Effect of the cyclic strain amplitude on the hot dynamic restoration of copper

Abstract

Federal University of MinasGerais, Department of Metallurgical and Materials Engineering, Rua Espi´rito Santo 35, s206,Centro, 30160-030, Belo Horizonte, Minas Gerais, Brazil(Received May 18, 2000)(Accepted in revised form July 20, 2000)Keywords: Dynamic phenomena; Recrystallization and recovery; Copper; Hot cyclic strainingIntroductionThe microstructures developed during hot and warm working of metals are profoundly affected byvariables such as the prevailing temperatures, strain rates and strains [1,2]. Another variable that hasbeen receiving attention is the strain path [3,4], with special emphasis on cyclic and multiaxial straining.This can be of particular importance in the industrial rolling of flat and long products. For both cases,there is a cyclic superficial shearing associated to the change of direction of the friction shear stressesas the material traverses the rolling gap [5]. For long products, there is also a 45° or 90° change in thecompression direction from one rolling pass to the next one. The situation is particularly relevant to thefinishing stands of wire rod mills, where interpass times are very low [6] and softening between passesis probably not important.Cyclic cold straining of metals leads to lower work hardening rates than monotonic deformation. Thedegree of hardening decreases as the strain amplitude is lowered [7]. It has also been shown [3] that theflow stress of copper in hot cyclic torsion is substantially lower than under monotonic torsion. Theeffect of strain amplitude was the same as for cold working, and the dynamic recrystallization peak waseliminated. These facts are illustrated in Figure 1.The effects of hot cyclic straining depend on the strain amplitude (De) in the cycle. The availabledata for copper at 500°C and a strain rate of 0.1s