Investigation of recrystallization kinetics of cold drawn electrolytic copper wires

In this study, the effects of cold-drawing of electrolytic copper wire obtained from SARKUYSAN in Gebze-Kocaeli on mechanical properties and recrystallization behavior were investigated. Cold drawing is a metal forming process to obtain dimensionally consistent product and clean surface without scale. Copper rods, bars and wires are manufactured by cold drawing. Metal forming processes are hot working at elevated temperatures and cold working at room temperature. In cold working of metals and alloys, not only the shape of the material changes, but also its microstructure and mechanical properties. Strength and hardness of metals increases due to the movement of dislocations and move and their collapse at grain borders during cold working. As depending on the degree of cold working, the shaped material becomes harder than the initial form and the cracks may begin to form because of low ductility. The initial 11 mm diameter electrolytic copper wire was cold drawn, first to 9.3 mm in diameter and then to 8 mm in diameter. The cold deformation rates of copper wire via cold drawing are 33.5% for first drawing stage and 63.7% for second drawing stage. In the metalographic examination of the samples, it was observed that the grains were elongated in the drawing direction with cold drawing. According to the results in the tensile tests of the samples, while the tensile strength of the initial copper wire was 255 MPa, this value increased to 355.4 MPa after 33.5% cold deformation and to 426.6 MPa for 63.7% cold deformation. Moreover the yield strength of the initial copper wire was 102.4 MPa, this value increased to 121.3 MPa after 33.5% cold deformation and to 168.3 MPa for 63.7% cold deformation. It was observed that the tensile strength and yield strength of electrolytic copper wire increased with increasing degree of cold drawn deformation. Likewise, the microhardness values of the samples increased from 93.5 HV to 116.8 HV for 33.5% cold deformation and 121.2 HV for 63.7% cold deformation. The ductility value of the samples decreased from 43% to 24.3% for 33.5% cold deformation and 20.6% for 63.7% cold deformation. As a result of work hardening, also known as strain hardening, of electrolytic copper wire, there has been an increase in internal energy associated with an increase in dislocation density. After cold working of metallic materials, their cell structures are mechanically stable, but they are not thermodynamically stable. These metallic materials can be returned to their original states by annealing that is a heat treatment process for further cold working. The annealing process ensures a strain-free structure and improves ductility and toughness. Annealing process has three steps; recovery, recrystallization and grain growth. This treatment is carried out at the temperature that is 30 – 60% of melting temperature of material. Recovery step is a low temperature process and it restores physical properties without changing in microstructure. There are entangled dislocation networks and point defects in the microstructure of a cold worked microstructure. During recovery step, dislocations in structure move and rearrange. Residual stresses decrease and remove in this step. The driving force for recovery and recrystallization in structure is the stored energy due to cold working. In the recrystallization step, dislocations in structure start to polygonized and annihilated. The dislocation density decreases during this step and this situation protomes higher ductility and lower strength in the material. The temperature, time, initial grain size, composition and the degree of deformation affect the crystallization temperature. The recrystallization process starts upon heating cold-worked metallic parts to temperatures in the range of 30 – 50% of the melting temperature of metal. In addition, the recrystallization temperature depends strongly on the purity of metals. Impure metals may not be recrystallized up to temperature of 50 – 70% of the melting temperature in Kelvin unit. The traditional methods of kinetic analysis is based on fitting data to reaction models via isothermal studies. In non-isothermal kinetics, the use of the traditional methods results in uncertain values of Arrhenius parameters that can not be compared meaningfully with isothermal values. The alternative model-free methods are based on the isoconversional method. The model-free methods for both isothermal and nonisothermal kinetics studies avoid the problems that come from the ambiguous evaluation of the reaction model. The model-free methods allows the dependence of the activation energy on the extent of conversion to be determined and this acceptation provides reliable reaction rate predictions. The non-isothermal kinetics is a thermoanalytical method based on differential thermal analysis data, thermogravimetric analysis data and differential scanning calorimetry data. Nowadays a detailed examination for reaction mechanism is less important than the process optimization by investigating the effects of experimental conditions on the reaction rates. In this study, the model-free non-isothermal recrystallization kinetics of cold drawn copper wires was performed by using differential scanning calorimetry (DSC) with different heating rates (10–15–20–25°C/min). The recrystallization temperatures of the cold-worked electrolytic copper wire reduced to 33.5% from its primary crosssection are between 269.9 and 287.7°C, corresponding to four different heating rates. These temperatures for the cold-worked electrolytic copper wire reduced 63.7% are between 256.9 and 268.9°C, corresponding to four different heating rates. The graphics of Kissenger, (ln(β/T2) – 1/T), Boswell (ln(β/T) – 1/T), Ozawa (ln(β) – 1/T) and Starink (ln(β/T1.92) – 1/T) were drawn using peak temperatures, where R is gas constant (J/mol.K), T is peak temperature (K) and β is heating rate (K/min). The slopes of the graphics are equal to –E/R, where E is activation energy (kJ/mol). The calculated activation energies for 33.5% of cold-worked copper wire are 118.1 kJ/mol for Kissenger method, 122.6 kJ/mol for Boswell method, 120.2 kJ/mol for Ozawa method and 110.5 kJ/mol for Starink method. The calculated activation energies for 63.7% of cold-worked copper wire are 171.8 kJ/mol for Kissenger method, 176.2 kJ/mol for Boswell method, 171.9 kJ/mol for Ozawa method and 172.1 kJ/mol for Starink method. As a result, the activation energy of recrystallization for 33.5% cold deformed copper wire is in the range of 110 – 123 kJ/mol and this value for 63.7% cold deformed copper wire is in the range of 171 – 177 kJ/mol. It is understood from this study that as the amount of cold deformation increases, the strength and hardness of the electrolytic copper wire increases and its ductility decreases. In addition, the amount of stored energy increased depending on cold deformation, so the recrystallization temperature decreased slightly and the activation energy increased.