Preparation of CuCr25 alloy by the hottest vacuum

CuCr25 alloy prepared by vacuum induction melting

Abstract: in this paper, cucr25w1ni2 alloy was prepared by vacuum induction melting. The effects of different compositions (25%cu and 75%cu) and Cr grain size on physical properties and breakdown voltage were studied, and the effects of alloy elements and microstructure on electrical breakdown properties of cucr25w1ni2 alloy were discussed. The results show that W powder can significantly refine the CR phase and strengthen the CR phase at the same time; After electric breakdown, the very fine W powder in the molten layer can play the role of non spontaneous nucleation core, and the surface of cucr25w1ni2 molten layer is more flat; On the other hand, Ni can promote the mutual dissolution of Cu and Cr and strengthen the alloy as a whole. These two elements can significantly improve the voltage strength of the alloy

key words: voltage withstand strength of CuCr contact material by vacuum induction melting 1 Introduction

experiments show that increasing the Cu content in CuCr alloy can increase the thermal conductivity of the material, reduce the temperature of the contact surface during the breaking process and reduce the thickness of the melting layer, so as to reduce the reignition of the arc when the current crosses zero [1, 2]

at present, the international production of CuCr25 later mainly includes powder mixing sintering and vacuum consumable smelting [3]. CuCr25 alloy prepared by mixed powder sintering method of Westinghouse is not of ideal quality, such as high oxygen content, low density and low production efficiency. Siemens adopts vacuum consumable smelting method to prepare CuCr25 alloy, which has excellent product performance, but the preparation process is very complex, and CuCr25 consumable electrode needs to be made in advance, with long production cycle and high product cost. Recently, CuCr25 and CuCr25 w1co1 alloys [4, 5] have been prepared by vacuum arc melting with an increasing number of automobiles. However, the characteristic structure of these two alloys is Cr dendrite, which is not conducive to the further improvement of voltage withstand strength

CR phase in CuCr contact material is the weak phase of electrical breakdown. The research shows that the microstructure of contact material has a great influence on its electrical breakdown behavior. For example, the refinement of Cr phase size can improve the voltage withstand strength and reduce the cut-off value. At the same time, there is no significant change in breaking performance and fusion welding resistance [6 ~ 8]. The vacuum induction melting method of CuCr25 series alloy is studied in this paper. As a heterogeneous nucleating agent, W powder refined and strengthened the CR phase, and Ni strengthened the alloy as a whole through solid solution. As a result, the voltage strength increased significantly. The raw material used is Cr block instead of Cr powder, and its content is reduced from 50% to 25%, so the cost is reduced. The whole production cycle is about 2 hours, and the output is large, which is suitable for large-scale production of contact materials

2 test process

cucr25w1ni2 alloy is prepared by vacuum medium frequency induction melting. The raw materials are Cu block, Cr block and W powder (particle size is 0.6 ~ 3) prepared according to the mass ratio of 72 ∶ 25 ∶ 1 ∶ 2 μ m) And Ni block. Put Cu, Cr and Ni blocks into the crucible and pump high vacuum (10. Isolation method 1 × 10－2～3 × 10-2 PA) for melting, Archimedes drainage method is used to measure density, fqr-7501 eddy current conductivity meter is used to measure conductivity, Brinell hardness tester is used to measure hardness, and LECO infrared oxygen and nitrogen gas analyzer is used to measure oxygen and nitrogen content

process the alloy into 20 × The surface of a 5 mm sample is polished and loaded into a vacuum arc extinguishing chamber modified from a tdr-40a single crystal furnace. The sample is a cathode, and the polished w rod is an anode with a radius of 5 mm and an edge radius of 1 mm. In order to remove the gas adsorbed by the electrode, raise the temperature in the furnace to 500 ℃ and the vacuum degree to 1.5 × 10-3 PA, hold for 30 min. When the cathode is cooled to room temperature, apply a DC voltage of 8 kV between the electrodes, and let the cathode move towards the anode at the speed of 0.2 mm/min. when electric breakdown occurs, stop moving, and record the distance between the two poles to calculate the breakdown electric field strength. Then move down the cathode to prepare for the next test. Each sample is measured 100 times. In order to eliminate the influence of aging [9], the last 60 points are taken when calculating the breakdown field strength. The microstructure of the alloy was observed by scanning electron microscope, and the composition distribution of the alloy was determined by energy spectrum analysis

3 experimental results and discussion

3.1 composition and micro morphology

under the action of electromagnetic stirring during smelting, Cu and Cr are fully mixed, and the segregation tendency of Cr under the influence of gravity under the condition of rapid solidification is reduced. See Table 1 for the composition analysis

Figure 1 shows the typical structure of CuCr25 alloy prepared by VIM, showing CR dendrites. It can be seen that it is difficult to further refine C by water-cooling solidification alone. You may say that the R phase of the original six gear machine of a manufacturer, and most of it exists in large dendrites, which is not conducive to the heat dissipation of Cr phase to Cu matrix during the operation of the circuit breaker, so the voltage strength is reduced. Therefore, Cr grains must be further refined by alloying. Figure 2 shows the microstructure of CuCr25 w1ni2 alloy prepared by vim. It can be seen that the CR phase is uniformly distributed in the Cu matrix in granular form. Because W powder is used as the non spontaneous nucleation core during solidification, the obtained CR grain size has reached 5 ~ 15 μ m. Dendrites are eliminated. The gray white spots in CR grains are mainly W by energy spectrum analysis, and their boundaries are fuzzy. According to the Cr-W phase diagram, it is α Solid solution, w can effectively selectively strengthen CR phase

the characteristic morphology of Cr phase in cucr25w1co1 alloy prepared by vacuum arc melting is still dendrite [5]. Because the w sheet and Cu, Cr and co blocks are melted together by electric arc, the sheet like w does not play the role of heterogeneous nucleating agent

ni can reduce the conductivity to a certain extent, but it can strengthen the alloy by solid solution. According to thermodynamic calculation, Ni can promote the mutual solution of Cu and Cr. under rapid solidification, a large number of Cr rich particles precipitate from the Cu matrix, which can strengthen the whole alloy, as shown in Figure 2. Since Ni has a large solid solubility in both Cu and Cr, the addition of Ni can also improve the wettability of Cu and Cr phases, reduce the micropores generated during solidification, and improve the compactness of the material

3.2 physical properties and breakdown field strength

Table 2 shows the performance test results of cucr25w1ni2 and other alloys prepared by vim and other methods. It can be seen that cucr25w1ni2 alloy has fine CR grains, low oxygen content, good conductivity and significantly improved voltage withstand strength

for cucr25w1ni2 alloy, Cr phase is affected by α The solid solution is strengthened and evenly distributed. At the same time, Ni has strengthened the alloy as a whole. The hardness of CuCr25 w1ni2 is higher than CuCr25, and the ability to resist surface deformation caused by electric field is greatly enhanced, which can make the back arc