Hot deformation behaviour and flow stress prediction of 7075 aluminium alloy powder compacts during compression at elevated temperatures

Abstract

Abstract:In the present study, the hot deformation behaviour of 7075 aluminium alloy powder compacts was studied by performing hot compression tests on a Gleeble 3800 machine. The main objectives were to evaluate the effect of the relative green density on the hot deformation behaviour and to model and predict the hot deformation flow stress of powder compacts using constitutive equations. For this pur-pose, powder compacts with relative green densities ranging from 83 to 95%, which were prepared by uniaxial cold pressing a commercial pre-mixed powder, were hot compressed at temperatures ranging from 350 • Cto450 • C and at true strain rates ranging from 0.01 s −1 to 10 s −1 . The true stress-true strain curves of the powder compacts exhibited a peak stress at a critical strain after which the flow stress remained nearly constant. As the deformation temperature increased or the strain rate and green density decreased, a decrease in the peak stress level was observed. The relationship between deformation tem-perature, strain rate, and the peak flow stress of powder compacts was described by the Zener-Hollomon parameter in an exponential equation containing relative green density compensated material constants and the deformation activation energy. The peak flow stresses calculated from the proposed formula were in good agreement with the experimental results, which confirms the applicability of the employed method for the prediction of the hot deformation flow stress of porous materials with different relative green densities.

A Constitutive Equation for the Flow and Densification Behaviors of Powder Metallurgy Fe-0.5C-2Cu Steel at Elevated Temperatures

“…The flow stress of the compressed specimens increased rapidly with the increasing strain in the early stages and then slowly rose until the end of the tests, which indicated that PM Fe–0.5C–2Cu steel exhibited the expected characteristic of persistent hardening during the entire hot compression process, as presented elsewhere . The hardening mechanisms (matrix work hardening and densification hardening) and the dynamic softening mechanisms (dynamic recovery or recrystallization of the matrix) functioned simultaneously during the hot deformation process of porous metals at elevated temperatures . At the beginning of the deformation, the matrix work hardening as the result of the dramatic increase in the dislocation density and the densification hardening due to the compression of the volume led to a sharp increase in the flow stress.…”

“…As the deformation continued, the activation of the dynamic softening mechanisms partially neutralized the effects of both the matrix work hardening and densification hardening mechanisms, resulting in the gradual decrease of the stress rise. In some materials produced by powder metallurgy, dynamic softening can cause the flow stress to decrease gradually after reaching a peak value …”

“…However, the deformation behavior of PM materials is different from that of conventional wrought materials because of the presence of substantial porosity content . Recently, Jabbari Taleghani et al and Wolla et al investigated the hot deformation behavior of PM aluminum alloy with different initial relative densities at various deformation temperatures and strain rates. They revealed that the flow stress curves exhibited peak flow stresses, which decrease with the increasing deformation temperature and decreasing strain rate or initial relative density.…”

“…When the density of PM material increases, its strength will rapidly increase . Therefore, in addition to matrix work hardening, the densification during plastic deformation can further enhance the flow stress of a PM material, which is known as densification hardening . Therefore, the flow stress and relative density of a PM material during plastic deformation is often constantly changing.…”

Hot tensile deformation study and constitutive modeling of new high zinc containing hot rolled Al–Zn–Mg–Cu alloy

“…Various models have been proposed based on the results to predict flow behavior. In all those models, some equations are proposed which estimates the power or load required for the hot deformation process and flow behavior using true stress strain curve [8][9][10][11][12][13][14][15][16]. Some researchers have also made effort to simulate various test results.…”