Additive Manufacturing of Pure Copper: Technologies and Applications

*Tobia Romano and Maurizio Vedani*

### **Abstract**

The opportunity to process pure copper through additive manufacturing has been widely explored in recent years, both in academic research and for industrial uses. Compared to well-established fabrication routes, the inherent absence of severe design constraints in additive manufacturing enables the creation of sophisticated copper components for applications where excellent electrical and thermal conductivity is paramount. These include electric motor components, heat management systems, heat-treating inductors, and electromagnetic devices. This chapter discusses the main additive manufacturing technologies used to fabricate pure copper products and their achievable properties, drawing attention to the advantages and the challenges they have to face considering the peculiar physical properties of copper. An insight on the topic of recycling of copper powders used in additive manufacturing is also provided. Finally, an overview of the potential areas of application of additively manufactured pure copper components is presented, highlighting the current technological gaps that could be filled by the implementation of additive manufacturing solutions.

**Keywords:** additive manufacturing, copper, powder bed fusion, green laser, binder jetting, heat exchangers

### **1. Introduction**

Pure copper is one of the most widely employed materials in electronic, electromagnetic, and heat management applications because it combines superior electrical and thermal properties with high workability and solderability [1]. In recent years, the great advances in additive manufacturing (AM) technologies have shown the ability to create tortuous geometries inconceivable with traditional production methods. AM of pure copper has attracted the interest of both academic and industrial researchers because it now enables the fabrication of complex-shaped components, including novel-design antennas, inductors, radiators, and heat exchangers, with improved performance through topology optimization based on the specific requirements of the application considered.

AM technologies for the processing of pure copper can be roughly classified into powder bed-based and direct deposition technologies. In the first case, the powder is gradually deposited in successive layers and selectively consolidated using an energy source or a polymeric binder. At the end of the printing process, the part is surrounded by the unconsolidated powder bed, from which it is extracted for any subsequent processing steps. On the other hand, in direct deposition processes the material feedstock in powder or wire form is deposited according to the targeted geometry by a printing head (e.g., a laser torch) that enables material delivery and provides the heat required for its consolidation. AM of copper has been studied predominantly in the context of powder bed-based technologies rather than direct energy deposition (DED) methods because they provide higher dimensional accuracy and tolerances [2]. This is of major importance when building tiny intricate features, such as curved channels or lattice structures in complex heat exchangers. On the other hand, flexible DED systems, which use a robotic arm equipped with a nozzle for material dispensing, can more easily create multi-material parts with added functionalities, as each material can be conveniently deposited where needed. This would allow the one-step fabrication of components constituted by dissimilar materials, such as copper and steel, which currently can only be made by other routes through repeated joining operations with high costs and long lead times [3].

### **2. Additive manufacturing technologies for pure copper processing**

Various classes of AM technologies have been employed to successfully fabricate pure copper parts. However, significant processing challenges related to the physical properties of copper and the difficulty of achieving full density in the final parts still represent challenging issues that should be thoroughly investigated for these methods to become established.

### **2.1 Laser powder bed fusion**

Laser powder bed fusion (LPBF) is the most established AM technology for the processing of metallic materials [4]. It uses a laser source to selectively melt the powder spread by a roller system to generate a bed of controlled thickness. Once a laser scan is completed, the build platform is lowered by a distance equal to the layer thickness set for the process, and a new layer of powder is deposited. The process is then repeated layer by layer to build the desired geometry based on the designed STL model. All the procedure is carried out in a closed chamber filled with inert gas to avoid material contamination. The typical setup of LPBF is schematically illustrated in **Figure 1**.

A large number of process variables influence the quality of parts produced by LPBF [5]. Fine powders with an average particle size lower than 50 μm are typically employed [6–8] to allow the generation of thin layer thicknesses, leading to improved resolution and reduced staircase effects, which result in a better surface finish. A major role is also played by the characteristics of the laser and how the material interacts with it. Commercial LPBF machines are normally equipped with infrared laser sources with a wavelength of around 1 μm and power up to 500 W [9]. Although the effective absorbance of the powder bed in LPBF is higher than in bulk materials due to the multiple laser reflections induced by the closely spaced powder particles, it hardly exceeds 30% when processing pure copper powders with infrared lasers [10].

*Additive Manufacturing of Pure Copper: Technologies and Applications DOI: http://dx.doi.org/10.5772/intechopen.107233*

### **Figure 1.** *Schematic layout of the LPBF process.*

**Figure 2.** *Absorbance of metal powders in LPBF as a function of laser wavelength. Data extracted from Brandau et al. [10].*

This is clearly shown in **Figure 2**, which compares the absorbance of some of the most common metal powders used in LPBF. The poor absorption of the laser energy during the printing process causes incomplete fusion of the powder material. The result is a substantial residual porosity that affects the thermal and electrical performance of the components.

Two main approaches have been adopted to overcome this issue. On the one hand, some researchers [9, 11, 12] have employed high-power infrared lasers combined with low-scanning speeds to ensure complete melting of the powder material despite low absorption. Although relative densities exceeding 99% have been reported, the processing window available for parameter setting is very narrow, and it is difficult to sustain a stable melt track when such high powers are involved, resulting in lower resolution and poor surface finish. Also, the large amount of energy wasted during the printing process leads to high production costs, and the low-scanning speed

results in long lead times. The other strategy relies on the use of blue or green lasers with wavelengths of 450 and 515 nm, respectively, for which copper exhibits a higher absorption rate (**Figure 2**). TRUMPF (Germany) recently launched the first series of commercial LPBF machines equipped with a 515 nm wavelength laser for the manufacturing of copper parts [13]. Gruber et al. [14] achieved almost full density and electrical conductivity of 100% IACS in pure copper samples produced with the green laser-based process.

### **2.2 Electron beam powder bed fusion**

Electron beam powder bed fusion (EB-PBF) features a fabrication approach similar to LPBF but uses a high-energy electron beam as the heat source to selectively melt the powder material. The printing process is conducted inside a high-vacuum chamber to prevent electrons from scattering by collision with gas molecules. A schematic representation of the machine setup is illustrated in **Figure 3**.

Although its use has been limited to date due to a relatively lower number of systems available in industrial and academic research centers, the main advantage of EB-PBF for the processing of pure copper, as compared to its laser-based counterpart, is that the energy absorbed from the incident electrons is not affected by the optical reflectivity of the target material. This leads to highly efficient energy transfer during the printing operation (a rate of absorption of around 80% is estimated [15]). In addition, the high-vacuum environment protects the material from oxidation, ensuring high purity of the final parts.

One well-known issue of EB-PBF is the so-called smoking. Due to the incident electrons, a negative charge tends to accumulate at the surface of particles in the powder bed. The mutual repulsion among neighboring particles may cause them to suddenly jump from the powder bed, generating a cloud of powder inside the work chamber, which may impair the smooth processing of the material [16, 17]. Smoking is not a primary issue in the case of pure copper since its high electrical conductivity can prevent the buildup of a strong negative charge. Still, slightly coarser powders than in LPBF are generally used (in the range of 60–105 μm [15, 18, 19]) because they have a lower tendency to smoking compared to fine powders [17]. Also, each powder

**Figure 3.** *Schematic layout of the EB-PBF process.*

### *Additive Manufacturing of Pure Copper: Technologies and Applications DOI: http://dx.doi.org/10.5772/intechopen.107233*

layer is preheated around 400–500°C [18] and partially sintered by the beam itself before the actual scanning operation to provide an even improved electrical connection for electrostatic charge dissipation. One difficulty that arises from the high temperatures at which the deposited material is maintained is that copper particles tend to stick together during the raking operation owing to their high propensity to sinter. Particle clusters may impede the correct deposition of the following powder layers, possibly causing interlayer defects. Also, particles that are partially sintered on the external surface of the as-print parts generate a relatively high surface roughness [18]. The electron beam has a relatively large spot size of about 0.5 mm [20], while beams 30–100 μm in diameter are typically used in LPBF [21]. This restricts the capability of EB-PBF of producing minute details in complex parts, which is a nontrivial limitation, for example, in the fabrication of sophisticated heat exchangers made with pure copper. However, higher production rates can be achieved compared to LPBF because preheating, combined with the more effective energy input, ensures complete powder melting even when selecting high beam scanning speeds during the printing process [19].

Few studies [15, 18, 22] have been conducted so far to identify the main variables that affect the quality and reliability of pure copper parts produced by EB-PBF with commercial machines. Feedstock quality, preheating temperature, beam focal point, and scanning strategy are among the parameters that need to be optimized to ensure a robust production with consistent characteristics across runs. Guschlbauer et al. [15] obtained pure copper specimens with a relative density above 99.5% by conveniently combining the beam power and the scanning speed within the ranges 275–750 W and 250–1500 mm/s, respectively.

### **2.3 Binder jetting**

Binder jetting shares with PBF technologies the powder bed strategy for the creation of three-dimensional parts. However, during the printing process, the powder particles are glued together by water- or solvent-based polymeric binder selectively deposited by a print head instead of being melted by a high-energy beam. A curing treatment at moderate temperature is then applied to eliminate the volatile fraction of the binder and induce polymerization. The reticulated polymer provides the green part with sufficient strength to be removed from the unbound powder bed without breaking. The part is finally subjected to a debinding and sintering cycle to burn off the polymer fraction and densify the material by diffusion-assisted mechanisms promoted by the high temperature. Although there exist several sintering strategies for both metallic and ceramic materials, which may involve for instance the formation of a liquid phase to facilitate interparticle bonding, in the case of pure copper densification is achieved by solid-state diffusion only because no low-melting second phases are present. As-sintered parts normally possess a relatively high surface roughness, especially on vertical surfaces due to the effect of the distinct powder layers they are made of [23]. Postprocessing operations involving vibratory abrasion and chemical treatments [24] may be required to achieve a good surface finish in components with complex geometry and internal features. The flowchart of the binder jetting process is illustrated in **Figure 4**.

Feedstock powders with an average size exceeding 20 μm are normally preferred in binder jetting because their relatively low tendency to agglomerate facilitates the spreading operation [25]. However, the development of vibrating devices for powder sieving and dispensing has enabled the use of finer powders with an average size

**Figure 4.** *Schematic flowchart of the binder jetting process.*

lower than 5 μm [26, 27] for improved resolution and densification. Bimodal mixtures also showed the potential to improve powder flowability and reduce part shrinkage upon sintering, because particles with different sizes can tightly pack and result in high green part density [28].

The peculiar feature of binder jetting compared to the above-described PBF technologies is that the generation of the three-dimensional geometry and the bonding between powder particles occur in two distinct stages. In addition to those already mentioned, one challenge LPBF faces in processing pure copper is related to its high thermal conductivity. The heat provided by the energy source is rapidly dissipated by the surrounding material. This may cause poor interlayer adhesion and lead to delamination defects in the produced part. Binder jetting does not suffer from this phenomenon, because the material is homogeneously heated in a controlled environment by applying a proper sintering cycle.

Investigation on binder jetting of pure copper showed that the primary challenges consist of attaining complete density and high purity in the sintered parts. The powder particles in the green parts have a low tendency to sinter because they are covered by a low surface energy oxide layer and are not tightly compacted by the recoater during the printing process. Both the surface oxide layer and the large interparticle distance hamper neck formation and growth between neighboring powder particles by solid-state diffusion [25, 29]. In addition, carbon residues may result from incomplete combustion of the polymeric binder during the debinding stage [27], and uneven sintering may occur between the outer and the core regions of the parts due to temperature gradients along the material thickness. Indeed, the rapid stiffening of the outer zones exposed to a higher temperature may hinder inward shrinkage, thus leaving a central volume with a high residual porosity [27]. Therefore, the sintering parameters need to be carefully adjusted in terms of sintering atmosphere, heating ramp, peak temperature, and sintering time to minimize the residual porosity and carbon impurities, which adversely affect the mechanical, thermal, and electrical properties of the final parts [30, 31].

From a design perspective, binder jetting does not require support structures for overhanging features, because the powder bed itself serves as a support. In addition, several parts can be stacked in the vertical direction, allowing the entire volume of the working chamber to be utilized to print large series of parts [32, 33] that can then all be sintered in a single furnace cycle. However, the possible effects of creep activated

### *Additive Manufacturing of Pure Copper: Technologies and Applications DOI: http://dx.doi.org/10.5772/intechopen.107233*

by the prolonged exposure to high temperatures during sintering need to be taken into account to avoid undesirable deformations of cantilever elements in the final components [34]. In addition, parts shrink as a result of densification. Because backto-back powder layers are not well consolidated due to limited binder penetration, a larger amount of porosity is observed among them than within individual layers. This causes a larger shrinkage along the build direction than in the lateral directions upon sintering, which is further accentuated by gravity effects [29]. Therefore, a careful design is needed to compensate for these differential changes in the part dimensions upon sintering.

### **2.4 Laser metal deposition**

Laser metal deposition (LMD) belongs to the family of DED processes. It utilizes a coaxial nozzle equipped with a laser source to melt the feedstock material, while it is simultaneously deposited in a series of weld tracks to generate the designed threedimensional geometry. A carrier gas transports the powder through the outermost annular channel to the melting zone, while a shielding gas is used to avoid excessive oxygen pickup by the melt pool. The use of copper powders with size ranging from 30 to 110 μm have been reported in the literature [35–38]. The setup of the LMD process is schematically illustrated in **Figure 5**.

The main advantages of LMD over powder bed-based technologies are the high productivity and flexibility, the ability to work without closed chambers with protected environment, and the possibility to add features to the existing part. Also, machining tools can be integrated into the printing system to enable hybrid manufacturing [39]. The multi-axis configuration allows the construction of threedimensional elements even on non-flat surfaces.

LMD is not commonly employed to fabricate individual components from pure copper because it cannot provide the dimensional accuracy and resolution usually required for its typical applications (e.g., heat exchangers, inductors, and electromagnetic devices). In addition, the shielding gas can only partially prevent oxidation [39]. Oxides reduce the wettability of molten copper on solid surfaces [40], leading to poor adhesion between the substrate and the built features.

A great impetus to LMD of pure copper has been given by the recent introduction of green and blue laser sources, for which copper displays a relatively high absorptivity.

**Figure 5.** *Schematic layout of the LMD process.*

Higher process control can thus be achieved and lower powers are required compared to conventional infrared lasers, ranging from 200 W to 1 kW for green laser sources depending on the physical properties of the substrate material [41] and lower than 87 W for blue diode lasers [35, 36]. However, the real strength of LMD compared to powder-bed technologies is that it enables the relatively easy fabrication of multimaterial parts. In principle, different materials can be conveniently placed at specific locations by simply replacing and/or mixing the powder fed during the building process. Therefore, properties such as hardness, thermal and electrical conductivity, or corrosion resistance can be fine-tuned throughout a single component by conveniently customizing its constituent materials.

Copper has been investigated in the context of multi-material LMD mainly in combination with steel. The coupling of copper and tool steels has been proposed to fabricate molds and dies with improved cooling efficiency due to the high thermal conductivity of the copper portion [37, 42]. The more uniform and faster heat extraction would increase both the quality of the formed parts and the productivity. Multimaterials based on copper and stainless steel, on the other hand, may find use in highly demanding applications such as fusion reactors and high-field pulsed magnets [43]. However, the manufacturing of such structures still has to face significant challenges due to the discrepancy in laser absorption, thermal conduction, and thermal expansion behavior between copper and steel and their poor mutual solubility.

### **3. Recycling of pure copper powders in additive manufacturing**

High flowability and chemistry control are key requirements for metallic powders for AM. Gas atomization is normally employed to produce powders for AM, because it can provide higher sphericity, smoother surface morphology, and tighter control of the particle size distribution and the chemical composition, particularly in terms of oxygen content, compared to other processes such as water atomization [44]. Pure copper powders for AM are generally produced from oxygen-free electronic (OFE), oxygen-free, and electrolytic tough pitch (ETP) grades to provide feedstocks with low impurity content despite the tendency to oxidation caused by the large surface area of the fine powder particles [1, 45].

Both the use of high-purity raw materials and the complexity of the gas atomization process result in rather expensive powder feedstocks. While LMD can achieve high material utilization efficiency with optimized process parameters [46], in PBF and binder jetting, most of the powder is used to generate the powder bed. It is a common procedure to collect, sieve, and reuse the excess of feedstock material that is not consolidated during the building process in order to minimize material waste and keep manufacturing costs low. Also, the practice of powder recycling improves the environmental profile of these processes, since gas atomization is a very energyand material-intensive process [47]. Nevertheless, the powder is normally discarded after a certain number of cycles as it is subjected to various degradation phenomena, depending on the nature of the AM process, which can severely affect the quality of the produced parts. This is particularly critical for materials that are sensitive to oxidation and are used in applications that impose stringent limits on the impurity content, such as pure copper for thermal management and electrical applications.

The complex combination of factors involved in PBF processes can severely affect the quality of copper powders collected after several printing cycles compared to the virgin material. Oxygen pickup occurs due to residual oxygen and moisture in the

### *Additive Manufacturing of Pure Copper: Technologies and Applications DOI: http://dx.doi.org/10.5772/intechopen.107233*

work chamber and during part recovery at the end of the printing process [48] when the atmosphere control is turned off. As a result, the thickness of the oxide layer that naturally covers the surface of copper powder particles increases significantly [49]. Surface oxides are not easily removed during the fabrication process and may be incorporated into the final parts, affecting their mechanical, thermal, and electrical performance. Hence the need to reduce the oxygen content of the powder before reuse, typically by heating it in a reducing environment containing forming gas [20]. Speidel et al. [50] have proposed a less expensive and more readily scalable method for treating heavily oxidized copper powders for LPBF, based on chemical etching with a dilute solution of nitric acid.

In binder jetting, the powder in the bed is further subjected to oxidation during the curing stage [27], which is normally carried out in the air. In the case of copper, oxidation is additionally promoted when a water-based binder is employed, as copper is highly sensitive to moisture. Binder splashes may generate massive particle agglomerates in the regions of the powder bed next to the boundaries of the printed parts [51]. Also, the powder particles that remain sticked on the surface of cured parts are usually removed with pressurized air. The applied pressure may deform the highly ductile copper particles [52], reducing their sphericity [51]. However, such particles account for a minimal fraction of the total powder collected at the end of a printing cycle and should not affect the overall powder flowability in the next runs. In **Figure 6**, a comparison between fresh copper powder and the feedstock to be recycled after binder jetting is proposed. No significant morphological differences are observed between the two powder batches.

Sieving is normally conducted to break particle conglomerates and eliminate coarsened and partially sintered particles. However, a significant fraction of fine particles may escape the sieve and is lost by dispersion into the air [51]. This may lead to a nonnegligible shift in the median diameter toward higher values and narrower size distribution, as finer particles are preferentially removed from the feedstock. Therefore, the effect of sieving on powder granulometry should be considered when employing recycled powders in AM. In PBF, processing parameters should be adjusted to account for the varying powder bed density from one cycle to next. In binder jetting, a coarser and narrower powder size distribution results in lower green part density and, consequently, higher shrinkage upon sintering for the same final density. In addition, a higher sintering temperature may be required to attain satisfactory densities, because coarse powder particles have a lower tendency to sinter due to the reduced surface area compared to fine particles [26].

### **Figure 6.** *Copper powder in as-received conditions (left) and collected from the powder bed after binder jetting (right).*

### **4. Applications of additively manufactured pure copper components**

In recent years, AM processes have been applied to pure copper, particularly for the manufacturing of high-added-value components for advanced applications in various fields that require excellent thermal and electrical performances. The main barrier to the large-scale adoption of these technologies in the industry is the limited availability of regulations covering the different aspects of AM, including raw material quality, design guidelines, fabrication and postprocessing techniques, material testing, and inspection of the final components. This obliges companies to make considerable efforts in terms of investigation and testing to qualify and certify AM products prior to market launch, which may result in extremely high costs and long lead times [53]. In addition, compared to traditional fabrication methods, AM processes often exhibit poor repeatability and reproducibility [54, 55], and the relationship between manufacturing route, material characteristics, and final product quality still needs to be thoroughly explored [53].

While some standards have already been developed for the AM of metallic materials such as stainless steels and nickel- and titanium-based alloys [53], AM of copper is still in its infancy and is not yet targeted for standardization. However, several examples of additively manufacturing components made of pure copper have already been reported in the literature and in public technical expositions.

### **4.1 Automotive components**

Copper and copper alloys are widely used in the automotive industry to manufacture a variety of vehicle parts, such as electric motor components, ABS and power lock pumps, water and oil coolers, radiators, and heat exchangers for air conditioning [56]. The use of copper in this sector has significantly increased in the past few years with the spread of hybrid and electric vehicles [57].

Pure copper windings and rotors play a key role in the performance of electric motors. Proper winding design optimization can reduce AC and DC loss. However, this is restricted by the limited capability of established manufacturing methods of producing complex geometries with reasonable costs and lead times. Maxwell Motors, a startup based in the USA, recently developed a novel copper winding design to improve the performance of an electric motor that does not use rareearth-based magnets. They jointly developed and manufactured with ExOne (USA) a monolithic winding assembly by binder jetting, hence obviating the usual steps of manufacturing and welding the individual parts [58]. The same approach can be extended to the fabrication of customized shanks and adaptors for car chassis welding [59].

### **4.2 Thermal management devices**

The high thermal conductivity of pure copper makes it the ideal material for heat dissipation purposes in numerous fields including microelectronics, power plants, and transport. Topological optimization can improve the performance of thermal management devices by maximizing the efficiency of heat transfer. This can be achieved through AM, for instance by building intricate features or even lattice structures with very large specific surface area available for heat exchange [60].

*Additive Manufacturing of Pure Copper: Technologies and Applications DOI: http://dx.doi.org/10.5772/intechopen.107233*

**Figure 7.** *Columnar (left), helix (center), and bent tube (right) pure copper heat sinks. Adapted from [8].*

Constantin et al. [8] fabricated complex-shaped heat sinks consisting of helix and bent-tube structures through LPBF (**Figure 7**). They developed a special experimental setup to evaluate the performance of the additively manufactured heat sinks in comparison with a commercial device featuring a straight columnar structure. Each heat sink was connected to a memory card chip placed on a heater plate and air circulation was provided by a fan. The helix and bent-tube heat sinks exhibited a higher cooling efficiency owing to their larger surface area compared to the commercial device.

Various companies have showcased prototypes of pure copper cooling plates and heat exchangers featuring minute details and mesh structures made by PBF and BJ technologies with optimized processing parameters [61–64].

### **4.3 Electrical and electromagnetic devices**

Due to its remarkable physical properties, pure copper is the preferred material for the fabrication of electrical and electromagnetic devices.

Copper is commonly employed to produce inductors for heat treatments and other hot processes, enabling highly controlled and localized heat delivery to the parts. The shape and size of the coils can be tuned to meet the heat treatment specifications and ensure adequate productivity [65]. Copper inductors are conventionally made by hand-wrapping copper wires on blueprints. Then, individual coils are brazed to match the geometry of the workpiece. This manufacturing route is time-consuming and requires highly skilled labor, resulting in high production costs. Also, the devices have relatively low durability, due to the discontinuities at the brazing joints [66]. On the other hand, monolithic inductors with homogeneous properties could be directly fabricated by AM, resulting in increased productivity and service life. Silbernagel et al. [6] also demonstrated that the cross-section of coils made by LPBF can be conveniently varied to locally control the electrical resistivity. Such inductors may be used in treatments featuring a complex thermal profile or applied to components with variable wall thickness. Hollow structures for cooling fluid circulation can also be produced for applications where particularly high cooling efficiency is required.

The flexibility of AM processes also enables the one-step manufacturing of sophisticated antennas with tunable electromagnetic properties. This would avoid issues, such as uncontrolled shifts in the operating frequency band, which may be caused by imperfect alignment when soldering different pieces. Johnson et al. [67]

**Figure 8.** *Pure copper pyramidal fractal antenna made by green laser-based PBF [67].*

fabricated a copper pyramidal fractal antenna with an LPBF system equipped with a green laser (**Figure 8**). The radio frequency (RF) performance of the antenna was evaluated with a spectrum analyzer and it was found to be in good agreement with the results of simulations. A novel-design bullhorn antenna made of pure copper has also been manufactured with the high-precision binder jetting technology developed by Digital Metal (Sweden) [64].

### **4.4 Particle accelerator components**

The manufacturing of vacuum devices and particle accelerator components made of pure copper can also benefit from the AM's inherent design freedom and capability of one-step fabrication. Although the market size might appear limited, the sophisticated technologies involved in this sector represent an extremely important test bench for AM development. The layer-by-layer approach of AM technologies facilitates the creation of enclosed envelopes in RF cavities, eliminating the need for sophisticated techniques and highly specialized labor for the assembly of individual parts [66]. This was demonstrated by Mayerhofer et al. [68], who manufactured a monolithic copper RF cavity for a linear accelerator prototype using LPBF. Although the additively manufactured cavity exhibited a slightly lower performance compared to the reference cavity fabricated with conventional methods, the production costs were reduced to a third. Internal channels and lattice structures can also be implemented in RF devices to improve their cooling efficiency, hence eliminating current limitations on the duty cycle and average power [66, 69].

Frigola et al. [69] fabricated a pure copper cathode by EB-PBF and tested it in a RF photoinjector. The performance of the additively manufactured cathode was in line with that of conventionally machined cathodes and it could be further improved by integrating internal cooling channels for liquid helium circulation. Within the frame of the I.FAST project [70], aimed at boosting innovation in the field of particle accelerators, Torims et al. [71] prototyped the quarter sector of a pure copper radio frequency quadrupole (RFQ ) by green laser-based LPBF (**Figure 9**). They demonstrated the possibility of rapid manufacturing, avoidance of brazing operations, and improved cooling efficiency offered by AM compared to more restrictive conventional production methods. A full-size four-vane RFQ prototype (**Figure 10**) was recently presented at the 13th International Particle Accelerator Conference (IPAC22) [72].

*Additive Manufacturing of Pure Copper: Technologies and Applications DOI: http://dx.doi.org/10.5772/intechopen.107233*

### **Figure 9.**

*Manufacturing of a pure copper RFQ section by LPBF with a green laser source [71].*

**Figure 10.** *Additively manufactured pure copper RFQ [72].*

### **5. Conclusion**

Additive manufacturing of pure copper has been extensively investigated in the last decade because it has the potential to create complex components with improved thermal and electrical performance. Within this perspective, the component topology can be optimized according to specific needs and requirements, instead of being dictated by the technological constraints imposed by conventional manufacturing methods.

Pure copper has historically been a challenging material to be processed by additive manufacturing. The high reflectivity of pure copper makes the more established laser-based processes difficult to control and energy inefficient. On the other hand, when using an electron beam energy source, high resolution and dimensional accuracy can hardly be achieved due to the relatively large beam spot size and the tendency of copper powder particles to stick on the consolidated surfaces. Binder jetting has also exhibited some drawbacks related to the difficulty in accomplishing adequate density and material purity after sintering.

In the past few years, however, additive manufacturing technologies have made significant progress in the context of pure copper processing. More reliable processes have been developed by optimizing the operating parameters, such as power input, scanning strategy, or sintering setup. In addition, novel robust green and blue laser sources have recently been introduced, for which copper exhibits higher absorption rates. They are expected to extend the stability window for the processing of pure copper so as to create high-quality products competitive with their counterparts produced by more conventional routes.

Several companies and research groups have already showcased a variety of additively manufactured prototypes made of pure copper. These include complex high-added-value components such as heat exchangers, inductors, electromagnetic devices, and motor windings with optimized geometry and tailored functional characteristics. However, the full potential of additive manufacturing still needs to be explored for these methods to truly become an integral part of the industrial supply chain.

### **Acknowledgements**

This research was partially funded through the Horizon 2020 Research and Innovation program under grant agreement No 101004730 for the I.FAST project.

### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Tobia Romano\* and Maurizio Vedani Department of Mechanical Engineering, Politecnico di Milano, Milan, Italy

\*Address all correspondence to: tobia.romano@polimi.it

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Additive Manufacturing of Pure Copper: Technologies and Applications DOI: http://dx.doi.org/10.5772/intechopen.107233*

### **References**

[1] Davis DJR. ASM Specialty Handbook, Copper and Copper Alloys. ASM Materials Park, OH, USA: ASM International; 2001:3-9. DOI: 10.131/ caca2001p003

[2] Gruber S, Grunert C, Riede M, López E, Marquardt A, Brueckner F, et al. Comparison of dimensional accuracy and tolerances of powder bed based and nozzle based additive manufacturing processes. Journal of Laser Applications. 2020;**32**(3):032016. DOI: 10.2351/7.0000115

[3] Pascal C, Chaix JM, Dutt A, Lay S, Allibert CH. Elaboration of (steel/ cemented carbide) multimaterial by powder metallurgy. Materials Science Forum. 2007;**534-536**:1529-1532. DOI: 10.4028/www.scientific.net/ MSF.534-536.1529

[4] Papadakis L, Chantzis D, Salonitis K. On the energy efficiency of pre-heating methods in SLM/SLS processes. International Journal of Advanced Manufacturing Technologies. 2018;**95**:1325-1338. DOI: 10.1007/ s00170-017-1287-9

[5] Hanzl P, Zetek M, Bakša T, Kroupa T. The influence of processing parameters on the mechanical properties of SLM parts. Procedia Engineering. 2015;**100**:1405- 1413. DOI: 10.1016/j.proeng.2015.01.510

[6] Silbernagel C, Gargalis L, Ashcroft I, Hague R, Galea M, Dickens P. Electrical resistivity of pure copper processed by medium-powered laser powder bed fusion additive manufacturing for use in electromagnetic applications. Additive Manufacturing. 2019;**29**:1-11. DOI: 10.1016/j.addma.2019.100831

[7] Qu S, Ding J, Fu J, Fu M, Zhang B, Song X. High-precision laser powder

bed fusion processing of pure copper. Additive Manufacturing. 2021;**48**:102417. DOI: 10.1016/j.addma.2021.102417

[8] Constantin L, Wu Z, Li N, Fan L, Silvain JF, Lu YF. Laser 3D printing of complex copper structures. Additive Manufacturing. 2020;**35**:101268. DOI: 10.1016/j.addma.2020.101268

[9] Colopi M, Caprio L, Demir AG, Previtali B. Selective laser melting of pure Cu with a 1 kW single mode fiber laser. Procedia CIRP. 2018;**74**:59-63. DOI: 10.1016/j.procir.2018.08.030

[10] Brandau B, Da Silva A, Wilsnack C, Brueckner F, Kaplan AFH. Absorbance study of powder conditions for laser additive manufacturing. Materials and Desing. 2022;**216**:110591. DOI: 10.1016/j. matdes.2022.110591

[11] Ikeshoji TT, Nakamura K, Yonehara M, Imai K, Kyogoku H. Selective laser melting of pure copper. The Journal of The Minerals, Metals & Materials Society (TMS). 2018;**70**(3):396-400. DOI: 10.1007/ s11837-017-2695-x

[12] Jadhav SD, Goossens LR, Kinds Y, Van Hooreweder B, Vanmeensel K. Laserbased powder bed fusion additive manufacturing of pure copper. Additive Manufacturing. 2021;**42**:1-15. DOI: 10.1016/j.addma.2021.101990

[13] TruPrint Serie 1000 Green Edition. TRUMPF. Available from: https://www. trumpf.com/en\_INT/products/machinessystems/additive-production-systems/ truprint-serie-1000-green-edition/. [Accessed: June 1, 2022]

[14] Gruber S, Stepien L, López E, Brueckner F, Leyens C. Physical and geometrical properties of additively manufactured pure copper samples using a green laser source. Materials. 2021;**14**(13):1-11. DOI: 10.3390/ ma14133642

[15] Guschlbauer R, Momeni S, Osmanlic F, Körner C. Process development of 99.95% pure copper processed via selective electron beam melting and its mechanical and physical properties. Materials Characterization. 2018;**143**:163-170. DOI: 10.1016/j. matchar.2018.04.009

[16] Gibson I, Rosen D, Stucker B, Khorasani M. Additive Manufacturing Technologies. 3rd ed. Cham, Switzerland: Springer; 2021. p. 159

[17] Chiba A, Daino Y, Aoyagi K, Yamanaka K. Smoke suppression in electron beam melting of inconel 718 alloy powder based on insulator–metal transition of surface oxide film by mechanical stimulation. Materials. 2021;**14**(16):1-24. DOI: 10.3390/ ma14164662

[18] Dadbakhsh S, Zhao X, Kumar P, Shanmugam V, Lin Z, Hulme C. Process and geometrical integrity optimization of electron beam melting for copper. CIRP Annals. 2022;**71**(1):1-4. DOI: 10.1016/ j.cirp.2022.03.041

[19] DebRoy T, Wei HL, Zuback JS, Mukherjee T, Elmer JW, Milewski JO, et al. Additive manufacturing of metallic components – Process, structure and properties. Progress in Materials Science. 2018;**92**:112-224. DOI: 10.1016/j. pmatsci.2017.10.001

[20] El-Wardany TI, She Y, Jagdale VN, Garofano JK, Liou JJ, Schmidt WR. Challenges in three-dimensional printing of high-conductivity copper. Journal of Electronic Packaging. 2018;**140**(2):1-12. DOI: 10.1115/1.4039974

[21] Shi W, Liu Y, Shi X, Hou Y, Wang P, Song G. Beam diameter dependence of performance in thick-layer and high-power selective laser melting of Ti-6Al-4V. Materials. 2018;**11**(7):1-17. DOI: 10.3390/ma11071237

[22] Lodes MA, Guschlbauer R, Körner C. Process development for the manufacturing of 99.94% pure copper via selective electron beam melting. Materials Letters. 2015;**143**:298-301. DOI: 10.1016/j.matlet.2014.12.105

[23] Mostafaei A, Elliott AM, Barnes JE, Li F, Tan W, Cramer CL, et al. Binder jet 3D printing — Process parameters , materials , properties, modeling, and challenges. Progress in Materials Science. 2021;**119**:100707. DOI: 10.1016/j. pmatsci.2020.100707

[24] Kumbhar NN, Mulay A. Post processing methods used to improve surface finish of products which are manufactured by additive manufacturing technologies : A review. Journal of The Institution of Engineers (India): Series C. 2018;**99**(4):481-487. DOI: 10.1007/s40032-016-0340-z

[25] Bai Y, Williams CB. An exploration of binder jetting of copper. Rapid Prototyping Journal. 2015;**21**(2):177-185. DOI: 10.1108/RPJ-12-2014-0180

[26] Miyanaji H, Rahman KM, Da M, Williams CB. Effect of fine powder particles on quality of binder jetting parts. Additive Manufacturing. 2020;**36**: 1-10. DOI: 10.1016/j.addma.2020.101587

[27] Romano T, Migliori E, Mariani M, Lecis N, Vedani M. Densification behaviour of pure copper processed through cold pressing and binder jetting under different atmospheres. Rapid Prototyping Journal. 2022;**28**(6): 1023-1039. DOI: 10.1108/RPJ-09- 2021-0243

*Additive Manufacturing of Pure Copper: Technologies and Applications DOI: http://dx.doi.org/10.5772/intechopen.107233*

[28] Bai Y, Wagner G, Williams CB. Effect of particle size distribution on powder packing and sintering in binder jetting additive manufacturing of metals. Journal of Manufacturing Science and Engineering. 2017;**139**(8):1-6. DOI: 10.1115/1.4036640

[29] Kumar A, Bai Y, Eklund A, Williams CB. Effects of hot isostatic pressing on copper parts fabricated via binder jetting. Procedia Manufacturing. 2017;**10**:935-944. DOI: 10.1016/j. promfg.2017.07.084

[30] Yegyan Kumar A, Wang J, Bai Y, Huxtable ST, Williams CB. Impacts of process-induced porosity on material properties of copper made by binder jetting additive manufacturing. Materials and Design. 2019;**182**:108001. DOI: 10.1016/j.matdes.2019.108001

[31] Jadhav SD, Dadbakhsh S, Vleugels J, Hofkens J, Van PP, Yang S, et al. Influence of carbon nanoparticle addition (and impurities) on selective laser melting of pure copper. Materials. 2019;**12**(15):1-17. DOI: 10.3390/ma12152469

[32] Lores A, Azurmendi N, Agote I, Zuza E. A review on recent developments in binder jetting metal additive manufacturing: Materials and process characteristics. Powder Metallurgy. 2019;**62**(5):267-296. DOI: 10.1080/00325899.2019.1669299

[33] Binder Jetting and FDM vs Powder Bed Fusion and Injection Moulding. Available from: https://www.metal-am. com/articles/binder-jetting-fdmcomparison-with-powder-bed-fusion-3d-printing-injection-moulding/. [Accessed: August 1, 2022]

[34] Bai Y, Williams CB. The effect of inkjetted nanoparticles on metal part properties in binder jetting additive manufacturing.

Nanotechnology. 2018;**29**(39):1-11. DOI: 10.1088/1361-6528/aad0bb

[35] Asano K, Tsukamoto M, Sechi Y, Sato Y, Masuno S, et al. Laser metal deposition of pure copper on stainless steel with blue and IR diode lasers. Optics and Laser Technologies. 2018;**107**:291- 296. DOI: 10.1016/j.optlastec.2018.06.012

[36] Ono K, Sato Y, Higashino R, Funada Y, Abe N, Tsukamoto M. Pure copper rod formation by multibeam laser metal deposition method with blue diode lasers. Journal of Laser Applications. 2021;**33**(1):012013. DOI: 10.2351/7.0000322

[37] Zhang X, Sun C, Pan T, Flood A, Zhang Y, Li L, et al. Additive manufacturing of copper – H13 tool steel bi-metallic structures via Ni-based multiinterlayer. Additive Manufacturing. 2020;**36**:101474. DOI: 10.1016/j. addma.2020.101474

[38] Zhang X, Pan T, Flood A, Chen Y, Zhang Y, Liou F. Investigation of copper/ stainless steel multi-metallic materials fabricated by laser metal deposition. Materials Science and Engineering: A. 2021;**811**:141071. DOI: 10.1016/j. msea.2021.141071

[39] Stepien L, Gruber S, Greifzu M, Riede M, Roch A. Pure copper: Advanced additive manufacturing. In: Shishkovsky IV, editor. Advanced Additive Manufacturing. London, UK: IntechOpen; 2022. DOI: 10.5772/ intechopen.103673

[40] Yadav S, Paul CP, Jinoop AN, Rai AK, Bindra KS. Laser directed energy deposition based additive manufacturing of copper: Process development and material characterizations. Journal of Manufacturing Processes. 2020;**58**:984- 997. DOI: 10.1016/j.jmapro.2020.09.008

[41] Siva Prasad H, Brueckner F, Volpp J, Kaplan AFH. Laser metal deposition of copper on diverse metals using green laser sources. The International Journal of Advanced Manufacturing Technologies. 2020;**107**(3-4):1559-1568. DOI: 10.1007/s00170-020-05117-z

[42] Polenz S, Kolbe C, Bittner F, López E, Brückner F, Leyens C. Integration of pure copper to optimize heat dissipation in injection mould inserts using laser metal deposition. Journal of Laser Applications. 2021;**33**(1):012029. DOI: 10.2351/7.0000303

[43] Liu ZH, Zhang DQ, Sing SL, Chua CK, Loh LE. Interfacial characterization of SLM parts in multimaterial processing: Metallurgical diffusion between 316L stainless steel and C18400 copper alloy. Materials Characterization. 2014;**94**:116-125. DOI: 10.1016/j.matchar.2014.05.001

[44] Abdelwahed M, Casati R, Bengtsson S, Larsson A, Riccio M, Vedani M. Effects of powder atomisation on microstructural and mechanical behaviour of l-pbf processed steels. Metals. 2020;**10**(11):1-21. DOI: 10.3390/ met10111474

[45] Ledford C, Rock C, Tung M, Wang H, Schroth J, Horn T. Evaluation of electron beam powder bed fusion additive manufacturing of high purity copper for overhang structures using in-situ real time backscatter electron monitoring. Procedia Manufacturing. 2019;**2020**(48):828-838. DOI: 10.1016/j. promfg.2020.05.120

[46] Mahamood RM, Akinlabi ET. Processing parameters optimization for material deposition efficiency in laser metal deposited titanium alloy. Lasers in Manufacturing and Materials Processing. 2016;**3**(1):9-21. DOI: 10.1007/ s40516-015-0020-5

[47] Azevedo JMC, CabreraSerrenho A, Allwood JM. Energy and material efficiency of steel powder metallurgy. Powder Technology. 2018;**328**:329-336. DOI: 10.1016/j.powtec.2018.01.009

[48] Nandwana P, Peter WH, Dehoff RR, Lowe LE, Kirka MM, Medina F, et al. Recyclability study on Inconel 718 and Ti-6Al-4V powders for use in Electron beam melting. Metallurgical and Materials Transactions B. 2016;**47**:754- 762. DOI: 10.1007/s11663-015-0477-9

[49] Bojestig E, Cao Y, Nyborg L. Surface chemical analysis of copper powder used in additive manufacturing. Surface and Interface Analysis. 2020;**52**(12):1104- 1110. DOI: 10.1002/sia.6833

[50] Speidel A, Gargalis L, Ye J, Matthews MJ, Spierings A, Hague R, et al. Chemical recovery of spent copper powder in laser powder bed fusion. Additive Manufacturing. 2022;**52**:1-13. DOI: 10.1016/j.addma.2022.102711

[51] Mirzababaei S, Paul BK, Pasebani S. Metal powder recyclability in binder jet additive manufacturing. The Journal of The Minerals, Metals & Materials Society (TMS). 2020;**72**(9):3070-3079. DOI: 10.1007/s11837-020-04258-6

[52] Roccetti Campagnoli M, Galati M, Saboori A. On the processability of copper components via powder-based additive manufacturing processes: Potentials, challenges and feasible solutions. Journal of Manufacturing Processes. 2021;**72**:320-337. DOI: 10.1016/j.jmapro.2021.10.038

[53] Chen Z, Han C, Gao M, Kandukuri SY, Zhou K. A review on qualification and certification for metal additive manufacturing. Virtual and Physical Prototyping. 2022;**17**(2):382- 405. DOI: 10.1080/17452759.2021.2018938 *Additive Manufacturing of Pure Copper: Technologies and Applications DOI: http://dx.doi.org/10.5772/intechopen.107233*

[54] Pereira T, Kennedy JV, Potgieter J. A comparison of traditional manufacturing vs additive manufacturing, the best method for the job. Procedia Manufacturing. 2019;**30**:11-18. DOI: 10.1016/j.promfg.2019.02.003

[55] Dowling L, Kennedy J, O'Shaughnessy S, Trimble D. A review of critical repeatability and reproducibility issues in powder bed fusion. Materials and Design. 2020;**186**:108346. DOI: 10.1016/j.matdes.2019.108346

[56] Lipowsky H, Arpaci E. Copper in the Automotive Industry. Hoboken, NJ, USA: John Wiley & Sons; 2007. p. 123-125. DOI: 10.1002/9783527611652

[57] Copper's Role in Growing Electric Vehicle Production | Paid for and posted by CME Group. Available from: https:// www.reuters.com/article/sponsored/ copper-electric-vehicle. [Accessed: June 7, 2022]

[58] ExOne and Maxxwell Motors develop Binder Jetting process for copper winding in electric motors | Metal Additive Manufacturing. https://www. metal-am.com/exone-and-maxxwellmotors-develop-binder-jetting-processfor-copper-winding-in-electric-motors/. [Accessed: June 7, 2022]

[59] Markforged Adds Pure Copper to its Metal X Rapid Additive Manufacturing system. Available from: https://www. metal-am.com/markforged-adds-purecopper-to-its-metal-x-rapid-additivemanufacturing-system/. [Accessed: June 7, 2022]

[60] Pelanconi M, Barbato M, Zavattoni S, Vignoles GL, Ortona A. Thermal design, optimization and additive manufacturing of ceramic regular structures to maximize the radiative heat transfer. Materials and Design. 2019;**163**:107539. DOI: 10.1016/j.matdes.2018.107539

[61] Farsoon Develops Advanced Pure Copper Additive Manufacturing Process. Available from: https://www.metal-am. com/farsoon-develops-advancedpure-copper-additive-manufacturingprocess/. [Accessed: June 7, 2022]

[62] Alloyed's AM Copper Cooling Plate Key Highlight at Formnext - Alloyed. Available from: https://alloyed.com/ alloyeds-am-copper-cooling-plate-keyhighlight-at-formnext/. [Accessed: June 7, 2022]

[63] Renishaw and nTopology Collaboration Produces Intricate, Pure Copper Structures. Available from: https://www.metal-am.com/renishawand-ntopology-collaboration-producesintricate-pure-copper-structures/. [Accessed: June 7, 2022]

[64] Digital Metal Adds Pure Copper to its Metals Range. Available from: https:// www.metal-am.com/digital-metaladds-pure-copper-to-its-metals-range/. [Accessed: June 7, 2022]

[65] Goldstein R, William Stuehr F, Black M. Design and fabrication of inductors for induction heat treating. In: Rudnev V, Totten G, editors. ASM Handbook: Induction Heating and Heat Treatment. Materials Park, OH, USA: ASM International; 2014;**4C**: 589-605. DOI: 10.31399/asm.hb.v04c.a0005839

[66] Horn TJ, Gamzina D. Additive manufacturing of copper and copper alloys. In: Bourell D, Frazier W, Kuhn H, Seifi M, editors. Additive Manufacturing Processes. Materials Park, OH, USA: ASM International; 2020

[67] Johnson K, Burden E, Shaffer M, Noack T, Mueller M, Walker J, et al. A copper pyramidal fractal antenna fabricated with green-laser powder bed fusion. Progress in Additive

Manufacturing. 2022. DOI: 10.1007/ s40964-022-00268-9

[68] Mayerhofer M, Mitteneder J, Dollinger G. A 3D printed pure copper drift tube linac prototype. Review of Scientific Instruments. 2022;**93**(2): 023304. DOI: 10.1063/5.0068494

[69] Frigola P, Harrysson OA, Horn TJ, West HA, Aman RL, Rigsbee JM, et al. Fabricating copper components with electron beam melting. Advanced Materials and Processes. 2014;**172**(7):20-24

[70] Home | IFAST. Available from: https://ifast-project.eu/. [Accessed: June 7, 2022]

[71] Torims T, Pikurs G, Gruber S, Vretenar M, Ratkus A, Vedani M, et al. First proof-of-concept prototype of an additive manufactured radio frequency quadrupole. Instruments. 2021;**5**(4):1-12. DOI: 10.3390/instruments5040035

[72] Torims T, Ratkus A, Pikurs G, Krogere D, Cherif A, Gruber SS, et al. Evaluation of geometrical precision and surface roughness quality for the additively manufactured radio frequency quadrupole prototype. Proceedings of IPAC2022. 2022:787-791. DOI: 10.18429/ JACoW-IPAC2022-TUOXSP3

### **Chapter 7**

## Thermal Tuning of Thermophysical Properties of Single Cu-Ni Alloy

*Yong W. Kim*

### **Abstract**

The great majority of metallic materials in use are not single crystals but disordered. We model such a material specimen as being composed of nanoclusters, each cluster being a small mutually interacting cluster of atoms. In this modeling, a material specimen is then treated as a mixture of nanocrystalline and glassy-state atoms. If we define the degree of crystallinity of the object by the probability that an atom is a member of a crystallite existing within the specimen, the probability would be smaller than unity. Structural disorder in such metallic alloys affects thermophysical properties of the alloy specimen in myriad ways. Transport properties in turn impact material utilization in significant ways to the extent that the specimen could behave as possessing completely different alloy properties. This approach to changing alloy properties can serve useful purposes. We show how one might approach such modification of alloy properties without changing alloy composition with a sample of copper-nickel alloy.

**Keywords:** nanocrystallites, structural disorder, copper-nickel alloy, thermophysical properties, thermal forcing

### **1. Introduction**

A general theory [1] has been developed for a wide class of metallic alloys. In this theory, such a specimen codifies the structural disorder by means of broad size distributions of nanocrystallites and population of glassy-state atoms of the given alloy elements. It has been established by experiment that the functional form of the nanocrystallite size distribution depends sensitively on the alloy's elemental composition. A model material specimen is then represented by a compactified mixture of nanocrystallites as large molecules, intermixed with constituent atoms in glassy state. A large system of reaction equilibrium equations is solved numerically iteratively to follow the growth or shrinkage of nanocrystallites as the temperature of the alloy medium is varied.

In this approach, an alloy specimen is regarded as a randomly close-packed (RCP) mixture of a population of nanocrystallites and constituent atoms in glassy state. The disorder is then represented by the size distribution function of the nanocrystallites. Under sustained exposure to thermal, stress, nuclear, or chemical forcing at an elevated temperature, the distribution function becomes modified, and this process is predictable for a given forcing condition and thus controllable. Transport of

excitations is affected by the detail of the distribution function, making it possible to control transport properties, all at a fixed alloy composition. The modeling and experimental support will be presented [2].

The reaction of 2-D randomly close-packed (RCP) structures to thermal forcing through a simulated oven experiment was described in previous work [1, 3]. The suggested dissociation of nanocrystallites seen in this experiment lent itself to a modeling approach for the dissociation of nanocrystallites in a RCP system. The treatment of this dissociation is handled by the law of mass action. To start, we must define the room temperature basis for our binary alloy. We model a metallic binary alloy as a mixture of nanocrystallites and glassy atoms. At room temperature, we define the structure of our binary alloy as having a certain mix of these two states. The degree of crystallinity at the binary alloy composition will determine the ratio of glassy matter to nanocrystallites. To define the structure of the nanocrystallites, we use the crystallite size distribution found by combination of our simulated experiment and numerical simulation study for the composition. These parameters for the structure of the binary alloy are compositiondependent, and we use the lessons learned from RCP modeling to determine the room temperature structure of each binary alloy we investigate.

In this study, the nanocrystallite size distribution is changed by sustained thermal forcing at a temperature close to but below the melting point of the given alloy. The specimen is quenched to capture the new equilibrium size distribution of the nanocrystallites at this temperature, and its thermophysical properties are measured. The protocol as described is used at different forcing temperatures for a single coppernickel alloy specimen at fixed elemental composition. In this chapter, theoretical modeling and forcing experiments will be presented together with measured thermophysical properties resulting from the forcing runs.

### **2. Thermal forcing**

When we acquire a sample of an alloy from a vendor, it is never clearly known what its thermal history is except that we do have reliable pedigree (composition of 55 W% *Cu* and 45 W% *Ni*). Experimentally, we do know from alloy structure study [1] that our new theory gives rise to the average value of the degree of crystallinity at room temperature [1]. **Figure 1** shows how the degree of crystallinity of the specimen would be expected to change after each round of thermal annealing. We define the degree of crystallinity by the average probability that an atom remains to be part of nanocrystallites within the specimen. **Figure 1** is obtained from our theory [1] making use of the Lennard-Jones constants [4, 5], available in the literature for *Cu* and *Ni*.

We note that we view specimen, which is in the form of a wire, 40.5000 cm long and 1.5875 mm in diameter, is often not straight. It contains many small curved segments, which result from unwinding out of the spool and putting in and out of an electric furnace (Fisher Muffle Furnace). For precise measurements, the specimen has been stretched between two end point locations on an optical bench as follows. The end of the copper-nickel wire specimen was flared by a small hammer to serve as an anchor by means of a metal-ceramic latch in an electrically insulating manner. The opposite end is held in a similar manner. One end of the specimen is struck by the bob of a charged pendulum made of a conductor to charge a grounded capacitor (ceramic capacitor, 0.02 microfarad), generating a short pulse from the pendulum in order to generate a narrow electrical pulse to trigger an oscilloscope (digital oscilloscope 400 MHz bandwidth at 400 M bites per second digitization rate). At the end of the

### **Figure 1.**

*The dimensionless number density* ρ *of glassy-state atoms, where ρ =* n/n0 *and* n *is given per unit volume, versus dimensionless temperature for the alloy T=Tmp. Tmp denotes the melting point of the alloy; the intersection of the green line with the blue line in the figure corresponds to unity in dimensionless temperature. n*<sup>0</sup> *denotes the number of single isolated atoms in glassy state at melting point per unit volume.*

wire, specimen is pressed by a spring-loaded transducer (piezoelectric), whose output is displayed in the oscilloscope, thus making the determination of the time of flight of the mechanical pulse (acoustic pulse) to travel the full length of the specimen [6].

On further consideration of any bends in the wire specimen, we have prepared a fixture for straightening the wire specimen. The fixture is designed not to compress nor to stretch the specimen after each annealing treatment, as follows. Two long slabs of an optical bread board made out of aluminum, each with a full row of a straight groove machined out between the two rows of threaded holes, and the heat-treated specimen is sandwiched along the full length of the boards, facing each other. The wire specimen is trapped within the two long 90-degree V-grooves facing each other, the two sides of the wire specimen pressed by aluminum walls gently against each other by means of long screws threaded into the rows of holes on either side of the long grooves. This arrangement allows the measurement of the length of the specimen before and after each annealing round. This type of fixtures can be readily prepared in a machine shop.

The interior of a furnace is typically three-dimensional chamber and cube-shaped, and, consequently, the specimen must be bent into a winding roll to be within a furnace. The above-mentioned wire straightener fixture resolves the chores of reshaping the wire specimen before and after the annealing run, while the annealing could be carried out in an inert atmosphere.

As the temperature the specimen may encounter the effect of oxidation as one approaches the melting temperature of the alloy, we developed a method of putting the specimen within a long stainless tubing capped with all-metal (stainless steel) Swagelok's (Parkin-Hannifin). The protective tubing used was 6 mm in outer diameter. The protective tubing can be evacuated and filled with argon and sealed after the wire specimen is inserted. The assembly may easily rolled up in order to insert into the furnace and retrieve the annealed specimen after annealing.

### **3. Theoretical considerations**

The size of nanocrystallites that make up a real alloy specimen changes as a result of annealing. The very interesting question is what happens to the length and volume of the specimen due to annealing and quenching of the specimen. It is also important to know how stable would the size distribution of nanocrystallites be because the new distribution function determines the thermophysical properties of the alloy specimen. The motion of an atom is governed by the size of the energy that binds it to a site relative to available thermal energy, whereas energy variation experienced in normal applications is usually very small.

Another to ask is how the thermal evolution of the nanocrystallite population impacts the thermophysical properties of disordered metallic solids as the temperature is raised. The model calculations have clearly shown that not only the number of the nanocrystallites diminishes in population but also the functional form of their size distribution undergoes significant changes at elevated temperatures. These changes lead to changes in packing fraction of the alloy specimen. From the definition of the degree of crystallinity, which measures the probability that an atom belongs to a nanocrystallite, we can derive the packing fraction of glassy matter, *η*gl, expressed in terms of the degree of crystallinity, *γ*, the packing fraction *η*cp when the alloy medium is crystalline and the packing fraction *η*rcp when the medium is disordered. We use the composite value for the average packing fraction as compiled by Berryman [7] for the latter. The expression for the packing fraction of fully glassy matter can be found as follows:

$$
\eta\_{gl} = \frac{(\mathbf{1} - \boldsymbol{\chi})\eta\_{rp}\eta\_{cp}}{\eta\_{cp} - \boldsymbol{\chi}\eta\_{rp}}.\tag{1}
$$

For a given alloy specimen *η*gl is always smaller than either *η*cp or *η*rcp, and a transition of crystalline particles into the glassy state of matter directly leads to a volume expansion. It thus impacts thermal expansion of the specimen; this contribution is new and additional to the usual definition of thermal expansion. We propose that the simplest way to incorporate the contribution into the alloy model is to treat the thermal expansion in nanocrystallites and in glassy matter separately from the change of the relative populations of atoms within nanocrystallites and atoms in glassy state of matter. Upon heating, the nanocrystallites within the specimen expand according to the crystalline expansion coefficient. Glassy matter of the specimen would also expand but according to the average expansion coefficient of metallic glass. Both of these expansion coefficients are knowable in principle from the literature.

In prescribing the linear expansion coefficient for a disordered alloy in a functional form, one can express the contributions from these effects to the volume of the specimen as a function of temperature in the following form:

$$V(T) = V(T\_o) \left[ \chi(T) \{ (1 + \beta\_{cr}(T)(T - T\_o)) \} + \{ 1 - \chi(T) \} \left\{ \left( 1 + \beta\_{gl}(T)(T - T\_o) \right) \right\} \right] \tag{2}$$

*V*(*T*) and *V*(*T*o) denote the volume of the specimen at temperature *T* and at a reference temperature *T*o, respectively. *βcr*ð Þ *T* and *βgl*ð Þ *T* are the volume expansion coefficient at temperature *T* of the crystallites and glassy matter, respectively. The volume expansion is written out as the sum of the expansions of the nanocrystallites and glassy matter. The new contribution is included into *γ*ð Þ *T* , which measures the movement of the population of ordered crystalline atoms in the specimen when the temperature is raised [3].

In the conventional treatment, no distinction is made of the nanocrystallites separately from the glassy matter in writing out the linear expansion coefficient; their relative populations are assumed to remain fixed. In the present treatment, the fraction of nanocrystallites in the specimen changes when the temperature is raised, and this change can be determined by first-principle calculation.

### **4. Concluding remarks**

Theoretical analyses of the above kind were carried out for the following singeatom metals: *Ag*, *Al*, *Au*, *Ca*, *Ce*, *Cu*, *Ir*, *Ni*, *Pb*, *Pd*, *Pt*, *Rh*,*Th*, *Ba*, *Cr*, *Fe*, *K*, *Li*, *Na*, *Nb*, *Rb*,*Ta*, *V*, *W*, *Be*, *Cd*, *Co*, *Dy*, *Er*, *Hf*, *Mg*, *Re*, *Ru*,*Ti*,*Tl*, *Y*, *Zn*, *and Zr*.. Similar calculations were carried out for binary disordered metallic alloys *AlTi*, *Al*3*Ti*, *AlTi*3*, AuCu*, and *AuCU*3*.* All calculations showed results of the types consistent with those shown in **Figure 1** with varying degrees of agreement for the melting points with known experimentally determined melting point data.

We had a successful thermal forcing run. A 0.382-M-long wire specimen of 55 W% in copper and 45%W% in nickel was used for this run. When forced at 940 K for 16.45 h and the specimen was quenched in water at 10°C, the measured speed of longitudinal sound pulse at room temperature was found to be increased to 5744.36 M/s from the speed of sound in untreated specimen, off the shelf, at 4235.03 M/s. This is an increase by 35.64 percent.

The change of metallic alloy's thermophysical properties stems from the modifications of the equilibrium distribution functions of nanocrystallites within the system by slow thermal forcing (over 15 hours at a selected temperature) at high temperatures (at temperatures above one half of the melting point), and therefore, it is reasonable to expect that the thermophysical property of the system will most likely to remain as modified throughout normal utilization of the material. The melting point of the alloy used here is given to be 1507 K.

The structural disorders that are pertinent in the context of the present discussion may arise partly from the rough (approximate) handling in material processing stages or partly from trace-level impurities that remain within the alloy-making, and these should be considered intrinsic to the industry, and therefore, the current state of the art may present many useful opportunities to be exploited.

### **Author details**

Yong W. Kim Department of Physics, Lehigh University, Bethlehem, USA

\*Address all correspondence to: ywk0@lehigh.edu

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Thermal Tuning of Thermophysical Properties of Single Cu-Ni Alloy DOI: http://dx.doi.org/10.5772/intechopen.108266*

### **References**

[1] Cress R, Kim YW. Statistical physics modeling of disordered metallic alloys. In: Glebovsky VG, editor. Progress in Metallic Alloys. IntechOpen, London, UK; 2016

[2] Cerny R. Crystal structures from powder diffraction: Principles, difficulties and progress. Crystals. 2017; **7**:142

[3] Kim YW, Raffield JH. Sound propagation in thermally-forced coppernickel alloy. High Temperatures-High Pressures. 2017;**46**:271-280

[4] Zhen S, Davies GJ. Calculation of the Lennard-Jones N-m potential energy parameters for metals. Physica Status Solidi. 1983;**78**:595-618

[5] Kong CL. Combining rules for intermolecular potential parameters. II. Rules for the Lennard-Jones (12–6) potential and the morse potential. The Journal of Chemical Physics. 1973;**59**: 5-23

[6] Goodrich CP et al. Jamming in finite systems: Stability, anisotropy, fluctuations, and scaling. Physical Review E. 2014;**90**:022138

[7] Berryman JG. Random close packing of hard spheres and disks. Physical Review. 1983;**A27**:1053

*Edited by Daniel Fernández-González and Luis Felipe Verdeja González*

Copper has been an important metal throughout history. Initially, it was used as raw material for the manufacture of tools, weapons, ornamental objects, and more. The later discovery of copper alloys, such as bronze and brass, extended the use of this metal alloy to many different fields based on its mechanical, corrosion, and wear resistance. Nowadays, copper is mainly used in the electrical and thermal conductivity fields, although new uses are being discovered. This book provides a comprehensive overview of copper in two sections on copper mining and processing and copper applications.

Published in London, UK © 2023 IntechOpen © FactoryTh / iStock

Copper - From the Mineral to the Final Application

Copper

From the Mineral to the Final Application

*Edited by Daniel Fernández-González* 

*and Luis Felipe Verdeja González*