and powder filled around it and the energy required to heat the stainless steel plate from To = 20°C (ambient temperature) to Tb = 750°C (initial build temperature) is calculated by Eq. 4: 𝑎𝑛𝑑 𝑝𝑜𝑤𝑑𝑒𝑟 𝑝𝑙𝑎𝑡𝑒 𝑅𝐸𝑡𝑖𝑡𝑎𝑛𝑖𝑢𝑚/𝑠𝑡𝑒𝑒𝑙 = 𝐸𝑐𝑜𝑚𝑚𝑜𝑛 𝑝𝑎𝑟𝑡 / 𝐸𝑠𝑡𝑒𝑒𝑙 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 =

=

𝐶𝑝(𝑇𝑖 ‒ 6𝐴𝑙 ‒ 4𝑉) ∗ 𝜌(𝑇𝑖 ‒ 6𝐴𝑙 ‒ 4𝑉) ∗ 𝐻𝑐 𝐶𝑝(316𝐿 𝑠𝑡𝑒𝑒𝑙) ∗ 𝜌(316𝐿 𝑠𝑡𝑒𝑒𝑙) ∗ 𝐻𝑝𝑙𝑎𝑡𝑒

𝐶𝑝(𝑇𝑖 ‒ 6𝐴𝑙 ‒ 4𝑉) ∗ 𝑚(𝑇𝑖 ‒ 6𝐴𝑙 ‒ 4𝑉) ∗ ∆𝑇 𝐶𝑝(316𝐿 𝑠𝑡𝑒𝑒𝑙) ∗ 𝑚𝑝𝑙𝑎𝑡𝑒 ∗ ∆𝑇

= 0.07 ∗ 𝐻𝑐

(4)

Where (Ti-6Al-4V) = 4.43 (g/cm3) and (316L steel) = 7.87 (g/cm3) corresponding to the density of Ti-6Al-4V and 316L steel; Hplate = 10 (mm) and Hc (mm) are respectively the plate height and the common part height. Thus, the energy consumption for heating the common part and powder is approximately calculated by Eq. 5: 𝑎𝑛𝑑 𝑝𝑜𝑤𝑑𝑒𝑟 𝑝𝑙𝑎𝑡𝑒 = 0.07 ∗ 𝐻𝑐 ∗ 𝐸𝑠𝑡𝑒𝑒𝑙 𝐸𝑐𝑜𝑚𝑚𝑜𝑛 𝑝𝑎𝑟𝑡 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 = 0.14 ∗ 𝐻𝑐

12

(5)

ACCEPTED MANUSCRIPT

The energy consumptions in the melting and cooling phases are estimated using three manufacturing cases: the height of the total build is varied in a range of {10; 35; and 59.25 mm}, while the cross-section area of parts has the same order of magnitude, and the same stainless steel plate is used. The energy consumptions measured in these cases are given in Table 4. Table 4. Energy consumptions measured in three cases of part build. Energy consumption (kWh) Build (1) (10 mm)

Build (2) (35 mm)

Build (3) (59.25 mm)

1.78

1.78

1.78

Estimated value (kWh) 𝐸𝑣𝑎𝑐𝑢𝑢𝑚 = 1.78

Heating

2.02

2.02

2.02

𝑝𝑙𝑎𝑡𝑒 𝐸𝑠𝑡𝑒𝑒𝑙 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 = 2.02

Melting

7.10

19.20

31.51

𝐸𝑚𝑒𝑙𝑡𝑖𝑛𝑔 = 0.4956 ∗ 𝐻𝑡𝑜𝑡𝑎𝑙 𝑏𝑢𝑖𝑙𝑑 + 2.0488

Cooling

0.494

1.6

2.42

𝐸𝑐𝑜𝑜𝑙𝑖𝑛𝑔 = 0.0391 ∗ 𝐻𝑡𝑜𝑡𝑎𝑙 𝑏𝑢𝑖𝑙𝑑 + 0.1448

Build steps in EBM Vacuum

It is found that the electricity energy consumption in the melting and cooling phases is properly linear with the total height of the part build (Fig. 6) even through cross-section contours of the parts in three cases are different. This is in line with the observation in the work of (Baumers et al., 2016), in which the authors

Energy consumed in the cooling (kWh)

Energy consumed in the melting (kWh)

found that energy consumptions of EBM systems were not driven by the shape complexity.

40 30 20 10 0 0

10

20

30

40

50

60

3 2.5 2 1.5 1 0.5 0

70

0

Total height of part build (mm)

10

20

30

40

50

60

70

Total height of part build (mm)

(a)

(b)

Fig. 6. Energy consumptions in the melting (a) and cooling (b) phases depending on the total height of part build.

Based on these results, the total energy consumption of EBM system in the case study can be determined by Eq. 6: 𝐸𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 (𝐸𝐵𝑀) 𝑝𝑙𝑎𝑡𝑒 𝑐𝑜𝑚𝑚𝑜𝑛 𝑝𝑎𝑟𝑡 𝑎𝑛𝑑 𝑝𝑜𝑤𝑑𝑒𝑟 = 𝐸𝑣𝑎𝑐𝑢𝑢𝑚 + 𝐸𝑠𝑡𝑒𝑒𝑙 + 𝐸𝑚𝑒𝑙𝑡𝑖𝑛𝑔 + 𝐸𝑐𝑜𝑜𝑙𝑖𝑛𝑔 = 1.78 + 2.02 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 + 𝐸 ℎ𝑒𝑎𝑡𝑖𝑛𝑔 + 0.14 ∗ 𝐻𝑐 + 0.4956 ∗ (𝐻𝑓 ‒ 𝐻𝑐) + 2.0488 + 0.0391 ∗ 𝐻𝑓 + 0.1448  𝐸𝑒𝑛𝑒𝑟𝑔𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 (𝐸𝐵𝑀) = 5.9936 + 0.5347 ∗ 𝐻𝑓 ‒ 0.3556 ∗ 𝐻𝑐

(6)

4.4. Energy and material consumptions in the machining process As mentioned before, the CNC machine is used to perform subtractive operations in both the innovative and conventional strategies. Roughing operations in the innovative strategy aim to achieve a horizontal planar surface of the common part for depositing materials in EBM process. Semi-finishing and finishing 13

ACCEPTED MANUSCRIPT

operations are executed after additive operations to achieve the geometry and required quality of the final part (Fig. 2a). Whereas, roughing and finishing operations in the conventional strategy allows achieving the final part from the workpiece (Fig. 2b). During machining operations the cutting fluid - a mixture of oil and about 70 – 90 wt. % of water, is used to improve surface quality and tool life. The elements to be calculated in machining are electricity, water and oil consumptions, as well as generated chips. In this study, the empirical model to calculate the specific energy consumption (SEC) of machine-tools developed by (Kara and Li, 2011) is used (Eq. 7): 𝑆𝐸𝐶 = 𝐶0 +

𝐶1

(7)

𝑀𝑅𝑅

Where SEC (kJ/cm3) presents the total energy consumption of machine-tools for removing 1 cm3 of materials; C0 and C1 are the machine specific coefficients. For the 3-axis CNC machine (Fadal VMC 4020), in a wet cutting condition, the specific coefficients of this machine are C0 = 3.082 (kJ/cm3) and C1 = 1.396 (kW) (Kara and Li, 2011). MRR (cm3/s) is the material removal rate, which depends on cutting parameters, and also inter-depends on workpiece machinability and tool capacity. MRR (cm3/s) is calculated from cutting parameters (Eq. 8): 𝑀𝑅𝑅 =

𝑎𝑝 ∗ 𝑎𝑒 ∗ 𝑉𝑓

(8)

60 ∗ 103

Where ap (mm) is the axial depth of cut; ae (mm) is the radial depth of cut (Fig. 7). Vf (mm/min) is the feed rate, which can be determined from cutting speed Vc (m/min), feed per tooth fz (mm/tooth), number of teeth (z) and diameter D (mm) of the cutting tool, as Eq. 9: 𝑉𝑓 =

1000 ∗ 𝑉𝑐 ∗ 𝑓𝑧 ∗ 𝑧 𝜋∗𝐷

(9)

Fig. 7. Illustration of the axial depth of cut (ap) and the radial depth of cut (ae) in milling operations.

Using the data given in Table 5 that is used for machining operations in the case study, the SEC for roughing operations, ½ finishing and finishing operations are calculated: SEC(roughing) = 10.39 (kJ/cm3) and SEC(finishing) = 44.83 (kJ/cm3). Table 5. Cutting parameters for roughing, ½ finishing and finishing operations in milling process. Cutting parameter Operation Roughing

ap (mm)

ae (mm)

4

15

14

fz

Vc

(mm/tooth)

(m/min)

0.075

40

D (mm)

z

20

4

MRR (cm3/s) 0.1911

½ Finishing and Finishing

ACCEPTED MANUSCRIPT 0.5

7.5

0.070

60

10

4

0.0334

The energy consumption (kWh) in machining operations can be expressed by Eq. 10: 𝐸𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 (𝑚𝑎𝑐ℎ𝑖𝑛𝑖𝑛𝑔) =

1000 ∗ 𝑆𝐸𝐶 ∗ 𝑀𝑐ℎ𝑖𝑝 3600 ∗ 𝜌

=

𝑆𝐸𝐶 ∗ 𝑀𝑐ℎ𝑖𝑝 3.6 ∗ 𝜌

(10)

Where Mchip (kg) is the mass of chips removed in roughing operations or ½ finishing and finishing operations;  (g/cm3) is the volumetric mass density of materials. In the case study, the material of parts is Ti-6Al-4V; thus  = 4.43 (g/cm3). According to Kellens et al. (2012a), the cutting fluid (water and oil) is lost during machining processes. The rate of oil and water losses have been estimated about 0.042 (g/s) and 0.238 (g/s), respectively. The losses of oil and water in this study is considered as water and oil consumptions during machining processes. Approximately, the time of cutting operations, tcutting (s), can be estimated from the mass of removed chips (Mchip) and MRR by Eq. 11: 𝑡𝑐𝑢𝑡𝑡𝑖𝑛𝑔 =

1000 ∗ 𝑀𝑐ℎ𝑖𝑝

(11)

𝜌 ∗ 𝑀𝑅𝑅

As a result, water and oil consumptions in cutting operations, Mwater consumption (g) and Moil consumption (g), can be calculated by Eq. 12 and Eq. 13, respectively: 𝑀𝑤𝑎𝑡𝑒𝑟 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 = 0.238 ∗ 𝑡𝑐𝑢𝑡𝑡𝑖𝑛𝑔 = 𝑀𝑜𝑖𝑙 𝑐𝑜𝑛𝑠𝑢𝑚𝑝𝑡𝑖𝑜𝑛 = 0.042 ∗ 𝑡𝑐𝑢𝑡𝑡𝑖𝑛𝑔 =

238 ∗ 𝑀𝑐ℎ𝑖𝑝 𝜌 ∗ 𝑀𝑅𝑅

42 ∗ 𝑀𝑐ℎ𝑖𝑝 𝜌 ∗ 𝑀𝑅𝑅

(12) (13)

In the innovative strategy, the quantity of chips produced in roughing operations, Mroughing chip (a) (kg), can be calculated from the existing part mass (Me) and the common part mass (Mc), as Eq. 14: 𝑀𝑟𝑜𝑢𝑔ℎ𝑖𝑛𝑔 𝑐ℎ𝑖𝑝 (𝑎) = 𝑀𝑒 ‒ 𝑀𝑐

(14)

In the conventional strategy, the quantity of chips generated in roughing operations is determined by Eq. 15: 𝑀𝑟𝑜𝑢𝑔ℎ𝑖𝑛𝑔 𝑐ℎ𝑖𝑝 (𝑏) = 𝑀𝑤 ‒ (𝑀𝑓 + 𝑀𝑓𝑖𝑛𝑖𝑠ℎ𝑖𝑛𝑔 𝑐ℎ𝑖𝑝 (𝑏)) = (𝐾 ‒ 1) ∗ 𝑀𝑓 ‒ 𝑀𝑓𝑖𝑛𝑖𝑠ℎ𝑖𝑛𝑔 𝑐ℎ𝑖𝑝 (𝑏) (15) Where Mw is the mass of the workpiece (Mw = K*Mf) and K is the ratio of material removal volume in machining (Eq. 16): 𝐾=

𝑀𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑤𝑜𝑟𝑘𝑝𝑖𝑒𝑐𝑒 (𝑀𝑤) 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑛𝑎𝑙 𝑝𝑎𝑟𝑡 (𝑀𝑓)

(16)

The energy and resources (water and oil) consumptions in roughing operations depend on either Mc in the innovative strategy, or Mf and K in the conventional strategy, Eq. (7) to (16). Due to the mass of chips generated in ½ finishing and finishing operations is fixed (Mfinishing chip (a) = Mfinishing chip (b) = 0.04 kg) electricity, water and oil consumed in this phase in two strategies are estimated by 0.112 (kWh) of electricity, 643 (g) of water, and 113 (g) of oil. 4.5. Environmental impact calculation and the metrics to compare the strategies To evaluate environmental trade-offs between the innovative and conventional strategies, the following scenarios are proposed (Table 6).

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 In the first scenario, the common part mass (Mc) is varied in a range of {0.4; 0.6; and 0.8 kg}; and its

corresponding heights are {8; 12; and 16 mm}. The final part mass and its height along the build direction are fixed (Mf = 1.2 kg and Hf = 20 mm). Additionally, the value of K in the conventional strategy is varied in a range of {3; 5; and 7}.  In the second scenario, the common part mass, the value of K and the final part mass are fixed (Mc = 0.8 kg; K = 7; and Mf = 1.6 kg), whereas the height of final part (Hf) is varied in a range of {20; 40; 60; and 80 mm}. Table 6. Two scenarios used to evaluate environmental trade-offs between two strategies. Scenario 1: Mf = 1.2 kg; Hf = 20 mm; K = {3; 5; 7} Mc (kg)

0.4

0.6

0.8

Hc (mm)

8

12

16

Scenario 2: Mc = 0.8 kg; Mf = 1.6 kg; K = 7 Hf (mm)

20

40

60

80

The first scenario allows investigating the role of Mc (i.e., amount of existing part material to be reused) and K on the environmental competition between two strategies. On the other hand, the second scenario enables studying the influence of the height of final part on environmental impacts. After calculating the inputs and outputs for each unit process in two strategies, these scenarios were modeled in SimaPro software (version 8.0.4.30) with Ecoinvent database (version 3.1). To calculate environmental impacts, CExD (version 1.04) and CML 2 Baseline 2000 (version 2.05) methods are used. The CExD method has been developed in order to quantify the life cycle exergy demand of a product or a process. This method is defined as the sum of exergy of all resources required for a process or a product (Bösch et al., 2007). On the other hand, in the CML 2 Baseline 2000 method, environmental burdens are aggregated according to their relative contributions to environmental impacts that they can potentially cause. The ratio R calculated by Eq. 17 for each environmental impact indicator is used to compare environmental impacts between two strategies. 𝐸𝑛𝑣𝑖𝑟𝑜𝑛𝑚𝑒𝑛𝑡𝑎𝑙 𝑖𝑚𝑝𝑎𝑐𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑟𝑒𝑝𝑢𝑟𝑝𝑜𝑠𝑖𝑛𝑔 𝑠𝑡𝑟𝑎𝑡𝑒𝑔𝑦

𝑅 = 𝐸𝑛𝑣𝑖𝑟𝑜𝑛𝑚𝑒𝑛𝑡𝑎𝑙 𝑖𝑚𝑝𝑎𝑐𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑐𝑜𝑛𝑣𝑒𝑛𝑡𝑖𝑜𝑛𝑎𝑙 𝑠𝑡𝑟𝑎𝑡𝑒𝑔𝑦

(17)

According to each indicator of environmental impacts, if the value of R is smaller than 1, the proposed strategy is more environmentally friendly than the conventional strategy. In the case where R is superior to 1, the conventional strategy is more environmentally friendly to produce the part. If R is equal to 1, both strategies are similar in terms of environmental impacts. 5. Results and discussion In this work, the results were obtained according to ten main environmental impact indicators selected from CExD and CML 2 Baseline 2000 methods. Four indicators coming from the CExD method: nonrenewable fossil (1), non-renewable nuclear (2), renewable potential (3), renewable water (4). Six indicators

16

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are selected from the CML 2 Baseline 2000 methods: abiotic depletion (5), acidification (6), global warming (7), human toxicity (8), fresh water aquatic ecotox (9), and marine aquatic ecotoxicity (10). Fig. 8 presents the contribution of processes in two strategies to generate environmental impacts. It is first observed that energy and argon consumptions mainly cause environmental impacts in the innovative strategy; whereas energy and cutting fluid consumptions are the main elements, which cause environmental impacts in the conventional strategy.

Fig. 8. Process contribution to generate the environmental impacts in the case of Mc = 0.4 kg; K = 7, according to the CML 2 Baseline 2000 method: (a) the innovative strategy, and (b) the conventional strategy.

Fig. 9 shows the evaluation of R in the first scenario according to the selected indicators and the ratio K (note that K presents the ratio of material volume to be removed by machining in the conventional strategy). It is found that the evaluation trend of R is similar across all of these indicators. The value of R decreases when Mc increases. R also decreases when the value of K increases. In particular, the value of R is always inferior to 1 when Mc = 0.8 (kg) and K is superior to 5 (Fig. 9a, b). These mean that the innovative strategy becomes more environmentally friendly than the conventional strategy when Mc and K are increasing. It is also shown that the environmental competition between two strategies can be evaluated based on the indicator (10) because the environmental impact based on this indicator seems the most important. Fig. 9c shows the evaluation of R depending to Mc and K based on this indicator. In this case, it appears that the innovative strategy is more environmentally friendly when K is superior to 4.6, or 6.2, or 7.6 for Mc corresponds to 0.8, or 0.6, or 0.4 (kg), respectively. Based on this observation, the innovative strategy becomes the best option in the environmental point of view when K is superior to 4.6 and Mc is equal or superior to 80% of the existing part mass. These are due to an amount of energy embodied in the common part is preserved. In addition, the quantity of powder required to build the final part is decreased when the common part mass is reused as much as possible. Whereas, at high values of K (e.g., K > 5) a significant amount of energy and cutting fluid is consumed to remove chips by machining and to recycle chips in the conventional strategy. The energy and cutting fluid consumptions are two main elements generating environmental impacts in the conventional strategy (Fig. 8). As a result, the conventional strategy generates much more environmental impacts.

17

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R

2 1.5

K=3 K=5 K=7

1 0.5 0 0.4 (1)

0.6

0.8

0.4 (2)

0.6

0.8

0.6

0.4 (3)

0.8

0.4 (4)

0.6

0.8

(Mc)

(1) non-renewable fossil; (2) non-renewable nuclear; (3) renewable potential; and (4) renewable water

(a) 3.00E+00

R

2.50E+00 2.00E+00 1.50E+00

K=3 K=5 K=7

1.00E+00 5.00E-01 0.00E+00 0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 0.4 0.6 0.8 (Mc) (5) (6) (7) (8) (9) (10) (5) abiotic depletion; (6) acidification; (7) global warming; (8) human toxicity; (9) fresh water aquatic ecotox; and (10) marine aquatic ecotoxicity

(b) Environmental impacts based on the indicator (10)

3

R

2.5 2 1.5

Mc = 0.4

1

Mc = 0.6

0.5

Mc = 0.8

0 0

1

2

3

4

5

6

7

8

9

K (c) Fig. 9. Environmental trade-offs of two strategies in the case of (Mf = 1.2 kg, Mc and K varied) according to (a) four indicators coming from the CExD method, (b) six indicators coming from the CML 2 baseline 2000 method, and (c) environmental trade-offs in detail according to the indicator (10).

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On the contrary, at lower values of K and Mc, the conventional strategy can reduce energy consumption and emissions on the environment. The reason is that less energy consumption is required to remove small quantity of chips by machining and to recycle chips by the material recycling process in the conventional strategy. On the other hand, in this case, a significant quantity of powder is required to build the final part. Consequently, the energy consumption in both EBM and atomization processes, and the argon consumption in the atomization increase. These elements mainly generate environmental impacts in the innovative strategy (Fig. 8). Thus, the innovative strategy generates much more environmental impacts than the conventional strategy. Fig. 10 shows environmental benefits of the innovative strategy compared to the conventional strategy

%

in the case of (Mc = 0.8 kg and K = 7).

100 90 80 70 60 50 40 30 20 10 0 (1)

(2)

(3)

(4)

(5)

(6)

(7)

(8)

(9)

(10)

(1) non-renewable fossil; (2) non-renewable nuclear; (3) renewable potential; (4) renewable water; (5) abiotic depletion; (6) acidification; (7) global warming; (8) human toxicity; (9) fresh water aquatic ecotox; and (10) marine aquatic ecotoxicity

Fig. 10. Environmental impacts of the innovative strategy (green) and the conventional strategy (red) in the case of Mc = 0.8 kg and K = 7.

Based on the indicators (1) to (4), which come from the CExD method, it is shown that the innovative strategy is much more energy- and resource-efficient for manufacture of part than the conventional strategy. The innovative strategy also generates fewer environmental impacts according to the indicators (5) to (10) of the CML 2 Baseline 2000 method. Looking at the results for the global warming, indicator (7), it is observed that the manufacture of parts by the innovative strategy produces fewer than 42% of the CO2 equivalent in comparison with the conventional strategy. It is also interesting to observe the human toxicity resulting in this case according the indicator (8). The manufacture of parts by the innovative strategy produces fewer than 55% of 1,4-DB (dichlorobenzene) equivalent. This suggests that the proposed strategy is noticeably better for human health than the conventional strategy. Fig. 11 presents the evaluation of R according to Hf in the second scenario. The results show that R increases when the height of final part along the build direction in EBM (Hf) is increasing. According to three indicators coming from the CExD method, indicators (1), (2) and (3), the innovative strategy is much more environmentally friendly and energy-efficient than the conventional strategy when Hf is inferior to 76, 78, and 82 (mm), respectively. In particular, the innovative strategy is always the best choice based on 19

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the indicator (4). Based on the indicators (9-10), (7), (6), (5) and (8), the innovative strategy is more interesting in terms of environmental impacts when the Hf is lower than 50; 58; 66 and 68 (mm), respectively. Respecting all of these indicators, the innovative strategy is the best option in the environmental aspect for Hf < 50 (mm). In the remaining cases, the conventional strategy is still interesting for manufacturing the final part from the EoL/existing part.

1.40E+00 1.20E+00 (1)

R

1.00E+00

(2) (3)

8.00E-01

(4) (5)

6.00E-01

(6) 4.00E-01

(7) (8)

2.00E-01

(9) (10)

0.00E+00 0

10

20

30

40

50

60

70

80

90

Hf (mm) Fig. 11. Environmental trade-offs between two strategies in the case: Mc = 0.8 kg; Mf = 1.6 kg; K = 7 and Hf varied.

The results obtained in the two scenarios suggest that the innovative strategy is the best option in terms of environmental impacts when the existing part material is reused as much as possible (about 60% to 80% of the existing part mass), and the material removal ratio (K) in the conventional strategy has a high value (K > 5). In these cases, the advantages of AM techniques to build complex shapes are also demonstrated. On the other hand, the conventional strategy is still an interesting option when the final part having a small material removal ratio in machining processes (e.g., K < 3), and the amount of existing part material to be reused is also small (inferior to 40% of the existing part mass). These findings are in line with the observation in previous published works (Morrow et al., 2007; Paris et al., 2016; Serres et al., 2011). In addition, in the case where the EBM is used to release additive operations in the innovative strategy, this strategy is more environmentally friendly when the height of parts along the build direction is inferior to 50 mm (in the second scenario). When the build height of the final part is higher the innovative strategy generates more environmental impacts even though the final part presents a high level of K and the existing part material is reused up to 80%. The reason is that with a high build height the EBM process consumes a significant amount of electricity energy (Section 4.3). However, it is importantly note that, in the second scenario, the mass of part to be added into the common part is fixed at a value of 0.8 kg. Thus, it is necessary to consider the height and the mass (or the shape) of parts simultaneously when selecting the suited strategy.

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6. Conclusions

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Combining additive and subtractive manufacturing technologies is today becoming a promising solution for issues in the manufacturing field. This technique combination allows the freeform production of parts through AM techniques and achieving desirable dimensional and surface qualities by CNC machining. Taking into consideration consolidated advantages of this technique combination the innovative strategy is proposed in this paper. The strategy allows the manufacture of parts directly from EoL parts (or existing parts), avoiding the material recycling phase. Thus, it has potential to replace the conventional strategy, in which the existing part is recycled into the workpiece, and then the final part is obtained from the workpiece by machining. This paper proposes a general methodology for the environmental assessment of the proposed strategy, and compare it with the conventional strategy in terms of environmental impacts. The proposed assessment framework is based on the LCA method. Energy and resource consumptions of each unit process in two strategies are calculated. Thereafter, the typical LCI is completed to assess environmental impacts of the manufacturing process. The case study has been done where the combination of EBM and CNC techniques was used in the innovative strategy. The environmental comparison between the proposed strategy and the conventional strategy is also performed by using the environmental impact ratio (R) and the material removal ratio (K). The results show that the innovative strategy is more environmentally friendly and also a better solution when the existing part material is reused as much as possible (e.g., up to 80% volume of the existing part), and the final part requires a great material volume to be removed by machining process (i.e., the value of K is high). In the case where the EBM is selected for additive operations, the height of final part along the build direction is also a factor that has influence on the environmental competition of the proposed strategy. On the other hand, in the case of manufacturing the parts with lower values of K and the existing material volume to be reused is small, the conventional strategy produces fewer environmental impacts. The environmental competition between two strategies are mainly related to the powder production and the EBM process in the innovative strategy, and the roughing operations and the workpiece production (i.e., material recycling) in the conventional strategies. These phases consume a significant amount of electricity energy and resources (e.g., water, oil and argon), and generates the main environmental impacts. In addition, environmental impacts relating to the EBM process are mainly due to energy consumed in this process and the argon consumption in the powder production. The methodology proposed in this work can be applied for the combination of CNC and other AM techniques; and can help designers and manufacturers to select the most suitable strategy for manufacturing the parts from the EoL/existing parts with minimum environmental impacts. However, in this work the key assumption is that the final part manufactured by two strategies has the same geometry and mass. Thus, the ability for freeform production of AM techniques is not taken into consideration yet. The geometry of parts to be added into the common part can be optimized by the topological optimization. Thus, the mass of powder required to build the final parts can be reduced. This would have potential to change the

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ACCEPTED MANUSCRIPT Environmental impact assessment of an innovative strategy based on an additive and subtractive manufacturing combination Van Thao Le, Henri Paris*, Guillaume Mandil Univ. Grenoble-Alpes, CNRS, G-SCOP, 38000 Grenoble, France *Corresponding author. E-mail addresses: [email protected] (V.T. Le); [email protected] (H. Paris); [email protected] (G. Mandil)

Highlights 

An “innovative strategy” based on combining additive and subtractive manufacturing technologies that can give a new life to end-of-life (EoL) components is proposed.



A methodology based on the life cycle assessment (LCA) method is developed to assess the proposed strategy in terms of environmental impacts.



The environmental trade-off between the proposed strategy and the conventional strategy is discussed in detail through the case study.



The proposed methodology can help designers and manufacturers to select the most suitable strategy to manufacture parts from the EoL parts with minimum environmental impacts.