J. Cent. South Univ. (2020) 27: 1134-1145

DOI: https://doi.org/10.1007/s11771-020-4354-6

Towards a circular metal additive manufacturing through recycling of materials: A mini review

XIA Yang(夏阳)¹, DONG Zhao-wang(董朝望)¹, GUO Xue-yi(郭学益)¹,TIAN Qing-hua(田庆华)¹, LIU Yong(刘咏)²

1. School of Metallurgy and Environment, Central South University, Changsha 410083, China;

2. State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract: Additive manufacturing is a new emerging technology which is ideal for low-to-zero waste production, and it is considered to be a green and clean process that has the potential to lower the cost and energy consumption of production. However, the cost of the feedstock for additive manufacturing and the additive manufactured parts is usually very high, which hinders the further application of additive manufacturing, especially for the metal additive manufacturing. The concept of circular metal additive manufacturing involves the recycling of the metal feedstock and the additive manufactured parts leading to the truly zero waste production and the most energy saving. This paper reviews the technologies that help the formation of a circular metal additive manufacturing through recycling of the feedstocks and the damaged metal parts. Reactive metals, such as titanium, tend to be contaminated easily during handling and production. Recycling of the titanium for achieving a circular titanium additive manufacturing is reviewed in detail.

Key words: recycling; additive manufacturing; titanium; powder

Cite this article as: XIA Yang, DONG Zhao-wang, GUO Xue-yi, TIAN Qing-hua, LIU Yong. Towards a circular metal additive manufacturing through recycling of materials: A mini review [J]. Journal of Central South University, 2020, 27(4): 1134-1145. DOI: https://doi.org/10.1007/s11771-020-4354-6.

1 Introduction

Additive manufacturing is a novel process for building a three-dimensional object from a computer-aided design (CAD) model, usually by successively adding material layer by layer [1-3], unlike conventional casting, forging, and machining processes, where material is removed from a stock item (subtractive manufacturing) or poured into a mold and shaped by means of dies and presses [4-6]. Metal additive manufacturing is the process by which metal parts are joined or solidified from a metal feedstock [7, 8]. The typical metal additive manufacturing includes powder bed fusion such as selective laser melting (SLM) [9], and electron beam melting (EBM) [10], direct energy deposition such as laser materials deposition [11], and binder jetting [12, 13]. The major differences between these types of metal additive manufacturing technologies are related to how they fuse the powder into metal parts. They vary greatly, from using high energy lasers to fuse loose powder to extruding bound metal powder filament. However, they all require spherical powder as the feedstock for a good flow ability. Therefore, a good metal feedstock is very critical to metal additive manufacturing. Since the materials utilization rate is high, additive manufacturing inherently produces much less waste than the conventional subtractive manufacturing [14, 15]. It has the potential to support the circular strategies，offer sustainable design and manufacturing by creating opportunities to extend a product’s lifespan (e.g. by enabling repair or upgrades of the products), and enable the feasibility of recycling of additive manufacturing feedstocks and products [16, 17].

Recycling is a process of transforming waste or no-value materials into new and useful materials, which is an alternative to traditional waste disposal that can save material and help to reduce the greenhouse gas emissions [18]. It can prevent the waste of potentially useful materials and decrease the consumption of fresh raw materials, thereby decreasing energy usage, air pollution from incineration, and water pollution from landfilling. It is a key component of waste reduction and aims at environmental sustainability by substituting raw material inputs into the economic system and redirecting waste outputs out of the economic system [19-21].

The combination of additive manufacturing and recycling has the potential to achieve 100% materials usage and 0% waste production, thus significantly decreasing the cost and energy consumption during production. This paper proposes a new concept of circular additive manufacturing, which can be achieved by recycling of the feedstocks and the damaged parts. The related research on the recycling and preparing of additive manufacturing feedstocks by recycling of the used metals, and the recycling of broken parts by repairing with additive manufacturing was reviewed.

2 Concept of circular additive manufacturing

Figure 1(a) shows the traditional linear additive manufacturing. The no-value products or by-products from extraction, manufacturing or the end of the life (EOL) products, or products with no functionality are usually disposed. Lots of energy and resources are wasted during the traditional manufacturing. Circular additive manufacturing (CAM) is a promising manufacturing strategy towards the sustainable development. Usually, when the product reaches EOL, there is no value from the point of view of application. With the CAM based manufacturing strategy, the lifetime of the product, which is determined by its physical, functional, technical, economical properties, can be extended, and the energy and resources needed for extraction of metals from the original ores, heat treating the intermediated materials, and machining to make the final products can be avoided or reduced. Figure 1(b) shows the flowchart of the circular additive manufacturing. The waste feedstocks or products are recycled in circular additive manufacturing for achieving 100% materials usage and 0% waste production, thus significantly decreasing the cost and energy consumption during production.

The combination of additive manufacturing and recycling has the potential to offer sustainable design and manufacturing by creating opportunities to extend the product’s lifespan. It is an alternative to conventional waste disposal that can save material and help to lower greenhouse gas emissions. By combination of recycling of resources and additive manufacturing, an enhanced manufacturing with the most energy-and-resources saving can be achieved. In the next section, the measures and processes, including recycling of the feedstocks, recycling of the scraps for making feedstocks and recycling of the damaged parts by repairing with additive manufacturing, for achieving the circular additive manufacturing will be reviewed.

Figure 1 Comparison of circular additive manufacturing with linear additive manufacturing:

3 Recycling of metal feedstocks

3.1 Recycling of used feedstocks

Material recycling enables manufacturers to recapture some of the energy in the material waste streams and increase the materials utilization rate. A large percentage of material throughout the feedstock manufacturing, printing and post processing can be reprocessed and reused. The powder-based additive manufacturing technologies have their own requirements for material properties in terms of chemistry, size distribution, porosity, flow-ability, morphology, and apparent density, which affect the properties of the final product [22-24]. However, powder degradation occurs during manufacturing [25]. Significant oxygen pick-up, morphology and flowability change are three typical degradation issues [26-28]. L.P.W. technology did a case study on the powder degradation [26]. It has been found that the oxygen content in the powder increased when increasing the number of printing cycles during selective laser melting [26]. The oxygen content in mass fraction was analyzed using inert gas fusion (IGF), and samples, taken after the first build, after sieving and after drying, showed no changing in oxygen content in the In718 powder. However, powder tested after subsequent builds showed an increase in oxygen with each build, as illustrated in Figure 2(a), an increase of 0.002 wt% over 14 builds. Further builds and tensile test on the printed samples were carried out with the results shown in Figure 2(b). It indicated that after 25 builds, there is a significant increase in ultimate strength and reduction in ductility from 20% to around 13%. Titanium powders are more susceptible to oxygen pick-up, and therefore can only be used in limited times before the powders fall out of specification due to the high oxygen content. Figure 2(c) shows that over 0.02 wt% of oxygen was increased after 10 builds [26].

Figure 2 Effect of printing cycle on:

When oxygen level does not meet the standard specification, the powder cannot be used any more. In that case, powder has to be retreated to drag the oxygen back to the range of the standard specification. A treatment such as deoxygenation is necessary. Recently, XIA et al [29-32] developed a low-temperature molten salt calcium deoxygenation process. This process can deoxygenate titanium- oxygen solid solution with the oxygen level from 0.1 wt% to 14.3 wt%. The deoxygenating agent was calcium in this process. The calcium halide bearing eutectic molten salt was used to decrease the deoxygenation temperature from above 850 °C to as low as 600 °C. The molten salt helps the dissolution of deoxygenation agent Ca at the temperature less than 850 °C. Ca⁺ ions are formed in the molten salt [31]. As shown in Figure 3, they react on the surface of the titanium particles to cause the deoxygenation reaction by the following reactions:

Ca+Ca²⁺→2Ca⁺                                         (1)

Ca⁺→Ca²⁺+e                                           (2)

2e+O=O^2-                                               (3)

O^2-+Ca²⁺=CaO                                    (4)

Detailed TEM analysis showed that the crystal lattice cell shrinks after the release of interstitial oxygen. The lattice parameter of a decreased to 0.294 nm from 0.302 nm and c decreased to 0.465 nm from 0.491 nm after deoxygenation [31].

Except the low temperature molten salt deoxygenation method, oxygen can also be removed by the DOSS process [33, 34], hydrogen assisted Mg deoxygenation process [35-38], the Ca vacuum process [39-42] and the electrolysis process [43-45]. A comprehensive review on the deoxygenation technologies can be found in our recent publication [46].

Morphology change is another degradation issue. As these materials melt at higher temperatures, the material surrounding the melt becomes distorted and sintered together, which can make powder particles larger and unusable [26]. Figure 4(a) shows the typical spherical morphology of the gas atomized metal powder [47]. After printing, some of the powders tended to distort and more satellite powders were observed as shown in Figure 4(b). One plausible technique to recover these powders is the induction plasma process. This process consists of in-flight heating and melting of the surface of the powder by the plasma. The powder characteristics such as morphology and flowability can be improved as shown in Figure 4(c) [26].

3.2 Recycling of scraps or undesirable powders for making additive manufacturing feedstocks

For titanium, the buy to fly (BTF) ratio is often more than 20:1, i.e., to produce a part weighing only a pound may require more than 9.07 kg of raw material, with a yield of only 5% [48]. As a result, most of the materials are wasted during the fabrication process. Therefore, lots of titanium scrap is produced during titanium part production. Based on the information of titanium scrap production and consumption from 2009 to 2018 from the United States Geological Survey statistics in the USA. In 2018, about 61500 t of titanium scrap metal was consumed. Most of the time, the Ti scraps are re-melted with the virgin metal to produce primary ingots of Ti or its alloys [46].

Recently, FANG and his coworkers [32, 49, 50] developed a novel process for making Ti alloy spherical powders from the Ti powder scrap.Figures 5-7 show the process flowchart and materials of the novel process. The milled fine titanium hydride particles are first agglomerated to form spherical granules in a desired size range with the assistance of binder and spray drying; then the granules are treated to obtain dense spherical powder; and finally, the densified spherical Ti powder is deoxygenated with a reducing agent to reduce oxygen content to meet the chemical composition requirements of industry standards. With this process, the waste titanium scrap can be reused and transferred to a high-value product which can be used as feedstock for additive manufacturing. The new process overcomes the two difficulties in the processing methods mentioned above, much higher yield than the conventional atomization techniques, and use of low cost feedstock materials [49]. The tensile properties of parts fabricated from spherical Ti-6Al-4V powder produced with the granulation-sintering- deoxygenation (GSD) process by selective laser melting are comparable with the typical mill- annealed Ti-6Al-4V alloy, and the characteristics of printed alloy from the powder are also compared with those of commercial materials [47]. It demonstrated that the process is capable of producing good powder for additive manufacturing application.

By using the similar method, some other spherical metal powder feedstocks can be produced for additive manufacturing by using the undesirable low-value and even waste elemental powders. Our recent study [51] showed that the spherical Co-Cr-Mo alloy powder for additive manufacturing could be successfully produced by granulation and sintering. Low-value elemental powder (Co, Cr, and Mo) was first mixed in water with PEG as the binder to make a slurry, and then granulated using a spray drying machine. The spray-dried porous powder was then densified in a hydrogen atmosphere to make a final powder. The produced powder meets the composition standard specification for Co-Cr-Mo alloy with the ASTM F75 standard [52]. The process flowchart and the product at each step are shown in Figure 8.

Figure 3 Deoxygenation treating of high oxygen-containing powder:

Figure 4 SEM images of:

Figure 5 Process flowchart for making spherical powder from Ti-6Al-4V hydride

Figure 6 SEM images of:

Figure 7 Particle size distribution of final powder reproduced with permission [49]

For some alloys such as Ti-Ta alloy powder, it is very difficult to produce because of the segregation problem during melting [49, 50]. XIA et al [32] recently found that the biomedical Ti-30Ta alloy spherical powder can be produced using this granulation-sintering-deoxygenation process as well. The elemental Ti and Ta powders were used as the starting materials. The results indicated that the composition segregation problem could be avoided. With an additional deoxygenation process, the critical intestinal element oxygen was controlled to be <0.04 wt% for the powder with particle size of <75 μm.

4 Recycling of damaged parts

Repair processes aim at returning the value of the product during its life cycle. Repairing the Figure 8 Process flowchart (a), products at each step of Co-Cr-Mo alloy spherical powder production starting from low-value elemental powder (b)-(e) [51] damaged parts is usually finished by welding a new piece of material that has to be machined to exact size. Additive manufacturing can also be used to build up a new structure or repair based on the damaged parts. The end-of-life of the product can be reversed by extension usage or giving a second life to the product. Additive manufacturing enables a free-form structure modelling and remanufacturing of complex geometry laser or electron beam additive repairs through sintering, deposition or melting. Additive manufacturing repair showed the high flexibility, efficiency, fast output, good quality and low cost [55]. Additive manufacturing technology offers an effective and attractive method for repair in the case of the chipped components, namely, the blade tip, impeller blade, sprocket, or gas turbine. It resolves the issue of direct build-up in the position of the broken part layer-by-layer, rather than requiring a manual build of the part and then attaching it to the position of the broken part [56-66]. A detailed review on additive manufacturing for repair and restoration in remanufacturing was recently published by DASS et al [67].

Figure 8 Process flowchart (a), products at each step of Co-Cr-Mo alloy spherical powder production starting from low-value elemental powder (b)-(e) [51]

Figure 9 SEM images of powder at each step for making Ti-30Ta spherical powder from low-value elemental powders:

Compared with the conventional repair welding technologies like tungsten inert gas or gas metal arc welding, the additive manufacturing repairing feature has several obvious advantages. First, the fine control over the processing parameters including the atmosphere, temperature, pressure, air flow, impurity (O, N, H, etc.) pick-up, the laser power and burn time, etc., could be possible since the entire process is conducted in an enclosed and controlled chamber. The homogenous deposition or materials addition with low defect occurrence can be ensured and the added portion can be completely merged with the underlaying base layers [66]. Second, heat input during repairing is usually very low, which leads to much lower distortion and lower thermal damage in the underlaying base material. Furthermore, finer microstructure with better performance can be obtainable. Lastly, the energy input can be controlled to be stable and repeatable, which grants a high reproducibility of the material deposition and therefore a high reliability and the opportunity to automate the processes can be achievable [66]. In addition, the additive manufacturing repairing process can be done with in a relative short time with a reasonable cost if a complex geometry is needed.

Therefore, additive manufacturing is expected to be an enabling technology for repairing, remanufacturing and even redesign in circular additive manufacturing. Many companies [68-70] have started offering the repairing and remanufacturing service based on the additive manufacturing technologies, such as laser cladding, laser melting, or laser engineered net shaping technologies. The missing part or the worn-out portion can be readily added to the base sample. If this issue breakthrough in feature comes with the support of government agencies and local research institutions, a greater confidence and acceptance of the repair innovation could be expected.

5 Summary and prospects

A circular additive manufacturing was proposed through recycling of the feedstocks and the damaged parts. With the circular additive manufacturing strategy, the lifetime of the intermediates or the final products, which is determined by its physical, functional, technical, economical properties, can be extended, and the energy and resources needed for extraction of metals from the original ores, heat treating the intermediated materials, and machining to make the final products can be reduced. In the case of titanium, the scrap and the waste feedstock can be recycled through several technologies. And the damaged or wore-out part can be returned to use after repairing using additive manufacturing such as laser cladding, laser melting, or laser engineered net shaping technologies. However, most of these technologies are still under development. To achieve the effective use of the resource and the sustainable development of the circular additive manufacturing, more researches on combination of recycling and advanced manufacturing like additive manufacturing are needed. In addition, the policy support from the government and financial support from the industry are necessary to ensure the steady and fast development of the circular additive manufacturing.

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(Edited by ZHENG Yu-tong)

中文导读

小综述: 通过材料回收实现金属增材制造的循环利用

摘要：增材制造是一种新兴的废物排放接近于零的生产技术，它被公认为是一种绿色清洁的生产工艺，具有降低生产成本和能耗的潜力。但是增材制造的原料成本和增材制造的零部件成本通常比较高，严重阻碍了增材制造，尤其是金属增材制造的进一步工业应用。循环金属增材制造的概念涉及到金属原料和零部件的回收利用，从而实现真正的零废物产生和最大程度的能量节省。本文综述了通过循环利用增材制造过程中的金属原料和金属零件形成循环的金属增材制造技术。活性金属，例如钛，在粉末处理和生产过程中很容易被污染，本文详细介绍了钛材增材制造过程中金属钛的综合利用。

关键词：循环利用；增材制造；钛；粉末

These authors contribute equally: XIA Yang, DONG Zhao-wang.

Foundation item: Project(51922108) supported by the National Natural Science Foundation of China; Project(2019JJ20031) supported by Hunan Natural Science Foundation, China; Project(2019SK2061) supported by Hunan Key Research and Development Program, China

Received date: 2019-12-25; Accepted date: 2020-03-23

Corresponding author: XIA Yang, PhD, Associate Professor; Tel: +86-731-88876089; E-mail: yang.xia@uqconnect.edu.au; ORCID: 0000-0001-9536-4445; GUO Xue-yi, PhD, Professor; Tel: +86-731-88879101; E-mail: xyguo@csu.edu.cn