Gradual formation of nano/ultrafine structure under accumulative press bonding (APB) process

Accumulative press bonding (APB) is a developed reuse of the oldest severe plastic deformation (SPD) process to produce nano/ultrafine grained (UFG) materials. In the present work, microstructural evolution and mechanical properties of AA1050 pure aluminum alloy have been investigated under APB technique up to 14 cycles. From scanning transmission electron microscopy characterization, it was found that continuous dynamic recovery was a dominant mechanism in grain refinement, and resulted in formation of nano/ultrafine grains with average diameter of 450 nm in pressing direction and 320 nm in transverse direction. After 14 APB cycles, almost 10% of all grains were less than 100 nm. Electron backscattered diffraction revealed that the fraction of high angle boundaries and the mean misorientation angle increased by increasing strain during APB process and reached a saturation value of ~ 78% and ~ 35°, respectively. By increasing the number of APB cycles, the tensile strength of the APB processed aluminum improved and reached 180 MPa after 14 cycles, which was two times higher than obtained values for initial materials, i.e. 88 MPa. On the other hand, the elongation dropped abruptly at the first cycle and then increased slightly. Strengthening in APB processed aluminum was attributed to strain hardening by dislocation accumulation dominant in primary cycles, and grain refinement which was dominant in final cycles. Scanning electron microscopy observations demonstrated that failure mode in APB processed aluminum was shear ductile rupture with small elongated dimples.