Environmental Performance and Key Characteristics

Available online at www.sciencedirect.com

ScienceDirect Procedia CIRP 69 (2018) 148 – 153

25th CIRP Life Cycle Engineering (LCE) Conference, 30 April – 2 May 2018, Copenhagen, Denmark

Environmental Performance and Key Characteristics in Additive Manufacturing: A Literature Review Yahya AL-MESLEMIa*, Nabil ANWERa, Luc MATHIEUa a

LURPA, ENS Cachan, Univ. Paris-Sud, Université Paris-Saclay, 94235 Cachan, France

* Corresponding author. Tel.: +33 1 47 40 22 20; fax: +33 1 47 40 22 15. E-mail address: [email protected]

Abstract Additive Manufacturing (AM) opens new opportunities for the economy and the society and the global market of this technology is growing rapidly. However, quality assurance remains the main barrier for a broader integration of AM in the industrial sector. Most quality-related problems of AM are caused by uncontrolled variations in the production chain. By identifying the key controlling parameters or the “Key Characteristics (KC)” and introducing the proper process control protocol for these parameters, the effect of these variations can be limited and expensive monitoring, rework, repair and quality-related problems can often be avoided. In addition, these KCs can also be used to achieve sustainability in AM. In addition of providing a product with a quality that satisfies the client’s requirements, the manufacturing resources usage (energy and material) can be minimized. The work presented in this paper reviews the recent literature related to sustainability in AM and proposes a new approach into how the key characteristics, which are normally used to reduce variations in production, can give an insight to a sustainable AM. © Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ©2018 2017The The Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference. Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference

Keywords: Additive Manufacturing; Quality Assurance; Sustainability; Key Characteristics

1.

Introduction

1.1. Additive Manufacturing Additive Manufacturing (AM) is defined as the process in which an object is produced by joining a number of layers with specific thickness using different bonding technologies. The production chain starts by creating the geometrical model using a CAD software or reverse engineering a physical product. This model is tessellated, sliced, and then transferred to the AM machine that must be properly set up prior to the build process. When the part is removed, it may require additional cleaning and post-treatment. There exist seven categories of AM processes and each process uses different bonding technology, suited for specific type of materials, and requires a different postprocessing [1, 2].

AM is fundamentally different from other manufacturing processes such as casting and machining. Adopting this technology can overcome some handicaps that traditional manufacturing suffers from and can bring huge changes to the industrial sector. AM offers the following: x A high customization and innovation possibilities: major interest of AM is to build parts or areas of parts that are not manufacturable by conventional means. x A high complexity level: the ability to design internal structures (lattice structures) within a part creates a high potential for lightweight and high performance components and devices. x A relative low associated cost: no tooling or assembly cost is required and less material is used compared to the alternative techniques. Depending on the production volume and the complexity of the product, AM can compete with traditional manufacturing techniques such as injection moulding [3].

2212-8271 © 2018 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 25th CIRP Life Cycle Engineering (LCE) Conference doi:10.1016/j.procir.2017.11.141

Yahya Al-Meslemi et al. / Procedia CIRP 69 (2018) 148 – 153

can reduce their severity, which can help achieve a certain level of sustainability. In this section, we will focus only on Metallic Powder Bed Fusion Process.

1.2. Sustainability in Additive Manufacturing The benefits of AM should be compared to other alternatives when choosing the manufacturing strategy. When considering AM, several decisive factors are put into question such as the added value, the effectiveness (regarding the lead-time, the cost, and the amount of material used), the quality desired, and the application field (prototyping, tooling, or end-to-end manufacturing). However, one overlooked factor is the level of sustainability (and precisely the environmental benefits) that can be offered by this technology. In general, this is because companies lack the incentive to implement any environmental protection measures. Depending on the size of the company, other forms of incentives can be implemented. Companies driven by environmental incentives usually aim to increase the profit margin, preserve the resources they use to manufacture their products, and increase their competitiveness. This is why policies are being adopted to evaluate the environmental performance in the industrial sector [4]. There is a big uncertainty regarding sustainability-related data and their representativeness [5, 6]. At present, only a limited researcher is available which compare AM to traditional manufacturing in terms of three aspects (Table.1). Table 1. Sustainability Aspects. Description Sustainability Aspects Social aspect

This is concerned with the health and the safety of workers and the working conditions. In addition, it is related to the consumption patterns of clients and the competitiveness between companies. This aspect is hard to measure since how a product benefits the society is relative and can be interpreted differently.

Economical aspect

This covers a wide variety of areas and many sustainability-related measures can be integrated in any part of the product’s lifecycle to target one objective: the reduction of the production cost.

Environmental aspect

This is where pollution, process emissions, and material usage are studied. Measuring the environmental impact is quantified throughout the product’s life cycle using a variety of tools and indicators.

Agencies and companies focus only on one aspect instead of evaluating the overall sustainability-related challenges and opportunities. A good product is that which can maximize the social and the economic aspects and minimize the harmful environmental impact [7]. The work presented here gives some insight into sustainability in additive manufacturing from the literature. The remainder of the paper is organized as follows: x Section 2 reviews the literature related to sustainability in all the AM processes and focuses only on the environmental and the economic aspects; x Section 3 describes the main quality problems in AM and how proper process control of the main key characteristics

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2.

Literature Review on Sustainability in Additive Manufacturing

The life cycle of any product can be divided into five phases [8]: material production, manufacturing, distribution, usage, and disposal phases. In the first phase, several atomization techniques are used to transfer the material into a usable form for manufacturing. Manufacturing phase includes all the successive processes that construct the production chain from creating the digital model to the postprocessing. Distribution deals with transportation and packaging of the manufactured product. Usage includes all the activities in which the product is used. Disposal phase begins after the usage phase is over. In this section, and for each lifecycle phase, we reviewed some sustainability-related literatures. 2.1. Material Production and the manufacturing Phase Producing the material powder for AM consist of three stages: ore extraction, atomization, and validation. Dawes et al. [9] reviewed the different atomization processes. The atomization is a process in which raw material is put under great heat and pressure and atomized using argon gas to obtain the powder used for AM machines. This process is essential to powder-based processes, which includes binder jetting, powder bed fusion, and direct energy deposition processes. The evaluation of sustainability is measured on function of the electricity, water, and fluids used in the process [10]. Ma et al. [11] provided an overview on life cycle analysis of metal AM emphasizing on the feedstock powder role. In literature, the manufacturing phase has attracted a special attention due to the many sustainability-related opportunities it offers regarding the efficient use of energy and material. Duflou et al. [12] suggested that the manufacturing phase can be decomposed into many processes which are working individually (units) or collectively (lines and factories) and several measures can be adopted depending on which configuration is implemented. Measures related to several aspects such as the energy flow, the production time and planning, the process nature, the process control, and the waste recovery can ensure a sustainable and an eco-friendly manufacturing phase. Energy consumption is one of the big sustainability questions. Regarding selective laser sintering (SLS), Sreenivasan et al. [13] estimated that there are five main power drains during this phase: laser, bed heaters, rollers, piston motors, and small machine systems. Mognol et al. [14] suggested that reducing the manufacturing time is a direct method to minimize energy consumption. The author described the manufacturing time as dependent on both the part’s height and the volume of support. Baumers et al. [15] studied the relation between energy consumption and the machine’s building capacity. The relation was examined across different platforms. Calculating the energy took in consideration three parameters: the build volume, stocking strategy, and the

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preheating process. It was concluded that the used capacity has an impact on the energy efficiency and this impact varies from one AM technique to another: for Laser Sintering (LS), due to the energy needed for warming and atmosphere creation, manufacturing multiple parts has a greater energy saving. However, for Fused Deposition Modeling (FDM), no efficiency can be noticed from both configurations (single or multiple parts production). Kellens et al. [16] focused more on Selective Laser Sintering (SLS) and studied different correlations between process parameters, precisely processing time, the energy and compressed air consumption, and the waste powder. In addition, the author proposed two factors for a potential reduction of the environmental impact: build design (including nesting strategy and layer thickness) and machine tool design (including chambers and bed platform). Paul et al. [17, 18] were interested in modeling and calculating the total energy for manufacturing a part. The authors established parametric correlations between the slice thickness, the part’s orientation and geometry, and the laser energy. Reducing energy consumption using support structures instead bed heaters was studied as well. In plastic laser sintering, this can circumvent one of the energy consumptionrelated problems since bed heaters are responsible for at least 20% of the energy consumption [16, 19]. Diaz et al. [20] studied the concept of highly stable bioinspired support structures. The environmental and the economic performances were calculated as a function of the power, process time, and the material used to manufacture a complete part with support. Around 50% material saving rate was possible. Similarly, part’s topology plays an important role in optimizing the material ecology and in pushing towards innovative solutions [21]. A powder delivery system was developed by Wang et al. [22] using ultrasonic vibration to optimize powder delivery in direct laser deposition AM. The system optimizes the powder flow and decreases the amount of waste powder. Although AM offers many advantages compared to conventional manufacturing, the quality and geometrical accuracy still suffers from problems. To enhance the surface properties, postprocessing is an essential step [23]. Postprocessing methods used after the layering process are different and choosing the proper method depend on the geometrical quality wanted, the powder material used, and the additive technique mechanics. Dotchev et al. [24] proposed a method to gather un-sintered powder depending on the Melt Flow Rate (MFR) index and powder deterioration. Based on this, powder with acceptable quality can be recycled and used again to provide consistent parts. Also, the quantity of the powder removed depends on several parameters, including but not limited to, the parts geometry, the raw material, the air pressure of the Powder Recovery System (PRS), and the removal strategy (duration) [25]. 2.2. Distribution, Usage and Disposal Depending on the type of the product, usage scenarios may differ. For products that do not consume energy, sustainability can be achieved if the usage period increased and the disposal phase is delayed. Tateno et al. [26] pointed to the importance of customization of additively manufactured product by

improving their aesthetic value which can lead to reusing products. Baumers et al. [27] concluded that there is no apparent relation between energy consumption and the product’s geometrical complexity. Thus, it is possible to increase the product’s serviceability using the customization abilities of AM. For products that are used for transportation purposes (such as the automotive parts), decreasing the energy used and the emissions they release can increase their level of sustainability. Morrow et al. [28] emphasized on the capacity of DMD to repair tools while being environmentally friendly. Also, sustainability can be reached using Maintenance, Repair and Overhaul (MRO) process flow. By profiting from the advantages AM offers, users can download CAD files directly to print / replace / optimize / merge parts bypassing original manufacturers. This can prove to be beneficial since the time and cost are reduced [29]. 3.

Key Characteristics for Additive Manufacturing

3.1. Quality Assurance There are still significant challenges that stem from the fact that the underlining physics of AM is still complex. Aspects such as the repeatability and the traceability are the major hinders facing a wider adoption of this technology. The lack of consistency can lead many AM manufacturers to supply parts with different characteristics, in terms of either mechanical properties or geometric tolerances. One solution that can limit these problems is to reduce the damage caused by these inconsistencies using corrective protocols. The manufacturing phase is composed of many successive processes which construct the production chain [1]. Process control is a set of measures that insures the process’s efficiency. The process that falls within the control limits (determined by the client’s requirements and the process capability) is considered under control. Otherwise, this indicates that a cause, or a combination of causes, is the source of a variation in the production [30]. Tapia et al. [31] reviewed the process monitoring and control efforts in literature. For an optimized control, an in-depth understanding to the AM’s informational flow is necessary. The informational flow is represented by a set of large volumes of data or parameters and it is not feasible to monitor or control all these parameters especially in complex products. One solution is to separate the useful parameters from the meaningless ones and to focus on them. These parameters are given the notion of ‘’Key Characteristics (KC)’’. 3.2. Key Characteristics (KCs) in Additive Manufacturing Many definitions for KCs are proposed in literature [32]. Although the “Key Characteristics” terminology and implementation may vary between corporations, but one common goal is shared: to identify a small set of critical features to focus on during manufacturing phases. Spears et al. [33] listed around 50 key parameters in SLM. These critical features have several points in common. Their variation causes the most harmful effects on the product and the process

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performance. They are unique and dynamic in nature and different experts and organizations may have different understandings of the meaning of the same KC depending on the quality system adopted. They are identified during any phase of the product’s lifecycle and their selection requires a thorough knowledge of the company’s value chain and customer needs [34]. 3.3. How to achieve Quality Assurance using Key Characteristics? Process variations can have different source and types. Variations require special control to reduce their effects. Proper process control can efficiently reduce the effect of these variations if the chosen monitored parameters are those whose impact is important. This means that identifying the main key characteristics of both the product and the process is an essential step to ensure the result approaches the client’s requirements [35]. After identifying the sources of the variation, the associated parameters are identified and listed hierarchically based on their influence on the product (or the process) when their status changes. Those ones on the top of the list are considered the most impacting parameters. As a result, the variation(s) in question will be verified (if the variation still respects the control limits), reduced, or monitored [32]. While this is the objective of quality assurance, sustainability searchers on how to reduce resources usage and the impact resulted. The next sub-section discusses how through using the same KCs, we can attain the sustainability main objective: minimize the usage of resources regarding the energy consumed and the material used. 3.4. How to achieve Sustainability using Key Characteristics?

Fig. 1. Powder Bed Fusion process [1]

From the review in section 2, we can conclude that sustainability focuses on the efficiency of using the resources and process control can be a way to attain a sustainable manufacturing phase. Table 2 summarises how AM can be sustainable (from an economic and environmental point of views) and the KCs associated with these sustainability goals. We chose only four methods to reach sustainability during the manufacturing phase: reducing the time of manufacturing; reducing the cost of manufacturing; reducing the material usage; increasing the parts functionality. Naturally, other sustainability-related solutions can be applied during the other lifecycle phases of additively manufactured products. However, there is no documented work on the KCs associated with those phases. Table 2. Sustainability solutions and related KCs Related KC How to achieve sustainability? Reduction of the manufacturing time

In this sub-section, we will limit ourselves to one category of metallic AM and one phase in the lifecycle: the manufacturing phase in Powder Bed Fusion (PBF). In literature, the manufacturing phase is regarded attentively by authors and most the identified KCs are process-related parameters. These KCs belong to one of four areas as shown in Fig.1: x Scanning System: which includes the laser system; x Powder Deposition System: which includes the powder delivery system, the roller, and the powder bed; x Building Environment: which involves the part’s layering strategy and conditions; x Meltpool Zone: which represent the interaction between the powder and the laser beam.

Scanning System

Scanning Speed Scanning Strategy (Layering of part and support)

Powder Deposition System

Roller Speed

Building Environment

Number of parts Spacing between parts Height of parts Layer thickness Nesting strategy

Reduction of the material usage

Scanning System

Scanning Strategy (Layering of part and support)

Powder Deposition System

Bed thickness


Number of parts Height of parts Nesting strategy Powder Bed (Powder Contamination)

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Reduction of production’s cost

Increasing the product’s functionality

Manufacturing Time and Material Usage Scanning System

Average Power


Powder Bed (Powder Morphology and Bed Temperature)

Scanning System

Scanning Strategy (Layering of part)

The table relates the manufacturing key characteristics to some of the sustainability’s solutions concluded from the review in section 2: x Reducing manufacturing time: the time is affected by three main factors. One factor is the manufactured part’s height and orientation on the building plate. Another one is the production volume, which is a function of the number of parts on the building plate, their position, and the nesting strategy. The last factor is the scanning pattern that take in consideration the customization and topology improvement abilities of AM. The machine operating time has a direct impact on the operational cost and the amount of emissions (economic and environmental aspects). x Minimizing powder material usage: this can be achieved either by recycling the un-sintered powder extracted from finished part and reusing it (if the percentage of contamination is low) or by optimizing the topology of manufactured parts (by using the proper scanning strategy). In addition, the amount of powder used is proportional to the number of parts on the building plate. x Reduce the manufacturing cost: the total cost of in the manufacturing phase is composed into three parts: o Material cost: which depends on the properties of powder; o Energy cost: which includes the energy needed of operating all the systems of the machine. Also, the longer the production time, the more energy used; o Indirect cost: such as labor cost, machine cost, etc. x Increase the functionality of part: this includes the serviceability time of the product. This can be done by increasing the aesthetic value via shape personalization. As the KCs can give an insight into how and where to reduce which variations in the manufacturing phase, they can also give an indication into the areas where sustainability is achievable by connecting the sustainability goals to the manufacturing phase parameters. Now, we can easily identify which parameters can help reduce the manufacturing cost, the material

usage, the production time, or increase the following usage phase Our proposed approach constitutes a new vision to implement a more sustainable manufacturing phase for AM. This approach is heavily dependent on the customer’s specifications, the AM process chosen, the company’s incentive, and the application field. For example, products designed for the aeronautics possess different product characteristics from those that are used in the medical sector. As a result, the process-related KCs will be different. Also, depending on the manufacturer’s motivation, either the quality aspect or the sustainability aspect is prioritized. However, depending on the part’s geometry and the proposed postprocessing, a compromise can be found to achieve the two goals. For example, setting the scanning strategy to optimize the part geometry using cellular structures. This will reduce the material usage (sustainability aspect) without compromising the part’s functionality (quality aspect). 4.

Conclusion

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