ABSTRACT: High Purity oxygen-free copper-silver Alloy with high conductivity, high softening temperature and high strength is a new material for series-excited motor commutator. The effects of chemical composition, melting process and processing technology on the properties of the alloy were studied. 1. The commutator is one of the important parts of DC motor. Copper (T1) and oxygen-free copper (TU1) are the traditional materials to manufacture the commutator. With the development of manufacturing technology of micro-motor, the speed of motor is higher and the volume is smaller. The electromagnetic load of the motor increases and the temperature rise increases, so the performance of the commutator material is more demanding. Due to the low strength and low softening temperature of red copper and oxygen-free copper, it is obvious that they can not meet the requirements of micro-special motor commutators, so new materials must be developed. High Purity oxygen-free copper-silver alloy o ≤10010-6, total inclusion ∑≤10010-6, Cu + Ag ≥99.99% (AG0.02% ~ 0.10%) , because the content of oxygen and inclusion is very low, the grain boundary is purified, and the probability of grain boundary embrittlement and crack is remarkably reduced, the toughness and electrical conductivity of the alloy were improved, and the mechanical properties and softening temperature of the alloy were significantly improved with little decrease in electrical conductivity due to the addition of a small amount of silver. The standards of oxygen-free Copper and copper-silver alloys in China are compared with those of Japan and the United States as shown in Table 1. As can be seen from Table 1, the allowable oxygen content and inclusions of oxygen-free copper in China are much higher than those in similar standards abroad, so it is very important to develop high quality oxygen-free copper. After many years of experimental research, we found that the main properties of high purity oxygen-free copper-silver alloy (Tuoag0.06 alloy) can catch up with the international advanced level and can replace the imported materials. Table 2 shows the comparison of chemical composition and main properties between Tuoag0.06 alloy and foreign oxygen-free copper-silver alloy. 2. Test method 2.1 range of silver content in order to improve the tensile strength and hardness of copper, alloy elements can be added and cold deformation degree can be increased, but the electrical conductivity of copper can be reduced to some extent. High Purity oxygen-free copper-silver alloy with AG0.02% ~ 0.10% was selected to maintain high electrical conductivity. 2.2 according to Gibbs adsorption theory, most inclusions with low melting point will be enriched at grain boundary or interface. [2] the local segregation concentration of Bi at grain boundary determined by tracer method is 103 times higher than that within the grain boundary. As the grain boundary itself is a transition zone with more defects, only 110-6 low melting point inclusions exist in the metal, the segregation of PB and Se at grain boundary is about 0.1% , which is similar to Bi, PB and Se are also easily segregated at grain boundary. In addition, the brittle compounds in copper (Cu2O, Cu2S, Cu2Se, etc.) tend to aggregate or precipitate at Grain Boundaries, which has a series of adverse effects on the properties of the alloy. Generally there are about 500 ~ 10 ~ 6 oxygen and inclusions in electrolytic copper, which are difficult to be removed by chemical methods. By smelting in vacuum, the harmful elements such as PB, Bi and SB can be removed by volatilizing the elements with higher vapor pressure. Therefore, ZG-25 vacuum induction furnace was used to melt and cast ingots, and high purity graphite crucible and iron mould were selected. Each furnace load 25kg, the copper one-time load into the crucible, AG load into the hopper, the raw materials must be loaded with high purity, clean without rust. When the vacuum degree reaches 0.6 PA, the power is sent, and the gas is discharged as far as possible during the melting period. After the charge is completely melted, the power and vacuum are increased, and the proper refining time is kept, so that the gas in the copper liquid is further discharged and the inclusion is accelerated to volatilize. Therefore,

The vacuum is controlled at 0.5 ~ 0.6 PA, the temperature is controlled at 1200 ~ 1270 °C, and the refining time is 30 ~ 40 min. In the pure copper liquid, good deoxidation, moderate temperature can be added in batches of AG material, high-power stirring, so that Ag homogenization. At about 1200 °C, pouring should keep the alloy liquid rising steadily. 2.3 before hot rolling, the riser and milling surface of the alloy ingot should be cut off and the surface treated alloy ingot should be heated in a resistance furnace (to keep the local reducibility atmosphere to prevent oxygen penetration) to 890 °C, after holding for 20 ~ 30min, hot rolling (two-high reversing mill) is carried out. The final rolling size should be controlled with 20% ~ 80% cold deformation (or 10% ~ 90%) . After hot rolling, copper-silver Alloy Strip with thickness of 2mm must be cold rolled by pickling and surface cleaning, and then cold rolled by two-high cold rolling mill. 3. Test results and discussion 3.1 control of chemical composition chemical composition analysis of 6 furnace alloy is carried out. The results are shown in table 3. The results show that o ≤510-6 and total inclusion ∑≤10010-6 of cu-ag alloy can be obtained by controlling the proper vacuum degree (0.6 ~ 0.7 PA) , refining temperature (1200 ~ 1270 °c) and refining time (30 ~ 40 Min) , the Standard of ASTMF68-77 in USA and JISH3510-86 in Japan has been met, and the recovery of AG is more than 96% . The results show that high purity oxygen-free copper-silver alloy can be produced by one-time vacuum melting using electrolytic copper as raw material. 3.2 The effect of Ag content on the electrical conductivity of the material required by the commutator of a series motor: soft I-ACS ≥99.5% , hard IACS ≥97.5% . Any element added to copper will reduce its electrical conductivity to some extent, and AG is the element that slowly reduces conductivity. Therefore, the content of AG in copper should be reduced as far as possible under the premise of ensuring the strength and hardness of the alloy. The electrical conductivity of a high purity oxygen-free copper-silver alloy (in soft state) containing 0.02%-0.09% agwas measured, as shown in Fig. 1. IACS ≥99.1% with AG0.09,% and IACS ≥99.5% with AG0.06,% . As a high conductivity alloy, the AG content should be controlled around 0.06% as TUOAG0.06.

3.3 effect of cold deformation degree on electrical conductivity high purity oxygen-free copper-silver alloy is used after cold work hardening. In order to compare the relationship between cold deformation degree and electrical conductivity, the TUOAG0.06 alloy is rolled with different cold deformation degree (10% ~ 90%) , its effect on conductivity is shown in figure 2. The cold deformation degree increases and the electrical conductivity decreases because of the change of the state of grain boundary, the increase of lattice distortion, dislocation and lattice length. When the cold deformation degree is 80% , the electrical conductivity is about iacs 97.5% ,

The cold deformation of high purity oxygen-free copper-silver Alloy Tuoag0.06 should not exceed 80% . 3.4 The tensile strength and vickers hardness of TUOAG0.06 alloy with 10% ~ 90% cold deformation were measured. The results are basically consistent with reference [5] . The hardness of pure copper is 124hv after cold deformation of 90% . When ag0.04% is added to copper, the hardness of pure copper can reach 137HV under the same cold deformation condition, which shows that the addition of micronutrient can improve the mechanical properties of the alloy significantly. The effect of cold deformation degree on mechanical properties of Tuoag0.06 alloy is shown in Fig. 3. When the cold deformation degree is below 40% , the hardness and strength increase slowly with the increase of the cold deformation degree; when the cold deformation degree is between 40% and 80% , the hardness and strength increase greatly, and the slope of the curve becomes steeper; The hardness and strength increase with the increase of cold deformation degree. Therefore, the strength and hardness of high purity oxygen-free copper-silver alloy can be improved effectively by cold deformation, and the new mechanical properties can be satisfied by controlling the cold deformation degree.

3.5 the effect of annealing temperature on the hardness of cold deformed TUOAG0.06 alloy (130HV5) was annealed at 50 ~ 350 °C for 2 H. The relationship between vickers hardness and annealing temperature is shown in Fig. 4, which also shows the annealing softening curve of Tuoag0.06 alloy after cold deformation. At 250 °C, the hardness of the alloy is 106HV5, which is equivalent to 80% of the hardness before annealing, and is consistent with the softening temperature of 250 °C.

[7] it is considered that the softening temperature of copper is mainly related to inclusions and micronutrient, and that the micronutrient of solid solution to copper matrix and the impurities that form dispersed precipitates can effectively block the movement of dislocations, thus increasing the softening temperature of copper, to a certain extent, it will increase with the increase of micronutrient. In general, the softening temperature of pure copper is 150 °C. after the copper contains 0.06% Ag, the softening temperature is increased to 250 °C. 4. Conclusion (1) high purity oxygen-free Cu-Ag alloy, O ≤510-6, total inclusion ∑≤10010-6, Cu + Ag ≥99.99% can be obtained by vacuum melting process with proper vacuum degree, refining temperature and refining time, the recovery rate of AG was over 96% . (2) when cold deformation is controlled at 80% , the electrical conductivity of TUOAG0.06 Alloy is iacs & GT; 98% , hardness is 130 HV5, B is 450 MPA, softening temperature is 250 °C. Its comprehensive performance reaches the advanced level of similar products abroad. 

Source: Chinanews.com, by Sun Xiangming

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