Mechanical behaviour of AZ91D and AZ31B magnesium alloys at wide range of strain rates and temperatures
Iram Raza Ahmad
Date of Issue2014
School of Mechanical and Aerospace Engineering
In many industries especially in automobile and aerospace, developments in near future are mainly concerned with the weight reduction to meet the global challenges of fuel economy and reduction of toxic emissions. Magnesium alloys being the lightest of all metals offer great potential to achieve that weight reduction by replacing the most commonly used materials, i.e. steel, aluminium and plastics. Magnesium alloy are being used in variety of non-structural and to some extent in semi-structural applications. However, their use in structural applications such as auto-body structure is very limited. Limited data available for high strain rate deformation of magnesium alloys combined with an insufficient understanding of the underlying deformation mechanisms adds to the reservations for their use as structural materials. For widespread usage of magnesium alloys, their dynamic behaviour must be determined to assess their performance during a crash event. In the present work, an experimental and constitutive study followed by the microstructural analysis has been carried out to investigate the dynamic behaviour of as-cast AZ91D and wrought AZ31B magnesium alloys. These alloys have been tested at strain rates in the range between 10-4s-1 and 4x103s-1 and at temperatures between -30oC and 200oC under compression and under tension between 10-4s-1 and 1500s-1 strain rates and at 25oC and at 250oC temperatures. Quasi-static tests were performed using universal testing machine and high strain tests were carried out with Hopkinson Bar apparatus. Dry ice was used to achieve below zero temperature while the elevated temperatures were obtained by using coil heater. Increasing stresses, larger strains and higher energy absorption are observed with increasing strain rate in both alloys under compression as well as in tension. On the other hand, lower stresses and relatively larger strains are observed at elevated temperatures. However, temperature has little effect on the mechanical behaviour of these alloys at dynamic strain rates. The alloys deform by dislocation glide and twining at room temperature. Number of twins decreases as the strain rate increases. Twins, voids, shear bands, grains refinement and elongation have a key role in alloys response such as strength, ductility, hardenability and energy absorption. AZ31B alloy is strongly anisotropic and shows a considerable tensile-compressive yield asymmetry. The experimental data was fit to the Johnson-Cook model and the results are in reasonable agreement except in the beginning portion of the flow curves, where the fitted curves deviate from the experimental data.
DRNTU::Engineering::Mechanical engineering::Mechanics and dynamics