ACTA METALLURGICA SINICA (ENGLISH LETTERS) Vol. 17 No. 2 pp April 2004

Transcription

1 ACTA METALLURGICA SINICA (ENGLISH LETTERS) Vol. 17 No. 2 pp April 2004 IMPROVING THE MECHANICAL PROPERTIES OF COPPER ALLOYS BY THERMO-MECHANICAL PROCESSING M.C. Somani and L.P. Karjalainen Department of Mechanical Engineering, University of Oulu, Oulu, Finland Manuscript received 11 October 2003 Systematic physical simulation of thermo-mechanical processing routes has been applied on a Gleeble 1500 simulator to four copper alloys (mass %) Cu-0.57Co-0.32Si, Cu-0.55Cr-0.065P, Cu-0.22Zr-0.035Si and Cu-1.01Ni-0.43Si aimed at clarifying the influences of processing conditions on their final properties, strength and electrical conductivity. Flow curves were determined over wide temperature and strain rate ranges. Hardness was used as a measure of the strength level achieved. High hardness was obtained as using equal amounts (strains 0.5) of cold deformation before and after the precipitation annealing stage. The maximum values achieved for the Cu-Co-Si, Cu-Cr-P, Cu-Zr-Si and Cu-Ni-Si alloys were 190, 165, 178 and 193 HV5, respectively. A thermo-mechanical schedule involving the hot deformation-ageing-cold deformation stages showed even better results for the Cu-Zr-Si alloy. Consequently, the processing routes were designed based on simulation test results and wires of 5 and 2mm in diameters have been successfully processed in the industrial scale. KEY WORDS copper alloys, thermo-mechanical processing, ageing, strength, flow stress, hardness 1. Introduction In electrical conductors for railways and contactors of various microelectronic devices, mobile phones, etc., materials with high strength and excellent electrical conductivity are needed in increasing amount, and certain precipitation strengthening copper alloys are candidates for these applications. The properties of the conventionally manufactured Cu- Zr alloy have been extensively studied and reported already in 1960s by Saarivirta [1] and Dies [2], for instance. It has been shown that the best properties are achieved by combining cold working and annealing. A more recent study on the thermo-mechanical treatments of four precipitation-strengthening copper alloys, viz., CuFe2.4, CuCr0.2, CuNiSi and Cu- NiSn, has demonstrated clearly how a combination of high strength and conductivity could be achieved through cold working and precipitation annealing stages [3]. The work described here formed a part of a large project, directed to developing advanced properties in dilute copper alloys. More detailed results can be found in Ref. [4]. Physical simulation was applied to four ternary copper alloys in order to optimise their processing as regards to their strength and electrical conductivity properties. A Gleeble simulator enabled well-controlled testing over wide temperature ranges and at different deformation rates, although the maximum true strain was limited to about The experiments were carried out essentially simulating the behaviour of the alloys at temperatures corresponding to the Rodex process (a range of C) [4,5], a potential processing method, largely similar as Conform TM extrusion process [6]. Deformation rates in Rodex process are high, typically of the order of s 1, so that in the simulation, the rates

2 112 were much lower. Finally, the results of the simulation tests were utilised in designing an appropriate industrial processing route involving the extrusion and wire drawing stages. 2. Experimental The experimental copper alloys, viz., (in mass %) Cu-0.57Co-0.32Si, Cu-0.55Cr-0.065P, Cu-0.22Zr-0.035Si and Cu-1.01Ni-0.43Si chosen were produced from high purity raw materials by vacuum casting at Outokumpu Poricopper, Pori, Finland. The continuous up-cast rods were hot-extruded into rods (at 900 C, about φ15mm), which were finally drawn to the diameter of 10mm. Table 1 shows the optimised solution and ageing conditions and the corresponding hardness and electrical conductivity data achieved in these four alloys without any thermo-mechanical processing. Specimens for compression testing were solution treated and loaded in axisymmetric compression at various temperatures from ambient to 800 C and strain rates in the range s 1 on a Gleeble 1500 simulator. The recrystallisation and precipitation kinetics were studied by employing the stress relaxation technique, developed earlier [7,8]. Vickers hardness with 5 or 2kg load was measured. The electrical conductivity of the specimens used in simulation tests was determined using a Sigma test device. A Megger BT51 digital milliohm meter was employed to determine the conductivity of the industrially-processed wires (φ5 and 2mm) over the 200mm lengths at +20 C. Table 1 Optimised solution and ageing parameters of the experimental copper alloys and corresponding hardness and electrical conductivity data Copper alloy Parameters and properties Solution Ageing Hardness Electrical treatment (in STA conductivity condition) C min C min HV5 %IACS Cu-0.57Co-0.32Si Cu-0.55Cr-0.065P Cu-0.22Zr-0.035Si Cu-1.01Ni-0.43Si STA: solution treated and aged condition IACS: International Annealed Copper Standard 3. Results in Simulation Tests 3.1. Flow stress of as solution-treated alloys An example of the true stress-true strain curves determined for all the copper alloys is shown in Fig.1 for the solution treated Cu-Zr-Si alloy at the strain rate of 0.25s 1. Flow stress gives the indication of the deformation resistance as well as the strength level obtained. The curves showed work hardening behaviour at all temperatures up to 800 C. As expected, the flow stress decreased steadily with increasing temperature. Similarly as the Cu-Zr-Si alloy, other copper alloys viz., Cu-Co-Si, Cu-Cr-P and Cu-Ni-Si, also exhibited work-hardening behaviour at all temperatures in the range RT-700 C up to strains about 1.0.

3

4

5

6 116 Table 2 Industrially-processed experimental copper alloy wires and their properties Copper alloy Processing stage Diameter, Average Electrical hardness, conductivity, mm HV5 %IACS Cu-0.57Co-0.32Si CD/aged CD/aged/cold drawn CD/aged/cold drawn Cu-0.55Cr-0.065P CD/aged CD/aged/cold drawn CD/aged/cold drawn Cu-0.22Zr-0.035Si CD/aged CD: Cold deformed (Extruded) CD/aged/cold drawn Based on the compression tests and hardness measurements of φ8.2mm (extruded and aged) and φ5mm (extruded/aged/cold drawn to ε 1) wires (see Table 2), yield stress vs. hardness relationship was obtained as follows σ = 2.93 HV 44.6 (R 2 = 0.95) where σ is the flow stress (MPa) at about plastic strain (i.e. equivalent to the yield strength). Equation above can be used to assess the yield strength values from hardness values determined in simulation tests. Electrical conductivity was measured for the φ5 and 2mm wires for the Cu-Co-Si and Cu-Cr-P alloys. For the former, the conductivity decreased from about 45% IACS (φ5mm wires) to 39% IACS (φ2mm wires) in the second cold drawing stage (Table 2). The level of the electrical conductivity of the φ2mm wires is almost identical to that obtained for CDaged-CD specimens (38.5% 40% IACS). Similarly, the Cu-Cr-P wires showed a decrease in the electrical conductivity due to the second cold drawing stage. The corresponding conductivity values are 89% IACS and 73% IACS for φ5 and 2mm wires, respectively (Table 2). In the simulation tests, the conductivity was 76% 77.5% IACS for the CDaged-CD specimens. 5. Summary and Conclusions Flow stress curves measured at 0.25s 1 for the four copper alloys showed essentially work hardening behaviour at all temperatures in the range RT-700 C (up to 800 C for Cu-Zr-Si alloy) to a strain of about 1.0. The strain rate has an effect on the flow stress of the solution treated copper alloys at 800 C (the upper limit in the Rodex process). While the Cu-Zr-Si alloy did not exhibit any peak stress typical of DRX, the Cu-Co-Si (at low strain rates 0.1s 1 ), Cu-Cr-P (at all strain rates) and Cu-Ni-Si alloy (at 0.1s 1 ) showed DRX. Successive CD-ageing-CD stages resulted in a significant increase in the hardness of all the alloys. The maximum hardness achieved for the Cu-Co-Si, Cu-Cr-P, Cu-Zr-Si and

7 117 Cu-Ni-Si alloys were 190, 165, 178 and 193 HV5, respectively, compared to the hardness without thermo-mechanical treatment (145, 113, 107 and 100 HV5, respectively). A special thermomechanical schedule involving the hot deformation-ageing-cd stages showed promising results in respect of the final hardness for the Cu-Zr-Si alloy. The hardness values (190 and 187 HV5) were markedly higher than the maximum hardness (178 HV5) achieved by the CD-ageing-CD schedule. However, overageing of the precipitates at deformation temperatures above 550 C adversely affected the hardness achieved in subsequent ageing (167 HV5). The stress relaxation technique is capable of revealing the occurrence of strain induced precipitation in these copper alloys. Although it is difficult to ascertain the precipitation start time, the completion of precipitation can be determined more distinctly. The electrical conductivity was not affected detrimentally by the thermo-mechanical processing routes used in the simulation tests. The properties of the industrially-processed Cu-Co-Si and Cu-Cr-P alloys in CD-agedcold drawn to 1.0 (φ5mm wires) are comparable with those obtained in simulation tests in the CD-ageing-CD schedules. In the case of Cu-Zr-Si alloy, the hardness (172 HV5) of φ11.5mm rods is somewhat lower than the hardness (178 HV5) obtained as using the CDageing-CD schedule. However, both the hardness values remained considerably lower than that obtained in the simulation tests for the alloy in hot deformation-ageing-cd schedule ( HV5). Acknowledgements The authors express their gratitude to National Technology Academy of Finland (TEKES) and Outokumpu Poricopper Oyj for the funding of the project. REFERENCES 1 M. Saarivirta, Trans. AIME 218 (1960) 431, K. Dies, Kupfer and Kupferlegierungen in der Technik, (Springer-Verlag, Berlin, 1967) p.638 (in German). 3 R. Sundberg and M. Sundberg, in Proc. Int. Conf. on Thermomechanical Processing in Theory, Modelling and Practice (TMP) 2 (The Swedish Society for Materials Technology, Stockholm, Sweden, 1996) p M.C. Somani and L.P. Karjalainen, Simulation of Thermomechanical Processing of High-Strength, High- Conductive Copper Alloys Report No. 115, Department of Mechanical Engineering, University of Oulu (Oulu, Finland, 2001) p P. Nordling, Master Thesis, University of Oulu,(Oulu) (1994) p.97 (in Finnish). 6 J. Lu, N. Saluja, A.L. Riviere and Y. Zhou, J. Mater. Processing Technol. 79 (1998) L.P. Karjalainen, Mater. Sci. Technol. 11 (1995) L.P. Karjalainen, J. Perttula, Y.R. Xu and J. Niu in Proc. 7th Int. Symp. on Physical Simulation of Casting, Hot Rolling and Welding, (National Research Institute of Metals, Tsukuba, Japan, 1997) p.231.