AN EXPERIMENTAL INVESTIGATION ON THE EFFECT OF ANNEALING TREATMENT ON STRAIN INHOMOGENEITY IN THE CROSS-SECTION OF DRAWN COPPER WIRES

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1 AN EXPERIMENTAL INVESTIGATION ON THE EFFECT OF ANNEALING TREATMENT ON STRAIN INHOMOGENEITY IN THE CROSS-SECTION OF DRAWN COPPER WIRES A. AKBARI 1, G. H. HASANI 2 & M. JAMSHIDI JAM 1 1 Department of Material Science and Engineering, Sirjan branch of Islamic Azad University, Sirjan, Iran 2 Department of Metallurgical and Material Engineering, University of Tehran, P.O. Box , Tehran, Iran Abstract The results of an investigation of annealing time and temperature on strain inhomogeneity of copper wires after drawing through dies with various die-angles and reduction of areas are presented in this paper. As an important industrial tool for the evaluation of strain, the distributions of the microhardness over the transverse cross-sections were measured. Inhomogeneity factors (), as a function of above parameters were determined. The results indicate that due to different grain growth kinetics of the coarse and fine microstructure both surface and center grains grow but with different rates in applied dies, and hence strain inhomogeneity decreases as time and temperature increase. Due to different recrystalization kinetics vs. time and temperature inhomogeneity is more sensitive to temperature rather than to time. Keywords: wire-drawing; strain inhomogeneity; annealing time; annealing temperature. 1. INTRODUCTION Drawing is a metalworking process of bars, rods and wires particularly used in the electric and automotive sectors. It is also applied prior to the heading of various fasteners such as bolts, nails, screws, and rivets. The process consists of reducing the cross-section by pulling the wire through series of conical dies. During the drawing operation, a plastic deformation is imparted to the material depending on the drawability of the material [1-4]. Deformation Strain caused by drawing differs from that associated with tensile deformation, for the same reduction of area in both cases. This is due to the deformation heterogeneity in the drawn product, associated with localized shears superimposed on the overall external strain [4, 5]. In wire drawing like other metal forming processes, shear strains due to deformation geometries or friction conditions at die-material interfaces play an important role in metal flow and in the formation of surface and internal defects [6]. Inhomogeneous deformation leads to a non-uniform distribution of dislocation stored energy being the driving force for recrystallization [7]. The strain distribution in the cross-section of drawn wires through conical dies affects the post-annealing microstructure, since static recrystallization phenomenon is influenced by the amount of strain within the material [8]. Inhomogeneity in wire microstructure would affect some of the physical properties such as; electrical conductivity, magnetic moments and current density [9-12] as well as the formability and mechanical properties of the wire in next drawing passes [3, 13]. On the other words, an inhomogeneous microstructure results to inhomogeneous properties. The factors that affect strain distribution in wire cross-section are die-angle, friction, reduction of area, number of drawing passes and the wire metallurgical history [14]. For a successful drawing operation, it is of considerable importance to material designers to predict the inhomogeneity of deformation in deformed workpiece and careful selection of such process parameters should be carried out. Since the cold deformation increases the dislocation density and stored energy of workpiece leads to a decrease of its ductility, post-deformation annealing is carried out to minimize the effect of these factors

2 Although, there are many works on the effects of annealing metals and microstructure evolution during annealing [15-17]., the effects of annealing parameters on the microstructure inhomogeneity achieved after annealing the deformed metal has not yet been quantitatively investigated. In this study, attempts were made to evaluate the optimum conditions resulting in less strain inhomogeneity in the cross-section of drawn copper wires which are the preferred and predominant choice in the electrical industry because of their high conductivity, both electrical and thermal [18] by hardness measurement which is an important industrial tool for the evaluation of strain and hardening levels in deformed products [19, 2]. In the literature, the relationship between hardness and strain is derived from the so called strain-hardness reference curves [19, 21]. Also investigated, is the combined effect of drawing and annealing parameters on the strain inhomogeneity. 2. EXPERIMENTAL DETAILS The starting material in the present investigation was a commercially pure (99.95 wt.%) copper wire, annealed at 485 C for 1 h. Immediately after annealing, the wires were quenched in water to remove the oxide layer produced in the annealing treatment. The wires were then cut into pieces to be drawn through a single-die draw bench at a speed of 45 mm s 1. A swaging machine was used to reduce the initial wire cross section to pass through conical dies. Tungsten carbide dies with nominal included angles of 5, 1, 2, 3 degrees were used for drawing process. Reductions of areas were in the range 11% to 38%. Prior to drawing, Commercial engine oil was applied to the surface of the wires as lubricant. Specimens for microscopic examinations and hardness measurements were prepared from drawn wires using an electrodischarge wire-cut machine. After cold mounting, grinding and polishing, a chemical etchant with the composition of 8 ml H 2 O 2 and 2 ml HNO 3 was used to reveal the microstructure. To obtain an image with a better contrast between the finer grains at the surface and coarser grains in the center of the wire, some of specimens were partially annealed. The microhardness measurements were performed by a Leitz hardness testing machine using.1 kg weights, which resulted in an applied load of about 1 N. Measurements of Hv.1 have been performed over two optionally perpendicular directions on the crosssection of wires after every drawing schedule and annealing process. The distance between adjacent indentations was approximately.4 mm. The annealing treatment was performed on the cold-drawn copper wires with different heating times of 5 min, 1 min, 2 min, 3 min, 4 min and 1h in a temperature range of 2 C to 6 C. 3. RESULTS AND DISCUSSION Figure 1 shows the cross-sectional microstructure of the wire drawn through 11% of area reduction before annealing and after annealing for 1h at 5 C, respectively. It is observed that due to the localization of strain in surface region of wire, grain size in this zone is smaller than that in the center part and this difference in grain size is indicative of inhomogeneous flow in the cross section of the drawn wire. The finer grain structure of the surface layers is a result of higher strain energy stored after cold working which acts as the driving force for recrystallization during subsequent partial annealing treatment. Regarding the Hall-Petch relationship between grain size and hardness, one may expect higher hardness values at surface layers of the wire. By the way, after annealing metallographic inspection confirms the occurrence of full recrystallization in the copper wire above 4 C. Grain growth prevails at higher annealing temperatures. On

3 the other words although the surface and center s grains have grown with different rates, but inhomogneity in the cross section of the wire has plummeted. Cente Surface (a (b 5µm Fig. 1. Microstructure of the drawn wire with 11% of area reduction, a) before annealing and b) after 1h annealing at 5 C. Typical hardness distribution across the diameter of the wires drawn in various routes of constant reduction of area (2%) or constant die-angle (2α=3 ) before annealing are illustrated in Fig. 2. As it can be observed the hardness profile is not uniform along the measured paths, being higher in the vicinity of free surface in both routes. This is directly related to the strain inhomogeneity in the studied cross sections, consistent with microstructural observations depicted in Fig. 1. Researchers [22] have characterized the hardness gradients in wire and strip drawing experiments by an inhomogeneity factor defined as: (1) where Hs and Hc are the Vickers hardness at the surface and center, respectively. To eliminate the effects of temperature rise in the surface of the wires, in the above equation, Hs represents the hardness of the points at a distance of 5 µm from the surface. 12 Microhardness (Hv) α=4, RA=2% 2α=3, RA=11% Distance along diameter (mm) Fig. 2. Hardness profiles across the drawn wires through constant reduction of area and constant die-angle routes.

4 The effects of annealing time and temperature on in various reductions of area and constant die-angle are illustrated in Fig. 3. It can be seen that inhomogeneity decreases with increasing reduction of area mainly due to lower structural changes or less inhomogeneity after considerable plastic deformation. This means that in the higher reduction the deformation is high and the difference between deformations in different regions is decreased. Also with increasing the parameters of annealing time and temperature the I.F is decreased when two parameters are constant. This means that with increasing annealing time and temperature, the difference between the grain sizes of different regions is decreased and thus the inhomogeneity in grain size distribution at the cross section of drawn wire is decreased. This can be related to the different grain growth kinetic of fine and coarse grains with annealing time and temperature [7, 23]. As it is known the grain growth kinetic of fine grains is higher than coarse ones [24]. Consequently, in annealing the finer grains grow faster than the coarse ones and finally the difference between their sizes is decreased. Similar to the variation in inhomogeneity with reduction of area, also the die-angle dependence of in Fig. 4, shows a second order polynomial relationship with annealing time and temperature, except that inhomogeneity increases with die-angle. Comparing figs. 3 and 4 one may observe that inhomogeneity is more sensitive to temperature rather than to time. This may be attributed to different recrystalization kinetics vs. time and temperature [7]..4.3 (a) 2α=3, T=4 'C RA=11% RA=2% RA=32% RA=38%.4.3 (b) 2α=3, t=3 min RA=11% RA=2% RA=32% RA=38% Annealing Time (min) Annealing Temperature ('C) Fig. 3. The effect of a) annealing time and b) temperature on the inhomogeneity factor (I.F) of drawn wire with different area reductions and constant die-angle of 2α=3. Moreover, with increasing annealing temperature and time the grain size of each region in the cross section of drawn wire is increased due to grain growth phenomenon after recrystallization and from Figs. 3 and 4 it can be concluded that in annealing temperature higher than 4 C the grain growth is faster at the surface of the wire where more deformation takes place and has more nucleation sites for recrystallization [25] than the center of the wire. As a result the inhomogeneity of grain size and hardness distribution is increased again. Comparing the convex curves in figures 3 and 4 one may also conclude that at high annealing times and temperatures the effects of wire drawing parameters on the amount of inhomogeneity decrease. From a practical point of view, it is always desirable to minimize the strain inhomogeneities in drawn wires. This can be achieved by working at lower die-angles, higher reduction per passes and high annealing time and temperatures.

5 .4.3 (a) RA=2%, T=4 'C 2α=5 2α=1 2α=2 2α=3.4.3 (b) RA=2%, t=3 min 2α=5 2α=1 2α=2 2α= Annealing Time (min) Annealing Temperature ('C) Fig. 4. The effect of a) annealing time and b) temperature on the inhomogeneity factor (I.F) of drawn wire with different die-angles and constant area reductions of RA=2%. 4. CONCLUSIONS To summarize, the following conclusions can be made based on the microstructural and hardness changes observed upon drawing and annealing of a commercially pure copper wire: 1. The inhomogeneity factor has a second order polynomial relationship with the annealing time and temperature. 2. Due to different recrystalization kinetics vs. time and temperature, inhomogeneity is more sensitive to temperature rather than to time. 3. The optimum condition for annealing the copper wire is performing it at 4 C for 1 hour. 4. To achieve less strain inhomogeneity in wire drawing process, a combination of high reduction per pass, low die-angle and high annealing time and temperature is suggested. ACKNOWLEDGEMENTS Thanks are given to the Research Council of Sirjan Branch of Islamic Azad University for providing financial support of this wok. LITERATURE [1] JO, H., LEE, S. KIM, M. Kim, B. Pass Schedule Design System in the Dry Wire-drawing Process of High Carbon Steel. J Eng Manufacture, 22, Vol. 216, page 365. [2] AVITZUR, B. Metal forming: process and analysis. McGraw Hill, New York, [3] RIENDEAU, M. MATAYA, M. MATLOCK, D. Controlled Drawing to Produce Desirable Hardness and Microstructural Gradients in Alloy 32 Wire, Metall Mater Trans A, 1997, Vol. 28, page 363. [4] CASTRO, A.CAMPOS, H. CETLIN, P. Influence of Die Semi-angle on Mechanical Properties of Single and Multiple Pass Drawn Copper. J Mat Proc Tech, 1996, Vol. 6, page 179. [5] CAMPOS, H. CETLIN, P. The Influence of Die Semi-angle and of the Coefficient of Friction on the Uniform Tensile Elongation of Drawn Copper Bars. J Mat Proc Tech, 1998, Vol. 8 81, page 388. [6] PARK, H. LEE, D. The Evolution of Annealing Textures in 9 Pct Drawn Copper Wire. Metallurgical and Materials Transactions A, 23, Vol 34A, page 531.

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