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ÓASM International 1059-9495/$19.00

Microstructure and Mechanical Properties of UltrafineGrained Al-6061 Prepared Using Intermittent UltrasonicAssisted Equal-Channel Angular Pressing Jianxun Lu, Xiaoyu Wu, Zhaozhi Wu, Zhiyuan Liu, Dengji Guo, Yan Lou, and Shuangchen Ruan (Submitted March 17, 2017; in revised form July 25, 2017; published online September 21, 2017) Equal-channel angular pressing (ECAP) is an efficient technique to achieve grain refinement in a wide range of materials. However, the extrusion process requires an excessive extrusion force, the microstructure of ECAPed specimens scatters heterogeneously because of considerable fragmentation of the structure and strain heterogeneity, and the resultant ultrafine grains exhibit poor thermal stability. The intermittent ultrasonic-assisted ECAP (IU-ECAP) approach was proposed to address these issues. In this work, ECAP and IU-ECAP were applied to produce ultrafine-grained Al-6061 alloys, and the differences in their mechanical properties, microstructural characteristics, and thermal stability were investigated. Mechanical testing demonstrated that the necessary extrusion force for IU-ECAP was significantly reduced; even more, the microhardness and ultimate tensile strength were strengthened. In addition, the IU-ECAPed Al alloy exhibited a smaller grain size with a more homogeneous microstructure. X-ray diffraction analysis indicated that the intensities of the textures were weakened using IU-ECAP, and a more homogeneous microstructure and larger dislocation densities were obtained. Investigation of the thermal stability revealed that the ultrafine-grained materials produced using IU-ECAP recrystallized at higher temperature or after longer time; the materials thus exhibited improved thermal stability. Keywords

Al alloy, ECAP, thermal stability, ultrafine-grained, ultrasonic vibration

1. Introduction Grain refinement of Al and its alloys using thermal treatment methods is generally difficult. Because of cross-slip of dislocations caused by their high stacking fault energy (SFE), Al and its alloys exhibit very high rates of dynamic recovery, which is generally expected to completely inhibit dynamic recrystallization (Ref 1, 2). In addition, Al alloys exhibit no phase change during heating. Therefore, ultrafine grains can generally be obtained using severe plastic deformation (SPD) processes. Equal-channel angular pressing (ECAP) is a SPD process that has received widespread attention because of its simplicity and practicality (Ref 3, 4). ECAP can be used to achieve SPD of materials, reducing grain sizes from the micrometer to nanometer level and resulting in significantly improved strength. Many researchers have applied ECAP and successfully obtained ultrafine-grained Al and its alloys. Iwahashi et al. Jianxun Lu, Xiaoyu Wu, Zhaozhi Wu, Zhiyuan Liu, Dengji Guo, and Yan Lou, Guangdong Provincial Key Laboratory of Micro/Nano Optomechatronics Engineering, Shenzhen University, Nan-hai Ave 3688, Shenzhen 518060, Guangdong, People’s Republic of China and Shenzhen K