Carlos CAMURRI, Claudia CARRASCO, Soraya ALBRETCH

Transcription

1 IMPURITIES ON CATHODIC COPPER: STUDY OF THEIR INFLUENCE ON THE DUCTILITY OF COPPER WIRES AND DEVELOPMENT OF MECHANICAL TESTS SENSIBLE TO SUCH IMPURITIES Carlos CAMURRI, Claudia CARRASCO, Soraya ALBRETCH University of Concepción, Materials Engineering Department, Edmundo Larenas 270, Concepción, Chile Abstract The main characteristic of cathodic copper is the concentration of impurities because it depends on mechanical characteristics, i.e. ductility, and the sales price. International standards permit impurities contents not exceeding 132 ppm, and 100 of them associated to the oxygen. The certification of impurities concentration of cathodes is made by means of in-situ chemical analysis and by mechanical tests to the wires obtained by melting of cathodes, continuous casting and wire drawing of the produced rods. These standard mechanical tests that evaluate ductility, are quick elongation and spiral elongation tests. The results of these essays shown that there is not a clear correlation among the impurities content in cathodes and wire ductility, ignoring which impurities are the source of the premature fracture in wires. So, it is common to see cathodes with higher impurities concentration and elevated ductility when wire, or notorious differences among ductility results of wires coming from the same cathodes but obtained by different laboratories. Also there is a poor correlation between these two tests determining ductility. In this way, it is not clear the maximum acceptable impurities level in cathodes from the point of view of drawability of copper rods; also, the mechanical tests actually used are not able to discriminate differences on the copper ductility associated to variations in impurities concentration, at ppm level. In this work, a design of new specimen for traction test that permits clearly discriminate differences on copper ductility associated to variations in impurities concentration is exposed, and preliminary and interesting results respect to the influence of different impurities in copper ductility are also shown. 1. INTRODUCTION Copper cathodes are the input material in the production of electrical wires. The maximum impurity levels associated to cathodic copper is regulated by international standards, ASTM B115 or its equivalent BSEN , where the maximum permitted concentration of impurities is 132 ppm, with 100 of them corresponding to the maximum oxygen content, as can be seen in table 1 [1]. These impurities concentration obey more to historic operational conditions in Copper refineries or growing restrictions of environmental type, than to the real problem that is lacks on the ductility during the drawing of copper wires. Prove of this situation is the considerably higher maximum admissible impurities on continued casting rods of 8 mm diameter used for the drawing of electrical wire, which are also shown in table 1. Since more than twenty years ago is well know the deleterious effect on copper ductility of impurities such as Selenium, Lead, Bismuth, Antimony, Oxygen and others [2-5]: Bismuth concentration over 20 ppm produces a severe brittleness manifested by a cohesion loss in the matrix when it segregates preferably in grain boundaries. The same behaviors have lead and antimony, both also practically unsolved in solid copper. Respect to the oxygen, contents over 36 ppm produce a cuprous oxide segregation onto grain boundaries of copper, diminishing its ductility. However, when oxygen concentration is higher and there are also other impurities elements, a positive effect in copper is generated. In fact, oxygen contents between 175 to 450 ppm, would be profitable for controlling the gas-metal reactions in foundry, due to the formation of unsolved

2 oxides with other present impurities and for inducing low grain size during recrystallization of copper when it is rolled into rods for a posterior wiredrawing [6]. On the other hand, higher oxygen content produces troubles in wiredrawing process of wires, when the reminder segregates in grain boundaries forming cuprous oxide and producing a premature brittleness of wire [6]. Nevertheless, when it is present at impurities level in Copper, at ppm level or traces, there are no conclusive studies that show its effect on copper ductility, and neither about the combined effect of other impurities. Table 1: International acceptable impurities contents in cathodic copper grade A and 8 mm copper rods. Impurity element International standard (max. ppm LME) Copper Cathodes Codelco Standard (max. ppm Chile) Copper Rods 8 mm diameter (ppm) Selenium (Se) 0,5 0,01 2,0 Tellurium (Te) 1,0 0,05 2,0 Bismuth (Bi) 0,1 0,005 1,0 Antimony (Sb) 0,5 0,02 4,0 Arsenic (As) 0,5 0,005 5,0 Tin (Sn)v 0,5 1,8 No information Lead (Pb) 0,1 0,01 5,0 Iron (Fe) 3,0 3,4 10,0 Nickel (Ni) 2,0 0,19 No information Sulphur (S) 12,0 3,4 15,0 Silver (Ag) 10,0 0,22 25,0 Oxygen (O) 100, ,0 Nowadays, the characterization and certification of impurities contents on cathodes is carried out by means of chemical analysis made in the refineries and, additionally, by mechanical essays done, in the case of Chilean copper refineries, at foreign laboratory, as quick elongation test (AR) and spiral elongation test (SEN), to wires obtained by melting of cathodes, continuous casting, wire drawing and annealing of the rods. The results of chemical analysis and mechanical tests made in several laboratories show that there is not clear correlation between impurities content in cathodes and wire ductility, ignoring which impurities or precipitates are the source of the premature fracture in wires. So, it is common to see cathodes, with high impurities concentration, afford mayor ductility at wire level, as can be seen in Fig. 1 a). Also it is common to verify notorious differences among the ductility results of wires that come from the same cathodes delivered by different laboratories. Likewise poor correlation exists among essays that determine ductility, normally between the quick elongation test and spiral elongation test, as is shown in Fig. 1 b) [7]. The mentioned facts lets to conclude that, from the point of view of later drawability of rods, it is ignored what should be really the maximum acceptable impurities level in cathodes; even more, the mechanical tests used are unable to discriminate differences on the copper ductility associated to concentration variations of the impurities at ppm order. In that sense, the objective of this work is to determine which impurities or precipitates are the real causes of the ductility loss in copper wires and to develop some mechanical test that allow to clearly discriminate differences on the copper ductility associated to concentration variations at ppm level as much for cathodes as for wires. Even when this is a current project, some interesting results will be show in the next sections.

3 a) b) Fig. 1. a) Data of Chilean laboratory showing no correlation between impurities concentration and ductility; b) Data correlation between quick elongation (AR) and spiral elongation (SEN) test from freeing laboratories. 2. EXPERIMENTAL PROCEDURE Started material in our investigation was copper cathodes grade A, with thicknesses varying from 5 to 8 mm, which were chemical and mechanically analyzed. Sampling was made in agree with international standard ASTM B115, in a commonly named X shape. Analyzed cathodes were melting in an induction furnace in a carbon - nitrogen protective atmosphere, and casting in a stainless steel permanent mould obtaining cylindrical samples of 20 mm diameter and 100 mm length. These samples were wiredrawing obtaining rods of 8 mm of final diameter, with reductions of %. Rods were also chemical and mechanically analyzed. Chemical analysis was made using X-Ray fluorescence, and for oxygen content determination, a specific LECO oxygen analyzer was used for obtain more accurate results. Mechanical analyses, consisted in traction test, were made in different specimen shape. For copper cathodes three different shapes were tested, for obtain the better specimen which permits discriminate small difference in ductility of the material. The three shapes, named standard, standard with neck and with reduced gage length specimens are shown in figure 3. From the obtained results with these specimens, those are showed later, tensile test of drawing rods were made using reduced gage length specimens. Traction test was made in an Instron machine, at strain rates of 10-3 seg-1, reporting as indicative of ductility the elongation to fracture (%). It is important to note that all the samples for traction test were previously annealed at 270ºC for 10 minutes as the habitual practice for mechanical tests of pure copper rods. Finally, fracture surfaces of the samples obtained from mechanical analysis test were analyzed by means of Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) to determine focus that could be generated during traction test, where impurities or precipitates can be found, causing the fracture. Figure 3: Three different samples for tensile tests. a) Standard specimen, b) standard with neck, c) reduced gage length specimen.

4 3. RESULTS AND DISCUSSIONS 3.1. Chemical analysis The results of chemical analysis were very similar for copper cathodes and copper rods for all present elements except oxygen; this means that the fusion and casting process was made rigorously and no new impurities were added to the liquid bath. Average chemical composition is shown in table 2, and oxygen concentration can be seen in table 3. As was expected, oxygen concentration was higher in copper rods because this element is incorporated to the liquid bath mainly during casting. Table 2: Chemical composition Element Ppm Fe 2 Pb 2 As 0.2 Sb 0.2 Bi 0.05 S 25 Table 3: Oxygen concentration in copper cathodes and rods (ppm). Measurement Number Lower zone Copper cathode Central zone Uper zone Rods Average Mechanical analysis of copper cathodes This analysis was made for determine a shape of the specimen which permits to discriminate small differences in ductility of the tested material. In this sense, the main result of traction test in this case is the elongation to fracture that can be seen in Table 4, including gage length of each specimen. Table 4: Gage length and elongation to fracture in different specimens taken from copper cathodes Specimen L 0 [mm] Elongation (%) Standard Standard with neck With reduced gage length From table 4 is clear that a specimen of 5 mm diameter and gage length of 10 mm has a notorious elongation to fracture and consequent is more discriminator from the point of view of the effect of the impurities on the ductility. In this way, it was concluded that all the mechanical essays made to wire rods will be made with specimens machined with this shape. As was seen in table 3, notorious differences in oxygen concentration were found in different zones in a same cathode. Figure 4 shown traction test curves obtained from specimens taken from two different zones of the same cathode, where is clear the deleterious effect of impurities concentration reveled in the higher difference in ductility of the specimens. In a more thoroughgoing analysis, it was found that the unique elements varying significantly its concentration were oxygen and sulphur. SEM analysis of the fracture surfaces evidenced the problem induced by differences in impurities concentration of these elements in copper. Figure 5 shown photographs of fracture surfaces and table 5 shown EDS analysis in these surfaces. From these data is clear that increasing concentration in O and S promotes a brittle behavior of specimens obtained from lower central zone of the cathodes.

5 Upper central zone Lower central zone 0,0 4,0 8,0 12,0 16,0 Deformation (%) Table 5: EDS analysis of fracture surfaces in cathodes specimens (weight percent) Cathode zone % O % S % Cu Upper central Lower central Figure 4: Stress strain curves for two different zones in a same copper cathode. Figure 5: SEM images of fracture surfaces. a) Typical ductile fracture upper central zone of cathode, b) Typical brittle fracture lower central zone of cathode 3.3. Mechanical analysis of copper wire rods Traction test results made in specimens taken from wire rods are shown in table 6, and SEM EDS analysis of the fracture surfaces of these specimens are shown in figure 6. Table 6. Elongation to fracture and oxygen content in wire rods samples. Specimen number Elongation to fracture (%) O content (ppm) From these results some interesting points can be discussed: Impurities elements found in sample 1, such as Aluminum, chlorine, tin and tellurium are typical in cathodes composition, and of course also in copper rods. The fact that only in this specimen a large amount of these elements have been found confirms the non-homogeneity of chemical composition of copper cathodes. On the other hand, all these impurities elements found in specimen number 1, regarding the elongation to fracture, have no influence in traction test results, i.e., on ductility. In this sense, from the point of view of ductility, the requirements of international

6 standards for guarantee no problems during wire drawing of the rods are excessive. Another important conclusion obtained from theses results is that oxygen contents in these rods are very similar on both samples, and in this way, no significant difference in copper ductility is appreciated. Our work group is working now in defining the real influence of oxygen and sulphur in ductility of wire rods, results that will be published promptly. Figure 6: SEM EDS analysis for two fracture surfaces of wire rods. CONCLUSIONS With the development of this work, it is possible to conclude: Actual mechanical tests for determining ductility of copper cathodes and copper wire rods are no adequate, and a new specimen shape has been designed for ductility measurements in a simple traction test. Whit the proposed design, small differences in ductility due to impurities contents can be discriminated. When cathodes, copper ductility is strongly affected by oxygen and sulphur contents at ppm level. When wire rods, impurities like Te, Sn, Cl Al have not considerable influence on copper ductility, which apparently is controlled only by oxygen and sulphur contents at ppm level. ACKNOWLEDGEMENT This work has been supported by The National Council of Research in Science and Technology of Chile, CONICYT (FONDECYT project no ). The authors gratefully acknowledge this support LITERATURE [1] Ignat M. Problems in fragilization and certification of copper. From Seminar Proceedings Minor Contaminants in copper, University of Concepcion, Chili, 2007, s [2] Sánchez M et al. Influence des additions de Pb et de Sn sur la ductilité du cuivre OFHC. Mémoires et ëtudes Scientifiques de la Revue de Métallurgie, 1981, 78, nr. 2, page [3] Nieh E. et al. Embritterment of copper due to segregation of oxygen to grain boundaries, Metallurgical Transactions, 1981, 12, pages [4] Keast VJ, Williams DB. Quantitative compositional mapping of Bi segregation to grain boundaries in Cu. Acta Materialia, 1999, nr.

7 15-16, pages [5] Nakahara S, Microscopics mechanism of the hydrogen on the ductility of electroless copper. Acta Metallurgica, 1988, 36, nr. 7, pages [6] Pops H. The metallurgy of copper wire. Innovations, Available on-line publications/newsletters/innovations/1997/12/wiremetallurgy.html visited march [7] Technical report from GMBH, 2002.