# Progress in the Preparation of Large-Size High-Performance CuCr Alloys

Authors: Jin-Ru Han; Zhi-He Dou; Ting-An Zhang; Wang An
Journal: Advances in Materials Science and Engineering
 (2022)

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## Abstract

CuCr contact material, whose mass fraction of Cr is generally in the range of 25% to 50%, is one of the most desirable contact materials for medium- and high-voltage vacuum switches. The homogeneity and denseness of CuCr alloy are the keys to its application. The infiltration and powder sintering have defects, such as uneven distribution of Cr phase, and great difficulties in preparing high-performance CuCr alloys with low gas content; the equipment investment in the vacuum consumable arc melting method is large and the production cost is high, while mechanical alloying, vacuum induction melting, and laser-selective melting as new technologies are still in the laboratory stage. From the perspective of surface modification treatment and multicomponent alloying, refinement of Cr phase grains can improve the properties of CuCr alloy. The new process of aluminothermic reduction-electromagnetic coupling refining-water and gas composite rapid cooling takes oxides as raw material and obtains a fully miscible CuCr alloy melt by aluminothermic reduction theoretically at 2848K, followed by secondary refining under electromagnetic field coupling and water-gas composite cooling and casting, which can obtain alloy ingots of 50 mm in diameter and 10 mm in height. The density and hardness of CuCr alloy prepared by this process as vacuum contact material are better than the requirements of GB/T 26867-2011, so this process has the potential to become a new-generation preparation method for large-size homogeneous high-performance CuCr alloy.


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## Body

## 1. Introduction

CuCr alloys with mass fraction of Cr that are generally in the range of 25% to 50% are widely used in 10∼40.5 kV medium-voltage vacuum interrupters due to their high mechanical strength and good electrical conductivity [1–7]. During the solidification of CuCr alloys, the process of two-phase separation in CuCr alloys consists of three stages, as shown in Figure 1. In the first stage, the nucleation of a small amount of Cr phase; in the second stage, the nucleation grows by various modes of material transport (diffusion, Marangoni motion, convection, etc.) and coarsens by collisional aggregation between nuclei; in the third stage, when the Cr-phase droplets reach a certain size, the density difference between Cu and Cr components and the Marangoni motion lead to the deposition or floating of the Cr-phase droplets, and the stokes condensation and Marangoni condensation also occur during this motion, so the smaller-size droplets are continuously captured, which eventually leads to the delamination of the melt [8]. With the increase of Cr content, the number of Cr phases crystallized increases greatly, resulting in less liquid phase present at the eutectic transition, which cannot fill all the voids after precipitation of Cr phases, leading to the low density of the alloy. Therefore, the difficulty in the preparation of high Cr content CuCr alloys lies in the densification and homogenization of the alloy, which is also the bottleneck limiting its performance enhancement and application.Figure 1
Three stages of two-phase separation of CuCr alloy [8].So far, the industrial production methods for CuCr alloys with diameters larger than 50 mm include the infiltration method [9–12], the powder sintering method [13–16], and the vacuum self-consumption arc melting method [17]. The CuCr alloys prepared by infiltration and powder sintering have defects, such as low density and uneven distribution of Cu and Cr phases, and great difficulties in preparing high-performance CuCr alloys with low gas content. The CuCr alloys prepared by the vacuum consumable arc melting method have low gas content and high purity, but the equipment investment is large, and the production cost is high. To achieve a breakthrough in the preparation of high-performance CuCr alloys, domestic and foreign scholars have developed preparation methods, such as mechanical alloying methods [18–25], vacuum induction melting [26–30], selective laser melting [31, 32], and aluminothermic reduction [33–43], and other preparation methods [44–46] at the laboratory stage [44–46].With the development of ultra-high-voltage transmission and other fields, the performance requirements of CuCr alloys are becoming higher. Therefore, to improve the basic performance of CuCr alloys, the research on surface treatment and alloying of CuCr alloys has become another hot topic, such as laser surface alloying [47–50], heat treatment, and alloying. With the significant improvement of CuCr alloy performance, CuCr contact materials also appear in 40.5∼126 kV high-voltage vacuum interrupters.In this paper, the research progress of large-scale high-performance CuCr alloys is reviewed, the characteristics and research status of various methods are analyzed, and the research status of modification of CuCr alloys is reviewed. Finally, the development trend of CuCr alloys is prospected.


## 2. Research Progress of CuCr Alloy Preparation Technology

### 2.1. Industrial Preparation Technology of CuCr Alloy

#### 2.1.1. Infiltration Method

The process of preparing CuCr alloy by infiltration method includes powder mixing, pressing, sintering and reduction of Cr skeleton, and infiltration of Cu, as shown in Figure2 [38, 51–53]. The oxygen-free copper (99.97% Cu, 20–50 μm) and chromium powder (99.9% Cr, 50 μm) [9] are used to prepare CuCr alloy contacts with a diameter larger than 50 mm. The key steps in the infiltration method are the sintering and reduction of the Cr skeleton and the infiltration of the molten Cu. The infiltration process is simple, and the oxygen content of the CuCr alloys can be reduced with the reduction of the Cr skeleton. The reduction of the Cr skeleton is done to reduce the oxygen content in the sintered skeleton, usually by H2 reduction or carbon thermal reduction. To form a continuous infiltrated Cr skeleton, the mass fraction of Cr in the alloy should not be less than 30%. However, the nondeformability of the Cr skeleton and its poor wettability with infiltrated Cu easily generate defects, such as micropores, shrinkage holes, and oxide residues, which reduce the voltage resistance and arc stability of the CuCr contact materials.Figure 2
Process route of CuCr alloy preparation by infiltration method [38].To reduce the microporous defects of the CuCr alloy prepared by the infiltration method, Ma et al. [9] conducted a high-pressure heat treatment of CuCr50 alloy prepared by the infiltration method and found that the microstructure of CuCr50 alloy prepared by the infiltration method did not change significantly before and after the 1 GPa high-pressure heat treatment, as shown in Figure 3, but the density of the alloy increased from 8.044 g/cm3 to 8.058 g/cm3. The high-pressure treatment bridged the internal micropores in the CuCr alloy, thus reducing the micropores and increasing the density of the alloy, as shown in Figure 4. Cao et al. [11] measured the chopping current of 3.5 A and the breakdown strength of 1.1 ∗ 108 V m−1 for the vacuum interrupter chamber assembled by the CuCr50 alloy prepared by the infiltration method.Figure 3
Metallographic photographs of CuCr alloy: (a) before and (b) after 1 GPa high-pressure treatment [9].
(a)(b)Figure 4
SEM photographs of CuCr alloy: (a) before and (b) after 1 GPa high-pressure treatment [9].
(a)(b)


#### 2.1.2. Powder Sintering Method

The powder sintering method prepares CuCr alloys by mixing, pressing, sintering, repressing of high-purity Cu and Cr powders, and the subsequent heat treatment, as shown in Figure5 [15]. The higher-purity chromium powders (99.95%, 40 μm) and copper powders (99.99%,42 μm) [15] are used in the powder sintering method to produce CuCr alloy contacts with a diameter larger than 50 mm.Figure 5
Process flow of CuCr alloy preparation by powder sintering [15].The composition of CuCr alloys prepared by powder sintering is easy to adjust and control, and the powder sintering is solid-phase sintering at 900°C, so the solid solution of Cr in Cu matrix is less than 0.2% as observed by phase diagram, which is beneficial to maintain the characteristics of Cu and Cr phase, but it is difficult to obtain CuCr alloys with higher density by solid-phase sintering. To improve the defects of the low density of CuCr alloys prepared by the powder sintering method, secondary treatment can be carried out by thermal processing processes, such as hot extrusion, hot forging, and hot isostatic pressing. The combination of Cu and Cr particles in CuCr alloys prepared by the powder sintering method is mechanically bonded by compression and friction, so the toughness of the material is poor, and its breaking performances are lower than those of CuCr alloys prepared by the infiltration method [54, 55].To investigate the factors influencing the electrical conductivity of CuCr alloys prepared by powder sintering method, Klinski-Wetzel et al. [14] prepared CuCr25 and CuCr43 alloys by powder sintering and developed a model for the calculation of conductivity and validated the model by comparing it with measured values, as shown in Figure 6. It was found that the geometry of the Cr phase was the main factor affecting the electrical conductivity of CuCr alloys, as shown in Figure 7, while the effects of porosity and impurity elements seem to be minimal. Li et al. [16] prepared CuCr alloys with Cr content varying from 5 wt.% to 75 wt.% by powder sintering and tested them in a 38 kV axial magnetic field vacuum circuit breaker. The electrical conductivity and the hardness of CuCr30 alloy were 40% IACS and 64 HB, respectively. The electrical properties of CuCr30 alloy achieved the best balance among all the requirements in vacuum circuit breakers with large current interruption capacity.Figure 6
Conductivity simulation model. (a) Spherical second-phase particles; (b) needlelike and flaky particles generated by scaling particles from (a); and (c) the measured values [14].
(a)(b)(c)Figure 7
Influence of the shape of Cr particles in the Cu matrix on the electrical conductivity: transition from platelet-like (h/d < 1) over spherical (h/d=1) to needle-like (h/d>1) particles for a simulation (a) parallel and (b) perpendicular to the main scaling axis [14].
(a)(b)


### 2.2. Vacuum Self-Consumption Arc Melting Method

Vacuum self-consumption arc melting is to process the isostatic vacuum sintered CuCr alloy into self-consumption electrodes and to remelt the self-consumption electrodes by vacuum self-consumption to obtain CuCr alloy ingots. The pure copper powders (99.9%, 75μm) and pure chromium powders (99.9%, 60–75 μm) [17] are taken in the vacuum self-consumption arc melting method to prepare CuCr alloy contacts with a diameter larger than 50 mm. The process combines the traditional casting technique with directional solidification, which effectively suppresses the precipitation and separation of the Cr phase, eliminates the macrosegregation and microsegregation of the alloy, and removes oxide inclusions and residues of elements with low melting point and high vapor pressure [56, 57]. CuCr electrical contact materials are prepared at gas pressure of 220 mbar–260 mbar; melting current of 2.5 KA; and melting voltage controlled at 22 V. The materials are free from defects, such as porosity, looseness, and inclusions, and the Cr microstructure is less than 20 μm. Liang et al. [17, 58] found that the CuCr50 alloy prepared by vacuum self-consumable arc melting had finer Cr phase grains, higher density and hardness, and lower oxygen content compared with powder metallurgy and infiltration methods, as shown in Figure 8 and Table 1.Figure 8
Microstructure of CuCr alloy prepared by different preparation methods [58].
(a)(b)(c)Table 1
Properties of CuCr alloy prepared by different preparation methods [17].
Preparation methodsAlloy gradesAlloy sizeDensity (g·cm−3)Hardness (HV)Electrical conductivity (m)·(Ω·mm2)−1Oxygen content (×10−6)InfiltrationCuCr50Φ35 mm∗ 4 mm7.8412319.1450Powder sintering7.8213417.6595Vacuum self-consumption arc melting7.9717417.8410


### 2.3. New Preparation Process of CuCr Alloy

#### 2.3.1. Mechanical Alloying Method

Mechanical alloying prepares CuCr alloys by subjecting high-purity Cu powder and Cr powder to high-energy ball milling, compacting, sintering, or pressure sintering. Mechanical alloying is an ideal method to prepare nanocrystalline CuCr alloys with Cr content in the range of 5% to 50%, and the refinement of Cr phase grains is beneficial to improving the properties of CuCr alloys, but the contamination of impurities may exist during the ball milling process, resulting in the reduction of properties of the alloy, especially electrical conductivity [59, 60]. Mechanical alloying method is one of the common methods used in the laboratory. The ratio of grinding volume to grinding balls is between 5 : 1 and 20 : 1, the ball milling speed of 300 rpm–600 rpm, and the ball milling time of 10 h or more during the ball milling process [25, 26].To improve the efficiency of mechanical alloying and reduce the contamination of impurities in the process, Zhao et al. [22] used Cu(OH)2 and Cr(OH)3 to replace high-purity Cu powder and Cr powder, used copper material in the grinding balls and tank lining to prevent contamination, and applied nitrogen to prevent oxidation when preparing CuCr alloy powders by the mechanical alloying method. The results showed that when the ball milling temperature was 325°C, the ball milling time was 3 hours, and the ball material mass ratio was 15 : 1, the pure CuCr alloy nanocrystalline powders were successfully prepared. Wei et al. [19] prepared Cu55Cr45 alloy by mechanical alloying + hot pressing sintering and compared it with Cu55Cr45 alloy prepared by vacuum induction melting. The Cu55Cr45 alloy was prepared by mechanical alloying + hot pressing sintering with the presence of finer Cr phase, as shown in Figures 9(a) and 9(b), where the dark dendrites and particles were Cr phase and the bright matrix was Cu phase. The liquid phase separation process of Cu55Cr45 alloy during vacuum breakdown was shown in Figures 10(a) and 11(b), where the Cr particles in the liquid-phase separation of Cu55Cr45 alloy after mechanical alloying + hot pressing sintering were significantly finer than those in the liquid-phase separation of Cu55Cr45 alloy prepared by vacuum induction melting.Figure 9
Microstructures of the Cu55Cr45 alloys. (a) Vacuum induction melting. (b) Mechanical alloying + hot pressing sintering [19].
(a)(b)Figure 10
The liquid-phase separation process of Cu55Cr45 alloy during vacuum breakdown. (a) Vacuum induction melting. (b) Mechanical alloying + hot pressing sintering [19].
(a)(b)


#### 2.3.2. Vacuum Induction Melting

The process flow of vacuum induction melting includes mixing, melting, and casting of Cu powder and Cr powder. The melting and casting processes are carried out under vacuum conditions, so the gas content in the alloy is greatly reduced, but the preparation of CuCr alloys with high Cr contents is demanding on the melting equipment due to the high melting point of the alloys [26–31].Xiu et al. [27] prepared CuCr25, CuCr30, and CuCr40 alloys by vacuum induction melting and investigated their microstructure and properties. As shown in Figure 11 and Table 2, with the increase of Cr content in the CuCr alloy, the microstructure of the Cr phase became more consistent, the electrical conductivity and density of the CuCr alloy decreased, and the hardness and tensile strength increased linearly. After assembling the vacuum interrupters with CuCr25, CuCr30, and CuCr40 alloys as contact materials, it was found that the ultimate breaking current of the vacuum interrupters assembled with CuCr25 contact material was 23.9 kA, while the ultimate breaking currents of the vacuum interrupters assembled with CuCr30 and CuCr40 alloys were 23.0 kA and 21.3 kA, respectively, so CuCr25 alloy has the best breaking current capability. Figure 12 showed the breaking current and arc voltage of the vacuum interrupters assembled with CuCr25, CuCr30, and CuCr40 alloy contacts, which found that the arc voltage of the vacuum interrupter assembled with CuCr25 contact material is the lowest, so the ignition arc energy is low, which is conducive to breaking. Garzón-Manjón et al. [26] successfully prepared Cu-32 at.%Cr alloy with nonequilibrium nanostructures by combining vacuum induction melting and rapid solidification. Zhang et al. [29] changed the morphology and distribution of Cr particles in the CuCr30 alloy prepared by vacuum induction melting and casting through deformation treatment, which resulted in the transformation of Cr phase from dendritic to longitudinally arranged fibrous, and the thermal conductivity of the extruded state along the deformation direction increased by 5.8% compared to the cast alloy.Figure 11
OM images of vacuum cast alloys. (a) CuCr25 alloy; (b) CuCr30 alloy; and (c) CuCr40 alloy [27].
(a)(b)(c)Table 2
Physical and mechanical properties of CuCr alloys [27].
MaterialElectrical conductivity (MS·m−1)Density (g·cm−3)Hardness (N·mm−2)Tensile strength (MPa)CuCr2530.68.4183.9319CuCr3028.08.2789.7327CuCr4023.88.1697.2342Figure 12
Breaking current and arc voltage at current peak [27].Zhang et al. [30] prepared CuCr25 alloy by vacuum induction melting, and the microstructure of the alloy consisted of the Cu-rich phase of the matrix and Cr-rich dendrites, as shown in Figure 13. The CuCr25 alloy prepared by this process had a relative density >99.00%, hardness of 80 HB, conductivity of 25 MS/m, oxygen content less than 500 ppm, and nitrogen content less than 20 ppm.Figure 13
SEM photos of CuCr25 alloy: (a) normal microstructure and (b) coarse Cr dendrite and Cr microsegregation [30].
(a)(b)


#### 2.3.3. Selective Laser Melting Method

Selective laser melting is a method of preparing the high-density alloy containing substable supersaturated solid solutions by rapid laser melting and rapid cooling of metal powders. The density of the CuCr alloy prepared by this process is nearly 100%, and its mechanical properties are comparable to those of CuCr alloys prepared by forging, but the preparation process of selective laser melting is complex, costly, and confidential [31].Chen et al. [32] prepared CuCr20 and CuCr25 alloys by selective laser melting and investigated the effect of subsequent aging treatment on their microstructure and properties. The results showed that the microstructure of the CuCr alloys prepared by the selective laser melting consisted of Cr-rich phases with diameters of ∼10 mm, fine spherical Cr-rich particles with diameters in the range of 50–800 nm, and Cu-rich matrix, as shown in Figure 14. The aging temperature significantly affected the electrical conductivity and hardness of the alloys, while the aging time had less effect on the electrical conductivity and hardness of the alloys, as shown in Figure 15. After aging treatment at 500°C for 1 h, the electrical conductivity of CuCr20 and CuCr25 contacts increased by 2.95 times (from 11 MS·m−1 to 32.5 MS·m−1) and 2.73 times (from 11 MS·m−1 to 30 MS·m−1), respectively, and the hardness increased by 60% (from 166 HV to 265 HV) and 51% (from 184 HV to 278 HV), respectively.Figure 14
Microstructures of as-fabricated CuCr20. (a) The overall distribution of Cr in Cu. (b) Further magnification of (a), A indicates the Cr-rich phase with a particle size of ∼10 mm, B indicates the Cu-rich phase surrounding A. (c) EDS results of A and B in (b) [32].Figure 15
Properties of SLM CuCr20 and CuCr25 aged under different conditions: (a) electrical conductivity and (b) hardness [32].
(a)(b)


## 2.1. Industrial Preparation Technology of CuCr Alloy

### 2.1.1. Infiltration Method

The process of preparing CuCr alloy by infiltration method includes powder mixing, pressing, sintering and reduction of Cr skeleton, and infiltration of Cu, as shown in Figure2 [38, 51–53]. The oxygen-free copper (99.97% Cu, 20–50 μm) and chromium powder (99.9% Cr, 50 μm) [9] are used to prepare CuCr alloy contacts with a diameter larger than 50 mm. The key steps in the infiltration method are the sintering and reduction of the Cr skeleton and the infiltration of the molten Cu. The infiltration process is simple, and the oxygen content of the CuCr alloys can be reduced with the reduction of the Cr skeleton. The reduction of the Cr skeleton is done to reduce the oxygen content in the sintered skeleton, usually by H2 reduction or carbon thermal reduction. To form a continuous infiltrated Cr skeleton, the mass fraction of Cr in the alloy should not be less than 30%. However, the nondeformability of the Cr skeleton and its poor wettability with infiltrated Cu easily generate defects, such as micropores, shrinkage holes, and oxide residues, which reduce the voltage resistance and arc stability of the CuCr contact materials.Figure 2
Process route of CuCr alloy preparation by infiltration method [38].To reduce the microporous defects of the CuCr alloy prepared by the infiltration method, Ma et al. [9] conducted a high-pressure heat treatment of CuCr50 alloy prepared by the infiltration method and found that the microstructure of CuCr50 alloy prepared by the infiltration method did not change significantly before and after the 1 GPa high-pressure heat treatment, as shown in Figure 3, but the density of the alloy increased from 8.044 g/cm3 to 8.058 g/cm3. The high-pressure treatment bridged the internal micropores in the CuCr alloy, thus reducing the micropores and increasing the density of the alloy, as shown in Figure 4. Cao et al. [11] measured the chopping current of 3.5 A and the breakdown strength of 1.1 ∗ 108 V m−1 for the vacuum interrupter chamber assembled by the CuCr50 alloy prepared by the infiltration method.Figure 3
Metallographic photographs of CuCr alloy: (a) before and (b) after 1 GPa high-pressure treatment [9].
(a)(b)Figure 4
SEM photographs of CuCr alloy: (a) before and (b) after 1 GPa high-pressure treatment [9].
(a)(b)


### 2.1.2. Powder Sintering Method

The powder sintering method prepares CuCr alloys by mixing, pressing, sintering, repressing of high-purity Cu and Cr powders, and the subsequent heat treatment, as shown in Figure5 [15]. The higher-purity chromium powders (99.95%, 40 μm) and copper powders (99.99%,42 μm) [15] are used in the powder sintering method to produce CuCr alloy contacts with a diameter larger than 50 mm.Figure 5
Process flow of CuCr alloy preparation by powder sintering [15].The composition of CuCr alloys prepared by powder sintering is easy to adjust and control, and the powder sintering is solid-phase sintering at 900°C, so the solid solution of Cr in Cu matrix is less than 0.2% as observed by phase diagram, which is beneficial to maintain the characteristics of Cu and Cr phase, but it is difficult to obtain CuCr alloys with higher density by solid-phase sintering. To improve the defects of the low density of CuCr alloys prepared by the powder sintering method, secondary treatment can be carried out by thermal processing processes, such as hot extrusion, hot forging, and hot isostatic pressing. The combination of Cu and Cr particles in CuCr alloys prepared by the powder sintering method is mechanically bonded by compression and friction, so the toughness of the material is poor, and its breaking performances are lower than those of CuCr alloys prepared by the infiltration method [54, 55].To investigate the factors influencing the electrical conductivity of CuCr alloys prepared by powder sintering method, Klinski-Wetzel et al. [14] prepared CuCr25 and CuCr43 alloys by powder sintering and developed a model for the calculation of conductivity and validated the model by comparing it with measured values, as shown in Figure 6. It was found that the geometry of the Cr phase was the main factor affecting the electrical conductivity of CuCr alloys, as shown in Figure 7, while the effects of porosity and impurity elements seem to be minimal. Li et al. [16] prepared CuCr alloys with Cr content varying from 5 wt.% to 75 wt.% by powder sintering and tested them in a 38 kV axial magnetic field vacuum circuit breaker. The electrical conductivity and the hardness of CuCr30 alloy were 40% IACS and 64 HB, respectively. The electrical properties of CuCr30 alloy achieved the best balance among all the requirements in vacuum circuit breakers with large current interruption capacity.Figure 6
Conductivity simulation model. (a) Spherical second-phase particles; (b) needlelike and flaky particles generated by scaling particles from (a); and (c) the measured values [14].
(a)(b)(c)Figure 7
Influence of the shape of Cr particles in the Cu matrix on the electrical conductivity: transition from platelet-like (h/d < 1) over spherical (h/d=1) to needle-like (h/d>1) particles for a simulation (a) parallel and (b) perpendicular to the main scaling axis [14].
(a)(b)


## 2.1.1. Infiltration Method

The process of preparing CuCr alloy by infiltration method includes powder mixing, pressing, sintering and reduction of Cr skeleton, and infiltration of Cu, as shown in Figure2 [38, 51–53]. The oxygen-free copper (99.97% Cu, 20–50 μm) and chromium powder (99.9% Cr, 50 μm) [9] are used to prepare CuCr alloy contacts with a diameter larger than 50 mm. The key steps in the infiltration method are the sintering and reduction of the Cr skeleton and the infiltration of the molten Cu. The infiltration process is simple, and the oxygen content of the CuCr alloys can be reduced with the reduction of the Cr skeleton. The reduction of the Cr skeleton is done to reduce the oxygen content in the sintered skeleton, usually by H2 reduction or carbon thermal reduction. To form a continuous infiltrated Cr skeleton, the mass fraction of Cr in the alloy should not be less than 30%. However, the nondeformability of the Cr skeleton and its poor wettability with infiltrated Cu easily generate defects, such as micropores, shrinkage holes, and oxide residues, which reduce the voltage resistance and arc stability of the CuCr contact materials.Figure 2
Process route of CuCr alloy preparation by infiltration method [38].To reduce the microporous defects of the CuCr alloy prepared by the infiltration method, Ma et al. [9] conducted a high-pressure heat treatment of CuCr50 alloy prepared by the infiltration method and found that the microstructure of CuCr50 alloy prepared by the infiltration method did not change significantly before and after the 1 GPa high-pressure heat treatment, as shown in Figure 3, but the density of the alloy increased from 8.044 g/cm3 to 8.058 g/cm3. The high-pressure treatment bridged the internal micropores in the CuCr alloy, thus reducing the micropores and increasing the density of the alloy, as shown in Figure 4. Cao et al. [11] measured the chopping current of 3.5 A and the breakdown strength of 1.1 ∗ 108 V m−1 for the vacuum interrupter chamber assembled by the CuCr50 alloy prepared by the infiltration method.Figure 3
Metallographic photographs of CuCr alloy: (a) before and (b) after 1 GPa high-pressure treatment [9].
(a)(b)Figure 4
SEM photographs of CuCr alloy: (a) before and (b) after 1 GPa high-pressure treatment [9].
(a)(b)


## 2.1.2. Powder Sintering Method

The powder sintering method prepares CuCr alloys by mixing, pressing, sintering, repressing of high-purity Cu and Cr powders, and the subsequent heat treatment, as shown in Figure5 [15]. The higher-purity chromium powders (99.95%, 40 μm) and copper powders (99.99%,42 μm) [15] are used in the powder sintering method to produce CuCr alloy contacts with a diameter larger than 50 mm.Figure 5
Process flow of CuCr alloy preparation by powder sintering [15].The composition of CuCr alloys prepared by powder sintering is easy to adjust and control, and the powder sintering is solid-phase sintering at 900°C, so the solid solution of Cr in Cu matrix is less than 0.2% as observed by phase diagram, which is beneficial to maintain the characteristics of Cu and Cr phase, but it is difficult to obtain CuCr alloys with higher density by solid-phase sintering. To improve the defects of the low density of CuCr alloys prepared by the powder sintering method, secondary treatment can be carried out by thermal processing processes, such as hot extrusion, hot forging, and hot isostatic pressing. The combination of Cu and Cr particles in CuCr alloys prepared by the powder sintering method is mechanically bonded by compression and friction, so the toughness of the material is poor, and its breaking performances are lower than those of CuCr alloys prepared by the infiltration method [54, 55].To investigate the factors influencing the electrical conductivity of CuCr alloys prepared by powder sintering method, Klinski-Wetzel et al. [14] prepared CuCr25 and CuCr43 alloys by powder sintering and developed a model for the calculation of conductivity and validated the model by comparing it with measured values, as shown in Figure 6. It was found that the geometry of the Cr phase was the main factor affecting the electrical conductivity of CuCr alloys, as shown in Figure 7, while the effects of porosity and impurity elements seem to be minimal. Li et al. [16] prepared CuCr alloys with Cr content varying from 5 wt.% to 75 wt.% by powder sintering and tested them in a 38 kV axial magnetic field vacuum circuit breaker. The electrical conductivity and the hardness of CuCr30 alloy were 40% IACS and 64 HB, respectively. The electrical properties of CuCr30 alloy achieved the best balance among all the requirements in vacuum circuit breakers with large current interruption capacity.Figure 6
Conductivity simulation model. (a) Spherical second-phase particles; (b) needlelike and flaky particles generated by scaling particles from (a); and (c) the measured values [14].
(a)(b)(c)Figure 7
Influence of the shape of Cr particles in the Cu matrix on the electrical conductivity: transition from platelet-like (h/d < 1) over spherical (h/d=1) to needle-like (h/d>1) particles for a simulation (a) parallel and (b) perpendicular to the main scaling axis [14].
(a)(b)


## 2.2. Vacuum Self-Consumption Arc Melting Method

Vacuum self-consumption arc melting is to process the isostatic vacuum sintered CuCr alloy into self-consumption electrodes and to remelt the self-consumption electrodes by vacuum self-consumption to obtain CuCr alloy ingots. The pure copper powders (99.9%, 75μm) and pure chromium powders (99.9%, 60–75 μm) [17] are taken in the vacuum self-consumption arc melting method to prepare CuCr alloy contacts with a diameter larger than 50 mm. The process combines the traditional casting technique with directional solidification, which effectively suppresses the precipitation and separation of the Cr phase, eliminates the macrosegregation and microsegregation of the alloy, and removes oxide inclusions and residues of elements with low melting point and high vapor pressure [56, 57]. CuCr electrical contact materials are prepared at gas pressure of 220 mbar–260 mbar; melting current of 2.5 KA; and melting voltage controlled at 22 V. The materials are free from defects, such as porosity, looseness, and inclusions, and the Cr microstructure is less than 20 μm. Liang et al. [17, 58] found that the CuCr50 alloy prepared by vacuum self-consumable arc melting had finer Cr phase grains, higher density and hardness, and lower oxygen content compared with powder metallurgy and infiltration methods, as shown in Figure 8 and Table 1.Figure 8
Microstructure of CuCr alloy prepared by different preparation methods [58].
(a)(b)(c)Table 1
Properties of CuCr alloy prepared by different preparation methods [17].
Preparation methodsAlloy gradesAlloy sizeDensity (g·cm−3)Hardness (HV)Electrical conductivity (m)·(Ω·mm2)−1Oxygen content (×10−6)InfiltrationCuCr50Φ35 mm∗ 4 mm7.8412319.1450Powder sintering7.8213417.6595Vacuum self-consumption arc melting7.9717417.8410


## 2.3. New Preparation Process of CuCr Alloy

### 2.3.1. Mechanical Alloying Method

Mechanical alloying prepares CuCr alloys by subjecting high-purity Cu powder and Cr powder to high-energy ball milling, compacting, sintering, or pressure sintering. Mechanical alloying is an ideal method to prepare nanocrystalline CuCr alloys with Cr content in the range of 5% to 50%, and the refinement of Cr phase grains is beneficial to improving the properties of CuCr alloys, but the contamination of impurities may exist during the ball milling process, resulting in the reduction of properties of the alloy, especially electrical conductivity [59, 60]. Mechanical alloying method is one of the common methods used in the laboratory. The ratio of grinding volume to grinding balls is between 5 : 1 and 20 : 1, the ball milling speed of 300 rpm–600 rpm, and the ball milling time of 10 h or more during the ball milling process [25, 26].To improve the efficiency of mechanical alloying and reduce the contamination of impurities in the process, Zhao et al. [22] used Cu(OH)2 and Cr(OH)3 to replace high-purity Cu powder and Cr powder, used copper material in the grinding balls and tank lining to prevent contamination, and applied nitrogen to prevent oxidation when preparing CuCr alloy powders by the mechanical alloying method. The results showed that when the ball milling temperature was 325°C, the ball milling time was 3 hours, and the ball material mass ratio was 15 : 1, the pure CuCr alloy nanocrystalline powders were successfully prepared. Wei et al. [19] prepared Cu55Cr45 alloy by mechanical alloying + hot pressing sintering and compared it with Cu55Cr45 alloy prepared by vacuum induction melting. The Cu55Cr45 alloy was prepared by mechanical alloying + hot pressing sintering with the presence of finer Cr phase, as shown in Figures 9(a) and 9(b), where the dark dendrites and particles were Cr phase and the bright matrix was Cu phase. The liquid phase separation process of Cu55Cr45 alloy during vacuum breakdown was shown in Figures 10(a) and 11(b), where the Cr particles in the liquid-phase separation of Cu55Cr45 alloy after mechanical alloying + hot pressing sintering were significantly finer than those in the liquid-phase separation of Cu55Cr45 alloy prepared by vacuum induction melting.Figure 9
Microstructures of the Cu55Cr45 alloys. (a) Vacuum induction melting. (b) Mechanical alloying + hot pressing sintering [19].
(a)(b)Figure 10
The liquid-phase separation process of Cu55Cr45 alloy during vacuum breakdown. (a) Vacuum induction melting. (b) Mechanical alloying + hot pressing sintering [19].
(a)(b)


### 2.3.2. Vacuum Induction Melting

The process flow of vacuum induction melting includes mixing, melting, and casting of Cu powder and Cr powder. The melting and casting processes are carried out under vacuum conditions, so the gas content in the alloy is greatly reduced, but the preparation of CuCr alloys with high Cr contents is demanding on the melting equipment due to the high melting point of the alloys [26–31].Xiu et al. [27] prepared CuCr25, CuCr30, and CuCr40 alloys by vacuum induction melting and investigated their microstructure and properties. As shown in Figure 11 and Table 2, with the increase of Cr content in the CuCr alloy, the microstructure of the Cr phase became more consistent, the electrical conductivity and density of the CuCr alloy decreased, and the hardness and tensile strength increased linearly. After assembling the vacuum interrupters with CuCr25, CuCr30, and CuCr40 alloys as contact materials, it was found that the ultimate breaking current of the vacuum interrupters assembled with CuCr25 contact material was 23.9 kA, while the ultimate breaking currents of the vacuum interrupters assembled with CuCr30 and CuCr40 alloys were 23.0 kA and 21.3 kA, respectively, so CuCr25 alloy has the best breaking current capability. Figure 12 showed the breaking current and arc voltage of the vacuum interrupters assembled with CuCr25, CuCr30, and CuCr40 alloy contacts, which found that the arc voltage of the vacuum interrupter assembled with CuCr25 contact material is the lowest, so the ignition arc energy is low, which is conducive to breaking. Garzón-Manjón et al. [26] successfully prepared Cu-32 at.%Cr alloy with nonequilibrium nanostructures by combining vacuum induction melting and rapid solidification. Zhang et al. [29] changed the morphology and distribution of Cr particles in the CuCr30 alloy prepared by vacuum induction melting and casting through deformation treatment, which resulted in the transformation of Cr phase from dendritic to longitudinally arranged fibrous, and the thermal conductivity of the extruded state along the deformation direction increased by 5.8% compared to the cast alloy.Figure 11
OM images of vacuum cast alloys. (a) CuCr25 alloy; (b) CuCr30 alloy; and (c) CuCr40 alloy [27].
(a)(b)(c)Table 2
Physical and mechanical properties of CuCr alloys [27].
MaterialElectrical conductivity (MS·m−1)Density (g·cm−3)Hardness (N·mm−2)Tensile strength (MPa)CuCr2530.68.4183.9319CuCr3028.08.2789.7327CuCr4023.88.1697.2342Figure 12
Breaking current and arc voltage at current peak [27].Zhang et al. [30] prepared CuCr25 alloy by vacuum induction melting, and the microstructure of the alloy consisted of the Cu-rich phase of the matrix and Cr-rich dendrites, as shown in Figure 13. The CuCr25 alloy prepared by this process had a relative density >99.00%, hardness of 80 HB, conductivity of 25 MS/m, oxygen content less than 500 ppm, and nitrogen content less than 20 ppm.Figure 13
SEM photos of CuCr25 alloy: (a) normal microstructure and (b) coarse Cr dendrite and Cr microsegregation [30].
(a)(b)


### 2.3.3. Selective Laser Melting Method

Selective laser melting is a method of preparing the high-density alloy containing substable supersaturated solid solutions by rapid laser melting and rapid cooling of metal powders. The density of the CuCr alloy prepared by this process is nearly 100%, and its mechanical properties are comparable to those of CuCr alloys prepared by forging, but the preparation process of selective laser melting is complex, costly, and confidential [31].Chen et al. [32] prepared CuCr20 and CuCr25 alloys by selective laser melting and investigated the effect of subsequent aging treatment on their microstructure and properties. The results showed that the microstructure of the CuCr alloys prepared by the selective laser melting consisted of Cr-rich phases with diameters of ∼10 mm, fine spherical Cr-rich particles with diameters in the range of 50–800 nm, and Cu-rich matrix, as shown in Figure 14. The aging temperature significantly affected the electrical conductivity and hardness of the alloys, while the aging time had less effect on the electrical conductivity and hardness of the alloys, as shown in Figure 15. After aging treatment at 500°C for 1 h, the electrical conductivity of CuCr20 and CuCr25 contacts increased by 2.95 times (from 11 MS·m−1 to 32.5 MS·m−1) and 2.73 times (from 11 MS·m−1 to 30 MS·m−1), respectively, and the hardness increased by 60% (from 166 HV to 265 HV) and 51% (from 184 HV to 278 HV), respectively.Figure 14
Microstructures of as-fabricated CuCr20. (a) The overall distribution of Cr in Cu. (b) Further magnification of (a), A indicates the Cr-rich phase with a particle size of ∼10 mm, B indicates the Cu-rich phase surrounding A. (c) EDS results of A and B in (b) [32].Figure 15
Properties of SLM CuCr20 and CuCr25 aged under different conditions: (a) electrical conductivity and (b) hardness [32].
(a)(b)


## 2.3.1. Mechanical Alloying Method

Mechanical alloying prepares CuCr alloys by subjecting high-purity Cu powder and Cr powder to high-energy ball milling, compacting, sintering, or pressure sintering. Mechanical alloying is an ideal method to prepare nanocrystalline CuCr alloys with Cr content in the range of 5% to 50%, and the refinement of Cr phase grains is beneficial to improving the properties of CuCr alloys, but the contamination of impurities may exist during the ball milling process, resulting in the reduction of properties of the alloy, especially electrical conductivity [59, 60]. Mechanical alloying method is one of the common methods used in the laboratory. The ratio of grinding volume to grinding balls is between 5 : 1 and 20 : 1, the ball milling speed of 300 rpm–600 rpm, and the ball milling time of 10 h or more during the ball milling process [25, 26].To improve the efficiency of mechanical alloying and reduce the contamination of impurities in the process, Zhao et al. [22] used Cu(OH)2 and Cr(OH)3 to replace high-purity Cu powder and Cr powder, used copper material in the grinding balls and tank lining to prevent contamination, and applied nitrogen to prevent oxidation when preparing CuCr alloy powders by the mechanical alloying method. The results showed that when the ball milling temperature was 325°C, the ball milling time was 3 hours, and the ball material mass ratio was 15 : 1, the pure CuCr alloy nanocrystalline powders were successfully prepared. Wei et al. [19] prepared Cu55Cr45 alloy by mechanical alloying + hot pressing sintering and compared it with Cu55Cr45 alloy prepared by vacuum induction melting. The Cu55Cr45 alloy was prepared by mechanical alloying + hot pressing sintering with the presence of finer Cr phase, as shown in Figures 9(a) and 9(b), where the dark dendrites and particles were Cr phase and the bright matrix was Cu phase. The liquid phase separation process of Cu55Cr45 alloy during vacuum breakdown was shown in Figures 10(a) and 11(b), where the Cr particles in the liquid-phase separation of Cu55Cr45 alloy after mechanical alloying + hot pressing sintering were significantly finer than those in the liquid-phase separation of Cu55Cr45 alloy prepared by vacuum induction melting.Figure 9
Microstructures of the Cu55Cr45 alloys. (a) Vacuum induction melting. (b) Mechanical alloying + hot pressing sintering [19].
(a)(b)Figure 10
The liquid-phase separation process of Cu55Cr45 alloy during vacuum breakdown. (a) Vacuum induction melting. (b) Mechanical alloying + hot pressing sintering [19].
(a)(b)


## 2.3.2. Vacuum Induction Melting

The process flow of vacuum induction melting includes mixing, melting, and casting of Cu powder and Cr powder. The melting and casting processes are carried out under vacuum conditions, so the gas content in the alloy is greatly reduced, but the preparation of CuCr alloys with high Cr contents is demanding on the melting equipment due to the high melting point of the alloys [26–31].Xiu et al. [27] prepared CuCr25, CuCr30, and CuCr40 alloys by vacuum induction melting and investigated their microstructure and properties. As shown in Figure 11 and Table 2, with the increase of Cr content in the CuCr alloy, the microstructure of the Cr phase became more consistent, the electrical conductivity and density of the CuCr alloy decreased, and the hardness and tensile strength increased linearly. After assembling the vacuum interrupters with CuCr25, CuCr30, and CuCr40 alloys as contact materials, it was found that the ultimate breaking current of the vacuum interrupters assembled with CuCr25 contact material was 23.9 kA, while the ultimate breaking currents of the vacuum interrupters assembled with CuCr30 and CuCr40 alloys were 23.0 kA and 21.3 kA, respectively, so CuCr25 alloy has the best breaking current capability. Figure 12 showed the breaking current and arc voltage of the vacuum interrupters assembled with CuCr25, CuCr30, and CuCr40 alloy contacts, which found that the arc voltage of the vacuum interrupter assembled with CuCr25 contact material is the lowest, so the ignition arc energy is low, which is conducive to breaking. Garzón-Manjón et al. [26] successfully prepared Cu-32 at.%Cr alloy with nonequilibrium nanostructures by combining vacuum induction melting and rapid solidification. Zhang et al. [29] changed the morphology and distribution of Cr particles in the CuCr30 alloy prepared by vacuum induction melting and casting through deformation treatment, which resulted in the transformation of Cr phase from dendritic to longitudinally arranged fibrous, and the thermal conductivity of the extruded state along the deformation direction increased by 5.8% compared to the cast alloy.Figure 11
OM images of vacuum cast alloys. (a) CuCr25 alloy; (b) CuCr30 alloy; and (c) CuCr40 alloy [27].
(a)(b)(c)Table 2
Physical and mechanical properties of CuCr alloys [27].
MaterialElectrical conductivity (MS·m−1)Density (g·cm−3)Hardness (N·mm−2)Tensile strength (MPa)CuCr2530.68.4183.9319CuCr3028.08.2789.7327CuCr4023.88.1697.2342Figure 12
Breaking current and arc voltage at current peak [27].Zhang et al. [30] prepared CuCr25 alloy by vacuum induction melting, and the microstructure of the alloy consisted of the Cu-rich phase of the matrix and Cr-rich dendrites, as shown in Figure 13. The CuCr25 alloy prepared by this process had a relative density >99.00%, hardness of 80 HB, conductivity of 25 MS/m, oxygen content less than 500 ppm, and nitrogen content less than 20 ppm.Figure 13
SEM photos of CuCr25 alloy: (a) normal microstructure and (b) coarse Cr dendrite and Cr microsegregation [30].
(a)(b)


## 2.3.3. Selective Laser Melting Method

Selective laser melting is a method of preparing the high-density alloy containing substable supersaturated solid solutions by rapid laser melting and rapid cooling of metal powders. The density of the CuCr alloy prepared by this process is nearly 100%, and its mechanical properties are comparable to those of CuCr alloys prepared by forging, but the preparation process of selective laser melting is complex, costly, and confidential [31].Chen et al. [32] prepared CuCr20 and CuCr25 alloys by selective laser melting and investigated the effect of subsequent aging treatment on their microstructure and properties. The results showed that the microstructure of the CuCr alloys prepared by the selective laser melting consisted of Cr-rich phases with diameters of ∼10 mm, fine spherical Cr-rich particles with diameters in the range of 50–800 nm, and Cu-rich matrix, as shown in Figure 14. The aging temperature significantly affected the electrical conductivity and hardness of the alloys, while the aging time had less effect on the electrical conductivity and hardness of the alloys, as shown in Figure 15. After aging treatment at 500°C for 1 h, the electrical conductivity of CuCr20 and CuCr25 contacts increased by 2.95 times (from 11 MS·m−1 to 32.5 MS·m−1) and 2.73 times (from 11 MS·m−1 to 30 MS·m−1), respectively, and the hardness increased by 60% (from 166 HV to 265 HV) and 51% (from 184 HV to 278 HV), respectively.Figure 14
Microstructures of as-fabricated CuCr20. (a) The overall distribution of Cr in Cu. (b) Further magnification of (a), A indicates the Cr-rich phase with a particle size of ∼10 mm, B indicates the Cu-rich phase surrounding A. (c) EDS results of A and B in (b) [32].Figure 15
Properties of SLM CuCr20 and CuCr25 aged under different conditions: (a) electrical conductivity and (b) hardness [32].
(a)(b)


## 3. Research Progress on Modification of CuCr Alloys

The modification research of CuCr alloys mainly focuses on surface modification [48, 61, 62] to obtain uniformly dispersed ultrafine grain microstructure and composition design of alloys [10–13, 19, 29].Surface modification mainly includes laser surface melting, surface mechanical attrition treatment, and other processes. Laser surface melting is to irradiate the surface of the alloy with a high-power laser beam to form a molten layer, followed by rapid cooling (103–108 K/s) of the molten layer to form a surface alloyed layer for surface modification treatment. The Cr phase grains in the surface alloy layer obtained by laser surface melting are fine, and the metallurgical bond between the alloy layer and the substrate is strong.Zhang et al. [48] performed laser surface melting of CuCr50 alloy by a high-power laser beam, and the process schematic is shown in Figure 16. The results showed that the thickness of the surface alloyed layer was 165 ± 20 μm, and the average diameter of the Cr phase within the alloyed layer was refined to several hundred nanometers with a spherical shape, as shown in Figure 17. It was found that the high cooling rate (5.98 × 106 K/s) effectively suppressed the coarsening effect of Cr particles during the liquid phase separation. The microhardness of the alloyed layer was significantly increased up to 232 HV, which was 2.8 times higher than that of the substrate. Compared with the untreated CuCr alloy contact material, the withstand voltage value of the LSM-treated CuCr alloy contact was increased by 35.4%, and the arc duration was increased by 18%.Figure 16
(a) Schematic diagram of experimental setup for LSM and (b) the principle diagram of interaction with Cu-Cr alloy [48].
(a)(b)Figure 17
(a) Untreated. (b) LSM-treated Cu-50Cr contacts [48].
(a)(b)Yang et al. [61] prepared gradient nanostructures in CuCr25 alloy by surface mechanical attrition treatment. As shown in Figure 18, the SMAT surface layer consisted of two sublayers, with a nanocrystalline layer (labeled I) and a submicrocrystalline layer (labeled II). The thickness of the nanocrystalline layer was about 20 μm, which contained nanoscale Cr grains with random crystal orientation, and the average size of the Cr grains was about 10 nm; the thickness of the submicrocrystalline layer is about 180 μm, and the average size of Cr grains within the layer is about 200 nm. Yang et al. [61] also concluded for the first time that the activity of high-density dislocations was related to the grain refinement mechanism in the binary alloy system. The variation of electrical conductivity and hardness of CuCr alloy with decreasing distance from the surface under different SMAT process conditions was shown in Figure 19. The surface hardness of CuCr alloy reached 475 HV after annealing at SMAT 5 h + 280°C for 2 h, and the conductivity of the alloy was 46.5% IACS. The CuCr alloy reached a surface hardness of 475 HV and electrical conductivity of 46.5% IACS after SMAT 5 h + 280°C 2 h annealing treatment.Figure 18
SEM micrograph of a cross section of CuCr alloys sample. (a) Original. (b) SMAT—5 h sample [61].
(a)(b)Figure 19
Variations of the microhardness and electrical conductivity with the depth on the cross section of each sample [61].
(a)(b)The multicomponent alloying of CuCr alloys is also a good way to improve and enhance the performance of CuCr alloys. The added elements can be divided into the following categories according to their different effects on the properties of CuCr alloys: (1) Fe, Co, Ni, Ti, Zr, and other elements can increase the wettability of Cu and Cr and improve the denseness of CuCr alloy; (2) metal carbides and oxides or moderate amounts of elements with high vapor pressure can reduce the chopping current of CuCr alloy; (3) W, Mo, Ta, Nb, and hard phases such as carbides, oxides, and borides can improve breaking capacity and compressive strength of CuCr alloy; and (4) grain boundary brittle elements such as Bi and Te can reduce the welding force of CuCr alloys [63–70].Cao et al. [10, 11] prepared CuCr50, CuCr31Mo19, and CuCr40Fe10 alloys by infiltration method. The research results showed that both Fe and Mo strengthened the Cr phase of CuCr alloy, which made the first breakdown of the alloy tend to transfer from the Cr to Cu phase, improved the arc stability, and reduced the chopping current. The addition of Mo and Fe led to the splitting and deviation of the cathode spot, and the pits were shallow and dispersed, which could avoid concentrated corrosion and prolong the life of CuCr alloy contact materials. Chang et al. [12] prepared CuCr50 alloy by infiltration with the addition of W (mass fraction in the range of 1–5.7 wt.%) and C (mass fraction in the range of 0.3–1 wt.%) elements. It was found that W and C elements significantly increased the hardness of CuCr alloy. The formation of brittle chromium carbide film and the nucleation of W particles increased the solidification rate, which was beneficial to its voltage resistance and breaking capacity as a contact material. Xie et al. [13] prepared Cu-Cr1.8 alloy, CuCr1.9La2.7 alloy, and CuCr1.3Y2.4 alloy by powder sintering and found that rare earth elements could refine Cr grains and effectively improve the electrical conductivity of CuCr alloys. Xiu et al. [28] found that the addition of the Te element could spheroid the Cr particles in CuCr25 alloy and improve the welding resistance of the alloy. Wei et al. [19] prepared CuCr45 and CuCr42.7Ni2.3 alloys by mechanical alloying + hot pressing sintering. As shown in Figure 20, when Ni was added to the CuCr alloy, the element Ni was mainly present in the Cr phase of the CuCr alloy, and the liquid-phase separation was suppressed during alloy breakdown and the Cr phase was significantly refined (arrow B and C). The Cr-rich particles from the liquid-phase separation are uniformly distributed with spherical nanometer dimensions.Figure 20
Microstructures of the Cu-Cr-Ni alloys and after the vacuum breakdowns (Cu = 55, Cr-Ni = 45, at.%) alloys: (a) Cu55Cr45. (b) Cu55(Cr95Ni5)45 [19].
(a)(b)Zhang et al. [23] prepared CuCr25 alloy by vacuum induction melting and investigated the effects of W and Ni elements on the microstructure and properties of CuCr25 alloy. The results showed that the addition of W and Ni to the CuCr25 alloy could refine and spheroid the Cr phase organization, eliminate the macroscopic segregation of Cr, and significantly improve the breakdown strength of CuCr25, as shown in Figure 21.Figure 21
Microstructure of CuCr alloy prepared by vacuum induction melting. (a) CuCr25; (b) CuCr25W1; and (c) CuCr25Ni1 [23].
(a)(b)(c)


## 4. Aluminothermic Reduction-Magnetoelectric Coupling Refining-Water and Gas Composite Cooling Method

During the study of SHS synthesis reactions, the reaction system must meet certain thermodynamic conditions for the reaction to self-sustain the combustion reaction process. One of the most basic thermodynamic parameters is the adiabatic temperature of the reaction (Tad). Combustion synthesis (or SHS) is a chemical reaction under special conditions. Calculation of combustion temperature and product equilibrium components is performed under adiabatic conditions, that is, when all the heat released by the reaction is used to heat the products synthesized during the reaction, based on the principles of conservation of mass and energy and chemical potential (Gibbs free energy) minimum. Thermodynamic calculations help to control the temperature and composition of the process products. The basic task of the kinetic study of the reaction process is to study the influence of various factors (e.g., temperature, pressure, concentration, medium, catalyst, etc.) on the reaction rate in order to reveal the relationship between the self-propagation reaction and the structure of the material for the purpose of controlling the self-propagation reaction. [71–74]. The flowchart of aluminothermic reduction-magnetoelectric coupling refining-water and gas composite cooling is shown in Figure 22. The raw materials of CuO (size 30–50 μm)and Cr2O3 (size 30–50 μm) and the reducing agent Al powder (size 0.2–3 mm) are taken to synthesize large-size homogeneous CuCr alloys in situ by aluminothermic reduction under the magnetic field stirring, and the magnetic field stirring equipment is shown in Figure 23 [41]. This process utilizes the chemical heat released by the aluminothermic reaction to instantly heat the reaction system to above 2000°C to obtain a completely miscible high-temperature CuCr alloy melt, eliminating the temperature gradient field caused by the slow heating rate in the traditional smelting process, suppressing the phenomenon of segregation that exists in the alloy by rapid cooling, and obtaining large-size CuCr alloy ingots without segregation, but with pores and inclusions in the alloy [75, 76], as shown in Figure 24.Figure 22
Flowchart for the preparation of CuCr alloy by aluminothermic reduction-magnetoelectric coupling refining-water and gas composite cooling [71–74].Figure 23
The applied magnetic field stirring equipment [41].Figure 24
Macroscopic and microscopic photographs of Cu-Cr alloy ingots prepared by aluminothermic reduction under magnetic field stirring [75, 76]. (a) CuCr25. (b) CuCr50 (magnification 500 times).
(a)(b)To eliminate the pores and inclusions present in the CuCr alloy, the magnetoelectric coupling refining is carried out to make the oxide inclusions and pores in the alloy fully float, and the water and gas composite cooling is used to rapidly solidify to prevent segregation [58, 77], and finally, dense and homogenized CuCr alloy ingots are obtained, as shown in Figure 25 [78, 79].Figure 25
Macroscopic photograph and microstructure of CuCr alloy by aluminothermic reduction-magnetoelectric coupling refining-water and gas composite cooling. (a) CuCr25. (b) CuCr50 [78, 79].
(a)(b)This process can obtain CuCr alloy melt with ultra-high-temperature mutual solubility, which together with the rapid solidification technique is the key to preparing large-size homogeneous CuCr alloy. The acquisition of CuCr alloy melt is completely dependent on the heat generated by reduction and does not require external heat. Therefore, large-size CuCr alloys can be quickly prepared at a low cost, thereby realizing the continuous production of large-size CuCr alloys. However, CuCr alloys are prepared by extracting metal elements from oxides, so the oxygen content in the alloy is difficult to control; the CuCr alloy prepared by aluminothermic reduction needs to be refined by magnetoelectric coupling, which also increases the preparation time and cost of the alloy. As shown in Table3, the oxygen content, nitrogen content, and electrical conductivity of the alloy are insufficient. The oxidation behavior of the second phase can have an important effect on the alloy properties [80, 81].Table 3
Chemical composition and mechanical and physical properties of CuCr contacts [58].
Contact productsChemical composition (mass fraction, %)Gas content (mass fraction, %)Density (g/cm3)Conductivity (MS/m)Hardness (HB)CuCrOxygen contentNitrogen contentGB/T 26867-2011CuCr25Residuals23∼27≤0.050≤0.003≥8.25≥28≥70CuCr50Residuals47∼52≤0.050≤0.004≥7.96≥16≥80New process based on aluminothermic reductionCuCr2575.1623.980.544%0.116%8.3924.3191.3CuCr5052.1947.260.437%0.078%8.1715.92108The CuCr alloys prepared by aluminothermic reduction-magnetoelectric coupling refining-water and gas composite cooling were assembled into 12 kV high-power vacuum interrupters, as shown in Figure26. The electrical performance test results were shown in Table 4. The power frequency withstands breakdown voltage of 12 kV high-power vacuum interrupters reached 75 kV/min, which was much higher than the rated power frequency withstand voltage requirement of 42 kV/min; the lightning impulse breakdown withstand voltage of positive and negative electrodes of 12 kV high-power vacuum interrupters reached 156 kV and 160 kV, respectively, which is much higher than the rated standard lightning impulse withstand voltage requirement of 85 kV; the chopping current of 12 kV high-power vacuum interrupters is only 0.40 A, much better than the world’s best level of 1.0∼1.5 A [63, 64].Figure 26
Vacuum interrupter assembled by CuCr alloy prepared by this process [77].Table 4
Electrical performance test results of vacuum interrupters assembled with CuCr alloy [77].
Contact productsRated requirementsMeasured valueVoltage resistanceImpact resistanceInterception capacityResistance to weldingVoltage resistanceImpact resistanceInterception capacityResistance to weldingRated power frequency withstand voltage (kV)Rated lightning impulse withstand voltage (kV)Chopping current (a)Welding force/NPower frequency withstand voltage (kV)Lightning impulse withstand voltage (kV)Chopping current (a)Welding force (N)PositiveNegativePositiveNegativeCuCr254285854100∼200741561600.40176CuCr504285854.5100∼200751301460.67184To improve the effect of slag-alloy separation, Shi et al. [82–89]studied different slag systems, as well as the viscosity, density, surface tension, and other properties of the slag system, analyzed the factors affecting the alloy-slag separation during the reduction, and considered whether a suitable slag system could be selected to improve the alloy-slag separation to realize the one-step direct preparation of CuCr alloys by aluminothermic reduction. The final study found that if Cr2O3 was coupled with reduction and slagging, the homogeneous dense CuCr alloys could be prepared directly by the aluminothermic reduction in one step. The one-step direct preparation of CuCr alloy by the aluminothermic reduction method could reduce the production cost and improve the continuity of the process.


## 5. Conclusion and Outlook

CuCr contact materials with high Cr content have become an essential key component of vacuum switches. The industrial preparation technologies of CuCr alloys are mainly infiltration, powder sintering, and vacuum self-consumption arc melting. The infiltration method combined with the subsequent high-pressure treatment can reduce the internal micropores of the CuCr alloy and significantly improve the density of the CuCr alloy; the geometry and content of the Cr phase in the preparation of CuCr alloy by powder metallurgy will significantly affect the electrical properties of CuCr alloy; the vacuum self-consumption arc melting method prepares CuCr alloy with a finer grain size of Cr phase and low oxygen content. Among the new technologies for the preparation of CuCr alloys, the mechanical alloying method can prepare nanocrystalline CuCr alloys with Cr content ranging from 5% to 50%; the vacuum induction melting method can prepare CuCr alloys with low gas content and high purity; the selective laser melting method can prepare CuCr alloys with uniform and fine Cr phases and density of nearly 100%, and the mechanical properties are comparable to those of CuCr alloys prepared by forging.The aluminothermic reduction-electromagnetic coupling refining-water and gas composite cooling can obtain a fully miscible high-temperature melt of CuCr alloy through aluminothermic reduction, followed by slag refining under electromagnetic coupling; finally, CuCr alloy is prepared by water-gas composite rapid cooling. This process can quickly and cost-effectively prepare large-size CuCr alloys, thereby realizing the continuous production of large-size CuCr alloys. To improve the performance of CuCr alloys and meet the requirements of ultra-high-voltage transmission, the research on surface treatment and alloying of CuCr alloys has become another hot topic. The alloy layer with nano-Cr phase grains can be formed on the surface of CuCr alloys to improve the alloy properties by laser surface melting method or surface mechanical attrition. The alloying of CuCr alloys improves the properties of CuCr alloys by adding trace amounts of other elements. The essence of surface treatment and alloying is to refine the Cr phase grains and improve the uniformity of Cr phase distribution to achieve the purpose of improving the properties of CuCr alloys. Therefore, in general, this process has the potential to become a new generation of preparation method for large-size, homogeneous high-performance CuCr alloys.



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*Source: 1333985-2022-06-29.xml*