High strain rate properties of selected aluminium alloys

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Abstract

The strain rate sensitivity of two commercial aluminium alloys AA6082 and AA7108 in peak temper T6 and overaged T79 condition is analysed. These alloys are precipitation hardenable and as such, are expected to have a relatively low strain rate sensitivity. Their response to rapid loading has been tested over a wide range of strain rates, from 0.1 to 3000 s−1 at room temperature, 375 and 515°C for AA6082 and room temperature, 280 and 340°C for AA7108. Specimens were cut from planar extruded sections and tested in uniaxial compression with the deformation axis parallel to the extrusion direction. Intermediate strain rate testing was carried out using a computer controlled servo-hydraulic testing machine and high strain rate testing above 500 s−1 on a Split Hopkinson Pressure Bar (SHPB). Owing to inaccuracies of the conventional Kolsky solution of the SHPB, a numerical simulation was performed. The flow stress at a plastic strain of 5% was plotted as a function of strain rate and temperature. Up to 2000 s−1, there is very little change of the activation volume with increasing temperature for both alloys, indicating that barriers to thermal activation are large and approximately the same. However, at higher strain rates, the rate controlling mechanism appears to change and at lower temperatures, a trend towards negative strain rate sensitivity was observed. Metallographic examination of AA7108 revealed strain localisation, seen as parallel deformation bands. The strain localisation could be the cause of the negative strain rate sensitivity.

Introduction

There are many practical applications where materials are subjected to high rates of straining, such as: high speed machining, impact conditions, rapid crack propagation, metal forming and others. For all of these, knowledge of a high strain rate response of the material is required. Recent years have seen great advances in the determination of the mechanical properties of materials subjected to high strain rate [1], [2], [3], [4].

An attempt has been made to determine the nature of strain rate dependence of two selected aluminium alloys: AA6082 in T6 and AA7108 in T79 condition, over the wide range of strain rates at three different temperatures. It is known that the strain rate sensitivity drops with an increase of the alloy content, but also with the tempering [2]. However, previous research carried out on these alloys is lacking and the knowledge is needed in order to predict the behaviour of those during rapid loading. The idea was to monitor the change of the strain rate sensitivity with an increase of temperature.

The selection of elevated testing temperatures for these alloys was made according to the significant points in the phase and temperature–time diagrams. Testing of both alloys was performed at room temperature, then the temperature corresponding to the position of the ‘nose’ of the temperature–time curves, where the minimum time for coarsening of the secondary phase hardening particles is needed and solvus temperature. It is important to emphasise that these temperatures are alloy dependent. The displacement of the so-called C-curve in temperature-time space is determined by many factors, one of which is the density of heterogeneous nucleation sites. Following the research carried out by Reiso et al. in [5] and Droenen and Ryum in [6] elevated testing temperatures were chosen as 375 and 515°C for AA6082 and 280 and 340°C for AA7108.

Section snippets

Mechanical equations of state

Rate effects are governed by different flow controlling mechanisms and the research carried out on widely different materials has revealed mainly three different regions, with the corresponding rate controlling mechanisms: athermal flow, thermally activated flow and phonon drag.

At high temperatures and low strain rates, a rate independent flow may be observed, attributed to athermal friction stress. Thermal vibrations of the lattice supply insufficient energy for overcoming of the long-range

Mechanical testing

Two commercial aluminium alloys AA6082 and AA7108, cast and homogenised using standard industrial practise were tested over the wide range of strain rates from 0.1 to 3000 strains per second (s−1) at three different temperatures. Specimens of cylindrical shape were cut from planar extruded sections and tested in uniaxial compression, as shown in Fig. 1. Compression axis was parallel to the extrusion direction. Specimens dimensions were: 10 mm diameter and 7.5 mm thickness for intermediate

Results and discussion

Following the experimental procedure described above, flow stress at 5% of plastic strain in compression was monitored over the wide range of strain rates at three different temperatures. Although it is a common procedure to assess the strain rate sensitivity by using the so-called steady state flow stress instead of flow stress at arbitrary chosen strain levels, there is a reasonable argument for not doing so here.

Firstly, at lower temperatures and higher strain rates, the inhomogenities in

Concluding remarks

The results obtained in this study show that strain rate sensitivity of AA6082 and AA7108 is very low. Since these are precipitation hardenable alloys, tested in the peak tempered and overaged condition, this is not surprising. A trend of negative strain rate sensitivity, seen at strain rates greater than 2000 s−1, could be a consequence of the strain localisation on a microscopic scale. Testing of different specimen geometries at high strain rates, yielded different stress–strain curves when

Acknowledgements

This work has been financed by Hydro Aluminium Metal Products. Thanks are due to Dr Einar Wathne for his special interest in this subject and constant support. Fruitful discussions and help in the field of extrusion technology we received from Antonie Oosterkamp. For useful input in the area of metallurgy we thank to Dr Trond Furu.

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    The work was carried out at Imperial College, Mechanical Engineering Department, London SW7 2BX, UK and Institut für Bildsame Formgebung, RWTH Aachen 52056, Germany.

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