Introduction
Silver Contact Rivet is a key component in low-voltage electrical appliances, and its performance directly affects the stability and reliability of electrical appliance operation. Among electrical contact alloy materials, silver alloy materials are the most important electrical contact materials with the largest amount of precious metals. In order to improve the performance of electrical contacts and achieve the purpose of saving silver, a series of silver-based electrical contact materials have been developed, including AgCdO, AgSnO2, AgZnO, AgNi, AgW, AgC, etc. Among many silver-based contact materials, AgCdO contact materials are widely used due to their many advantages such as arc resistance, welding resistance, electrical and mechanical wear resistance, corrosion resistance, and low and stable contact resistance. They can be used in a variety of low-voltage electrical appliances with currents ranging from a few amperes to several thousand amperes, and are called "universal contacts". However, since Cd is toxic and poses a hazard to the human body during manufacturing and use, the EU market has banned the use of AgCdO contact materials since June 2006.
AgZnO Electrical Silver Contact material is one of the alternative materials for AgCdO. It is an environmentally friendly electrical contact material developed in the late 1960s and early 1970s. AgZnO electrical contact material has the characteristics of resistance to burning, welding, electrical wear, low and stable contact resistance, resistance to large current impact, good breaking performance, short arcing time, electrical corrosion resistance, and non-toxicity. Therefore, it has been used in air circuit breakers, leakage circuit breakers, small circuit breakers, contactors, disconnect switches, transfer switches, and protection switches. The alloy powder pre-oxidation method produces environmentally friendly silver zinc oxide contact materials. It is easy to process and has excellent electrical properties. It is a new type of contact material with broad market prospects.
AgZnO alloy powders with different silver contents were prepared by alloy powder pre-oxidation process. Wires with the same state specifications were obtained after isostatic pressing, sintering, extrusion and drawing. The mechanical and physical properties, metallographic structures, etc. were compared, and the differences in metallographic structures and mechanical and physical properties of wires with different contents were analyzed. The electrical properties of the integral rivets made of wire were tested, and the electrical properties of AgZnO contact materials with different contents were analyzed, providing a reference for the development and application of contact materials of this system.
1 Experimental method
The test was prepared using 99.99% silver plates and 99.99% Zn ingots from the same batch. The samples were prepared by alloy powder pre-oxidation method, and processed into wires through atomization powder making, alloy powder pre-oxidation, isostatic pressing, sintering, extrusion, drawing and other processes. The mechanical and physical properties of the wires were tested and compared; the rivets were made into an integral Silver Electrical Contact manufacturer, and the specifications of the rivets were: dynamic point R3×0.5(0.25)+1.5×0.6SR10 static point F3×0.6(0.25)+1.5×0.6E, assembled into relays, and the electrical life was verified under AC 250 V/10 A.
The resistance of the samples was tested by TH2512B intelligent current low resistance group tester; the metallographic structure of the materials was analyzed by L150 metallographic microscope; the hardness of the samples was measured by DHV-1000Z video microhardness tester; the tensile strength of the samples was measured by an electronic universal testing machine; the microstructure morphology of the samples and the surface morphology of the rivet samples after the test were observed by scanning electron microscope (SEM); the electrical life was verified by AC resistive load test system.
2 Results and analysis
2.1 Metallographic structure analysis
Figure 1 shows the metallographic structures of the cross-section and longitudinal sections of the finished wires of AgZnO(8), AgZnO(10), and AgZnO(12) with different ZnO contents (a and b are AgZnO(8), c and d are AgZnO(10), and e and f are AgZnO(12)). By comparison, it can be seen that the alloy powder pre-oxidation method can successfully prepare uniform AgZnO(8-12). ZnO is dispersed and evenly distributed in the Ag matrix, but there is very little ZnO aggregation. With the increase of ZnO content, the number of ZnO particles per unit area increases, and the particle aggregation phenomenon inside the material tends to increase, but the overall tissue distribution is still relatively uniform.
2.2 Mechanical and physical properties analysis
Figure 2 shows the distribution probability of mechanical and physical properties of wires with a diameter of 1.920 mm in the annealed state. Figure 2(a) shows the resistivity distribution probability. It can be seen that with the increase of ZnO content, its resistivity has a significant increasing trend. The resistivity of silver metal oxide Silver Contact Points material is controlled by parameters such as material composition, oxide volume fraction, particle size and its distribution in the Ag matrix [10]. With the increase of ZnO content, the ZnO volume fraction increases, the increase of particle interfaces leads to increased electron scattering inside the material, and the material body resistance gradually increases; Figure 2(b) shows the hardness distribution probability. It can be seen that with the increase of ZnO content, the hardness has a significant increasing trend. This is because the content of metal oxides distributed in the Ag matrix increases, and the particle dispersion strengthening effect is enhanced. Similarly, dispersion strengthening leads to a significant increasing trend in tensile strength, as shown in Figure 2(c). In summary, with the increase of ZnO content in AgZnO material, the resistivity, hardness and tensile strength of the material have a significant increasing trend.
2.3 Electrical life verification
Rivets were made from 1.920 mm diameter annealed wire, with the specifications of Silver electrical contacts: dynamic point (R3×0.5(0.25)+1.5×0.6SR10) and static point (F3×0.6(0.25)+1.5×0.6E). The rivets were post-processed and assembled into relays for electrical life verification. The test conditions are shown in Table 1. Figure 3 shows the electrical life data of relays made of AgZnO(8), AgZnO(10), and AgZnO(12). It can be seen that under the conditions of 250 V and 10 A, within the 95% confidence interval, the electrical life of AgZnO(8) material is the longest, with an average electrical life of 202,029 times; the electrical life of AgZnO(10) material is between AgZnO(8) and AgZnO(12), with an average electrical life of 149,941 times; the number of evaluated electrical life of AgZnO(12) material is the least, at 98,665 times.
Comprehensive comparison shows that under the condition of small current within 20 A, all three materials can meet the electrical life requirement of 100,000 times, but with the increase of ZnO content in AgZnO contact material, its Silver contacts for Relay electrical life shows a downward trend.
2.4 Analysis of the appearance of failed contacts
During the contact closing and disconnecting process, due to the influence of arc discharge and Joule heat, the contact surface undergoes a partial melting and solidification process, resulting in the failure of the contact to disconnect normally, which is called contact welding [10]. Figure 4 shows the appearance and energy spectrum components of failed contacts under 250 V/10 A conditions. Figures 4 (a, d, g) are SEM photos of the contact appearance morphology of AgZnO (8), AgZnO (10) and AgZnO (12) at the end of their life. Figures 4 (b, e, h) are the corresponding failure positions, and Figures 4 (c, f, i) are the energy spectrum component data of the failure area. By comparison, it can be seen that the failure position of the AgZnO (8) contact is at the edge of the contact, which contains a high content of Cu. At the end of the contact life, the silver layer has been completely consumed, and the copper layer participates in the contact, which eventually leads to contact welding failure. The failure position of the AgZnO (10) contact is close to the edge of the contact, which contains a high content of Cu. The failure position of AgZnO (12) is located inside the working surface, and the bonding position contains a high content of Cu. As the ZnO content in the contact material increases, the viscosity of the molten pool increases, which is not conducive to flow. The failure position tends to move from the outside of the contact working surface to the inside.
Arc erosion occurs on the surface of the contact during the closing and opening process, that is, the material loss caused by the evaporation and splashing of the material due to local overheating of the contact under the action of the arc. Arc erosion is essentially a physical metallurgical process such as rapid heating, melting, vaporization, flow, and solidification on the contact surface, resulting in softening, splashing, flow, cracks, etc. on the contact surface [10-12]. Contact arc erosion is mainly affected by the melting, vaporization and solidification processes. In the melting process, the micro-area of the contact surface melts and changes the original structure. Driven by the arc force and mechanical force, the molten metal flows at a certain flow rate, causing splashing and causing material loss.
As can be seen from Figure 4 (a, d, g), after the AgZnO (8) test, the contact surface was ablated relatively flat and uniform, with a few pores, and there were a lot of splashes around the working surface, which accumulated around the contacts. Because the number of tests was the largest, the splashing was serious, resulting in the complete loss of the silver layer on the working surface of the Silver contacts for Relay, and the copper layer failed after the contact. After the AgZnO (10) test, there were obvious pores on the contact surface, and there were fewer splashes around the contacts; after the AgZnO (12) test, the contact working surface was severely cracked, and the melted copper matrix splashed to the working surface, causing the welding failure. Comparing Figures 4 (a, d, g), it can be seen that with the increase of ZnO content, the cracking trend of the contact failure surface increases, which is caused by the cooling and shrinkage of the contact. After the arc is extinguished, the contact surface cools rapidly, the surface molten pool solidifies, and the liquid phase is transformed into a solid phase, and the surface solidifies and shrinks. Studies have shown that cracks and holes formed on the surface of silver metal oxide contacts will inevitably cause the surface area structure to become loose, which in turn increases the amount of arc erosion and the contact resistance. With the increase of ZnO content, the tendency of cracks and pores increases, the amount of arc erosion increases, the contact resistance becomes higher, the temperature rise is abnormal, and the loose internal structure leads to contact failure.
Comprehensive comparison shows that with the increase of ZnO content, when the AgZnO (8-12) contact material fails, the contact position moves from the outside to the inside of the working surface, and the tendency of cracks and pores on the contact surface increases, resulting in a decrease in the electrical life of the contact.
3 Conclusions
The alloy powder pre-oxidation method can successfully prepare electrical contact materials with a ZnO content of 8% to 12%. With the increase of ZnO content, the resistivity, hardness and tensile strength tend to increase, and the aggregation of ZnO particles inside the material tends to increase; under the condition of small current within 20 A, with the increase of ZnO content, the electrical life tends to decrease, and the electrical life verification performance of AgZnO(8) material contacts is the best, which can reach more than 200,000 times; with the increase of ZnO content, under the action of arc, the surface cracking and porosity of Silver electrical contacts increase, and the electrical life tends to decrease.
Our Silver electrical contacts are carefully crafted high-quality products. They are made of high-purity silver and have excellent electrical conductivity, which allows smooth current transmission and greatly reduces energy loss. The exquisite manufacturing process makes the contacts closely combined with the base material, firm and reliable, able to withstand frequent opening and closing operations and not easily deformed or damaged. In various complex electrical environments, they can function stably, whether in high temperature, high humidity or vibration conditions, they can ensure the safe and stable operation of electrical equipment.
