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Arc erosion behavior of Ag/Ni(12) electrical contacts

Electrical contact physical phenomena

Electrical contact physical phenomena, including arc energy, arc time, and fusion welding force, change under the action of arc erosion due to changes in the microstructure and composition of the contact surface.

the average values of arc energy, arc time, and fusion welding force per 100 operations for Ag/Ni(12) electrical contact materials prepared by the SE process at 50,000 operations are shown in Figure 2-8. during the 50,000 operations, the Arc time and arc energy average change trend is similar, but different from the fusion welding force; when the number of operations is less than 8000 times, the arc time, arc energy and fusion welding force average value increase with the number of operations; when the number of operations increases from 8000 times to 15000 times, the arc time and arc energy average value decreases with the number of operations;

when the number of operations increases from 15000 When the number of operations increases from 8000 to 15000, the average values of arc time and arc energy decrease with the number of operations; when the number of operations increases from 15000 to 50000, the average values of arc time and arc energy remain stable; when the number of operations increases from 8000 to 10000, the average values of welding force decrease with the number of operations; when the number of operations increases from 10000 to 30000, the average values of welding force remain stable;

however, when the number of operations increases from 30000 to 40000, the average values of welding force first increase with the number of operations. However, when the number of operations increases from 30,000 to 40,000, the average welding force first increases and then decreases; when the number of operations increases from 40,000 to 50,000, the average welding force remains stable.

Thus, the physical phenomena of electrical contact (arc time, arc energy and welding force) change with each operation due to arc erosion.Under the action of arc erosion, the quality of the cathode and anode electrical contacts will change due to evaporation and splash erosion. In addition, during the arc operation, the material transfer between the cathode contact and the anode contact will also cause the change of its quality. The quality test results show that after 50,000 operations, the Ag/Ni(12)SE
mass on both cathode and anode contacts decreased (2.7 mg mass loss on cathode contacts; 4 mg mass loss on anode contacts). The mass loss of the anode contact was greater than that of the cathode contact, indicating that the arc erosion of the anode contact was more serious than that of the cathode contact under the same service conditions.

The two-dimensional macroscopic morphology of Ag/Ni(12)SE electrical contact material after 50,000 operations is shown in Figure 2-9, where the surface morphology of both cathode and anode contacts were changed due to arc erosion. On the one hand, both cathode and anode contact surfaces were severely deformed due to arc erosion and contact forces; on the other hand, splash erosion was also observed on the cathode and anode contact surfaces (see circles on Figure 2-9(a) and (b)). In addition, the morphological changes on the surface of the anode contacts were more severe than those on the cathode contacts, indicating that the arc erosion on the anode contacts was more severe than that on the cathode contacts, which is consistent with the previous results on mass loss.

Arc erosion causes not only changes in the morphology of the electrical contact surface, but also changes in the composition due to physical metallurgical reactions on the contact surface of the electrical contacts. Table 2-1 shows the elemental composition content of different areas of the electrical contact surface (Figure 2-9(a) and (b)) as measured by EDS. From Table 2-1, it can be seen that the Ag/Ni(12)SE electrical contact material by arc erosion
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After the arc erosion, the silver content (mass fraction) is less than 88% and the nickel content (mass fraction) is more than12%. In addition, a large amount of oxygen and a small amount of carbon were detected on the contact surface of the electrical contacts, which indicated that oxygen and carbon in the air were dissolved in the silver matrix during the arc erosion. The temperature at the root of the arc is high due to the arcing action. The solubility of nickel and oxygen in silver increases with increasing temperature. When the temperature is higher than the nickel melting point (1453°C), some nickel particles also dissolve in the melt pool. These nickel particles melt and cool in a short time to form secondary crystals. In addition, during the arc erosion process, silver has a low melting point (961°C) and melts and evaporates easily. On the one hand, the melting of silver increases the solubility of oxygen in silver, leading to an increase in the oxygen content of the contact surface; on the other hand, the evaporation of silver reduces the silver content of the contact surface. In addition, since the density of nickel particles is lower than that of silver (Ni
8.9g/cm, Ag:10.5g/cm3), so the nickel particles gather and float on the melted silver, which leads to the increase of nickel content on the contact surface. Therefore, when the Ag/Ni(12)SE electrical contacts undergo arc erosion, the content of oxygen and nickel increases, while the content of silver decreases.

The two-dimensional microscopic morphologies of Ag/Ni(12)SE cathode and anode contacts after 50,000 operations are shown in Figure 2-10 and Figure 2-11, respectively. It can be seen that the surface morphology of the cathodic and anodic contacts after arc erosion is similar. The “crater” type erosion craters were observed on the surface of both cathode and anode contacts (see Figure 2-10(a) and Figure 2-11(a)), but more spattered erosion was observed near the “crater” of the anode contact. Under certain conditions, silver will melt and evaporate under the action of arc erosion, especially at high currents, silver vapor will flow from the arc root, silver vapor from the arc root, due to the splash of large droplets, the contact surface will leave a “crater” type of erosion crater. In the cathode contact and anode contact surface also observed island-shaped silver-rich band (see Figure 2-10 (b) and Figure 2-11 (b)). When the silver melted under the action of arc energy, the melted silver could not diffuse on the contact surfaces in time, resulting in the formation of island-like structures on the surfaces of the cathode and anode contacts. Also, pores and cracks were observed on the cathode contact and anode contact surfaces

Molten metal absorbs large amounts of gas from the air under the action of an electric arc. The solubility of oxygen in liquid silver (0.3%) is 40 times greater than the solubility of solid silver (0.008%).

Therefore, molten silver contains a large amount of oxygen under the action of an electric arc. After the arc is extinguished, part of the oxygen dissolved in the molten silver escapes into the air due to the change in oxygen pressure, and another part of the oxygen does not have time to escape from the molten silver due to rapid solidification, which leads to the formation of holes on the contact surface and inside the contact.

The cracks produced on the surface of the anode contacts are more severe than those of the cathode contacts, indicating that arc erosion is more severe on the anode contacts than on the cathode contacts. Cracking is a dangerous form of arc erosion.

The causes of crack formation are very complex and depend mainly on the structure and properties of the electrical contact material, arc energy and external working conditions.

There are inevitably some defects inside or on the surface of the material (micropores, microcrack inclusions, grain boundaries and interfacial dislocation groups, etc.), which are missing as the root cause of surface crack formation. On the one hand, under the high temperature of the arc, the silver on the contact surface will melt; but due to the short duration of the arc (less than 1ms layer in this work will be rapidly cooled and solidified, the rapid solidification of the clad layer leads to an increase in the density of vacancies and dislocations in the organization of the clad layer, the increase in the density of vacancies and dislocations will reduce the strength of grain boundaries, increasing the possibility of grain boundary crack formation under the action of force (see the white circle in Figure 2-11()) .

On the other hand, porosity decreases the mechanical strength of the material and tends to cause crack formation or promote crack development. For example, the pores on Figure 2-10(b) lead to the formation and development of cracks (see the white circle on Figure 2-10(b)). In addition, coral-like structure spatters were observed on the surfaces of the cathode contacts and anode contacts (see Figure 2-10(d) and Figure 211(d)). The coral-like structure spatter is a particle accumulation with particle size between 200 and 500 nm that occurs mainly on the edge portion of the electrical contact surface due to the occurrence of splash erosion. The vaporization and liquid evaporation spattering during the electric arc erosion are the main reasons for the formation of coral-like structure particles. On the one hand, the electric contact surface material changes from solid to liquid under the action of arc energy, and then becomes gas to escape from the electric contact material surface, and finally the gaseous silver absorbs a large amount of oxygen in the air and solidifies rapidly on the electric contact contact surface, forming coral-like structure particles; on the other hand, the electric contact surface under the action of arc energy. Formation of silver molten pool, silver molten pool of tiny droplets under the action of various forces (such as electrostatic field force, electromagnetic force, the reaction force of material movement, surface tension, etc.) from the molten pool splash off, therefore, coral-like structure particles are the product of evaporation and splash erosion under the action of electric arc.

Cross-sectional organization and elemental surface distribution

Cross-sectional organization

The metallographic microstructures of the cross-sections of Ag/Ni(12)SE cathode contacts and anode contacts after 50,000 operations are shown in Figure 2-12. Under the effect of arc erosion, the cathode contact surface changed from convex to flat (see Figure 2-12(a))
and material transfer was observed on the cathode contact surface (see the white circle on Figure 2-12(a)). Under the action of arc erosion, the anode contact surface changed from convex to concave (see Figure 2-12(b1)), and material transfer and splashing were observed on the anode contact surface (see white circle on Figure 2-12(b)). A small number of erosion pits appeared on both the cathode contact and anode contact surfaces, and melt pools formed within the erosion pits (see Fig. 2-12(a2) and (b₂)

The elemental distribution in the melt pool after 50,000 operations on Ag/Ni(12)SE cathode contacts and anode contacts are shown in Figures 2-13 and 2-14, respectively. From the figures, it can be seen that the element distribution in the melt pool on the cathode and anode contacts is different for the same number of operations. The melt pool area on the cathode contact is smaller than that on the anode contact (see Figure 2-13(a) and Figure 2-14(a)), which indicates that the area affected by arc erosion on the anode contact is larger than that on the cathode contact, and also indicates that the arc erosion on the anode contact is more severe than that on the cathode contact. The backscattered electron image shows three different phases of white, black and dark gray distributed on the gray silver substrate (see Figure 2-13(a), Figure 2-14(a)). The compositions of the different regions in Figures 2-13(a) and 2-14(a) measured by electron probe spectrometry (WDS) are shown in Table 2-2. The results in Table 2-2 indicate that the white phase contains 87% to 97% (mass fraction) of elemental W, the black phase contains about 98% (mass fraction) of elemental Ni, the gray phase contains about 99% (mass fraction) of elemental Ag, and the dark gray phase contains about 80% ( In addition, the results of the elemental facet analysis also show that the gray phase contains mainly elemental Ag (see Figures 2-13(b) and 2-14(b)), the black phase contains mainly elemental Ni (see Figures 2-13(c) and 2-14(c)), the white phase contains mainly elemental W (see Figures 2-13(d) and 2-14(d)) , the dark gray phase contains mainly Ni and W elements. The shape of the melt pool on the cathode contact and the anode contact resembles an inverted magic hat (see Figures 2-13(a) and 2-14(a)).

The results of elemental surface distribution of Ag/Ni(12) RE electrical contact materials show that the distribution of Ag and Ni elements in the molten pool of cathode contacts is different from that of Ag and Ni elements in the molten pool of anode contacts. In the cathode contact melt pool, Ni elements are mainly distributed at the bottom of the melt pool, and Ag elements are mainly distributed inside the melt pool; while the distribution of Ni and Ag elements in the anode contact melt pool is layer by layer.

Therefore, the formation mechanism of cathode contact and anode contact melt pool is different. Figure 2-17 shows the melt pool formation of Ag/Ni(12)SE electrical contact material after 50,000 operations. Under the action of the arc energy, the temperature of the contact surface of the cathode contact and anode contact material will increase, and when the temperature of the cathode and anode contact surface reaches 960°C, the silver melts, while the nickel particles remain in the solid state due to the high melting point (1453°C).

The melting of silver forms a molten pool of silver containing nickel particles near the surface layers of the cathode and anode contacts (see Figure 2-17(b)). Under the effect of gravity, the Ni particles in the molten pool of electrical contacts move from bottom to top while the silver moves from top to bottom due to the fact that the density of Ni (8.9 g/cm3) is less than that of silver (10.5 g/cm3) (see Figure 2-17(c)). Therefore, when the silver melt cools rapidly, the nickel particles are mainly distributed above the silver layer.

When the Ni particles in the molten pool of cathode contact material move from bottom to top, the Ni particles in the molten pool of cathode contact move to the bottom of the molten pool because the bottom of the molten pool is located on top of the molten pool, resulting in the final distribution of Ni particles mainly at the bottom of the molten pool of cathode contact and silver particles mainly inside the molten pool of cathode contact (see Figure 2-13(c) and Figure 2-17(d)). On the anode contact, when the Ni particles inside the molten pool move from the bottom to the top, the Ni particles move to the surface of the molten pool because the surface of the molten pool is located on top of the molten pool, making the Ni particles mainly distributed on the upper surface of the molten pool, while the silver is mainly distributed inside the molten pool. However, the elemental surface distribution results show that the Ni and Ag elements are distributed in layers on the anode contacts (see Figure 2-14(b), Figure 2-14(c) and Figure 2-17(e)), which may be the result of the action of contact pressure cycling.

 

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