Microestrutura e propriedades mecânicas da liga AA6061 processada por deformação plástica severa em temperatura criogênica

Abstract

The present work aimed two fronts: to characterize the mechanical behavior of the AA6061 alloy at cryogenic temperature, and to study the microstructural and properties evolution after cryogenic severe plastic deformation. Samples of AA6061 alloy were solution heat-treated and processed by conventional rolling, asymmetric rolling and equal-channel angular pressing (ECAP), both at room temperature and cryogenic temperature. After processing, such samples were treated for precipitation at 25 °C and at 100 °C, for different times. On the cryogenic mechanical behavior it was observed the suppression of the dynamic strain aging, which also allowed the ECAP processing without cracking. Uniform and total elongation and mechanical strength were always higher at -196 °C than at room temperature. The strain-rate sensitivity, despite having low values, becomes positive for cryogenic deformation, directly affecting the deformation capacity. Regardless of the processing route employed, a saturation of the hardness around 125 HV has been seen. The microstructures after cryogenic ECAP were very refined in relation to the starting material (average grain size up to 500 nm). In addition, it was observed a homogeneous strains distribution after cryogenic processing, with detection of several new fine grains from grain boundaries, which suggests recrystallization. The tensile tests results, after cryogenic processing in combination with artificial aging at 100 °C (48 hours), indicated an increase in uniform elongation. Regarding mechanical strength, the highest values were observed for extrusion at room temperature combined with artificial aging. In addition, there was to some static recovery during artificial aging, especially for cryogenic processing, which generates thermodynamically unstable microstructures. The density of dislocations, in fact, decreases with the prolongation of artificial aging time. Thus, the present study was based on a study of the cryogenic mechanical behavior of the AA6061 alloy, as well as the possibility of processing by cryogenic severe deformation.