Flow softening of 7075 aluminum alloy under hot compression

Abstract

Hot deformation of 7075 aluminum alloy was studied in compression in the temperature range of 723 to 803 K and at constant true strain rates from 5.4 × 10-6 to 0.8 s-1. The flow characteristics were classified into three types and analyzed in each region of testing conditions, as follows; L, M, and H. In region H where flow stresses were above 30 MPa, high temperature yield point phenomenon appeared. The flow in H is suggested to be controlled by the dragging motion of dislocations. The stress of 30 MPa was equal to about the Orowan stress σor resulting from the dispersion of insoluble compound particles. In region M where flow stresses were between 15 and 30 MPa, flow softening took place clearly at high strains. The flow in M is suggested to be controlled by the climb-by-pass motion of dislocations against particles, which can cause yield stress to decrease from σor to about 0.5σor. In region L where flow stresses were below 15 MPa, flow softening mentioned above occurred more clearly and flow stress depended sensitively on strain rate and grain size. These suggest that grain boundary sliding takes place frequently during deformation. It is concluded that the flow softening in the regions of L and M results from the apparent grain refinement based on the folding of pancaked-shape grain structures and grain boundary sliding.