ABSTRACT: two kinds of alloys, copper (s) and copper (Te) , were prepared. The effects of extrusion parameters and cold deformation on mechanical properties, electrical properties and microstructure of the two alloys were studied, it provides a reliable basis for the technological parameter design of copper bar and tellurium bar. As-cast sulfur and tellurium alloy elements form compounds sulfur Cu2S and Cu2Te with copper, which form eutectic with copper at higher temperatures and distribute in the copper matrix as dispersed and soft second phase particles to refine the grains, the cutting performance of the alloy is improved obviously, and the effect on the electrical conductivity is small. 

Key Words: Second Phase; dispersion; extrusion; classification number: 146.11 Ref. ID: A

Precise conductive devices not only require high conductivity, because of the complex shape, but also require a precise size, smooth surface, most of them have to be machined, so also require materials have excellent machinability. Although pure copper has excellent electrical and thermal conductivity, its cutting performance is poor, so the most important conductive device is usually made of copper alloy containing tellurium. While retaining the excellent properties of copper alloys, the conductivity and thermal conductivity are not much reduced, and can be produced by conventional methods. S, se, Te have been successfully added to copper and copper alloys in the United States. They can not be dissolved in the base metal and are dispersed in the intercrystalline or intracrystalline phase in the form of a second phase, in addition, the second phase formed by them is very soft (the microhardness is lower than Cu2O)[2] , which makes the chip easy to break and improves the material cutting performance. As a free-cutting material, chalcogen (14720) tellurium (14500) alloy has not been studied sufficiently in our country at present. A large amount of technical data has been accumulated in the trial production of Non-ferrous metal, which provides a reliable basis for the process parameter design of chalcogenide and telluride.

1. Objective and methods to explore the change law of microstructure and properties of Chalcogen (C14720) and tellurium (C14500) and the reasonable process parameters, and to work out the production process of bar which meets the technical requirements of C14720 and C14500 in Astm B124. Trial-produced product specification and state: 30 ± 0.06H50(drawn) S25.4 ± 0.10H50(drawn)1.1 alloy preparation (cast)(1) using equipment: power frequency 600kg electric furnace;

(2) Ingot Size: 223(370 ~ 400) mm;

 (3) casting temperature: 1200 ~ 1220 °C; 

(4) covering agent: flue-cured Ash;

 (5) adding P-Cu master alloy as Deoxidizer, phosphorus content C14720 is 0.02% , C14500 is 0.01% . (6) due to the easy burning of sulfur, sulfur powder shall be inserted into the sealed ends of the thin-walled copper tubes, and te shall be wrapped with a thin strip of red copper and then added. C 14720 s: 0.45% and C 14500 te: 0.55% were used to observe the microstructure of the cast samples, and the metallographic images were taken.

1.2 to observe and test the difference of mechanical properties and metallographic structure between water-sealed extrusion and air-cooled after extrusion, and to carry out comprehensive mechanical properties test after extrusion, including tensile strength, hardness, conductivity, elongation, grain size. Size of extruded blank: 33(3000 ~ 3500) MM1.3 draw hardening curve draw the extruded blank into bar, draw metallographic specimen and comprehensive mechanical property specimen according to drawing pass, carry out material hardening property experiment, draw material hardening curve. Cold working test procedure for extruded blank:

2. Experimental results 2.1 study on extrusion process parameters

The extrusion cooling condition has no obvious effect on the electrical conductivity, grain size and mechanical properties of the extruded bar. The conductivity of tellurium copper was 89% IACS in water quenching and air cooling, and the conductivity of chalcogen was 84% iacs in air cooling. The reason why the conductivity of Chalcos and tellurium is not affected by the cooling method is that the solubility of chalcos and tellurium in copper is very small, and they are dispersed in the form of the second phase, which results in the relatively small lattice distortion, the low strength of the stress field results in the excellent conductivity of the alloy. Of particular importance is the fact that the solubility of sulphur and tellurium is very small even at high temperatures, that of tellurium does not exceed 0.01 per cent at 800 °C and that of sulphur in copper is even less than 0.002 per cent at 800 °C, therefore, these two alloys can not be hardened by quenching and aging heat treatment. The eutectic temperature of the two alloys is 1067 °C for chalcos and 1084 °C for tellurite, which will not cause hot embrittlement due to eutectic melting, and the cooling method will not affect the properties of the alloys. From the data of extrusion test, it can be observed that the strength and hardness of the two alloys have no obvious change, but the extensibility decreases slightly, however, the grain size of extruded cu-s and cu-te alloys is obviously lower than that of pure cu, which shows that alloying elements can obviously increase the recrystallization temperature of the alloy matrix.

2.2 metallographic observation

The metallographic microstructure is observed in Fig. 1. Photo 1 shows the metallographic structure of cast tellurium copper (uneroded) . The White Matrix is a solid solution, and the blue-gray precipitate is a compound of tellurium and copper (Cu2Te) , which forms a eutectic with the solid solution at 1084 °C, and forms a second phase (+ Cu2Te) in copper. Photo 2 shows the metallographic structure of cast sulfur copper (uncorroded) . The White Matrix is solid solution, and the precipitates with blue-gray granular dispersion distribution are eutectic (+ Cu2S) . It can be seen from the photographs that the particles in the as-cast microstructure are relatively large, and the second phase particles are spherical in shape with a maximum particle size of 10 m. It is pointed out in reference [4] that the effective strengthening particles must be uniformly distributed and of sub-micron size, so the second phase particle fineness of copper telluride and copper sulfide is not enough and the strengthening effect is small. Photographs 3 and 4 microstructure of copper-sulphur and copper-tellurium alloys as extruded, the Matrix has been recrystallized, and the second phase particles are chain and strip. Photographs 5 and 6 are metallographic photographs of deformed bar specimens of copper sulfide and copper tellurium. The microstructure of the deformed alloy consists of the matrix phase elongated along the processing direction and the chain-like and strip-like second phase particles formed by compounds Cu2Te and Cu2S. According to the strengthening theory, if the second phase particle is a soft phase, when the dislocation moves, the dislocation line can easily get rid of the pinning of the particles and climb over the particles, and continue to slide forward, so that the second phase is dragged into a chain-like distribution of particles, that is, the second phase particles are easily cut or even crushed by dislocations, and deform together with the Matrix. Therefore, the strengthening effect on the material is very weak. This is consistent with the conclusion from the property tests that the properties of chalcogenide and tellurium differ very little from that of pure copper, causing only a small loss of extensibility.

Cold working properties of 2.3 alloy

Sulfur-tellurium alloy has good cold working ability, it can be directly processed from 34mm extruded bar to 18mm by multi-pass cold drawing without intermediate annealing, and the working hardening rate is lower than other copper alloys. The Comprehensive Mechanical Properties of Chalcogen (C14720) and tellurium (C14500) are shown in Fig. 2. Figure 2 shows the hardening behavior of chalcogenide and chalcogenide. It is clear that the coincidence of the two curves is quite good, indicating that chalcogenide and chalcogenide have relatively uniform properties and hardening rates. The rate of hardening is even, the strength and hardness increase slowly, but the elongation decreases rapidly to a lower value at the beginning of cold deformation, and the rate of ductility loss is obviously higher than that of pure copper. There is no obvious difference in the properties of the bar produced by the extrusion blank with quenching and air cooling. The hardening speed of chalcos and tellurium is similar to that of pure copper, but the elongation is slightly affected. Large cold working can only decrease the conductivity of the alloy by about 2% , and the decrease of the conductivity is small, which indicates that the conductivity of the alloy is not sensitive to cold working.

2.4 according to Literature 5, there are many factors affecting the cutting performance of materials. In addition to the correct selection of cutting conditions, the recrystallized grains are uniformly fine, and the second phase is fine and uniformly distributed, the effect of the second phase is even better than that of increasing the content of the alloy. Free-cutting copper for tensile processing, can increase the shear force, coupled with flexible

The influence of soft particle can greatly reduce the cutting resistance and is beneficial to improve the cutting performance of materials to a certain extent. But the cold working capacity is not too large. From the test data of our factory, the cutting performance is best when the cold working capacity is less than 50% under the condition of the alloy strength. The cupric sulfide and Cupric tellurium produced in our factory are drawn products with fine microstructure, the grains elongate along the processing direction, and the second phase also deforms along with them, compared with the free-cutting brass (61% cu, 3% pb, 36% zn) , the free-cutting brass (61% cu, 3% pb, 36% zn) has a strip or chain distribution, the turning rate of pure copper is only 20% , and the turning rate of Chalcos and tellurium can reach above 80% . 3. Conclusion 

(1) The cooling condition has no obvious effect on the electrical conductivity, grain size and mechanical properties of extruded bar. The strength and hardness of extruded bar are similar to that of pure copper, but the ductility decreases slightly and the grain size is refined obviously. 

(2) tellurium and sulfur both form stable intermetallic compounds Cu2Te and Cu2S in copper, which are dispersed in network and point shape. The second phase particles are very soft and can be deformed into strips and chains together with the Matrix. The strengthening effect on the alloy is weak and the recrystallization temperature is higher than that of the Matrix metal. 

(3) the alloy has excellent cold working property, uniform working hardening speed, rapid elongation loss, and the conductivity is not sensitive to cold working. (4) excellent cutting performance of finished products.

 Source: Chinanews.com, by Wang Yanjie

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