Open access peer-reviewed chapter - ONLINE FIRST

Pure Copper: Advanced Additive Manufacturing

Written By

Lukas Stepien, Samira Gruber, Moritz Greifzu, Mirko Riede and Aljoscha Roch

Submitted: January 14th, 2022 Reviewed: February 11th, 2022 Published: May 5th, 2022

DOI: 10.5772/intechopen.103673

IntechOpen
Advanced Additive Manufacturing Edited by Igor Shishkovsky

From the Edited Volume

Advanced Additive Manufacturing [Working Title]

Prof. Igor V. Shishkovsky

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Abstract

This book chapter elaborates on different additive manufacturing (AM) processes of copper and copper alloys. The scope is to give the reader a basic understanding of the state-of-the-art of copper additive manufacturing by different AM technologies, such as laser powder bed fusion (LPBF), laser metal deposition (LMD), binder jetting (BJ), and metal-fused filament fabrication (M-FFF). Furthermore, we want the reader to be able to use this knowledge to find and assess potential use cases. Recently, with the commercial availability of green laser sources, the difficulties for laser processing of pure copper were overcome, which gave AM technologies, such as LPBF and LMD new momentum and increased interest. AM technologies involving a subsequent sintering step. They are relatively new and gained interest due to fast build-up rates (BJ) or ease of operation (M-FFF). We will cover important material-related properties of copper and its implications for manufacturing and application (e.g. absorption, sinterability, conductivity, and its dependency on impurities). Further, we address applications for AM copper, present the state-of-the-art for above mentioned AM technologies and share our own recent research in this field.

Keywords

  • additive manufacturing
  • copper
  • electrical conductivity
  • applications

1. Introduction

Copper and copper alloys are one of the major groups of commercial metals. Pure copper is defined as having a minimum copper content of 99.3% [1].

While pure copper is used extensively for electrical components, such as cables and contacts, alloys like brass or bronze are used for thermal energy transfer applications, such as radiators and heat exchangers [2].

While the laser-based additive manufacturing of alloyed coppers, such as brass or bronze, was successfully done, approaches in processing pure copper with, at the time, available infrared laser sources were not satisfying in terms of electrical conductivity, density, and process stability. Electron beam-based AM technologies overcame this and reached densities close to 99.8% [3], however, the coarse powders combined with the high-thermal conductivity resulted in higher surface roughness and hindered de-powdering of fine channels. Sintered-based AM technologies recently reached densities above 95%, but as in metal injection molding (MIM), their mechanical properties are behind their laser or electron beam-melted counterparts.

With the availability of a powerful green laser source, some of the drawbacks in terms of the processing could be overcome resulting in highly dense and conductive parts. However technological aspects, such as a bigger laser spot diameter reduces the ability to produce for instance thin-walled or other intricate features.

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2. Physical properties and their effects on manufacturing

Copper has unique properties that make it an outstanding engineering material, however, those properties can make the processing a particular challenge in the context of additive manufacturing and demands specific approaches.

2.1 Physical properties of pure copper

Copper possesses the second highest electrical and thermal conductivity of all metals. The high-thermal conductivity of copper is a particular challenge during welding processes whether it is during the direct laser metal deposition or powder bed laser processing. For powder bed, this results in higher surface roughness, because the heat zone (due to the heat spreading into the powder bed) is wider causing particles to partially sinter to the consolidated body.

Due to its crystalline structure (fcc), pure copper also has high ductility. This also remains after the processing of pure copper parts from powders. Internal stresses, typically a problem for additively manufactured materials, are very low. This is beneficial since process and geometry-induced distortions are usually not a big problem. Further, an stress-relief annealing, is in most cases, not necessary but may be useful for the homogenization of the microstructure. Table 1 gives a brief overview of some physical properties of pure copper. While, based on the definition of pure copper, the absolute values often show deviations, however, the table should give an orientation.

Melting point1083°C[4]
Density8.94 g/cm3 @ 20°C[4]
Coef. thermal expansion17.0 x 106 /C (20–100°C)[4]
Thermal conductivity401 W/mK @ 20°C[4]
Electrical conductivity59.6 MS/m @ 20°C[5]
Ultimate tensile strength210–390 MPa[6]
Young’s modulus120 GPa[6]

Table 1.

Physical properties of pure bulk copper.

The electrical conductivity of copper and its alloys is often given relative to a copper wire test sample (international annealed copper standard, IACS) which was established in 1914. For comparison, 100% IACS is defined as 58×106 S/m at 20 °C, while the absolute maximum electrical conductivity measured for pure copper (Cu-ETP-1 or Cu-OF-1) is 58.58×106 S/m at 20°C (referring to 101% IACS). Thus, some electrical conductivity values may also exceed 100% IACS [7]. For pure copper (99.999%) value is 103.06% IACS and for pure silver it is 106% IACS [2].

2.2 Absorption

The absorptivity of electromagnetic radiation into the material is wavelength-dependent. For all materials, the absorptivity generally increases with smaller wavelengths (Figure 1). For copper as a reflective material, there is a huge increase in absorptivity at 515 nm (green wavelength) compared to 1064 nm (infrared wavelength). This can be used for laser-based AM processes to change the laser source to smaller wavelengths to increase the absorptivity, thus energy can be transferred more effectively resulting in higher efficiency.

Figure 1.

Absorption of different solid metals. Data extracted from Spisz et al. [8].

To mitigate the low absorptivity in the infrared region, higher laser power can be used. Recently this approach become more attention due to the well-developed system technology, especially for big build sizes. However, the higher energy input into the powder bed can lead to smaller processing windows.

2.3 Sintering capability and impurities

While the absorptivity of copper does not affect the sintering capabilities of the copper powder, binder jetting and metal fused filament fabrication can be well compared to other powder metallurgical processes since a sintering step is clearly necessary to obtain functional metallic parts. For powder metallurgy of pure copper, the Copper Development Association Inc., an industrial board for copper, copper alloys, and their applications, mentions that “it is impractical to achieve a density of 8.94 g/cm3 by pressing and sintering alone” [9]. To achieve high density, in classical powder metallurgy, non-spherical powders are used and pressure for compaction of 207–248 MPa is recommended. Pre-compaction at higher pressures of up to 730 MPa can further increase the sintered density of simple geometries up to 97.6% [10] but might be impractical for parts that are more complex. The sintering density of the parts is then a function of sintering time and temperature, as shown in Figure 2. To show a more recent example, hot pressing of copper for 4 minutes at 600, 700, and 800°C at 50 MPa resulted in density values between 97.9 and 99.1% [11]. Interestingly, also at the highest measured density, electrical conductivity was corresponding to 90.2% IACS. This example may illustrate, that even achieving high physical density is still no guarantee to achieve high electrical conductivity, too. Besides pressurized sintering, also sintering atmosphere or other modifications are mentioned to influence the sintering activity positively, as the use of reactive gases ore use of powders having a thin oxide layer.

Figure 2.

Dependency of physical density from sintering temperature and time for copper powder compacts [9].

Ott et al. investigated the heat conductivity of pressureless sintered Cu-powders and analyzed the influence of residual porosity, but also elemental impurities on that physical parameter and backed their analysis with simulated data. The conclusion of that study was, that impurities, especially Fe, cause a stronger depression of thermal conductivity than pores. According to that group, porosity of 2–5% causes loss of 10 W/mK, while 200 mg/kg Fe cause ~40 W/mK [12]. Due to the connection between thermal and electrical transport, known as Wiedemann–Franz law, also the electrical conductivity is strongly affected by impurities (Figure 3).

Figure 3.

Relation between electrical conductivity and concentration of impurities [13].

During processing, oxygen from the ambient atmosphere or processing gas is the main contaminant. Fortunately, its effect on the conductivity is relatively small compared to other elements. However, using high-quality process gas (e.g. Argon with 99.999% purity) is highly recommended. Electrolytic-tough Pitch copper is allowed to have max. 400 ppm of oxygen. During LPBF processing we did not observe an additional rise in oxygen content for oxygen levels of 100 ppm in the processing gas during printing.

Ambient control with LMD is more challenging since normal shielding gas is often not enough to protect the part from oxidizing. Especially hot sections outside the working zone. Reasons are turbulences in the shielding gas stream down to the part. Technical solutions, such as a dedicated modular gas-shielding unit (e.g. COAXshield), showed good efficiency for Ti4Al4V but have to be verified for copper.

During the sintering of BJ and M-FFF parts, one can utilize hydrogen gas for the reduction of oxides and binder residue. However, during debinding carbon can potentially dissolve in copper causing a decrease in electrical conductivity.

Jadhav et al. showed (here in the case of nanoparticle addition for LPBF) that small impurities of 0.055 wt.-% carbon in the printed part can also reduce the electrical conductivity to 22.7 ×106 S/m (or 39.2% IACS) [14]. This explains the relatively lower electrical conductivity of binder-based sinter processes where complete binder burnout is often difficult to achieve.

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3. State-of-the-art laser powder bed fusion

Laser powder bed fusion is an AM process with the following repeating process steps—metal powder particles are spread evenly onto a substrate with a recoating system, then a laser source selectively melts the metal powder with specified parameters according to a previously prepared computer file with scanning strategy and laser parameters, such as laser powder, scanning velocity, and distance of single scanning tracks. Then the substrate plate is lowered by a specific layer thickness, a new powder layer is spread, and the process is repeated until the part is finished. Commercially available systems range in build volume, maximum laser power, amount of used laser sources, and laser type. Since the absorption of pure copper is poor in the infrared wavelength and commonly, LPBF machines were equipped with infrared fiber lasers, the processing of pure copper with LPBF was challenging in the past [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25]. The energy input into the material was insufficient for complete melting leaving a lack of fusion defects. The highest achievable density for pure copper parts when using a common 200 W infrared laser source was 83–88% [23, 24]. There have been two approaches in LPBF of pure copper to increase the density and subsequently the electrical conductivity—increase the infrared laser power to above 1 kW or switch to a green laser source. Colopi et al. and Ikeshoji et al. [19, 25] have used infrared laser powers of 1 kW and were able to increase the relative densities to 99.1 and 99.6%. However, melt-pool instabilities were observed due to the high difference in absorptivity in the solid and molten state of the pure copper which led to parts with low surface quality. Also, the high reflectivity can harm the optic system of the machine. TRUMPF has released an LPBF machine with an integrated green laser and could prove that high electrical conductivity can be achieved with such a system around 100% IACS [26]. With this machine, complex-shaped pure copper parts can be manufactured with high quality regarding density and electrical conductivity, and therefore, the technology is now ready to produce parts for various applications.

At Fraunhofer IWS such a TruPrint1000 Green Edition machine, equipped with a TruDisk1020 frequency-doubled laser emitting 515 nm wavelength, is available since mid-2020. The characteristics of the laser machine include a maximum laser power of 500 W, a spot diameter of 200 μm and a build volume of 100 mm diameter with 100 mm build height. Ongoing research concentrates on the following:

  • process parameter development for pure copper and copper alloys to increase the build rate while maintaining the high part quality, such as density and electrical conductivity

  • different post-processing techniques and their effects on surface quality and geometrical accuracy

  • pure copper and copper alloy applications

The density of pure copper parts is above 99.5% and the electrical conductivity was proven to be above 100%IACS. The oxygen content in the final part is below 400 ppm.

As can be seen in Figure 4, the surface quality shows the high roughness of the pure copper parts. Therefore, the surface needs smoothening. With two benchmark geometries developed by Fraunhofer IWS (Figure 5) specific feature sizes and overhang angle roughness can be analyzed via 3D scan and tactile measurements. The effect of different post-processes, such as sandblasting, abrasive flow machining, or chemical processes, such as plasma or electropolishing can improve the surface quality. However, material removal can be irregular, and therefore the process itself and applied parameters must be adapted to each geometry and particular application (Figure 5).

Figure 4.

Microsection of a density cube of pure copper (left), etched microstructure in the x-z axis (right) ©IWS.

Figure 5.

Benchmark for resolution and different features (left), benchmark for overhang angles (right) ©IWS.

Currently, possible applications investigated are components for the nuclear accelerator community, such as radiofrequency quadrupoles or nozzle geometries for laser metal deposition. Individualized inductor coils are also a field predestined for AM (Figure 6).

Figure 6.

Example of pure copper inductor coil geometry ©IWS.

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4. State-of-the-art laser metal deposition

Laser metal deposition (LMD) is an AM process that is assigned to the DED processes. Laser metal deposition is a well-established technology for coating and repair of metal components for more than a decade. Recently, it has been utilized for manufacturing metallic parts from micro to macro scale without any support structures. Compared to the well-known powder bed fusion process, LMD enhances manufacturing possibilities to overcome AM-specific challenges such as process inherent porosity, minor build rates, and limited part size. Moreover, the advantages aforementioned combined with conventional machining enable novel manufacturing approaches in various fields of applications.

For small and filigree additive manufactured components, LPBF is usually considered due to the freedom of design and short-lead times [27]. However, even this innovative technology has manufacturing constraints, such as the need for support structures or high build-up times. That affects cost efficiency and process stability. In contrast to powder bed processes or competing direct methods (e.g. WAAM and EBAM), additive manufacturing via powder LMD provides

  • support-less manufacturing (cf. PBF),

  • high productivity (cf. PBF),

  • high flexibility due to local shielding (cf. PBF, EBAM),

  • precise energy input—beneficial microstructure (cf. WAAM, EBAM),

  • low porosity—HIP not needed (cf. PBF) and

  • hybrid manufacturing in one machine (cf. PBF, EBAM)

That makes this technology suitable for the realization of high-performance component designs. Besides, a further advantage of LMD is that conventionally manufactured semi-finished parts can be used adding new features via LMD. This approach decreases manufacturing time and potentiates the advantages of hybrid AM processes. Hence, powder LMD has been established in several branches, e. g. aerospace, medical, or tooling industry for the production of components for jet engines, implants, or drilling tools [28]. To deposit material on a substrate, the powder material is blown into the process zone by a nozzle, partially preheated in the laser beam, and finally reabsorbed in the laser as illustrated in Figure 7.

Figure 7.

Principle of laser metal deposition (LMD) using powder ©IWS.

During the manufacturing process, the bulk material is melted using a laser as a heat source and powder is transported via a carrier gas, like helium or argon [29], into the melting pool using a coaxial nozzle. The powder interacts there with the melting pool and gets absorbed to manufacture the desired part. To fully absorb the powder into the melting pool minimal energy is needed, which can be called line energy. The Marangoni effect causes a strong melt pool movement, which is driven by the surface tension of the melt and leads to a strong mixing of the filler (powder) and part of the substrate material [30]. That also results in potential pores being discharged, improved density, and increased building rates. The subsequent formation of a certain microstructure during solidification is mainly driven by the material selection and the local and temporal gradient, which is affected by process parameters, material, and boundary conditions. When the powder is deposited, heat transfer through prior layers can result in an additional modification of the microstructure.

By tailoring energy input and distribution as well as powder particle size, a wide range of materials could be applied even on various substrate materials (e.g. Stellite on Inconel 718, Brass on Steel, Al2O3 on Al-Alloy) [31, 32]. However, the processing of pure copper using established infrared laser sources has been associated with major challenges. Low absorptivity ends up in a lack of fusion and high porosity [33]. High reflection can damage the laser source or may cause overheating of applied nozzles.

The use of green (495–570 nm) and blue (~445 nm) laser sources can increase the laser absorption of pure copper by a factor of 10 [34].

Specialized processing heads enable dense cooper parts manufactured on substrates, as well as complex prototypes [28, 35, 36].

Moreover, in contrast to powder bed-based additive manufacturing, LMD enables hybrid manufacturing (additive, subtractive) approaches and multi-material processes. Various powders could be applied, exchanged, and mixed in situto achieve multi-material components with localized material properties. The latter recently was applied by IWS to significantly increase the performance of mold inserts by the local implementation of copper features and thus reduced cycle times [37] (Figure 8). In further developments, the essential intermediate and final machining could be fully incorporated in the LMD process chain, resulting in production tools close to industrial needs.

Figure 8.

Laser Metal Deposition with a green laser to build up multi-material mold inserts (pure Cu/steel 1.2764) ©IWS.

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5. State-of-the-art binder jetting

Binder Jetting of pure copper has been intensively studied by Virginia Polytechnic Institute and State University. The main question of this research was how to increase the physical density of copper parts produced by binder jetting. Different approaches to achieve high density were taken under investigation of the influence of particle size of the feedstock (D50 = 15 μm or 75 μm) [38], including bimodal powder compositions [39] where a small fraction of very fine powder should fill up the spaces between the larger particles. Different sintering atmospheres (Ar and H2) were also part of the analysis. Modified binders were investigated, comprising MOD (metal–organic decomposition) inks [40] and nanoparticle [41] enhanced binders. The expectation for the latter both approaches is that introduction of nanoparticles will decrease the temperature for the sintering process to start, but also introduce additional copper into the green body. Also, the influence of HIP post-treatment was investigated [42]. For all the previously described experiments sintering temperature was quite high 1075–1080°C compared to the theoretical melting point of copper 1084°C. Dwell time was varied between 2–10 h, most sintering regimes employed H2 as reducing atmosphere.

The following Table 2 sums up the results of that group.

Use of powder with D50 = 15 μm in comparison to D50 = 75 μm leads to 85.5 % instead of 63.2% of the theoretical density of copper, applying a 4 h @ 1080°C sintering regime[38]
Using bi modal powders (30 μm + 5 μm with a mixing ratio of 17% + 73% respectively) results in a density of 92.3%. All bimodal compositions show significantly less shrinkage[39]
By using HIP treatment of test samples from bimodal powders, the density could be further increased to 99.7%.[42]
Using Metal-Organic-decomposition inks, the part density of the core section could be increased. The overall density however was lower as 73.3% in comparison to non-modified binder (80.8%).[40]
By using nanoparticle loaded inks, the sintered part density is 86.1% compared to 80.9%, when using a neat binder,[41]
Use of fine copper powders (~5 μm) with new recoating equipment.[43]

Table 2.

Effect of different approaches by the Virginia polytechnic group on the relative density of BJ copper parts.

Additionally, the same group published work using a copper feedstock that incorporates a foaming agent introduced by mechanical milling for modification of the porosity of printed parts [44].

It should be also mentioned that companies, active in the development of binder jetting machines, try to qualify materials to be processed on their equipment. Currently, DigitalMetal [45] and ExOne [46] have announced qualified processes with pure copper for applications, such as antennas, heat exchangers, and windings for electric drives.

To further investigate the influence of bimodal powder compositions on the electrical properties of binder-jetted parts, two powder feedstocks were selected, printed, and compared regarding the final part electrical conductivity at IWS. The powders were a monomodal and a bimodal composition, the latter consisted of 73% coarse and 27% fine powder. The powder size distribution of the feedstock is shown in Table 3.

PowderD10 [μm]D50 [μm]D90 [μm]
m4p PureCu.043815
m4p PureCu1.0182638

Table 3.

D10, D50, and D90 of the fine (m4p PureCu.04) and coarse powder (m4p PureCu1.0).

Parts were printed on an ExOne binder jetter (model MFlex). After optimizing the parameters of roller speed, roller transverse speed, layer thickness, and binder saturation, a set of flat samples (25 × 25 × 1 mm3) and cubes (10 × 10 × 10 mm3) for measuring electrical conductivity, physical density, and dilatometry were printed. Although the focus was on the influence of the powder composition, also three different dwell times for sintering, and two different layer thicknesses during printing were compared. The density is analyzed by standard metallography, the electrical conductivity is measured by the eddy current test method (Sigmascope 350, Karl-Fischer), and the dilatometry was done with a DIL 402 Expedis Classic (Netzsch). As expected, longer dwell times lead to higher conductivity (Figure 9). The achieved maximum is found at 84.7% IACS for the bimodal powder and a layer thickness of 80 μm, while for the same configuration the monomodal sample led to 52.6%. For all sintering times, samples made of bimodal powder delivered better conductivity. The observation for the influence of layer thickness is that for monomodal powders, 50 μm leads to the same or slightly better results, while for the bimodal configuration the better values are found for 80 μm. Though, at 12 h that difference disappears.

Figure 9.

Graph showing the relation between sintering time and electrical conductivity for mono- and bimodal feedstock and different layer thicknesses during the printing process.

The differences between the mono and bimodal powder distribution are apparent in Figure 10. The shrinkage of the mono (black) and bimodal (green) sample over the time of the applied temperature profile during sintering is shown. The plots do not contain any compensation for thermal expansion. Two main information can be extracted from the dilatometer experiment. First, the overall shrinkage for the bimodal powder is much lower (12.4%) than for the monomodal powder (17.3%). Second, the onset temperature for begin of shrinkage is ~37 K less for the bimodal powder at 987.5°C.

Figure 10.

Dilatometer plot, comparing shrinkage of mono and bimodal samples during the debinding and sinter profile.

In Figure 11, two etched cross sections of bimodal samples, sintered for 2 h (left) and 12 h (right) are shown. After 2 h of sintering, the density is clearly still low as it seems necking is just about to begin. After 12 h instead, a quite dense microstructure can be seen, nonetheless showing a lot and partially also quite large (> 50 μm) pores at the grain boundaries.

Figure 11.

Metallographic comparison of 2 h (left) and 12 h (right) sintering at 1080°C of bimodal samples.

One of the main challenges in binder jetting obviously remains to achieve high sintered density since compaction of parts is not possible as in classic press and sinter processes. Bimodal powder compositions enhance green part density and stability, lead to higher sintered density, earlier sintering activity, and in the case of copper better electrical conductivity.

The sinter activity of shown samples is clearly low, as for comparison from Figure 2 after 2 h about 90% relative density should be achievable in classic press and sinter. Using bimodal powder compositions seems to be one possible way to tackle that challenge even though 12 h sintering time is still very long. Possible reasons for the poor sintering activity might be insufficient powder bed compaction during the printing process, an incomplete debinding process, or sinter impeding surface oxides on the copper particles.

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6. State-of-the-art metal-fused filament fabrication

Fused filament fabrication (FFF) belongs to the extrusion-based AM technologies. It was usually used for printing polymers, such as Acrylonitrile butadiene styrene (ABS) or Polylactide (PLA) [47], and became the most used AM technology worldwide due to its user-friendly handling [48, 49]. During the printing process, a filament is melted in a print head and extruded onto a build platform [50]. Layer after layer of molten filament is added to create a prototype or product. A sketch of the overall concept is shown in Figure 12.

Figure 12.

Schematic representation of fused filament fabrication method [47].

Today, FFF is well established in many industries, such as the automotive sector [51, 52], in aviation (Airbus) [53], and the medical sector (printing biomedical implants, scaffolds, or other applications) [54]. The cost-efficiency of the FFF process suggested using FFF beyond polymers also for printing other materials.

Meanwhile, the upcoming metal FFF has demonstrated its capability in manufacturing sophisticated structures through a variety of materials [55, 56, 57, 58, 59]. Besides stainless steel (17-4PH) or titanium alloy (Ti6Al4V) [55, 60, 61, 62, 63], Fe-parts for electrical engines or glass-ceramic scaffolds for medical application were printed [64]. Recently was published a multi-material approach by printing and sintering 17-4PH and ZrO2 together [65, 66].

During the process, a filament based on a polymer-binder, containing thermoplastic polymers [55, 56], infiltrated with metal powder, is fed into a print head where the binder is melted, and the material is extruded onto a building platform (Figure 13). After having printed, a so-called green-part layer-by-layer, a catalytic debinding step or solvent debinding step is required for removing a certain fraction of the binder. The solvent debinding step creates pores in the green part. These pores allow gases to escape during the thermal debinding of the remaining binder in a furnace. The polymer that remains after solvent debinding, stabilizes the structure as backbone until sintering of the particles takes place. The thermal debinding of the backbone by pyrolysis is crucial because escaping gases can cause deformations and cracks.

Figure 13.

FFF process, left to right: shaping the part by deposition of filament; two-step debinding process involving solvent extraction and thermal decomposition; finally sintering in a furnace, after [47,67].

The part shrinks during sintering usually around 13–20% in x-, y- and z-direction, which needs to be predicted for near net shape fabrication.

Significant advantages of FFF are as follows:

  1. All kinds of powder materials and even nanoparticles can be utilized

  2. Multi-material can be deposited by using different print heads

  3. Microstructures related anisotropic mechanical behavior can be avoided due to homogeneously sintering [56]

  4. Little investment costs and cost-efficient printing and sintering of metal and ceramic parts at atmospheric pressure

  5. No powder particles are airborne, causing potential health problems for operators.

Additionally, high material throughput (1–10 g/min), material efficiency (no material waste), design freedom for printing even hollow structures, and the competitive material properties make FFF a highly competitive AM technology [56, 57, 65].

Nowadays companies, such as AM Extrusion GmbH [68] or BASF [69] offer an open filaments system for printing and sintering metal parts, such as copper, 316L, 17-4PH, or carbon steels, such as 440C, M2, or H13. Even filaments with unique materials can be prepared exclusively for customers.

Copper filaments by AM Extrusion GmbH (filled with 63 vol.% copper powder) can be printed with a modified BondTech extruder. Nozzle and print bed temperatures are 120 and 70°C. The recommended nozzle is a 300 μm hardened steel nozzle. The standard layer height is 80–200 μm [68]. Using a 300 μm nozzle line, the width is 360 μm and print speed 1000–3000 mm/min.

After solvent debinding in acetone at 45°C and sintering at 950°C in H2, a relative density of 96% can be obtained [68]. The shrinkage during sintering is 13% in x-, y-, z-direction. A final part accuracy of < ± 80 μm can be obtained [68]. Material properties of FFF printed Cu and printed parts are shown in Figure 14.

Figure 14.

(left) Properties of FFF printed copper measured by accredited test laboratory, (middle) cross section of sintered copper (@950°C, 90 min, H2), (right) sintered copper robot gripper [68].

Compared to powder-bed technologies, FFF is safe and user-friendly. During the FFF process, no powder can be airborne, which may cause health issues for employees. In general, FFF is capable of manufacturing medium-sized complex metal and ceramic structures in small serial production.

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7. Conclusion

Additive manufacturing of copper is emerging and additive fabrication methods, such as laser powder bed fusion, laser metal deposition, binder jetting, fused filament fabrication, or electron beam melting become more refined.

Recently, it is possible to fabricate complex copper parts with an electrical conductivity of 100% IACS. In addition, the fabrication of hybrid material parts, including copper, is possible. Thus, additive manufacturing of pure copper keeps up and excels conventional manufacturing methods in terms of geometrical complexity.

Due to its unique properties, copper is primarily used for electrical or thermal applications. Already realized use cases are components for electric vehicles by LPBF [70], cooling sockets for milling tools by FFF [71], or a horn waveguide antenna [72].

Also increased research interest in found in the manufacturing of complex propulsion systems, such as aerospike thrusters, made from alloyed copper [73].

Further, printed heat sinks, heat pipes, and complex coils are already demonstrated.

Further improvements, especially impeccable material properties in combination with new fabrication approaches, are pursued. For instance, the modification of the copper powder feedstock with a coating of metal oxides or metal hydroxides (approx. 5–30% coverage) increases the absorptivity, especially when using standard infrared laser sources [74].

Another approach to utilize infrared lasers for the processing of pure copper is to use high laser power of 600–1000 W. Researchers from Politecnico di Milano achieved a density of ~ 97% using a 600 W laser on pure copper [75]. Yet this approach, in contrast to using green laser sources, has the advantage of using bigger build chambers. However, this advantage will disappear, since bigger LPBF setups with green laser sources are under development.

In addition, polymeric coatings of copper powder are under development for use in selective laser sintering machines. This process is advertised as cold metal fusion (or Metal SLS). Using this approach, lower laser powers are necessary to consolidate the powder, since only the polymer coating will be molten and sintered. Further, the commonly used infrared lasers can be used effectively. The printed part, however, needs to undergo a thermal sintering step though, comparable to binder jetting or FFF, to burn out the polymer and sinter the metal powder together [76].

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Conflict of interest

All Authors declare that there is no conflict of interest.

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Notes/thanks/other declarations

This research was conducted within the High-Performance Center »Smart Production and Materials« and partially funded by the Fraunhofer-Gesellschaft, the German Federal Ministry of Education and Research and the State of Saxony.

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Written By

Lukas Stepien, Samira Gruber, Moritz Greifzu, Mirko Riede and Aljoscha Roch

Submitted: January 14th, 2022 Reviewed: February 11th, 2022 Published: May 5th, 2022