A Review on the Additive Manufacturing of W-Cu Composites

Abstract

In recent years, W-Cu composite systems have become very interesting subjects due to good electrical and thermal conductivity, high-temperature strength, certain plasticity, and excellent radiation resistance. W-Cu composites are a very important class of materials in applications like PFM (plasma facing materials), functional graded materials (FGM), electronic packaging materials, high-voltage electrical contacts, sweating materials, shaped charge liners, electromagnetic gun-rail materials, kinetic energy penetrators, and radiation shielding/protection. There is no possibility of forming a crystalline structure between these two materials. However, due to the unique properties these materials possess, they can be used by preparing them as a composite. Generally, W-Cu composites are prepared via the conventional powder metallurgy routes, i.e., sintering, hot pressing, hot isostatic pressing, isostatic cold pressing, sintering and infiltration, and microwave sintering. However, these processes have certain limitations, like the inability to produce bulk material, they are expensive, and their adoptability is limited. Here, in this review, we will discuss in detail the fabrication routes of additive manufacturing, and its current progress, challenges, trends, and associated properties obtained. We will also explain the challenges for the additive manufacturing of the composite. We will also compare W-Cu composites to other materials that can challenge them in terms of specific applications or service conditions. The solidification mechanism will be explained for W-Cu composites in additive manufacturing. Finally, we will conclude the progress of additive manufacturing of W-Cu composites to date and suggest future recommendations based on the current challenges in additive manufacturing.

1. Introduction

Additive manufacturing (AM) involves creating 3D components by depositing multiple layers of material based on CAD models. The layers are produced using a CNC positioning system, often involving laser or arc deposition processes. AM is also known as 3D printing, additive processing, and free-form fabrication [1,2,3]. Mostly, AM parts are manufactured through laser selective melting; however, AM production through arc processes like GMAW, GTAW, and plasma have become increasingly important [1].

The main advantages of AM include design flexibility though modeling, the ability to create complex shapes using CAD geometries, and the use of various available materials. It finds applications in industries like aerospace, where it is used for intricate titanium parts, and automotive manufacturing, especially for small batches of specific components. Other sectors, like medical implants, tooling, nuclear, and defense industries, also benefit from AM technology [4,5,6,7,8]. The current market of AM grows with a CAGR of 21%, having USD 44 billion, and it is forecasted to have USD 35 billion in 2027 [9].

W-Cu alloys systems are very interesting subjects due to their appeal in the field of nuclear, high-temperature, and wear applications due to their high mechanical strength, high abrasion resistance, high thermal stability (under high gamma and neutron fluxes), and thermal conductivity [5,10,11]. Because of the difference of properties like melting point, electronegativity, atomic size, and crystal structure, there is a large mismatch in the properties, making it difficult for any chemical bond formation and fabrication via conventional casting method, as there is no formation of any bond between W and Cu atoms [12,13].

Because of the low heat input and better thermal management, W-Cu composites are mostly fabricated by conventional powder metallurgy routes like sintering and infiltration, liquid-phase sintering, microwave sintering, hot pressing, and injection molding [14,15,16,17,18]. Some of the current additive manufacturing routes include laser melting, selective laser melting, and electron beam melting [10,19,20,21]. In this review paper, we will discuss in detail the fabrication techniques, and we will focus on the additive fabrication of W-Cu composites, current trends, modifications, challenges, and future aspects of the composite material.

2. Applications for W-Cu

Additive manufacturing or 3D printing is well suited for rapid manufacturing due to automated high-speed production of complex geometries through computer aided manufacturing (CAM) systems. Some of the main applications of W-Cu composites are given in Table 1.

3. Consideration for Fabrication of W-Cu

Starting from selection of composition to the fabrication equipment and, finally, moving to characterization, there are many things to be considered. Some of them are given in the following.

3.1. Selection of Powder Composition

Selection of powder composition plays a key role not only in the selection of parameters; the mechanical and metallurgical properties are also greatly affected. The reason behind this is understandable, as the W and Cu have different properties, as shown in Table 2 [36,37]. Normally W-Cu composites are prepared with Cu% greater than 50%, while the % of W varies between 50 and 90%. The reason behind this is the softness of Cu, which can deteriorate the properties of the composite. The thermal conductivities of mild and stainless steel are 45 and 15 W/(mK) [38], while the electrical resistivity of mild and stainless steel are 9.6 and 68.97 Ω·m [39]. This shows that both the thermal and electrical conductivity of W are higher than that of mild and stainless steel. For these reasons, W plays a very important role in electrical and thermal conduction compared to mild and stainless steel in structural applications, while providing strength and hardness at the same time. Until now, there has been no standard adopted for the qualification of W-Cu composites except for applications in electrical contacts by ASTM B702-93 (2024) [40]. However, properties obtained after fabrication through different routes can be compared and justified. Table 3 shows the general trend of the most common properties of W-Cu composites prepared by Cu infiltration for electrical contacts [41].

For additive manufacturing, due to the non-melting nature of W, the size of initial W should be such that steady flow is maintained during welding. Usually, high-purity commercial powders obtained through gas atomization technique in the size range of 20–200 µm are used for the composite [20,42,43,44]. Meanwhile, small nano-sized powders are well used in conventional powder metallurgy due to the excellent control on the process. A small size in the conventional fabrication routes has better homogeneity regarding both properties and microstructure. Meanwhile, the additive fabrication route can produce certain inhomogeneity in the fabricated composite depending on the welding parameters and, hence, flow kinetics of the molten pool. To improve the flow kinetics of the W particles in the molten pool, spherical powders are preferred.

3.2. Solubility W-Cu Explained

From the data available in Table 1, there is no possibility of a solid solution between these two elements. Before the W starts melting, the Cu will start vaporizing at 2562 °C. In other words, their melting points do not overlap, as shown by the phase diagram in Figure 1 [45].

From the phase diagram, we can deduce that conventional methods of alloy production cannot be adopted to produce W-Cu alloys. M. Nastasi, et al. [46] tabulated the heat of formation for various W-Cu alloys in the range of 16–24 kJ/mole. They also showed the Gibbs free energy to be positive, meaning that it is impossible to form a thermodynamically stable alloy of the system, as shown in Figure 2. Hongbo Zhang et al. [47] reported the formation enthalpy of a W-Cu system at +35.5 kJ/mol as the highest in the bimetallic immiscible nanoparticle system. They also suggested a critical size of 10 nm of Cu at which the enthalpy of formation becomes negative.

C.P. Liang et al. [48] reported the heat of formation of W-Cu system to be +33 kJ/mol in both solid and liquid states using CE, SQS and Miedema’s model, as shown in Figure 3.

The surface energy, or surface free energy, of a material is the tendency of its molecules to attract the molecules within or from other sources. Generally, if the surface energy is high (like metals), a good wetting phenomenon occurs, as shown in Figure 4 [49].

With an increase in the atomic number (as W have), the surface energy decreases, as shown in Figure 5 [50].

A better way to increase the surface energy and fairly reduce the enthalpy of formation is to reduce the size of the powder for the W-Cu system, as shown in Figure 6 for the possible two BCC and FCC structures [47,51,52].

Researchers have established an intermediate phase of binary pseudo-alloy of W-Cu using non-equilibrium processes like high-current pulsed electron (HCPE) and ion sputtering beam, resulting in the formation of a long-period super-lattice structure [43,53,54]. TEM analysis has confirmed these phases with a regular pattern of arrangement (d-spacing) of W and Cu atoms. However, these processes are surface-modification techniques, which provide rapid heating and cooling of a surface, producing high thermal stresses and shocks. These methods are only suitable to produce a few microns of effects on the surface but not the bulk material [55,56].

Due to the above-mentioned challenges, most of the W-Cu composites are fabricated through convention metallurgy routes. Other fabrication routes, like laser, electron, and arc methods, have also been used to fabricate this composite material.

As discussed, there is no possibility of formation of a crystalline structure between these two materials. However, due to the unique properties of these materials, they can be made useful by preparing them in the form of a composite. The new composite material will have the properties of excellent mechanical, thermal, and electric properties depending upon the composition of the system [57]. For example, a 50–50% system will have moderate mechanical, thermal, and electric properties. With the increase in Cu content, the shift from good mechanical properties to low/medium mechanical properties will occur. However, the thermal conductivity, coefficient of thermal expansion, and electrical conductivity will increase [57,58,59,60]. An excellent approach, depending on the application, is to produce functionally gradient materials (FGMs) [61]. These materials possess both of the extreme properties (different) at their ends/faces. A general explanation for both continuous and non-continuous FGMs is given in Figure 7 [62]. In discontinuous FGM, the microstructure changes stepwise, usually through an interface, while in continuous FGM, the microstructure changes smoothly, without any interface [60,62].

3.3. Ductile-to-Brittle Transition Temperature (DBTT)

DBTT is the temperature at which the sudden loss of ductility occurs. The DBTT of metals is very important in terms of their practicality. One of the challenge with W is its high DBTT temperature (500–700 K) causing high residual thermal stresses at the interfaces [63]. FCC structures have their slip plane of (111), and they have a large number of slip systems, so lowering the temperature does not affect their ductility. Metals like Cu and Al have FCC structures. HCP metals also possess ductility at lower temperatures, like titanium and magnesium do. BCC materials, like W, undergoes DBTT at higher temperature, as there are no further movements of dislocations, as shown in Figure 8 [64,65].

Chai Ren et al. [66] reported the addition of Re as having a very good impact on the DBTT of W, as shown in Figure 9. The addition of Re introduces an extra slip plane for skew dislocation of W atoms and a reduction in the staking fault energy [67]. They documented the DBTT to be 113 °C for a 1.9 wt.% addition and −101 °C for a 26 wt.% addition. They also stated that Re affects above 6 wt.% on the grain size. However due to the high cost and transmutation of Re into osmium under high neutron radiation, its practical use in fusion reactors is limited [68].

3.4. Modeling and Simulations of Additive Manufacturing

The availability of different advanced modeling and analysis software, including mathematical modeling, has made it easy for the designers to analyze the additive manufacturing for the process, structure, properties, and performance [69,70]. The accuracy of the model (physical and numerical) is dependent on the material, process properties, assumptions, and temporal and spatial discretization. Different tools are used to model and perform the analysis. For example, Finite-Element Analysis (FEA)-based Computational Fluid Dynamics (CFD) is used for thermomechanical modeling of heat transfer and deformation in solids [70]. Truchas, which is an analysis tool, is use for the process-parameter optimization in laser power systems [71]. The ALE3D multiphysics code is vastly applied to the melt pool characteristics, including solidification behavior and solid mechanics [72]. The powder feeding before melting can be modeled using LIGGGHTS, LAMMPS, and GEODYN-L. For microstructure and phase evolution, Tusas with CALPHAD and SPPARKS based on the Potts Kinetic Monte Carlo technique are used. Finite-Element Analysis (FEA)-based ABAQUS, and Diablo can be used for the performance analysis of the weld and, hence, the part. PLATO tool, Tosca, and ParetoWorks are employed for the topological analysis of the AM parts [69]. Figure 10 shows a graphical representation of the four-modeling representation, along with their length scale [70].

Michael M. Gasik et al. [73] used mathematical modeling of W-Cu FGM to evaluate the thermal and mechanical model. Meng Li et al. [74] performed a post-fabrication analysis for internal stresses, strains, interfacial forces, and densification mechanisms of a W-Cu composite. Alessandro Zivelonghi et al. [75] evaluated the performance characteristics of W-Cu composites under thermomechanical loading. Ding, Xiaoyu et al. successfully fabricated an FGM based on their FE model and performance optimization. From the literature, enough data are available on the post-fabrication analysis of W-Cu composites. However, the process modeling of W-Cu is very limited.

4. W-Cu Fabrication Routes

The fabrication routes for W-Cu composites can be divided into (1) conventional and (2) additive fabrication routes. Due to the low melting and boiling point of Cu compared to W, casting is impossible, as Cu will start evaporating before the melting of W. Hence, the only possibility is to melt the Cu phase and surround W particles with a composite form. This kind of approach is well suited to the traditional powder metallurgy routes, and that is why it is mostly adapted.

4.1. Conventional Routes

4.1.1. Infiltration Techniques

In this method, the W powder is pressed at high pressures, up to 450 MPa, in the range of 2000 °C, and a W skeleton ID formed. The skeleton is infiltrated with molten Cu at a temperature higher than 1300 °C, and the molten Cu is allowed some time to reach the vacant spaces inside the W structure, as shown in Figure 11 [25]. The infiltration technique is still widely adopted for producing composites having more than 60% of W content. H₂ or vacuum is maintained during infiltration [58]. The W skeleton preform can be obtained from particles, as well as in the form of fibers [12,15]. The relative density of composite can be increased by reducing the size of W particles. However, reducing the size also brings difficulty in the flow of Cu, and porosity can occur in the final structure. To lower the residual thermal stresses and improve the wetting between Cu and W phases, other alloying elements, like Ni, Sb, and Fe, can be added. The infiltration technique is relatively a slow process, but researchers have tried the use of super gravity at a lower temperature to increase the speed of infiltration and obtained the composite in only 5 min [76]. To reduce the sintering time, wire crumpling has been proposed, along with subsequent compaction and infiltration by molten Cu [77]. This technique allows for the elimination of typical steps involved in powder metallurgy, like powder treatment, and the density obtained was excellent.

4.1.2. Liquid-Phase Sintering

In this method, the green densities are produced via compaction after the mixing of the powders in the relevant ratios. The green densities are heated above the melting point of Cu and sintered, as shown in Figure 12 [78]. The composite produced is highly dense, and the Cu in the structure is well distributed. Because the process of liquid-phase sintering is time-consuming; hence, the grains grow to bigger sizes. Because of the grain growth in the composite, thermal expansion occurs in the composite. During cooling, the composite must accommodate the shrinkage, and hence, the formation of cracks can occur in the material. Due to these reasons, the composite is often retreated with post-fabrication techniques like hot pressing, repressing, and hot swage. Although the initial process is quick and simple, the application of these post-fabrication processes and techniques increases the overall cost of fabrication. Researchers have tried to use the LPS combined with infiltration to reduce the cost and obtain fully dense material [79]. Mixing of the powders can be performed either physically in a mixture or mechanical allying can be applied using ball milling. Ball milling of W and Cu powders has a profound effect on the thermal, electric, and melting point of the composite [80]. Fe, Ni, and Co can be used to improve the sintering behavior of the composite with the sacrifice of certain properties in a process called activated liquid-phase sintering [81].

4.1.3. Microwave Sintering

In this method, the heat energy used to melt the Cu part of the composite is provided by the heat generated by the eddy current due to the electromagnetic induction on the surface of the metallic particles. The schematic of the process is shown in Figure 13 [82]. Generally, metals are good reflectors of electromagnetic radiation. However, at high frequencies, generation of current densities due to electron disturbance at the surface of metal can interact with the electromagnetic field of the microwave and produce an eddy current, according to the Lorentz force law. This is further favored by increasing the surface-to-volume ratio of the metal by using powders in the microwave. Most research and industrial activities involve microwaves only at 2.45 GHz and 915 MHz frequencies. Uniform volumetric heating up to 100 °C/min can be achieved. Using microwave technology, fully densified microstructures have been obtained [14,83].

4.1.4. Hot Pressing

A thermomechanical process which combines both pressure and heating of the powder can be performed to obtain the desired shape of the mold. Usually, the sintering temperature is kept below the melting temperature of copper and above the recrystallization temperature. Usually, the temperature adopted for this is 950 °C. Before sintering, the powders are cleaned, usually in a H₂ atmosphere and at a high temperature (250 °C) for 2 h. After being mixed for hours, the powder is subjected to graphite dies and hot-pressed at a high pressure (usually 45–70 MPa) [60,84]. A typical graphical representation is given in Figure 14 [60].

As can be seen in Figure 14, hot pressing can take a long time and is dependent on the pressure applied. The process is limited to the shape of the die. Better mechanical and electrical properties can be obtained, as the application of external pressure for consolidation of the composite can induce hardening and dislocation densities, which can improve the strength and fatigue resistance of the composite [85,86].

4.1.5. Spark Plasma Sintering

In this method, also known as pulsed electric current sintering (PECS), the powder is passed through a direct pulsed current, while, at the same time, a lower mechanical pressure compared to hot pressing is applied on the powder. The mechanism of this process is shown in Figure 15 [82]. Initially, the electric discharge creates sparks between the gap of the particles’ contacts by passing the particles through a high current (DC or AC). The result is high-temperature micro plasma generated on the surface of particles which causes a sharp rise in surface temperature of the particles. In the second stage, the flow of current through the particles melts the particles by joules heating. Since the charge passes through the particles uniformly, the heat generated is homogeneous throughout the particles, and a dense structure is obtained of more than 99% theoretical density. W-Cu composites have been fabricated with a heating-rate range from 100 to 200 °C, temperature range from 950 to 1300 °C, and pressure range from 30 to 75 MPa [25]. The relative density obtained in one case was 90.5% for a fully graded composite, while in another case, it was 96.53% for a partially graded composite structure [87,88]. The reason behind this is the gap that remains during the short period of time between the W particles, as they cannot be fully melted, and hence, only partial melting occurs between the W particles.

4.1.6. Metal Injection Molding

Because of the low melting temperature and plastic nature of Cu, W-Cu composites can also be prepared via this means. In one of the methods, the initial powder is mixed and injected through a mold with a binder. After the green compact has been obtained, the precursor is heated to remove the bonding material and then sintered. In another method, a precursor of the W in billet form is prepared using injection molding and then infiltrated sintered. This method is suitable for mass production of small, uniform parts. The schematic is shown in Figure 16 [25].

4.1.7. Mechanical Alloying and Sintering

In this method, ball milling is used to produce nanoparticles of the powders, and after that, the particles are sintered to obtain maximum relative density. The main objective of this method is densification of W-Cu composites using small particles, a result which is not possible with the traditional sintering and infiltration technique. However, ball milling can introduce impurities into the composite and is very time-consuming. The grain size of the composite and, hence, the mechanical, thermal, and electrical properties are all dependent on the milling and sintering time [89]. Mechanical alloying also introduces imperfections into the microstructure of the alloying particles, like vacancies, dislocations, dislocation densities, and refinement of grains, which can further impact the properties [90,91]. The schematic of mechanical alloying is shown in Figure 17 [92].

4.2. Additive Fabrication Routes

The additive fabrication routes for W-Cu composites can be divided into the following:

4.2.1. Laser Melting and Selective Laser Sintering

Laser additive manufacturing (LAM) can be divided into three types, namely stereolithography (SLA), selective laser sintering (SLS), and selective laser melting (SLM). SLA is mostly applied for resins with an ultraviolet source of beam. SLS is applied to polymers which mostly use gas-type lasers. While fiber lasers are usually recommended for metal powder melting. In this method, a laser beam is used to melt the powder that is injected into the focal point of the beam while the molten pool is covered with shielding gas. The process is like the arc process, except for the use of a laser. The laser-equipment parameters are adjusted accordingly. Layer-by-layer additive manufacturing can be performed. The schematic of the process is shown in Figure 18 [93].

The quality of the weld by LAM is controlled by the laser power, scanning speed, and environment of the welding. The flow of the powder can be controlled individually, or a pre-mixed ratio of the powder can be used. Before the experiments are performed, the substrate is cleaned properly with acetone, and different substrates can be used. LM can be further divided into selective laser melting in which only a selective portion of the pre-deposited powder is melted and bonded together. This works the same as powder bed fusion. For W-Cu composites, both methods can be applied. LM can be used to produce additive parts, cladding, and coatings.

Zheng Hong Liu et al. [94] fabricated W-Cu composites using different scanning speeds and laser-power levels, and he concluded that the quality of weld is strongly affected by the operating parameters as shown in the Figure 19. Specifically, the balling phenomenon is observed because of the mismatch in the shrinkage and filling of voids between two scans that go side by side [93]. Producing the composite by increasing the laser power can create more chances for the melting and formation of nanocrystals of W in the composite. Similarly, the recrystallization of W grains can occur, and the hardness can change [95]. Xiaoyu Ding et al. [61] prepared FGM, using the SLM method, on SS316 stainless steel. However, porosities and micro cracks were observed in the FGM, greatly affecting the thermal conductivity and thermal shock resistance.

4.2.2. Electron Beam Melting

In this approach, an electron beam is used in a vacuum chamber to melt the powders and perform the welding. The beam can be focused on a small area for melting. The power of the equipment controls the velocity of electrons and, hence, the energy required for melting, depending on the size of the weld zone. However, the size of the weld is greatly constrained by the size of the equipment’s chamber. Due to high capital and operating costs, this technology is not much applied in welding. The schematic of the process is shown in Figure 20 [96].

Zongwen Fu et al. [97] prepared W-Cu FGM using an electron beam. In another study, W skeletons were prepared by electron beam melting, followed by infiltration of Cu to prepare the composite [98]. The strength of the composite was mainly dependent on the construction of the skeleton W fabricated by electron beam melting.

A modified approach to the use of electron beam melting is to use a high-pulsed electron beam for joining dissimilar materials, like W and Cu, and produce a diffusion region between them. However, this process is mostly used in the surface modifications for better diffusion between W and Cu atoms [43,99]. While electron beam melting is used in the welding of other materials, its use in the production of W-Cu still needs to be explored further [100].

4.2.3. Arc Fabrication Routes

The arc process using the general plasma produced by highly charged ionized gases during welding is common in the other welding processes. Usually, W and Cu in powder form are used in plasma-transferred arc welding in which the feedstock powder is fed into the center of the arc and is deposited onto the substrate plate. Feeding with wire is also an option when using this route. Until now, the research on W-Cu composite has been in the initial phase, and more work is required to explore this route. Jiri et al. [44] used the PTA route to prepare W-Cu FGM. However, their work still needs further investigations, as there was high dilution from the base material which distorted the main constitution of the W-Cu composite.

5. Limitations of Conventional Fabrication Routes

While there are some process advantages of traditional powder metallurgy routes, like low operating cost, near net shape, better material utilization, close tolerance, surface finish, small-to-moderate-level production, energy efficiency, and environmental friendliness, there are also some metallurgical advantages, like homogeneity, chemical composition control, controlled porosity, and special microstructures. However, despite the many advantages of powder metallurgy, there are some disadvantages also related to powder production, handling, sintering, compaction, part size/shape, and bulk production [101]. Some of the limitations associated with each of the powder metallurgy routes for W-Cu composites are given below.

5.1. Infiltration Method

One of the limitations of this method is that the tungsten skeleton should be at least 50% [102]. However, this is, in turn, beneficial, as most of the commercial W-Cu composites are fabricated with W starting from 50 wt.%. The size, shape, and sintering pressure of the powder have a great effect on the final density of the composite. Another limitation of this process is the % content of W, which produces problems for the flow of molten Cu as the content increases. The production of green density (W-skeleton) of W also increases the cost and reduces the process efficiency of the composite. Sometimes, extra densification is also required for higher contents of W at higher temperatures [15,103].

5.2. Liquid-Phase Sintering Method (LPS)

In this method, the green density, which consists of W and Cu compact powder, is heated above the melting point of the Cu. One of the key issues with this method is the shrinkage of the composite as Cu melts and the evolution of long grains in the composite. Due to this, cracks can develop in the composite, and the theoretical density achieved is lower, typically in the range of 90–95% [25]. To overcome this problem, post-processing or sintering with fine powders can solve this problem. However, this will further increase the cost of fabrication. To improve the sintering ability and densification, elements like Fe, Ni, Co, and Pd are useful, but the addition of these elements can deteriorate thermal and electrical properties [95,104,105,106,107].

5.3. Microwave Sintering

This method can produce a theoretically 100% dense microstructure. However, the process is still on the lab scale and has not been fully employed on a commercial scale [25]. The size and shape of the product is limited by the equipment itself, and the more fine the powder is, the more suitable it is for the electromagnetic waves’ absorption [108].

5.4. Hot-Pressing and Sintering (HPS)

One of the main disadvantages of hot pressing is the requirement of a mold and its associated cost, as the mold must be able to withstand the applied pressure during the process [85,109].

5.5. Spark Plasma Sintering (SPS)

This method for producing components also has challenges, like size and shape constraints [110]. Grain coarsening in Cu and recrystallization in W can occur due to the high holding time at higher temperatures [18].

5.6. Metal Injection Molding

W-Cu composites cannot be directly prepared using this method. This is due to the flow characteristics that are disturbed during the pouring of a molten mixture of W-Cu into the mold. To overcome this problem, binders with a mixture of the powders are used in the injection molding process. After that, debonding, sintering, and infiltration are used to shape the final composite [111,112]. Thus, the time and cost of the process are significant.

5.7. Mechanical Alloying and Sintering

The main disadvantages of mechanical alloying are the time required for mechanical alloying and the impurities from the process towards the final sintering. Also, the cold sintering agglomeration of the particles during the process poses a challenge [89,113]. Similarly, the W-Cu composite may have less ductility, electrical/thermal conductivity, and toughness because of the cold working in its final form [114]. Sometime, because of the possible oxidation of W during ball milling, WO₃ is directly used with Cu in the presence of a H₂ atmosphere to obtain the mechanically alloyed composite [115].

6. Challenges in Additive Manufacturing

6.1. Laser Melting and Selective Laser Sintering

Apart from the capital and running costs of laser additive manufacturing, there are challenges with the use of Cu powder also. One of the main problems with welding the W-Cu composites is the reflectivity of Cu of laser light [94,116]. This is one of the main concerns during welding, as the reflected light can damage the machine parts. Because of the high conductivity and reflectivity of the Cu particles, the melting is difficult. However, as the melting of Cu occurs, the reflectivity decreases from 95% to 7%, and the process can be smoothly run. Operating a fiber laser near the infrared region of 1070 nm of wavelength, using high-intensity laser pulses, protecting the equipment using a coating, and the use of a specially designed nozzle will make possible the fabrication of W-Cu composites [117]. With W-Cu composites, a balling phenomenon can be observed because of the mismatch between the shrinkage and filling of voids between two scans that go side by side in powder bed fusion melting [93]. Porosity in the microstructure, as shown in Figure 21, is another problem that can arise during laser additive manufacturing due to various reasons, like improper parameters, lack of fusion, keyhole effect, and vaporization of Cu [42,118,119]. Inflight particles characteristics like speed, temperature, shape, along with carrier-gas speed, and inflight distance all play an important role in determining the turbulency and quality of the weld [120,121]. The size of the particles in terms of handling and feeding is crucial for the continued operation and efficiency of the process [122].

6.2. Electron Beam Melting

Electron beam welding is one of the advanced welding technologies that can be applied to weld W-Cu composites. However, challenges like setup costs, operator costs, size limitations on the weld part, and X-ray radiations from the part’s surface are common issues with the process [123,124]. Electron beam welding has not been fully established to additively manufacture W-Cu composites with high W content, except for welding of interface of Cu and W-Cu and surface modifications [21,43,99,125]. Interaction of high-energy electrons with W atoms can produce radiation effects, which is a great challenge for producing the composite with electron beam melting [126]. Figure 22 shows the microstructure of a W-Cu composite prepared through the use of a high-current pulsed electron beam [43].

6.3. Arc-Based Processes

Arc-based additive manufacturing can be performed in either wire or powder form. In both cases, poor thermal management can cause residual stresses, distortion, pores, and large grains, and a poor dimensional and surface finish can be observed [127]. W particles, because of their non-melting nature in the arc process, can produced turbulency in the melt pool and can cause porosity and exudation in the composite [128,129]. To improve the flow kinetics of the molten Cu and partially molten W, other elements, like Fe, Ni, Cr, and Co, are added [130,131]. However, the addition of these particles severely affects the other properties, like the thermal/electrical conductivity [44]. Similarly, plasma, which is composed of superheated charged particles, can be entrapped in the keyhole made by the falling W heavy particles, which can be entrapped in the Cu molten pool and carried by hot molten flow under the effect of forces acting during welding. The plasma entrapped can cause boiling of the Cu phase in the composite, which may appear as gas porosity [132]. The particle size is one of main factors determining the flow characteristics of the feeding and process efficiency [133,134]. Control of the distribution of W particles in the composite is also one of the great challenges for the arc-based processes because of the homogeneity loss during feeding, buoyancy forces, and remelting of some of the earlier deposited beads. This effect is enhanced by the introduction of activator elements, which increase the fluidity of the weld pool [135]. Figure 23 shows the microstructure of a W-Cu composite prepared by plasma-transferred arc welding [44].

7. Discussion on Fabrication of W-Cu Composite Through Additive Manufacturing Route

As we discussed earlier, conventional fabrication routes are mostly applied for fabrication of W-Cu composites. However, conventional fabrication has its own limitations, and as the time passes, the demand for quick production with low-cost increases. Additive manufacturing is well suited for such requirements of fabrication. However, as almost all the additive manufacturing techniques are high-temperature processes, special consideration must be given to control the weld pool kinetics. Because the W particles can only be partially melted, it can greatly disturb the weld pool kinetics. Inflight melting, temperature rise (of the W and Cu particles in case of powder inflow towards the melting zone), speed, shape, size, and inflight distance all determine the quality of weld at the end. Carrier gas for injecting the particles into the melt zone can further increase the temperature of the inflight particles, as well as the temperature of the melt pool. To avoid the excessive swelling, residual stress, and boiling inside the composite, thermal management during welding is required. Usually, it is controlled by the proper selection of welding parameters and baseplate for deposition. Usually, Cu substrate is employed to remove the heat quickly from the additive fabrication processes [136,137]. Carbon steel has also been reported to be used as substrate material [94,106]. However, the size in terms of height obtained for the composite is small, and the power of laser used is on low side, as there is no dilution observed from the base metal as in the microstructure. From the literature, the amount of heat input during welding should be enough to melt the Cu phase. However, this is one of the difficult tasks to control during the AM process, as the process is continuous and as the time passes the material is heated more and more and finally the temperature is above the boiling point of Cu, which causes the exudation of Cu from the composite. For this reason, intermittent welding with control of interpass temperature can greatly reduce this. Using high heat input can cause partial melting of W, production of nano and micro-sized particles of W, and other intermetallics via the reaction of W with other elements from the baseplate [106].

Some researchers have tried to produce 3D shapes of W-Cu composites using semi-additive techniques which use the oxides of W in combination with the printer ink to produce the 3D shape and then reduction, followed by sintering and infiltration [138]. The techniques, however, are costly and complex for large-scale manufacturing.

Conventional post-fabrication techniques like hot rolling, hot isostatic pressing, hot press, and hot extrusion can be adopted to remove or minimize the defects in the fabrication material [109,139,140,141]. Although these processes can be used to fabricate the composite, the additive manufacturing, due to its high heat input and partial melting of the W particles, can have advantages in terms of mechanical properties.

8. Microstructure of W-Cu

The microstructure of W-Cu composite consists of W embedded in a molten Cu structure. Almost all the W-Cu systems prepared by different techniques possess the same microstructure. XRD and EDX analyses show two distinct phases of W and Cu, as shown in Figure 24 [19,142]. However, the only difference is the different defects associated with each of the techniques and the evolution of grains during the process. For example, laser bed fusion and sintering/infiltration of W and Cu powder produce some of the pores due to the entrapped gases, which can lead to the formation of cracks under loadings [18,19].

Since in W-Cu composites, only the Cu part melts, to improve the densification, mechanical, and metallurgical properties, fine- and ultrafine-sized powders are used [89]. One of the methods to reduce the grain size and, hence, increase the overall hardness and strength of the composite is to reduce the powder size to the nano level [59,85,89,143]. However, increasing the number of grain boundaries will reduce the thermal and electrical conductivity of the composite. Another method to reduce the grain size is rapid solidification during and after melting [43]. However, rapid solidification will also cause residual stress at the interface of W and Cu. Reducing the grains by reducing the size of the initial powder has challenges too, as it requires a long time for processing and can also introduce some impurities, as discussed earlier.

The addition of foreign elements for the purpose of property improvements in W-Cu composites requires study of the interaction of those particles with either W or Cu. Any foreign element will either interact with Cu or W. Because of the hard and brittle nature of W, most of the interacting of elements like Ni, Fe, Co, and TiC forms complex intermetallic around the interface of W [95,104,105,106,107]. Most of these intermetallic elements are harder than W and have complex crystal structures, so using only XRD and EBSD is insufficient for complete analysis. TEM analysis can confirm the d-spacing and crystal structure configuration. Depending upon the formation mechanism and morphology of these intermetallics, residual stresses may cause cracking in the structure. For example, liquid-phase sintering in which sufficient time is allowed for the composite to consolidate produces no hot cracks due to Fe and Ni in W-Cu [144,145]. However, high-temperature welding can cause thermal and residual stresses in these intermetallics, and cracks can occur similar to the additive high-temperature manufacturing processes [146]. Similarly, to increase the strength and hardness of Cu, elements like Sb and Zn can be added to form a solid solution with Cu. Meanwhile Be, Cr, Ni, and Zr can also be used, as they can precipitate out after quenching and aging at low temperature for long time. These elements decrease the thermal and electrical conductivity of the composite but increase the strength and hardness of the overall composite [147]. Alloying elements can increase the strength of the composite through any of the solution-strengthening, second phase-strengthening, and interfacial-strengthening mechanisms.

The interface strength of W and Cu in the microstructure is very important for the overall properties of the composite in the use of structural materials. The composite must withstand thermal and mechanical stresses [11,58,148]. To decrease thermal expansion and improve the bonding strength at the interface, decreasing surface energy by reducing the particles size is one method that was discussed earlier. Another method is to physically increase the diffusion layer by mechanical, interlocking, and deformation mechanisms. Direct bonding between W and Cu was obtained, producing a diffusion layer of 22 nm after 3 h at 980 °C, under a pressure of 106 MPa [149]. Similarly, producing dents and pits on the surface of W to increase the reaction surface area and treatment with Cu for long hours can also produce the diffusion [150,151]. TEM analysis confirm a diffusion layer of 32 nm at the interface of W and Cu, as shown in Figure 25 [151]. Dislocations, fine grains, and subgrains near the interface can rise during the fabrication process of W-Cu. As these dislocations can be beneficial in terms of strengthening the composite, at the same time, they can also reduce thermal and electrical properties [139,152].

9. W-Cu Mechanical Properties

A summary of the mechanical properties of W-Cu follows:

• Hardness: Hardness shows the stiffness and the ability to resist wear. Generally, with an increase in the content of W% in the composition, the hardness values increase [60,144]. However, the hardness of the W-Cu system depends upon various factors, like the inclusion of other elements [84,144,153,154,155], initial particle size [12,47], and grain size [29]. Hardness values of 275 HV, 357 HV, and 500 HV have been reported as the % of W increases in the UFG composite [59]. In another study, the comparison was made with coarse grain particles, and a similar trend was observed, with the compressive strength being ~1.55 times, ~1.72 times, and ~1.80 times than that of coarse grain particles.

• Fracture toughness: This property is an important parameter to measure the resistance of a flawed material to withstand loads. To check material performance under high temperature, fracture toughness tests are performed under vacuum. Likewise, W-Cu material is mainly used in high-temperature environments; hence, it is very useful to evaluate this property. To produce the notch, either a laser or wire EDM is used. The fracture toughness of the W-Cu system reported in the range of room temperature to 800 °C mostly remains on the higher side (17.6 to 21 MPa·m^1/2) below 350 °C, after which, it decreases with increase in temperature, Cu increase, and the addition of other impurities [11,58,156].

• Flexural strength: The three-point bending test is usually applied to check the material strength under bending loading conditions. The flexure strength of the W-Cu system strongly depends upon the appropriate elemental contributions [5,155]. An addition of 14% Zn can almost double the flexure strength to a maximum of 960 MPa [82]. For a 50%W–50%Cu composite material, the flexure strength has been reported to be above 1000 MPa and more than 600 MPa at a temperature of 425 °C, which is comparable to the values of plain steels.

• Tensile strength: The graph of W-Cu in the tensile testing shows an elastic, plastic, and fracture point [5]. The elasticity of the W-Cu system mainly depends upon the interface bonding of W and Cu and the testing temperature. During testing, the load is taken by W particles. However, as the load is increased, any weak point at the W-W interface may break, and the intergranular fracture can start, and then it can propagate the weak interfaces [11]. The tensile strength of a W-30Cu composite material decreases from 550 MPa at room temperature to 100 MPa at 800 °C [11].

• Elastic modulus: The elastic modulus of W-Cu system decreases with increases in the composition of Cu and testing temperature [142]. This shows one of the major concern factors in practical high-temperature applications. The elastic modulus for a W-30Cu system has been reported to slightly decrease from 175 GPa at room temperature to 110 GPa at 800 °C [11].

• Thermal expansion coefficient (CTE): The W-Cu system increases with an increase in Cu content, and at higher temperatures, it remains almost constant [11,142]. The CTE only slightly changes with an increase in the temperature from room temperature to 800 °C. A relative change of 1.6 × 10⁻³ °C⁻¹ has been reported for a W-30CU system.

• Compressive strength: Compressive strength also increases with increases in the W content. However, internal stresses, grain morphology, and dislocations also play an important role. For example, the compressive strength reported in UFG 1200 MPA is 2.1 times greater than that of the respective coarse grain particles [59].

10. Defects in W-Cu Alloys

Because of the parameters’ dependency on the morphology of the powder and non-melting nature of W in the W-Cu composite, we can expect to have some difficulties not only associated with the process but also inside the material. Some of the defects in the deposited layer of W-Cu are the following:

• Pores: Pores can form due to either vacant spaces remaining during the alloy formation or the blockage of channels to be connected by small W particles [157,158]. These are mostly observed in Cu infiltration because Cu does not reach some of the areas in the W-Cu system [18]. The pore formation decreases with the increase in sintering temperature, as a good wetting between Cu and W can be obtained by decreasing the wetting angle in the formula of capillary action [15]. Porosity levels of 6% and 4% have been reported in sintered and hipped pure W materials [159]. In arc welding processes, the porosity is mainly caused by the rapid solidification and entrapment of gas prior to escape due to an increase in the viscous drag force [160].

• Copper lakes: Cu can accumulate at certain areas in the matrix in the form of large islands around W or separate form [18,158].

• Cu agglomeration: Inflight melting of Cu can cause agglomeration of Cu particles around W particles [18]. With less heat input from the plasma, these particles can go into the melt pool without melting [158].

• Cu exudation: A high heating rate can cause Cu exudation from the matrix of the W-Cu system [19,83].

Some microstructure defects related to W-Cu are the following:

• Micro holes: An optical microscope can detect small micro holes in the W-Cu systems. Zhenlu Zhou et al. [19] detected a 0.2% area of micro holes occupied in the selected region of 200 µm of micrograph for a W-Cu composite. Adjustment of the correct parameters can greatly reduce the amount of pore formation [154]. On the other hand, for a self-assembled lamellar (SAL) structure, the micro cracks formed due to pore formation can be helpful in the release of stress concentrations and strain localization, resulting in large plastic deformation and delay in fracture [161].

• Segregation at grain boundaries: Even the pure W obtained from Plansee SE contains impurities like H, N, O, P, S, and Si, and these elements are considered one of the main reason for the brittle nature of W that favors intergranular fracture in up to 95.9% of surfaces analyzed [159,162].

• Inhomogeneity: Inhomogeneity can occur especially at high currents in which high temperatures are generated, causing high-density particles to flow freely to the bottom [163,164]. W in PTA tends to fall near the substrate, while Cu floats at the top.

• Micro cracks: can arise due to the rapid solidification caused by Cu content. They are more visible and associated with pores [165]. During FGM preparation through SLM, severe internal stresses caused by high-temperature melting and rapid cooling can generate micro cracks in the composite, and these micro cracks are more visible in the regions with a lower % of weak Cu. Micro cracks can also generate during in-service conditions, resulting in the interfacial cracks between Cu and W [61].

11. Substrate Material

The choice of substrate depends on certain factors, like the metallurgical compatibility of the coating and substrate, melting point of the substrate, purpose of substrate (thermal spraying or hard facing), and process (heating or cooling) [10,166,167]. Graphite substrates have been widely used for deposition of the W-Cu layer as a means of easy removal of the layer [142]. Cu substrate is employed in most of the FGM specimens, as it can provide a source of pre-heating and post-cooling [10]. A steel substrate (EUROFER97) was used in an experiment using the W powder deposition via laser beam for armor and heat sink applications in a DEMO limiter project of fusion reactors [4]. A.K. Lakshmi Narayanan et al. [168] used boiler-grade carbon steel as a substrate for hard facing using PTA route and satellite. Kaushal Kishore et al. [169] used AISI 304L austenitic stainless steel and 9Cr-1Mo tempered martensitic steel to study the effect of these substrates of satellite 6 cladding. Marcin Adamiak et al. [170] used NiSiB + 60% WC on structural-steel substrate using laser and PTA deposition techniques. The cooling rates of the base plate greatly control the microstructure characteristics of the weld. The cooling rate, in terms of the base plate, depends on the thickness, type of material of the base metal, and environmental conditions. Radiation, conduction, convection, and evaporative heat loss also account for the cooling of the bead [171]. Martin Malý et al. [165] studied the effect of substrate heating on pure copper through LBPF and concluded that the porosity was greatly reduced at 400 °C as compared to 200 °C with 99% of relative dense material.

Substrate plays a very important role in the solidification mechanism of the weld puddle. Figure 26a shows the solidification of the Cu substrate being heated to different molten temperatures and in Figure 26b the cooling rates of different substrates [172]. Figure 26b shows that the cooling rate of the Cu substrate is higher than that of the SS and Al.

12. Other Materials Available as Competitors to W-Cu Based on Application

Some of the materials available which can compete with W-Cu based on their applications are given in Table 4.

13. Solidification Behavior in Additive Manufacturing and Solidification Mechanism of W-Cu Composite

Laser welding can be achieved through either keyhole or conduction mode depending on the energy supplied by the laser power. In keyhole mode, a small keyhole is generated in the material upon melting by the laser source. The surrounding metal is also heated up by conduction, followed by rapid solidification. A small amount of plasma can be seen at the surface of the welding area due to the heating and boiling of the material. Laser melting can create heating rates up to >109 K·s⁻¹ [177]. Throughout the thickness of the material being welded, the energy absorbed decreases exponentially. Keyhole mode is one of the common approaches, as it creates small HAZ regions. However, instabilities and keyhole oscillations can occur and can badly affect the quality of the weld. A schematic of the laser-welding molten pool formation is shown in Figure 27 [178]. The porosity was greatly reduced, and thickness was reduced by increasing the laser power, as the parameters of the laser source have a direct effect on the quality of the W-Cu composites [10].

In electron beam welding, the fast accelerated electrons lose their kinetic energy through inelastic collision with the atoms on the area of focus. The area is melted and depending on the energy density of the electron beam, a molten pool is formed in a narrow area of interest. Since electron beam uses a very precise and narrow beam, this method is mostly applied to the joining of similar and dissimilar metals [179]. The schematic of electron beam welding is shown in Figure 28 [179]. The depth of penetration strongly depends on the density and energy carried by the electron beam. Initially, the interaction of electrons occurs with the surface of material producing different kinds of radiation. However, as the intensity of the electrons increases to a certain level, the heat obstruction is surpassed, and the melting and boiling of the material occurs. Different kinds of forces can be observed during the welding. The main forces involved during the welding are vapor pressure, friction forces, surface tension, electromagnetic force, thermocapillary, electron pressure, hydrostatic pressure, and surface tension.

As shown in Figure 29, the forces involved during arcing in the final bead shape of the molten pool depend upon the four types of forces, i.e., electromagnetic or Lorentz force (F_em), buoyancy or gravity force (F_b), surface tension or Marangoni forces (F_s), and impinging or plasma forces [180,181]. The electromagnetic force is the result of interaction of the current with its own magnetic field. Buoyancy forces are caused by the temperature slopes in molten pool and the difference in the densities, composition, and gravity. Surface tension or Marangoni forces are due to the temperature differences at the molten pool surface. The imping forces are caused by the interaction of momentum carried by the plasma particles with the atoms/particles in the molten pool. The resultant forces acting on the weld pool causing either a poloidal or convergent flow are the result of welding parameters, material characteristics, and environment. Since the current is the cause of producing the electromagnetic forces, increase in the arc current also increases the plasma pressure to the square of the current, causing more depression and distortion in the weld puddle [182]. The plasma fabrication route is a direct energy deposition process with a rapid solidification mechanism of the melt pool [183].

The additive manufacturing process featuring W-and Cu can be classified as a rapid solidification (RS) process. In W-Cu composites, Cu is one of the main sources of rapid cooling mechanism. Cu composites have been reported to undergo a temperature change of around 10⁴ K/s to 10³ K/s from molten state to a stable solid state by giving up both super heat and latent heat [184]. Although W has less thermal conductivity as compared to Cu, it has also been reported to undergo RS in liquid state [26]. The RS phenomenon can be beneficial in terms of grain refinement, intermetallic segregation, non-equilibrium metastable crystalline phases, disordered crystalline structure, compositional flexibility, and solid solubility. Undercooling of a material is the rate at which the latent heat is released to the environment below the solidification temperature, while the material is still in the liquid state. The process of recalescence will bring back the temperature to the solidus temperature, as shown in Figure 30 [185].

As discussed, RS is not only one of the important factors in determining important properties of the material, but it can also bring some important crystalline defects, like high dislocation densities, voids or micro-cavitation, residual stresses, and mismatch in lattice orientations [186]. RS can lead to shrinkage of the material, which can further lead to the development of stresses in the microstructure. The material must accommodate these shrinkages (stresses) during solid formation (plastic deformation) that are called geometrically necessary dislocations (GNDs). The other type of dislocations, called statistically stored dislocations (SSDs), are formed due to the random formation of the crystal structure, as shown in Figure 31 [161,186,187].

EBSD analysis can identify DNDs or SSDs based on the resolution of the system. However, EBSD will be insensitive to the dislocations which have net-zero Burgers vector. Generally, SSDs have net-zero Burgers vector and are difficult to identify. However, we can overcome this situation by reducing the length of measurement.

To study the microstructure dislocations, the most-adopted technique is EBSD. Other techniques, like TEM and ECCI, are also used but with certain limitations: in TEM, distortion during sample preparation can destroy the results; ECCI cannot identify individual dislocations at high dislocation densities [188].

The grain growth mechanism is dependent upon the maximum temperature achieved during the process and the time allowed to melt the W or Cu [154]. Epitaxial columnar growth of grains is a phenomenon rise due to the directional cooling after the partial melting of the substrate (hard facing) in additive manufacturing [19,189,190]. The grain growth during the solidification process largely depends upon the cooling rate of the system. For W-Cu system, due to the high thermal conductivity of Cu in both liquid and solid phase as compared to W, it takes away most of the heat from the W as well as from the W-Cu system. The cooling in a W-Cu melted system may be divided into two phases. In the first phase, at a temperature greater than the boiling point of Cu, because of the high temperatures of the processes (nearly 6000–7000 °C), some evaporation in Cu may occur, causing high cooling rates and, hence, refined grain size of the W and Cu. In the second stage, when the temperature of the W-Cu system has dropped below the boiling point of Cu, Cu which starts to solidify can still take a lot of heat from the W. However, at this point, the temperature gradient direction is changed from the melting position to the interface of W and Cu. At this stage, the size of Cu grains are relatively larger as compared to the first stage because Cu is still taking heat away from W [19]. The mechanism is shown in Figure 32.

Different base plates made of Cu, mild steel, and stainless steel can be used for experimentation. The interaction of Cu and W with other alloying elements in the substrate is of great importance for microstructure evolution and mechanical properties.

14. Conclusions

Some of the conclusions that can be drawn from this study are the following:

1. Fabrication of W-Cu composite material is very attractive, and it is a new field in terms of additive manufacturing that has not been fully explored by the researchers. The simplicity and the economic efficiency of the additive manufacturing processes can give us great value advantages over the conventional powder metallurgy routes.

2. Because of the high temperature involved in the additive manufacturing processes, evaporation of Cu can occur due to lower boiling point of Cu. At the same time, due to the unmalting nature of W, the flow characteristics of molten pool are highly disturbed, and due to these reasons, the defects within the composite are common in the additively manufactured composites.

3. The current stage of W-Cu composites for additive manufacturing is established enough to support the future research in terms of economic and process feasibility. However, more work still needs to be performed to establish standard mechanical, electrical, and thermal properties.

15. Future Recommendations

Conventional fabrication techniques have already been established for W-Cu fabrication. However, to utilize the material in different applications effectively, as given in Table 1, with low cost and bulk production, the production techniques need to be improved and potentially changed to additive manufacturing. With the advancement of new techniques in additive manufacturing, the limitations on the process can be greatly reduced, and, hence, better-quality W-Cu composites can be obtained. The most important challenges for the additive manufacturing of W-Cu composites are the residual stresses, distortion, pores, Cu exudation, poor dimensional, and surface finish due to the high temperatures involved during the welding process. Control of heat input by the right selection of input parameters, thermal management of molten pool, and flow dynamics of molten pool have great influence on the quality of deposition, and these studies must be explored to fully adopt the additive manufacturing. Post-fabrication techniques can be used to minimize the defects and improve the properties. The appeal to fabricate W-Cu composites lies in the cost, fabrication time, and improved mechanical properties compared to conventional fabrication routes.

Author Contributions

M.H., conceptualization, methodology, investigation, data analysis, and writing and editing—original draft; B.D., resources, data validation, data curation, methodology, review, and editing; Z.Q., external supervision, review and editing, project administration, external funding acquisition, and resources; U.G., project administration, resources, visualization, external funding acquisition, external supervision, and resources; Z.P., conceptualization, investigation, supervision, and project administration; H.L., conceptualization, investigation, supervision, and project administration. All authors have read and agreed to the published version of the manuscript.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures and Tables

Figure 1. Phase diagram of W and Cu. Reprinted with permission from Ref. [45].

Figure 2. Gibbs free energy vs. alloy composition, Reprinted with permission from Ref. [46]. 1985, AIP Publishing.

Figure 3. Formation enthalpy calculated vs. compositional ratios using MD simulations, reprinted with permission from ref. [48], 2017, Elsevier.

Figure 4. Surface energy of different materials studied in the research, reprinted from [49].

Figure 5. Surface energy of different transition metals and W (The dotted line shows results based on FCD-LMTO calculations), reprinted with permission from ref. [50], 2018, Elsevier.

Figure 6. Effect of nano-sized particles on the formation enthalpy for W50Cu50 system, reprinted from Ref. [47].

Figure 7. (a) Discontinuous FGM and (b) continuous FGM, (c–e) schematic diagrams showing discontinuous FGMs that contain interfaces with gradual change in composition, grain orientation and volume fractions of two types of second-phase particles, respectively. (f–h) schematic diagrams showing continuous FGMs in absence of interfaces and with gradual change in grain size, fiber orientation and volume fraction of second-phase particles. Reprinted with permission from Ref. [62], 2019, Elsevier.

Figure 8. Shift (red arrow) in Ductile-to-brittle transition temperature (DBTT) of W tested in cold-rolled, hot-rolled, and annealed conditions in LS and TS orientations. Reprinted with permission from Ref. [65], 2016, Elsevier.

Figure 9. Effect of Re addition on DBTT in W, reprinted with permission from Ref. [66], 2018, Elsevier.

Figure 10. Mechanistic models of heat and mass transfer and fluid flow in AM. reprinted with permission from Ref. [70].

Figure 11. (a) Infiltration method for W-Cu production. (b) W-Cu microstructure obtained, reprinted with permission from Ref. [25], 2018, Elsevier.

Figure 12. Liquid-phase sintering mechanism, reprinted with permission from Ref. [78], 2009, Springer Nature.

Figure 13. (a) Vacuum and (b) microwave heating (the intensity of color represents the heat energy), reprinted with permission from Ref. [82], 2024, Wiley.

Figure 14. Schematic of hot-pressing process, reprinted with permission from Ref. [60], 2008, Elsevier.

Figure 15. Spark plasma sintering setup and the heating mechanism, reprinted with permission from Ref. [82], 2024, Wiley.

Figure 16. Schematic of injection molding, reprinted with permission from Ref. [25], 2018, Elsevier.

Figure 17. Schematic of mechanical alloying: (a) process and (b) mechanism. Reprinted with permission from Ref. [92], 2013, Elsevier.

Figure 18. A schematic of fabrication of W-Cu composite using laser technology, reprinted with permission from Ref. [93], 2020, Elsevier.

Figure 19. W-Cu composite prepared by laser additive manufacturing technology with different scanning and laser-power levels, reprinted with permission from Ref. [94], 2018, Elsevier.

Figure 20. Schematic of electron beam melting, reprinted from Ref. [96].

Figure 21. SEM surface morphology of the W-(Ni)-Cu composites. (a) W-Cu40 wt.%; (b) W-Cu30 wt.%; (c) W-Cu25 wt.%; and (d) W-Ni5 wt.%–Cu15 wt.%, reprinted with permission from Ref. [42], 2016, Elsevier.

Figure 22. Surface SEM images of Cu-15W sample before and after HCPEB irradiation. (a) Initial, (b) 5 pulses, (c) 10 pulses, and (d) BSE image of 10-pulsed sample, reprinted with permission from Ref. [43], 2018, Elsevier.

Figure 23. Cross-sections of three-layer W–Cu cladding on a steel substrate: (a) one layer (75% W), (b) two layers (75% + 95% W), and (c) three layers (75% + 95% + 98% W), reprinted from Ref. [44].

Figure 24. Typical (a,b) microstructure and (c) XRD of W and Cu system, reprinted with permission from Ref. [19], 2022, Elsevier.

Figure 25. TEM observations at W/Cu interface: (a) HAADF image; (b) EDX line-scanning profile along the red arrow marked in (a); (c,d) element mapping; (e) HAADF image of the W/Cu interface; and (f) HR-TEM image of the interface, reprinted with permission from Ref. [151], 2022, Elsevier.

Figure 26. Colling rate on (a) Cu substrate; and (b) Cu, Al, and SS substrate, reprinted with permission from Ref. [172], 1995, Elsevier.

Figure 27. Schematic of laser welding and molten pool formation, reprinted from Ref. [178].

Figure 28. Schematic of electron beam melting, reprinted with permission from Ref. [179], 2016, Elsevier.

Figure 29. (a) Schematic of the welding principle, (b) Material states, fluid flow and four types of forces involved during the welding process, reprinted from Ref. [181].

Figure 30. The degree of undercooling explained (a) Hypocooling, (b) Critical undercooling, (c) Hypercooling, reprinted with permission from Ref. [185], 2010, Springer.

Figure 31. Geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs), reprinted with permission from Ref. [187], 2019, Elsevier.

Figure 32. Solidification mechanism of W-Cu system explained in four steps, reprinted with permission from Ref. [19], 2022, Elsevier.

Table 1

W-Cu applications.

Application Type	Scope	Reference
Electrodes (in EDM)	- Minimization of arc ablation rate and better material removal efficiency, compared with another electrode.	[10]
Functional graded materials	- In PFM in fusion reactors. Gradual transition from pure W (plasma facing, sputtering, and low CTE to high) to % of Cu (heat sink).	[22,23]
Electronic packaging materials	- High-power microelectronic device heat sinks for stable mechanical support, higher heat dissipation and larger electrical interconnection. - Warping and reducing thermal stresses. - Low CTE and high thermal conductivity.	[24,25,26]
High-voltage electrical contacts	- Contacts for circuit breakers operating in oil or SF6 or vacuum. - Continuous development of switches to higher power voltage, larger capacity, and miniaturization; novel W-Cu alloys with improved performance are urgently demanded.	[27,28]
Sweating materials	- Field of aerospace and military. - Erosion mechanisms usually consist of an inter-granular fracture of W skeleton, evaporation of Cu, transpiration cooling, extra sintering of W skeleton, and penetration of alumina droplets.	[24,29]
Shaped charge liners	- For armor penetration as it gets high speed.	[30,31]
Electromagnetic gun-rail materials	- Rails experience high electrical current density and huge mechanical and thermal shock.	[29,32]
Kinetic energy penetrators	- Warhead materials, kinetic projectiles, and armor penetrators.	[33]
Radiation shielding/protection	- W (19.3) is 1.7 times denser than lead (11.34 g/cm3); hence, due to this property of tungsten, it can be used in radiation shielding. However, due to the high cost of W, lead is preferred. However, this can be accommodated by Cu in W by compromising the density and obtaining an optimal shielding.	[7,22,26,34]
Nozzle throat of a solid/liquid fuel rocket	- At a high temperature and pressure, the throat area of the nozzle has been reported to be made of W-Cu alloy system.	[25,26]
Integrated gate bipolar transistor	- Used in electric and hybrid vehicles for conversion of one form of energy into other.	[35]

Table 2

Different properties of W and Cu.

Sr#	Property	W	Cu
1	Melting point	3370 °C	1083.2 °C
2	Boiling point	5555 °C	2562 °C
3	Density (near RT)	19.25 g/cm3	8.96 g/cm3
4	Heat of fusion	184.2 J/g	204.8 J/g
5	Heat of vaporization	4009 J/g	5234 J/g
6	Specific heat capacity	0.134 J/g-°C	0.385 J/g-°C
7	Thermal expansion	4.4 µm/(m·K) (at 25 °C)	16.4 µm/(m·K) (at 25 °C)
8	Thermal conductivity	163.3 W/(m·K)	398 W/(m·K)
9	Electrical resistivity	5.65 Ω·m (at 20 °C)	1.67Ω·m (at 20 °C)
10	Electro negativity	1.7	1.9
11	Young’s modulus	400 GPa	110 GPa
12	Shear modulus	156 GPa	46 GPa
13	Bulk modulus	310 GPa	140 GPa
14	Poisson ratio	0.28	0.343
15	Mohs hardness	7.5	3
16	Vickers hardness	480 HV	50 HV

Table 3

Properties of W-Cu composites for contact switches.

Composition	Density	Electrical Conductivity	CTE	Thermal Conductivity	Hardness	Bending Strength	Tensile Strength
	g/cm3	IACS% Min	10−6 K−1	W/m · K−1	HRBMin	Mpa	Mpa
WCu 50/50	12.2	66.1	12.5	310	81	-	344–413
WCu 60/40	13.7	55.2	11.8	280	87	-	379–448
WCu 70/30	14.0	52.1	9.1	230	95	667	516–585
WCu 75/25	14.8	45.2	8.2	220	99	706	585–654
WCu 80/20	15.6	43	7.5	200	102	736	620–689

Table 4

Materials that can challenge W-Cu.

Material Name	Application Type	Advantages over W-Cu	Disadvantages of the Material	Can W-Cu Cover the Disadvantages	References
Copper, brass, tungsten, molybdenum, coated (zinc-coated and diffusion-annealed), and steel-core wires	Wire EDM	Low cost and easy availability	High wear of the wire	yes	[10,173]
Beryllium and carbon-fiber composites	As PFM	Be can help in conditioning the carbon fiber surface. Good thermal conductivity comparable to Cu	Be is toxic and has a melting point of around 1287 °C. Transmutations can occur in Be, producing He and tritium.	Yes, but transmutations, as well as H and He retention, can occur in W to Re (ductile) and Os (brittle)	[148]
Al2O3, BeO	Electronic packaging and heat sink	Low cost	Thermal fatigue and expansion	Yes, but fine particles are required	[174]
Chrome zirconium (CuCrZr)	High-voltage electrical contacts	Low cost, non-arcing, and enough strength	Low strength and life compared to W-Cu	Yes	[28]
Lead (Pb)	Radiation shielding/protection	Better efficiency and easily available	Toxic and not useful in case of high energy beta rays	Yes	[7]
Depleted uranium with small fractions of titanium	Kinetic energy penetrators	Readily available and cheaper	Reactive to corrosion and cracking due to residual stresses	Yes	[175]
Refractory metals, refractory-metal carbides, graphites, ceramics	Nozzle throat of solid/liquid fuel rocket	Easily available and cheap	Erosion problem in almost all the materials	W-Cu can perform better in certain conditions as compared to other materials	[176]

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