| Wire is produced by drawing, which consists of pulling a rod of the material to be processed through a series of funnel-shaped holes, or dies, of decreasing size. Drawing involves plastic deformation and friction within the contact zone between the wire and the die. These occurrences contribute to the generation of heat in the wire, which may affect its final properties. High drawing speeds and the use of several dies in one production line—to improve productivity—may accentuate heating to a point where recrystallization of the wire material sets in and its properties are affected. Also, excessive temperatures may lead to an accelerated deterioration of the lubricants used, to the point that the optimum lubrication mechanism is no longer prevailing, with consequent negative effects on the surface quality of the processed wire.; A number of analytical models estimating the temperature increase in drawn wires, due to the dissipation of plastic deformation energy and to the friction between wire and die, were reviewed. While all models agree on the contribution of plastic deformation to heating, some discrepancies exist as to the effect of friction. The original treatment, developed in Germany in 1943, provided slightly higher peak temperature values than other models for the parameters used in the calculations, which are simulative of commercial practice on copper wire.; A finite element study conducted in the framework of this research showed that the axial heat flow within the wire, which was assumed to be negligible in the reviewed analytical theories, could, in fact, not be disregarded. It also showed that a purely isothermal process cannot be obtained in practice, whatever adjustment of the process parameters is undertaken.; In order to unequivocally correlate the effects of heating on the properties of wire and the temperature it experiences during commercial drawing, electrolytic tough pitch copper wire was drawn under independently controlled, quasi-isothermal temperature conditions at 25°C, 100°C, 150°C, 200°C, and 250°C using a custom designed apparatus. The surface appearance, together with the calculated coefficients of friction, gave indications as to the prevailing lubrication mechanism, which turned out to be of the boundary type in most cases, except at 100°C, where surface shaving was observed. Hardness measurements, tensile testing, metallographical investigations, as well as texture analysis indicated that at least partial recrystallization occurs when the wire is exposed to temperatures of 200°C and beyond during drawing. An account was also given on the annealing response of the as-drawn wire, by applying the concept of annealing index, a parameter that encompasses annealing time and temperature. |
