M. El Mehtedi

A New Constitutive Model for the Plastic Flow of Metals at Elevated Temperatures

A new constitutive model based on the combination of the Garofalo and Hensel-Spittel equations has been used to describe the plastic flow behavior of an AA6005 aluminum alloy tested in torsion. The analysis of the experimental data by the constitutive model resulted in an excellent description of the flow curves. The model equation was then rewritten to explicitly include the Arrhenius term describing the temperature dependence of plastic deformation. The calculation indicated that the activation energy for hot working slowly decreased with increasing strain, leading to thermally activated flow softening. The combined use of the new equation and torsion testing led to the development of a constitutive model which can be safely adopted in a computer code to simulate forging or extrusion.

High Temperature creep and superplasticity in a Mg-Zn-Zr alloy

Creep and superplasticity were investigated by testing a fine-grained extruded Mg–Zn–Zr magnesium alloy under a wide range of applied stress in the temperature range between 100 and 300 °C. Grain boundary sliding became the dominating mechanism at 200 °C, leading to a true superplastic behaviour at 300 °C, where superplasticity was attained even under relatively high strain rates (5×10 −3 s −1 ). By contrast, for lower temperatures, the straining process was controlled by dislocation climb. A comprehensive model, taking into account the simultaneous operation of the different mechanisms, was developed to describe the strain rate dependence on applied stress.

Mechanical And Microstructural Aspects Of High Temperature Formability Of AZ31 Sheets

The high temperature formability of AZ31 magnesium alloy sheets has been investigated in the temperature from 150 to 350°C. The different straining conditions have been studied by using sheet blanks with several length to width ratios, and Forming Limit Diagrams were then obtained. As expected, an increase in temperature was observed to enhance the formability of the alloy. These differences in formability observed in the warm‐ and hot‐forming regions were related to the microstructural evolution during deformation. Optical microscopy revealed the occurrence of recrystallization, and the volume fraction of recrystallized microstructure was found to increase with temperature. FEM simulation of the Nakazima test were also obtained by using flow curves obtained in torsion between 100 and 400°C as input data. Correlation between FEM modeling results and experimental data was considered to be excellent.

High temperature deformation of IN718 superalloy: use of basic creep modelling in the study of Nickel and single-phase Ni-based superalloys

Abstract A basic model was applied to pure Ni and then to a single-phase superalloy. The high temperature deformation of the superalloy, a solution treated IN718, was investigated by torsion testing in a high-temperature regime (1000–1100 °C) where no precipitation of secondary phases was expected. The material exhibited the classical behaviour of alloys which undergo dynamic recrystallization. The peak-flow stress dependence on strain-rate and temperature was described by a physically-based set of constitutive equations, which took into account both dislocation hardening and solid solution strengthening, and was previously successfully used for the description of creep deformation of Cu, Al alloys and austenitic steels. The model provided an excellent description of the experimental data and for this reason can be considered an excellent basis for further development, which should take into account the precipitation of secondary phases.

Identification of Plastic Behaviour and Formability Limits of Aluminium Alloys at High Temperature

In order to simulate accurately the stamping process of sheet metals, their constitutive behaviour, as well as their formability limits, must be accurately evaluated. In this work, tensile and Nakazima tests are conducted at high temperature on aluminium alloys of the 5000 series. The tensile tests are mainly used to determine the flow stress curve of the material, while the Nakazima tests, are used to determine the admissible elongation before failure at different strain ratios. The raw load-displacement curves and the deformation of the samples, measured by the optical grid method, are included in an inverse FEM procedure to best identify the real elasto-plastic law of the material and its formability limits.